CA1106044A - Scanning beam radio navigation method and apparatus - Google Patents
Scanning beam radio navigation method and apparatusInfo
- Publication number
- CA1106044A CA1106044A CA278,250A CA278250A CA1106044A CA 1106044 A CA1106044 A CA 1106044A CA 278250 A CA278250 A CA 278250A CA 1106044 A CA1106044 A CA 1106044A
- Authority
- CA
- Canada
- Prior art keywords
- sector
- signals
- angle
- scan
- antenna array
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S1/00—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith
- G01S1/02—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith using radio waves
- G01S1/022—Means for monitoring or calibrating
- G01S1/024—Means for monitoring or calibrating of beacon transmitters
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S1/00—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith
- G01S1/02—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith using radio waves
- G01S1/08—Systems for determining direction or position line
- G01S1/38—Systems for determining direction or position line using comparison of [1] the phase of the envelope of the change of frequency, due to Doppler effect, of the signal transmitted by an antenna moving, or appearing to move, in a cyclic path with [2] the phase of a reference signal, the frequency of this reference signal being synchronised with that of the cyclic movement, or apparent cyclic movement, of the antenna
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S1/00—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith
- G01S1/02—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith using radio waves
- G01S1/08—Systems for determining direction or position line
- G01S1/44—Rotating or oscillating beam beacons defining directions in the plane of rotation or oscillation
Abstract
SCANNING BEAM RADIO NAVIGATION
METHOD AND APPARATUS
Abstract of the Disclosure:
High accuracy guidance signals are produced over a wide angle by mechanically rotating a narrow angle electrically scanned transmitter through a series of narrow angular sectors while electrically scanning the transmitted signals within each sector.
METHOD AND APPARATUS
Abstract of the Disclosure:
High accuracy guidance signals are produced over a wide angle by mechanically rotating a narrow angle electrically scanned transmitter through a series of narrow angular sectors while electrically scanning the transmitted signals within each sector.
Description
1~27-F
~!'6~ `4 This invention relates to radio navigation systems em-ploying directional radio signals, and more particularly to radio guidance systems which are especially useful for aircraft.
In radio navigation systems for providing guidance signals, it is common to provide guidance signals which simply define a desired approach direction to a landing runway. How-ever it is very desirable to provide precise directional "bearing"
and glide path elevation information to an aircraft approaching an airport from any direction which gives the aircraft its precise angles of approach to a ground based transmitter at the airport. For azimuth, this requires 360 degree azimuth signal coverage. Providing 360 degree azimuth coverage with a high accuracy of signal resolution usually presents formidable problems, including potentially high cost and elaborate antenna structures.
Accordingly, it i5 one object of the present invention to provide a very accurate guidance signal which is availa~le over a wide angle, and which may encompass the entire 360 degrees of azimuth with a high resolution and with a simple and very economical structure.
The invention is carried out by a method of providing accurate directional information to an aircraft in any angular position about a predetermined axis over a wide angle sector with respect to a ground station comprising transmitting radio guidance signals in a sequence of separate electrically scanned `4~
sector signals in limited angle sectors within said wide angle sector, the sector signals being transmitted by mechanically rotating about said predetermined axis a ground station trans-mitter antenna array which is capable of producing dlrectional radio guidance signals by an electrical scan through only one of said limited angle sectors for each rotational position thereof, and transmitting from the antenna array said sequence of separate electrically scanned sector signals during successive mechanical rotation through a plurality of separate rotational positions, transmitting radio signals in conjunction with each of said sepa-rate electrically scanned sector signals to uniquely identify the particular sector, the identification radio signals transmitted in conjunction with each of said electrically scanned sector sig-nals being operable to uniquely identify the rotational position of said antenna array corresponding to a particular sector, the number of said separate sectors at which electrically scanned sector signals are emitted being sufficient to cover all angular positions within said wide angle sector by the combination of all of the separate electrically scanned sector signals.
2Q The invention may further provide that said predetermined axis is a horizontal axis and said angular positions are elevation angles.
Alternatively, the invention may provide that said pre-determined axis is a vertical axis and said angular positions are azimuth angles.
In another aspect, the invention may further provide that said wide angle sector encompasses the entire 360 degree azimuth.
In still another aspect, the invention may further pro-vide that said radio signals transmitted in conjunction with each ~ ~'J~6~
of said electrically scanned sector signals to uniquely identify the rotational position of said antenna array are transmitted prior to the transmission of the associated electrically scanned sector signals as a part of an information preamble to the sector signals, and wherein the information preamble also includes other identifying data distinguishing the wide angle sector angular position function from other functions for which similar signals are transmitted.
The invention may further provide that the information preamble is transmitted by binary digital coding of the carrier signal.
In another aspect, the invention may further provide that said rotational position identification signals are trans-mitted over a limited sector generally corresponding to the sector covered by the sector signals.
In still another aspect, the invention may further pro-vide that additional radio signal energy is transmitted in clearance beams and in side lobe suppression signals in order to enable the receiver to discriminate between valid sector signals for the sector in which the receiver is positioned and sector signals radiated for other sectors.
The invention may further provide that the scanned sector signals are each emitted as a time reference scan group including a "to" scan and a "fro" scan, with the angular position of the receiver being determined on the basis of the time interval between reception of the "to" scan and "fro" scan signals.
In addition to the features of the preceding paragraph, the invention may provide that the electrical scanning of the sector signals is carri~d out in a substantially circular sector and in which the effective axis of the electrical scan is displaced .~' from the axis of mechanical rotation of the antenna array in order to provide for multipath discrimination as the signals are received.
The invention may further provide that the electrical scanning rates of the "to" and "fro" scans are different to pro-vide rotational scan rate compensation for the mechanical rotation of the antenna array so as to provide "to" and "fro" scans having the same absolute rate of rotation in the two different scan direc-tions.
In addition to the features of the preceding paragraph, the invention may provide that the "to" and "fro" scans are posi-tion compensated so that the angles at which the electrical scans respectively for "to" and "fro" are commenced at different angles with respect to the rotating antenna array in order to compensate for the mechanical displacement of the mechanically rotating antenna array in the interval between the commencement of the "to"
scan and the commencement of the "fro" scan.
In another aspect, the invention may further provide that the electrically scanned sector signals are scanned by the Doppler method wherein the radiated signal is switched in sequence from one end to the other of an array of radiating elements to convey the impression of radiation from a moving source which moves from one end to the other of the array, and which creates a frequency shift in a receiver dependent upon the orientation of the receiver with respect to the array.
Additionally, the invention may provide for receiving said directional radio guidance signals with a receiver in an aircraft to be guided, storing the true angular position of the receiver relative to the ground station as indicated by at least the last previous radio guidance signal, and processing the next subsequent guidance signal by decoding the sector identification .~
/~ ' information associated with that guidance signal, subtracting the characteristic sector angle associated with the identified sector from said stored true relative angular position and using the difference to set angle limit txack gates indicating the angle limits within which a valid sector scan signal should appear within the identified sector, then using said track gate limits to confirm the validity of the subsequent sector scan signal indicating the angle within the identified sector, and then adding to the confirmed sector scan signal the characteristic sector angle of the identified sector to provide the txue relative angular position as an output to the operator.
The invention may further provide for receiving said directional radio guidance signals with a receiver in an aircraft to be guided, storing the true angular position of the receiver relative to the ground station as indicated by each of a repre-sentative plurality of said signals received in sequence, computing an average value of said true relative angular positions, setting said receiver into a track mode, then processing the first subse-quent guidance signal by decoding the sector identification infor-mation, subtracting the characteristic sector angle associatedwith the identified sector from said average value and using the difference to set angle limit track gates indicating the angle limits within which a valid sector scan signal should appear within the identified sector, then using said track gate limits to confirm the validity of the subsequent sector scan signal indi-cating the angular position within the identified sector, and then adding to the confirmed sector scan signal the characteristic sector angle of the identified sector to provide the true relative angular position as an output to the operator.
4~
In addition to the features of the preceding paragraph, the invention may provide that second and later subsequent guid-ance signals are processed by storing the last previous true relative angular position output and using that stored output in substitution for said average value in setting the angle limit track gate after subtracting the characteristic sector angle of the identified sector.
The invention may further provide that the scanned sector signals are monitored by a fixed monitor receiver which is arranged to receive scanned sector signals in only one of the antenna sectors.
The invention may be carried out by apparatus for pro-viding accurate orientation information to an aircraft in any angular position over a wide angle sector with respect to a predetermined axis at a ground station comprising a ground station including a transmitter and an associated antenna array ~hich is capable of producing directional radio guidance signals by an electrical scan through only a limited angle sector, plat-~ form means for supporting and mechanically rotating said antenna ~ 20 array about said predetermined axis to thereby change theorientation of said antenna array, said ground station being capable of transmitting radio guidance signals from said antenna array in a sequence of separate electrically scanned sector signals during successive mechanical rotation through a plurality of different rotational positions, the number of said different rotational positions at which electrically scanned sector signals are emitted being sufficient to cover all anyles within said wide angle sector by the combination of all of the electrically scanned sector signals, said apparatus including means for trans-~, .
6~
mitting radio signals in conjunction with each of said elec-trically scanned sector signals to uniquely identify the rota-tional position of said antenna array corresponding to a particular sector.
In addition to the features of the preceding paragraph, the invention may provide that all of the directions about said axis are divided into a predetermined number of fixed sectors having predetermined sector width, and wherein there is provided a means for detecting the rotation of said antenna array through said different sectors, said transmitter being connected to said last-named means and being operable in response to detection of the rotation of said antenna array from one sector to another to initiate a sector scan signal for the new sector.
In addition to the features of the preceding paragraph, the invention may provide that said transmitter is rotatable with said antenna array, and wherein said means for detecting the rotation of said antenna array from one sector to another comprises a photo responsive pick-up means rotatable with said transmitter and said antenna array ~nd a cooperating fixed optical track having optical characteristics which are different at different sectors.
In the accompanying drawings:
FIG. 1 is a schematic top view of a transmitter with an associated antenna arrangement mounted upon a rotatable platform for carrying out the present invention.
FIG. 2 is a reduced scale representation of the appara-tus of FIG. 1 showing the individual sectors electrically scanned by the apparatus and illustrating the arrangement of a monitor for the apparatus.
FIG~ 3 is a schematic representation of an aircraft re-ceiver for receiving and processing signals from the ground station of FIGS. 1 and 2.
FIGS. 4A and 4B together form a flow chart, sometimes referred to collectively as FIG. 4, illustrating the logical sequence of steps in the operation of the receiver of FIG. 3.
FIG. 5 is a schematic representation of an alternative : 10 embodiment of the transmitter and antenna arrangement which is employed for scanning signals.using Doppler principles.
Referring more particularly to FIG. 1 there is illus-trated a narrow sector scanning beam antenna array mounted for rotation upon a platform indicated at 20. By mechanically rotating this narrow sector antenna array and electrically scanning individual sectors during mechanical rotation the entire 360 degree azimuth range is effectively covered by the combination of the:signals of all of the individual sector scans.
The antenna array includes a reflector 22 which is a sector of a horizontally aligned cylinder for shaping the beams in elevation and a plurality of antenna elements direct-ing beams to the reflector 22 for subse~uent reflection and us~e by aircraft receivers. The antenna elements may include a phased array 24 for producing a beam which is phase scanned ' --,3' --1527A ~ 44 in azimuth,cleaxance elements 26 and 28 for providing clearance signals, side lobe suppression ~lements 30 and 32, and a sector omni horn radiator 34. Tl1ese elements are all connected to receive signals from a transmitter 36.
Because of the geometry of the antenna s~ructure, the apparent center of radiation of the phase scanned beam is at a point 37 behind the reflector 22 and offset from the center of rotation of platform 20. This is significant and is referred to more fully below.
As the entire array is rotated to cover different sectors, the sector omni horn 34 emits a signal for each sector which identifies that sector to the receiver and which may t me-share the same carrier signal used for the navigation signals.
In a preferred signal format the signals emitted from the sector omni horn 34 are digitally coded by simply keying the carrier by phase reversals to provide a group of binary digital bits identifying ~he ~60 degree azimuth function and also iden-tifying which sector the antenna platform 20 and the antenna array has been mechanically rotated to. The entire 360 degree azimuth is preferably divided into a number of predetermined sectors.
me sector identification is preferably a coded representation of the particular sector which must be decoded by the receiver.
m e data just mentioned above is followed by the navi-gational signal information which may be transmitted preferably by a simple to and fro scanning beam arrangement in which the aircraft receiver determines its position by measuring the time interval between successive receptions of the scanning beam on the to and fro scans. This system i~?
_ ,~ _ ~6~44 of scannin~ is disclosed for instance in connection with ~IG. 7 of U.S. Patent 3,757,337 issued September 4, 1973 on an invention by George Lichford entitled "Aircraft Instru-ment Landing System." The signal format described above ~including the to and fro scan) has been proposed for use in a new microwave landing system which is presently under development by the United States Federal Aviation Adminis-tration of the Department of Transportation. The proposed new system is described in various publications, including a report dated December 1975 entitled "Time Reference Scanning Beam Microwave Landing System" issued by the U.S.
Department of Transportation, Federal Aviation Administra-tion and submitted to the International Civil Aviation Organization. This report is hereinafter referred to simply as "the FAA reportl'. Details of the present system are pre-; ferably carried out in accordance with the teachings of the FAA report.
In the present invention, in order to provide to and fro scans of substantially equal net velocity, the elec-tronic scanning s~eed in the "fro" direction, opposi~e to the direction of mechanical rotation, is increased to compen-sate for the subtraction in scan velocity due to the mechanical rotation. By this means, the net rotational scan rates in the two directions tthe "to" direction in which the mechanical rotation adds to the electrical scan, and the "fro" direction in which the mechanical rotation subtracts from the electrical scan) are made to be equal, and thus no error is introduced `6~
into the to and fro scan information. Rotational position compensation is also introduced to compensate for the dif-ference in rotational position of the antenna array from the beginning of the "T0" scan to the beginning of the "FR0" scan.
The signals are then directly analogous to those which would have been emitted from a stationary electrical sector scan antenna which scans electrically at equal speeds in the to and fro directions.
By designing the sector scan antenna (F~G. 1) for a limited scan angle such as, for instance, plus or minus 16 degrees ~a total of 32 degrees in azimuth), the antenna design may economically produce high resolution signals.
The scanning beam is preferably quite narrow in azimuth, being in the order of 3 degrees wide.
With a 32 degree scan, it is practical to cover the entire 360 degree azimuth range with a total of sixteen 22.5 degree sectors. These scan sectors are indicated sche-matically in FIG. 2 of the accompanying drawing~ and respec-tively numbered 1-16. I'he entire group of signals for a particular sector scan, is sometimes referred to hereinafter as simply~"sector signal", or a radio guidance signal. This entire group of signals may also be referred to as carrying a "set of guidance information" for a particular sector.
As explained more fully below, the sector signal from the transmitter enables an aircraft receiver to determine the true bearing of the aircraft based upon the resultant re-ceived signal(set of signal information).
,~_ ~527 6~4~
In the ~ield of one of the sectors lO there is pro-vided a monitor 41 including a plurality of monitor antennas 38 arranged to pick up the signals at various angles within that sector. By monitoring the scanning beam within the single sector lO on every mechanical revolution of the antenna array, su~stantially continuous monitoring of the operation of the system is accomplished in a very simple manner. Since each sector scan overlaps the boun~aries of its sector into the adjacent sectors, provision is made for monitoring outside the boundaries of sector lO by means of monitor antennas 39 .
The correct sector position signal emitted by the horn 34 for each individual sector is preferably obtained by : means of a switching device which detects the rotational position of the antenna platform 20.
This switching device is illustrated as a photoelectric pickup 40 in FIG. l which is electrically connected to the ;~ transmitter 36 and mechanically mounted for rotation upon the platform 20. As schematically indicated at 42, the photoelectric pickup 40 is arranged to pick up optical sig-nals from a stationary non-rotating optical track 44. Track 44 has alternating light and dark sectoLs corresponding to the sectors 1-16. Accordingly, as the platform 20 rotates, the photoelectric pickup cell 40 indicates, by a transition between a light and a dark sector of track 44, whenever a new sector is entered .
The track 44 and optical pickup 40 are shown schematically ,~ ~
1~6~
in FIG. 1 at the periphery of the platform in order to illustrate the principles of operation in a simple plan view. For depend-able operation, a light source would be required in conjunction with this optical pickup device, but is not shown. Furthermore, the optical track 44 and the optical pickup device 40 are prefer-ably arranged beneath the platform 2Q, rather than around the periphery of the platform, as schematically shown in the drawing.
It is an important advantage that the sector omni horn 34 only needs to cover the sector currently in use, with a much lower total power input than would be required with a unitary (non-sectorized) 360 scan having a 360 omni signal radiator.
The platform 20 is driven by an electric motor indicated at - 46 through a drive shaft schematically indicated at 48, a worm gear indicated at 50, and a cooperating worm wheel 52 which is attached to the underside of the platform 20. Preferably, the drive motor 46 is a constant speed motor, such as an alternating current synchronous motor. With such an arrangement, it is preferred that the signals provided by the photoelectric pickup device 40 and the photo optical track 44 in response to rotation of the platform 20 are used as the clock signals for the system.
Thus, the entire system functions in synchronism with the mechan-ical rotation of the plat~orm 20. Where the 360 azimuth function time-shares the use of the same radio carrier frequency with other navigation signal functions, the time intervals used by all of the functions are clocked to be in synchronism with the rotation of the platform 10.
Alternatively, if it is not convenient to synchronize the entire system in response to rotation of the platform, - a variable speed motor is used for motor 46, and the motor ~6~4~
speed is continuously adjusted in order to synchronize the rotation of the platform with the timing clock of the sys-tem.
While fixed predetermined angular sectors are preferred, it is also possible to employ additional optical pickups, such as pickup 40, and additional optical tracks such as track 44 to provide digitally coded indications of the exact angles of rotation of the platform 20 ~within the resolution provided by the smallest subdivisions on the optical tracks) and to operate the system without fixed sectors, determining the sectors on the basis of control by the system clock, rather than on the basis o~ the position of plat~orm 20.
However, the fixed angle sectors are preferred because of the resultant simplification of the system, and particularly of - the receiver.
While varicus speeds o~ operation are possible, it is proposed to have as many as three complete 360 degree azimuth coverages per second. This requires a mechanical rotational speed of 180 rev~lutions per minute for the platform 20.
For applications where a lower data rate is permissible, the mechanical speed can be correspondingly decreased.
With the above mentioned mechanical rotational speed of 180 revolutions per minute, and typical sector scan cycle times for the individual sector scans having a total inter--val of about 11-2/3 milliseconds (consistent with the proposed back azimuth function timing in the above mentioned .-FAA report), the "dead" time interval between individual sector scans is approximately 9-1/6 milliseconds. Thus, "~
6~4~
these "dead" time intervals may be employed for other navi-gation functions,such as elevation for instance. If de-sired, more frequent sector scans may be provided on some sectors than ~>n others where some sect~rs aLe more important because they serve a particular runway. This can be done by increasing the platform speed, reducing the dead time between scans, and then scanning the most important sectors on every rotation, and scanning the other sectors only on every other rotation of the platform.
The monitor 41 is preferably provided with a signal or signals, on a connection indicated at 54 in FIG. 2 indicating when the platform 20 has been rotated into posi~ion to transmit signals into the sector or sectors being monitored.
This sector signal may preferably be provided by means of a photoelectric pickup 56 which can pick up optical signals reflected frsm a track ~not shown), which rotates with the platform, having sectors of different colors or differ-ent reflectivity. Each of the monitor antennas 38 and 39 may operate a separate receiver channel within the monitor 41 following along the lines of the receiver described below in connection with FIGS. 3 and 4. For reasons of economy, adequate monitoring may be obtained by employing a single receiver channel within the monitor_l, and successively switching that receiver to the different antennas 38 and 39 to monitor scans on successive rotations of the platform 20.
A receiver which is ~uitable for incorporation with the present invention is illustrated~schematically in FIG.
~6~44 3, and a flow chart illustrating the operation of the re-ceiver, in greater de~ail, is illustrated in FIG. 4.
The construction of the receiver, in many basic res-pects, conforms with the receiver proposed in the FAA report.
Referring more specifically to the receiver as illus-trated in FIG. 3, the receiver includes an antenna sche-matically shown at 57 which receives the scanning beam signals, which are transmitted preferably at C-band micro-wave frequencies. Antenna 57 supplies those signals to the early sta~es of the receiver indicated at 58, which include radio frequency amplifiers, filters, and intermediate frequency circuits. These circuits amplify the received signal, filter out unwanted adjacent signals, and convert the microwave signal to an intermediate frequency signal at connection 59, and a "video" signal at 60. A local oscilla-tor frequency synthesizer is provided, as indicated at 61, which supplies suitable local oscillator frequencies to the mixers within 58 for accomplishing the frequency conversions.
The intermediate frequency signal on connection S9 is supplied to a data processor circuit 62 which detects and decodes the digital parts of the signals, usually consisting of the digital preamble to each set of navigation signals.
The video signal is supplied on connection 60 to the acquisition and validation circuits 63 and to the envelope processor 64. The acquisition and validation circuits 63 assess the quality of the navigation signal and thereby de-termine whether or not the resulting data is to be displayed and used. The envelope processor 64 precisely determines .~ . .
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1527 ~ 44 when the "to" and "fro" beams scan by the aircraft. The resultant signals are supplied to an angle decoder 65 which determines azimuth angle based upon the time interval between the ''to" and "fro" scans. Such angle determination signals appear at the output 66, and are ultimately gated, such as through the gate 67 to indicators or angle data utilization devices, such as the navigation azimuth angle indicator shown at 68.
The time gate tracker circuit 69, which is connected between the angle decoder 65 and the envelope processor 64 designates which of the "to" and "fro" scans are to be processed.
All of the circuits of FIG. 3 described above are pre-ferably used in common for a number of different navigation information signal functions, in addition to those of the present invention.
When a 360 degree azimuth function scan group of "to"
and "fro" scans is detected by the data processor 62 by means of the digital data preamble accompanying those scans, a resultant signal appears at the data processor output con-nection 70 to a logic AND gate 71. When -the acquisition and validation circuits 63 determine that all of the other conditions necessary for valid reception of data exist on a particular data cycle, an output signal appears on connec-tion 72, providing a second input to the AND gate 71, causing the delivery of a signal from ~hat AND gate to a counter 73. The data cycles are repeated in a sequence, over and over, and include interspersed data scans for different ;. , :IX~6~4~l functions. However, an entire series of data cycles is re-peated with considerable rapidity, with one or mDre localizer scans included in each series. The reliability of the data is checked a number of times, on a number of separate localizer scan cycles so as to increment the count stored in the counter 73 up to a level above a predetermined count, before the guidance information is actually used. When that count, or any count above that level, is achieved, it is detected by a decoder 74 which provides an output at con--~ 10 nection 75. The resultant output signal at connection 75 operates visible flag device 76 which forms a part of the navigation indicator 68, and tells the pilot of the aircraft that true data is available. The signal on connection 75 ` from the decoder 74 is also supplied to open the gate 67 to allow the navigation information from the angle decoder 66 to be supplied to the indicator 68.
If the aircraft is too far away from the ground sta-- tion to receive navigation signals of sufficient strength, the acqulsition and validation circuits will not incre-ment the counter 73, and the flag 76 will not be operated so that the pilot will know that he does not have reliable navigation information. When "operated", the flag is pre-ferably withdrawn from view. Thus, if the flag is not operated, it is visible to the pilot. If the signals are marginal, and fading in and out, so that the counter 73 is incremented occasionally, but not consistently, the counter 73 will never reach the required count because the counter 73 is automatically counted down from time to time to keep /~
,~
marginal signals, which are only occasionally validated, from being used.
The data processor 62 not only determines the presence of the 360 degree azimuth function, it also identifies, from the preamble, which sector of the 16 sectors the particular signals are coming from. This information is available from the data processor 62 on a connection 79 and is effective to store an angle which is characteristic of the identified sector in a storage device indicated at 77. Preferably, this characteristic sector angle is a representation of the center line of the iden-tified sector in terms of a true bearing. Since the output angle ;`~
information at indicator 68 provides 360 degree azimuth informa-tion, the indicator also preferably indicates the azimuth bearing ~ in the form of a true bearing related to North. Thus, North is ;~ zero, East is 90 degrees, South i5 180 degrees, West is 270 degrees, etc. Since the navigation angle information at connec-tion 66 from the angle decoder 65 is only an indicator of an ~; angle within the identified sector, tsometimes referred to here-, .
. , ` inafter as the "scan angle") that output must be added in the adder 78, with the characteristic identified sector angle from store 77 in order to provide the true bearing required for indicator 68.
The receiver is actually much more complicated than indicated by the preceding description related to FIG. 3. It is believed that the details of the receiver are most easily comprehended by presentation of the functions of the receiver in _ ,~ _ ;
6~14 a flow chart. Accordingly, FIG. ~ is presented as a flow chart indicating the details of the receiver operation. FIG. 4 refers collectively to FIG. 4A and FIG. 4B combined as shown in the drawing labeled FIG. 4.
FIG. 4 is a flow chart illustrating the fucntional logic operation of the aircraft receiver in receiving and processing the signals transmitted from the ground station as represented by FIGS. 1 and 2. The incoming information at 82 in the upper right hand corner of the diagram, representing a single set of signals, first has a digital preamble, which begins with a "Barker" code, decoded at 84, in the presence of a "ready" con-dition as indicated at 86. The decoded preamble information is then checked for the ~unction identification at 88, and that information is checked at 90 to determine whether or not the set of information relates to the 360 azimuth function. If not, as signified by branch 92, other subroutines are followed relating to other signal functions as indicated at 94. If the 360 azimuth function is positively identified, then the sector angle informa-tion is decoded, as indicated at 96, to determine which of the sixteen sectors the signal infoxmation came from.
Next, the signal is tested, as indicated at 97, to deter-mine if it is acceptable, that is, to determine whether it is stronger than the side lobe suppression signals and the clearance pulses. The side lobe suppression signals are strong enough in all of the other fourteen or fifteen sectors to cause a rejection, at this point of all signals except those for the sector or sectors at which the receiver is located. If the answer is "No"
trejection), the signal is to be ignored, as indicated at 99, and not processed further. Accordingly, "ready" information is sent 6~'44 back as indicated by the connection to 86. On the other hand, if the answer is "Yes", the next processing of the information is determined at 98 based upon whether or not it has previously been determined that the entire receiver is operating within an "in track" mode, meaning that it has been determined, as further explained below, that a sequence of consistent valid signals has been received. If the receiver is "in track", the next test is, as indicated at 101, to determine if the system was in "track"
mode during the processing of the last previous signal. If the answer is "yes", the last previous output angle information is to be used for setting the track gates, as indicated at 103. The supplying of that angle information is indicated by the dotted line 105 from the output box 112 described further below. The last output angle information is in the form of an absolute cor-rected relative bearing of the receiver with relation to the transmitter. Next, as indicated at 109, the sector angle is subtracted from the angle value which is to be used to set the track gates. The sector angle information i5 supplied, as indi-cated by the dotted line 111, from the decoded sector angle as determined at the previously described step 96. The resultant remainder represents the expected scan angle within the sector being scanned. Next, the remainder angle is used, as indicated at 102, to set the txack gates.
However, going back to box 101, if the system was not in the "track" mode during processing of the last previous signal, then there will be no valid output angle at 112. Accordingly, as indicated at 107, an updated average of previously taken tentative angle measurements is to be used to set the track ~! 1527 1~6~4~
gates. This average value is available, as indicated b~ the dotted ; line input at 113 from an average measured angle determination as indicated by box 144, to be described more fully below.
The updated average angle used at 107 is also in the form of an absolute corrected compass bearing, and the sector angle is also subtracted from this heading, at 109.
After the track gates are set at 102, either on the basis of the last output, or on the basis of the updated average, the scan angle is decoded, as indicated at 104, to determine the actual scan angle within the designated sector. Then a test is made at 106 to determine whether the scan is within "track", based upon whether or not the scan is within the preset tracking gate limits.
The tracking gates are set on the basis of the scan angle within the sector scan range. While out of sector signals are rejected at 97, the tracking gates provide another discrimination function, for these gates cannot be set outside the normal sector scan range.
If the result is "Yes", at branch 110 the sector angle is added to the scan angle at 108 and the result is supplied as the output angle information at 112 for indication of the bearing of the aircraft relative to the ground station to the operator of the aircraft. This also results in "ready" information provided at con-nection 114 to again ready the s~stem at 86 for processing of new information.
It is one of the most important features of the inven- -tion that because of the logic set forth above, the system is capable of dealing accurately with successive sets of scanning information which may be derived from scans from different sectors. This follows from the fact that the output angle '~ _ ~, ' 4~
information available at 112 is in the foxm of an absolute compass bearing related to a single reference direction, such as north, and that output is used at 103 as the data ~or setting the track gates after subtracting the unique sector location angle for the sector within which the current scan occurs (at 109). Thus, the track gates are set, at 102, at limits indicative of where the sector scan should appear in relation to the center of the currently scanned sector, even though the currently scanned sector may be different ~rom the previously scanned sector ~h~ch established the output angle infoxmation available at 112. Thus, the aircraft carrying the receiver can fly from one sector to another, without losing "track", and while maintaining up-to-date, and constantly updated, angle-bearing information.
Referring back again to box 106, the indication of a valid scan within track at connection 110 also serves, as indi-cated at 116, to rein~orce the information that the receiver is "in track" by incre~enting a ~alidity counter. On the other hand, if it is determined that the scan is not within track, at 115, then the validity counter is decremented, as ind~cated at 118. Also, the signal is ignored, as indicated at 117 and ready infoxmation is sent back, as indicated at 114 to ready block 86.
A determinat~.on is then made as to whether the validity count is great~r than the minimum, as indicated at 120. If the answer is "Yes", an indicatox flag is held back, indicating to the operator that valid signals are being received, as signified . _ ., .~?, -- ,W--6~44 at 122. Also, the receiver is maintained in the "in track"
status, as signified by box 123. If the answer is "No", then the indicator flag is shown, as signified at 124. Also, as signified at box 125, the recei~er s~stem drops the "in track"
status. From the above, it is ev~dent that the indicator flag, which is visible to the aircraft operator, generally indicates whether or not the recei~er is in the "in track" status.
Referring back again to the box 118 indicating the operation "decrement validity counter", there is preferably provided a timer 119 which provides validity counter decrement pulses at timed intervals. Thus, in circumstances where val~d signals are received with insufficient frequency, the timer 119 will operate to decrement the val~dity counter, even in the absence of invalid (out of track~ signals, so as to bring the validity count below the minimum value required for maintenance o~ the "in track" status. Thus, the "in track" status will be dropped.
If the system is not in the "track" mode, it must operate in a signal acquisition mode. This operation is deter-mined, in terms of the logic flow chart, by the test previouslydescribed back at block 98. Thu~, ~f the result of the test at 98 is "No", ~ndicating that the receiver is not "in track", that information appears at 128, and then the track gates are set to a wîde aperture, as indicated at 138, the scan angle within the sector is decoded, as indicated at 140, the sector angle is added to the scan angle, as indicated at 142, and o ~
~ 1527 ~1~6~44 then the resultant sum, which is the absolute relative beariny angle measurement,is averaged with previous angle measurements as indicated at 144. The number of samples entering into the average, and the average value, are stored, as indicated at 146.
Next a decision is made as to whether a sufficient number of signals have been received within a prescribed tim~ interval to accumulate an acceptable average angle value, as indicated at 146. This test is carried out on the basis of the recorded number of previously received signals which have been averaged, as indicated by connection 162 from box 146. If the answer is "No", the "not in track" status is maintained, as indicated at 148, and information is supplied, as indicated at 114 to ready : the system, at 86, for the next information.
; If the average is determined to be valid, as indicated by a "Yes" answer at connection 152,~then the "in track" flag is set, as indicated at 154, and the ready information is again sent on connection 114 to ready box 86.~ The output information from box 144 also serves to reset a timer 158. If the valid average condition is not updated with sufficient frequency by informa-tion from box 144, the timer 158 times out, and through a connec-tion 160, causes the previously stored average to be dumped from storage. The system must then begin a new attempt to build up a : valid average.
Upon the setting of the "in track" status, as indicated at box 154, the validity counter is set to a predetermined initial value above the minimum value. This is indicated by the dotted connection 156 from box 154 to box 120. `
~ , . .
66~4~
Referring again to FIG. 1, one of the important advan-tages of the invention is that there is inherently provided multipath discrimination in the signals because of the feature that the center of mechanical rotation of the antenna array is different from the apparent center of radiation 37 (the apparent center of the scan of the scannin~ beam signal).
The term "multipath", as used in the above statement, refers particularly to signals which may be reflected from other fixed or movable objects near the transmitter, and which create 1~ additional signal paths for the transmitted signals to the receiver, with the possible effect of confusing the recei~er as to the correct directional information imparted by the signals. The multipath discrimination arising from the lack of concentricity of the mechanical rotation and the center of radiation arises from a space diversity effect (also sometimes referred to as a motion averaging effect~ which has the result of reducing the amplitude of any spurious multipath signals arising from any one signal reflective object, or any other one cause of multipath signals.
, .
~1~6~4 ~ _ . . . ~
As mentioned aboYe, one of the most important features of the invention is in the ability of the system to permit the aircraft which i5 being guided to fly from one sector to another without losing "track", and while continuously receiving guidance signals. This is made possible, in part, by the substantial overlap between the sector scan signals of adjacent sectors.
Thus, in the exf~mple given earlier in connection with FIG. 2, the individual sectors ma~ be, for instance, 22-1/2 degrees in angular width, whereas the indi~idual sector scans may be in the `:
10~ order of 36 degrees in width. Thu3, there may be as much as 13-1/2 degrees of overlap of adjacent sector scans.
:~ : As a specific example of what this overlap means, . particularly in relation to the operation of the receiver, suppose the aircraft, with its receiver, is located at the boundary be-tween sectox 2 and sector 3 tn FIG. 2. This corresponds to a true bearing of 45 with respect to the transmitter, assuming that the boundary between sectors 1 and 16 represents a true bearing of true North~ Assumlng a sector scan in sector 2, an~
referring back to FIG. 4, at box 96, the sector angle will be 20 decoded as the center line angle for sector 2, which is 33-3/4 degrees. Later, when the scan angle is decoded at 104, that scan ~1~6~
angle is detected as 11-1/4 degrees. When those two angles are added at 108, the resultant output angle, the true bearing, is correctly 45~ Then, if sector 3 is next scanned, the charac-teristic sector angle for sector 3 is decoded at 96 as the sector 3 center line bearing angle which is 56-1/4 degrees.
Later, when the scan angle is decoded at box 104, that scan angle is minus 11-1/4 degrees. Thus, when the two angles are added at box 108, the correct output angle (true bearing~ is again available at 112 which is 45. Thus, the sector signals for both of the adjacent sectors are available to provide a true bearing output. Furthermore, since the true bearing output is available at 103 for determining the setting of the track gates, the track gates are correctly adjusted for either sector by subtracting the correct sector angle at box 109. Thus, whether the aircraft receiver remains on the 45 true bearing, or whether it flies away from the 45 bearing towards the center of either sector 2 or sector 3, there is a continuity in tracking, and directional bearing in~ormation is continuously available to the operator of the aircraft.
Thus far, the invention has been described in terms of an implementation with "to and fro" scanning. However, the principles of the invention are also applicable to other scanning methods. For instance, the scanning of individual sectors may be accomplished by means of a so-called Doppler system as illustrated, for instance, in FIG. 5 of the drawings.
'- ~7d?
_"~-- 7 152i 6~
FIG. 5 is a simplified representation of a Doppler system for carrying out the present invention in which a rotating platform 2OA is provided to rotate a Doppler antenna array 166 with its associated transmitter 168 through different sectors. This Doppler s~stem may operate in a conventional Doppler mode in which transmitted signal radiations are switched in sequence from one radiation element to another and from one end of the array to another to create what appears to be a radiation signal from a source which is moving along the line of the array. A properly equipped receiver can then determine its bearing with respect to the line of the array based upon the apparent frequency shift due to the apparent movement of the source of the radiation along the line of the array. A sepa-rate antenna element, such as the element illustrated at 170, is provided for the purpose of issuing a standard frequency signal which has no apparent motion and which serves as a stan-dard for determination of the frequency shift due to the appar-ent motion of the slgnal from the arxay 166. Element 170 is preferably located at the center of mechanical rotation of platform 20A, but is not shown in that position in order to promote clarity.
- Insofar as applicable, all of the features described above in connection with the embodiment of FIGS. 1-4 are employed in this alternative Doppler embodiment of FIG. 5. Thus the system and the signal format are directly analogous to those used in the embodiment of FIGS. 1-4, with the exception that the .. - ~ ,~;,.
_ ~ _ scanning for each sector is carried out by the Doppler prin-ciple rather than by the "to and fro" principle.
Navigation systems employing the Doppler s~stem are well known, and accordingly, it is not believed to be necessary to further spell out the details of the embodiment of FIG. 5.
Furthermore, the details of the Doppler embodiment of ~IG. 5 may be carried out in a manner consistent with, and compatible with, a proposal entitled "DOPPLER MICROWAVE LANDING GUIDANCE
SYSTEM FOR A NEW NON-VISUAL PRECISION APPROACH AND LANDING
GUIDANCE SYSTEM" submitted by the United Kingdom to the International Civil Aviation Organization in November 1975.
As mentioned above, all of the details of the embodi-ment of FIG. 5 which are applicable may be carried out in accordance with the teachings of the embodiment of FIGS. 1-4.
For instance, detection of rotation of the platform 20A from one sector to another may be accomplished by means of a photo-electric pickup carried by the platform which detects different positions from a stationar~ optical track, as illustrated in FIG. 1. Furthermore, the Doppler signals may be monitored by a monitor receiver which is located to pick up the scanning signals within one predetermined sector.
In accordance with one of the features of the present invention, where a serious multipath signal problem is present in a particular sector on a permanent or frequent basis, and where it is not essential to have guidance signals within that sector, the regular guidance signals to that sector may be blanked out, and a special data meSsage~may be incorporated in .,j ~'' .
- ,a~ --'44 the preamble of a set of signal information which is sent out instead of the guidance signals telling the aircraft receiver that this is a blanked sector. A second flag may be provided in the visual angle indicator 68 of FIG. 3 (not shown~ to signal the operator of the aircraft that the aircraft is in a sector corresponding to a blanked signal. me aircraft operator will then know that he should change course to a new bearing, to fly into a sector where correct a~imuth information is availableO
This sector blanking arrangement for avoiding multipath problems is particularly applicable to the Doppler system.
While the disclosure of the embodiments of FIGS. 1 and 5 refers to a rotating platform 20 or 20A as a part of the physical structure, it is to be understood that the platform need not necessarily constitute a disc-like element. ~hus, it may be a completely open framework for rigidly supporting the rotating components in an arrangement such as could be provided by a solid platform, but without th~ need for providing a con-tinuous flat circular platform surface.
'rhe description of the invention given above has been entirely in terms of providing azimuth guidan~e signals for an aircraft with respect to a ground station. However, it is obvious that the invention is also useful for the purpose of providing elevation signals by using a platform for support of the antenna which _rQtates on a horizontal axis, and by providing sector scans generally about that horizontal axis.
In such an arrangement, since elevations only above the horizon are generally required, 180 degrees of total angular coverage will generally be the maximum required.
`44 In the foregoiny description, the angular position determined by the system is generally described as a "bearing angle". However, when used for elevation, this angle will generally be referred to simply as "elevation", or as the "elevation angle". In the material which follows, these angles are generically referred to as simply the "angular position".
The foregoing description is generally directed to providing a wide angle sector coverage which may include the entire 360 degree azimuth. However, it is obvious that the system also may be advantageously employed for coverage of any selected combination of sectors. As pre~iously mentioned above, when used for elevation, sectors may be selected which cover only the 180 degrees above the horizon. Similarly, when used in azimuth, the system need not necessarily cover the entire 360 degree azimuth.
~!'6~ `4 This invention relates to radio navigation systems em-ploying directional radio signals, and more particularly to radio guidance systems which are especially useful for aircraft.
In radio navigation systems for providing guidance signals, it is common to provide guidance signals which simply define a desired approach direction to a landing runway. How-ever it is very desirable to provide precise directional "bearing"
and glide path elevation information to an aircraft approaching an airport from any direction which gives the aircraft its precise angles of approach to a ground based transmitter at the airport. For azimuth, this requires 360 degree azimuth signal coverage. Providing 360 degree azimuth coverage with a high accuracy of signal resolution usually presents formidable problems, including potentially high cost and elaborate antenna structures.
Accordingly, it i5 one object of the present invention to provide a very accurate guidance signal which is availa~le over a wide angle, and which may encompass the entire 360 degrees of azimuth with a high resolution and with a simple and very economical structure.
The invention is carried out by a method of providing accurate directional information to an aircraft in any angular position about a predetermined axis over a wide angle sector with respect to a ground station comprising transmitting radio guidance signals in a sequence of separate electrically scanned `4~
sector signals in limited angle sectors within said wide angle sector, the sector signals being transmitted by mechanically rotating about said predetermined axis a ground station trans-mitter antenna array which is capable of producing dlrectional radio guidance signals by an electrical scan through only one of said limited angle sectors for each rotational position thereof, and transmitting from the antenna array said sequence of separate electrically scanned sector signals during successive mechanical rotation through a plurality of separate rotational positions, transmitting radio signals in conjunction with each of said sepa-rate electrically scanned sector signals to uniquely identify the particular sector, the identification radio signals transmitted in conjunction with each of said electrically scanned sector sig-nals being operable to uniquely identify the rotational position of said antenna array corresponding to a particular sector, the number of said separate sectors at which electrically scanned sector signals are emitted being sufficient to cover all angular positions within said wide angle sector by the combination of all of the separate electrically scanned sector signals.
2Q The invention may further provide that said predetermined axis is a horizontal axis and said angular positions are elevation angles.
Alternatively, the invention may provide that said pre-determined axis is a vertical axis and said angular positions are azimuth angles.
In another aspect, the invention may further provide that said wide angle sector encompasses the entire 360 degree azimuth.
In still another aspect, the invention may further pro-vide that said radio signals transmitted in conjunction with each ~ ~'J~6~
of said electrically scanned sector signals to uniquely identify the rotational position of said antenna array are transmitted prior to the transmission of the associated electrically scanned sector signals as a part of an information preamble to the sector signals, and wherein the information preamble also includes other identifying data distinguishing the wide angle sector angular position function from other functions for which similar signals are transmitted.
The invention may further provide that the information preamble is transmitted by binary digital coding of the carrier signal.
In another aspect, the invention may further provide that said rotational position identification signals are trans-mitted over a limited sector generally corresponding to the sector covered by the sector signals.
In still another aspect, the invention may further pro-vide that additional radio signal energy is transmitted in clearance beams and in side lobe suppression signals in order to enable the receiver to discriminate between valid sector signals for the sector in which the receiver is positioned and sector signals radiated for other sectors.
The invention may further provide that the scanned sector signals are each emitted as a time reference scan group including a "to" scan and a "fro" scan, with the angular position of the receiver being determined on the basis of the time interval between reception of the "to" scan and "fro" scan signals.
In addition to the features of the preceding paragraph, the invention may provide that the electrical scanning of the sector signals is carri~d out in a substantially circular sector and in which the effective axis of the electrical scan is displaced .~' from the axis of mechanical rotation of the antenna array in order to provide for multipath discrimination as the signals are received.
The invention may further provide that the electrical scanning rates of the "to" and "fro" scans are different to pro-vide rotational scan rate compensation for the mechanical rotation of the antenna array so as to provide "to" and "fro" scans having the same absolute rate of rotation in the two different scan direc-tions.
In addition to the features of the preceding paragraph, the invention may provide that the "to" and "fro" scans are posi-tion compensated so that the angles at which the electrical scans respectively for "to" and "fro" are commenced at different angles with respect to the rotating antenna array in order to compensate for the mechanical displacement of the mechanically rotating antenna array in the interval between the commencement of the "to"
scan and the commencement of the "fro" scan.
In another aspect, the invention may further provide that the electrically scanned sector signals are scanned by the Doppler method wherein the radiated signal is switched in sequence from one end to the other of an array of radiating elements to convey the impression of radiation from a moving source which moves from one end to the other of the array, and which creates a frequency shift in a receiver dependent upon the orientation of the receiver with respect to the array.
Additionally, the invention may provide for receiving said directional radio guidance signals with a receiver in an aircraft to be guided, storing the true angular position of the receiver relative to the ground station as indicated by at least the last previous radio guidance signal, and processing the next subsequent guidance signal by decoding the sector identification .~
/~ ' information associated with that guidance signal, subtracting the characteristic sector angle associated with the identified sector from said stored true relative angular position and using the difference to set angle limit txack gates indicating the angle limits within which a valid sector scan signal should appear within the identified sector, then using said track gate limits to confirm the validity of the subsequent sector scan signal indicating the angle within the identified sector, and then adding to the confirmed sector scan signal the characteristic sector angle of the identified sector to provide the txue relative angular position as an output to the operator.
The invention may further provide for receiving said directional radio guidance signals with a receiver in an aircraft to be guided, storing the true angular position of the receiver relative to the ground station as indicated by each of a repre-sentative plurality of said signals received in sequence, computing an average value of said true relative angular positions, setting said receiver into a track mode, then processing the first subse-quent guidance signal by decoding the sector identification infor-mation, subtracting the characteristic sector angle associatedwith the identified sector from said average value and using the difference to set angle limit track gates indicating the angle limits within which a valid sector scan signal should appear within the identified sector, then using said track gate limits to confirm the validity of the subsequent sector scan signal indi-cating the angular position within the identified sector, and then adding to the confirmed sector scan signal the characteristic sector angle of the identified sector to provide the true relative angular position as an output to the operator.
4~
In addition to the features of the preceding paragraph, the invention may provide that second and later subsequent guid-ance signals are processed by storing the last previous true relative angular position output and using that stored output in substitution for said average value in setting the angle limit track gate after subtracting the characteristic sector angle of the identified sector.
The invention may further provide that the scanned sector signals are monitored by a fixed monitor receiver which is arranged to receive scanned sector signals in only one of the antenna sectors.
The invention may be carried out by apparatus for pro-viding accurate orientation information to an aircraft in any angular position over a wide angle sector with respect to a predetermined axis at a ground station comprising a ground station including a transmitter and an associated antenna array ~hich is capable of producing directional radio guidance signals by an electrical scan through only a limited angle sector, plat-~ form means for supporting and mechanically rotating said antenna ~ 20 array about said predetermined axis to thereby change theorientation of said antenna array, said ground station being capable of transmitting radio guidance signals from said antenna array in a sequence of separate electrically scanned sector signals during successive mechanical rotation through a plurality of different rotational positions, the number of said different rotational positions at which electrically scanned sector signals are emitted being sufficient to cover all anyles within said wide angle sector by the combination of all of the electrically scanned sector signals, said apparatus including means for trans-~, .
6~
mitting radio signals in conjunction with each of said elec-trically scanned sector signals to uniquely identify the rota-tional position of said antenna array corresponding to a particular sector.
In addition to the features of the preceding paragraph, the invention may provide that all of the directions about said axis are divided into a predetermined number of fixed sectors having predetermined sector width, and wherein there is provided a means for detecting the rotation of said antenna array through said different sectors, said transmitter being connected to said last-named means and being operable in response to detection of the rotation of said antenna array from one sector to another to initiate a sector scan signal for the new sector.
In addition to the features of the preceding paragraph, the invention may provide that said transmitter is rotatable with said antenna array, and wherein said means for detecting the rotation of said antenna array from one sector to another comprises a photo responsive pick-up means rotatable with said transmitter and said antenna array ~nd a cooperating fixed optical track having optical characteristics which are different at different sectors.
In the accompanying drawings:
FIG. 1 is a schematic top view of a transmitter with an associated antenna arrangement mounted upon a rotatable platform for carrying out the present invention.
FIG. 2 is a reduced scale representation of the appara-tus of FIG. 1 showing the individual sectors electrically scanned by the apparatus and illustrating the arrangement of a monitor for the apparatus.
FIG~ 3 is a schematic representation of an aircraft re-ceiver for receiving and processing signals from the ground station of FIGS. 1 and 2.
FIGS. 4A and 4B together form a flow chart, sometimes referred to collectively as FIG. 4, illustrating the logical sequence of steps in the operation of the receiver of FIG. 3.
FIG. 5 is a schematic representation of an alternative : 10 embodiment of the transmitter and antenna arrangement which is employed for scanning signals.using Doppler principles.
Referring more particularly to FIG. 1 there is illus-trated a narrow sector scanning beam antenna array mounted for rotation upon a platform indicated at 20. By mechanically rotating this narrow sector antenna array and electrically scanning individual sectors during mechanical rotation the entire 360 degree azimuth range is effectively covered by the combination of the:signals of all of the individual sector scans.
The antenna array includes a reflector 22 which is a sector of a horizontally aligned cylinder for shaping the beams in elevation and a plurality of antenna elements direct-ing beams to the reflector 22 for subse~uent reflection and us~e by aircraft receivers. The antenna elements may include a phased array 24 for producing a beam which is phase scanned ' --,3' --1527A ~ 44 in azimuth,cleaxance elements 26 and 28 for providing clearance signals, side lobe suppression ~lements 30 and 32, and a sector omni horn radiator 34. Tl1ese elements are all connected to receive signals from a transmitter 36.
Because of the geometry of the antenna s~ructure, the apparent center of radiation of the phase scanned beam is at a point 37 behind the reflector 22 and offset from the center of rotation of platform 20. This is significant and is referred to more fully below.
As the entire array is rotated to cover different sectors, the sector omni horn 34 emits a signal for each sector which identifies that sector to the receiver and which may t me-share the same carrier signal used for the navigation signals.
In a preferred signal format the signals emitted from the sector omni horn 34 are digitally coded by simply keying the carrier by phase reversals to provide a group of binary digital bits identifying ~he ~60 degree azimuth function and also iden-tifying which sector the antenna platform 20 and the antenna array has been mechanically rotated to. The entire 360 degree azimuth is preferably divided into a number of predetermined sectors.
me sector identification is preferably a coded representation of the particular sector which must be decoded by the receiver.
m e data just mentioned above is followed by the navi-gational signal information which may be transmitted preferably by a simple to and fro scanning beam arrangement in which the aircraft receiver determines its position by measuring the time interval between successive receptions of the scanning beam on the to and fro scans. This system i~?
_ ,~ _ ~6~44 of scannin~ is disclosed for instance in connection with ~IG. 7 of U.S. Patent 3,757,337 issued September 4, 1973 on an invention by George Lichford entitled "Aircraft Instru-ment Landing System." The signal format described above ~including the to and fro scan) has been proposed for use in a new microwave landing system which is presently under development by the United States Federal Aviation Adminis-tration of the Department of Transportation. The proposed new system is described in various publications, including a report dated December 1975 entitled "Time Reference Scanning Beam Microwave Landing System" issued by the U.S.
Department of Transportation, Federal Aviation Administra-tion and submitted to the International Civil Aviation Organization. This report is hereinafter referred to simply as "the FAA reportl'. Details of the present system are pre-; ferably carried out in accordance with the teachings of the FAA report.
In the present invention, in order to provide to and fro scans of substantially equal net velocity, the elec-tronic scanning s~eed in the "fro" direction, opposi~e to the direction of mechanical rotation, is increased to compen-sate for the subtraction in scan velocity due to the mechanical rotation. By this means, the net rotational scan rates in the two directions tthe "to" direction in which the mechanical rotation adds to the electrical scan, and the "fro" direction in which the mechanical rotation subtracts from the electrical scan) are made to be equal, and thus no error is introduced `6~
into the to and fro scan information. Rotational position compensation is also introduced to compensate for the dif-ference in rotational position of the antenna array from the beginning of the "T0" scan to the beginning of the "FR0" scan.
The signals are then directly analogous to those which would have been emitted from a stationary electrical sector scan antenna which scans electrically at equal speeds in the to and fro directions.
By designing the sector scan antenna (F~G. 1) for a limited scan angle such as, for instance, plus or minus 16 degrees ~a total of 32 degrees in azimuth), the antenna design may economically produce high resolution signals.
The scanning beam is preferably quite narrow in azimuth, being in the order of 3 degrees wide.
With a 32 degree scan, it is practical to cover the entire 360 degree azimuth range with a total of sixteen 22.5 degree sectors. These scan sectors are indicated sche-matically in FIG. 2 of the accompanying drawing~ and respec-tively numbered 1-16. I'he entire group of signals for a particular sector scan, is sometimes referred to hereinafter as simply~"sector signal", or a radio guidance signal. This entire group of signals may also be referred to as carrying a "set of guidance information" for a particular sector.
As explained more fully below, the sector signal from the transmitter enables an aircraft receiver to determine the true bearing of the aircraft based upon the resultant re-ceived signal(set of signal information).
,~_ ~527 6~4~
In the ~ield of one of the sectors lO there is pro-vided a monitor 41 including a plurality of monitor antennas 38 arranged to pick up the signals at various angles within that sector. By monitoring the scanning beam within the single sector lO on every mechanical revolution of the antenna array, su~stantially continuous monitoring of the operation of the system is accomplished in a very simple manner. Since each sector scan overlaps the boun~aries of its sector into the adjacent sectors, provision is made for monitoring outside the boundaries of sector lO by means of monitor antennas 39 .
The correct sector position signal emitted by the horn 34 for each individual sector is preferably obtained by : means of a switching device which detects the rotational position of the antenna platform 20.
This switching device is illustrated as a photoelectric pickup 40 in FIG. l which is electrically connected to the ;~ transmitter 36 and mechanically mounted for rotation upon the platform 20. As schematically indicated at 42, the photoelectric pickup 40 is arranged to pick up optical sig-nals from a stationary non-rotating optical track 44. Track 44 has alternating light and dark sectoLs corresponding to the sectors 1-16. Accordingly, as the platform 20 rotates, the photoelectric pickup cell 40 indicates, by a transition between a light and a dark sector of track 44, whenever a new sector is entered .
The track 44 and optical pickup 40 are shown schematically ,~ ~
1~6~
in FIG. 1 at the periphery of the platform in order to illustrate the principles of operation in a simple plan view. For depend-able operation, a light source would be required in conjunction with this optical pickup device, but is not shown. Furthermore, the optical track 44 and the optical pickup device 40 are prefer-ably arranged beneath the platform 2Q, rather than around the periphery of the platform, as schematically shown in the drawing.
It is an important advantage that the sector omni horn 34 only needs to cover the sector currently in use, with a much lower total power input than would be required with a unitary (non-sectorized) 360 scan having a 360 omni signal radiator.
The platform 20 is driven by an electric motor indicated at - 46 through a drive shaft schematically indicated at 48, a worm gear indicated at 50, and a cooperating worm wheel 52 which is attached to the underside of the platform 20. Preferably, the drive motor 46 is a constant speed motor, such as an alternating current synchronous motor. With such an arrangement, it is preferred that the signals provided by the photoelectric pickup device 40 and the photo optical track 44 in response to rotation of the platform 20 are used as the clock signals for the system.
Thus, the entire system functions in synchronism with the mechan-ical rotation of the plat~orm 20. Where the 360 azimuth function time-shares the use of the same radio carrier frequency with other navigation signal functions, the time intervals used by all of the functions are clocked to be in synchronism with the rotation of the platform 10.
Alternatively, if it is not convenient to synchronize the entire system in response to rotation of the platform, - a variable speed motor is used for motor 46, and the motor ~6~4~
speed is continuously adjusted in order to synchronize the rotation of the platform with the timing clock of the sys-tem.
While fixed predetermined angular sectors are preferred, it is also possible to employ additional optical pickups, such as pickup 40, and additional optical tracks such as track 44 to provide digitally coded indications of the exact angles of rotation of the platform 20 ~within the resolution provided by the smallest subdivisions on the optical tracks) and to operate the system without fixed sectors, determining the sectors on the basis of control by the system clock, rather than on the basis o~ the position of plat~orm 20.
However, the fixed angle sectors are preferred because of the resultant simplification of the system, and particularly of - the receiver.
While varicus speeds o~ operation are possible, it is proposed to have as many as three complete 360 degree azimuth coverages per second. This requires a mechanical rotational speed of 180 rev~lutions per minute for the platform 20.
For applications where a lower data rate is permissible, the mechanical speed can be correspondingly decreased.
With the above mentioned mechanical rotational speed of 180 revolutions per minute, and typical sector scan cycle times for the individual sector scans having a total inter--val of about 11-2/3 milliseconds (consistent with the proposed back azimuth function timing in the above mentioned .-FAA report), the "dead" time interval between individual sector scans is approximately 9-1/6 milliseconds. Thus, "~
6~4~
these "dead" time intervals may be employed for other navi-gation functions,such as elevation for instance. If de-sired, more frequent sector scans may be provided on some sectors than ~>n others where some sect~rs aLe more important because they serve a particular runway. This can be done by increasing the platform speed, reducing the dead time between scans, and then scanning the most important sectors on every rotation, and scanning the other sectors only on every other rotation of the platform.
The monitor 41 is preferably provided with a signal or signals, on a connection indicated at 54 in FIG. 2 indicating when the platform 20 has been rotated into posi~ion to transmit signals into the sector or sectors being monitored.
This sector signal may preferably be provided by means of a photoelectric pickup 56 which can pick up optical signals reflected frsm a track ~not shown), which rotates with the platform, having sectors of different colors or differ-ent reflectivity. Each of the monitor antennas 38 and 39 may operate a separate receiver channel within the monitor 41 following along the lines of the receiver described below in connection with FIGS. 3 and 4. For reasons of economy, adequate monitoring may be obtained by employing a single receiver channel within the monitor_l, and successively switching that receiver to the different antennas 38 and 39 to monitor scans on successive rotations of the platform 20.
A receiver which is ~uitable for incorporation with the present invention is illustrated~schematically in FIG.
~6~44 3, and a flow chart illustrating the operation of the re-ceiver, in greater de~ail, is illustrated in FIG. 4.
The construction of the receiver, in many basic res-pects, conforms with the receiver proposed in the FAA report.
Referring more specifically to the receiver as illus-trated in FIG. 3, the receiver includes an antenna sche-matically shown at 57 which receives the scanning beam signals, which are transmitted preferably at C-band micro-wave frequencies. Antenna 57 supplies those signals to the early sta~es of the receiver indicated at 58, which include radio frequency amplifiers, filters, and intermediate frequency circuits. These circuits amplify the received signal, filter out unwanted adjacent signals, and convert the microwave signal to an intermediate frequency signal at connection 59, and a "video" signal at 60. A local oscilla-tor frequency synthesizer is provided, as indicated at 61, which supplies suitable local oscillator frequencies to the mixers within 58 for accomplishing the frequency conversions.
The intermediate frequency signal on connection S9 is supplied to a data processor circuit 62 which detects and decodes the digital parts of the signals, usually consisting of the digital preamble to each set of navigation signals.
The video signal is supplied on connection 60 to the acquisition and validation circuits 63 and to the envelope processor 64. The acquisition and validation circuits 63 assess the quality of the navigation signal and thereby de-termine whether or not the resulting data is to be displayed and used. The envelope processor 64 precisely determines .~ . .
/~
1527 ~ 44 when the "to" and "fro" beams scan by the aircraft. The resultant signals are supplied to an angle decoder 65 which determines azimuth angle based upon the time interval between the ''to" and "fro" scans. Such angle determination signals appear at the output 66, and are ultimately gated, such as through the gate 67 to indicators or angle data utilization devices, such as the navigation azimuth angle indicator shown at 68.
The time gate tracker circuit 69, which is connected between the angle decoder 65 and the envelope processor 64 designates which of the "to" and "fro" scans are to be processed.
All of the circuits of FIG. 3 described above are pre-ferably used in common for a number of different navigation information signal functions, in addition to those of the present invention.
When a 360 degree azimuth function scan group of "to"
and "fro" scans is detected by the data processor 62 by means of the digital data preamble accompanying those scans, a resultant signal appears at the data processor output con-nection 70 to a logic AND gate 71. When -the acquisition and validation circuits 63 determine that all of the other conditions necessary for valid reception of data exist on a particular data cycle, an output signal appears on connec-tion 72, providing a second input to the AND gate 71, causing the delivery of a signal from ~hat AND gate to a counter 73. The data cycles are repeated in a sequence, over and over, and include interspersed data scans for different ;. , :IX~6~4~l functions. However, an entire series of data cycles is re-peated with considerable rapidity, with one or mDre localizer scans included in each series. The reliability of the data is checked a number of times, on a number of separate localizer scan cycles so as to increment the count stored in the counter 73 up to a level above a predetermined count, before the guidance information is actually used. When that count, or any count above that level, is achieved, it is detected by a decoder 74 which provides an output at con--~ 10 nection 75. The resultant output signal at connection 75 operates visible flag device 76 which forms a part of the navigation indicator 68, and tells the pilot of the aircraft that true data is available. The signal on connection 75 ` from the decoder 74 is also supplied to open the gate 67 to allow the navigation information from the angle decoder 66 to be supplied to the indicator 68.
If the aircraft is too far away from the ground sta-- tion to receive navigation signals of sufficient strength, the acqulsition and validation circuits will not incre-ment the counter 73, and the flag 76 will not be operated so that the pilot will know that he does not have reliable navigation information. When "operated", the flag is pre-ferably withdrawn from view. Thus, if the flag is not operated, it is visible to the pilot. If the signals are marginal, and fading in and out, so that the counter 73 is incremented occasionally, but not consistently, the counter 73 will never reach the required count because the counter 73 is automatically counted down from time to time to keep /~
,~
marginal signals, which are only occasionally validated, from being used.
The data processor 62 not only determines the presence of the 360 degree azimuth function, it also identifies, from the preamble, which sector of the 16 sectors the particular signals are coming from. This information is available from the data processor 62 on a connection 79 and is effective to store an angle which is characteristic of the identified sector in a storage device indicated at 77. Preferably, this characteristic sector angle is a representation of the center line of the iden-tified sector in terms of a true bearing. Since the output angle ;`~
information at indicator 68 provides 360 degree azimuth informa-tion, the indicator also preferably indicates the azimuth bearing ~ in the form of a true bearing related to North. Thus, North is ;~ zero, East is 90 degrees, South i5 180 degrees, West is 270 degrees, etc. Since the navigation angle information at connec-tion 66 from the angle decoder 65 is only an indicator of an ~; angle within the identified sector, tsometimes referred to here-, .
. , ` inafter as the "scan angle") that output must be added in the adder 78, with the characteristic identified sector angle from store 77 in order to provide the true bearing required for indicator 68.
The receiver is actually much more complicated than indicated by the preceding description related to FIG. 3. It is believed that the details of the receiver are most easily comprehended by presentation of the functions of the receiver in _ ,~ _ ;
6~14 a flow chart. Accordingly, FIG. ~ is presented as a flow chart indicating the details of the receiver operation. FIG. 4 refers collectively to FIG. 4A and FIG. 4B combined as shown in the drawing labeled FIG. 4.
FIG. 4 is a flow chart illustrating the fucntional logic operation of the aircraft receiver in receiving and processing the signals transmitted from the ground station as represented by FIGS. 1 and 2. The incoming information at 82 in the upper right hand corner of the diagram, representing a single set of signals, first has a digital preamble, which begins with a "Barker" code, decoded at 84, in the presence of a "ready" con-dition as indicated at 86. The decoded preamble information is then checked for the ~unction identification at 88, and that information is checked at 90 to determine whether or not the set of information relates to the 360 azimuth function. If not, as signified by branch 92, other subroutines are followed relating to other signal functions as indicated at 94. If the 360 azimuth function is positively identified, then the sector angle informa-tion is decoded, as indicated at 96, to determine which of the sixteen sectors the signal infoxmation came from.
Next, the signal is tested, as indicated at 97, to deter-mine if it is acceptable, that is, to determine whether it is stronger than the side lobe suppression signals and the clearance pulses. The side lobe suppression signals are strong enough in all of the other fourteen or fifteen sectors to cause a rejection, at this point of all signals except those for the sector or sectors at which the receiver is located. If the answer is "No"
trejection), the signal is to be ignored, as indicated at 99, and not processed further. Accordingly, "ready" information is sent 6~'44 back as indicated by the connection to 86. On the other hand, if the answer is "Yes", the next processing of the information is determined at 98 based upon whether or not it has previously been determined that the entire receiver is operating within an "in track" mode, meaning that it has been determined, as further explained below, that a sequence of consistent valid signals has been received. If the receiver is "in track", the next test is, as indicated at 101, to determine if the system was in "track"
mode during the processing of the last previous signal. If the answer is "yes", the last previous output angle information is to be used for setting the track gates, as indicated at 103. The supplying of that angle information is indicated by the dotted line 105 from the output box 112 described further below. The last output angle information is in the form of an absolute cor-rected relative bearing of the receiver with relation to the transmitter. Next, as indicated at 109, the sector angle is subtracted from the angle value which is to be used to set the track gates. The sector angle information i5 supplied, as indi-cated by the dotted line 111, from the decoded sector angle as determined at the previously described step 96. The resultant remainder represents the expected scan angle within the sector being scanned. Next, the remainder angle is used, as indicated at 102, to set the txack gates.
However, going back to box 101, if the system was not in the "track" mode during processing of the last previous signal, then there will be no valid output angle at 112. Accordingly, as indicated at 107, an updated average of previously taken tentative angle measurements is to be used to set the track ~! 1527 1~6~4~
gates. This average value is available, as indicated b~ the dotted ; line input at 113 from an average measured angle determination as indicated by box 144, to be described more fully below.
The updated average angle used at 107 is also in the form of an absolute corrected compass bearing, and the sector angle is also subtracted from this heading, at 109.
After the track gates are set at 102, either on the basis of the last output, or on the basis of the updated average, the scan angle is decoded, as indicated at 104, to determine the actual scan angle within the designated sector. Then a test is made at 106 to determine whether the scan is within "track", based upon whether or not the scan is within the preset tracking gate limits.
The tracking gates are set on the basis of the scan angle within the sector scan range. While out of sector signals are rejected at 97, the tracking gates provide another discrimination function, for these gates cannot be set outside the normal sector scan range.
If the result is "Yes", at branch 110 the sector angle is added to the scan angle at 108 and the result is supplied as the output angle information at 112 for indication of the bearing of the aircraft relative to the ground station to the operator of the aircraft. This also results in "ready" information provided at con-nection 114 to again ready the s~stem at 86 for processing of new information.
It is one of the most important features of the inven- -tion that because of the logic set forth above, the system is capable of dealing accurately with successive sets of scanning information which may be derived from scans from different sectors. This follows from the fact that the output angle '~ _ ~, ' 4~
information available at 112 is in the foxm of an absolute compass bearing related to a single reference direction, such as north, and that output is used at 103 as the data ~or setting the track gates after subtracting the unique sector location angle for the sector within which the current scan occurs (at 109). Thus, the track gates are set, at 102, at limits indicative of where the sector scan should appear in relation to the center of the currently scanned sector, even though the currently scanned sector may be different ~rom the previously scanned sector ~h~ch established the output angle infoxmation available at 112. Thus, the aircraft carrying the receiver can fly from one sector to another, without losing "track", and while maintaining up-to-date, and constantly updated, angle-bearing information.
Referring back again to box 106, the indication of a valid scan within track at connection 110 also serves, as indi-cated at 116, to rein~orce the information that the receiver is "in track" by incre~enting a ~alidity counter. On the other hand, if it is determined that the scan is not within track, at 115, then the validity counter is decremented, as ind~cated at 118. Also, the signal is ignored, as indicated at 117 and ready infoxmation is sent back, as indicated at 114 to ready block 86.
A determinat~.on is then made as to whether the validity count is great~r than the minimum, as indicated at 120. If the answer is "Yes", an indicatox flag is held back, indicating to the operator that valid signals are being received, as signified . _ ., .~?, -- ,W--6~44 at 122. Also, the receiver is maintained in the "in track"
status, as signified by box 123. If the answer is "No", then the indicator flag is shown, as signified at 124. Also, as signified at box 125, the recei~er s~stem drops the "in track"
status. From the above, it is ev~dent that the indicator flag, which is visible to the aircraft operator, generally indicates whether or not the recei~er is in the "in track" status.
Referring back again to the box 118 indicating the operation "decrement validity counter", there is preferably provided a timer 119 which provides validity counter decrement pulses at timed intervals. Thus, in circumstances where val~d signals are received with insufficient frequency, the timer 119 will operate to decrement the val~dity counter, even in the absence of invalid (out of track~ signals, so as to bring the validity count below the minimum value required for maintenance o~ the "in track" status. Thus, the "in track" status will be dropped.
If the system is not in the "track" mode, it must operate in a signal acquisition mode. This operation is deter-mined, in terms of the logic flow chart, by the test previouslydescribed back at block 98. Thu~, ~f the result of the test at 98 is "No", ~ndicating that the receiver is not "in track", that information appears at 128, and then the track gates are set to a wîde aperture, as indicated at 138, the scan angle within the sector is decoded, as indicated at 140, the sector angle is added to the scan angle, as indicated at 142, and o ~
~ 1527 ~1~6~44 then the resultant sum, which is the absolute relative beariny angle measurement,is averaged with previous angle measurements as indicated at 144. The number of samples entering into the average, and the average value, are stored, as indicated at 146.
Next a decision is made as to whether a sufficient number of signals have been received within a prescribed tim~ interval to accumulate an acceptable average angle value, as indicated at 146. This test is carried out on the basis of the recorded number of previously received signals which have been averaged, as indicated by connection 162 from box 146. If the answer is "No", the "not in track" status is maintained, as indicated at 148, and information is supplied, as indicated at 114 to ready : the system, at 86, for the next information.
; If the average is determined to be valid, as indicated by a "Yes" answer at connection 152,~then the "in track" flag is set, as indicated at 154, and the ready information is again sent on connection 114 to ready box 86.~ The output information from box 144 also serves to reset a timer 158. If the valid average condition is not updated with sufficient frequency by informa-tion from box 144, the timer 158 times out, and through a connec-tion 160, causes the previously stored average to be dumped from storage. The system must then begin a new attempt to build up a : valid average.
Upon the setting of the "in track" status, as indicated at box 154, the validity counter is set to a predetermined initial value above the minimum value. This is indicated by the dotted connection 156 from box 154 to box 120. `
~ , . .
66~4~
Referring again to FIG. 1, one of the important advan-tages of the invention is that there is inherently provided multipath discrimination in the signals because of the feature that the center of mechanical rotation of the antenna array is different from the apparent center of radiation 37 (the apparent center of the scan of the scannin~ beam signal).
The term "multipath", as used in the above statement, refers particularly to signals which may be reflected from other fixed or movable objects near the transmitter, and which create 1~ additional signal paths for the transmitted signals to the receiver, with the possible effect of confusing the recei~er as to the correct directional information imparted by the signals. The multipath discrimination arising from the lack of concentricity of the mechanical rotation and the center of radiation arises from a space diversity effect (also sometimes referred to as a motion averaging effect~ which has the result of reducing the amplitude of any spurious multipath signals arising from any one signal reflective object, or any other one cause of multipath signals.
, .
~1~6~4 ~ _ . . . ~
As mentioned aboYe, one of the most important features of the invention is in the ability of the system to permit the aircraft which i5 being guided to fly from one sector to another without losing "track", and while continuously receiving guidance signals. This is made possible, in part, by the substantial overlap between the sector scan signals of adjacent sectors.
Thus, in the exf~mple given earlier in connection with FIG. 2, the individual sectors ma~ be, for instance, 22-1/2 degrees in angular width, whereas the indi~idual sector scans may be in the `:
10~ order of 36 degrees in width. Thu3, there may be as much as 13-1/2 degrees of overlap of adjacent sector scans.
:~ : As a specific example of what this overlap means, . particularly in relation to the operation of the receiver, suppose the aircraft, with its receiver, is located at the boundary be-tween sectox 2 and sector 3 tn FIG. 2. This corresponds to a true bearing of 45 with respect to the transmitter, assuming that the boundary between sectors 1 and 16 represents a true bearing of true North~ Assumlng a sector scan in sector 2, an~
referring back to FIG. 4, at box 96, the sector angle will be 20 decoded as the center line angle for sector 2, which is 33-3/4 degrees. Later, when the scan angle is decoded at 104, that scan ~1~6~
angle is detected as 11-1/4 degrees. When those two angles are added at 108, the resultant output angle, the true bearing, is correctly 45~ Then, if sector 3 is next scanned, the charac-teristic sector angle for sector 3 is decoded at 96 as the sector 3 center line bearing angle which is 56-1/4 degrees.
Later, when the scan angle is decoded at box 104, that scan angle is minus 11-1/4 degrees. Thus, when the two angles are added at box 108, the correct output angle (true bearing~ is again available at 112 which is 45. Thus, the sector signals for both of the adjacent sectors are available to provide a true bearing output. Furthermore, since the true bearing output is available at 103 for determining the setting of the track gates, the track gates are correctly adjusted for either sector by subtracting the correct sector angle at box 109. Thus, whether the aircraft receiver remains on the 45 true bearing, or whether it flies away from the 45 bearing towards the center of either sector 2 or sector 3, there is a continuity in tracking, and directional bearing in~ormation is continuously available to the operator of the aircraft.
Thus far, the invention has been described in terms of an implementation with "to and fro" scanning. However, the principles of the invention are also applicable to other scanning methods. For instance, the scanning of individual sectors may be accomplished by means of a so-called Doppler system as illustrated, for instance, in FIG. 5 of the drawings.
'- ~7d?
_"~-- 7 152i 6~
FIG. 5 is a simplified representation of a Doppler system for carrying out the present invention in which a rotating platform 2OA is provided to rotate a Doppler antenna array 166 with its associated transmitter 168 through different sectors. This Doppler s~stem may operate in a conventional Doppler mode in which transmitted signal radiations are switched in sequence from one radiation element to another and from one end of the array to another to create what appears to be a radiation signal from a source which is moving along the line of the array. A properly equipped receiver can then determine its bearing with respect to the line of the array based upon the apparent frequency shift due to the apparent movement of the source of the radiation along the line of the array. A sepa-rate antenna element, such as the element illustrated at 170, is provided for the purpose of issuing a standard frequency signal which has no apparent motion and which serves as a stan-dard for determination of the frequency shift due to the appar-ent motion of the slgnal from the arxay 166. Element 170 is preferably located at the center of mechanical rotation of platform 20A, but is not shown in that position in order to promote clarity.
- Insofar as applicable, all of the features described above in connection with the embodiment of FIGS. 1-4 are employed in this alternative Doppler embodiment of FIG. 5. Thus the system and the signal format are directly analogous to those used in the embodiment of FIGS. 1-4, with the exception that the .. - ~ ,~;,.
_ ~ _ scanning for each sector is carried out by the Doppler prin-ciple rather than by the "to and fro" principle.
Navigation systems employing the Doppler s~stem are well known, and accordingly, it is not believed to be necessary to further spell out the details of the embodiment of FIG. 5.
Furthermore, the details of the Doppler embodiment of ~IG. 5 may be carried out in a manner consistent with, and compatible with, a proposal entitled "DOPPLER MICROWAVE LANDING GUIDANCE
SYSTEM FOR A NEW NON-VISUAL PRECISION APPROACH AND LANDING
GUIDANCE SYSTEM" submitted by the United Kingdom to the International Civil Aviation Organization in November 1975.
As mentioned above, all of the details of the embodi-ment of FIG. 5 which are applicable may be carried out in accordance with the teachings of the embodiment of FIGS. 1-4.
For instance, detection of rotation of the platform 20A from one sector to another may be accomplished by means of a photo-electric pickup carried by the platform which detects different positions from a stationar~ optical track, as illustrated in FIG. 1. Furthermore, the Doppler signals may be monitored by a monitor receiver which is located to pick up the scanning signals within one predetermined sector.
In accordance with one of the features of the present invention, where a serious multipath signal problem is present in a particular sector on a permanent or frequent basis, and where it is not essential to have guidance signals within that sector, the regular guidance signals to that sector may be blanked out, and a special data meSsage~may be incorporated in .,j ~'' .
- ,a~ --'44 the preamble of a set of signal information which is sent out instead of the guidance signals telling the aircraft receiver that this is a blanked sector. A second flag may be provided in the visual angle indicator 68 of FIG. 3 (not shown~ to signal the operator of the aircraft that the aircraft is in a sector corresponding to a blanked signal. me aircraft operator will then know that he should change course to a new bearing, to fly into a sector where correct a~imuth information is availableO
This sector blanking arrangement for avoiding multipath problems is particularly applicable to the Doppler system.
While the disclosure of the embodiments of FIGS. 1 and 5 refers to a rotating platform 20 or 20A as a part of the physical structure, it is to be understood that the platform need not necessarily constitute a disc-like element. ~hus, it may be a completely open framework for rigidly supporting the rotating components in an arrangement such as could be provided by a solid platform, but without th~ need for providing a con-tinuous flat circular platform surface.
'rhe description of the invention given above has been entirely in terms of providing azimuth guidan~e signals for an aircraft with respect to a ground station. However, it is obvious that the invention is also useful for the purpose of providing elevation signals by using a platform for support of the antenna which _rQtates on a horizontal axis, and by providing sector scans generally about that horizontal axis.
In such an arrangement, since elevations only above the horizon are generally required, 180 degrees of total angular coverage will generally be the maximum required.
`44 In the foregoiny description, the angular position determined by the system is generally described as a "bearing angle". However, when used for elevation, this angle will generally be referred to simply as "elevation", or as the "elevation angle". In the material which follows, these angles are generically referred to as simply the "angular position".
The foregoing description is generally directed to providing a wide angle sector coverage which may include the entire 360 degree azimuth. However, it is obvious that the system also may be advantageously employed for coverage of any selected combination of sectors. As pre~iously mentioned above, when used for elevation, sectors may be selected which cover only the 180 degrees above the horizon. Similarly, when used in azimuth, the system need not necessarily cover the entire 360 degree azimuth.
Claims (20)
1. A method of providing accurate directional informa-tion to an aircraft in any angular position about a predeter-mined axis over a wide angle sector with respect to a ground station comprising transmitting radio guidance signals in a sequence of separate electrically scanned sector signals in limited angle sectors within said wide angle sector, the sector signals being transmitted by mechanically rotating about said predetermined axis a ground station trans-mitter antenna array which is capable of producing directional radio guidance signals by an electrical scan through only one of said limited angle sectors for each rotational position thereof, and transmitting from the antenna array said sequence of separate electrically scanned sector signals during successive mechanical rotation through a plurality of separate rotational positions, transmitting radio signals in conjunction with each of said separate electrically scanned sector signals to uniquely identify the particular sector, the identification radio signals transmitted in con-junction with each of said electrically scanned sector signals being operable to uniquely identify the rotational position of said antenna array corresponding to a particular sector, the number of said separate sectors at which electri-cally scanned sector signals are emitted being sufficient to cover all angular positions within said wide angle sector by the combination of all of the separate electrically scanned sector signals.
2. A method as claimed in claim 1 wherein said predetermined axis is a horizontal axis and said angular positions are elevation angles.
3. A method as claimed in claim 1 wherein said predetermined axis is a vertical axis and said angular positions are azimuth angles.
4. A method as claimed in claim 3 wherein said wide angle sector encompasses the entire 360 degree azimuth.
5. A method as claimed in claim 1 wherein said radio signals transmitted in conjunction with each of said electrically scanned sector signals to uniquely identify the rotational position of said antenna array are transmitted prior to the transmission of the associated electrically scanned sector signals as a part of an informa-tion preamble to the sector signals, and wherein the information preamble also includes other identifying data distinguishing the wide angle sector angular position function from other functions for which similar signals are transmitted.
6. A method as claimed in claim 5 wherein the information preamble is transmitted by binary digital coding of the carrier signal.
7. A method as claimed in any one of claims 1, 3, or 5 wherein said rotational position identification signals are transmitted over a limited sector generally corresponding to the sector covered by the sector signals.
8. A method as claimed in any one of claims 1, 3, or 5 wherein additional radio signal energy is transmitted in clearance beams and in side lobe suppression signals in order to enable the receiver to discriminate between valid sector signals for the sector in which the receiver is positioned and sector signals radiated for other sectors.
9. A method as claimed in claim 1 wherein the scanned sector signals are each emitted as a time reference scan group including a "to" scan and a "fro" scan, with the angular position of the receiver being deter-mined on the basis of the time interval between reception of the "to" scan and "fro" scan signals.
10. A method as claimed in claim 9 wherein the electrical scanning of the sector signals is carried out in a substantially circular sector and in which the effective axis of the electrical scan is displaced from the axis of mechanical rotation of the antenna array in order to provide for multipath discrimination as the signals are received.
11. A method as claimed in claim 9 wherein the electrical scanning rates of the "to" and "fro"
scans are different to provide rotational scan rate compensa-tion for the mechanical rotation of the antenna array so as to provide "to" and "fro" scans having the same absolute rate of rotation in the two different scan directions.
scans are different to provide rotational scan rate compensa-tion for the mechanical rotation of the antenna array so as to provide "to" and "fro" scans having the same absolute rate of rotation in the two different scan directions.
12. A method as claimed in claim 11 wherein the "to" and "fro" scans are position compensated so that the angles at which the electrical scans respectively for "to" and "fro" are commenced at different angles with respect to the rotating antenna array in order to compensate for the mechanical displacement of the mechanically rotating antenna array in the interval between the commencement of the "to"
scan and the commencement of the "fro" scan.
scan and the commencement of the "fro" scan.
13. A method as claimed in any one of claims 1, 3, or 5 wherein the electrically scanned sector signals are scanned by the Doppler method wherein the radiated signal is switched in sequence from one end to the other of an array of radiating elements to convey the impression of radiation from a moving source which moves from one end to the other of the array, and which creates a frequency shift in a receiver dependent upon the orientation of the receiver with respect to the array.
14. A method as claimed in any one of claims 1, 5, or 9 including receiving said directional radio guidance signals with a receiver in an aircraft to be guided, storing the true angular position of the receiver relative to the ground station as indicated by at least the last previous radio guidance signal, and processing the next subsequent guidance signal by decoding the sector identification information associated with that guidance signal, subtracting the characteristic sector angle associ-ated with the identified sector from said stored true relative angular position and using the difference to set angle limit track gates indicating the angle limits within which a valid sector scan signal should appear within the identified sector, then using said track gate limits to confirm the validity of the subsequent sector scan signal indicating the angle within the identified sector, and then adding to the confirmed sector scan signal the characteristic sector angle of the identified sector to provide the true relative angular position as an output to the operator.
15. A method as claimed in claim 1 including receiving said directional radio guidance signals with a receiver in an aircraft to be guided, storing the true angular position of the receiver relative to the ground station as indicated by each of a representative plurality of said signals received in sequence, computing an average value of said true relative angular positions, setting said receiver into a track mode, then processing the first subsequent guidance signal by decoding the sector identification information, subtracting the characteristic sector angle associated with the identified sector from said average value and using the difference to set angle limit track gates indicating the angle limits within which a valid sector scan signal should appear within the identified sector, then using said track gate limits to confirm the validity of the subsequent sector scan signal indicating the angular position within the identified sector, and then adding to the confirmed sector scan signal the characteristic sector angle of the identified sector to provide the true relative angular position as an output to the operator.
16. A method as claimed in claim 15 wherein second and later subsequent guidance signals are processed by storing the last previous true relative angular position output and using that stored output in substitution for said average value in setting the angle limit track gate after subtracting the characteristic sector angle of the identified sector.
17. A method as claimed in any one of the preceding claims 1, 9, or 12 wherein the scanned sector signals are monitored by a fixed monitor receiver which is arranged to receive scanned sector signals in only one of the antenna sectors.
18. Radio guidance apparatus for providing accurate orientation information to an aircraft in any angular position over a wide angle sector with respect to a predetermined axis at a ground station comprising a ground station including a transmitter and an associated antenna array which is capable of producing direc-tional radio guidance signals by an electrical scan through only a limited angle sector, platform means for supporting and mechanically rotating said antenna array about said predetermined axis to thereby change the orientation of said antenna array, said ground station being capable of transmitting radio guidance signals from said antenna array in a sequence of separate electrically scanned sector signals during successive mechanical rotation through a plurality of different rotational positions, the number of said different rotational positions at which electrically scanned sector signals are emitted being sufficient to cover all angles within said wide angle sector by the combination of all of the electrically scanned sector signals, said apparatus including means for transmitting radio signals in conjunction with each of said electrically scanned sector signals to uniquely identify the rotational position of said antenna array corresponding to a particular sector.
19. Apparatus as claimed in claim 18 wherein all of the directions about said axis are divided into a predetermined number of fixed sectors having predetermined sector width, and wherein there is provided a means for detecting the rotation of said antenna array through said different sectors, said transmitter being connected to said last-named means and being operable in response to detection of the rota-tion of said antenna array from one sector to another to initiate a sector scan signal for the new sector.
20. Apparatus as claimed in claim 19 wherein said transmitter is rotatable with said antenna array, and wherein said means for detecting the rotation of said antenna array from one sector to another comprises a photo responsive pick-up means rotatable with said transmitter and said antenna array and a cooperating fixed optical track having optical characteristics which are dif-ferent at different sectors.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US68815976A | 1976-05-20 | 1976-05-20 | |
| US688,159 | 1976-05-20 | ||
| US747,244 | 1976-12-03 | ||
| US05/747,244 US4080600A (en) | 1976-05-20 | 1976-12-03 | Scanning beam radio navigation method and apparatus |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1106044A true CA1106044A (en) | 1981-07-28 |
Family
ID=27104173
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA278,250A Expired CA1106044A (en) | 1976-05-20 | 1977-05-12 | Scanning beam radio navigation method and apparatus |
Country Status (8)
| Country | Link |
|---|---|
| JP (1) | JPS53991A (en) |
| AU (1) | AU503018B2 (en) |
| CA (1) | CA1106044A (en) |
| DE (1) | DE2722217A1 (en) |
| FR (1) | FR2352308A1 (en) |
| GB (1) | GB1523861A (en) |
| NL (1) | NL7705381A (en) |
| SE (1) | SE7705831L (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2924847A1 (en) * | 1979-06-20 | 1981-01-22 | Standard Elektrik Lorenz Ag | MICROWAVE LANDING SYSTEM THAT WORKS BY THE JET SWIVELING PROCESS |
| JPS5653532U (en) * | 1979-10-02 | 1981-05-11 |
-
1977
- 1977-05-12 CA CA278,250A patent/CA1106044A/en not_active Expired
- 1977-05-12 GB GB19925/77A patent/GB1523861A/en not_active Expired
- 1977-05-16 NL NL7705381A patent/NL7705381A/en not_active Application Discontinuation
- 1977-05-16 FR FR7714893A patent/FR2352308A1/en active Granted
- 1977-05-16 AU AU25158/77A patent/AU503018B2/en not_active Expired
- 1977-05-17 DE DE19772722217 patent/DE2722217A1/en not_active Withdrawn
- 1977-05-17 SE SE7705831A patent/SE7705831L/en unknown
- 1977-05-19 JP JP5817177A patent/JPS53991A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| SE7705831L (en) | 1977-11-21 |
| NL7705381A (en) | 1977-11-22 |
| FR2352308B3 (en) | 1980-02-15 |
| AU2515877A (en) | 1978-11-23 |
| JPS53991A (en) | 1978-01-07 |
| DE2722217A1 (en) | 1977-12-01 |
| AU503018B2 (en) | 1979-08-16 |
| FR2352308A1 (en) | 1977-12-16 |
| GB1523861A (en) | 1978-09-06 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |