GB2181317A - Providing positional information - Google Patents

Abstract

In a method and apparatus for providing positional information to a radiation receiver, particularly relating to a Microwave Landing System of the Time Reference Scanning Beam kind, a beam of radiation is first swept over a range of angles in one sense (TO sweep) by one (U/R) of a pair of spaced apart scannable radiators (U/R, U/L) and then in the opposite sense (FRO sweep) by the other radiator (U/L). The interval between reception of the TO and FRO sweeps indicates the angular position of the receiver with respect to the mid-point of the line joining the radiators (U/R, U/L) and to a reference plane passing through that point, with an error that in practice is mostly very small and in particular is zero in the reference plane. The radiators (U/R, U/L) may be situated close together, suitably on the centre-line (RCL) of a runway (RY), to provide duplication in the event of failure of one radiator, or may be substantially separated, suitably symmetically about the runway centre-line (RCL), for situations in which it is undesirable or impracticable to use a conventional single scanning radiator on the runway centre-line (RCL) beyond the stop end (SE) of the runway (RY). <IMAGE>

Description

SPECIFICATION Providing positional information The invention relates to a method of providing positional information to a radiation receiverbyoperating radiating means comprising two scannable radiators first to sweep a beam of radiation over a first range of angles in a first plane in one sense (TO sweep) and then to sweep a beam of radiation overa second range of angles in said first plane in the other sense (FRO sweep),the second range of angles overlapping the first range of angles, wherein the interval between reception by a receiver within the region of overlap ofthe ranges of angles of the beam of radiation on a TO sweep and ofthe beam of radiation on the next FRO sweep is ideally at least approximately representative of the angular position of the receiver with respect to the radiating means and with reference to a second plane normal to said first plane. The invention further relates to apparatus operable to perform such a method.

The invention is especially applicable to providing navigational guidance information, and particularly but not exclusively to providing azimuth guidance foraircraftwith a Microwave Landing System (MLS) oftheTime Reference Scanning Beam (TRSB) kind.

The term "ideally" is to be understood to mean in the absence of disturbing phenomena (generally of a random nature) which may occur in practice and which maydegradethe accuracy ofthe positional information,for example noise in the receiver or signals received both directly and after a reflection (multipath), such phenomena being additional to any broad, consistent geometrical considerations which result in said interval being only approximately representative ofthe true angular position ofthe receiverwith respect two a given origin.The effect of disturbing phenomena on the accuracy of the positional information may be reduced by averaging successive representations of angular position derived by the receiver.

International standardsforTRSB MLS have been established in recent years, in particular by the International Civil Aviation Organisation (ICAO), and systems ofthis kind are expected to come into increas inglywidespread use from now on; forexample,the Federal Aviation Authority has already ordered more than 200 systems, the first batch of some 1250 systems which are intended to be installed at airports in the USA.In such a system, azimuth guidance is nor mallyprovidedbya phased a rray which sca ns a beam having an azimuth beamwidth of 1 or 2 de grees over a range of azimuth angles of up to app- roximately t60 degrees with respect to the vertical plane including the centre line ofthe runway; the antenna array typically is located on said centre line several hundred (for example seven hundred) feet beyond the stop end ofthe runway.

A method and apparatus as set forth in the opening paragraph, for azimuth guidance for aircraft, are disclosed in UK Patent Specification 1 416364 in the name ofthe Commonwealth Scientific and Industrial Research Organization, a Body Corporate ofAustr- alia that played an important part in establishing the TRSB kind of MLS. This specification discloses (see particularly page 5, lines 66-88, and Claim 9) the use of two scanning aerials located at the stop end ofthe runway one on either side thereof; the aerials are used on alternate scanning cycles. Thus if a scanning cycle includes a TO scan and a FRO scan, both the TO scan and the FRO scan ofany one cycle are radiated by the same aerial.Such an arrangement has the ser iousdisadvantagethat unlessthe aerials are im- mediately adjacent one another, the angular positions derived by a receiver from the scanning cycles radiated respectively by the two aerials differ significantly, especially at fairly close range and in part ocular in the critical region oftouchdown on the runway. As a result, it is not intended that such an arrangementshould be used to provide azimuth guidance with TRSB MLS. Furthermore, it has consequ entry been generally considered by those skilled in the art that providing azimuth guidance in a MLS using two transversely-spaced antennae would not be practicable.

There are however situations in which the conventional single-antenna arrangement is disadvantageous or impossible. It may be impracticable to instal a suitable antenna beyond the stop end of the runway,forexample because of a sharp dip in the ground level or, in the case of a coastal airport, because the runwayterminates at the sea. At some airports, there may be a hump of ground between the ends of the runway that at least partly obscures thetouchdown region from an antenna mounted in the conventional position; raising the height ofthe antenna may not be permissible. Alternatively, duplication of equipment may be desirable, while substantially maintaining the accuracy of the positional information,for maximum safety in category Ill (minimum visibility) operation.

According to a first aspect ofthe invention, a method as setforth in the opening paragraph ofthis specification is characterised in that the beam of radiation is radiated on a TO sweep by a first of said rad iatorsand on the next FRO sweep bythe second of said radiators, whereby said angular position is with respect to the mid-point ofthe line joining thetwo radiators, said second plane including said point.

Analogously, according to a second aspect of the in vention,apparatus for providing positional information to a receiver, the apparatus having radiating means comprising two scannable radiators operable first to sweep a beam of radiation overafirst range of angles in a first plane in one sense (TO sweep) and then to sweep a beam of radiation over a second range of anglers in saidfirst plane in the othersense (FRO sweep), the second range of anglers overlapping thefirst range of angles, wherein the interval between reception by a receiver within the region of overlap ofthe ranges of angles of the beam of radiation on a TO sweep and ofthe beam of radiation on the next FRO sweep is ideally at least approximately representative of the angular position ofthe receiver with respect to the radiating means and with referenceto a second plane normal to said first plane, is characterised in that the apparatus is operable to radiate the beam of radiation on a TO sweep with a first of said radiators and on the next FRO sweep witch the second of said radiators, whereby said angular position is with respect to the mid-point ofthe line joining the two radiators, said second plane including said point.

The invention provides the advantagethatthe spacing of the radiators can be selected from a range of values in accordance with operational criteria, while the accuracy of the positional information derived bythe receiver will generally be significantly betterthan could be derived using two radiators in accordance with the disclosure oftheabove- mentioned UK Patent Specification 1 416364. For receivers lying substantially in the reference second plane bisecting the line joining the two radiators (suitably perpendicularly), the error is substantially zero, which may be particularly advantageous.

Forsimplicity, successive TO sweeps may beradiated by said first radiator. As an alternative, successive TO sweeps may be radiated alternately by said first radiator and by said second radiator, the representations of angular position derived by the receiver on successive pairs of TO and FRO sweeps being averaged. This has the advantage that the ef fect on the accuracy of the angular representation of phenomena such as multipath will in general be reduced.

The two radiators may be disposed closely adjacent to one another and the beam of radiation on both a TO sweep and the next FRO sweep be radiated by one ofthe radiators in the event offailure ofthe other radiator. This aspect ofthe invention enables equipment to be duplicated while providing substan tiallysimilaraccuracyin normal operation to that which would be provided buy a single radiator, the accuracy being significantly degraded only in the emergency situation of failure of one of the radiators.

Alternatively, the two radiators may be substantially spaced from one another.This is particularly de- sirable for situations in which the conventional use of a single radiator is impracticable or disadvantageous.

A method and apparatus embodying the invention mayrelatetoproviding azimuth guidanceinforma- tion with a Microwave Landing System for use by a receiver mounted in an aircraft, the radiators being ground-based antennae adjacent a runway and said second plane being vertical. Suitably, the centre-line ofthe runway passes substantiallythrough said point.

An embodiment of the invention will now be described, by way of example, with reference to the dia grammaticdrawings, in which: Figure 1 illustrates schematically the disposition of the Azimuth units and monitors of a Microwave Landing System embodying the invention; Figure2 is a block diagram ofthe Azimuth units, showing also the monitors and their interconnection; Figures3and 4are respectively two graphs of angular error against angular position in relation to a reference plane, and Figures5and 6depicta particular disposition of an airborne receiver in relation to two different runway configurations respectively.

Referring to Figure 1, two Azimuth units U/R and U/L respectively of a Microwave Landing System are located to the right and left respectively of an airport runway RY, as seen by aircraft landing on the runway, the aircraft touching down close to the runway threshold RT. In orderforthe System to continue to provide guidanceto aircraft aftertouchdown, the Azimuth units mayforexample be located in a region between approximately three-quarters ofthe length ofthe runway along itfrom its threshold RTto somewhat beyond the stop end SE of the runway, being in this instance approximately aligned with the stop end.The Azimuth units are disposed in such a mannerthatthe runway centre-line RCL (indicated by a dash-dot line) perpendicularly bisects the line joining the units, so that approaching aircraftcande- rivetheirangular position with respect to the point on the centre-line mid-way between the units with reference to a vertical plane including the centre-line.

The units may for example be spaced some 600 feet apart.

In a conventional Microwave Landing System, azimuth guidance is provided by a single Azimuth unit which typically is located on the runway centreline several hundred feet beyond the stop end. To provide positional information to approaching aircraft, the Azimuth unit sweeps a beam of radiation back and forth over a range of angles. The beam suitably is narrow in azimuth, having a 3dB beamwidth offorexample 2 degrees, and extends in elevation approximatelyfrom just above ground level to,for example, 20 degrees. The range of angles of both sweeps may for example be approximately +40 de- grees with reference to the runway centre-line.The beam is first swept in one sense (TO sweep, normally clockwise) at a uniform rate, for example 0.02 degrees per microsecond, and then after an interval is swept in the other sense (FRO sweep) at the same rate. Scanning cycles, each cycle comprising a TO sweep followed by a FRO sweep, are performed at a rate of either 13 Hz or 39Hz, successive cycles being spaced by intervals during which which data istransmitted and elevation scans are performed by an Elevation unit (not shown) ofthe Microwave Landing System.In accordance with the general principle of TRSB M LS, the scanning is such thatthe interval between reception buy a receiver disposed in a vertical reference plane including the runway centre-line of the centre of the beam of radiation on a TO sweep and ofthe centre ofthe beam on the next FRO sweep is a predetermined amount, for example 4800 microseconds. For receivers disposed otherthan in the reference plane, the interval between reception of a TO sweep and the next FRO sweep differs from the pred etermined amount; since the rate of scan is known, the receiver can by measuring the interval derive its angular position, the difference from the predetermined amount being directly proportional to the angletothe reference plane. If for example thereceiveris located to the rightofthe reference plane, it will receive the TO sweep earlierthan if itwere in the reference plane and will receive the FRO sweep later than it if were in the reference plane, so that the interval between reception ofthesweeps is largerthan the predetermined amount.

The invention as embodiment in the system of Fig ures 1 and 2 differs from the conventional arrangement in that in each scanning cycle, the TO sweep is radiated by one of the offset Azimuth units U/R, U/L and the FRO sweep is radiated by the otherAzimuth unit. It might well be thought thatthe relationship between angular position and the time interval between reception of the TO and FRO sweeps would not hold ifthe TO and FRO sweeps are respectively radiated by transversely-spaced antenna.However, a detailed investigation shows that for reasonable spacings of the antennae (in relation to the region of space in which guidance is to be provided), the dif ferenceintherepresentation of angular position from that provided by a conventional arrangement (or error) is very small and falls within acceptable limits; it is particularly advantageous that the error tendstozero asthe angletothe reference plane tends to zero, and therefore aircraft correctly positioned fortouchdown on the runway centre-line will derivetheirangular position as being the same as it would be with a conventional single Azimuth unit.It mayforexamplebe noted thatwheretheTO sweep is radiated by the right-hand Azimuth unit U/R and the FRO sweep by the left-hand Azimuth unit U/L (as indicated in Figure 1), a receiver disposed in the re ferenceplanewill receivetheTOsweeplaterthan if it had been radiated by an antenna conventionally located in the reference plane, and will also receive the FRO sweep later by the same amount: the interval between reception ofthe two sweeps is unchanged.

In this embodiment, successive TO sweeps are radiated bythe same unit, U/R, and successive FRO sweeps by the other unit, U/L. As an alternative, successive TO sweeps might be radiated alternately by one unit and the other. This would introduce slightly greater complexity in the Azimuth units, but would provide the advantage that averaging in the receiver of successive representations of angular position would tend to reduce errors due to multi path. However, this might not suit receivers which rely on the interval between the beginning of a scanning cycle and reception ofthe beam on the TO sweep being fairly consistent from one cycle to the next: this interval would vary for a receiver not disposed in the reference plane.

Figure 2 is a schematic block diagram ofthe Azimuth units U/R and U/L (each bounded by lines of long dashes)togetherwith monitors which are used to ascertain whether the Azimuth units are operating correctly. In this embodiment, the right-hand unit U/ R is a master unit, and the left-hand unit U/L is a slave unit. Each unit comprises a C-band (approximately 5 GHz) source OSC, an on/off RF switch S, a Differential Phase-shift Keying (DPSK) Modulator MDLTR (as also indicated bythe symbol ), an RF power amplifierAMP, and RFselectorswitch SS, and an antenna assembly comprising a power divider DIV, a set of phaseshiftersPS (as also indicated bythesymbol N) and a phased antenna arrayANThaving an integral monitor RF manifold.The switch S, the mod ulator MDLTR and the selector switch SS are controlled by a respective control microprocessor FP (as indicated by lines of short dashes); the phase shifters PS ofthe antenna assemblyare controlled bythe microprocessor FP via SCAN LOGIC. A respective first monitor MNTR 1/R receives RFsignalsfrom the RF manifold integral with the respective phased antenna array ANT, the monitorofthe slave unit U/L supplying video and logic signalsto the monitor of the master unit U/R, and the latter in turn being coupled to an executive monitor EXEC M NTR common to the whole Microwave Landing System.

Associated with each oftheAzimuth units is a re spective field detector FD/R, FD/L supplying RF signals to a respective second monito r M NTR 21R, 2/ L, the left-hand monitor supplying video and logic signalstothe right-hand monitor. Commontothe two Azimuth units is a far-field detector FFD supplying RFsignalsto a third monitor MNTR 3; the second monitor ofthe master unit, MNTR 2/R, and thethird monitor are coupled to the executive monitor. The field detectors FD/R, FD/L may be positioned directly in front of their respective phased antenna arrays, for example about a hundred feet away, and thefar-field detector FFD may be positioned on the runway centre-line RCL before and neartothethreshold RT (as depicted in Figure 1).

The selector switch SS in at least the master Azimuth unit U/R is selectivelyoperableto couple RF powerfrom the output ofthe amplifierAMP eitherto the azimuth scanning beam antenna assembly orto any of a plurality of other outputs which areconnec- tedto other respective antennae (not shown).As indicated in Figure 2, these other outputs may for example comprise DATA, whereby information encoded by DPSK modulation may be radiated over a range of t +60 degrees about the reference plane; clearance right and left (CL/R and CL/L respectively) which may be radiated over sectors extending app roximatelyfrom 40 degreesto 60 degrees one on each side ofthe reference plane; and Out-of Coverage Information (OCI) which may be radiated over angular ranges extending beyond the clearance sectors.

In operation, the Microwave Landing System, which will normally also include an Elevation unit (not shown), radiates with its various antennae in ac cordance with a standardised sequence oftrans missionswhich may be controlled bythemicro- processor ofthe Azimuth units. Considering the operation ofthe master Azimuth unit U/R, attimes when no RF energy is to be radiated by any ofthe antennae associated with the Azimuth unit, the RF source OSC is decoupled bythe switch S. Attimes when data isto be transmitted, the RFsignal is DPSKmodulated bythe modulator MDLTR. When an azimuth beam TO scan is to be performed, the output ofthe amplifier AMP is coupled bythe switch SSto the phased antenna array and the phase shifters PS thereof are controlled in the appropriate manner by the Scan Logic underthe overall control of the microprocessor FP. The switch Sand the phase shifters PS in the slave Azimuth unit U/L are similarly controlled by its respective microprocessorfor performing the FRO scan.Synchronisation of the TO and FRO scans is achieved by virtue of one unit being slaved to the other; a link mayforexample be provided between the microprocessors of the two Azimuth units, as indicated in Figure 2, for example by coaxial cable or fiber-optic link, or the slave unit may time its scan ning beam from a suitable pulse (for example a synchronising signal) of the RF radiation transmitted by the master unit, a separate receiving antenna and detector (not shown) coupled to the slave unit then being providedforthis purpose.

The DPSK modulator MDLTR and the selector switch SS in the slave unit U/L are not required in normal operation; however, they may be provided for exampleto enablethe DATA, clearance right and left (CL/R and CL/L), and out-of-coverage (OCl)functionsto be provided by the slave unit U/L in the event of a fault in the master unit U/R.

The various monitors checkwhetherthe system is operating correctly by measuring various operating parameters. If the value of any essential parameter falls outside acceptable limits, the executive monitor, which is coupled to the microprocessor of each Azimuth unit, may operate to transfer operating functionsfrom one Azimuth unittotheotherortoshut down the system.

Two aspects of monitoring that particularly relate to embodiments of the invention as compared with the conventional use of a single Azim uth unit are (i) Where the two Azimuth units are substantially separated, the use of a far-field detector (FFD, as shown in Figures 1 and 2)which receives both theTO sweep and the FRO sweep is especially desirable (each of the field detectors FD/R and FD/L receiving onlythe sweep radiated by its associated unit). Monitoring of the far-field detector should take into accountthe possibilitythatthe scanning beam may be temporarily obscured by, for example, an aircraft on the runway.

(ii) Some kinds of aircraft receiver may depend, in their determination of the centre ofthe scanning beam, on the peak received power being similar on the TO sweep and the FRO sweep. It maytherefore be desirableforthe output powers of the respective power amplifiers in the two Azimuth units to beadjustable relativeto one another (by means notshown) sothatthe powers radiated respectively bythetwo units do not differ by more than,for example, 2dB.

The accuracy ofthe positional information obtainable with embodiments ofthe invention will now be considered in some detail. The separation ofthetwo scannable radiators tends to introduce a negative bias error E by comparison with a conventional arrangement using a single scannable radiator. The error is zero for a receiver located in the reference plane: as the angle 0 between the reference plane andthe line joining the receivertothe mid-pointof the line joining the radiators increases, the error increases progressively to a maximum and then decreases again to zero at 0 = 90 degrees.The error is negative for 0 < 0 < 90 because the interval between reception ofthe TO sweep and the FRO sweep is slightly less than would occur witch a conventional single radiator. For a given value of 8, the error increases as the separation S ofthe two radiators increases (the distance D between the receiver and said mid-point remaining constant) and decreases as D increases (S remaining constant).Thevariation of the error E with the angle 0 may conveniently be con sidered for different val ues of the ratio 0 = D/S.In Figure 3, the negative error E (in degrees) on along arithmicscale is plotted against0 (in degrees) on a linearscaleforvalues of0 upto 60" and for Qvalues of 2,5, 10,20 and 40 respectively. It will be seen that for a given value of Q, E increases with 0 rapidly at first and then progressively more slowly, reaching a broad maximum at around 0 = 45" (higherfor small 0), after which it decreases again.The error is small even at low values of O (for example remaining below 0.30 for Q = 5) and becomes very small indeed as Q increases, being for example less than 0.005" for Q = 40. Figure 4shows in more detail the variation of E with 0 orthe same values of Q as in Figure 3for 0.25 S 0 S 10 (+ 10 being the smallest range over which the azimuth guidance beam is scanned in MLS). As in Figure 3, E is on a logarithmic scale and n on a linear scale).It can be seen from Figures 3 and 4 that accuracywhich will probably bequite accept- able is obtainable over a substantial range of situations that are likely to arise in practice.

Some examples of such practical situations in the use ofthe invention for MLS wil now be given with reference to Figures Sand 6, which are not to scale and in which the receiver is designated Rx.

1(a) SpacingSoftheAzimuth units500feet,dis- tance D of the receiver from the mid-point between the units 10,000 feet, angle 8 10 degrees. This come responds, for example, to Azimuth units situated symmetically about a runway centre-line, 700 feet beyond the stop end, with the aircraft situated in a plane 9150 feet before the stop end (e.g. aligned with the threshold of a fairly long runway) atthe 10 degree limit of proportional guidance: see Figure 5. The magnitude ofthe error in the transverse position amounts to about 1.1 feet, which compares with a permitted error of 10 feet on the centre-line atthe runwaythreshold.

(b) With the same disposition of the Azimuth units, when the aircraft is at the so-called "pointE" on the runway (designated E in Figure 5) feet before the stop end (corresponding to the closest to the guidance system that guidance must be provided by ILS), atthe edge ofthe runway 75 feet from the centre-line, the error amounts to about 0.7 feet.

2(a) Avery short runway of 4000 feet, the Azimuth units being located mid-way along it with a spacing S of 600 ft: see Figure 6. When an aircraft is aligned with thethreshold at an angle (3 oflOde- grees, it is 350 feetfrom the runway centre-line, corresponding to the limit of the ILS sector; the error then amounts to about 7.5 feet. (This is of course generally a highly unsuitable position for an aircraft intending to land on the runway.) (b) With the same disposition of Azimuth units, when o is 10 degrees and D is 8000 feet (i.e. the aircraft is approximately 1 nautical mile from the threshold), the error amounts to about 2.2 feet.

A Microwave Landing System having Azimuth units embodying the invention, the units being sub stantially spaced from one another, is particularly suitablein,forexample,thefollowingcircumstances: 1) where it is impracticableto mount a single Azimuth unit in the conventional position beyond the stop end ofthe runway,forexample owing tothe ground configuration (as mentioned above) orto dif ficulties due to co-location with the ILS localiserat airports where both systems are required during the transition from ILSto MLS; 2) where it is desired to improve reception of the scanning beam in the region ofthe runwaythreshold and along the runway; raising an antenna on the runwaycentre-lineto an adequate height may well not be permissible, but raising antennae well to the side ofthe centre-linewill probably be acceptable (ICAO standards allow a progressively increasing obstacle height four navigational aids with offsets of progressively more than 200 feet from the runway centre-line); 3) where it is desired not to present a physical obstacle in the direct path of an aircraftwhichover-runs the runway, for example following braking failure when landing oran aborted take-off; 4) where it is desired to provide a clea rway to reduce take-off restrictions.

Alternatively, an embodiment wherein the Azimuth units are closely adjacent one anotherand nearthe runway centre-line can provide suitable duplication ofequipmentforstringentoperating conditions, e.g.

category Ill landings.

Regarding the practical utilisation ofthe invention for MLS, minimum operational performance standards currently laid down for MLS airborne receivers include various guidance signal validation criteria to try to ensure that the receiver will provide positional information onlywhen suitable signals are received.

One criterion is that the mid-point in time between reception oftheTO scan and reception ofthe FRO scan should not differ by more than a specified amountfrom a specified time afterthe preamble of a signal transmission. This would preclude operation of a receiver using this validation criterion with substantially spaced Azimuth units for certain positions ofthe receiver relative to the units, in particularwhen the receiver is relatively close to the units, i.e. when 0 has a small value.This problem could be overcome by,forexample, including in the basic data transmitted from the ground that defines the position from which the Azimuth beam is being transmitted an indication that the beam is being transmitted from two spaced Azimuth units (split-Azimuth). An existing spare bit in the appropriate data word could be used forthis purpose. The receiver could then respond by, for example, omitting the use of this validation criterion altogether, or by using itonlywhen the receiver is initially acquiring guidance art a distance sufficiently great (i.e. Q sufficiently large) for the criterion still to be satisfied and then dropping the use ofthe criterion as the receiver approaches the antennae.

While an embodiment ofthe invention has been described in relation to a Microwave Landing System, itwill be appreciated that the invention is not restricted to such a system and thatthefollowing features amongst other of TRSB MLS are notes- sential to the invention: (a) The radiated beam is not restricted to an RF beam of electromagnetic energy. Any scannable beam of energy whose time of transit past a receiver located in space can be defined and measured with reasonable accuracy could be used.

(b) The ranges of angles overwhich the beam is swept own the TO sweep and on the next FRO sweep respectively need not be coextensive. Provided that the ranges overlap, a receiver disposed within the re gion of overlap will receive both the TO sweep and the FRO sweep.

(c) The angular rate of sweep ofthe beam need not be constant: provided the receiver has sufficient information on the rate of sweep, it can calculate its position.

(d) The plane perpendicularly bisecting the line joining the radiators need not coincide with a re ctilinearpath it is desired that the receiver shou Id take (the runway centre-line in the MLS embodiment described with reference to the drawings). If such a path exists, it may for example extend through the mid-pointofthe line joining the radiators, but nonperpendicularly to that line: provided the angle between the second plane (with reference to which the angular position of the receiver is measured) and the desired path is known, the angular position referred to the desired path can be derived. Furthermore, some offset ofthe desired path from said mid-point may be acceptable, depending on the desired ac cu racy ofthe positional information;: offset ofthe effective origin of the angular position from the runway centre-line may for example be acceptable with MLS for category I operation.

Claims (18)

1. A method of providing positional information to a radiation receiver by operating radiating means comprising two scannable radiatorsfirstto sweep a beam of radiation over a first range of ngles in a first plane in one sense (TO sweep) and then to sweep a beam of radiation overafirst range of angles in afirst saidfirstplane inthe othersense (FRO sweep),the second range of angles overlapping the first range of angles, wherein the interval between reception by a receiverwithintheregion of overlap of the ranges of angles of the beam of radiation on a TO sweep and of the beam of radiation on the next FRO sweep is ideally at least approximately representative of the angular position ofthe receiver with respecttothe radiating means and with reference to a second plane normal to said first plane, characterised in that the beam of radiation is radiated on a TO sweep by a first of said radiators and on the next FRO sweep by the second of said radiators, whereby said angular position is with respect to the mid-pointofthe line joining the two radiators, said second plane including said point.

2. A method as claimed in Claim 1 wherein successive TO sweeps are radiated by said first radiator.

3. A method as claimed in Claim 1 wherein suc cess.ve TO sweeps are radiated alternately by said first radiator and by said second radiator, the repre sentations ofangular position derived by the re receiver on successive pairs of TO and FRO sweeps being averaged.

4. A method as claimed in Claim 1,2 or3wherein the two radiators are disposed closely adjacent to one another, and wherein the method further com prises radiating the beam of radiation on both a TO sweep and the next FRO sweep with one of the radiators in the event offailure ofthe other radiator.

5. A method as claimed in Claim 1,2 or3 wherein the two radiators are substantially spaced from one another.

6. A method of operating a Microwave Landing System to provide azimuth guidance information to a receiver mounted in an aircraft, comprising a method as claimed in any preceding claim, the radiators being ground-based antennae adjacent a runway, and said second plane being vertical.

7. A method of operating a Microwave Landing System as claimed in Claim 6 wherein the centre-line ofthe runway passes substantially through said point.

8. Apparatus for providing positional information to a receiver, the apparatus having radiating means comprising two scannable radiators operable first to sweep a beam of radiation over a first range of angles in a first plane in one sense (TO sweep) and then to sweep a beam of radiation over a second range of angles in said first plane in the other sense (FRO sweep), the second range of angles overlapping the first range of angles, wherein the interval between reception by a receiver within the region of overlap ofthe ranges of angles ofthe beam of radiation on a TO sweep and of the beam of radiation on the next FRO sweep is ideally at least approximately representative ofthe angular position ofthe receiver with respect to the radiating means and with reference to a second plane normal to said first plane, characterised in that the apparatus is operable to radiate the beam of radiation on a TO sweep with a first of sa id radiators and on the next FRO sweep with the second of said radiators, whereby said angular position is with respect to the mid-point ofthe line joining the two radiators, said second plane including said point.

9. Apparatus as claimed in Claim 8 operableto radiate successive TO sweeps .:;th said first radiator.

10. Apparatus as claimed in Claim 8 operableto radiate successive TO sweeps alternately with said first radiator and with said second radiator, the representations of angular position derived by the receiver on successive pairs of TO and FRO sweeps being averaged.

11. Apparatus as claimed in any one of Claims 8 to 10 wherein the two radiators are disposed closely adjacent to one another wherein the apparatus is further operable to radiate the beam of radiation on both a TO sweep and the next FRO sweep with one of the radiators in the event of failure ofthe other radiator.

12. Apparatus as claimed in anyofClaims8to 10 wherein the two radiators are substantially spaced from one another.

13. A Microwave Landing System comprising apparatus as claimed in any of Claims 8 to 12 for providing azimuth guidance information to a receiver mounted in an aircraft, the radiators being groundbased antennae adjacent a runway, and said second plane being vertical.

14. A Microwave Landing System as claimed in Claim 13 wherein the centre-line ofthe runway passes substantially through said point.

15. A Microwave Landing System as claimed in Claim 13 or 14 comprising two monitoring means re- spectively associated with the two antennae and each disposed so asto receive the beam radiated by the associated antenna.

16. A Microwave Landing System as claimed in anyofClaims 13to 15 comprising monitoring means which are common to the two antennae and which are disposed so as to receive the beam radiated by each of the antennae.

17. A method ofoperating a Microwave Landing System to provide Azimuth guidance information to a receiver mounted in an aircraft substantially as herein described with reference to the drawings.

18. A Microwave Landing System comprising apparatus for providing Azimuth guidance information to a receiver mounted in an aircraft substantially as herein described with reference to the drawings.