US2421017A - Communication and guiding system - Google Patents

Description

7, 1947. E. M. DELORAINE ETAL 1,017

COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 13 Sheets-Sheet 1 7, 1947- E. M. DELORAINE ETAL 1 COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 15 Sheets-Sheet 2 24 will? )3; 25

76 W557 TEFM/NHZ 5727770 mam/1 2 G'AMMVEL N DEL/4 Y DEV/CE CHM/A/EZ Na I N VEN TORS May 27, 1947. E. M. DELORAINE ET AL 2,421,017

COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 13 Sheets-Sheet 5 A TTOF/VEY y 27, 1947- E. M. DELORAINE ETAL 2,421, 17

COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 13 Sheets-Sheet 4 INVENTORS EDMO/VJ M aim/24w: BY 510A 1?. A04M5 ATTORNEY y 27, 1947- E. M. DELORAINE ETAL 2,421,017

commumcuxou AND GUIDING SYSTEM Filed May 5, 1944 13 sh'eets-sheet 6 2 221 22.? 22a a w c: q:

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E. M. DELORAINE ETAL 2, 17

COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 15 Sheets-Sheet 8 Pl/lf Gil/19470? 3 1 700044701? savammk A TmR/VEY y 7 1947- E. M. DELORAINE ETAL 2,421,017

COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 13 Sheets-Sheet 9 t m 3 m IF] M20 P a ga 6 m m w y m w W M m a IN V EN TORS [ONO/V0 lW. 0540/?4/11'5 P401. IF. 404/116 BY y 27, 1947- E. M. DELORAINE ETAL 2,421,017

COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 13 Sheets-Sheet 10 syn/c 5455 WI] V5 6 JOURCE 418 E M00. M00- comma: saw/ 51? l cam mum] 405 M00 4 5) P/l/ISE INVENTORS fiomo/vo M 0510/?4/11 5 BY PHI/L 1?. flD/IMJ A TT GENE Y May 27, 1947. E. M. DELORAINE ETAL 1,

COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 13 Sheets-Sheet ll 7 6 0 2 4 m V i 2 6 .u 2 F F a W m 2 g r Ami MD M T. 2 mm wm m 2 w N Z 5 MM mm 2 p z m o u M M74 s 7 1 0 w 4 fi) 4 m lulu I r m M 5 2w M a f 4 4 MM rfl -I m E s 4 F: W H R 3 q M 4 2 H .w M W 4 m M l m ms a w a 4 a a e f f mu m 2% re 5 E01 2 J 4. 7 8

y 27, 1947- E. M. DELORAINE ETAL ,4

COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 15 Sheets-Sheet 12 A T TORNE Y 27, 1947. E. M. DELORAINE ETAL 2,421,017

COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 13 Sheets-Sheet l3 E *Jno A T TGRJVE Y Patented May 27, 1947 FICE COMIWUNICATION AND GUIDING SYSTEM Application May 5, 1944, Serial No. 534,284

37 Claims. 1

This invention relates to communication and guiding systems and more particularly to a system for communicating with a moving craft or other mobile communication unit and for guiding a craft or unit along a particular route.

Systems have been previously proposed for simultaneously guiding a craft along a given route and communicating with the craft from the beacon or guiding station. The systems of beacons for guiding crafts along a given course may comprise a series of radio beacons overlapping in space at points along the course with provisions, such as marker beacons, intermediate the radiation patterns of the separate beacons in the region of the overlapping patterns warning when to change over for reception from a new beacon system. In one prior art arrangement, there is provided a system in which the receiver on the craft is tuned in response to a change of frequency from one beacon to another so that the indicator will switch over without special attention of the pilot. However, in this type of beacon system, no provision is made to cause the craft indicatorto change its reception condition from one beacon to another at substantially the same point regardless of the direction of travel. Because of the inherent bias or sensitivity level needed to effect the control for changing the receiver tuning, this type of proposed beacon arrangement will cause the craft receiver to shift its reception condition at difierent points along the course depending on the direction of travel of the craft.

It is an object of our invention to provide a radio guiding beacon system in which a receiver carried by a craft following the beacon course from one beacon radiation pattern to another automatically changes its reception condition without special attention of the pilot and in which the change-over from reception of energy from one beacon to the other will take place at substantially the same point, regardless of the direction of travel along the beacon path.

It is a further object of our invention to provide a radio beacon system operative in combination with a radio craft receiver which will provide on the craft a uniform directional indication when passing from one beacon to another and in which the receiver is made to be selectively responsive to the separate beacons without special attention from the pilot and will change in selective response along the course at substantially the same point, regardless of the direction of travel of the craft.

It is contemplated that, along a given course there may be a plurality of different craft travelling and it is, therefore, desirable to be able to communicate selectively with the difierent craft from a central station at any point along the beacon path. In a copending application of E. M. Deloraine, Serial No. 531,851, filed April 20, 1944, entitled Repeater link system, is disclosed a repeater system for multiplex communication between two remote points by way of a plurality of directive repeaters together with the provision of arrangements for broadcasting or directively transmitting certain of the channels from each of the towers so that craft travelling along a line within range of these towers may communicate selectively with a base station or with each other.

It is a further object of this invention to provide a combined multiplex communication system of the general type disclosed in the aforementioned patent application, in combination with a beacon guiding system for guiding craft along a path defined by the line of radio towers.

It is a further object of our invention to provide a receiving circuit for use on a craft which will selectively receive multiplex communication channels and simultaneously receive beacon indicating signals.

It is a still further object of our invention to provide a combination communication system and beacon system in which a craft may follow along a line of towers provided with radio beacon radiators operating at alternately difierent radio frequencies and in which there is provided aboard each craft means for simultaneously receiving communication signals and guiding beacon signals and for automatically rendering the receiver operative selectively to the energy radiated from the alternate towers.

It is a still further object of our invention to provide, in a system as outlined above, means for assuring that the receiving equipment on board the craft changes from one alternate signal to the other substantially at the same point, regardless of direction of travel of the craft.

It is a still further object of our invention, to provide a system for communication with a craft and a guiding beacon therefore, in which communication and guiding equipment is arranged at spaced points along a course to be followed by a craft, and in which the equipment is rendered operative in response to signals from the craft in the vicinity of one of the stations, or to special control signals from a separate control station.

In accordance with a feature of our invention we may provide a multiplex communication system in which a plurality of channels are transmitted from a single terminal station over a plurality of successive towers. At each of the towers, or at selected ones of the towers, are arranged radio beacons so aligned that energy from said beacons will slightly overlap the energy radiated from each succeeding beacon. Preferably the energy from successive towers is radiated at slightly different frequencies to avoid danger of du licate reception. In the simplest form only two difierent frequencies alternate along the course. However, more frequencies may b used if desired, the frequency distributions to the different towers preferably being repeated successively along the course. Also, at each of the towers or at selected ones of the towers are branched off one or more communicating channels for broadcasting to the craft travelling along the course. The craft travelling along the course are each provided with receiving equipment for simultaneously receiving the branched off channel signals as well as the guiding beacon signal and fo segregating these signals to provide separate indications. Also, on the craft is provided some means responsive to the received energy for rendering the receiving circuit responsive only to one of the successive frequencies.

In order that the switching may take place when the craft is in substantially the same area, regardless of the direction of travel of the craft. a separate marker beacon is provided adjacent the beginning of one of the beacon courses in the region of overlap of the two adjacent beacons. This marker beacon preferably transmits a relatively narrow fan shaped beam of energy upwardly, the energy being characterized by the same frequency as the beacon whose radiation pattern overlaps the pattern of the next necessary beacon. Thus, in one direction of travel, the response of the craft receiver will be changed immediately after this marker beacon is passed due to the energy received from the beacon. In the other direction of travel, energy from the marker beacon will serve to switch the receiver to respond to energy from the next radio beacon. Accordingly, the switching over of the craft of these alternate frequencies will occur in substantially the same area.

Furthermore, the craft is preferably provided with other communication equipment permitting transmission of signals from the craft to the towers along the course for communication with the base station or for communication between separate craft travelling along the course.

In the preferred form, the multiplex communication channels will each consist of a series of pulses separated properly in time so that they will interleave one with another. These pulses may be modulated in any suitable manner to provide pulse amplitude modulation or time displacement modulation Or any other desired type of modulation. The particular type of modulation circuit used is not a feature of the present invention.

Likewise, the communication between the tower and the plane and the signals sent over the communication system may be of any desired form. Preferably, however, the beacon signals are of a relatively low frequency, for example, the normal 90-150 cycle tone frequencies or the known form of A-N keyed signals. The communication channels are preferably modulated pulse channels which may be readily separated from the guiding signals in the aircraft receiver by means of a clipper device so that segregation of the two channels may be accomplished without heavy and complicated equipment.

According to a further feature of our invention, the successive towers and beacons are normally inoperative to broadcast to the craft or to transmit beacon guiding signals and at each tower or station is provided means responsive to signal from a craft in the vicinity thereof to transmit beacon signals, and/or to broadcast communication signals. Alternately, special signals from the base station may be transmitted to the towers for the purpose of rendering the beacon and/or broadcast communication operative. Both these selective systems may be incorporated in a common system, if desired.

While the system of our invention is particularly useful for the guiding of aircraft, it is not to be considered as limited thereto. The beacon may be used for guiding and/or establishing communication with any type of mobile unit in the vicinity of the system.

A better understanding of our invention and the object and features may be had from the particular description of certain embodiments thereof, made with reference to the accompanying drawing, in which:

Fig. 1 is a diagrammatic view in perspective i1- lustrating two terminals of a multi-channel communication and guiding system together with a chain of relay stations therebetween;

Fig. 2 is a block diagram of the west terminal station of a system such as shown in Fig. 1;

Fig. 3 is a diagram partly in block of circuits illustrating the repeater tower equipment shown in Fig. 1;

Figs. 3A and 3B are block circuit diagrams of portions of the equipment shown in Fig. 3;

Fig. 4 is a block circuit diagram of a craft receiver circuit for use with the system shown in Figs. 2 and 3;

Fig. 4A is a curved used in explaining the operation of the selector feature of Fig. 4;

Fig. 5 is an elevational diagram of a beacon system in accordance with our invention illustrating the distribution of the various radiation field patterns;

Fig. 6 is a diagrammatic sectional plan view along line 6--6 of the radio beacon field patterns illustrated in Fig. 5;

Fig. 7 is a set of curve diagrams illustrating operation of equipment shown in Figs. 2, 3 and 4;

Fig. 8 is a wiring diagram of a time modulator of the type which may be used with our system;

Fig. 9 is a wiring diagram of selector and demodulator circuits usable with a system in accordance with our invention;

Fig. 10 is a circuit diagram of a pulse width selector which may be used in Figs. 2 and 3;

Fig. 11 is a graphical illustration used for illustrating the operation of the circuit of Fig. 10;

Fig. 12 is a block diagram of a modified form of equipment which may be used at a terminal station;

Fig. 13 is a block diagram of a tower equipment circuit arranged for use with a terminal station, such as shown in Fig. 12;

Fig. 14 is a block diagram of an aircraft communication equipment arranged for cooperation with the system of Fig. 13;

Fig. 15 is an elevational diagram of a modified beacon arrangement similar to that shown in Fig. 5;

Fig. 16 is a block circuit diagram illustrating the modified transmitter equipment used to produce the beacon arrangement of Fig. 15; and

Fig. 1'7 is a block circuit diagram of a receiver arrangement for response to signals such as transmitted from the circuit of Fig. 16.

Referring to Fig. 1, there are provided two terminals l and I2 (west and east) interconnected by a chain of relay tower stations l3, l4 and I5. Of course, there may be many other repeater towers, only three being shown for purposes of simplifying illustration. Each of the towers is provided with a plurality of antennas 2|, 22 and 2|A, 22A, being directional, as indicated. Energy is transmitted from west to east with different characteristics between alternate towers, for example, at Slightly different radio frequencies Fl, F2, as shown in the drawing.

In the opposite direction, east to west, alternate frequencies, F3 and F4, are used. By the use of these different radio frequencies, interference of the signals by reason of overlapping of energy between the repeater towers is minimized. Antennas 2|, 22 and 2|A, 22A are preferably made sharply directional. At each of the towers are provided other antenna units 23 and 24 for transmitting and receiving communication signals to craft in the neighborhood of the various stations. The transmitting antennas 23 may transmit energy at the same frequencies Fl and F2, as used in the radio link system. However, the inbranching or incoming signals from the various craft are preferably at a different common frequency, herein shown as frequency F5.

Also mounted on each of the towers are a plurality of guiding beacon transmitting antennas 26 preferably provided with a directive reflector arrangement 26A. These beacons transmitters may operate at the alternate frequencies corresponding to the alternate frequencies Fl and F2, transmitted from the towers in the west-east direction. It should be clear that, if desired, the beacons may be arranged to cooperate with the east-West channels instead. At each of the intermediate towers in the zone of overlap of the directive radio beacon signals are provided separate marker beacon transmitting antennas 25 operating at a particular signal characteristic, for example, at the frequency corresponding to the next preceding radio beacon pattern. The arrangement of the guiding beacon and marker beacon system is provided for assuring that craft travellin along a line defined by the beacons will be enabled to switch from the reception of one of the frequencies to the next adjacent one in substantially the same area, regardless of the direction of travel of the craft. This function will be explained more fully in detail later.

A better understanding of the system shown in Fig. 1 may be had from the particular description of one embodiment of our invention as illustrated in Figs. 2, 3 and 4.

Referring to Fig. 2, the west terminal is shown to include a switchboard 50 containing jack connections for outgoing and incoming channels l-N and la-Na, respectively. The switchboard is of known character provided for connectin telephone circuits or trunk lines to the proper channel jacks and also for connecting together incoming and outgoing channels under certain circumstances described hereinafter. Channel includes a modulator 5| coupled to terminal I. The modulator 5| and also all others from modulator 52 to the modulator for channel N may be of any desired type but are preferably of the character for producing push-pull time displacement modulation of pulses. In order to effect this time modulation, energy from a base wave source 60 is applied over individual phase shifters 62-41, etc., for channels 2 through N, respectively. Channel I need not have an individual phase shifter since the phase of the wave from source 60 may be applied directly without phase shift to the modulator 5|. The phase shifters for the other channels are adjusted so that the individual trains of pulses for the separate channels will be displaced in time to provide a single resultant train in the common output 10.

Channel I has been chosen as the synchronizing and monitoring channel and, as such, is given a particular width Wl by shaper 58. The pulses of the other channels may be given a difierent width W2 in shaper 58a.

All the trains of pulses produced by the different channel modulators are combined, as indicated by the output connections at 10, and applied to an R.--F. translator l4 and radiated by transmitting antenna 2|. The R.-F. translator in this instance will provide a carrier wave of frequency Fl, as indicated in Fig. 1. This train of multiplex channel pulses is east bound, channel operating as a synchronizing and monitoring channel for controlling the separation of channel pulses at the receivers whether in a plane or at the east terminal l2.

In order that selected ones of the channels may be broadcast at terminal l0, another modulator I6 is provided. This modulator is coupled to the selected channels to be broadcast transmitted at a wider angle for communication with craft. Separate couplers 7-8 are provided to prevent the other channels coupled in parallel to line 10 from also feeding to modulator 16. These branched signals are then transmitted from antenna 23 for a purpose of communication with craft as will be more readily understood when the particular description of Fig. 4 is given.

Before discussing the equipment and operation of the relay stations, let it be assumed insofar as the west base is now concerned that a west bound train of channel pulses la, 2a, 3a Na is being received on antenna 2lA, Fig. 2, and that the pulses thereof are detected into video form at receiver 80. The video pulses of the incoming channels are applied to a pulse width selector 8|. The video pulses passed by the selector 8| are applied to a demodulator 84, the output of which is applied to the corresponding switchboard terminal.

Energy from the pulse width selector is also applied to the deblocking selectors of the other receiving channels, the energy being applied through suitable delay devices for proper timing. Referring to channel 2a, for example, the deblocking pulse is applied to a delay device and thence to deblocking selector 92 where it controls the selection of the pulses of channel 2a, as received over line 9|, the selector excluding all other channel pulses. The channel pulses passed by the selector 92 are applied to demodulator 94 whereby the time displacement of the video pulses is translated into an audio signal wave which, in turn, is applied over line 95 to the proper jack on the switchboard 50. This selector control is the same for the additional channels 311 to Na and therefore need not be illustrated or described in detail.

The return communication signals from the craft in the vicinity from the west base terminal are received on antenna '24 and detected to audio frequencies in a suitable detector arrangement l5. Detector 15 is provided with a channel selector system for separating the received signals in accordance with the selected channel and applying these signals to individual lines 11. A suitable selector for this purpose may be made in accordance with the teachings hereinafter more fully explained in connection with Fig. 3B. The separate output lines are connected over a cable 19 to the individual terminals Ia, 3a, 5a, Ia, etc., of switchboard 50.

In Fig. 3 is disclosed in more detail a typical block circuit diagram of the equipment provided with the repeater tower. Thus, as preferably shown, there are provided four directive antenna arrangements 2|, 22, '2IA, 22A, coupled to the receiver- transmitter units 32, 3| and 43, 41, respectively, of the repeater circuit. From the antenna 22 are applied pulses to the outbranching circuit I which serves to select the channels it is desired to transmit from antenna 23 for reception on the separate craft. These output signals are applied to antenna 23 over line IOI. Carrier source 36 supplies energy at frequency F2 to energize beacon modulator 31 and the outbranching circuit I00. The energy received from the craft is applied from antenna 24 over line I03 to inbranching circuit I05 from whence it is supplied to antenna 22A together with signals from west bound transmitter 41. Energy from oscillator 48A is applied at frequency F3 to transmitter 41 and inbranching circuit I05. The inbranching circuit is timed by energy from receiver 43 as indicated by line I05a. The marker beacon antennas 25 are energized by transmitter I02 operating at the frequency FI, the same as that received on receiving antenna 22. For communication with aircraft antennas 23, 24 and 25 are preferably mounted above a shielding sheet I04 mounted on the associated tower. This sheet serves as an artificial ground for these antennas.

At the tower, the guiding beacon antennas 26 are provided within an angular reflector 26A which may be mounted on the tower in the manner illustrated in Fig. 1. The reflector arrangement is adjusted to cause the beacon antenna system to operate substantially unidirectionally. Furthermore, the horizontal portion of reflector 26A serves as an artificial ground to prevent much of the beacon energy from being radiated earthward to cause troublesome reflections and the consequent false course indications. It should be understood that, if the spacing above ground causes an unduly large number of lobes in the vertical plane, or too much trouble from ground reflection, the beacon antenna may be mounted directly adjacent the ground and the area therearound suitably cleared to prevent undesired refleeting action. The beacon antenna is illustrated only diagrammatically since the specific form of beacon used is not material to the principles of our invention. It is preferable to make this beacon as simple as possible and of such construction that little service is needed. Likewise, it is clear that the modulation may be produced either by keying the antennas alternately to shift the pattern or by modulating with different tone frequencies, such as 150 and 90 cycle tones commonly used in radio course beacons at the present. Moreover, each beacon may be caused to transmit identifying signals if desired as well as the directive indications.

Turning now to Fig. 3A, a more particular explanation of the out-branching circuit I00 will be given. The energy from the antenna 22 is applied to a detector I06 which detects the incoming waves to a video frequency. Since detection will generally take place in receiver 3I if the pulses are to be transmitted at a different R. F. detector I06 may be omitted and the signal taken from the output of this receiver. The detected pulse envelope is applied to pulse width selector I08 which serves to select out the shaped pulses of channel I. These pulses are then reshaped, if necessary, in shaper I I0 after which they are applied over line I33 to modulator I34. The selected pulses from the output of I08 are also branched over delay circuits I I3, H5, H1, I I9 and I2I and shapers H4, H6, H8, I20 and I22, respectively, and applied to channel selectors I24, I26, I28, I30 and I32, respectively with the channel Signals from detector I06 to select the pulses corresponding to channels 3, 5, 1, 9 and I I, for example. These selected channels properly timed with respect to channel I are then applied from the output of the respective selectors to line I33 from whence they are all applied to modulator I34. The output of modulator I34 is connected by means of line IOI to antenna 23, as shown in Fig. 3. Thus, the selected channels are broadcast for selection and reception by craft within the field of the communication system.

The operation of the in-branching circuit I05 may best be understood by reference to Fig. 3B. In this figure, the input line I03 is shown as connected to a detector I40 which serves to detect the received radio frequency to video frequency producing in the output a plurality of pulses corresponding to the channels received by antenna 24. In the form illustrated, each of these channels is represented by pulses of different widths. Accordingly, separate width selectors and demodulators are provided, as shown at I4I, I43, I45, I41, I49 and I5I. Just above selector MI is provided a base Wave generator I60 which is controlled by selector synchronzer I60a to produce a suitable base wave for timing these received channels for insertion into gaps in the east-west multiplexing channel of the system. Energy from this base wave generator is applied over separate phase shifters I6I, I63, I65, I61, I69 and Ill to the separate pulse modulators I42, I44, I46, I48, I50, I52, respectively. In these separate modulators are produced time modulation pulse trains corresponding to channels Ia, 3a, 5a, 1a, 9a, and I la, respectively. The output of all of these modulator circuits are combined and applied to modulator I12 which serves to modulate the carrier frequency F3. The resulting carrier frequency pulses are applied to the output of transmitter 41 for transmission together with the other multiplex channels. It should be understood, likewise. that in this particular case, instead of providing R.-F. modulator I12 in the in-branching circuit itself, these separate pulses could be applied to the input of the modulator in transmitter 41 used for the normal repeated channels.

In Fig. 4 is illustrated a typical receiver circuit for use on a craft, such as an aircraft, for receiving the communication and guiding signals from the tower of the system in accordance with our invention. In this circuit, there is provided aircraft receiving antenna I which receives the beacon signals and the communication signals transmitted from the tower by means of antennas 23 and 26. Antenna I80 is preferably coupled to a broad band receiver I8| tuned sufficiently broadly to receive both carrier frequencies FI and F2. In the output of receiver I 8I are provided amplifying filter arrangements I82, I84 tuned to pass frequencies FI and F2, respectively. When the craft is in such a position that energy at F2 predominates, the output from I84 may be applied to an automatic volume control circuit I86 which serves to bias I82 so that substantially no energy at FI passes. Similarly, when the craft passes to a region where Fl predominates A. V. C. circuit I88 in the output of I82 biases circuit I84 so that substantially no energy at frequency F2 is passed. Thus, as the craft travels along a course defined by the radio beacons of different frequencies, the receiver on the craft is made to respond to one of these signals to the exclusion of the other. It should be understood that other signal characteristics different from frequency discriminations may be used to operate the receiver switching if desired. The receiver energy is then detected in detector I81 to provide an envelope of the form shown in Fig. 4A. This envelope has a low frequency wave component 2I5 corresponding to the beacon signals and a pulse component 2I8 corresponding to the pulse communication signals. This composite wave from the output of detector I81 is applied to a limiter I90 which serves to clip or limit the envelope at a level 2I1 preferably substantially at twice the average incoming carrier frequency so that the low frequency envelope 2I5 will be passed but the high energy pulses will be limited in amplitude. The output of limiter I90 is then applied to a selective filter I9I which serves to remove the higher frequency pulse components still remaining and this output signal is then applied to a guide indicator I92 or to other visual or audible indicators, if desired.

The envelope wave is also passed to a clipper I93 which serves to remove the portion of the envelope below clipper level 2" to separate the incoming pulses from the lower frequency envelope and apply them to the selective circuit. These pulses are applied to a pulse width selector I94 which serves to select the pulses of channel I and apply them to a demodulator circuit I95 from which they are applied to headphones or other audible signal source I96. These pulses of channel I are preferably order channel impulses used for communicating to all of the planes or other craft in the region and also may serve for selecting another channel. Accordingly, the pulses from the output of pulse width selector I94 are applied to an adjustable delay circuit I91 which serves to adjust the delay of the pulses in the output of this selector to a proper position for selection of other communication channels from among those received at I8I. These output pulses are applied to a deblocking selector I98 together with the clipped pulses from the output of clipper I93. In selector I98 the desired channel is selected by adjustment of delay circuit I91 and these selected pulses are applied to demodulator circuit I99 and from there to headphones 289.

For the return communication a microphone 20I is associated with the other channel receiver I96 and applies pulses to a pulse modulator 282. A control wave source 293 is provided for properly timing modulator 292. If desired, the control wave source may be synchronized with the output pulses from adjustable delay network I91 or directly from the output of selector I94. This control wave, together with the audio signals from 281, produce in the output of modulator 222 pulses time displaced in accordance with the signalling energy applied. These output pulses are then shaped to a desired Width in pulse shaper circuit 284 and applied to radio fre-.

quency modulator 2. A wave of carrier frequency F5 is also applied to modulator '2 from a source 2I8- so that the combined signals may be radiated from antenna 2 I2 for reception on antennas 24 of the link channel. In addition to the control channel transmitter, is provided a communication channel transmitter including a microphone 206 and a pulse generator and modulator 201. Output energy from control wave generator 283 is applied over a phase shifter 205 to pulse generator 201 for the purpose of properly timing these pulses to prevent overlapping of the pulses from modulator 282. The output pulses from modulator 281 are applied to a pulse shaper 208 which serves to adjust the pulses in width to correspond with any one of the channels Ia, 3a, 5a, 1a, 9a and Ila, which are used for communication between the plane or other craft and base station. These pulses are then also applied to modulator 2I I for transmission over antenna 2I2.

It is clear that the signals transmitted from the craft equipment will be received in the tower equipment, such as shown in Fig. 3, and demodulated and properly timed for application to the multiplex channel in proper timed relationship.

A better understanding of the beacon operation may be had from the following description made with reference to Figs. 5 and 6 of the drawing. As shown, each of the communication mediums for the two directions comprises a series of repeaters having field patterns shown diagrammatically in Fig. 5 at 220, 22I, 222, 223 and 224 for the eastbound traffic and 240, 24I, 242, 243 and 244 for the westbound traffic. In addition, the beacon signals transmitted from the succession towers are shown at 225, 226, 221 and 228 of Fig. 5. This unidirectional pattern is preferably made to have considerable energy component in the vertical plane so that aircraft may follow the course at many different altitude levels. Preferably, each of these guiding beacon patterns is produced by two relatively narrow overlapping patterns 225, 225a, 226, 226a, 221, 221a, 228 and 228a, as can be more clearly seen in Fig. 6.

It should also be realized that there may be a number of vertical maxima and minima points in the radiation lobes of the beacon patterns when the antennas are mounted high above the earth. For this reason, it is desirable that the receiving switching system be not made too sensitive since, at the fringes where the succeeding patterns tend to overlap one another, there might be caused a number of reversals and switches from one pattern to the other before sufiicient domination of one of the radiation patterns was achieved. Because of the difficulty in maintaining perfect alignment of the beacon stations, this switching action would be very confusing on the pilot, as it would tend to show him suddenly bearing a considerable distance from the course. By making the receiver switching less sensitive so that considerable amplitude difference is required for the switching action, this oscillation of the receiver circuit between the two conditions at the points of overlap of the patterns may be avoided.

Because of the safety margin provided in sensitivity, an aircraft such as shown at 222A proceeding from the west to east would have to cross the boundary of the overlapping zone of radiation patterns 228 and 221 to some point such as indicated by the dot 225A before the automatic switching could take control. On the other hand, an ai fq fafil such as 2223 travelling in the 0pposite direction, would have to pass out of the influence of radiation pattern 227, for example, and considerably into the field of the next succeeding radiation pattern 226 to some point, such as shown at 225B before the switching over of the receiver would be effective. This would result in aircraft travelling in opposite directions switching over their control at different points along the path. Such variation in switching points is generally not desirable particularly in view of the communication with the base station mounted on similar type receivers. In order to assure that aircraft travelling in opposite directions will be switched over for reception of the next succeeding pattern in substantially the same zone or area, regardless of the direction of travel the additional marker beacons 230, 23!, 232 and 233 may be provided. These beacons, as shown, are made to have patterns slanting forward in the direction of the repeater stations because of the particular characteristic shape of the guiding beacon radiation patterns. Thus, craft 222A travelling west to east from one beacon to another will still be switched at substantially the same point 225A. However, craft 222B travelling in the opposite direction will have its receiver switched by means of the energy in the marker beams 230 to 233, respectively, so that the switching over will take place in substantially the same area as that of planes travelling west to east. In the case of plane 222B, for example, it will be switched over at about point 225C. These marker beams are preferably made relatively narrow in the line of travel of the craft and relatively wide transversely thereto as shown in Figs. and 6.

Fig. 6 may be considered as a plan view in cross section along lines 6-6 of Fig. 5. This type of illustration is preferable since the marker beams 230-233 are arranged at an an e d a accurate plan view would therefore be less clear.

While we have shown preferably the use of unidirectional radio beacons, it is clear that bidirectional beacons may be used, if desired. Likewise, it is clear that the beacons need not be provided at each repeater station but may be provided at discretely placed intervals so that adequate overlapping patterns are produced.

In order more clearly to understand the operation of the multiplexing system, as shown in the preceding figures, reference is made to Fig. 7. In this figure, curve a represents several trains of pulses corresponding to the respective channels. It will be seen that the succeeding channels I to l2 for example, are spaced in time so that they do not overlap. Likewise, channel I is shown to be composed of pulses of a different width from the other channels for the purpose of selectively timing the remote receiving equipment. The timing of the channels at the transmitting end, for example, at the west base, may be accomplished by means of apparatus described more fully in connection with Fig. 8. A rectified wave 250 may be provided to produce channel I and by phase shift a different rectified wave I used for production of the next succeeding channel. The progressive phase shift of the control wave may be used successively to time the other channels of the system in a similar manner.

In curve 0 the selected channels I, 3, 5, 1, 9 and H are illustrated in extended form with respect to the showing of curve (1. These pulses may, for example, represent the substantially evenly spaced pulses branched out at the control towers for communication with the craft. Separation or selection of the channels may be obtained by delaying pulses l different amounts so that they coincide in position with the pulses of successive channels, 3, 5, l, 9 and l I, as shown in curve d. If these pulses are then combined and clipped at level 252, the channels may be separated out from the entire group of channels. For example, if a single channel is to be selected as shown in curve e pulse I will be delayed to position it in time to coincide with channel 5 for the selection of this particular channel.

In curve f are shown pulses of different width which may be used on the craft to indicate separate reply channels lb, 3b, 5b, 1b, 9b and llb which may correspond to the channels I, 3, 5, 1, 9 and II transmitted to the craft. These separate channels are selected at the tower in accordance with pulse width, as was explained more particularly in connection with Fig. 3B. A more complete description of the pulse width selector apparatus is made hereinafter in connection with Figs. 10 and 11.

In order more clearly to disclose the structural examples of a pulse generator and modulator suitable for use in the transmitter circuits of Figs. 2, 3 and 4, reference may be had to the circuit diagram of Fig. 8. Signal energy for channel I, Fig. 2, for example, is applied to modulating circuit 5|, the output of which is coupled to the outgoing line 10. The input signals are applied to the grid circuit of coupling tube 300. The wave from tube 300 is applied over separate transformer windings 304 and 305 to a mixing transformer arrangement 303. Simultaneously, energy from base wave generator 60 is applied to mixing transformer 303 by means of coil 306. Secondary coils 301 and 308 are also coupled to transformer 303 to extract from there the mixed signals. Coils 301 and 308 are coupled to a pair of triode amplifiers 309 and 3l0 asymmetrically biased by batteries 3! I, 3l2 for operation in the manner of an offset full-wave rectifier. The base wave of generator 60 preferably has a frequency corresponding to the desired cadence frequency T of the pulses to be produced.

Since tubes 309 and 3l0 operate in efiect as an offset full-wave rectifier, an output wave of the form shown at 3|4a will be produced in the ab sence of any input signals. This wave is applied to a pulse shaper 314 which clips and shapes the cusps of wave 3! to produce a plurality of unevenly spaced pulses (see dotted line pulses of pulse train 3| 5) having cadence frequency T. Upon application of energy over channel connection 311, the effective bias of the push-pull circuit of tubes 309, 3| 0 is changed producing pulses having different displacement in pushpull as indicated by curve 3| 3 and the solid line pulses of pulse train 3l5. This displacement represents applied energy of one sign, for example, positive. The displacement will also take place in the other direction for applied energy of different sign so that the pulses will be displaced further from symmetry. Thus, as the signals are applied to modulate the base wave, a time displacement of the pulses in the output of pulse shaper 3M is provided. While Fig. 8 discloses a modulator circuit for block 5| of Fig. 2, it is clear that the same type of modulator may be used in all of the different pulse generator and modulator circuits of the system. For a further description of this type of modulator, reference may be made to the copending application of E. Labin and D. D. Grieg, Serial No. 455,897, filed August 24, 1942.

A typical demodulator circuit for time modulated pulses is shown in Fig. 9. This circuit corresponds to the combination of deblocking selector BI and demodulator arrangement 84 hereinbefore referred to in Fig. 2. It should be distinctly understood, however, that the demodulator 84 of this circuit may be used for demodulation in the other parts of the system.

According to the arrangement of Fig. 9, the train of pulses 32I is fed over line 83a to mixer tube 320 of the deblocking selector 92. At the same time, deblocking pulses 323 from delay circuit 90 (Fig. 2) are fed to another separate grid of tube 320 over line 322. These deblocking pulses serve to produce, in conjunction with the selected incoming pulses timed to add hereto, an output series of pulses from tube 320. Assume, for example, that channel pulses of 32l are applied to the selector 9|, the delay line will operate to provide a series of rectangular pulses, 323 timed in spaced relation according to the pulses of channel 3. These combined pulses may have the form shown in curve 6, Fig. 7. Tube 320 is also biased to serve as a clipper so that only the boosted pulses of the selected channel appear in the output thereof. A D. C. restorer rectifier tube 330 may be provided across the input of the deblocking pulses 323 to assure that the incoming deblocking pulses are of a proper level to work with the clipper circuit of tube 320 to leave only the desired output pulses above clipping level.

The output pulses from tube 320 have the same cadence frequency T as the original modulated pulses 3l5 from modulator 303, Fig. 8. The pulses are applied to the control grid of a demodulator tube 349 and cause tuned circuit 3M connected to another grid of this tube to oscillate at the tuned frequency producing in the tube 349 a combined grid potential in the form of a combination of the wave generated in 3H and the incoming pulses. Circuit 34l is preferably tuned to some harmonic of the cadence frequency of the input pulses so that as the repetition rate is varied, due to the modulation signals, the output pulses will be raised to different levels depending upon their time displacement. Accordingly, in the output of tube 340 will appear a modulation envelope of pulses carrying signal modulations thereon. For a further understanding of the principles of this type of demodulator, reference may be had to the copending application of D. D. Grieg, S. N. 459,959, filed September 28, 1942.

A low-pass filter 345 is provided to remove from this demodulated signal envelope the pulses of higher frequencies that define the signal envelope.

While the pulse width selectors of Figs. 2 and 3 may comprise any circuit arrangement capable of segregating a pulse of a given width from pulses of greater and/or lesser widths. we have shown in Fig. 10, for purposes of illustration, a suitable circuit which, depending upon selection of circuit constants, is capable of distinguishing between two pulse widths difiering by as little as one hundredth of a microsecond. Such fine distinction, however, is not necessary except where selection among a very large number of difierent pulse widths are required between close limits.

The circuit of Fig. 10 preferably includes a limit clipping stage 353 as an input coupler which limits all input pulses to substantially the same amplitude. Should the input pulses be of a positive polarity as indicated by the pulses of curve Ila in Fig. 11, the coupled stage 350 also serves to reverse the polarity as indicated by the pulses of curve I lb. This output pulse energy from stage 359 is applied through a resistor R to a shock excitable L-C circuit 355. Connected across the tunable circuit 355 is a vacuum tube 360, the cathode 36l of which is connected to the input side of the circuit 355, while the anode 362 is connected to the opposite side 363 of the tunable circuit. The side 363 is also connected to a source of anode potential 364. The pulse energy, curve I lb, from the anode connection 354 is applied to the grid 365 of the tube 380 so as to block the conduction between the cathode 36l and the anode 382 while pulse energy is applied to the circuit 355. The undulations produced in the circuit 355 in response to pulse energy over anode connection 354 are taken oil through a connection 3'10 for application to a threshold clippin amplifier stage 315. The bias on the grid 376 is controlled by adjustment of resistor 311. In the output 318 of stage 215 is a pulse width shaper 389, the operation of which is hereinafter described.

Assume, for purposes of illustration, that the widths of the pulses of curves Ho and llb correspond, respectively, to channels I, 3, 5, l and 9. as indicated by the width reference characters Wl, W3, W5, W1, and W9. Assume also that the circuit 355 is tuned for selection of pulse width W5. Curve I I0 represents the output of the circuit 355 when the circuit 355 is tuned for selection of pulse width W5, illustrating the different output undulations for the difierent pulse widths of curve Ila. When the leading edge 38l of the pulse W5 is applied at negative polarity, as indicated by curve llb, to the circuit 355, an initial undulation 382 is produced which is normally followed by undulations 383, and so on in the form of a damped wave. When the circuit 355 is tuned to a frequency, the period of which is exactly twice the width W5, the trailing edge 386 occurs where the initiated oscillatory energy crosses the zero axis from undulation 382 to undulation 383. Since the trailing edge 386 shock excites the circuit in the same direction at this point, an undulation 381 coinciding substantially with 383 produced thereby in the circuit 355 adds algebraically to the undulation 383 to produce undulation 399. The next succeeding pairs of undulations produced by the leadin and trailing edges of pulse width W5 would normally tend to produce a negative undulation which would continue as a damned wave. The damping tube 360, however, eliminates the trailing oscillations so that they do not interere wit the undulations produced by subsequent nulses a plied to the circuit 355.

A pulse width less than nulse width W5. such. for example. as pulse widths WI and W3, will -ot pro ce maximum undu ations as great as the undulation 398 for the tuning adjustment corres onding to pulse width W5. This is illustrated by the undulat ons 393 and 4 produced in response to the pulse widths W! and W3, res ectively. The reason for this is readily a parent because the shock excitations nroduced bv the leading and trailing edges of the pulses of lesser width than W5 are, in part. opposed to each other. as indicated y the broken lines associated with t e undulations 393 and 394. The undulations 395 and 396 produced in re ponse to the greater pulse widths W! and W9 are likewise smaller than the undulation 39!! since here again the oscillations produced in response to the leading and trailing edges of the greater

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