GB1605283A - Vehicle identification - Google Patents

Description

(54) VEHICLE IDENTIFICATION (71) We, A.C. COSSORLIMITED, a British Company, of The Pinnacles, Harlow, Essex, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an IFF (Identification Friend or Foe) system suitable for use by tanks and other military vehicles.

The prime requirements of an IFF system are firstly that it shall provide rapid recognition of a friend and secondly that it shall have a high degree of security. The first requirement is acute in close combat situations where there may be a weapon reaction time of only 2 seconds. The second requirement is essential in order that the enemy may not be able to locate vehicles from the interrogating or reply signals employed in the system.

The object of this invention is to provide a system which meets these requirements to a very high degree and is of such a nature that it can be developed to allow the exchange of large amounts of secure information between vehicles.

According to the present invention there is provided an IFF system wherein each of a plurality of vehicles is equipped with a highly directional transmitting aerial steerable in azimuth, means operable in an interrogating mode to transmit an interrogating signal via the transmitting aerial, an interferometric receiving aerial system coupled to means adapted to decode the bearing of an interrogating vehicle, control means operative in a receiving mode to steer the transmitting aerial to the reciprocal of the decoded bearing, and a transponder responsive to a received interrogating signal to transmit a reply signal on the reciprocal bearing by way of the transmitting aerial.

The interferomic receiving aerial system and the decoding means can be constructed in the manner disclosed in the specification of our British patent application No. 1401273 30703/71. This prior specification describes a secondary radar system operative in the L-band with interferometric arrays some hundreds of feet in extent. However, the principles are exactly the same at higher frequencies and it is preferred to operate in the region of 20 to 50 GHz. In the first place, such high frequencies bring the dimensions required for interferometric arrays and narrow beam width linear arrays down to the region of say six feet. Equally important, however, is the degree of security obtainable for the following reasons: 1. At such high frequencies the frequency bracket which has to be searched by an enemy listening post is very large indeed.

2. It is possible to make the transmitted beam very narrow, e.g. a 3 dB beam width less than 0.50. Both interrogations and replies are effected by way of narrow beams.

3. It is possible to make the message times very short and to employ frequency multiplexing.

The transmitting aerial is preferably an electronically steered, multi-aperture, linear phased array. In order to obtain full 360" coverage, one such array and a corresponding interferometric receiving array may be mounted along each side and across the front and rear of each vehicle.

In order to enhance the usefulness of the system, it can be arranged to operate in accordance with a mutual recognition dialogue.

Normally all vehicles are in a standby, ready-to-reply mode. When a first vehicle wishes to interrogate a second vehicle the operator switches his equipment to the interrogate mode and transmits an interrogation, having steered his transmitting aerial to the correct bearing, e.g. by feeding in this information from a visual sighting or automatically from a primary radar sighting.

The interrogated vehicle makes an immediate reply, assuming that it recognizes a valid interrogation. The first vehicle has now recognized the second as a friend. For mutual recognition it is arranged that, when a vehicle has made a reply, it shall immediately switch to the interrogate mode and transmit an interrogation on the same bearing as the reply.

Furthermore, each vehicle automatically reverts to standby mode when it has completed an interrogation. Therefore the second vehicle will now interrogate the first and effect recognition thereof. Termination of the dialogue is discussed below.

Provided the transmitting aerials and their feed circuits are made reciprocal it is possible to provide a communication channel between any two vehicles which have achieved mutual recognition since their aerials have been directed to reciprocal bearings. Because of the high frequency and narrow beamwidth the channel is reasonably secure.

It is furthermore possible, as explained below, to equip a command vehicle to operate in a surveillance mode in which it plots all vehicles in its vicinity by means of primary radar, plots all friendly vehicles using its IFF system and thereby knows which of the vehicles detected by primary radar are foes.

Security is naturally lessened by the use of any surveillance mode, but this may be an acceptable risk when the platoon commander is faced with extricating his vehicles from an involved situation which has developed during an engagement.

Once a vehicle has identified a friend it may effect pretty secure communication therewith by means of highly directional, low level transmissions via its steerable transmitting aerial.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein: Fig. 1 is a plan view of a tank showing the disposition of the aerial arrays thereon; Fig. 2 is a perspective view of the aerial arrays on one side of a tank; Fig. 3 is a schematic diagram of part of a transmitting array and the feed arrangements therefor; Fig. 4 is an exploded perspective view showing the construction of the transmitting array; Fig. 5 is a block diagram showing the transmitting array and transmitter with circuits allowing two-way communication; Fig. 6 is a more detailed diagram of the transmitter, omitting the circuits which allow two-way communication; and Fig. 7is a diagram of part of a receiving interferometric array and the circuits associated therewith.

Fig. 1 shows a tank 10 on which are mounted four aerial arrays 11 each covering a sector of 46" either side of the broadside direction of the array, whereby full 360" coverage is achieved.

Each array 11 (Fig. 2) is mounted in an armoured case 12 and comprises a transmitting array 13 and an interferometric array 14.

Assuming that the transmitting array 13 has a binary corporate feed, i.e. a feed which repeatedly bifurcates to branch out to the apertures, it must have a number of apertures which is a power of two, which must be large enough to yield the required resolution (beamwidth). 512 apertures will yield a 3 dB beamwidth in azimuth of 0.39 . The beamwidth in elevation may be 11", which is sufficient to cater for differences in elevation between vehicles and the tilting of vehicles. The operating frequencies are assumed to be around 35 or 40 GHz which provides a usable band of 350 or 400 MHz assuming a 1% bandwidth for the aerial systems. Within this band a set of frequencies and a different set of frequencies are assigned for interrogation and reply respectively.Frequency diversity techniques may be employed to render the system immune to jamming.

Code formats for interrogations and replies may be established using conventional pulse trains or using a block of contiguous frequency shifted pulses, such as may result from a Barker code sequence. Frequency multiplexing is necessary when employing a Barker sequence but optional when employing conventional pulse trains.

Fig. 3 shows four of the 512 apertures 15 of the array 13. These are coupled to a binary corporate feed 16 through individual ferrite phase shifters 17 which enable the beam to be steered. Fig. 4 shows one form of construction in which the apertures 15 are provided by vertical horns milled in an aluminium block 18 and are protected by a glass or reinforced plastics cover plate 19. Behind the block 18 is a block 20 milled with 512 channels 21 in which are disposed the ferrite phase shifters. Behind the block 20 is a stack of eight plates 22 in each of which is milled a series of bifurcating wave guides 23. These waveguides thus bifurcate from left and right corporate feeds 24 and 25 (Fig. 5) to the 512 apertures, 256 = 28 apertures from each feed 24 and 25.

In an alternative construction of the transmitting array, strip line techniques are employed; other distribution techniques are possible.

The transmitted energy is horizontally polarized, i.e. the E vector is horizontal, and the apertures 15 may have horizontal and vertical dimensions of 0.318 cm and 4.75 cm respectively. The horizontal pitch of the apertures is approximately 3.57 cm and the overall length of the array is 183 cm. These dimensions apply at a frequency around 40GHz.

If the transmitting arrays are to be used for two-way communication as well as for IFF transmissions, the arrangements shown in Fig.

5 may be employed which also allow use of ISLS (interrogator side lobe suppression) techniques. These techniques are very well known in conjunction with secondary radar systems and will be described very briefly only here. It is necessary to prevent a near transponder being triggered by energy in a side lobe of the transmitting array, and this is achieved by transmitting a less directional control pulse which, at any transponder, has an amplitude less than an interrogating pulse in the main beam but greater than an interrogating pulse in a side lobe. The transponder only replies when the interrogating pulse is larger than the control pulse.

In Fig. 5, the left and right corporate feeds 24 and 25 are connected to sum and difference channels 26 and 27 by means of a hybrid 28.

The transmitter 29 is connected to the channels 26 and 27 by way of a latching ferrite switching circulator 30 controlled by a switching waveform on line 31. The control pulse is routed to the difference channel 27 by switching the circulator 30 to anti-clockwise circulation in Fig. 5. Since the two halves of the aerial are excited out of phase, a broad control pattern is achieved (with a notch centred on the main beam). The interrogating pulses are routed to the sum channel 26 by switching the circulator to clockwise circulation. The full resolution of the energy is now attained.

A three-port circulator 32 is interposed in the sum channel 26 to route received signals to a line 33 connected to a communications receiver system (not shown). In order to effect two-way communication, the transmitter 29 is provided with means for effecting modulation in accordance with speech signals (e.g. utilizing any convenient form of PCM) as well as in accordance with IFF interrogating codes.

Fig. 6 shows the transmitter circuits in more detail, omitting for simplicity the sum and difference feed arrangements of Fig. 5. At the frequencies involved, and given the obvious requirement for robust construction, the transmitter is preferably based on solid-state devices such as LSA or TRAPATT oscillators (see for example 'Pulsed LSA and TRAPATT Sources for Microwave Systems, W.E. Wilson, Microwave Journal, Vol. 14, No. 8, August 1971). The circuits of both Fig. 6 and Fig. 7 (described below) are preferably all contained in the case 12 of the corresponding array 11 (Fig. 2), to which end a local power unit 34 is provided in Fig. 6. The heart of the transmitter is a diode mount 35 preceded by a video low pass filter 36 and followed by a harmonic filter 37, all of which elements are built into an appropriate waveguide construction.The output from the filter 37 is coupled to the array 13 through a circulator or isolator 38.

The interrogating transmission can be composed of a sequence of 0.1 jaS pulses framed by a pulse pair F1 and F2 and occupying a selected combination out of say 13 equally spaced pulse positions, thus utilizing a format similar to that of normal IFF/SSR practice. In such trains the interrogations and replies may be respectively at fl and fl' or may be multiplexed in any desired sequence of f and f2 (for transmission) or of fl' and f2) (for reception). The interrogation is preferably in a Barker code, in which case the frequency multiplexing is essential.Barker codes employ sequences of 5,7 or 13 bits and exhibit an extremely pronounced auto-correlation function which enables the transponder to effect absolutely reliable discrimination between valid and spurious interrogations and enables the framing pulses F1 and F2 to be omitted. Normally Barker codes are delineated by the use of phase shift modulation (keying) but, in the present suggestion, frequency shift keying is substituted, requiring the use off1 and f2 (or f11 and f2,) but allowing the transmission of the pulses to be contiguous in time.

Pulses at f, and f2 are triggered by signals on lines 39 and 40 connected to a modulating pulse amplitude programmer 41. AFC feedback is also applied to this point in the circuit from two cavities 42 and 43 turned to fl and f2 respectively, and having probes 44 coupled to the waveguide which incorporates the diode mount 35. The programmer 41 is followed by a modulating pulse driver 45 whose output drives the oscillator through a pre-distortion filter 46 provided to eliminate chirp effects.

Also mounted within the case 12 is an eight bit register 47 which controls the ferrite phase shifters 17, the steering data being fed serially into the register 47 on line 48. Digitally controlled ferrite phase shifters are known and current developments are extending availability into the region of tens of Gigahertz.

Various forms of ferrite phase shifters exist, some of which have latching properties and some of which are reciprocal (as is necessary if two-way communication, as provided by the inclusion of element 32 and data line 33, is to be employed).

In view of the information available in the above-mentioned patent specification, only a very brief description of the receiving interferometric array 14 will be given. As in the said specification, each such array may consist of three sets of three interferometer pairs, one set being shown in Fig. 7. The three pairs of horn apertures 50 are connected to individual mixers 51 fed from a common local oscillator 52. The mixers are followed by IF amplifiers 53. The outputs of each pair of amplifiers are applied to a corresponding phase-sensitive detector 54 which provides digital signals on lines 55 as follows: +1 = left aerial phase advanced relative to right -1 = left aerial phase retarded relative to right 0 = signals at both aerials in phase within a predetermined tolerance.

As explained in the aforementioned specification, these digital signals can be decoded into the sine of the angle of reception of the signal, with a response time fast enough to ascertain the angle on the basis of a single 0.1,aS pulse. If the nine aerial pairs have spacings of approximately 180 cm, 90 cm, 45 cm and so on, it is possible to resolve the bearing of the interrogating tank to approximately 0.25 unambiguously within a sector of 45".

Fig. 8 shows in outline the overall system.

Each vehicle is equipped with the same system but the operations thereof depend upon whether the vehicle is interrogating or interrogated. To assist in understanding this, the following paragraphs are headed First Vehicle and Second Vehicle, to relate the parts of the system of Fig. 8 clearly to the different operations in the two vehicles.

First Vehicle. A bistable circuit 56 selects the interrogate or standby mode. When set to the interrogate mode by a signal on line 56A, a digitally coded interrogation is transmitted via an encoder 57 and that transmitter 29 and array 13 which is selected by a quadrant selector 58.

The quadrant selector and the digital phase shifter driver 47 are controlled from a bearing register 59 into which the required bearing of interrogation is set manually or possibly automatically from a primary radar system. At the end of the interrogation the encoder resets the circuit 56 to restore the standby mode.

Second Vehicle. When a vehicle is interrogated by another vehicle, the signal on a line 60 from one element of that interferometric array 14 which faces the correct quadrant, passes to a transponder comprising a receiver 61 and decoder 62 whose output triggers the bistable circuit 56 to the interrogate state and also causes the encoder 57 to modify the transmitted code, whereby a reply code, rather than an interrogation code, is now radiated.

The digital signals on lines 55 (Fig. 7) and 60 pass to a bearing decoder 63. As explained in the aforementioned specification, the digital signals decode directly into the sine of the bearing; this quantity can be further decoded into the sine of the reciprocal angle by a converter 64. This angle is fed into the bearing register 59, whereby the reply signal is transmitted back to the first vehicle.

First Vehicle. The reply signal is received and, if identified as a correct reply, (a reply having the established format for a reply from a friendly vehicle), the decoder 62 signals as much to a unit 65 which identifies friendly vehicles with their range and azimuth.

Azimuth is fed in from the bearing decoder 63.

Range is determined conventionally in dependence on the round trip delay from the initiation of an interrogation (line 66) until the decoding of a reply. Suitable allowance is made for the transponder delay in the second vehicle and any other system delays.

It will be appreciated that the reception of the reply signal corrects the bearing in the bearing register which may not have been entered completely correctly in the first instance. Indeed, with such an exceedingly narrow transmitting beam, it is possible that the first vehicle will miss the second vehicle with its interrogation. In view of this dangerous possibility, the system can be programmed initially to interrogate on X -0.75 where X is the bearing entered in the register 59 on line 67 and then to repeat interrogations at XO - 0.5 , X" - 0.25 and so on until a reply is obtained, when the correct bearing will automatically be put in the register 59 and further interrogations will be made on that bearing alone.

Second Vehicle. The signal from the decoder 62 on line 68 triggers a reply transmission as described above. In addition, to provide for mutual recognition it can be arranged that, immediately after the reply transmission has been made, an interrogating transmission shall be initiated from the second vehicle to the first.

This transmission is initiated from the line 68 via a delay line 69. With this provision, the two vehicles will interrogate each other alternately until some action is taken to interrupt the dialogue. To this end a counter may be provided to intermpt the re-triggering circuit via the delay line 69 after a suitable number, e.g. ten, of interrogations have been made.

The system described will normally be capable of resolving two tanks 50 m apart at 5 Km (which is the probable limit of usability), each interferometric array giving resolution to a minute of an arc within 1 cue S.

If operation is required in a surveillance mode, as mentioned above, the command vehicle is additionally equipped with a non-directional transmitting aerial and a transmitter of increased power, enabling the transmission of an interrogating signal over a large sector in azimuth. The non-directional aerial can be omni-directional but preferably comprises say four horns viewing corresponding fractions of 360" and excitable in turn to cover the whole 360". This simplifies the decoding of the azimuths of replies and reduces power requirements.

When such a transmission is made, all friendly vehicles in the relevant sector reply; their replies will get back to the command vehicle at different times because the different ranges and the range resolution will be good because of the extremely short message times employed. The likelihood of replies garbling each other is therefore small.

At the command vehicle the azimuths and ranges of all replying vehicles are decoded and corresponding displays may be created on a PPI for example. Assume that all vehicles, friend and foe, are displayed as dots on the PPI in response to a primary radar, and that the dots are modified to crosses in respect of those vehicles which show up on the IFF interrogation. The officer commanding will now have a complete picture of the battle and can establish communication with any selected vehicle under his command by directing a directional transmitting array 13 to that vehicle and can issue such orders as are appropriate.

WHAT WE CLAIM IS: 1. An IFF system wherein each of a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. bearing of the interrogating tank to approximately 0.25 unambiguously within a sector of 45". Fig. 8 shows in outline the overall system. Each vehicle is equipped with the same system but the operations thereof depend upon whether the vehicle is interrogating or interrogated. To assist in understanding this, the following paragraphs are headed First Vehicle and Second Vehicle, to relate the parts of the system of Fig. 8 clearly to the different operations in the two vehicles. First Vehicle. A bistable circuit 56 selects the interrogate or standby mode. When set to the interrogate mode by a signal on line 56A, a digitally coded interrogation is transmitted via an encoder 57 and that transmitter 29 and array 13 which is selected by a quadrant selector 58. The quadrant selector and the digital phase shifter driver 47 are controlled from a bearing register 59 into which the required bearing of interrogation is set manually or possibly automatically from a primary radar system. At the end of the interrogation the encoder resets the circuit 56 to restore the standby mode. Second Vehicle. When a vehicle is interrogated by another vehicle, the signal on a line 60 from one element of that interferometric array 14 which faces the correct quadrant, passes to a transponder comprising a receiver 61 and decoder 62 whose output triggers the bistable circuit 56 to the interrogate state and also causes the encoder 57 to modify the transmitted code, whereby a reply code, rather than an interrogation code, is now radiated. The digital signals on lines 55 (Fig. 7) and 60 pass to a bearing decoder 63. As explained in the aforementioned specification, the digital signals decode directly into the sine of the bearing; this quantity can be further decoded into the sine of the reciprocal angle by a converter 64. This angle is fed into the bearing register 59, whereby the reply signal is transmitted back to the first vehicle. First Vehicle. The reply signal is received and, if identified as a correct reply, (a reply having the established format for a reply from a friendly vehicle), the decoder 62 signals as much to a unit 65 which identifies friendly vehicles with their range and azimuth. Azimuth is fed in from the bearing decoder 63. Range is determined conventionally in dependence on the round trip delay from the initiation of an interrogation (line 66) until the decoding of a reply. Suitable allowance is made for the transponder delay in the second vehicle and any other system delays. It will be appreciated that the reception of the reply signal corrects the bearing in the bearing register which may not have been entered completely correctly in the first instance. Indeed, with such an exceedingly narrow transmitting beam, it is possible that the first vehicle will miss the second vehicle with its interrogation. In view of this dangerous possibility, the system can be programmed initially to interrogate on X -0.75 where X is the bearing entered in the register 59 on line 67 and then to repeat interrogations at XO - 0.5 , X" - 0.25 and so on until a reply is obtained, when the correct bearing will automatically be put in the register 59 and further interrogations will be made on that bearing alone. Second Vehicle. The signal from the decoder 62 on line 68 triggers a reply transmission as described above. In addition, to provide for mutual recognition it can be arranged that, immediately after the reply transmission has been made, an interrogating transmission shall be initiated from the second vehicle to the first. This transmission is initiated from the line 68 via a delay line 69. With this provision, the two vehicles will interrogate each other alternately until some action is taken to interrupt the dialogue. To this end a counter may be provided to intermpt the re-triggering circuit via the delay line 69 after a suitable number, e.g. ten, of interrogations have been made. The system described will normally be capable of resolving two tanks 50 m apart at 5 Km (which is the probable limit of usability), each interferometric array giving resolution to a minute of an arc within 1 cue S. If operation is required in a surveillance mode, as mentioned above, the command vehicle is additionally equipped with a non-directional transmitting aerial and a transmitter of increased power, enabling the transmission of an interrogating signal over a large sector in azimuth. The non-directional aerial can be omni-directional but preferably comprises say four horns viewing corresponding fractions of 360" and excitable in turn to cover the whole 360". This simplifies the decoding of the azimuths of replies and reduces power requirements. When such a transmission is made, all friendly vehicles in the relevant sector reply; their replies will get back to the command vehicle at different times because the different ranges and the range resolution will be good because of the extremely short message times employed. The likelihood of replies garbling each other is therefore small. At the command vehicle the azimuths and ranges of all replying vehicles are decoded and corresponding displays may be created on a PPI for example. Assume that all vehicles, friend and foe, are displayed as dots on the PPI in response to a primary radar, and that the dots are modified to crosses in respect of those vehicles which show up on the IFF interrogation. The officer commanding will now have a complete picture of the battle and can establish communication with any selected vehicle under his command by directing a directional transmitting array 13 to that vehicle and can issue such orders as are appropriate. WHAT WE CLAIM IS:

1. An IFF system wherein each of a

plurality of vehicles is equipped with a highly directional transmitting aerial steerable in azimuth, means operable in an interrogating mode to transmit an interrogating signal via the transmitting aerial, an interferometric receiving aerial system coupled to means adapted to decode the bearing of an interrogating vehicle, control means operative in a receiving mode to steer the transmitting aerial to the reciprocal of the decoded bearing, and a transponder responsive to a received interrogating signal to transmit a reply signal on the reciprocal bearing by way of the transmitting aerial.

2. An IFF system according to Claim 1, wherein the interrogating and reply signals are transmitted at frequencies in the band of 20 to 50 GHz.

3. An IFF system according to Claim 1 or 2, wherein the transmitting aerial of each vehicle comprises at least one electronically steered, multi-aperture, linear phased array.

4. An IFF system according to Claim 3, wherein each vehicle has a transmitting phased array and a corresponding interferometric receiving array mounted along each side of the vehicle and across the front and the rear of the vehicle.

5. An IFF system according to any preceding claim, wherein all vehicles are normally in a standby, ready-to-reply mode to which they revert upon completing an interrogation and wherein a vehicle which has replied to an interrogation on a reciprocal bearing is arranged automatically then to transmit an interrogating signal on that same bearing.

6. An IFF system according to any preceding claim wherein the transmitting aerial and the feed circuit thereof are reciprocal and wherein each vehicle comprises means for effecting two-way communication with another vehicle by way of their transmitting aerials.

7. An IFF system according to any preceding claim, wherein one vehicle is further equipped with a non-directional transmitting aerial capable of transmitting an interrogating signal over a large sector in azimuth.

8. An IFF system according to Claim 7 wherein the said one vehicle is further equipped with a primary radar, display means for displaying all vehicles detected by the primary radar, and means responsive to reply signals received from those vehicles which respond to an interrogating signal transmitted by way of the non-directional transmitting aerial to apply a distinguishing feature to the displays of such vehicles.

9. An IFF system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.