GB2073530A - Secondary radar - Google Patents

Abstract

In an airfield surveillance radar system, use is made of individual low power radars located along the lengths of the pathways (runways or taxiways) to be monitored. Each such radar is a low power device which radiates directionally so as to define a block on the pathway. The radar emits P1-P3 pulse pairs from one aerial such as used to interrogate aircraft equipped with ICAO standard SSR transponders, and each pulse pair generates a response which is received via that radar's receiver. In a first variant a suppression pulse P2 is sent from another aerial at the same station but floodlighting space not covered by its own P1-P3 pulse pair. The radars are on both sides of the runway and staggered. In a second variant all radars are on one side of the runway while opposed P2 transponders, triggered by respective P1 pulses are on the other side. In both cases the radars are cable-controlled from the central equipment to interrogate as requested. To deal with vehicles (including aircraft) not fitted with secondary radar the vehicles can have passive or semi-passive responders of the type used in anti-shoplifting systems. <IMAGE>

Description

SPECIFICATION Secondary radar This invention relates to a vehicle detection sys tem for the detection and, in some cases, identification of vehicles on predetermined surface pathways such as airport runways and taxiways. The vehicles may include both aircraft and the other vehicles pre sent at an airport.

In a secondary radar system as used with aircraft, the return signal can be used as a data link, e.g. for a coded representation of the aircraft identity. In an I.C.A.O. secondary surveillance radar (SSR) system, the ground station sends pairs of pulses, known as P1 and P3, the pulses of which are 8 microsecs. apart on 1030 MHz, and the aircraft transponder replies on 1090 MHzwith a 20 microsec. duration pulse pattern which identifies the aircraft. The ground station receiving equipment includes the normal radar receiver, and pulse pattern decoders so that the aircraft identity can be displayed.

To overcome difficulties due to spurious responses to side-lobes of the main beam, the ground station emits a further pulse, known as P2, between P1 and P3, this P2 pulse being weaker than P1 or P3 but stronger than the side-lobe signals. P2 is transmitted omni-directionally so that "on beam" we get a relatively weak P2 between P1 and P3, while "off beam" we have a relatively strong P2 between the weak P1 and P3. The aircraft-borne transponder responds to the former but not the latter. Some signal garbling can occur when the separation bet ween targets is small.

For vehicle identification at airports such a system suffers from garbling so it is necessary to confine the P1 -P3 sequence to a small area or block which would contain one target only. In one such system, see Application No. 79 31771 (A. M. Levine-F. D.Wad- doups 47-4), very low power (by comparison with the usual main radar systems) fixed beam radars are equally spaced along a pathway to be monitored, and are cable-interconnected. Each radar has a wide beam in azimuth (e.g. +80 ) "aimed" at the centreline of the pathway to define a fan-shaped block.

These radars are enabled in turn via the cable, and each detects the presence or absence of a vehicle in its block, plus the vehicle's identity if it has a secondary radartransponder. This information is sent via the cable to the central control equipment for display, usually at the control tower. In practice two such radar chains are used, one on each side of the runway.

In one such system there are two sets of radars facing each other, one set on each side of the runway. One radar of each pair is an interrogating station which when enabled sends a 0.8 + 0.1 microsec.

pulse P1 at 1030 + 0.2 MHz. The radar across the pathway receives this P1 pulse and sends another pulse P3 at the same frequency. Due to the various delays due to transmission and in the second (transponder) radar, there is simulated at the centre-line of the pathway the "standard" P1-P3 pulse pair. In response to this, an aircraft transponder replies with its identity on 1090 + 3 MHz, 3 t 0.5 microsecs. after P3, the pulse pattern being 20.75 microsecs. long.

The interrogating station receives the signal, which it sends via the cable to the central equipment. The radars may be enabled according to a pattern or singly on demand.

The above system has certain limitations due to the risk that the receivers may pick up and respond to the wrong signals, and due to the fact that accurate maintenance of the correct pulse amplitudes on the centre-line of the pathway may be difficult.

An object of the invention is to provide a system in which the above disadvantages are minimised or overcome.

According to the present invention, there is provided a vehicle detection system for detecting vehicles present on predetermined surface pathways such as airport runways and taxiways, including interrogating stations spaced along the length of the pathways to be monitored, each said station being arranged when enabled to transmit a pulse pair such as needed to interrogate a secondary radar of the type used in an aircraft and each such station radiating directionally over a portion of its one of the pathways so that its signals define a block of said pathways to be monitored, means under control of a central eq u ipment fo r enablin g the interrogating stations as required for monitoring purposes, receiving means responsive to the responses produced by vehicles in a said block being interrogated, and display means adapted to display such responses at a control position.

In such a system, a block is a portion of a pathway to be monitored, e.g. a runway. Radar sensors radiate the P1-P3 pulse pairs and the P2 pulses, each of which occur temporarily between P1 and P3, using different directional patterns and with different power. The patterns and the powers are such that P1 and P3 by receiver on an aircraft with the block being monitored with larger signal strength than P2. The minimum value of the difference between signal strengths is such that the receiver recognises an SSR interrogation. This condition is only met in the block to be monitored. In one variant of the system local receivers are not used as in some cases the relatively high power level of an aircraft's responses allow the use of a centralised arrangement, which makes the cable which interconnects the radar sensors a oneway arrangement.

The above system is an improvement over its predecessors, but it is desired to extend the usefulness of such a system to the detection of vehicles not equipped with secondary radartransponders. Hence the invention also provides a vehicle detection system for detecting vehicles present on predetermined surface pathways such as airport runways and taxiways, including interrogating stations spaced along the pathways to be monitored, each said station including an interrogator for generating a pulse pair of the type needed to interrogate a secondary radar such as used in an aircraft and each such station radiating directionally over a portion of a said pathway to be monitored so that its signals define a block of that pathway, transponder stations each located on the opposite side of a said pathway from one of said interrogating stations, a said transponder station being responsive to the reception ofthe first pulse ofthe pulse pair from its one ofthe interrogating stations to radiate a suppression pulse, which suppression pulse occurs between the pulses of the said pulse pair and is radiated over an area not including the block within which its said- interrogating station radiates, receiving equipment associated with each said interrogating station, and connections from all of said interrogating stations to a central equipment via which any one of the interrogating stations may be enabled to check whether there is a vehicle in its block of a said pathway, said connections also serving to convey the responses from any vehicles to the central equipment, and wherein the receivers of said interrogating stations are simple and relatively insensitive so that a said receiver is substantially unresponsive to signals other than those due to or in response to pulse pairs from itself.

This exploits the fact that the transponders used radiate a relatively high power, bearing in mind the relatively short ranges involved. Thus a receiving antenna of relatively modest gain is adequate, so that a receiver which consists of little more than a cavity-tuned diode detector may be used. The output of such a receiver is adequate to drive a simple amplifierto provide the line drive to get the transponder responses to the central equipment. Further, a threshold can be set to exclude pulses of less than a predetermined amplitude, so that the receiver is insensitive to garbling due to distant SSR transmission. It is also possible for the detection circuitry of the receivers to be based on square-law detectors, as in such case sensitivity falls off rapidly.

To ensure that there are no difficulties due to amp litudethresholds it is preferred to "flood-light" the area which is not intended to be covered by the P1-P3 radiation with a P2 pulse, whereby there is little or no P2 over the pathway to be monitored.

However, part of the area covered by the P2 radiation is also covered by P1-P3, but this is unwanted.

Forthis purpose a subsidiary transmitter on the far side of the pathway from the P1 -P3 interrogating station produces the P2 pulse 2 microsecs. after it receives P1 . Thus outside the monitored area any P1-P3 "overspill" is accompanied by a P2 pulse large enough to act as an effective indication that responses from outside that area should be suppressed. The correct P1-P2 relation applies along the line between the two transmitters, and although the relation differs somewhat off this line, P2 is still effective for suppression in the "non-monttored" region.

The P2 pulse can be produced by the "re broadcasing" method used in the first mentioned system for P3, or triggered by a cable-distributed pulse. Such P2 transmitters can also be used to help in the suppression of unwanted responses by adja cent radars, the P2 transmitter pointing in the correct direction to cover the "overspill" areas of the adjacent transponder. To do this, the P2 transmitter emits two pulses, the first of which (P2) is sent 2 microsecs. after its own P1 signal for the suppression of any P1-P3 "backbeam". The second signal is sent at a time T slightly less than 2 microsecs. after itsnefghbou?s P1-P3 reachesthe far side of the pat-hway., T being sufficient to ensure that the limits prescribed for complete suppression are maintained overras much as possible dthse"overspill" area.

Suoh a system can be based on a relatively simple devise such as a radio pagermodule suitably modified, without scanning the irdividual radars all the time. Such transmitters are each separately addressable, and each one only interrogates when "told"fo do so by a central pmcessor oran operator.

Such an address, e.g. one usire pager code, can be arranged:to interrogate a particular block or group of blocks as required. Thus an operstorcan key in the identities ofthetransmitterto be baactiva- ted, their operational order and the numberoftimesto be enabled. The necessary codes are then sent over the cable in the correct order. Each transmitter has a decoder, and is passive until it receives its own address code. It then prepares to send, and sends in response to a following trigger pulse or pulse pair, the code "telling" itwhetherto send P1-P3 or P2.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 represents the location of a number of interrogation stations along a pathway, plus their polar diagrams, i o one embodiment of the invention.

Fig. 2 is a highly schematic representation of the apparatus at one of the interrogation stations in a system such as that of Fig. 1.

Fig. 3 shows the polar diagrams of the-two waves emitted from a station such as that of Fig. 2.

Fig. 4 shows schematically how the interrogation station and other stations are located with respect to a pathway to be monitored in another embodiment of the invention.

Fig. 5 is highly schematic representation of one of the interrogation stations of a system such as that of Fig. 4.

Fig. 6 is.a perspective view of one of the interrogation stations.

Fig. 7 shows schematically another embodiment of the invention.

As will be seen from Fig. 1, the individual interrog ation stations 1, Z 1,2,3 are locatedion opposite sidesof the pathway to be monitored; and each has a palar diagram such as to define one of a number of adjacentand slightly overlapping blocks on that pathway. As shown schematically each such block is roughly triangular as "seen" from the pathway. Note tttat Fig. 1 does not attempt to define polar diagrams, but merely indicates the runway blocks monitored by the stations 1, 2, 3.

Fig. 2 shows one of the stations used in the system of Fig. 1, but, like Fig. 1 it does not include the cable via which these stations are coupled to the central equipment. The station has a transmitter 10 which transmits the P1-P3 pulse pairs, which are generated under control of a pulse generator 11. This pulse generator also controls the transmission of the P2 pulses, each of which as described above, occurs between P1 and P3.

The output of the transmitter 10 is connected via a switch 11A, which is an electronic device although for simplicity shown as a mechanical switch. This switch is so controlled that P1 and P3 pulses are applied to a T-R switch 12 whereas P2 is applied to an antenna 13. Hence during each P1-P2-P3 sequence, switch 1 1A is reversed twice.

The P1-P3 pulses pass, as mentioned above, to the TR switch 12 from which they pass to an antenna shown schematically at 14. This antenna is provided with a reflection arrangement 15, shown as a corner reflector, which gives the sort of polar diagram appropriate to Fig. 1. This polar diagram is indicated at 16 in Fig. 3, where the polar diagram 17 is that for the aerial 13 which emits the P2 pulse. Thus we have P1 -P3 sent over the station's block of the monitored pathway and P2 over the rest of the space. Hence the suppression pulse is broadest in the areas not effectively covered by P1 -P3, although on the fringes of the P1-P3 area there is some overlap with P2, butthis is not harmful.

The transponder responses are received at the antenna 14, and pass to the T-R switch 12, which is now (after it has passed the P1-P3 pulse pair) in its "receive" condition. Thus the received response passes to the receiver 18 which, as already mentioned, includes the normal radar functions and may include decoders to decode a responding aircraft's identity code. Preferably, however, the code groups can be sent to the central station for decoding by a common processor. Each such code group is there associated with a specific coverage block by its time of reception. Such a code, after it has been decoded, causes the responding vehicle's identity to be displayed atthe central equipment. Here the received signals may be used for control purposes, e.g. to control traffic lights.

Although in the system described with reference to Figs. 1 to 3, the individual interrogation stations are coupled to the central station via electrical cables, other coupling techniques may be used. Thus if circumstances such as frequency allocations permit, this coupling could use radio links. Another possibility is to use optical fibre links although at the time of writing this would cost more than conventional cables.

The arrangement to which Fig. 4 relates differs from that of Figs. 1-3 in that, although P1 and P3 are sent from the same interrogation stations, e.g. 20, 21, P2 is sent from separate stations, e.g. 22; 23, on the opposite side of the pathway. Thus when a station such as 20 is addressed from the central equipment, it emits a P1 pulse followed by a P3 pulse. Of these pulses P1 on arrival at the corresponding station 22 causes the transmission of a P2 pulse. As will be seen the polar diagrams of the stations such as 20, 21, each define a block which is part of the pathway to be monitored such as a runway, in which respect the arrangement resembles that of Figs. 1-3.

However, the arrangement differs in that P2 is sent separately, its polar diagram being roughly as shown, so that P2 "floodlights" the non-monitored area.

The P2 pulse transmission is used to limit the block to be monitored in the direction normal to the axis of the runway. The limitation in the direction of the runway is caused by the radiation pattern used for the transmission of P1 and P3. It is also possible to radiate another P2 pulse from station 20, similarto the example of Fig. 1.

In Fig. 5 we see, in highly schematic form, an interrogating station such as 20 or 21 in Fig. 4. This station is connected to a cable indicated as 25 which interconnects a number of stations along one side of the pathway. When this station is addressed in the manner described above, the address is detected by a code detection unit 26 which responds to that station's code. When it responds it prepares the trans mitter 27 forthe transmission of the P1-P3 pulse pair, which transmission occurs when the following trigger pulse is detected by a detector 28.

The station such as 22 or 23 is relatively simple: as mentioned above it can be a transponder which generates P2 on reception of P1. Alternatively it can be triggered by a pulse sent along another cable which interconnects the stations such as 22, 23.

Returning to the interrogation stations, each of these also include a receiver 30 which responds to the response from the vehicle being detected. This is a simple and cheap receiver, and its output is fed via a line driver amplifier 31 to the cable 25 fortransmission to the central equipment.

The radars, each of which includes the transmitter for the P1-P3 pulse pair and the receiver, are relatively simple units and each of them is based on the technique used in the production of a radio pager.

Thus the device has a decoder which follows the basic principles of the code detector in a radio pager exceptthat it is cable fed instead of being radio fed.

When it responds it triggers its own transmitter, which is also relatively simple. The "P2 transponder" is similarly a simple unit in a low-profile housing, but on the other side of the pathway.

It will be appreciated that the reception of the responses from the interrogated aircraft or other vehicles can be dealt with at the central station, instead of atthe local station.

At this point some further comment on the use of the P2 emission seems to be useful. Thus the "overspill" due to a given station is limited by the use of its neighbour's back-beam P2 transmitter at the appropriate time. Thus if we have three stations N 1, N and N + 1 arranged as shown in Fig. 1, we have for each such station a P1 -P3 emission across the runway and a P2 back-beam emission. This will have the effect that for station N the following conditions apply: (a) Across the runway there are, effectively, only P1 and P3 pulses, which is an acceptable interrogation.

(b) Off the runway, and on the same side thereof as Ni, we have relatively weak P1 and P3 pulses, plus a strong P2 pulse generated by N1 own backbeam transmission. This will be an unacceptable interrogation, and so will not be effectively responded to.

(c) Off the runway, but on the opposite side thereof from N1, a similar unacceptable signal combination is produced, but with the P1-P3 pulses from N1 and the P2 pulse from (N-IJls transmitter, which points in the same direction.

Fig. 7 shows another form which a local radar sensorcantake. Here we have a post four feet high and six inches in diameter, with a horn radiator 50 whose "mouth" is 12" x 4". The upper portion 51 contains the receiver unit and the T-R switch, while a lower portion 52 contains the transmitter unit. The whole is on the top end of a frangible post 53 with an anchor 54 at its bottom. Connection to the buried cables is via input and output cables 55.

We now consider, with reference to Fig. 7, the arrangements of paragraphs (a), (b) and (c) in somewhat more detail. Fig. 7, like Fig. 6, showsthree interrogator stations 1,2 and 3 on opposite sides of the runway. Two stations at a time radiate signals when receiving the same address code, but when thus enabled radiate signals with different radiation levels and with different radiation patterns. Station 1 radiates P2 pulses with a very high power level and in such a way that on the side on which station 1 is located, the P2 pulse is longerthanthe P1, P3 pulses from station 2 on the opposite side of the runway.

Station 1 atthistime radiates no P1-P3 pulses.

Station 2 radiates P1, P2 and P3 pulses as described with reference to Fig. 1, and the next address code is received by stations 2 and 3. When these two stations have received this address code, they radiate signals in a similar way to stations 1 and 2 when they received the other address code. Thus in this case station 2 only radiates P2. In this case, unlike Fig. 4, no additional stations are needed to limitthe block in the direction normal to the runway.

In all cases in which the radiation pattern via which the P2 pulse is radiated does notcoverthe block to be monitored, it is possible to radiate a P1-P2 pulse pair, separated by 2 ,us. In this case the P1 pulse must have a much lower power than the P2 pulse.

When a receiver receives such a pulse pair it is blocked for 25 ,us, so that it cannot receive a P1 -P3 pulse pair. This is useful in regions in which, because of special circumstances, P1-P3 pulses are received with a largeramplitudethan P2.

Note that when address codes are sentto enable the interrogation stations, they are also sent toe central receiver and the evaluation unit. Then they can be further evaluated for time gating or antenna selection, which allows the transponder replies to be correlated with the block of origin.

Claims (10)

1. A vehicle detection system for detecting vehicles present on predetermined surface pathways such as airport runways and taxiways, including interrogating stations spaced along the length of the pathways to be monitored, each said station being arranged when enabled to transmit a pulse pair such as needed to interrogate a secondary radar of the type used in an aircraft and each such station radiating directionally over a portion of its one of the pathways so that its signals define a block of said pathways to be monitored, means under control of a central equipment for enabling the interrogating stations as required for monitoring purposes, receiving means responsive to the responses produced by vehicles in a said block being interrogated, and dis play means adapted to display such responses at a control position.

2. A system as claimed in claim 1, and in which each said interrogating station when enabled also radiates a suppression pulse, which suppression pulse is radiated: over substantially all of the monitored area exceptthat over which the same station radiates its said pulse pair.

3. A vehícle detection system for detecting vehicles present on predetermined surface pathways such as airportrunways and taxiways, including.

interrogating stations spaced along the length of the pathways to bernonitored, each said station being arranged when enabled to transmit a pulse pair of the type needed ta-interrogate a secondary radar of the type used in an aircraft and each such station radiating directionallyovera portion of its one of the pathways to be monitored so that its signals define a block of said pathway to be monitored, means under control of a central equipment to enable the interrogating stations as required for monitoring purposes, receiving means responsive to the responses produced by vehicles in the blocks being interrogated, display means adapted to display such responses at said central equipment, and means at each said interrogating station for radiating a suppression pulse when that station is enabled, said suppression pulse occurring in time between the pulses of said pulse pair and being radiated over substantially all of the monitored area exceptthat over which that station radiates its said pulse pair.

4. A system as claimed in claim 1, and which includes transponder stations each located on the opposite side of a said pathway from one of the interrogating stations, each said transponder station being adapted to radiate a suppression pulse when it receives the first pulse of its associated interrogating station's pulse pair, a said suppression pulse occurring in time between the two pulses of the pulse pair and being radiated over an area which excludes the block defined by the radiation of the pulse pair.

5. Avehicle detection system for detecting vehicles present on predetermined surface pathways such as airport runways and taxiways, including interrogating stations spaced along the pathways to be monitored, each said station including an interrogator for generating a pulse pair of the type needed to interrogate a secondary radar such as used in an aircraft and each such station radiating directionally over a portion of a said pathway to be monitored so that its signals define a block of that pathway, transponder stations each located on the opposite side of a said pathway from one of said interrogating stations, said transponder station being responsive to the reception of the first pulse of the pulse pair from its one of the interrogating sta tionsto radiate a suppression pulse, which suppression pulse occurs between the pulses of the said pulse pair and is radiated over an area not including the block within which its said interrogating station radiates, receiving equipment associated with each said interrogating station, and connections from all of said interrogating stations to a central equipment via which any one of the interrogating stations may be enabled to check whether there is a vehicle in its block of a said pathway, said connections also serv ing to convey the responses from any vehicles to the central equipment, and wherein the receivers of said interrogating stations are simple and relatively insensitive so that a said receiver is substantially unresponsive to signals other than those due to or in response to pulse pairs from itself.

6. A vehicle detecting system substantially as described with reference to Figs. 1,2 and 3 of the accompanying drawings.

7. A vehicle detecting system substantially as described with reference to Figs. 4 and 5 of the accompanying drawings.

New claims filed on 23.4.81.

New claims: 8, 9, 10

8. A vehicle detection system for detecting vehicles present on predetermined surface pathways such as airport runways and taxiways, including interrogating stations spaced along the length of the pathways to be monitored, each said station being arranged when enabled to transmit a pulse pair such as needed to interrogate a secondary radar of the type used in an aircraft and each such station radiating directionally over a portion of its one of the pathways so that its signals define a block of the pathway to be monitored, means associated with said interrogating station and responsive to the generation of the first pulse of each said pulse pair to generate a suppression pulse which is radiated over at least a major portion of the monitored area other than that defined by the said block of the pathway, means under control of a central equipment for enabling one or more of the interrogating stations as required for monitoring purposes, receiving means responsive to the responses to said pulse pair of any vehicle in each said block which is being interrogated, display means adapted to display such responses at a control position, and means under control of said suppression pulses for ensuring that responses due to signals other than those from the block currently being interrogated are suppressed.

9. A system as claimed in claim 1, and in which each said means for generating a said suppression pulse is co-located with its said interrogating stations, the said interrogating stations being on both sides of the pathways, with the station on one side staggered with respect to the stations on the other side.

10. A system as claimed in claim 1, in which the interrogating stations are all located along one side of a said pathway to be monitored, and in which each said means for generating a said suppression pulse is a transponder on the opposite side of the pathway from its said interrogating stations, each said transponder operating its suppression pulse in response to the first pulse of the pair from its said interrogating station.