GB2261344A - Radar transponder - Google Patents

Abstract

A radar transponder apparatus comprises a signal receiver means (3), a received signal processor means (5) to which the output of the receiver means is applied, a signal modulator and transmitter means (4) responsive to an output of the processor means, a common receive/transmit antenna (1) and a diplexer (2) arranged to feed signals from the antenna to the receiver means and signals from the transmitter means to the antenna. <IMAGE>

Description

Radar transponder This invention relates to a radar transponder apparatus.

A radar transponder is a device which will transmit r.f. signals in response to signals received from an interrogating radar. Such apparatus may be used in conjunction with a pulse modulated radar to enable the presence, position and identity of a vehicle or vessel equipped with the device to be determined. Particular applications include the use of such transponders for marine search and rescue, landing of helicopters on offshore platforms, etc.

It is known to use so-called "radar corner reflectors" to enhance radar signal returns or "echoes". However, it is difficult to obtain a sufficiently strong and reliable echo from such a passive reflector except at close range, unless its size is inconveniently large. A transponder, being an active device, can generate a simulated radar echo of strength equivalent to that which would arise with a physically large reflector at a considerable range whilst the device itself is comparatively smalL Moreover, the transponder can be constructed so as to generate signals which have special characteristics not obtainable in echoes from passive reflectors, such as frequency changes or offsets, and may also include coding to provide identity. One such transponder is disclosed in British patent specification No. 1 552 044.

British patent specification 1 584 937 discloses a radar beacon apparatus comprising a slot antenna for receiving radar waves; a receiving unit which receives the signal received by the receiving antenna; a transmitting unit which is controlled by a control signal from the receiving unit to transmit a signal responding to the received radar wave; a slot antenna for transmitting a responding signal and wherein the antennae and the transmitting and receiving units are arranged to form an assembly mounted on a base member to which is attached a radar dome case made of radio transmissive material which encloses the said assembly.

According to the present invention there is provided a radar transponder apparatus comprising a signal receiver means, a received signal processor means to which the output of the receiver means is applied, a signal modulator and transmitter means responsive to an output of the processor means, a common receive/transmit antenna and a diplexer arranged to feed signals from the antenna to the receiver means and signals from the transmitter means to the antenna.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a block schematic diagram of circuit elements of a transponder, and Fig. 2 is an illustrative cross-section of a transponder apparatus.

The transponder elements shown in Fig. 1 comprise within a main unit 100 a common receive/transmit antenna 1, a ferrite circulator diplexer 2, a high frequency receiver 3 and transmitter 4, a signal modulator 5 and signal conditioner 6. Radar signals received at the antenna from a remote interrogating radar are fed via the diplexer 2 to the receiver 3 where they are demodulated. After detection and demodulation in the receiver the demodulated signal is either routed to the signal conditioner 6 or else it is routed directly to the transmitter 4 via the modulator 5. The signal conditioner includes a means for avoiding triggering transponder replies due to transponder receiver internally generated noise. The signal conditioner may also include a decoding means where required.The modulator 5 drives the transmitter 4 which incorporates a high frequency oscillator (not shown) the output of the transmitter being fed via the diplexer 2 to the antenna 1. The circuits may be connected directly to an external power source, such as a ship's power supply. Alternatively the circuits can be connected to a separate power supply. In some implementations the circuits are connected to a separate power and control unit 200 which may house batteries 7 and, where required, monitoring facilities 10. The unit 200 also includes (not shown) ON/OFF and mode selection switches and external supply power conditioning to remove voltage fluctuations, surges etc. as required.

The unit 200 can also include control circuits 11 for a built-in self-test equipment 12 housed in the main unit 100 which, when initiated by an operator, can perform automatic checking of the proper functioning of the various transponder modules.

In operation the form of transmitted signal, also known as the "reply" transmission, may take one of several alternatives, depending on the particular application.

In some applications such as that of Search And Rescue, the frequency of the reply transmission is swept rapidly back and forth in a cyclic manner so as to ensure that the transponder reply transmission will be received by an suitable radar within operating range of the transponder.

Since this method of operation involves a characteristic element of uncertainty as to the relationship between the actual position of the transponder in relation to a given observing radar and the apparent position of the first observed signal of the transponder reply, one implementation of the transponder involves its integration or co-location with a passive radar reflector whose purpose is to provide an accurate range reference, since such an accurate reference is only required at relatively short radar-to-transponder ranges where the echo from such a passive reflector would be adequate for reliable radar observation.

Such a combination may also be of operational value where it is required to enhance the weak radar echo which sometimes characterises small craft such as fibre glass yachts, inflatable craft, etc.

In some applications such as helicopter homing, etc, the observing radars are fitted with a fixed tuned receiver channel specifically for the reception of transponder reply signals.

In this case, the transponder reply can be fixed in frequency and may be pulse or otherwise coded for identification purposes or in order to use the radar-to-beacon link for communication.

An operational advantage of this form of operation is that permanent echoes such as sea or other clutter may be removed from the radar display, leaving the transponder signals uncluttered by radar echoes.

In yet another implementation, the transponder reply frequency may be made equal to that of a particular observing radar, so as not to be received by other radars and to avoid interfering with them.

In one embodiment of the invention the receiver input may be protected against damage from nearby high power radar transmissions by the incorporation of a signal power limiter in the receiver front-end. The receiver can include a high frequency signal amplifier in the front end if it is anticipated that only weak signals will be received. If the reply transmission is required to be coded or otherwise characterised the modulator 5 can include the necessary coding or other characterising circuitry. This circuitry can be incorporated in a sub-modulator (not shown) which in turn feeds the modulator 5. The circuitry may also include a receiver suppression gate whose purpose is to de-sensitise the receiver for the duration of the reply transmission to avoid self-triggering.

In some implementations such as that for Search And Rescue (SART) the receiver is of the crystal video type having a wide instantaneous bandwidth, while in other cases, such as that for helicopter navigation, the receiver may be of the superheterodyne type.In one embodiment of the invention the reply transmission is radiated in response to proper received signals for a predetermined period followed by a second period in which no reply transmission is made.

At the end of the second period transmissions are resumed in an intermittent periodic manner so as to enable any radar echoes asdisplayed on the observing radar, which may be overlaid by the radar transponder reply, to be seen during the periods when no transponder replies are made.

The transponder electronics, which in themselves are constructed with known circuitry and technology, are housed in a robust housing 20 with the antenna at least being enclosed in a radome 21 made from material which is transparent to the high radio frequencies used by radars.

Claims (11)

1. A radar transponder apparatus comprising a signal receiver means, a received signal processor means to which the output of the receiver means is applied, a signal modulator and transmitter means responsive to an output of the processor means, a common receive/transmit antenna and a diplexer arranged to feed signals from the antenna to the receiver means and signals from the transmitter means to the antenna.

2. A transponder according to claim 1 wherein the diplexer is a ferrite circulator diplexer.

3. A transponder according to claim 1 or 2 wherein the signal processor means includes means for decoding received signals.

4. A transponder according to any one of claims 1 to 3 including means for causing the frequency of transmissions from the signal transmitter to be swept back and forth over a predetermined range in a cyclic manner.

5. A transponder according to any preceding claim including means for inhibiting reception of signals in the receiver during periods when signals are being transmitted from the transmitter.

6. A transponder according to any preceding claim further comprising a passive signal reflection means co-located with the transponder.

7. A transponder according to any preceding claim including means for alternating periods of transmission of signals with periods in which no signals are transmitted.

8. A transponder according to any preceding claim including means for encoding transmissions of signals from the transmitter.

9. A transponder according to any preceding claim having a local battery power supply and means for audio/visual monitoring the operation of the transponder.

10. A transponder according to any preceding claim including means for setting the transmission frequency of the transmitted signals dependent on the frequency of the received signals.

11. A radar transponder apparatus substantially as described with reference to the accompanying drawings.