GB2265523A - Radar transponder for echo enhancement - Google Patents

Abstract

A radar transponder beacon arranged to transmit a distinctive signal whereby the beacon functions as an active radar echo enhancer. The beacon response comprises a first short frequency sweep followed by a second long frequency sweep. In one embodiment the beacon can be switched in an emergency to operate in a 'search and rescue' (SART) mode.

Description

RADAR TRANSPONDER FOR ECHO ENHANCEMENT.

This invention relates to a radar transponder apparatus for use in small craft.

A radar transponder is a device which is able to transmit RF 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, in particular for search and rescue, landing of helicopters on off-shore platforms, etc.

It is already known to use so-called 'radar 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 strenght equivalent to that which would arise with a physically large reflector at a considerable range whilst the device itself is comparatively small. Moreover, the strength of the signal received by an observing radar, from a transponder, falls as the inverse of the square of the increase in range whereas the strength of the signal received from a passive radar reflector falls as the inverse fourth power of the increase in range.

The object of the present invention is to provide a transponder suitable for use in such active radar echo enhancement, in applications where poor radar reflecting properties are commonly encountered, such as fibre glass yachts and inflatable boats.

According to the invention there is provided a radar transponder beacon apparatus comprising a radar RF signal receiver means, a received signal processor to which the output of the receiver means is applied, signal modulator and transmitter means responsive an output of the processor means, a receive/transmit antenna, means for coupling the antenna to the input of the receiver means and to the output of the transmitter means, the processor including means for generating a reply signal, in response to a received radar RF signal, for transmission by the transmitter wherein the reply signal has a unique format distinct from the format of a search and rescue radar transponder.

In a preferred embodiment of the invention the reply signal format comprises first and second bursts of RF signal, the first burst consisting of a frequency sweep of short duration and the second burst consisting of a similar slower frequency sweep of longer duration.

In a further embodiment of the invention the apparatus includes means for suppressing response signals when the receiver is in receipt of radar signals radiated in the sidelobes of an observing radar antenna.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a block schematic diagram of circuit elements of a transponder beacon apparatus; Figure 2 is a cross-section of a transponder beacon apparatus; Figure 3 illustrates a typical echo enhancement response signal format of the apparatus shown in Figures 1 and 2; Figure 4 is a circuit diagram of an arrangement for discriminating sidelobe signals, and Figure 5 illustrates waveforms associated with the circuit of Figure 4.

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 a signal conditioner 6. Radar signals received at the antenna from a remote radar are fed via the diplexer 2 to the receiver where they are detected and 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 via the modulator 5. The signal conditioner includes a means for avoiding triggering transponder replies due to internally generated noise in the receiver. 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 vessel's power supply.

Alternatively the circuits can be connected to a separate power supply. In some implementations they can be connected to a separate power and control unit 200 which houses batteries 7 and, where required, monitoring facilities 10.

The unit 200 also includes ON/OFF and mode selection switches (not shown), and can also include if desired external power supply conditioning to remove voltage fluctuations, surges etc. 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 various transponder modules.

In operation as an echo enhancement transponder the transmitted signals have a unique format distinctive so that they cannot be confused with a 'search and rescue transponder' (SART) response. Typically the echo enhancement response signal comprises a first short duration frequency sweep over a specified frequency range followed by a second longer duration frequency sweep over the same frequency range,without repetition. The frequency range swept may be 9200 MHz to 9500 MHz, or such other range as may be required for the particular application, for responding to radars operating in the marine X-band, or other frequency bands, the short duration sweep lasting for, say, 0.5/Us and the long duration sweep lasting for, say 7.Sfis approximately.This signal format is clearly differentiated from the SART response of a sequence of 12 repetitive frequency sweeps each comprising a forward sweep of 0.4as followed by a reverse sweep of 7.5//so the total duration of a SART beacon reply being approximately 100//s. In one implementation provision is made for both the echo enhancement mode and the SART mode of operation to be combined such that, in normal circumstances the echo enhancement is in operation but that, when actuated, operation of the SART response is effected.

This should be done by means of a switch of a type which cannot be accidentally actuated. This change of operation is readily achieved by extending the duration of the transponder echo enhancement reply transmission from the single sweep cycle of approximately 8 s duration to the 12 sweep cycle reply of approximately 100/is duration required for SART operation. In as much as both the modes of operation can be effected by additional integrated circuitry in the modulator the combination of both modes in the one unit confers economic and structural advantages.

When operated in close proximity to an interrogating radar, the transponder replies can be triggered by transmissions radiated via the sidelobes of the radar antenna radiation pattern. The resulting transponder replies may be received by the radar, again via the sidelobes of the antenna pattern. This can cause erroneous replies at bearings in addition to that received via the antenna main beam, resulting in a series of arcs on the radar display which, at very close range, can coalesce into one or more concentric rings (known as 'ring around'). In the case of the SART this is recognised and is regarded as an advantage in as much as the presence of such side lobe responses indicates that the SART equipped liferaft is close at hand.

Where sidelobe response free operation is required at short range in the case of the echo enhancement mode two methods of eliminating such sidelobe responses are available.

The first method is known as the 'long time constant! method of interrogation path sidelobe suppression.

In this method a receiver of wide instantaneous dynamic range is used such that the receiver is still operating in a nonhard-limiting condition when as close as required operationally to the observing radar. The pulse transmission received from the radar as it appears at the output of the receiver is used to charge a capacitor C2 (Fig. 4). The resulting charge is allowed to leak away at a set rate between the reception of repetitive radar transmissions, such that a signal received at a level corresponding to those radiated via the sidelobes will be of lesser amplitude than the voltage remaining across the capacitor with the result that the side lobe signal will be of insufficient amplitude to cause further charging of the capacitor.Since it is the charging current flowing into the capacitor which provides the video signal which triggers transponder replies, suppress ion of the charging of the capacitor by sidelobe signals also suppresses transponder replies to such signals.

A potential disadvantage of this method is that very strong signals from a radar close at hand can prevent mainbeam signals from more distant radars from overcoming the charge on the capacitor, with the result that the transponder is 'captured' by the local radar.

The second method of sidelobe response suppression uses large scale semiconductor memory coupled with sophisticated computer data processing to recognise the sequence of signals received from each observing radar.

Observation of the amplitudes of signals received from a particular radar, transmitted via the antenna main lobe, are identified and replies are made to these interrogations only and not to the lower amplitude sidelobe signals which are, as a consequence, suppressed. Since large scale memory together with the high power computing capability enables many radars to be recognised and the sidelobe transmissions of each to be suppressed, transponder 'capture' by the radar whose signal as received at the transponder is the strongest is avoided.

Claims (5)

Claims:

1. A radar transponder beacon apparatus comprising a radar RF signal receiver means, a received signal processor to which the output of the receiver means is applied, signal modulator and transmitter means responsive an output of the processor means, a receive/transmit antenna, means for coupling the antenna to the input of the receiver means and to the output of the transmitter means, the processor including means for generating a reply signal, in response to a received radar RF signal, for transmission by the transmitter wherein the reply signal has a unique format distinct from the format of a search and rescue radar transponder.

2. An apparatus according to claim 1 wherein the reply signal format comprises non-repetetive first and second bursts of RF signal, the first burst consisting of a frequency sweep of short duration and the second burst consisting of a similar slower frequency sweep of longer duration.

3. An apparatus according to claim 1 or 2 includes means for suppressing response signals when the receiver is in receipt of radar signals radiated in the sidelobes of an observing radar antenna.

4. An apparatus according to any preceding claim further including means for generating an alternative reply signal being a 'search and rescue' (SART) response signal and means for switching the transponder from a normal mode of transmitting the distinct signal to a SART response mode.

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