Active target with height diversity

1. An active radar target comprising a plurality of antennas comprising a plurality of receive antennas and a plurality of transmit antennas arrangeable into pairs of transmit and receive antennas, each pair comprising a transmit antenna and a receive antenna the active radar target further comprising a first amplifier and a first modulator arranged to amplify and modulate an incoming signal prior to retransmitting it, wherein at least one antenna in one of the pairs of transmit and receive antennas is vertically displaced with respect to at least one antenna in a different one of the pairs of transmit and receive antennas and wherein the pairings are alterable in an automated manner.

2. An active radar target as claimed in claim 1 , in which multiplexers responsive to a controller change the antenna pairings.

3. An active radar target as claimed in claim 1 , where at least three antennas in the plurality of antennas are vertically displaced with respect to one another.

4. An active radar target as claimed in claim 1 , where all of the antennas in the plurality of antennas are vertically displaced with respect to one another.

5. An active radar target as claimed in claim 1 , in which the first amplifier and the first modulator are provided in a first channel, and a second amplifier and a second modulator are provided in a second channel, and the first channel processes a signal for a first receive antenna and transmit antenna pair, and the second channel processes a signal for a second receive antenna and transmit antenna pair.

6. An active radar target as claimed in claim 1 , in which the first modulator is configured to frequency shift an output signal away from the input signal.

7. An active radar target as claimed in claim 6 , in which the first modulator is configured to frequency or phase shift encode data onto the signal or some of the signals transmitted by each transmit antenna.

8. An active radar target as claimed in claim 7 , in which the data includes one or more of: (i) an identity code, (ii) data provided by external equipment, and (iii) an estimate of relative direction of the incoming radar signal.

9. An active radar target as claimed in claim 1 , in which the target has a detector for detecting when the target is illuminated by a radar signal above a predetermined intensity, and the target is adapted to energise the first amplifier and the first modulator in response to being illuminated by the radar signal above the predetermined intensity.

10. An active radar target as claimed in claim 1 , further including a direction detector for detecting a direction of an illuminating radar signal with respect to the active target.

11. An active radar target as claimed in claim 10 , in which the direction detector comprises first and second antennas and a phase detector for detecting the relative phases of signals arriving at the first and second antennas.

12. An active radar target as claimed in claim 1 , in which the receive and transmit antennas are configured to receive and transmit, respectively, orthogonally polarised radar signals.

13. A radar system in combination with an active radar target as claimed in claim 1 .

14. A radar system as claimed in claim 13 , wherein the radar system further includes at least one of: (i) two or more receive antennas from the plurality of receive antennas, the two or more receive antennas being vertically displaced with respect to one another, and (ii) two or more transmit antennas from the plurality of transmit antennas, the two or more transmit antennas being vertically displaced with respect to one another.