Abstract:
An active radar target comprising: a receive antenna for receiving a radar signal, an amplifier for amplifying the signal, and a transmit antenna for retransmitting the amplified signal, characterised in that the active radar target further includes a memory for storing an identity code or means for receiving an identity code, a modulator for receiving the identity code and modulating the radar signal with at least the identity code prior to retransmission of the amplified signal, and wherein the modulator is a single sideband modulator.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of United Kingdom Patent Application No. GB1004964.1, filed Mar. 25, 2010, the entire disclosure of which is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an active radar target, and in particular one that can return a data payload in excess of a target identification. 
       BACKGROUND TO THE INVENTION 
       [0003]    Radar can be used to measure range and distance to a reflective object, which is often referred to as a “target”. However, a return may be achieved from all possible reflecting surfaces, which in the case of a large structure such as a ship or oil rig can give a large return signal, of extended angular range and a range of distances. This is generally acceptable for standard collision avoidance applications. 
         [0004]    However such diverse returns of the radar signal are not useful if it is desired to use radar to measure distance to a target, such as a vessel position relative to an oil rig, to a few metres or less. 
         [0005]    It is known to provide active radar reflectors. An example of such a reflector is the “See me” (trademark) radar target enhancer which detects when a vessel carrying the target enhancer is illuminated by a radar system. It amplifies the incoming signal and retransmits it, and in so doing synthesises a radar cross section of around 34 square metres. 
       SUMMARY OF THE INVENTION 
       [0006]    According to a first aspect of the present invention there is provided an active radar target for use with an FMCW radar comprising:
       a receive antenna for receiving a radar signal,   an amplifier for amplifying the signal, and   a transmit antenna for retransmitting the amplified signal,   a memory for storing an identity code or means for receiving an identity code,   a single sideband modulator for receiving the identity code and modulating the radar signal with at least the identity code prior to retransmission of the amplified signal, and   wherein the receive and transmit antennas operate with orthogonally polarised radar signals.       
 
         [0013]    The identity code may, for example, be associated with the active radar target, and hence may be stored in a memory within the target. However, the identity code might relate to another item, such as a bracket or other structure on part of a further structure, such as a ship or an oil rig, and the identity code may be read by the active target or entered into the active target when the active target is being positioned. The means for receiving the identity code may comprise a keyboard, but other items such as bar code readers, RF tags or near field readers might be used when a RF or other memory tag is attached to the bracket or structure. Alternatively, where the active target includes a data input for receiving data from other devices, the identity may be sent to the active target via the data input. 
         [0014]    Preferably the modulator is a single sideband suppressed carrier (SSBSC) modulator. This is known to be efficient as signal power is not wasted on a carrier, which consumes power but conveys little information—except from identifying the presence of the carrier. However, in the context of the present invention and ranging systems using an active target constituting an embodiment of the present invention, the SSBSC modulation conveys additional advantages. 
         [0015]    Advantageously a frequency shift keying (FSK) scheme is employed to encode a digital word or words conveying at least the identity. FSK has the advantage that the radar signal is modulated in a form that is quite easy to detect, and which exists irrespective of the value of the data payload. This is to be compared with, for example, AM modulation where “1” might represent returning 90% of the transmit power and a “0” might represent returning 10% of the transmit power of the amplified signal. In such a scheme it becomes difficult to distinguish between a low signal return strength because the signal is encoding a “0” and low signal return because of multi-path destructive interference. Other modulation schemes, such as phase shift keying, PSK, might equally be employed to encode the identity signal. PSK provides similar advantages to FSK modulation. 
         [0016]    Preferably the identity is retransmitted periodically whilst the active target is illuminated by a radar signal above a predetermined intensity. Thus if for example the active target is powered by one or more batteries, then it can de-power into a resting state when not illuminated by a radar and hence conserve battery life. 
         [0017]    Advantageously the active target can transmit additional data back to a suitable radar system such that additional data in excess of the identity, which may be the identity of the active target, can be provided to the radar system. Such additional data may be provided by an external or internal data source. Data may include GPS position, vessel speed, vessel heading, sea state parameters, data from strain gauges or other external devices, and so on. Such external data may be transmitted to the active target wirelessly or sent over a databus. Similarly the identity may be provided to the active target wirelessly or via a databus. 
         [0018]    The target may also include internal sensors for detecting the direction of the incoming radar with respect to a reference direction. The reference direction may be defined by the shape of the target. Thus, if the target has a planar surface or some other direction defining the “front” of the target, then the direction of illumination may be measured with respect to the front. For embodiments which operate in a roughly 2D space, i.e. at fixed height with respect to the earth or sea, then a measured azimuth is generally sufficient. 
         [0019]    In a preferred embodiment the active target includes means for providing signal path diversity. 
         [0020]    According to a second aspect of the present invention there is provided a radar system comprising a radar transmitter and receiver adapted to co-operate with an active radar target according to the first aspect of the present invention so as to extract at least an identity from a signal returned by the active target. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The present invention will now be described, by way of example only, with reference to the accompanying Figures, in which: 
           [0022]      FIG. 1  schematically illustrates a vessel illuminated by a radar and a plurality of radar returns R 1  to R 4  which may result, each having a different range from the radar antenna; 
           [0023]      FIGS. 2   a  and  2   b  schematically illustrate how a chirped radar signal can provide very accurate distance and relative speed data; 
           [0024]      FIG. 3  schematically illustrates a circuit for reading an identity code from a memory and producing a baseband frequency shift keyed signal; 
           [0025]      FIG. 4  illustrates the spectrum of a frequency shift keyed signal when used to modulate an RF carrier; 
           [0026]      FIG. 5  illustrates the spectrum of  FIG. 4  when a SSBSC modulator is used; 
           [0027]      FIG. 6  illustrates a configuration of a SSBSC modulator in an active target constituting an embodiment of the present invention; 
           [0028]      FIG. 7  illustrates an arrangement for measuring the azimuth of an incoming radar beam; 
           [0029]      FIG. 8  illustrates an embodiment of the present invention incorporating two receive/transmit channels at different heights; 
           [0030]      FIG. 9  schematically illustrates a radar system operating in accordance with the present invention; 
           [0031]      FIG. 10  illustrates a further embodiment of the invention; 
           [0032]      FIG. 11  shows the magnetic input of  FIG. 10  in greater detail; and 
           [0033]      FIG. 12  illustrates a further embodiment of the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0034]    As noted hereinbefore, it can be difficult to obtain a very precise radar range to a complex target.  FIG. 1  illustrates a situation where a radar, schematically represented by a parabolic dish  2  (although no inference should be taken from this as the invention is equally suitable for use with arrays of antennas operable to form beam steered arrays) which transmits a radar signal that illuminates a complex target  4 , such as a large ship. The ship has a hull  6 , a superstructure  8 , a funnel  10  and an item of cargo  12 . Each of these items may give a radar return, and as shown each reflecting item has a slightly different distance R 1  to R 4  respectively from the radar  2 . 
         [0035]    This is not a problem if one merely wishes to know that he ship is there and to get a rough distance to it, e.g. 1.5 nautical miles (1 nautical mile=1854 m). 
         [0036]    However, some navigation, such as pipe laying and oil rig positioning requires the distance between objects to be measured to around 1 metre or less. 
         [0037]    Frequency modulated continuous wave (FMCW) is a known technique which can achieve measurements to within this required accuracy. This technique has the advantage over pulsed radar techniques in that the target can be continuously illuminated by the radio frequency energy of the radar, thereby allowing additional signal and data processing techniques to be used in conjunction with the radar ranging. 
         [0038]    Although FMCW is known, its basic principles will be briefly discussed for completeness, with respect to  FIGS. 2   a  and  2   b . FMCW is often driven with a triangle frequency change waveform. However, for convenience, and to demonstrate the power of this technique, we will consider a simplified example where the radar is modulated with a saw-tooth chirp, and a gap exists between each chirp. 
         [0039]    The radar frequency is frequency modulated such that it starts at frequency f 0  at time t o  and then increases at a known rate up to a maximum frequency f 1  at time t c . This is generally known as a ‘chirp’ and the chirp extends from t o  to t c , as shown in  FIG. 2   a.    
         [0040]    Now suppose at time t o  the radar is illuminating a target. At time t o  the radio signal having a frequency f 0  exits the radar antenna, and then travels to the target, is reflected and returns. The journey takes a round trip travel time such that at time r 1  the signal, having a frequency f 0  is received at the radar. 
         [0041]    In the meantime the radar frequency has been changing with the chirp, at a rate Δ f /T c . The difference between the instantaneous transmission frequency and the instantaneous return frequency is proportional to the journey time of the radar signal to the target and back. These frequencies can be mixed together by a mixer and produce a down converted beat frequency F b  that is proportional to the distance to the target, as illustrated in  FIG. 2   b.    
         [0042]    In fact, ignoring all other factors, such as Doppler shift 
         [0000]    
       
         
           
             Range 
             = 
             
               
                 
                   F 
                   b 
                 
                 × 
                 C 
                 × 
                 T 
               
               
                 2 
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                 Δ 
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         [0043]    where
       C=speed of light   T=t c −t 0      Δf=f 1 −f 0          
 
         [0047]    It can be seen that in this example, apart from the time period  20  where the beat frequency is nominally constant and is a function of the distance to the target, there is also another period  22  where the transmit and return frequencies are nominally the same, so any frequency shift here is a Doppler shift allowing relative velocities to be identified. 
         [0048]    Of course, with multiple reflections occurring there is still the potential for several return frequencies, rather than the monotone F b  described here. 
         [0049]    In order to facilitate ranging, an active target can be used, but to distinguish itself from the background, the active target needs to stand out, and it can do this by transmitting a modified signal. 
         [0050]    The modified signal may be modified by one or more of
       transmitting an identity signal   a distinctive modulation scheme   transmitting other data—such that the signal could not be a “simple” radar reflection.       
 
         [0054]    It is useful to consider these approaches in turn. 
         [0055]    Transmitting an Identity 
         [0056]    Each active target may be given an identity code that it uses to modulate the returning signal such that the target can be identified from other returns to the radar system. 
         [0057]    Simplistic schemes might include a periodic amplitude modulation (as might be achieved by a rotating reflector being covered and uncovered) or a periodic frequency shift, as could also be achieved by a rotating reflector giving a doppler shift. However these schemes are inflexible and rely on rotating mechanical parts. 
         [0058]    In a preferred embodiment, a digital identity code is used to modulate the return signal. A particularly useful approach is to amplitude modulate the returning radar signal, with the modulation occurring at rates M 1  and M 0  to represent digital “1,” and “0 s ” respectively of a digital word. This is also known as binary frequency shift keying. However, a greater number of frequencies could be used to transmit more complex symbols, thereby increasing the information content for a given symbol rate. 
         [0059]    Thus a modulation is always returned, irrespective of the digital word or words, but the digital signal can also convey a target identity, and also other data. 
         [0060]    In an exemplary embodiment modulation frequencies of 1.75 MHz and 2.25 MHz are chosen to represent the digital 1&#39;s and 0&#39;s.  FIG. 3  illustrates a simple circuit to read out an identity from a memory, such as a shift register  40  that gets reset to a predetermined bit pattern representing the identity. 
         [0061]    A clock  50  provides a timing and frequency reference. The clock has an output that is provided as an input to dividers  52 ,  54  and  56  dividing by M, N and P respectively. Outputs of the dividers  52  and  54  are provided to respective inputs of a 2 channel multiplexer, which has a selection input connected to an output of the shift register  40 . 
         [0062]    If, for example, the clock runs at 15.75 MHz, then if M=9, the output of divider  52  is a 1.75 MHz signal and in N=7 then the output of the divider  54  is a 2.25 MHz signal. 
         [0063]    If P is much larger than 7 or 9, say 128, then a digital word represented by 1.75 or 2.25 MHz modulation patterns can be transmitted at reasonable data rates, i.e. over 100 Kbits per second. 
         [0064]    Once the register  40  is enabled, it successively clocks out the target identity, and the pattern of 1&#39;s and 0&#39;s is used by the multiplexer to select the correct frequency to be output at node  62 . 
         [0065]    If it is desired to add further data after transmission of the identity signal then it is easy to modify the circuit to pass the signal from the register  40  via an AND gate as shown in the outline, and to arrange the register to repeatedly output an “1” after the identity has been transmitted. 
         [0066]    Thus the AND gate can receive a 1 at its second input whilst the identity is being transmitted, and then subsequently one or more data words that are to be transmitted to convey other information. The sequence is repeated automatically. 
         [0067]    The register could have its initial values hard wired, or they could be set by a keyboard, switches, or via any other suitable arrangement. Where a keyboard or switches are used to set the identity then the identity can be that a mounted bracket or similar whose position on a structure is well determined. 
         [0068]    A mounting bracket may carry mechanical protrusions, or galvanic connections which allow the identity of the mounting bracket to be passed to the active target when the target is attached to the bracket. The bracket might, in some embodiments, carry a near field memory device, such as an RF ID tag, which contains the identity information, and which is read by the active target. Similarly the active target may include a slot to accept a memory card or USB stick to provide identity information, or it may receive it from a non-contracting memory, or via a data connection from a further device. 
         [0069]    The invention has been described in the context of a frequency shift keying modulation scheme. However other schemes, such as phase shift keying could also be used to indicate the identity and/or to convey other information. For example, rather than using two frequencies to indicate a “0” or a “1”, a single frequency could be used, 2.25 MHz for example, and a “0” may be represented by a phase change of zero degrees and a “1” may be represented by a phase change of 180°. In such a system a preamble might be included such that the receiver could identify the preamble, which is a known bit sequence, to recover the phases correctly. 
         [0070]    Modulating the Return Signal with the Identity 
         [0071]    The simplest approach to returning the signal would be simple amplitude modulation of the radar signal. However this is a poor approach as a significant amount of the transmit power would be in the carrier frequency, and as this is identical to the frequency used to illuminate the target, then oscillation may result. The need to avoid oscillation would require the use of only a low gain amplifier, and probably physically separate and widely spaced apart receive and transmit antennas. This is at odds with the requirement to provide a relatively discrete physical target so as to achieve the desired range discrimination of 1 metre or less. 
         [0072]    The inventors realised that the oscillation risk could be much reduced by offsetting the returned data from the carrier frequency and/or suppressing the carrier. 
         [0073]    Representing the data by frequencies M 0  and M 1  has the effect of moving the return data, in the frequency domain, into sidebands positioned around the instantaneous carrier frequency. This is shown in  FIG. 4  where upper  80  and lower  82  sidebands each have signals for “1” and “0” spaced 1.75 and 2.25 MHz away from a carrier  84 . 
         [0074]    Additionally, at the receiver harmonics (not shown) of the sidebands are received, together with carriers that are displaced in frequency because they were reflected from a reflector at a different distance from the radar. 
         [0075]    These sidebands are unwanted as they have the potential to generate signals within the receiver that can interfere with a wanted signal. 
         [0076]    This is overcome in one embodiment of the invention by using a signal sideband suppressed carrier modulation scheme. Ideally only one sideband is selected for transmission, although as shown in  FIG. 5  in reality the carrier  84   a  and the upper sideband  80   a  can be significantly suppressed, say by 36 dB or so, compared to a lower sideband  82   a.    
         [0077]    Several approaches are known for SSBSC modulation, although for microwave frequencies the phasing method implemented using a Hartley modulator is appropriate. An embodiment of a modulator in accordance with the present invention is shown in  FIG. 6 . 
         [0078]    The modulator comprises a receive antenna  100  that provides an input to a first amplifier  102 . A power detector  104  follows the amplifier  102  and forms part of a power control loop with the amplifier  102  so as to control the signal at the output the amplifier to lie with a target power range, and/or reduce power consumption when the active target is not being illuminated by a radar. 
         [0079]    The amplifier output is provided as an input to a Hartley modulator  106 . The Hartley modulator is well know to the person skilled in the art, but the following brief explanation is given to assist the reader. An input signal S is passed through a 90° coupler  110  which provides two output signals S 1  and S 2 . S 1  is in phase with S, i.e. has a 0° phase shift whereas S 2  is shifted by 90°. Thus two signals out of phase are produced. 
         [0080]    These can be represented as 
         [0000]      S1=sin W m T 
         [0000]      S2=cos W m T 
         [0081]    The signals S 1  and S 2  are provided to first inputs of balanced mixers  112  and  114 . The mixers also receive in-phase and 90° shifted versions of the modulating signal, i.e. the 1.75 and 2.25 MHz signals generated by the circuit of  FIG. 3 . This can be achieved either by a phase shifting filter, or running the circuit at higher frequencies and then using dividers to divide down to the desired frequencies and to control the phases. 
         [0082]    Thus the signal inputs, labelled “MODI” and “MODQ” are provided to the mixers  112  and  114  respectively. 
         [0000]      MODI=sin W s T 
         [0000]      MODQ=cos W s T 
         [0083]    The outputs of the mixers are summed in a summing coupler  120 . 
         [0084]    The outputs of the mixers can be represented, using the trigonometric identities. 
         [0000]      sin  A ·sin  B =½cos( A−B )−½ cos( A+B )
 
         [0000]      cos  A ·cos  B= ½cos( A+B )−½cos( A−B )
 
         [0085]    to give 
         [0000]    
       
         
           
             
               
                 
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         [0086]    The lower sideband signal is then amplified by a further amplifier  122  and then provided to transmit antenna  124 , which may be a patch array. 
         [0087]    Thus the modulation scheme in conjunction with the FSK or PSK encoding gives a frequency shift between the input and the output frequencies of at least 1.75 MHz, and also suppresses the carrier, thereby reducing the risk of self sustaining oscillation and enabling a higher gain to be applied by the amplifiers  102  and  122 . The isolation can be further improved by using polarisation sensitive receive and transmit antennas and arranging for them to work with orthogonal polarisation states, e.g. vertically and horizontally linearly polarised radiation, but circular polarisations may also be used. Thus the receive antenna may be responsive to vertically polarised RF energy whereas the transmit antenna transmits horizontally polarised RF energy or vice versa. 
         [0088]    Thus the active target can transmit an identity, and with increased gain, so as to help it stand out against other radar returns. Signal processing at the radar can be used to extract the target identity, thereby allowing multiple targets to be used, and potentially to be quite closely spaced. 
         [0089]    Other Data 
         [0090]    Returning to  FIG. 1 , it can be seen that the ship  4  has a “forward” direction in which it travels. Consequently to know about the dynamics of motion of the ship it is advantageous to know more than just the range to the ship. It is desirable to have other data such as the relative direction of the ship to the radar beam. 
         [0091]    This can be achieved by measuring the angle of illumination that the incoming radar makes with the active target. This, combined with knowledge about placement of the target on the vessel allows the relative angle of the vessel to be determined. 
         [0092]    There are several approaches that could be used, such as a mechanically swept narrow beam antenna; multiple fixed narrow bean antennas, each with a respective direction of look, or phased array synthesis of a narrow beam antenna. 
         [0093]    A suitable approach is a phase comparative approach as this can be performed with a mixer. 
         [0094]      FIG. 7  illustrates an active target constituting an embodiment of the invention and comprising first and second receive antennas  130  and  132  that are spaced apart by a known distance D within a housing  134 . Although the drawing is schematic, the housing is shown to highlight that the target can be relatively thin, i.e. be quite planar, and hence easy to attach to surfaces. 
         [0095]    If the incoming radar beam is normal to the plane  135  containing the antennas  130  and  132 , as illustrated by beam B 1 , then a notional wave front arrives at both antennas at the same time. Hence the phases of the signals output by the antennas are identical. 
         [0096]    However if an oblique beam B 2  illuminates the active reflector then, as shown, a notional wave front W 2  arrives at antenna  130  before it arrives at antenna  132 . This gives rise to an extra “time of flight” which is proportional to the distance D and the sine of the angle of illumination θ. 
         [0097]    This in turn gives rise to a phase charge which depends on the wavelength of the radiation used by the radar. 
         [0098]    As shown the output of each antenna is provided to a respective amplifier  136 ,  138  which advantageously both amplifies and limits the signal amplitude. The outputs of the amplifiers are then provided to a mixer  140  which multiplies the signals together, forming a sum frequency, and more importantly a difference frequency which is a DC representation of the phase difference, and which is digitised by a digital to analog converter  142 . The DAC output can be placed in a data word and retransmitted to the radar, using the circuit shown in  FIG. 3 . 
         [0099]    Returning to  FIG. 7 , we have a choice of whether the normal illumination gives a maximum or a minimum in the output signal of the mixer  140 . 
         [0100]    If a maximum is required, then all the path lengths are matched within the receiver, or differ from each other by a multiple of λ/2 where λ is the wavelength of the radar used to illuminate the target. If however we want a null, then an additional path length of λ/4 (or 3, 5, 7, 9 etc λ/4) as indicated by 144 can be inserted into one of the signal paths. 
         [0101]    Other path delays could be selected, such as λ/8 so that a signal from the mixer would be ½ amplitude for radio waves arriving perpendicular to plane  135 , and would increase if, for example, the beam swings towards the first receive antenna  130 , and would decrease if the beam swings towards the second antenna  132 . Thus angle and direction can be resolved simultaneously. 
         [0102]    Signal Path Diversity 
         [0103]    The radar beam travelling to a target and/or the signal returning may, in fact, comprise multiple signals travelling via dissimilar paths. Thus one portion of the signal may travel directly to the target whilst another portion may reflect from the surface of the sea. The signals interfere with one another, and the interference may be constructive or destructive depending on the relative path lengths. 
         [0104]    Destructive interference may cause a return from the target to disappear. In order to address these multiple effects it is desirable to add diversity to the active target. The diversity may be one or more of a distance diversity, and a height diversity. If it is desired to keep the reflector “thin”, then height diversity is preferred and hence a second receive and a second transmit antenna are provided, vertically displaced (and optionally horizontally displaced) from the first transmit and receive antennas. In other configurations one of the receive or transmit antennas may be omitted, although this does allow destructive interference in one of the paths between the interrogating radar and the active target to effect the performance of the system compared to providing multiple receive and multiple transmit antennas. 
         [0105]    This could be done with two physically separate active targets, but this does not allow synergies to occur from building height diversity into a single unit. The single unit approach allows the height diversity to be controlled to a sensible range as defined by the case of the active target to around 0.5 to 1 metre. Furthermore the antennas can share the same target identity, and can return the same additional data. 
         [0106]    The returns from the antennas can be frequency or time multiplexed in order that they themselves do not interfere. 
         [0107]    Frequency multiplexing can occur by choosing different modulation frequencies to represent the “1”s and “0”s of the digital identity. Thus in the arrangement described hereinbefore the frequencies of 1.75 and 2.25 MHz can be regarded as being centred around 2 MHz. For the second antenna frequencies of 2.5 and 3 MHz may be chosen, giving signals centred around 2.75 MHz. 
         [0108]    Alternatively the antennas can be operated in a time multiplexed mode such that for a while the first transmit antenna is active, but the second transmit antenna is not, and then the second transmit antenna is active, but the first is not. This swapping is repeated in a cyclical manner. 
         [0109]    Finally a time delay in retransmit times—which could simply comprise an additional path length of several metres or 10&#39;s of metres can be added to one of the antennas. The delay may simply be a predetermined length of cable. This causes the returns to appear to come from different ranges, and hence stops them interfering with each other as they have different instantaneous frequencies. 
         [0110]      FIG. 8  schematically illustrates an active target in which signal path diversity is provided by having two separate receive and transmit channels physically separated from one another. Thus a first channel comprises a patch antenna  160 , amplifiers and single sideband suppressed carrier modulators within a block  162  vaguely corresponding to components  102 ,  106  and  122  of  FIG. 6  and a transmit antenna  164 . A second channel comprises receive antenna  170 , an amplification and modulation block  172  and a transmit antenna  174 . Both channels share the same identity memory and frequency shift keying circuit, for example as shown in  FIG. 3 , albeit with a further frequency translation being applied to the second channel, such that they transmit the same data but at slightly different frequencies. It therefore becomes much more unlikely that both channels will simultaneously suffer degradation due to destructive interference as a result of multiple propagation paths. Additional transmit or receive antennas may be added to further reduce the potential for destructive interference to adversely impact on operation of the radar ranging system. 
         [0111]    In the arrangement shown in  FIG. 8  each of the receive antennas is uniquely associated with a transmit antenna. This association need not be permanent and, for example, a multiplexer or other cross coupling circuit can be provided to change the connections on an automated basis such that sometimes receive antenna  160  is in a signal path that leads to transmit antenna  164  and at other times the receive antenna  160  is associated with the transmit antenna  174 .  FIG. 12  shows, in schematic form, a further embodiment where the number of receive antennas and transmit antennas are dissimilar and where time division multiplexing is used to vary the association between input antennas and output antennas. 
         [0112]    In the arrangement shown in  FIG. 12  three receive antennas RX 1 , RX 2  and RX 3  are provided each at a different height, as schematically indicated in  FIG. 12  by them being vertically displaced from one another. The horizontal spacing between the receive antennas may also vary, as the antennas may be used as part of the beam angle measurement system disclosed hereinbefore and different separations allow different angular sensitivities to be obtained. Each of the antennas RX 1 , RX 2  and RX 3  provides its signal to an associated amplifier  350 ,  352  and  354 , the outputs of which are connected to first and second multiplexers  360  and  362 . Each of the multiplexers  360  and  362  is responsive to a receive and transmit antenna pair control unit  364  which selects signals from the receive antennas, RX 1 , RX 2  and RX 3  to be supplied to a first modulator or a second modulator. The same or different input signal may be supplied to each of the modulators, labelled MOD 1  and MOD 2 . Each modulator may also receive a data payload encoded by frequency shift keying as described hereinbefore with respect to  FIG. 3 . The modulators MOD 1  and MOD 2  are preferably single sideband suppressed carrier modulators of the type generally designated  106  in  FIG. 6 . The output of each modulator MOD 1  and MOD 2  is provided to two further multiplexers  370  and  372  which can select the modulator outputs for output to amplifiers  374  and  376  which are themselves connected to transmit antennas TX 1  and TX 2  which are again disposed at different heights, as schematically illustrated by transmit antenna TX 1  being illustrated above transmit antenna TX 2 . Thus in this arrangement there are six possible height combinations which can be selected in a time multiplexed manner with two different height combinations being active concurrently. It is thus possible to provide height diversity such that destructive interference in the path from the radar to the receive antenna or in the path from the transmit antenna to the radar does not cause the data payload from the active target to become lost. 
         [0113]      FIG. 9  schematically shows a radar ranging and positioning system in which a radar, generally designated  200  comprises a chirped frequency source  202  which provides radio frequency energy, generally in the radar X band (wavelength=3 cm), or in similar sub 25 cm wavelengths. The radar signal is transmitted towards an active target  210  constituting an embodiment of the invention. As discussed hereinbefore the target amplifiers, modulates and returns the radar signal and the return signal is received by a receive antenna  204 , amplified by an amplifier  206 , and mixed with the output of the transmit oscillator  202  by mixer  208  to get a beat frequency which is then digitised by an analog to digital converter  210  and provided to a data processor  212 . The data processor  212  is adapted to calculate the range to the target based on the beat frequency, but further to take into account further frequency translations as a result of the frequency shift keying scheme introduced at the active target in order to modulate the signal being returned to the radar  200 . The data processor is also arranged to analyse the digitised signal to look for signal patterns representative of the target identity, and further data which may also be modulated onto the radar signal. Where the further data may be any one of a plurality of data types a header is advantageously attached to the data in order to signify the type of data that follows the header. 
         [0114]    It is also possible for the pairings of receive and transmit antennas to be arranged to transmit respective identities, rather than the same identity. Advantageously the respective identities are related to one another such that the task of setting up the respective identities is simplified. Thus, for example, where an identity is programmed in to one channel of active target having height diversity, the other identity may be generated automatically from the first identity. 
         [0115]    Height diversity may also be provided in the radar  200 . Generally the radar system has a rotating transmit antenna and a rotating receive antenna in synchronism with the transmit antenna. Additional receive or/and transmit antennas can be provided, and the transmit antennas may, for example, be driven in a time multiplexed manner. Signals received at the receive antennas may be selected on the basis of whichever signal is the strongest, or could be selected in an alternating fashion. Alternatively each receive signal may be down converted individually and then the signals combined in a non-destructive manner. 
         [0116]    In the embodiment of the active target described with respect to  FIG. 3 , the identity of the active target was held within register  40 . In some instances it may be preferable for the identity of the active target to be held by a mounting bracket for the target rather than the target itself. This means that, where the target is battery powered, a target whose batteries are becoming depleted may be removed from a mounting bracket and a new target with fresh batteries attached to the mounting bracket. Once the new target is attached it takes the same identity as the target that has just been removed because that identity is passed to it from the mounting bracket. Such an arrangement is schematically illustrated in  FIG. 10  where an active target  220  comprises receive and transmit antennas  222  and  224 , respectively, co-operating with a modulator  226  and enclosed within a case  230 . The case is selectively mountable and demountable inside a mounting frame  240  which in this example is shown having a hinged lid  242  such that the active target  220  can be quickly but securely fixed within the frame. The active target includes a battery  260  which powers the active target. This has the advantage that no external power needs to be routed to the target, but has the disadvantage that the target will need maintenance from time to time in order to replace the battery, which might be done simply by swapping out the active target with a new one. In order to facilitate this the active target includes means for receiving its target identity. In the embodiment shown the active target includes a tag reader  264 , such as an RF ID tag reader which is positioned such that it would be able to interrogate a corresponding RF ID tag  266  located in the bracket. This has the advantage of protecting the RF ID tag from being lost although similarly the RF ID tag, or some other removable memory device could be attached to the active target around the time that it is installed on the bracket. An alternative approach might be to include an array of magnets  280  to  284  whose presence, or absence, with respect to a corresponding magnetic switch, such as a read relay, allows the target identity to be passed to the active target. This arrangement is shown in more detail in  FIG. 11  where magnet  280  aligns with an associated reed relay  300  in the active target causing the reed relay to close whereas a further reed relay  302  aligns with the gap between magnets  280  and  281  and therefore remains open. It can therefore be seen that by suitable positioning of the magnets with respect to the relay positions a digital identity can be easily and robustly passed between the mounting bracket and the active target. 
         [0117]    It is thus possible to provide an improved active target, and a radar system which can benefit from the augmented information that can be returned by the active target.