GB2080070A

GB2080070A - Pulse Doppler Radar Apparatus

Info

Publication number: GB2080070A
Application number: GB8021753A
Authority: GB
Original assignee: Decca Ltd
Current assignee: Decca Ltd
Priority date: 1980-07-02
Filing date: 1980-07-02
Publication date: 1982-01-27

Abstract

A pulse doppler radar has an adaptive thresholding arrangement to provide positive radar returns or alarms only if a peak in the doppler spectrum of a range cell exceeds a threshold which is dependent on the mean power in at least a part of the spectrum of the range cell. The analysis signal from a SAW spectrum analyser (34) is integrated (39) and sampled to provide the threshold level for comparison with the original of the analysis signal after a delay (43). The threshold may be modified in response to the power at frequencies in the doppler spectrum immediately adjacent the datum frequency of a possible alarm. <IMAGE>

Description

SPECIFICATION Pulse Doppler Radar Apparatus This present invention is concerned with pulse doppler radar apparatus. In pulse doppler radar, the received radar signals (either at radio frequency (RF) or intermediate frequency (IF) are mixed with a reference frequency to provide signals representing the doppler frequency shift in the received signals. In practice, the doppler frequencies of interest, i.e. indicative of desired moving targets, may be low relative to the pulse repetition frequency (prf) of the radar apparatus.

Thus, the output of the mixer in response to a single received echo pulse from a target would be a substantially constant voltage level representative of the phase of the received echo relative to the reference frequency. By comparing the phases represented by the output of the mixer for the same target on successive pulse repetition intervals (pri) the doppler frequency of the target can be ascertained.

Typically, the above is achieved by range gating the mixer outputs to provide for each pri output voltage levels indicative of the respective doppler phases in successive range cells. The outputs for corresponding range cells of successive pri's are then fed to banks of contiguous filters to isolate the desired doppler frequency content of the returns in the particular range cell.

It has also now been proposed to perform spectral analyses on the doppler signals of the different range cells using Discrete Fourier Transform (DTF) or Fast Fourier Transform (FFT) techniques, or surface acoustic wave (SAW) spectrum analysers. Any of these techniques can provide an indication of the relative power across the doppler spectrum of the received doppler radar signals in a particular range cell. It will be appreciated that such a spectrum analysis signal for a particular range cell is generated from the doppler radar returns over several successive pri's and normally the radar apparatus is arranged so that the number of pri's used to generate the spectrum analysis signal corresponds to the doppler velocity resolution required of the system within the limits set by the azimuth width of the transmitted radar beam, the "dwell time" on a target and the prf.

The spectrum analysis signals for the various range cells can be used to generate target alarms in response to indications in the spectrum analysis signals of received power at doppler frequencies other than those corresponding to clutter. Typically, received power indications in the spectrum analysis signals at DC and frequencies corresponding to the prf of the radadar apparatus and harmonics thereof can readily be ignored or eliminated from the spectrum analysis signals and received power indications at other frequencies can be used to trigger target alarms if they exceed predetermined threshold levels. Typically this threshold level can be pre-set manually so as to give target indications at a constant power level across the spectrum of target doppler returns.

In the following description of the present invention the term "spectrum analysis signal" is used to described any time varying (i.e. serial) signal, or any set of signals in parallel, whether digital or analogue, which can be indicative of the relative power across the doppler spectrum in the received doppler radar signals in a particular range cell. Thus, for example, the spectrum analysis signal may be constituted by the output or outputs of the above mentioned DFT, FFT or SAW spectrum analyser devices, or the outputs of the above mentioned banks of contiguous filters which effectively divide the doppler spectrum into adjacent frequency channels.

According to the present invention, a pulse doppler radar apparatus has range gating means for dividing received doppler radar signals in successive pulse repitition intervals into predetermined range cells; doppler signal processing means for generating for each range cell a spectrum analysis signal representative of the relative power across the doppler spectrum in the received doppler radar signals in the range cell; an adaptive threshold generator responsive to the spectrum analysis signal of each range cell to generate a threshold level related to the mean power in at least a selected part of the spectrum of the doppler radar signals in the range cell; and comparator means arranged to compare the spectrum analysis signal of each range cell with the threshold level generated from said analysis signal of the respective range cell and to provide a target alarm indication when said analysis signal exceeds the threshold level. In this way, the threshold level is adjusted automatically for each range cell in response to the mean power level of received signals across at least a portion of the doppler spectrum. Normally, the part of the doppler spectrum containing primarily clutter returns is not used for generating the threshold level so that the level is related only to the mean power in the remaining part of the doppler spectrum in which moving targets may occur.

This arrangement can most usefully ensure that the threshold is set automatically to a level which prevents the production of an excessive number of target alarms which might overload further data handling equipment in the radar apparatus.

The achievement of a "constant false alarm rate" CFAR is known to be desirable in radar systems.

Normally, said received doppler signals comprise signals indicative of the amplitude and phase of received radar signals relative to a reference frequency and the doppler signal processing means is arranged to accumulate values of said doppler signals gated at corresponding range cells in a plurality of successive pulse repetition intervals, whereby to recreate the doppler frequency information of said doppler signals in each range cell.

The present invention can be used with any doppler signal processing means which produces the required spectrum analysis signals so that the relative power across the doppler spectrum can be ascertained. In one example, said received doppier signals are digital and said processing means comprises digital Fourier transformation apparatus operative to perform a Fourier transformation on the received doppler signals of each range cell to provide, as said spectrum analysis signal, digital outputs indicative of the Fourier frequency components of said received signals. So called Fast Fourier Transform (FTT) apparatus is known for this purpose. The adaptive threshold generator and comparator means of the present invention would then normally also operate digitally.

However, in a preferred example, said doppler signal processing means comprises a surface acoustic wave (SAW) spectrum analyser and storage means adapted to store the values of the range gated received doppler signals over said plurality of pulse repetition intervals and then, for respective range cells, to read out said stored values serially for successive said pulse repetition intervals and supply a respective time varying analogue signal representative of said read values to the SAW analyser for spectrum analysis thereby. The SAW spectrum analyser then produces a time varying output in response to the input from each range cell which represents power variations of the received doppler signal in the range cell across the doppler spectrum.

Typically, the SAW analyser output signal for a particular range cell starts with the power at doppler frequencies corresponding to fast approaching targets, proceeds with the powers of approaching targets of lesser speeds down to zero doppler returns and then the powers of doppler frequenies of receding targets of increasing veiocities.

Preferably, the adaptive threshold generator comprises an integrator to integrate separately the spectrum analysis signals of respective range cells from the SAW analyser, a timed gate to block from the integrator components of the spectrum analysis signals representing zero and very low doppler frequencies corresponding to clutter returns, and a hold circuit to hold a signal at a level dependent on the integrator output at the end of the preceding analysis signal at least until the end of the next analysis signal; and the apparatus includes a delay unit to delay each spectrum analysis signal, by a time at least as long as the duration of each analysis signal, for comparison with the held signals generated from the respective analysis signal.By delaying the spectrum analysis signals in the main signal path, each analysis signal can be compared with a threshold level formed by integrating over the same analysis signal.

In a further preferred embodiment, the threshold generator is arranged to modify the threshold level for comparison with different frequency components of the analysis signals, whereby the threshold level for comparison with a respective said frequency component is dependent on the power in selected frequency components relative to said respective component. Said selected frequency components may be adjacent said respective component. With this arrangement, the threshold level can be different for different frequency components of the analysis signal. This may be desirable because the background noise or clutter response in the doppler spectrum may be "coloured" i.e. not uniform over the spectrum. For example, the background noise and clutter response may be higher to one end of the doppler spectrum than at the other.If the threshold level for the particular range cell is set so as to be uniform across the spectrum and to cut the background noise and clutter at its higher level at one end of the spectrum, then true target signals at the other end of the spectrum might not exceed the threshold level, even though they stand out clearly from the noise and clutter background at immediately adjacent frequencies.

With the arrangement of the above preferred embodiment, the threshold level can be modified over the frequency spectrum to improve detectability of targets at frequencies amid relatively low clutter and noise background frequencies, whilst stili cutting out higher level clutter and noise background powers at other frequencies of the spectrum.

With the arrangement described above using the SAW spectrum analyser, the delay unit may include a tapped delay portion having an intermediate tap connected to supply said delayed analysis signals for comparison and one or more further taps on each side of the intermediate tap, each connected to modify said held signal in predetermined response to the output signal of the respective tap. Conveniently, the output signal of each said further tap is given a predetermined weighting and the apparatus has an adder connected to add the weighted tap output signal to the held signal to provide said modified threshold level. The weighting applied to each of the further tap outputs can be adjusted to suit the particular parameters of the radar apparatus.

An example of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic block diagram of part of a pulse doppler radar apparatus illustrating an embodiment of the present invention: Figure 2 is a schematic circuit diagram illustrating in more detail part of the apparatus of, Figure 1; Figure 3 is a timing diagram of the gating and control wave forms applied to the circuit of Figure 2: Figure 4 is a schematic block diagram illustrating an improved form of the invention; and Figure 5 is a graphical representation of the performance of the improved form illustrated in Figure 4.

Referring to Figure 1 , the illustrated example employs a SAW spectrum analyser to produce a spectrum analysis signal as described above for each range cell. Radar signals at intermediate frequency (IF) from a radar transmitting and receiving apparatus which is not illustrated in the figure are applied on a line 10 to mixers 11 and 1 2. A reference frequency from a stabilised local oscillator is suppled on a line 13 to a phase splitter 1 4. The reference frequency is normally equal to the IF equivalent of the carrier frequency of the radar system.The phase splitter 14 produces two reference signals on lines 1 5 and 1 6 each at the reference frequency but having a phase difference of 90 . The reference signals on lines 1 5 and 1 6 are supplied to the mixers 11 and 12 respectively where they are multiplied with the radar IF on line 10. It will be appreciated that if the radar IF returns are of constant phase relative to the reference frequency, the mixers 11 and 12, when suitably balanced, will produce DC outputs on lines 17 and 1 8 which together represent the amplitude and phase of the radar IF relative to the reference frequency. In practice, the radar If signals on the line 10 have phase dependent on the distance of the target generating the radar return.As is known in the art, pulse doppler radar operates to identify targets with a component of motion towards or away from the radar apparatus by determining whether the radar returns from such targets have varying phase relative to the reference frequency. Accordingly, for a typical radar IF signal on line 10 representing returns during one pulse repetition interval (pri), the signals on lines 17 and 18 are varying voltages which in combination define the amplitude and phase of the received radar signal at any moment.

The signals on lines 1 7 and 18 may be considered as the dopler radar signals since they define the doppler content of the received radar.

As is known for pulse doppler radar systems, the pulse repetition frequency of the radar together with the radar carrier frequency are usually selected so that atypical phase variations from moving targets of interest are slower than the radar prf. In accordance with normal practice in pulse doppler radar, the doppler radar signals on lines 1 7 and 18 are range gated, that is to say sampled at intervals corresponding to predetermined range increments along each range scan in a pri. Thus, a range gating unit 19 generates range gating pulses to gate the signals on lines 17 and 1 8 through analogue gates 20, 21.In the present example, the sampled values of the signals on lines 17 and 18 in each range cell defined by the range gating signals are substantially constant voltage values in view of the slowly varying nature of the signals on lines 17 and 1 8. These constant voltage values are converted into digital form by analogue to digital converters 22 and 23. The outputs of the analgue to digital converters 22 and 23 are in the present example, each 12-bit words defining the voltage level of the doppler radar signal in successive range cells.

The range gated doppler signals are supplied on 12-bit buses 24 and 25 to respective digital clutter cancellers 26 and 27. Such cancellers are known in the art and operate to reduce the content of the doppler radar signals generated by stationary and slow-moving targets which are considered to comprise primarily clutter. Thus, the digital clutter cancellers are arranged to reduce the DC and very low frequency components of the doppler signals at each range cell. It will be appreciated that the doppler frequency of radar returns in any one range cell can be determined by comparing the range gated doppler signals at the particular range cell on successive pri's. In radar systems, the transmitted beam width is such that a number of successive radar pulses will impinge on a target during the azimuth scan so that a target should appear in the same range cell on successive pri's.

Each of the digital clutter cancellers 26 and 27 act as a filter on the doppler signal returns of each range cell separately, looking at the range gated doppler signals for each range cell on successive pri's. For example, the doppler radar signals may be gated into sixteen range cells. The digital filters operate to substantially cancel any DC content in the doppler signals of each range cell, representing stationary target echoes, and also doppler signals having very low doppler frequencies which typically are also generated by various forms of clutter. It will also be apparent to those experienced in this art that clutter returns also appear with doppler frequencies at and immediately around the radar prf and its harmonics. Accordingly, the digital filters also effectively reduce doppler frequencies in the gated doppler signals at and adjacent to the radar prf and its harmonics.

The filtered and gated digital doppler radar signals are supplied on lines 28 and 29 to a digital store 30. The store 30 comprises a matrix of size NRxN,, where NR is the number of range cells in each pri and Nl is the number of pri's to be stored to determine the doppler content at each range cell. Thus, the two 1 2 bit words of data defining in effect the doppler vector, are stored in the matrix store 30 for each of the, for example 16 range cells of a pri, and this is done for several consecutive pri's (for example No=32). The store can be viewed, therefore, as containing 32 rows of data with each row comprising 1 6 sets of two 12 bit words.

As the data is written into the store 30 row by row, i.e. pri by pri, the data is read from the store column by column (or range cell by range cell).

Thus, the data of corresponding range cells is read out serially from the store for all 32 consecutive pri's. The two 1 2 bit words defining the doppler vector of each range cell are supplied from the store on a pair of buses 31 to digital to analogue converters 32. It can be seen that for each range cell, a total of 32 pairs of 12 bit words are supplied along the buses 31 in series and the converters 32 are arranged to generate from these respective series of twelve words a pair of time varying analgue signals which have frequency and phase content corresponding to the doppler content for the particular range cell. In practice, the information is read from the store 30 at a speed considerably higher than the writing speed so that the frequency information in the analogue signals generated by the converters 32 is considerably compressed in time.The analogue signals from the converters 32 are supplied on a line 33 to a SAW spectrum analyser 34. Such analysers are known and operate to produce a spectrum analysis signal on an output line 35 which represents the relative power content of the input signal across its frequency spectrum.

Thus, the output signal on the line 35 of the analyser is also an analogue time varying signal with its time axis corresponding to the frequency spectrum of the input signals on line 33 and its amplitude corresponding to the power in the input signals at the particular frequency. The use of SAW spectrum analysers for producing a spectrum analysis signal in pulse doppler radar processing is discussed in the article "A New Approach to Pulse Doppler Processing" by Roberts, Eames, McCaughan and Butler, published in the IEE Conference Publication No.

1 55, Radar 77, page 358.

Typically, the output of the analyser 34 for a particular range cell indicates initially the power at doppler frequencies representing fast approaching targets and subsequently indicates powers of approaching targets of lesser speeds, then the power at zero and very low frequency doppler and then the power at doppler frequencies of receding targets of increasing speeds. Since doppler frequency power responses in the signal from the analyser are spread out in time, it is a relatively simple matter to time gate the spectrum analysis signal on the line 35 to remove power indications at undesired doppler frequencies, e.g. zero and very low doppler and, typically dopplers in excess of half the radar prf.

Having gated out unwanted doppler responses from the spectrum analysis signal, the gated signal would normally be compared with a threshold level and target alarms generated when the gated analysis signal indicated power at a desired doppler frequency above the threshold.

The present invention is concerned with a technique of automatically adjusting the threshold level.

In the present example, the spectrum analysis signal from the analyser 34 is split into two paths along lines 36 and 37. Line 36 feeds the analysis signal to a gate 38 which is timed to remove from the analysis signal those components corresponding to undesired doppler frequencies as explained above. The gated analysis signal is then integrated in an integrator 39 to generate at the end of each analysis signal a mean power signal indicative of the mean power across the complete doppler spectrum (apart from the unwanted parts removed by the gate 38). It will be appreciated that a separate analysis signal is produced for each range cell and at the end of each analysis signal, the output of the integrator 39 is held by an analoue hold circuit 40. The circuit 40 may also scale the output suitably to generate the desired threshold level.The scaled and held output signal is supplied on a line 41 to a comparator 42. The signal on the line 41 is held by the circuit 40 following the end of one analysis signal for a duration of at least one analysis signal.

The analysis signal from the analyser 34 is fed along the line 37 on the other path to a delay unit 43 which is arranged to delay the analysis signal by a time at least as long as the duration of each analysis signal. This enables a delayed version of a particular analysis signal from the delay unit 43 to be fed to the other input of the comparator 42x after the threshold level on the line 41 has already been set to correspond to the mean power level of the respective analysis signal. In this way each analysis signal, corresponding to each successive range cell, is compared in the comparator 42 with a threshold level which is determined in accordance with the mean power across the doppler spectrum of the particular analysis signal.

This enables the threshold level to be substantially optimised in accordance with varying conditions from one range cell to the next, and of course with changing azimuth angle.

The output of the comparator 42 comprises target alarms whenever the instantaneous value of the analysis signal exceeds the threshold on the line 41. Since the entire analyser output signal is supplied via the delay unit 43 to the comparator 42, the output from the comparator will include alarms in response to unwanted doppler frequencies. However, the alarms during the course of each analysis signal are still spaced in time in accordance with the doppler frequency.

Thus, the alarms from the comparator 42 are supplied to a gate 43 where they are gated with a clutter gate signal on a line 44 to gate out alarms in response to unwanted doppler frequencies. The resulting gated alarms on the line 45 can then be supplied to the rest of the radar apparatus for further analysis or display.

Figure 2 illustrates a preferred form of the gate 38 integrator 39 and analogue hold circuit 40 from Figure 1. In Figure 2, the spectrum analysis signal from the analyser 34 is fed in on a line 50 to an analogue integrator circuit indicated generally by the reference 51. The incoming analysis signal is effectively gated by a gate control signal on a line 52 connected to one input of the integrator circuit 51. It will be appreciated that the integrator 51 is required to integrate each analysis signal separately and must therefore be, arranged to "dump" any previous integration voltage prior to starting the integration of a newa analysis signal. Accordingly, a switch 53 is connected in parallel with the integration capacitor 54 and is opened and closed in accordance with a DUMP control signal on a control line 55. The output of the integrator 51 is scaled as desired by an amplifier 56 and supplied to a hold circuit 57 which is triggered to store the output of the scaling amplifier 56 by a sample hold pulse on a control line 58. The voltage level stored and held by the hold circuit 57 is supplied on a line 59 which is equivalent to the line 41 of Figure 1.

Figure 3 illustrates the timing pulses of the control signals used in Figure 2. The gating signal is supplied to gate the integrator 51 is illustrated at 60 and has a central notch 61 which corresponds in time to the zero and low frequency doppler response in the analysis signal. The sample hold pulse 63 is generated shortly after completion of integration of the analysis signal and the DUMP control signal 64 is arranged to hold the switch 53 open from the start of the gate pulse 60 until after the sample hold pulse 63.

The adaptive threshold generator described with reference to Figures 1, 2 and 3 can operate most satisfactorily especially in circumstances where the frequency spread of background noise or clutter in the doppler radar signals is substantially uniform. However, quite frequently the background noise is "coloured", that is there is more noise at certain doppler frequencies that at others. Referring to Figure 5, a typical frequency spread of doppler returns for a particular range cell is illustrated. For simplicity the doppler frequencies on one side only of zero doppler are illustrated. The trace 70 is representative of the output of the SAW spectrum analyser 34 and shows the background noise level rising at higher doppler frequencies. A desired target return having a specific doppler frequency ft is illustrated at 71.It can be seen that if a uniform threshold level across the doppler spectrum is employed, i.e. that illustrated by the line 72, a false targer response is generated by the noise hump 73 if the threshold level is set sufficiently low to be sure of detecting a target such as 71.

Referring now to Figure 4, a modification of the apparatus of Figures 1 and 2 is illustrated to alleviate the above problem. The analysis signal from the analyser 34 is supplied on a line 80 and fed along one signal path as before to the integration scaling and hold circuits illustrated in the present example by the box 81. The output of these circuits 81 on a line 82 is a base threshold signal which is constant for each analysis signal in the same way as the threshold level generated by the example of Figures 1 and 2.

In the second signal path, the spectrum analysis signal is supplied to an analogue delay unit 83 and thence to a further tapped analogue delay unit 84. The main signal line is taken from a central tap 85 of the tapped delay 84 and fed as before to one input of a comparator 86. The total delay of the analogue delay 83 and of the tapped delay 84 up to the tap 85 is similar to the delay of delay unit 43 illustrated in Figure 1 and sufficient to permit integration of the analysis signal in the circuits 81 so that the base threshold level on line 82 is prepared before the delayed analysis signal is supplied to comparator 86.

On either side of the central tap 85 further taps 86 and 87 sample the analysis signal at times slightly earlier and slightly later than the datum time supplied from the central tap 85 to the comparator 86. In effect, these taps 86 and 87 sample the power in the doppler spectrum at frequencies immediately adjacent to the frequency being supplied to the comparator 86 at any particular time. The samples from the taps 86 and 87 are passed through respective weighting circuits 88 and 89 to an adder 90. The adder 90 adds the weighted tap outputs to the base threshold level on the line 82 to produce a modified threshold level which is dependent on the power in the doppler spectrum immediately adjacent the frequency of interest at any moment.

The weighting applied by the circuits 88 and 89 can be adjusted to suit the particular parameters of the radar apparatus in which the present system is employed. The selection of suitable weighting values can be determined by computer analysis of the specific parameters or empirically.

Referring again to Figure 5, the resultant frequency adaptive threshold from the adder 90 is illustrated by the line 74. As can be seen in Figure 5, the target return 71 is still readily detectable but the threshold level successfully "climbs" over the noise hump 73.

The frequency adaptive threshold level from the adder 90 is applied as before to the second input of the comparator 86 for comparison with the analysis signal to generate alarms. As before, the alarms from the comparator 86 are gated in a gate 87 to remove alarms corresponding to unwanted doppler frequencies.

Although the above described examples of the invention use a SAW spectrum analyser to generate the spectrum analysis signal, other analysing techniques can be employed, for example Fast Fourier Transform (FFT) circuits which produce digital outputs representative of the power content across the doppler frequency spectrum. Then, the adaptive threshold level is generated by digital techniques and the frequency adaptive threshold level can similarly be generated digitally.

Claims

1. A pulse doppler radar apparatus having range gating means for dividing received doppler radar signals in successive pulse repetition intervals into predetermined range cells; doppler signal processing means for generating for each range cell a spectrum analysis signal representative of the relative power across the doppler spectrum in the received doppler radar signals in the range cell; an adaptive threshold generator responsive to the spectrum analysis signal of each range cell to generate a threshold level related to the mean power in at least a selected part of the spectrum of the doppler radar signals in the range cell;; and comparator means arranged to compare the spectrum analysis signal of each range cell with the threshold level generated from said analysis signal of the respective range cell and to provide a target alarm indication when said analysis signal exceeds the threshold level.

2. Apparatus as claimed in claim 1, wherein said received doppler signals comprise signals indicative of the amplitude and phase of received radar signals relative to a reference frequency and the doppler signal processing means is arranged to accumulate values of said doppler signals gated at corresponding range cells in a plurality of successive pulse repetition intervals, whereby to recreate the doppler frequency information of said doppler signals in each range cell.

3. Apparatus as claimed in claim 2, wherein said received doppler signals are digital and said processing means comprises digital Fourier transformation apparatus operative to perform a Fourier transformation on the received doppler signals of each range cell to provide, as said spectrum analysis signal, digital outputs indicative of the Fourier frequency components of said received signals.

4. Apparatus as claimed in claim 2, wherein said doppler signal processing means comprises a surface acoustic wave (SAW) spectrum analyser and storage means adapted to store the values of the range gated received doppler signals over said plurality of pulse repetition intervals and then, for respective range cells, to read out said stored values serially for successive said pulse repetition intervals and supply a respective time varying analogue signal representative of said read values to the SAW analyser for spectrum analysis thereby.

5. Apparatus as claimed in claim 4, wherein the adaptive threshold generator comprises an integrator to integrate separately the spectrum analysis signals of respective range cells from the SAW analyser, a timed gate to block from the integrator components of the spectrum analysis signals representing zero and very low doppler frequencies corresponding to clutter returns, and a hold circuit to hold a signal at a level dependent on the integrator output at the end of the preceding analysis signal until at least the end of the next analysis signal; and the apparatus includes a delay unit to delay each spectrum analysis signal, by a time at least as long as the duration of each analysis signal, for comparison with the held signal generated from the respective analysis signal.

6. Apparatus as claimed in any preceding claim wherein the threshold generator is arranged to modify the threshold level for comparison with different frequency components of the analyser signals, whereby the threshold level for comparison with a respective said frequency component is dependent on the power in selected frequency components relative to said respective component.

7. Apparatus as claimed in claim 6, wherein said selected frequency components are adjacent said respective component.

8. Apparatus as claimed in claim 7, where claim 6, is dependent on claim 5, wherein the delay unit includes a tapped delay portion having an intermediate tap connected to supply said delayed analysis signals for comparison and one or more further taps on each side of the intermediate tap each connected to modify said held signal in predetermined response to the output signal of the respective tap.

9. Apparatus as claimed in claim 8, wherein the output signal of each said further tap is given a predetermined weighting and the apparatus has an adder connected to add the weighted tap output signals to the held signal to provide said modified threshold level.

10. Pulse doppler radar apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.