GB2134741A

GB2134741A - Radar apparatus

Info

Publication number: GB2134741A
Application number: GB08302586A
Authority: GB
Inventors: Philip David Lane Williams
Original assignee: Decca Ltd
Current assignee: Decca Ltd
Priority date: 1983-01-31
Filing date: 1983-01-31
Publication date: 1984-08-15
Also published as: GB2134741B

Abstract

Radar apparatus functions by transmitting (10, 12, 13) cyclically repeated sequences of pulses having mutually different frequencies, received (13) echo pulses being separated (12) into respective reception channels (TR1TRN) in dependence upon their frequencies, and there mixed (18 to 21) with local oscillator signals (26) to give signals at a common intermediate frequency, which signals are delayed ( tau 1- tau N) such that delayed signals corresponding to echoes at different frequencies from a single target coincide in time. The delayed signals then provide range information to a maximum range corresponding to the maximum unambiguous range for radar apparatus with a pulse repetition frequency equal to the rate of succession of the sequences of pulses. <IMAGE>

Description

SPECIFICATION Radar apparatus The present invention relates to radar apparatus. In pulse radar apparatus there is a conflict between the need to emit signals having a high mean power to ensure adequate signal strength in echos from distant targets, with the need to have a relatively low prf to obtain unambiguous range information of such distant targets. Quite apart from the need for higher powers at greater ranges resulting from the finite divergence of the transmitter beam, greater energy per transmitted pulse is required as the prf is reduced. This no'rally leads to the use of relatively longer pulses in low prf long range radar so as to keep peak power at reasonable levels.

Pulse compression techniques are then sometimes used to recover range discrimination.

On the other hand, a relatively high prf radar can readily provide unambiguous range information only for ranges out to a limited maximum dependent on the period between successive pulses.

Pulse to pulse frequency agility in pulse radar systems is also known and used both for confusing possible enemy electronic countermeasures and for decorrelation of certain types of clutter.

According to the present invention, there is provided radar apparatus having a frequency agile transmitter arranged to transmit successive sequences of pulses at different frequencies, receiving apparatus comprising receivers separating received signals at said different frequencies into respective reception channels and delay means linked to the receivers and arranged to delay the signals in the respective channels such that the delayed signals corresponding to echoes at said different frequencies from a single target substantially coincide in time, and indicator means responsive to the delayed signals to provide unambiguous range information of targets out to a maximum range corresponding to the normal maximum umambiguous range for a radar with a pulse repetition frequency equal to the rate of succession of said sequences of pulses.

The successive sequences of pulses may be at a repeated predetermined series of pulse frequencies. The pulses of corresponding frequency in successive sequences may vary in frequency over a respective distinct narrow band.

In one arrangement, the pulses of each sequence are irregularly time spaced.

Conveniently the time spacing between corresponding pairs of adjacent pulses in successive sequences remains constant.

By way of example, the transmitter may transmit a series of ten pulses at different frequencies with an interval between pulses of say 1 20 microseconds. Thus the series repeats every 1200 microseconds. Then the apparatus can provide a maximum unambiguous range coverage of about 100 miles with the energy per transmitted pulse considerably less than that required for similar range coverage of a radar with an interpulse period of about 1 200 microseconds.

Conveniently, the receiving apparatus includes local oscillator means arranged to generate a plurality of local oscillator signals at frequencies respectively corresponding to the pulse frequencies so as to generate received signals at a common intermediate frequency when mixed with the received echo signals at said different pulse frequencies. In this way, by feeding the mixer outputs corresponding to these different local oscillator signals, to a bank of identical intermediate frequency amplifiers, amplified IF signals are then produced corresponding to radar echo signals at the different transmitted frequencies. The various signals can then be delayed appropriately so that the returns at the different pulse frequencies corresponding to echoes from a single target are arranged to coincide in time.The coinciding signals can then, for example, be displayed on a normal PPI display with a range sweep corresponding to the repetition period of successive series of the transmitted pulses.

Conveniently also the transmitter comprises a waveform generator to generate said repeated series of pulses at different frequencies, and a radio frequency power amplifier to amplify said pulses for radiation at an antenna.

A master clock may be provided with the local oscillator means and the waveform generator being synchronized to a clock signal from the master clock. In this way the output from the local oscillator mixers in the bank of receivers can retain the phase information of the originally received radio frequency signals. Furthermore, the delay units may be non dispersive delay lines arranged to delay said received signals at the common intermediate frequency and the receiving apparatus may then include means to combine the delayed signals coherently to preserve phase information in the combined signal and a detector to detect radar returns in the combined signal.

Instead, receiving apparatus may include separate detectors for the undelayed signals corresponding to the different frequencies and the delay units may comprise video pulse delay units for delaying the detected radar returns. These pulse delay units may comprise digital shift registers, such as charge coupled devices (CCD) which preserve amplitude information in the return signals being delayed. In another embodiment, analogue to digital converters may be provided to produce multi-level digital signals representative of the detected radar returns, the digital shift registers then comprising multi-level shift register arrays.

Examples of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a block schematic diagram illustrating an example of the present invention; Figure 2 is a block schematic diagram of part of the apparatus illustrating a different embodiment of the invention; Figure 3 is a graphical representation illustrating functioning of the invention to produce a full range sweep from successive transmitter pulses at different frequencies in the series; and Figure 4 is a graphical representation illustrating the functioning of modified form of the invention to provide increase range performance.

Referring to figure 1, a transmitter waveform generator 10 generates radar transmitter pulses which are amplified by an RF power amplifier 11 and fed via a circulator 12 to an antenna 13 for radiation. The pulses generated by the generator 10 comprise a predetermined sequence of pulses at different frequencies. The period between successive pulses in the sequence may be constant and the generator 10 is arranged to repeat the complete sequence in a cyclic fashion.

The RF power amplifier 11 may be constituted by any known form of micro-wave power amplifier such as a travelling wave tube or broad band Klystron or a hybrid thereof such as a coupled cavity TWT. The circulator 12 is arranged to couple the power output from the RF amplifier 11 to the antenna 13 for radiation in the usual way. It will be appreciated that reflecting targets illuminated by the radiated pulses will produce echo signals for reception by the antenna 13 at the different frequencies in the series of transmitted pulses.

The circulator 1 2 is a multiport circulator having, in addition to port 1 for connection to the output of the power amplifier 11 and port 2 for connection to the antenna 13, one port for each frequency of pulses in the sequence of pulses from the generator 10. Thus if the generator 10 produces a sequence of N pulses of different frequencies, there are a total of N+2 ports in the circulator 12 distributed as shown in the drawing.

Each of the additional ports 3 to N+2 are connected to a respective Transmit/Receive cell TR1 TR2, Tor,,... TRN,. The TR cells are each arranged to be resonant at a respective band of frequencies about a respective one of the pulse frequencies and arranged such that the respective bands about the other frequencies of the pulse series are reflected back from the cell into the circulator 12. It can be seen therefore, that received echo signals from the antenna 13 entering the circulator at port 2 will be distributed amongst the remaining ports 3 to N+2 according to their frequency band. As a result, echo signals corresponding to the different transmitter pulse frequencies are passed by the respective TR cells on lines 14, 15, 1 6 and 17 to respective mixers 18to21.

The mixers 18 to 21 are fed with local oscillator signals at respective different frequencies fL1, fL2 f,,. .. f,, on lines 22 to 25 respectively from a local oscillator generator 26.

The local oscillator generator 26 is arranged to generate the different frequencies f,1 . . . f,N which when mixed with the received radar echo signals on lines 14 to 1 7 at the different transmitter pulse frequencies produce IF signals on lines 27 to 30 which are all at a common intermediate frequency.

The signals on lines 27 to 30, which though at the same intermediate frequency, still correspond to echo signals at the different pulse frequencies of the radar, are then amplified in respective tuned IF amplifiersRX1... RXN. The amplified IF signals are then supplied to respective delay units 31 to 34.

In the present example, the transmitter pulses generated by the waveform generator 10 and the local oscillator signals from the local oscillator generator 26 are each controlled by clock signals from a master clock 35, typically a crystal controlled clock generator. As a result, the transmitter pulses from the generator 1 0, subsequently radiated from the antenna 13, and the local oscillator frequency signals supplied to the mixers 1 8 to 21 are all coherent so that the phase information in the received echo signals are preserved in the IF signals on the lines 27 to 30 which are amplified and supplied to the delay units 31 to 34. The delay units are selected to be non-dispersive so that they to preserve the relative phase information of the IF signals being delayed. Surface acoustic wave (SAW) delay lines are known for this purpose.The delays of units 31 to 34 are selected to correspond to the periods between successive pulses of the repeated sequence of transmitted pulses such that echos at the different pulse frequencies from a single target coincide in time at the outputs of the delay units.

The differentially delayed signals from the delay units 31 to 34 can then be coherently combined in an analogue fashion as illustrated diagrammatically at point 36. Because phase information has been preserved in each of the received echo signals, full coherent combination can be achieved producing a combined return signal on line 37 which still contains the phase characteristics of the echo signals. The combined signals are then further amplifed in IF amplifier 38 before supplying to a detector 39, typically a superhet detector, connected to the usual integrator circuit 40 which presents radar return signals to a video amplifier 41 for supply to a display unit 42. The display unit 42 may be a PPI type display arranged to have a range sweep of duration corresponding to the repetition period of the complete series of transmitter pulses from the generator 10. As will become apparent, the unambiguous range of targets may be displayed out to a range corresponding to the normal unambiguous maximum range of a pulsed radar with a prf corresponding to the rate of repetition of the complete sequence of pulses from the generator 10.

Referring now to figure 3, the lower part of the graph illustrates successive transmitter pulses from the generator 10. There are shown as constituting a cycle of only 4 frequencies f1, f2, f3, f4, with constant repeating period T between successive pulses. If the echo signals at frequency f1 are delayed in the receiver for this frequency by the time 4T, echo signals at frequency f2 by 3T, echo signals at f3 by 2T and echo signals at f4 by T, it can be seen that echos at all four frequencies from a single target (at a particular range) will then coincide in time after being delayed. The upper graph in figure 3 illustrates a range sweep and is superimposed on the timing of the successive transmitter pulses.In fact the combined radar returns on each range sweep correspond to the echo signals of transmitter pulses actually transmitted during the preceding range sweep. Nevertheless it can be seen that, by so delaying the different received signal frequencies, umambiguous range information can be displayed out to the equivalent of 4T.

In one example, transmitter pulses may be generated with period between pulses 1 20 microseconds. If there are ten pulses at different frequencies in the repeating series, then the total series length is 1200 microseconds. The normal maximum unambiguous range for a constant prf radar with a pulse period of 120 microseconds is just 10 miles. With the present arrangement a range out to 100 miles can be achieved.

It will be appreciated that there will be a loss of receiver sensitivity at all frequencies for each transmitter pulse, as a result of the action of the TR cells. These will produce low sensitivity regions during each range sweep as illustrated at 44 in figure 3. However, the regions of low sensitivity need not constitute more than 10% of the total range sweep and for a mobile radar apparatus, eg. on an aircraft, the low sensitivity regions do not persist in any one place.

The arrangement described above has various advantages. By using a relatively high prf, the mean power of the radar can be high without the need for excessively high peak power in each pulse or excessively long pulses, and this without compromising unambiguous range performance.

The use of several different frequencies provides advantages in terms of ECM resistance, and decorrelation of various clutter returns such as sea clutter. Because the transmitter pulses and local oscillator frequencies are coherent, being locked to a single master clock, the radar returns at the different frequencies can be coherently combined before detection, enabling maximum use to be made of the phase information of the received signals.

Referring now to figure 2, another arrangement is illustrated in which the IF signals from the IF amplifiers RX1 to RXN are each detected in separate detectors 46 to 49. The resulting radar return signals on lines 50 to 53 are digitized in respective multilevel A to D converters 54 to 57.

The digital signals from the converters 54 to 57 are then supplied on respective data buses 58 to 61 to respective banks on shift registers 62 to 65.

The shift registers 62 to 65 are all clocked simultaneously at a common clocking frequency but are of different lengths so as effectively to delay the digital radar returns appearing at the outputs of the shift registers on data buses 66 to 69 so that signals corresponding to echoes at different transmitter pulse frequencies from a single target now coincide in time. The digital signals appearing on the buses 66 to 69 are combined digitally as schematically illustrated at 70 to provide a single multi-level digital signal on a bus 71 which may correspond to the digital sum of the signals on buses 66 to 69. The summed signal on bus 71 is then supplied for display in the usual way eg. by D to A conversion and modulation of the beam of a cathode ray tube providing a PPI display.This arrangement does not preserve the phase information of the radar returns at the different transmitter frequencies but simply combines the returns after detection. This may be advantageous for dealing with deeply fluctuating return signals which may appear in only some of the return frequencies at any one time. Instread of A to D converters 54 to 57, the return signals on lines 50 to 53 may be fed directly to Charge Coupled Devices (CCD's) forming delaying shift registers of appropriate lengths but preserving the amplitude information of the delayed returns. The delayed return signals from the CCD shift registers may then be combined in analogue fashion.

The separate detectors 46 to 49 may each be arranged to provide respective video signals representing the in phase and quadrature components of the respective undelayed IF signal.

Then these in phase and quadrature signals may be delayed in separate shift registers to preserve the phase information. The delayed component signals for each received frequency channel may then be independently M.T.I. processed and the resulting targets from the various M.T.I.

processors may be incoherently combined to reduce blind spot problems.

Modifications of the arrangement described above may include the provision of pulse compression techniques such as emission of the chirp pulse for all or some of the pulses in the transmitted series. If all the pulses at the different frequencies are "chirped", pulse compression can be applied to the received signals corresponding to each of the transmitted pulse frequencies. It will be appreciated that the channel spacing of the centre frequencies of the different pulses in the series must be sufficiently wide so that the "chirped" pulses at the different centre frequencies can be received by their respective receivers whilst simultaneously rejecting the chirped transmitted pulse of an adjacent channel (closest in centre frequency).

It may not be necesssary to apply this pulse compression technique to all of the successive pulses in the series. Pulse compression techniques may be desirable for relatively close range work, e.g. for rejection of area and volume clutter. If only one of the pulse frequencies is "chirped" a separate channel with pulse compression may be employed for this one frequency providing signal returns for close-in ranges. This will involve reduced total power on close range targets, but this need not be a disadvantage. All frequencies are then combined in the manner described above for longer range targets.

In the above described example, it is suggested that the sequence of pulses from the waveform generator are at equally spaced frequencies and the sequence is cyclic i.e. continuously repeating.

This is not essential. A random frequency sequencer such as indicated in ghost lines at 75 in figure 1, may be provided to cause the waveform generator 10 to generate pulses at a random or pseudo-random sequence of frequencies, which need not repeat. The frequencies will, of course, normally be contained within a specific band and the same predetermined set of frequencies may be employed in each sequence of pulses.

Simultaneously, the local oscillator generator 26 is also controlled by the random frequency sequence 75 to produce local oscillator frequencies on line 22 to 25 at corresponding local oscillator frequencies for mixing with return signals at the frequencies from the waveform generator 10. With this arrangement there should be no discrimination between received signal frequencies before the mixers 1 8 to 21. Instead, however the received signals may be discriminated into respective channels by the TR cells as before, but then the delay units 31 to 34 are programmable in response to the sequencer 75 to provide appropriate delays.

The frequencies of the set employed in successive sequences may not remain precisely constant but may each vary from sequence to sequence over a narrow band. The band of variation of each frequency must be narrow enough for the various bands of the pulses of different frequencies to remain distinct from one another so that the received return signals in the different bands can be separated into the different delay and processing channels.

The sequence of pulses from the waveform generator 10 need not be regular in time. The time spacing of the pulses of each sequence may vary from one pulse to the next in a manner which repeats in successive sequences.

In one further example the pulses may form a cyclic series of frequencies (or bands) but time spaces between successive pulses have a small 'jitter" from one sequence to the next so that the pulse at a particular frequency (or band) in successive sequences can appear at different positions in a small time slot. The "jitter" may be controlled by a random number generator.

The delay units of the different frequency channels may then be programmable in response to the random number generator to compensate for the timing "jitter" in each channel.

Alternatively, received video signals in the various channels may be digitally stored over several successive pulse sequences. The stored signals may then be digitally remapped to normalise the timing.

It may also be appreciated that there may be sufficient total power in the transmitted pulses for echoes to be received from targets which are more distant than the maximum range defined by the total radar pulse "flight time" corresponding to the normal period of repetition of the series of pulses. For example for a series of 10 pulses with a pulse to pulse period of 120 microseconds the maximum unambiguous range would be only 100 miles. Referring to figure 4, the waveform generator 10 may be arranged to delay the beginning of each subsequent series of pulses for an extra period R following the last pulse of the series. Thus, instead of the first pulse of the next series following times after the last pulse of the previous series, there is a total delay of z plus R.

Considering a series of just four pulses, the extra delay R enables returns from increased ranges to be displayed as can be seen from the graphical representation in figure 4 of a range sweep. In another technique, continuous regular transmission of pulses may be provided but with the transmission divided into two separate successive sequences. Then with two separate banks of delay units, full advantage may be obtained of long range performance with high data rates at shorter ranges.

Claims

1. Radar apparatus having a frequency agile transmitter arranged to transmit successive sequences of pulses at different frequencies, receiving apparatus comprising receivers separating received signals at said different frequencies into respective reception channels and delay means linked to the receivers and arranged to delay the signals in the respective channels such that the delayed signals corresponding to echoes at said different frequencies from a single target substantially coincide in time, and indicator means responsive to the delayed signals to provide unambiguous range information of targets out to a maximum range corresponding to the normal maximum unambiguous range for a radar with a pulse repetition frequency equal to the rate of succession of said sequences of pulses.

2. Radar apparatus as claimed in Claim 1 wherein the successive sequences of pulses are at a repeated predetermined series of pulse frequencies.

3. Radar apparatus as claimed in Claim 2 wherein pulses of corresponding frequency in successive sequences vary in frequency over a respective distinct narrow band.

4. Radar apparatus as claimed in any preceding claim wherein the pulses of each sequence are irregularly time spaced.

5. Radar apparatus as claimed in any of Claims 2 to 4 wherein the time spacing between corresponding pairs of adjacent pulses in successive sequences remains constant.

6. Radar apparatus as claimed in any of Claims 2 to 5 wherein the receiving apparatus includes local oscillator means arranged to generate a plurality of local oscillator signals at frequencies respectively corresponding to the pulse frequencies so as to generate received signals at a common intermediate frequency when mixed with the received echo signals at said different pulse frequencies.

7. Radar apparatus as claimed in Claim 6 wherein the transmitter comprises a waveform generator to generate the sequences of pulses at said repeated series of pulse frequencies, and a radio frequency power amplifier to amplify said pulses for radiation at an antenna.

8. Radar apparatus as claimed in Claim 7 and including a master clock, said local oscillator means and the waveform generator being synchronised to a clock signal from the master clock.

9. Radar apparatus as claimed in Claim 8 as dependant from Claim 5, wherein the delay means comprises a respective delay unit for each channel and the delay units are non dispersive delay lines arranged to delay said received signals at the common intermediate frequency and the receiving apparatus includes means to combine the delayed signals coherently to preserve phase information in the combined signal and a detector to detect radar returns in the combined signal.

10. Radar apparatus as claimed in Claim 5 or any of Claims 6 to 8 as dependant from Claim 5 wherein the receiving apparatus includes separate detectors for the undelayed signals corresponding to the different frequencies and the delay means comprise respective video pulse delay units for delaying the detected radar returns from the separate detectors.

11. Radar apparatus as claimed in Claim 10 wherein said pulse delay units comprise digital shift registers.

12. Radar apparatus as claimed in Claim 11 wherein the digital shift registers are charge coupled devices which preserve amplitude information in the return signals being delayed.

13. Radar apparatus as claimed in Claim 11 and indsiuding analogue to digital converter to produce multilevel digital signals representative of the detected radar returns, the digital shift registers comprising multi-level shift register arrays.

14. Radar apparatus as claimed in any of Claims 11 to 13, wherein said separate detectors each provide respective video signals representative of the in phase and quadrature components of the respective undelayed received signal and said digital shift registers are arranged for delaying both said in phase and quadrature signals.