GB2504251A

GB2504251A - Radar

Info

Publication number: GB2504251A
Application number: GB9011379.6A
Authority: GB
Inventors: Alistair Robin Faulkner
Original assignee: BAE Systems Electronics Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1990-05-22
Filing date: 1990-05-22
Publication date: 2014-01-29
Anticipated expiration: 2010-05-22
Also published as: GB2504251B; ITTO920066A0; GB9011379D0; ITTO920066A1

Abstract

In a digitally coded radar, in which a frequency ramp carrier wave generator 1 is modulated by code generator 2 with a code comprising a sequence of digits, the ambiguities resulting from the range det ermination based on demodulating at 8 the returns with a reference code generator generating an identical code but at a relative time delay, are resolved by thereafter demodulating the resulting sign al at 12,13 with the frequency ramp carrier t o produce an independent determination of range. Thus a relatively short code may be used without range ambiguity. The code reference generator 9 may provide a selectable delay or alternatively generators for a plurality of delays may be provided.

Description

RADAR

This invention relates to radar, particularly digitally coded radar.

In digitally coded radars, a microwave carrier wave is modulated with a code comprising a repeatedly generated sequence of digits. The modulation may be phase modulation e.g. bi-phase modulation.

This technique has been applied to coding within a pulse in pulsed carriers in order to improve resolution by pulse expansion and compression, as an alternative to frequency modulation chirp expansion and compression.

However, the range sidelobe performance of such short codes is not good and hence clutter is a problem.

It has also been proposed (the Applicant's patent application number 8512730) to use a very long pseudo random code typically tens of thousands of bits long on an interrupted continuous wave carrier, and to correlate (rather than compress) the radar returns with a signal synchronised to the transmitted coded carrier over a range of time delays. A correlation peak is only obtained at the time delay corresponding to the time of flight of the beam to the target and back, and this therefore identifies the range of the target. An advantage of such a long code is that the unambiguous range of the radar i.e. the range of targets up to that at which returns of the first bit of the code are received during the next repeat of the code, consequently, causing range ambiguity, is correspondly long e.g. several kilometers.

A disadvantage of the system is that it requires a separate correlator for each range cell to be examined, and there could be tens of thousands of these. Alternatively, one, or several, correlators, can be swept across many range cells in a sequential manner. However, this latter process takes time. To enable the processed results to be available in real time, a shorter code length can be used.

A disadvantage of using a shorter code length is that the unambiguous range is correspondly reduced, resulting in a multiplicity of range ambiguities, and also the range sidelobe performance obtained is degraded.

The invention provides radar comprising means fcr modulating a carrier wave with a code comprising aseqiience of digits, means for de-modulating the radar retuEns. with the code at varying delays relative to the code in the transmitted waveform, and including means for ramp frequency modulating the carrier waveform.

The resolution advantages of the coded carrier waveform can be retained without the penalty of a multiplicity of range ambiguities, since the ambiguities can be resolved using the independent range determination resulting from the use of an FM ramp carrier waveform.

A suitable code length would be 63 bits, 127 bits, or 255 bits4 In order to demodulate the radar returns with the code, two code generators synchronised to each other and generating the same code may be provided, one for modulating the ramped carrier to be transmitted, and the other for modulating the radar returns. Thereafter, the resulting ramped form of the returns can be mixed with a signal derived from the ramped carrier in order to obtain an independent determination of range and resolve ambiguities associated with the short code.

A digitally coded radar constructed in accordance with the invention will now be described, by way or example, with reference to the accompanying drawing, which is a block diagram of the radar.

A radar comprises a carrier wave generator 1 and a code generator 2 which modulates the carrier wave at a modulator 3. The coded carrier is amplified at amplifier 4 and pulsed at pulse modulator 5, before being fed to an antenna (not shown) for transmission.

Radar returns are received at an antenna (not shown), and passed by a pulse modulator 6 during the intervals between the transmitted pulses, to an amplifier 7.

The output of the amplifier is demodulated at demodulator 8 by the output of a reference code generator 9, which generates an identical code to that of the transmitter code generator 2, being synchronised to it by a variable and selectable time delay.

The code is a pseudo random sequence of bits, typically 127 bits in length, which is repeatedly generated, and the carrier is bi-phased modulated by the modulator 3 i.e. for a bit of one value, the phase is unchanged, but for a bit of the other value, the phase is altered by 1800.

The result of this is that when the time of flight of the radar signals from the transmitter to a target and from the target to the receiver, it precisely equal to the time delay between the reference code generator 9 and the transmitter code generator 2, the reversals of phase no longer appear at the output of the demodulator, which now becomes transparent to the code.

The output of the demodulator 8 is filtered by a bandpass filter 10, corresponding to the band of the carrier, as opposed to the wide band of the bi-phase modulated carrier. Consequently, radar returns from other targets which will produce coded carrier outputs from the demodulator, will be filtered out and the filter output will only contain signals corresponding to a certain time difference between the transmitted and reference codes.

However, the code may be relatively short, and a particular time difference between the transmitted and reference codes could correspond to a multiplicity of distances. For example, a time difference of 1/2 the code length could correspond to a time of flight corresponding to /2 the code length, 1'!? times the code length, 21/2 times the code length etc. It is for this reason that the carrier wave generator 1 is arranged to generate a linear ramp frequency modulated carrier. The resulting ramped f on of the returns can be mixed with a signal derived from the ramped carrier in order to obtain an independent determination of range and resolve ambiguities associated with the short code.

Thus, the output of code demodulator 8 is split into I and Q channels by splitter 11, and the signals are mixed in mixers 12, 13 with quadrature outputs from the frequency ramped carrier which is split in a 900 splitter 14. The outputs of the two mixers pass through bandpass filters 15, 16, amplifiers 17, 18, and are digitised by analogue-to-digital converters 19, 20. The digital outputs are fed to a digital signal processor 21 for frequency analysis.

As is known, when returns of a linear ramp frequency modulated radar are mixed with the transmit signal, the time delay due to range will result in a frequency difference.

Hence frequency analysis of the mixer outputs gives range information. This then enables the ambiguities in the range determination from the codes to be resolved.

Of course, as described, the radar is only capable of detecting targets in a single range cell, corresponding to the time delay between the code generators 2 and 9. This can be remedied by stepping through various delays corresponding to different range cells in a sequential manner.

As an alternative, the reference code generator 9 could have multiple outputs, and the bandpass filter 10, the splitter 11, the mixers 12, 13, the bandpass filters 15, 16, the amplifiers 17, 18, and the analogue-to-digital converters 19, 20 could be duplicated, one set for each output. For example, one set could be provided for each bit of the code, i.e. one for each range cell.

The repeating pseudo-random code may be produced with logic shift registers, read only memory (ROM) devices or random access memory (RAM) devices. In the case where the code generator has multiple outputs, these could be generated by feeding the code to a shift register and taking the parallel outputs of such a device. Other codes may be used instead of pseudo-random codes.

Instead of bi-phase modulating the carrier, the carrier may be poly-phase modulated by the code, and in either case modulator 3 may comprise a mixer, multiplier or switched phase-change device.

A suitable bit rate (clock rate) with a code would be MHz to 1 GHz, typically 400 -800 MHz, and the carrier may be any radar band e.g. UHF, SHF or EHF. A suitable ramp time would be from 0.1 to 10 milliseconds, typically 1 millisecond, with a corresponding frequency ramp of 1 to 100 MHz, typically 14 14Hz. The ramp could be linear, which gives the advantage that the difference frequency produced on demodulation relates linearly to distance, but a non-linear ramp could be used if desired. Other code lengths e.g. 63 or 255 bits could be used if desired.

The timing between the transmit pulses from the pulse modulator 5 and the receive periods defined by the pulse modulator 6 may be varied by pulse timing control 22. The pulse repetition frequency and pulse width could be related or unrelated to the ramp length. Equally, the code length may be, but need not be, related to the ramp length. If desired, the carrier could be transmitted as a continuous wave. Although as described above, the radar returns are code demodulated before they are de-ramped, if desired, the radar returns -could be de-ramped before they are code demodulated.

Among the advantages of the invention are that it provides the range resolution of coded modulation radar without the processing disadvantages attendant on the use of a long code, since a short code is used and the linear ramp frequency modulation is used to resolve range ambiguities.

The high range resolution gives better target discrimination against clutter. The coded modulation transmits power across a broad spectrum of frequencies, and the possibility of continuous wave (CW) or near continuous wave operation, results in a lower probability of intercept. The reduced processing corresponding to a shorter code permits the radar to be used as a scanning (surveillance) radar.

Claims

-10 -CLAIMS1. A radar comprising means for modulating a carrier wave with a code comprising a sequence of digits, means for de-modulating the radar returns with the code at varying delays relative to the code in the transmitted waveform, and including means for ramp frequency modulating the carrier waveform.
2. A radar as claimed in claim 1, in which there is provided a first code generator for modulating the carrier wave and a second code generator for generating an identical code for demodulating the radar returns, the second code generator being synchronised to the first code generator but there being a time delay between the two outputs.
3. A radar as claimed in claim 1 or claim 2, in which means is provided for mixing the radar returns with the output of the second code generator.
4. A radar as claimed in any one of claims 1 to 3, in which there is provided means for mixing the code demodulated radar returns with a waveform derived from the ramp frequency modulating means.
5. A radar substantially as hereinbefore described with reference to the accompanying drawing.Amendments to the claims have been filed as follows 1. A radar comprising means for ramp frequency modulating a carrier waveform, means for modulating the ramp frequency modulated carrier wave with a code comprising a sequence of digits, means forde-modulating the radar returns with the code at various delays relative to the code in the transmitted waveform and for de-modulating the radar returns with the ramp frequency modulation, means for using the independent range determination resulting from the use of the ramp waveform to resolve range ambiguities associated with the use of the code, and a bandpass filter having a bandwidth corresponding to the band of the carrier, for filtering the radar returns after demodulation with the code.2. A radar as claimed in claim 1, in which there is * provided a first code generator for modulating the carrier wave and a second code generator for generating an identical code for demodulating the radar returns, the second code generator being synchronised to the first code generator but there being a time delay between the two outputs.3. A radar as claimed in claim 1 or claim 2, in which means is provided for mixing the radar returns with the output of the second code generator.4. A radar as claimed in any one of claims 1 to 3, in which there is provided means for mixing the code demodulated radar returns with a waveform derived from the ramp frequency modulating means.5. A radar as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawing.