GB2269069A

GB2269069A - A radar for use on a platform which may be subject to movement

Info

Publication number: GB2269069A
Application number: GB8715135A
Authority: GB
Inventors: Alfred Brian Edwin Ellis
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1987-06-27
Filing date: 1987-06-27
Publication date: 1994-01-26
Anticipated expiration: 2007-06-27
Also published as: GB2269069B; GB8715135D0

Abstract

A radar for use on a platform which may be subject to movement transmits pulses into an area of the ground or sea at two frequencies and mixes the return signals to produce an intermediate frequency signal having a phase dependent on the range to the area, and being used to measure the movement of the platform relative to the ground or sea. The two frequencies may be transmitted as simultaneous pulses, or respective sets of interleaved pulses (Figs 2 and 3) with appropriate delays before phase comparison. Filters 9, (16A, 16B Fig 2, 21A, 21B Fig 3) restrict attention to originals corresponding to movement within a range of azimuth about the boresight. <IMAGE>

Description

A Radar for use on a platform which may be subject to movement This invention relates to a radar for use on a platform which may be subject to movement. It is particularly applicable to a synthetic aperture radar but could also be of use in airborne MTI radars.

A problem associated with most synthetic aperture radars is that unwanted movement of a platform in a direction having a component along the boresight of the radar results in defocussed signals. Attempts to deal with this in the past have involved the use of an accelerometer which requires a double integration to produce a measure of displacement in the direction of the boresight. This method gives rise to two problems. The first problem is the introduction of a time delay due to build up in the integrator outputs. The other problem is that of drift in the integrator. Furthermore, when the platform changes attitude as in a turn a component of the earth's gravity becomes a part of the measured acceleration. To overcome this one may use three orthogonal accelerometers, or mount a single accelerometer on a stable platform.

This invention provides a radar for use on a platform which may be subject to movement relative to an area of ground or sea comprising; a transmitter capable of transmitting signals on two frequencies to the area; receiver means for receiving the two frequencies after reflection from the area and for producing two respective received signals; and mixing means for mixing the two received signals to produce an intermediate frequency signal whose phase is related to the range of the aforementioned area and changes in which are therefore indicative of the movement.

By employing the invention it is thus possible, simply by sensing phase changes in the intermediate frequency signal, to obtain an indication of the unwanted movement of a synthetic aperture radar in the direction of the boresight; without the use of any accelerometer.

It is envisaged that the primary value of the invention will be in a synthetic aperture radar for the purpose of measuring sideways movement of the platform i.e. movement having a component perpendicular to the direction of flight. It is however possible that a system in accordance with the invention could be arranged with the boresight of the radar arranged in the direction of flight so that a measurement could be obtained of wanted rather than unwanted movement.

Means is preferably included downstream of each receiver for passing components of the received signal derived from a limited area of the terrain illuminated by the transmitted signal.

Whilst it is possible that the invention could be applied to a continuous wave radar the present plans are to use it with a pulse radar. In a pulse radar, means is preferably included for selecting two sets of interleaved pulse returns from the output of the mixing means, each set representing displaced positions in the terrain. In this arrangement some means is associated with each set of pulses for passing components of the received signal derived from a limited area of terrain illuminated by the transmitted signal.

The transmitter is preferably designed to transmit the two frequencies in the form of sequential pulses; and the receiving means includes means for delaying one receiver output to coincide with the other before they pass to the mixing means.

Whilst it is possible to achieve the required measurement with just two different frequencies the possibility is envisaged of employing three or more frequencies in which case a corresponding number of receivers would need to be included.

A number of ways of performing the invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 illustrates a radar constructed in accordance with the invention; Fig. 2 illustrates an alternative construction also constructed in accordance with the invention; and Fig. 3 illustrates yet another possible embodiment of the invention.

Referring to Fig. 1, a synthetic aperture radar 1 is mounted in an aircraft and transmits simultaneous pulses of two different frequencies and at a constant pulse rate towards an area of terrain or sea and then receives return signals reflected from this area. The return signals have their frequencies changed slightly by doppler shift due to relative movements of the radar and the aforementioned area.

Return signals at the first frequency, 1OGHz, are fed down a first line 2 and return signals at the second frequency, 1O.1GHz, are fed down a second line 3.

The two signals are then mixed with a 10.05GHz reference frequency produced by a local oscillator 4 in mixers 5 and 6 respectively. This mixing produces intermediate frequency (I.F.) signals at +50MHz from the first signal and -50MHz from the second signal. These I.F. signals are then mixed in a mixer 7 to produce a 100MHz I.F. signal.

This I.F. signal is then passed through a low pass filter 8 to remove any high frequency components produced in the mixers 5,6 & 7, and then through a narrow band-pass filter 9 which passes only the parts of the two signals with a doppler shift corresponding to a small region of interest in azimuth about the boresight direction of the radar 1.

The I.F. signal is then fed to a phase comparator 10 and to a delay 11 which delays the signal for a period equal to the period of repetition of the transmitted pulses and then feeds it to phase comparator 10.

The I.F. signal from the filter 9 will have a phase dependent on the distance between the radar and the aforementioned area from which the signal was reflected.

The phase comparator 10 produces an output dependent on the phase difference between the signal produced by the most recent pulse and the signal produced by the previous purse. The output of phase comparator 10 is fed to a computing system 12 which calculates the movement in the boresight direction of the S.A.R. system, of the aircraft by integrating the movement between each pulse to give the total movement over a period of time.

In figure 2 a synthetic aperture radar 13 transmits a first set of alternate pulses at 1OGHz and a second set of alternate pulses, interleaved with the first set, at 1O,1GHz. The return signals at 1OGHz are supplied on line 2 and those at 1O.1GHz are supplied on line 3.

The two signals are mixed with a 10.05GHz reference frequency produced by a local oscillator 4 in mixers 5 and 6 respectively. This mixing produces I.F. signals at +50MHz from the first set of return signals and at -50MHz from the second set of return signals.

The I.F. signals derived from the first set of alternate pulses are passed through a low pass filter 14A to remove any high frequency components produced by mixing and then converted to a digital data stream in an analogue to digital converter 15A. The digital data is then passed through a digital filter 16A which acts as a narrow bandpass filter and gives data with a doppler shift corresponding to a small region of interest in azimuth about the boresight direction of the radar 13 to phase comparator 17.

The I.F. signals derived from the second set of alternate pulses are similarly processed by a low pass filter 14B, an analogue to digital converter 15B and a digital filter 16B.

A phase comparator 17 contains two delays 17A and 17B and a processing unit 17C. Each delay 17A and 17B has a delay equal to the pulse repetition period. Therefore, when data relating to a pulse arrives at the processor directly from digital filter 16A; data relating to the previous pulse will arrive at the processor from delay 17B. Similarly, when data arrives from digital filter 16B, data relating to the previous pulse arrives from delay 17A. The processor compares the phase of each pulse with the phase of the previous pulse and produces an output signal dependent on this phase difference.

This signal is supplied to a computing system 12 as before.

In figure 3 the synthetic aperture radar 13 transmits a first set of alternate pulses at 1OGHz and a second set of alternate pulses, interleaved with the first set, at 1O.1GHz. The return signals at 1OGHz are supplied on line 2 and those at 1O.1GHz are supplied on line 3.

The two signals are mixed with a 10.05GHz reference frequency produced by a local oscillator 4 in mixers 5 and 6 respectively. This mixing produces I.F. signals at +SOMHz from the first set of return signals and at -SOMHz from the second set of return signals.

The I.F. signals derived from the two sets of signals are then converted to digital form by an analogue to digital converter 18. These digital signals are then stored in a toggle store 19.

The signals received by the synthetic aperture radar 13 will include information from the whole of the field of view of the S.A.R. 13. A sample control 20 controls the analogue to digital converter 18 and store 19 so that only the parts of these signals corresponding to certain preselected range bands are stored in the store 19.

The sample control 20 then selects from the store 19 the return signals produced by two successive pulses, one at the first and one at the second of the two frequencies, and supplies them to the digital filters 21A and 21B respectively. The digital filters 21A and 21B operate as band pass filters and pass only the parts of the signals corresponding to a small region in azimuth about the boresight direction of the S.A.R. 13. These filtered signals are then supplied to a phase conmparator 22 which compares their phase and supplies a signal giving the relative phase of the two signals to a computing system 12 which operates as before.

Claims

1. A radar for use on a platform which may be subject to movement relative to an area of ground or sea comprising: a transmitter capable of transmitting signals on two frequencies to the area; receiver means for receiving the two frequencies after reflection from the area and for producing two respective received signals; and mixing means for mixing the two received signals to produce an intermediate frequency signal whose phase is related to the range of the aforementioned area and changes in which are therefore indicative of the movement.

2. A synthetic aperture radar according to claim 1 in which said movement is a movement having a component along a boresight.

3. A synthetic aperture radar according to claim 2 including narrow bandwidth filter means downstream of the receiver for passing components of the received signal derived from a limited part of the area illuminated by the transmitted signal.

4. A pulse radar according to any preceding claim including means for selecting two sets of interleaved pulse returns from the output of the mixing means, each set representing displaced positions on the terrain; and means associated with each set for passing components of the received signal derived from a limited area of the terrain illuminated by the transmitted signal.

5. A radar according to any preceding claim in which the transmitter is designed to transmit the two frequencies in the form of sequential pulses and including means for delaying one received signal to coincide with the other before they pass to the mixing means.

6. A radar according to any preceding claim in which the transmitter is designed to transmit signals on more than two frequencies and in which a corresponding number of receivers are included.

7. A radar substantially as shown in and substantially as described with reference to figure 1 of the accompanying drawings.

8. A radar substantially as shown in and substantially as described with reference to figure 3 of the accompanying drawings.

8. A radar substantially as shown in and substantially as described with reference to figure 2 of the accompanying drawings.

9. A radar substantially as shown in and substantially as described with reference to figure 3 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. A radar for use on a platform which may be subject to movement relative to an area of ground or sea comprising: a transmitter capable of transmitting signals on two frequencies to the area; receiver means for receiving the two frequencies after reflection from the area and for producing two respective received signals; mixing means for mixing the two received signals to produce an intermediate frequency signal, and narrow bandwidth filter means downstream of the mixer for passing components of the intermediate frequency signal derived from a limited part of the area illuminated by the transmitted signal, the phase of the components being related to the range of the limited area and changes in phase being indicative of the movement.

3. A pulse radar according to claim 1 or 2 including means for selecting two sets of interleaved pulse returns from the output of the mixing means, each set representing displaced positions on the terrain.

4. A radar according to any preceding claim in which the transmitter is designed to transmit the two frequencies in the form of sequential pulses and including means for delaying one received signal to coincide with the other before they are passed to the mixing means.

5. A radar according to any preceding claim in which the transmitter is designed to transmit signals on more than two frequencies and in which a corresponding number of receivers are included.

6. A radar substantially as shown in and substantially as described with reference to figure 1 of the accompanying drawings.

7. A radar substantially as shown in and substantially as described with reference to figure 2 of the accompanying drawings.