GB2183117A

GB2183117A - Compensating for radar motion

Info

Publication number: GB2183117A
Application number: GB08432611A
Authority: GB
Inventors: Malcolm Geoffrey Cross
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1984-12-22
Filing date: 1984-12-22
Publication date: 1987-05-28
Also published as: FR2583173A1; GB2183117B; IT8568067A0; IT1199919B; DE3545029A1

Abstract

A radar transceiver 3, mounted on an aircraft, has associated with it at least two perpendicular accelerometers 12 and 13 the outputs of which are integrated at 14 and 15 to give measurements of motion. These are used to generate at 16 waveforms representing the Doppler shift in input 4 which when mixed at 5 with the received signal 4 from the radar cancel Doppler effects present therein and due to unwanted sideways movement of the radar. <IMAGE>

Description

SPECIFICATION Radar apparatus for use on a platform subject to unwanted movement This invention relates to radar apparatus for use on a platform subject to unwanted movement. Such a platform could be for example a helicopter attempting to hover at a fixed position or a fixed wing aircraft attempting to move along a linear path at a constant velocity but in fact also moving in directions radially with respect to the intended flight path and being subject to velocity variations along the flight path.

Figure 1 illustrates a fixed wing aircraft 1 carrying a radar and following an intended flight path along a line perpendicular to the plane of the paper as illustrated. The radar illuminates an area of the ground surface indicated at 2 and this area is assumed to be narrow in the direction of movement of the aircraft. Movement of the aircraft 1 in any radial direction imposes a Doppler shift on the received energy and this shift is different for energy received from different parts of the illuminated area 2. If the radar detections are performed on the assumption that the aircraft is stationary over each integration period the pulses will not be coherently integrated and signal energy is therefore lost. It is therefore necessary to adjust the received signal in some way so that it is compensated for the movement of the aircraft.Such movement seriously effects the processing of signals in a synthetic aperture radar and so the invention is of particular relevance to synthetic aperture radars.

This invention provides radar apparatus for use on a platform subject to unwanted movement, the apparatus comprising a radar transceiver arranged to illuminate a finite area with a pulse of radiation, means for deriving a measurement of motion of the radar and means for deriving from that measurement phase values corresponding to the expected Doppler effects on parts of the received signal from different parts of the illuminated area, and means for adjusting the received signal by the derived phase values to cancel the Doppler effects.

The means for deriving a measurement of motion preferably includes an accelerometer (in practice two perpendicularly configured accelerometers) and an integrator. The use of an accelerometer followed by an integrator provides a measure of velocity which is accurate for short term velocity variations. However it is not satisfactory for long term velocity variations because the integrator has a long term drift in its output. To eliminate this problem the invention preferably includes means which provides, from the adjusted received signal, a correction signal which corrects the output of the integrator.

The means for adjusting the received signal preferably comprises means for generating a waveform whose phase variations define the Doppler phase variations expected across the illuminated area and means for mixing that waveform with the received signal.

One way in which the invention may be performed will now be described with reference to Figures 2 to 6 of the accompanying drawings in which: Figure 2 is a block diagram of a radar constructed in accordance with the invention mounted on the aircraft shown at 1 on Figure 1; Figure 3 shows detaiis of the components indicated at 5 and 16 on Figure 2; Figure 4 shows an alternative construction for the components 5 and 16 of Figure 2; Figure 5 illustrates the waveforms (eight shown for the purposes of example) on the individual output lines from waveform generator 16 on Figure 3; and Figure 6 illustrates the waveforms (nineteen shown for the purposes of example) on the output lines from waveform generator 16 of Figure 4.

Referring to Figure 2 a radar transceiver 3 is shown mounted on the aircraft 1. Received signals at IF are applied on line 4 to a mixer 5 where they are mixed with a signal specially generated for the purpose of removing, from the received signals, any Doppler effects due to unwanted movement of the aircraft in radial directions i.e., in directions of the plane of the paper as shown in Figure 1. Details of the mixer 5 will be described later.

The output of the mixer 5 is applied to a synthetic aperture beam forming integrator 6 as is known per se. It is also applied to two radial velocity measuring circuits indicated on Figure 2 by the single block 7.

Such velocity measuring circuits are known per se, a suitable circuit being described for example in our co-pending Patent Application entitled 'A Sideways Looking Radar' dated 13th November, 1984. Velocity measurements made at 7 measure the error of cancellation of the mixer 5, and correspond in range to the extremities of the swath shown at 2 on Figure 1 as defined by range gate 8 and 9 on Figure 2. The error velocity measurements from 7 are, after processing at 7A, added at 10 and 11 to respective outputs of mutually perpendicular accelerometers 12 and 13. The calculator 7A serves to calculate long term velocity corrections in the directions of the axes of accelerometers 12 and 13.

The outputs from adders 10 and 11 are integrated by integrators 14 and 15to give signals which represent the components of the instantaneous velocity of the aircraft 1 in the directions of the accelerometer axes. These signals are fed to a waveform generator 16 which produces a waveform representing the variations of Doppler phase across the swath 2 at a rate equal to the real time range rate of the received signals from across that swath.

Athird accelerometer 17 is arranged with its linear axis along the intended direction of flight of the aircraft. Its output is added at 18 to a correction signal and then integrated at 19. The output of the integrator 19 represents the instantaneous forward velocity of the aircraft and is applied to the radar beam forming integrations of 6 to enable the correct phase compensations to be made during synthetic beam forming. The output from mixer 5 is used at 20 for long term measurement of forward velocity. The block 20 can be as described in our Patent Application No. 8423308. This long term measurement is used as the second input to the adder 18.

The waveform generator 16 and cancellation mixer 5 are shown on Figure 3 from which it will be noted that the mixer 5 includes a number of separate mixers e.g., as shown at 5A, 5B and 5C, each fed from line 4 via a range gate 5D, 5E, 5F defining different range cells within the swath 2. The outputs of mixers 5A, 5B, 5C are added at 5G to give an output signal which is applied to the synthetic aperture beam former 6 of Figure 2.

The waveform generator 16 takes the form of a plurality of controlled frequency oscillators 16A, 16B, 16C each controlled to generate a waveform corresponding to the Doppler frequency expected from a range cell 1A of Figure 1. The oscillators are controlled in frequency according to computations performed at 16D.

The construction shown in Figure 3 for components 5 and 16 is relatively complex. A simpler possibility is shown in Figure 4.

Referring to Figure 4 the waveform generator 16 is shown as including a calculator 16E which calculates phase increments 16F representing Doppler phase shifts at each range cell 1A over each interval between radar pulses. The phase increments are used to update, at the pulse repetition frequency rate, a series of phase accumulators 16G. The outputs from the phase accumulators 1 6G are fed to a series of phase-toamplitude convertors 16H which hold amplitude values corresponding to the current content of the phase accumulators 16G.

Range gates 161 are opened sequentially during an interval between radar pulses at times corresponding to the ranges from which a signal is being received. The outputs are combined in an adder 16J. Discontinuities in the output of the adder due to the discrete sampling effect of the electronic scan performed by the range gate 161, are removed buy a low pass filter 16K which produces the required cancellation waveform. The phase accumulator 16G is reset by a signal on line 1 6L at times corresponding to the end of each radar synthetic beam integration interval performed at 6. In the arrangement of Figure 4 the correction waveform at the output of circuit 16 is applied to a simple mixer 5 which does no more than to mix the two signals appearing at its inputs unlike the more complex mixer shown in Figure 3.

Figure 5 illustrates the waveforms on the individual output lines from the waveform generator 16 on Figure 3. From an inspection of Figure 5 it will be noted that the Dopplerfrequencies are different at the different ranges.

Figure 6 illustrates the waveforms at the output of the low pass filter 16K of Figure 4 during a number of pulse repetition frequency intervals (19 as illustrated), 15 of which constitute one beam forming integration time of processor 6. From Figure 6 it is intended that, for a given range, the samples of phase at pulse repetition frequency increments vary in the manner of Figure 5. Also to be noted is the build up of phase values stored at 16G. In the case of Figures 5 and 6 the departures of the continuous line waveforms from the broken lines in a direction perpendicular to the latter represents instantaneous amplitude of the compensating waveform.

Claims

1. Radar apparatus for use on a platform subject to unwanted movement, the apparatus comprising a radar transceiver arranged to illuminate a finite area with a pulse of radiation, means for deriving a measurement of motion of the radar and for deriving from that measurement phase values corresponding to the expected Doppler effects on parts of the received signal from different parts of the illuminated area, and means adjusting the received signal by the desired phase values to cancel the Doppler effects.

2. Radar apparatus according to claim 1 in which the means for deriving a measurement of motion comprises an accelerometer and means for integrating a signal derived from it to give a velocity measurement.

3. Radar apparatus according to claim 1 or 2 in which the means for adjusting the received signal comprises means for generating a waveform whose phase variations define the Doppler phase variations expected across the illuminated area and means for mixing that waveform with the received signal.

4. Radar apparatus according to any preceding claim comprising Doppler frequency measurement means which receives the adjusted received signal to provide a correction signal and means for using the correction signal to adjust the said measurement of motion.

5. Radar apparatus substantially as described with reference to Figures 2 and 3 of the accompanying drawings and substantially as illustrated therein.

6. Radar apparatus substantially as described with reference to Figures 2 and 4 of the accompanying drawings and substantially as illustrated therein.