GB2189362A

GB2189362A - A frequency domain correlation system

Info

Publication number: GB2189362A
Application number: GB08609225A
Authority: GB
Inventors: Alfred Brian Edwin Ellis
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1986-04-16
Filing date: 1986-04-16
Publication date: 1987-10-21
Anticipated expiration: 2006-04-16
Also published as: GB8609225D0; GB2189362B

Abstract

In a frequency domain correlation system the reference pattern is modified in the frequency domain thus avoiding the need to reprocess the original reference data. In a synthetic aperture radar, the reference pattern stored 4 represents typical returns from a point target at various ranges. An inertial navigation system 12 along with target information 13 modifies 10 the subsequently produced Fourier transformed 6 components to take account of platform and target motion. The modified components are used as the reference input to a usual frequency domain correlator 1-9 to identify targets. <IMAGE>

Description

SPECIFICATION Afrequency domain correlation system This invention relates to a frequency domain correlation system.

Frequency domain correlation systems are, in general, superiorto the more conventional time domain correlation systems because they produce more results in a given time. The construction of a known frequency domain correlation system is described for example in the book "The Fast Fourier Transform" by O. Brigham published by Prentice-Hall. Briefly, it comprises a multiplier and means for supplying to one input of it a set of samples, taken one at a time, representing frequency components of an inputsignal. The otherinputofthe multiplier receives a set of reference samples, also taken one ata time, representing frequency components of a reference pattern.

The output of the multiplier is applied to an inverse Fouriertransform device. The set of reference samples can either be fixed, in which case they may be held permanently in a store or can be generated from a reference pattern using a fast Fourier transform device. In the latter case the reference pattern can be modified but this calls for a very fast acting Fouriertransform circuit The present invention arose with a view to overcoming this difficulty.

The invention provides a frequency domain correlation system comprising: means for supplying to one input of a multiplier a set of samples, taken one at a time, representing frequency components of an input signal; means for supplying to the other input of the multiplier a set of reference samples, taken one at a time, representingfrequency components of a reference pattern; and means for performing an inverse Fouriertransform on the set of products from the multiplier; characterised by means for receiving the reference samples and for receiving control data, for modifying the former in dependence on the latter, and for passing the modified reference samples to the multiplier.

Because, in acco rda nce with the i nvention, the modification is performed directly on the reference samples, representing frequency components of the reference pattern, instead of on the reference pattern, from which the reference samples are derived, the aforementioned need for a veryfast acting Fo u ri er tra nsfo rm device for the reference pattern does not arise.

One way in which the invention may be performed will now be described by way of example with reference to the accompanying drawing which shows a synthetic aperture radar employing a correlation system constructed in accordance with the invention.

Atranceiver 1 is carried on a moving platform (not shown) such as an aircraft and transmits pulses of microwave energy broadside to the track of the aircraft towards the ground. On reception these pulses contain a Doppler spectrum which is analysed by a fast Fouriertransform device 2, the result ofthe analysis being stored in a store 3. The store 3 contains a large number of spectrum samples. In the illustrated example there are 2.048 (i.e. 2") such samples.

The ideal waveform expected from the output of the receiver 1 for a point target swept by the radiated beam for different ranges and velocities is stored in a reference pattern store 4. The pattern at 4 is processed by a fast Fouriertransform device 5 and the result is stored in a reference waveform store 6.

In a conventional frequency domain system the content of the store 6 would be passed direct to a second inputofa multiplier7,thefirstinputofwhich receives the content of store 3. This is indicated by the broken line shown on the drawing. The multiplier multiplies, in turn, each pair samples presented at its inputs and passes the result of multiplication to an inverse fast Fouriertransform device 8. The output of the latter represents a set of correlations of the reference pattern with the signal and this may be passed for presentation on a display as shown at 9 or to perform some control function.

In the illustrated system the output of the reference waveform store 6 is modified at 10 and the modified samples are stored at 11 before being passed to the multiplier 7. The modifier 10 receives information from a navigation system 12 concerning the movement of the aircraft e.g. its velocity and acceleration and modifies the spectrum received from 6 in a way which reflects the movement of the platform. For example, referring to Figure 2 the continuous line represents the basic reference spectrum stored 6for an aircraft moving ata particular speed. Figure 2 shoes, in continuous and in broken lines a shift in the spectrum which would be required for a cross track movement of the aircraft.Figure 3 shows, in broken and dotted lines, modifications which would be required for changes in forward velocity of the aircraft above and below the velocity assumed by the original waveform held in store 6. Obviously combinations ofthe situations depicted in Figures 2 and 3 can be accommodated requiring a modification perhaps as shown in Figure 4.

If it is required to search fortargets having particular additional movement characteristics a facility 13 can be provided to generate signals representing such movements. These are applied in turn to perform further modifications at 10 whidh are superimposed on the information from the navigation system 12.

It will be appreciated that the illustrated system has been described just as one example of howthe invention can be performed. In a modification ofthe illustrated system the multiplier7 could be designed to perform simultaneous multiplications of the contents of corresponding cells of the stores 3 and 11 instead of performing these multiplications in turn.

In another possible modification the reference problem store 4 and the Fouriertransform device 5 are located at a base station to load the appropriate data into store 6: but are not included as part ofthe airbourn system. These parts are therefore not essential integers of the invention. It is also pointed out that the invention is not applicable only to synthetic aperture radar. The reference pattern could be used to modulate a transmitted pulse in,for example, a conventional pulse radar employing pulse compression; or in a sonar system.

Claims

1. Afrequencydomain correlation system comprising: meansforsupplying to one input of a multiplier a set of samples, taken one at a time, representing frequency components of an input signal; means for supplying to the other in put of the multiplier a set of reference samples, taken one at a time, representing frequency components of a reference pattern; and means for performing an inverse Fouriertransform on the set of products from the multiplier; characterised by means for receiving the reference samples and for receiving control data, for modifying the former in dependence on the latter, and for passing the modified reference samples to the multiplier.

2. A synthetic aperture radar system incorporating a correlation system constructed in accordance with claim 1.

3. A frequency domain correlation system substantially as described with reference to Figure 1 oftheaccompanying drawings.