GB2129935A

GB2129935A - An adaptive filter

Info

Publication number: GB2129935A
Application number: GB08308829A
Authority: GB
Inventors: John Arthur Glasgow
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1982-03-30
Filing date: 1983-03-30
Publication date: 1984-05-23
Also published as: GB2129935B

Abstract

A received signal which may exhibit frequency variations, e.g., caused by variations of the Doppler shift, is detected by correlating the signal with a number of reference waveforms. The illustrated filter is a digital filter. The signal to be filtered, from a digitizer 5, is sampled and successive blocks of samples are entered into a correlator at 6A. Each such block is correlated in turn with a number of reference waves generated at 7 and placed in turn in the reference channel 6B of the correlator. The output of the correlator, together with information from the reference wave generator 7 and a timer 1 is passed to a phase corrector 8 which calculates, for each correlation, the phase and rate of change of phase of that component of the signal block which correlates with the appropriate reference channel. These phase and rate of change of phase values are stored, together with values denoting the degree of correlation in a signal characteristic store 9. A combiner 10 searches the information in the store 9 for correlation results, from signal block to signal block, which display phase continuity thereby filtering off the remaining information which is attributed to noise. The filter may be used in sonar or acoustic radar systems. <IMAGE>

Description

SPECIFICATION An adaptive filter This invention relates to a filter which is defined for the purpose of this Specification as a device which separates wanted and unwanted components of an incoming signal. The invention is thus applicable both to analogue and to digital filters. It arose during a study of sonar systems where a received signal needs to be filtered to remove components not derived from targets of interest.

In a sonar or acoustic radar system the frequency of echoes from a target varies with time due to changes of target speed, the speed of the observer, and changes in the direction of the target relative to the observer. This frequency variation, caused by the Doppler effect, is more significant with sonar waves than with conventional electromagnetic waves because the target speeds relative to the observer are iikely to be significant compared with the speed of propagation of the waves. Filters used in conventional sonar equipment have had to have a sufficiently wide bandwidth to embrace the whole range of the expected variations in the Doppler frequencies.Thus, at any one time, such filters pass both the wanted return signal, having a Doppler shift, and noise at other frequencies which the signal might have had, had the Doppler shift been different.

This invention provides a filter comprising: a correlator for correlating each of adjacent blocks of a signal to be filtered with a plurality of reference waveform blocks to give a measure in each case of the degree of correlation; and means for selecting a reference waveform block for each signal block in accordance with a preset selection procedure which is preferential to reference waveform blocks which correlate relatively strongly with the signal blocks.

The selection procedure can be improved by making it preferential to reference waveform blocks which are relatively coherent i.e. have continuity of phase from block to block. A further advantage can be obtained by making it preferential to reference waveform blocks which have relative continuity of rate of change of phase from block to block. Another possibility is to perform the selection procedure on the basis of continuity of frequency, or on the basis of continuity of amplitude and rate of change of amplitude.

One preferred form of the invention includes means for generating reference waveforms, each being longer than each signal block; and means for passing the signal block and/or the reference waveform progressively through the correlator so that the signal block is correlated progressively with different overlapping blocks of the reference waveform.

The wanted part of a signal can usually be assumed to be continuous from block to block in phase, rate of change of phase, amplitude, rate of change of amplitude, and frequency; whilst unwanted noise is usually non-continuous (i.e.

there are discontinuities of phase etc. over a period of time; or varies in frequency in a random fashion and therefore correlates relatively poorly with the reference waveforms. The selected reference waveform blocks, therefore constitute an approximation to the wanted part of the signal with the noise filtered off.

Reference waveforms will normally be chosen to include a range of fixed frequency signals and preferably also a selection of varying frequency signals such as might be produced by variations of the Doppler effect in a received signal.

One way in which the invention may be performed will now be described with reference to the accompanying schematic drawing of an acoustic radar system constructed in accordance with the invention.

Referring to the drawing the timer 1 controls an acoustic transmitter 2 so that the latter generates pulses of acoustic energy of fixed frequency which are transmitted towards a target which is assumed to be travelling relative to the radar system in the direction indicated by the arrow at a velocity which is significant compared with the velocity of acoustic waves in a medium between the radar system and the target. The transmitted acoustic waves are coherent within each pulse and from one pulse to the next, i.e., the phase in different pulses is the same as a continuous wave of the same frequency.

The acoustic pulses, after reflection from the target 3 are received by a receiver 4. A digitizer 5, controlled by the timer 1 samples short periods of the received signal forming blocks of samples (there could be many such blocks within each pulse period) and for each such period produces a block of n digital numbers representing, in digital form, the signal received during that period. Each block of digital numbers is entered in a register 6A, which constitutes a "signal channel" of a correlator 6 and has n memory cells.

A reference wave generator 7 contains a store of information defining respective different reference waves each being substantially longer than the signal block stored in the signal channel.

Some of the reference waves have a constant frequency covering the whole range of Doppler frequencies to be expected from a type of target of interest; and some have varying frequencies which vary at rates also to be expected from targets of interest.

Under the control of the timer 1 the generator 7 generates these reference waves in succession in digital form and feeds the digits to one end of a register 6 which constitutes a "reference channel" of the correlator 6. The whole succession of reference waveforms is generated and fed to the reference channel 6B for each block of numbers in the register 6A. Each block of numbers (representing the signal) are thus cross-correlated in turn with different overlapping blocks of each reference wave as the latter is fed in digital form into the channel 1 6. For each reference waveform the correlator produces a) a measure of the degree of correlation between the signal block in channel 6A and the reference wave and b) a count C of the number of timing signals which have been used to feed the reference signal into the correlator at the time of maximum correlation.

Each correlation result is fed to a phase calculator 8 together with a code representing range derived from the timer 1, it being understood that the time interval between transmission and reception of the pulse is proportional to range. The phase calculator also receives, from the reference wave generator, information defining the different reference waveforms.

For each reference waveform the phase calculator 8 identifies the block of that reference waveform which correlated best with the signal block in channel 6A and the phase and rate of change of phase at the beginning and end of that reference waveform block. This information is stored in a signal characteristic store 9 together with a measure of the degree of correlation.

The contents of the store 9 thus consist of, for each block of the input signal, a number of different sets of values, one for each reference wave, each set consisting of the phase and rate of change of phase at the beginning and end of the block of the reference waveform which correlates best with the signal block; and the degree of correlation with the reference waveform. A combiner 10 searches the store and selects an aforementioned set of values for each block of the input signal such that each selected set (a) has a high correlation value (b) displays continuity of phase with respect to the selected sets of the preceding and following blocks and (c) displays continuity of rate of change of phase with respect to the selected sets of the preceding the following blocks. These three factors all need to be taken into consideration by the combiner 10 when making the selection.When the factors (a), (b) and (c) are, in combination, considered to be good for all the selected sets according to some assessment factor defined by the combiner 10, a target is assumed to be present. When a target is thus detected the combiner 10 produces an output indicating the presence of a target and the behaviour of it, this behaviour being derived from the properties of the different blocks of the reference waveforms which correlated closely with the different blocks of the received signal and which displayed continuity from block to block.

The invention is also applicable to the filtering of continuous (i.e. not pulsed) received signals such as in a passive system that receives the noise radiated by a submarine or other target but does not itself radiate signals.

In the particular embodiment of the invention which has been described the correlation is performed in the time domain. However, other forms of correlation can be used. For example, a correlation that operates in the frequency domain may be used. In such a system the received signal and the reference waves are separately filtered into their frequency components and the two sets of filter outputs then correlated. The technique of correlation in the frequency domain is well known, being described for example in "Modulation Noise and Spectral Analysis" by P. F. Panter McGraw Hill 1905.

Claims

1. A filter comprising: a correlator for correlating each of adjacent blocks of a signal to be filtered with a plurality of reference waveform blocks to give a measure in each case of the degree of correlation; and means for selecting a reference waveform block for each signal block in accordance with a preset selection procedure which is preferential to reference waveform blocks which correlate relatively strongly with the signal blocks.

2. A filter according to Claim 1 in which the preset selection procedure is preferential to reference waveform blocks which have relative coherence from block to block.

3. A filter according to Claim 2 in which the selection procedure is preferential to reference waveform blocks which have relative continuity of phase and rate of change of phase from block to block.

4. A filter according to any preceding claim in which the present selection procedure is preferential to reference waveform blocks which have relative continuity of frequency from block to block.

5. A filter according to any preceding claim including means for generating reference waveforms, each being longer than each signal block; and including means for passing the signal block and/or the reference waveform progressively through the correlator so that the signal block is correlated progressively with different overlapping blocks of the reference waveform.

6. A filter substantially as described with reference to the accompanying drawing and substantially as described therein.