GB2359134A

GB2359134A - Signal processing apparatus for radar or sonar systems

Info

Publication number: GB2359134A
Application number: GB0028459A
Authority: GB
Inventors: Ronald Barnes; Peter Cooper
Original assignee: Systems Engineering and Assessment Ltd
Current assignee: Systems Engineering and Assessment Ltd
Priority date: 1999-11-22
Filing date: 2000-11-22
Publication date: 2001-08-15
Also published as: AU1531301A; WO2001038899A1; GB0028459D0; GB9927462D0

Abstract

The apparatus comprises inputs 13,14 for receiving at least the transmitted pulse signal and the received reflected signal. Signal analysing means 23 is provided for analysing at least some of the characteristics eg. pulse length of the transmitted pulse signal and to vary the operation of the apparatus in dependence thereon. Another apparatus comprises means for detecting the pulse length of the transmitted signal as an indication of the likely bandwidth of the reflected signal, and means for filtering the reflected signal with a band pass filter the pass band of which is varied in dependence on the anticipated bandwidth of the reflected signal. This improves the signal-to-noise ratio of the received reflected signal for further processing.

Description

2359134 1 SIGNAL PROCESSING APPARATUS The present invention relates

generally to signal processing apparatus, and particularly to apparatus for use in the processing of signals reflected from a region exposed to a transmitted pulse signal.

Such apparatus finds particular application in connection with so-called echo sounders, namely acoustic systems acting to transmit a sonic signal into a medium, generally a body of water, and to receive a signal reflected from a reflecting surface within the body of water, such as the bottom of the body of water, or fish, marine mammals or other life forms, submerged equipment, submarines or other vessels. Although primarily directed to this use, the present invention is not so limited, and may be used in any field in which a reflected signal is received from a region of interest, after a pulse has been transmitted towards the region of interest by a transducer. Such applications include medical, diagnostic and exploratory systems, and systems utilising other signals than acoustic, for example radar signals.

Echo sounders are widely used for detecting the depth of the body of water beneath a vessel, or for tracking fish 2 stocks by discriminating between the reflected or echo signals arriving from the seabed and those from shoals of fish. A wide range of equipment operating on broadly similar principles, but with different particular characteristics is available. Such equipment is relatively unsophisticated in that it primarily detects the presence of the echo signal and its time of transit in order to determine no more than the distance from the vessel of the reflecting surface. Attenuation of the signal during its transmission through the medium (usually water) is compensated by applying a timevarying gain to the received reflected signal so that signals from deeper in the body of water, indicating a greater depth, are more strongly amplified than signals arriving from shallower depths in order to provide sufficiently strong signals for secure detection.

In addition to this fundamental depth-finding function, it has been established that certain characteristics of the reflected echo signal represent information concerning the nature of the seabed. European Patent 0156636 describes a system in which the leading edge of an echo signal is isolated by means defining a narrow time slot for detection so that characteristics of the reflected echo signal and its shape can be analysed to 1 3 extract information concerning the nature of the reflecting surface. This earlier apparatus acts, in particular, on the so-called "second bottom echo" namely the reflected signal which, upon its return from the 5 seabed, is reflected from the surface of the sea (sometimes also from the bottom of the vessel) back to the seabed, and subsequently returns a second time, greatly attenuated, but having picked up additional information by virtue of its double reflection at the bottom of the seabed. Information on the nature of the seabed is also available from the first echo signal but since this is dominated by specula reflection, whereas the second bottom echo is a primarily a diffuse or scattered reflection, the first echo is unreliable, especially when the vessel is experiencing heavy weather, so that due to the analysis to abstract this information is more difficult.

In the earlier patent EP 0 156636 it is described how the area under a return pulse edge contains information characteristic of the nature of the body reflecting the transmitted signal, and further explains that this area may be displayed on an analogue meter, or digitally, in such a way as to provide the user with a display the value of which changes with the nature of the reflecting r, -1 1 4 body. In particular it has been established that the leading edge of the first or second bottom echo signal varies in dependence upon the nature of the material close to and at the seabed so that, for example, a very steeply rising leading edge in the return signal indicates a hard seabed on which there is no significant mud, whereas a leading edge having a shallower inclination indicates that a layer of mud may exist on the seabed at which reflection is taking place.

Integrating the timed leading edge part of the reflected signal thus provides a relatively reliable characterising value which can be used to represent the nature of the seabed whilst the timing of the arrival of the echo signal represents the depth of the seabed.

US patent 4648081 is also based on this realisation, and provides apparatus for automatic discrimination of the nature of the seabed over which a vessel is passing, in particular to provide an alarm signal for fishermen when the nature of the seabed changes so that appropriate action can be taken to prevent damage to fishing nets. This prior art apparatus includes an adjustable threshold circuit connected to the input at which the echo signal is received, and is arranged to provide an output when the input echo signal exceeds a selected threshold level, signal processing means being provided to remove from the output signal of the threshold circuit those components thereof derived from the transmission component, the first bottom echo component and any echo components representing intermediate bodies such as fish in the echo signal leaving only that component, if any, of the threshold circuit output signal which is derived from the second bottom echo component in the echo signal.

This is based on the realisation that the harder the seabed bottom the greater the amplitude of the second bottom echo component for a given depth of water. BY adjusting a threshold level to correspond to the expected amplitude of the second bottom echo component corresponding to the hardness at which action must be taken, it is possible to use a simple discrimination circuit to provide the alarm signal.

The signal processing provided by the apparatus described in these and other patents provides a highly valuable addition to the amount of information which can be extracted from an echo sounder signal. The additional circuitry required for the signal processing to achieve these ends is, however, not available in a standard echo sounder and in order to enjoy the benefits of the 6 additional knowledge which can be extracted from the echo sounder signal a significant expense would have to be born in discarding existing equipment and replacing it with new improved equipment incorporating such circuitry. Attempts to provide a general purpose circuit for use with known echo sounders has encountered difficulties due to the wide variety of parameters with which such equipment operates. Because of the environment in which echo sounders operate, namely a moving body of water supporting a moving platform (the vessel on which the equipment is mounted) the signal-to-noise ratio in the return echo is very low and the difficulty of extracting the required, somewhat weak, information signal from the strong and strongly varying noise signal, compounded by the fact that different items of echo sounder equipment may operate at different frequencies, utilising different pulse lengths with a different overall energy level, has resulted in the need for equipment to be adjustable by the operator in a manner which is more delicate and/or requires greater skill and knowledge than can readily be found on board.

Moreover, it is known that the absorption of the acoustic signal by sea water is different at different frequencies so that the return echo signal must be compensated not d i 11 - 7 only by the time-varying gain needed to take account of the depth of the water, but also by a factor dependent on the absorption rate of sea water at the frequency of the pulse signal utilised.

The major factors which affect the first and second bottom echo signal are the pulse length, which has an influence on the bandwidth of the received signal, the energy in the transmitted pulse signal (which is not independent of the pulse length), the central frequency of the transmitted pulse signal and the pulse repetition frequency of the signals. This latter also has to be related to the depth of the water since the speed of sound through water is relatively slow such that, at depths of the order of 1000 metres or more, the separation between a transmitted pulse and a received echo signal may be several seconds so that the pulse repetition frequency needs to be less than this in order to avoid the transmission of a second pulse masking or preceding the arrival of the echo (especially the second bottom echo) from a first pulse. However, when analysing the seabed, particularly for surveying purposes, a better spatial resolution is given by a faster pulse repetition frequency and the present invention seeks to allow a higher pulse repetition frequency to be utilised by 8 interleaving transmission and reception pulses in deep water without suffering from ambiguity in the analysed results.

In particular the present invention seeks to provide apparatus which enables analyses of the received reflected signal in an echo sounder to be undertaken using any known previously-installed echo sounder without requiring long and complex setting up operations to match the analysing apparatus to the operating characteristics of the echo sounder by providing a system which automatically adjusts itself to the nature of the echo sounder equipment and its signal. The apparatus of the present invention is thus readily usable with a wide range of previouslyinstalled echo sounders avoiding the need for their replacement whilst nevertheless obtaining the beneficial effects of the more sophisticated analyses which may be undertaken utilising the equipment.

This represents a significant improvement over previous implementations of such analysis equipment as described in the prior art. In particular, automatic adaptation of the equipment to variations in power level which are a result of either a change in the echo sounder itself, or of manually changing the power level (which may be done

1 11, c c 1 9 in different circumstances to take account of, for example, the nature of the seabed bottom, relative motion due to waves, turbidity in the water due to particles in suspension therein, and also from an automatic variation of the power within the echo sounder or drif t in the electronic components. Apparatus formed as an embodiment of the invention may simply be connected to a "host" echo sounder and will automatically adapt itself to the operating parameters so that no manual adjustment of the equipment is necessary in order to put it into use.

The present invention also seeks to provide apparatus for analysing a signal reflected from a region exposed to a transmitted pulse in which automatic adaptation of the equipment to take account of the frequency of the signals (and here signal frequency is intended to mean the frequency of oscillation of the transmitted signal, pulses of which are transmitted at a (lower) pulse repetition frequency). Typically, the signal frequency may vary between 20 kHz and 200 kHz, although signal frequencies lower than 20 kHz may sometimes be used for specialised purposes. The apparatus of the present invention is capable of detecting the signal frequency of the host echo sounder and automatically adjusts itself to this frequency thereby enabling it to ignore signals from 1 other echo sounders in the immediate vicinity as well as other sources of interference. As mentioned above differential absorption of the sound waves at different frequencies by the water, especially sea water, can also 5 be compensated or accommodated.

According to one aspect of the present invention, therefore, signal processing apparatus for analysing a signal reflected from a region exposed to a transmitted pulse signal, comprising an input stage having inputs for receiving the transmitted pulse signal and the received reflected signal, and means sensitive to at least some of the characteristics of the transmitted pulse signal and operative to vary the operation of the signal analysing apparatus in dependence thereon.

The signal processing apparatus of the invention preferably has means sensitive to at least one characteristic of the transmitted pulse signal which may be sensitive to the frequency of the transmitted pulse signal and the signal analysing apparatus preferably compensates for variations in the frequency of the transmitted signal such that the analysis of the reflected signal is substantially unaffected by variations in the frequency of the transmitted signal.

11 The compensation for variations in the frequency of the transmitted signal is preferably independent of the absolute value of frequency of the transmitted signal.

The frequency of the detected transmitted signal may, for example, by shifted by heterodyning to a standard frequency for use in the signal analysing apparatus.

Preferably the signal analysing apparatus operates 10 digitally, and the heterodyning of the signal shifts the received reflected signal to a standard frequency of 0 Hz (that is a complex signal centred at DC).

Signal processing apparatus designed for use with echo sounding equipment usable to obtain an echo signal from the bottom of a body of water such as the sea may further be provided with means for accommodating variations due to the differential absorption of the water dependent on the frequency of the signal and the varying depths of water through which the transmitted signal and the reflected signal travel, whereby effectively to make the signal analysis independent of the absolute frequency of the transmitted signal or the depth of the water. In such a system the depth of the water is determined and the anticipated absorption factor at that depth 12 calculated on the basis of a nominal or average characteristic thereof in order to cancel the factor by multiplying by a signal representing the estimated absorption to the received signal.

The absorption factor may be determined as being proportional to the exp(-%(f)2d) where:

f is the frequency of the transmitted signal d is the depth of the body of water % is a standard absorption function for compensation applied to the reflected signal received from the bottom of the body of water as the first bottom echo signal received.

is If the compensation is applied to the echo signal in order to compensate the second bottom echo signal received after transmission of the transmitted pulse the absorption factor may be calculated as being proportional to: exp(-%(f)4d) where % is the standard absorption function referred to above f is the frequency of the transmitted signal and d is the depth of the body of water.

1 13 The length of the transmitted pulse is preferably determined so that compensation therefor can be applied to the received signal whereby to maximise the signal-tonoise ratio in the received signal.

According, therefore, to another aspect of the present invention, signal processing apparatus for analysing a signal reflected from a region exposed to a transmitted pulse signal, includes means for detecting the pulse length of the transmitted signal as an indication of the likely bandwidth of the reflected signal, and means for filtering the reflected signal with a band pass filter the pass band of which is varied in dependence on the anticipated bandwidth of the reflected signal, whereby to improve the signal-to-noise ratio in the received reflected signal for further processing.

In such signal processing apparatus there may be further provided means for dividing the reflected signal by a value dependent on the detected transmitted pulse length whereby to obtain normalised values for further processing. Preferably, the integral value of the reflected signal is divided.

Embodiments of the present invention will now be more 14 particularly described; by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram illustrating the basic components of a system forming an embodiment of the 5 present invention; Figure 2 is a diagram illustrating the treatment of the transmitted and received signals; Figure 3 is a graph illustrating the variation of absorption with frequency used in compensating for 10 frequency variations; and Figure 4 is a diagram illustrating the interleaving of pulses transmitted and received in deep water.

Referring first to Figure 1 the apparatus shown comprises is a transmitter/receiver transducer 11 from which acoustic signals are transmitted at a voltage typically up to a maximum of 1.5 kV and at which received reflected signals are received which generate signals in the order V. The host apparatus for generating the transmitted signal is conventional and illustrated in the drawing as echo sounder 16. The transducer is resonant at the frequency of the transmitted signal such that acoustic radiation transmits from it into the medium (typically a body of sea water) and is propagated until reflected by a reflecting surface 12, typically the seabed. The is reflected acoustic signal is received, after transmission and reflection, back at a transducer 11 and typically has a power sufficient to generate signals in the order of pV to V. The electrical signal generated by the transducer 11 in response to the reflected acoustic signals is fed on lines 13 and 14 to a self-switching circuit 15 which also receives, along lines 13 and 14, the transmitted signal applied to the transducer 11 by the host echo sounder apparatus 15 and 16. The amplitude dependant self-switching circuit and directs the transmission pulse signal arriving from the echo sounder 16 to an attenuator circuit 17 and the received signal arriving from the transducer 11 to an amplifier 18. Outputs from the attenuator 17 and the amplifier 18 are fed to an analogue switch 19 controlled by a switch control circuit 20 and the analogue signals are fed to a filter 21 before being passed to an analogue-to- digital converter 22 for conversion into digital form for further processing. once converted into digital form the signals are further processed in software represented 23.

Figure 2 represents the shape of a signal centred for example at 50 kHz showing the distribution of frequencies within the pulse signals. The width of the spread is proportional to the inverse of the pulse length. Namely, 16 the longer the pulse length the more "pure" the signal, that is the fewer components it has other than the nominal frequency. An infinitely long pulse at, for example 50 kHz would have a single 50 kHz spike whereas a very short pulse at 50 kHz would also contain significant components on either side of 50 kHz. In the signal processing it is required that the compensation for frequency is achieved regardless of the actual frequency of the signal. This is done by heterodyning to shift the frequency to a standard. transducer is multiplied (that is mixed with) an internally generated signal at the sensed nominal frequency in order to provide a standard at 0 Hz thereby moving the signal down to a complex (real and imaginary) 15 base band.

The signal from its It is also necessary to compensate for differential absorption of the water at different frequencies. Figure 3 illustrates a typical relationship between frequency and absorption expressed in dB/metre. In order to achieve this the received reflected pulse timing (that is the delay between the transmission of the transmitted pulse and the arrival of the received echo pulse, is used to determine the depth of the water and then the received signal is multiplied by a factor based on this in order 9 a 1 4 1:.

17 to cancel the absorption factor on the basis of a nominal or average characteristic. The absorption varies as exp(-%(f)2d) for the first echo signal and as exp(%(f)4d) for the second echo. Since information can be extracted from both echo signals the apparatus is set up to respond to both.

The apparatus also automatically compensates for the pulse length so that it is capable of being fitted to any echo sounder without requiring complicated or sensitive tuning operations. Pulse length compensation is effected at two levels. First, in a manner similar to the frequency compensation, the transmitted pulse length is determined by a threshold discrimination and a counter to determine the length of the pulse. The bandwidth of a band pass filter is then set to a value which maximises the signal-to-noise ratio in the received signal. The other compensation takes account of the fact that a longer pulse delivers overall more energy to the transmission medium (the water) and this is compensated by normalising the output of the apparatus by a scalar factor as a function of the transmitted pulse length and amplitude whereby to influence the output of the whole system. Effectively the input signal is divided by the 18 pulse length to normalise the signal. The integral values from the first and second echoes are divided by the transmitted pulse length and amplitude for normalisation.

is In essence, then, the system is tuned by measuring the frequency of the transmitted pulse and is made more selective by measuring the pulse length. This is achieved digitally by sampling the received pulse signal over time at a rate commensurate with the bandwidth. Sampling takes place at, for example 250 kHz, then the samples are multiplied by a carrier frequency to shift them down to base band by heterodyning as before, and then added. The addition of samples effectively constitute the filtering; this varies in dependence on the number of samples added together.

when conducting seabed analysis from the received signal it is desirable for the pulse repetition frequency to be as great as possible so that the resolution of the information can be as accurate as possible. However, in deep water when the echo signal may be returning several seconds after the transmitted signal eclipsing or masking of the second echo, which as mentioned above contains important information not readily available from the c 1 1 19 first echo, may occur if the pulse repetition frequency is too great. The apparatus of the present invention acts to compensate for depth by multiplying the signal by a number proportional to depth. It is also necessary to check for large changes in depth. If the depth and the pulse repetition interval is known it can be determined whether an ambiguous position is likely to arise position due to transmitted/received pulse interference. The apparatus then divides by the apparent depth and multiplies by the true depth to compensate by interleaving. In this respect a depth correction is equal to the apparent depth to which is added a depth scale factor d which is equal to (speed of sound) C multiplied by (pulse repetition interval)/2 for the first echo. The expression is divided by 4 for the second echo.

It is, of course, only necessary to interleave in deep water because the echo arrives more quickly in shallow water and the pulse repetition interval can therefore be short without being ambiguous due to interference of the reflected signal and a subsequent transmitted pulse. Figure 4 illustrates the form of a detected signal where interleaving takes place. Here, it can be seen that tl results after time x in a first echo pulse e', and after a further period of time x in a second echo pulse e'2.

1 t.

With a pulse repetition interval of t the second pulse t2 2 gives rise to a first echo pulse e 1 which arrives after the second echo pulse e'2 from the first transmission 2 pulse. Correspondingly, before the second echo pulse e' can arrive the third transmission pulse t3 is transmitted.

In preferred embodiments, the frequency and pulse length compensation processes are implemented in software 23, that is to say software 23 comprises signal processing algorithms for each of the compensation processes described, namely: frequency compensation for base band normalisation (heterodyning); frequency compensation for differential absorption; pulse length compensation for signal filtering; pulse amplitude compensation for input signal division and, pulse length compensation for input signal division (attenuation). Software 23 comprises control algorithms for controlling the switch control circuit 20 and algorithms for extracting the processed, or normalised, signals which relate to seabed characteristics, whereby to identify the seabed type being monitored.

The signal processing apparatus determines integral 25 values for the first and second echo signals reflected 21 from the seabed. The integral values correspond to the area under the respective time domain reflected echo signal waveforms. For variation in the characteristics of the echosounder, the two integral values may be used 5 to characterise the nature of the sea bed.

Q,.

1 22

Claims

1. Signal processing apparatus for analysing a signal reflected form a region exposed to a transmitted pulse signal, comprising an input stage having inputs for receiving the transmitted pulse signal and the received reflected signal, means sensitive to at least some of the characteristics of the transmitted pulse signal and operative to vary the operation of the signal analysing 10 apparatus in dependence thereon.

2. Signal processing apparatus as claimed in Claim 1 in which the means sensitive to at least one characteristic of the transmitted pulse signal is sensitive to the frequency of the signal a pulse of which is transmitted, and the signal analysing apparatus compensates for variations in the frequency of the transmitted signal such that the analysis of the reflected signal is substantially unaffected by 20 variations in the frequency of the transmitted signal.

3. Signal processing apparatus as claimed in Claim 2, in which the compensation for variations in the frequency of the transmitted signal is independent of the absolute value of the frequency transmitted signal.

23

4. Signal processing apparatus as claimed in Claim 2 or Claim 3 in which the frequency of the detected transmitted signal is shifted by heterodyning to a standard frequency for use in the signal analysing apparatus.

5. Signal processing apparatus as claimed in Claim 4, in which the said standard frequency is OHZ.

6. Signal processing apparatus according to any preceding claim, in which the signal processing apparatus is designed for use with echo sounding equipment usable to obtain an echo signal f rom the bottom of a body of water, in which there are provided means for compensating for the differential absorption upon variation in the frequency of the signal and with varying depth of water through which the transmitted signal and the reflected signal travel, whereby effectively to make the signal analysis independent of the absolute frequency of the transmitted signal or the depth of the water, in which the depth of the water is determined and the anticipated absorption factor at that depth is calculated on the basis of a nominal or average characteristic thereof in order to cancel the factor by adding a signal - 1 . t.

1 1 r 24 representing the estimated absorption to the received signal.

7. Signal processing apparatus as claimed in Claim 6, in which the absorption factor is determined as being proportional to exp (-% (f) 2d) where: f is the frequency of the transmitted signal, d is the depth of the body of water, and V is the absorption factor for compensation applied to the reflected signal received from the bottom of the body of water in synchronisation with the first bottom echo signal received.

is

8. Signal processing apparatus as claimed in Claim 6, in which the compensation is applied to the echo signal in order to compensate the second bottom echo signal received after transmission of the transmitted pulse and the absorption factor is calculated as being proportional to:

exp (-% (f) 2d) Where: V is the absorption factor F is the frequency of the transmitted signal i d is the depth of the body of water.

9. Signal processing apparatus as claimed in any preceding Claim in which the length of the transmitted pulse is determined and compensation therefor applied to the received signal whereby to maximise the signaltonoise ratio in the received signal.

10. Signal processing apparatus as claimed in any 1:0 preceding claim, in which the transmitted pulse signal is sampled by a threshold discrimination circuit and timer to determine the presence and length of a transmitted pulse.

11. Signal processing apparatus for analysing a signal reflected from a region exposed to a transmitted pulse signal, including means for detecting the pulse length of the transmitted signal as an indication of the likely bandwidth of the reflected signal, and means for filtering the reflected signal with a band pass filter the pass band of which is varied in dependence on the anticipated bandwidth of the reflected signal, whereby to improve the signal-to-noise ratio in the received reflected signal for further processing.

26

12. Signal processing apparatus as claimed in Claim 7, further including means for dividing the reflected signal by a value dependent on the detected transmitted pulse length whereby to obtain normalised values for 5 further processing.

13. Signal processing apparatus as claimed in Claim 7, further including means for dividing the reflected signal by a value dependent on the detected transmitted pulse amplitude whereby to obtain normalised values for further processing.