The processing of reflected signals

Abstract: A circuit is disclosed for displaying data relating to the nature of the seabed from echoes of hydro-acoustic echo equipment. Both the first and second echo components are extracted and integrated (C1,C1′) and the results are combined (62,63,64) in additive relation and displayed (65).

Description: This invention relates to the processing of reflected, or echo, signals obtained by the use of signal transmitter-receiver equipment and has particular applications in the fields of hydro-acoustic transceivers, such as sonar equipment and echo sounders. The terms transceiver and transmitter-receivers when used herein are intended to mean equipment for transmitting pulses and for receiving consequent echoes of the transmitted pulses, and is not limited, unless it is otherwise expressely stated, by the type of pulse (e.g. it may be sonic, supersonic or electromagnetic) nor is it limited to equipment which shares transmitting and receiving parts. In the field of sonic and supersonic waves, equipment is available, such as sonar and echo sounders, which gives audible and visible indications of the existence of return echoes. Echo sounders, for example, give results recorded on recording paper. In order to improve such equipment, circuitry has been designed for processing the return echo signal in order to extract from it more information than has heretofore been readily available with this type of equipment. Such a circuit is shown in U.K. specification No. 8221670 in which means are provided to extract the second bottom echo component of the return echo signal in order to provide information about the nature of the seabed. In that specification, it was disclosed that the nature of the seabed was revealed by sensing whether or not the second bottom echo component had an amplitude above a preset threshold level. It has now been discovered that further useful information is present in the return echoes of equipment such as echo sounders, specifically in both the first and second bottom echoes. EP-A-0049644 describes a circuit for determining reflection coefficients from multiple reflections. Specifically, EP-A-0049644 describes a circuit for use in detecting the nature of the seabed from the return echo signal of hydro-acoustic transmitter receiver equipment and comprising means for defining a time slot to extract from the return echo signal a first component which is a portion of the first bottom echo component, means for defining a time slot to extract from the return echo signal a second component which is the second, or later, bottom echo component, and means for providing a first signal from the extracted first component. According to the present invention, such a circuit has means for integrating the extracted second component to provide a second signal giving an indication of the energy content of the second component, and means for providing a display representing the individual values of the first and second signals. The integration of an echo signal is known from FR-A-2121555. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a general block diagram of echo pulse processing circuitry; Figure 2 shows a series of wave forms illustrating the operation of circuitry within Figure 1; Figure 3 shows more detailed wave forms to a longer time scale; Figure 4 is a diagram of a combined meter and alarm circuit; Figure 5 is a diagram of a ground metering circuit; Figure 6 shows a series of waveforms illustrating the operation of the circuit of Figure 5; and Figure 7 shows a circuit of a second bottom echo timing circuit. Referring to Figure 1, this shows circuitry designed specifically to process the return echo signals A from a conventional echo sounder S with transmitter/receiver transducer T and described in detail in European Patent Application No. 0156636. The analog return echo signal, after time varied gain (TVG), is supplied to input 1 and the form of this signal is shown generally at line A of Figure 2. It is a repetitive signal comprising, in each repetition, a large transmission component TC, a first bottom echo component E1 and a second bottom echo component E2, all of successively decreasing amplitude. In between the transmission component and the first echo component there may well be a fish echo as shown at EF. Various noise and other echo signals might also be present but their magnitudes are insignificant in the present context and are not shown in Figure 2. If the circuitry were employed in the context of a forward-looking sonar or marine radar, a cyclic pattern of pulses would similarly occur but the form of the signal would be different, nevertheless including pulses such as that shown at E1 when the environment includes any reflective "body" such as a boat or shoal of fish. Even using conventional echo sounder TVG some disortion or saturation may occur in some conditions and it may therefore be beneficial to provide an alternative TVG circuit to overcome saturation and so make more information available. For example, a microprocessor with a mathematical algorithm could provide the TVG process. The echo sounder S additionally provides a pulse TX at an input 2 in Figure 1, this pulse defining the time at which the transmission pulse is emitted by transducer T. From input 2, the pulse TX is passed to a Schmitt trigger circuit 3 for shaping and then to a monostable circuit 4 providing a stretched pulse B (line B of Figure 2) which is used in the circuitry as a mask to filter out the TX component of the echo signal and acoustic noise close to the transmitter. Returning now to the analog signal provided at input 1, this is applied to three analog amplifiers 5, 6 and 7 having a gains which may be varied by the user at variable resistors 8, 9 and 10. The amplifiers have capacitors 11, 12 and 13 connected across their outputs which are connected to Schmitt trigger circuits 14a, 14b and 14c. The capacitors provide smoothing to smooth the leading edges of echoes in an attempt to avoid lead detecting processes, to be described, being falsified by distorted leading edges. Circuit 14a feeds a fish alarm circuit 15 via a NOR gate 16 fed by signal B to blank transmission pulses. Circuit 14b feeds a second echo ground alarm 17 via a NOR gate 18 also fed by signal B. The second echo ground alarm 17 may be as described in British Patent Application No.8221670 (2102573). Amplifier 6 also feeds its analog signal (corresponding to A in Figures 2 and 3) without smoothing to fish (19) and ground (20) metering circuits to be described hereinafter. Circuit 14c supplies a bottom pulse detection circuit and this will be described with reference to Figure 3 which shows more detailed wave forms over several cycles of the return echo signal A, including signals B, C, D and E of Figure 2. Schmitt trigger circuit 14c provides at its output a digitised version of signal A, i.e. with its pulses converted to a series of square pulses of uniform amplitude. These pulses are passed through a series of NOR gates 21, 22 and 23 and inverters 24 and 25 to a latch 26 the Q output of which is signal F shown at line F of Figure 3. This signal F is a pulse commencing from a given amplitude point on the rising edge of the first echo and terminating on the commencement of signal B, fed to the reset input of the latch 26. The threshold for the rising edge of the echo signal E1 is selected by adjustment of the resistor 10. The repetitive signal F is fed to a light emitting diode (LED) at 27 which will be seen to be flashing in normal use of the equipment. The threshold, during set up, will be increased until that flashing stops and then decreased again until the flashing just starts in order to set a threshold for this channel appropriate to detecting a point high up on the rising edge of the echo pulse concerned. In order to prevent response of the channel to other pulses, the transmission pulse itself is masked by feeding signal B to NOR gate 23 and other signals can be masked by applying further signals via NOR gates 21 and 22. Signal F is fed to a monostable circuit 28 which provides a 40 millisecond pulse G (line G of Figure 3). With a switch 29 in the position B indicated, the pulse G is fed as an input to a monostable circuit 30 producing at its Q output a marker pulse having a duration determined by the user at variable resistor 31. Capacitors can also be switched into and out of circuit with circuit 30 by a section of switch 29 to vary the timing range of adjustment produced by resistor 31. This pulse from the Q output of monostable circuit 30 is signal H in Figure 3 and will be seen to commence on the falling edge of pulse G and to stop some later time, adjustable by the user, so as to be the desired amount ahead of the leading edge of the next first bottom echo pulse E1. As described so far it will be appreciated that there is provision for several gating signals to define ranges over which an echosounder signal can be processed, these including signal G defining the sea bed, i.e. the bottom pulse, and signal H defining a point an adjustable height above the sea bed. Switch 29 provides other gating signals when in its other position P. In that case, monostable circuit 30 is fed by signal TX after shaping from circuit 3 and so provides a version of signal H defining an adjustable depth below the transducer. Similarly, a second monostable circuit 32 provides a second signal to define another adjustable depth below the transducer. Together these signals provide an adjustable range, e.g. for the fish alarm circuit 15. Before discussing the details of the remaining circuitry the concepts behind fish and ground metering circuits will now be described. These circuits comprise integrating or averaging means for averaging the areas under the return echo for specific time slots within each pulse repetition period, i.e. cycle. With regard to the fish metering circuit one may use a time slot such as shown at C in Figure 2 to define a lower area of the sea, terminating at or near the top of the leading edge of the bottom pulse E1. The output goes to meter 38 which will show a rising value as one travels into an area of softer ground and/or of fish near the sea bed. By contrast the ground meter 20 will integrate or average over a time slot to include substantially the whole of a bottom echo (first and/or second and/or third etc). As one traverses into hard ground, the reading of meter 38′ will rise. Thus, if meter 38 rises and meter 38′ falls, then this is normally indicative of soft ground. If meter 38 rises and meter 38′ is constant, this is normally indicative of fish. These meters are thus complementary, and useful not only for fisherman but also in hydrographic survey work. In the fish and ground metering circuits of Figure 1 (and Figure 5 to be described later) a meter is used to generate a visual indication of the conditions. An audible alarm circuit can be added to both circuits, if desired, as shown in Figure 4. In each case, there is provided at the output of switch 35 (which is used in both the fish meter and ground meter (Figure 5) an alternative R2, C2 combination, referenced R2″ and C2″. Instead of feeding a meter, this additional circuit feeds a load resistor R3 across which is connected an audible alarm circuit (shown only for fish meter 19 in Figure 4). A significant point is that resistor R2" can be adjusted to give a different time constant from those set by resistors R2. Thus the meters may give readings averaged over, say, 8 or more cycles but this may give too slow a response for an alarm. This is resolved by adjusting R2″ to give an alarm in response to the average value of, say, two cycles. R2″ may be buffered as necessary from R2. The following components are placed in parallel with the load resistor R3 in order to generate an alarm corresponding to a selected level: two op-amps 49 and 50; five resistors (incluing two potentiometers 51 and 52): and one monostable circuit 53. Potentiometer 52, with vernier knob, sets the voltage threshold on the negative terminal of comparator op-amp 50. If the amplified voltage from R3 exceeds the voltage on the negative input by a small amount an audible alarm is generated. The alarm will continue to ring until the amplified voltage is slightly less than the theshold voltage on the negative input to op-amp 50. This hysteresis is inherent in the circuit as shown. Op-amp 49 provides a non-inverting D.C. amplifier with a gain of approximately 42 which is selected by potentiometer 51 so that the voltage at the output of op-amp 49 equals 10v when the meter is reading full scale i.e. 1ma. Hysteresis is required in the comparator as the voltage across the meter varies slightly before settling. Figure 5 shows one form of the ground metering circuit 20. It will be appreciated that it uses a sample-and-hold technique. The analogue signal from amplifier 6 is fed to the base of transistor 54 from where the signal is taken to first and second bottom echo component processing circuits 55 and 56. The first bottom echo component processing circuit 55 comprises semiconductor switches 33, 35 and 39 (type 4016), integrating capacitor C1, a leaky integrator and storage means (composed of variable resistor R2 and capacitor C2), a resistor 57 and an amplifier 37. Switch 33 is controlled by signal G (Figures 1 and 3) which is a signal of 20 to 40 milliseconds commencing when the first bottom echo component has reached a given amplitude. When signal G is not present, switch 33 is open and switch 39 closed, so that the charge on capacitor C1 tends towards zero. On the rising edge of signal G, switch 33 closes and switch 39 opens, thereby allowing capacitor C1 to charge in proportion to the quantity of the first echo component signal strength (an approximation to the integral of the signal). On the negative-going edge of signal G, switch 33 opens to halt the integration process and a one-shot multivibrator 41 is activated to produce a short signal M (Figure 6) which closes switch 35, allowing the integrated first echo signal to charge capacitor C2 via resistor R2. A delay circuit 40 composed of a diode, resistor, capacitor and inverter produces a signal N from signal G to open switch 39 and then to close it only after switch 35 has opened. The elements R2 and C2 provide an adjustable sample-and-hold circuit. If resistor R2 is set to zero ohms, capacitor C2 holds the charge from the one previous echo, so that the voltage across the capacitor C2 might then vary suddenly, e.g. drop almost to zero should the echo component not be received during one cycle of the system. By increasing the value of resistor R2, the capacitor stores the average of more and more cycles (upto 85 with the resistor R2 at 100 kohms). Typically one might set the resistor at an intermediate value to store the average of the last, say, ten to fifteen cycles. The second bottom echo circuit 56 is similar in that it comprises components corresponding to components of circuit 55. The difference relates to additional elements which are a NOR gate 58, a latch comprising NOR gates 59 and 60 and a one-shot multivibrator 61 of 25 ms duration. NOR gate 58 is fed by signal G and also by a signal O in Figure 6 which is a pulse train identifying echoes (particularly bottom echoes) of sufficient amplitude. NOR gate 58 causes that portion of signal O occurring during the first echo to be eliminated, leaving its subsequent portion, occurring at the second echo and shown as signal P in Figure 6. Signal P will thus occur at each second bottom echo of sufficient amplitude Signal P sets the latch 59, 60 which is reset by signal B (the transmission pulse - Figure 2). The output signal Q of the latch fires the multivibrator 61 to produce signal R which thus starts when the second bottom echo reaches a given amplitude and stops 25 milliseconds later. Signal R is used in place of signal G in the second circuit 56 and so extracts the second bottom echo. The two circuits 55 and 56 are coupled by switches 62 and 63 to an amplifier 64 and meter 65, the meter being as described above in relation to Figures 1, 4 and 6. With switches 55 and 56 in their upper positons, meter 65 displays the first bottom echo processed signal. In their downward positions, the second bottom echo processed signal is displayed. With one switch up and the other down the sum of the signals is displayed. It will be observed that, in Figure 6, four cycles are shown in which, in the second, the second bottom echo is too small to be captured and in the third, both bottom echoes are missing. As shown by the other waveforms, the various switches are operated to prevent these low or absent signals from significantly reducing the meter reading in view of the sample-and-hold features. Figure 7 shows a modification, which is a digital method of identifying second or third bottom echoes, to produce a signal corresponding to signal P in Figure 6, thus replacing gates 58 to 60 of Figure 5. This modification uses the transmission pulse received at input 2 in Figure 1 and the first bottom echo pulse signal F produced by monostable circuit 26 also shown in Figure 1. It comprises an up/down counter 83, a clock 84, a comparator 85, an AND gate 86 and NOR gate 87. The counter is designed to count up when signal F goes low (when TX occurs) and count down when F goes high (when the bottom is acquired). Also, the counter is reset by monostable circuit 3, i.e. every time there is a transmission, in case the first echo is lost. The parallel output of the counter is compared with 'zero' at the comparator which issues a '1' on detecting 'zero'. At transmission the counter is reset, so the comparator issues a '1' but this is blocked by signal F at gate 86, keeping Q low at that stage. The counter increments until F goes high, when the counter decrements to reach zero, causing the inputs to the AND gate 86 both to be high, resulting in Q going high, thereby commencing gating for the second echo. If F does not go high (first echo lost), the counter will continue to increment until the next transmission. When Q does go high, the counting is stopped by gate 87. As the second echo is slightly more than double the depth of the first echo (this due to the depth of the transducer on the hull), gating for the second echo will commence slightly before it arrives, even if the maximum error of one clock pulse occurs when measuring the interval from transmit to the bottom pulse. The maximum error in depth is designed to be not more than half a meter. Hence a vessel with a draft of more than half a meter will begin processing the second echo slightly before it arrives. The LSB of the counter should represent not more than half a metre of water; this determines the lowest clock frequency, which should preferably be greater than or equal to 1.5khz. The maxiumum operating depth can be calculated from the clock frequency and the number of bits in the counter. The maximum operating depth for the counter should preferably be equal to or greater than that corresponding to the period of the longest pulse repetition period, e.g. for a longest period of 3 seconds, there will be a maximum depth of 2250 m. In this example 12 bits would be required for the counter. In order to extract the third bottom echo component, the circuit of Figure 11 can be modified to operate its described count sequence again to define the expected time of arrival of the third bottom echo. The maximum count reached will be stored and either the counter will be reset to recount down from that value or will be configured to count up to that value. The embodiments disclosed above use logic components hard-wired to achieve the desired functions with switches where necessary to change the mode of operation. Alternative circuitry could be used to achieve the same and similar effects. Microprocessor circuitry plus software may also be used to implement at least some of the described functions. For example, waveforms may be digitised, stored in memory and analysed by software, e.g. to locate and identify the various echoes, especially the bottom echo components and their leading edges. References herein to time slots may in some cases, especially with regard to the leading edges of bottom echo components, be considered to include memory address ranges. Such ranges correspond to time slots and may be identified by analysing the values of, and changes in values of, memory locations rather than by real time processing relative to bottom pulses and transmitted pulses.

Claims: A circuit for use in detecting the nature of the seabed from the return echo signal of hydro-acoustic transmitter receiver equipment (S) and comprising means (28) for defining a time slot to extract from the return echo signal (A) a first component which is a portion of the first bottom echo component (E1), means (58,59,60) for defining a time slot to extract from the return echo signal a second component (E2) which is the second, or later, bottom echo component, and means (55) for providing a first signal from the extracted first component, characterised by means (C1′) for integrating the extracted second component to provide a second signal giving an indication of the energy content of the second component, and means (62,63,64,65) for providing a display representing the individual values of the first and second signals. A circuit according to claim 1, wherein the means for providing the first signal comprises integrating means (C1). A circuit according to claim 1 or 2, wherein the means for providing a display are arranged to be operable to display a value which is an additive function of the values of the first and second signals. A circuit according to claim 1, 2 or 3, wherein the second component is the second bottom echo component (E2). A circuit according to any one of the preceding claims for use with equipment wherein pulses are transmitted repetitively so that cycles of return echo signals are received, wherein there are means (C2,R2,C2′,R2′) for holding said first and second signals produced in one cycle for display in a subsequent cycle when, in that subsequent cycle, said time slot is not definable in relation to the return echo pulse owing to poor signal reception. A circuit according to claim 5, and comprising sample-and-hold means provided to hold said first and second signals and including switching means (35,35′) responsive to a return echo component having an amplitude below a given level to prevent updating of the output of the sample-and-hold means. A circuit according to any one of the preceding claims, wherein to define a time slot to extract the second bottom component, there is provided a digital counter (83) to count in one direction from an initial transmission until a first bottom echo component (E1) is received and then count in the other direction substantially to define the expected time of occurrence of the second bottom echo component (E2).

Inventors: BURNS DAVID, BURNS, DAVID
Applicants: SALUBRE INVESTMENTS, SALUBRE INVESTMENTS LIMITED
Application year: 1985
Main classification: G01S7
IPC codes: G01S15, G01S7