GB2094010A - A detector for periodic signal bursts - Google Patents

Abstract

A video scan signal (20) for a SONAR monitor display, is compared with a reference level by a comparator 22 and an accumulator 24 incremented if the reference level is exceeded. A count relating to each sample of each scan is accumulated and stored in a shift register (28). When a predetermined number of consecutive such samples exceeds a predetermined number via a comparator (31) an indication of correlated burst reception is provided (40) for onward transmission to means for providing an output when burst reception is periodic. In this way periodically transmitting objects can be detected in scan signals which would otherwise be obscured in a monitor display, and an indication of presence and bearing provided. <IMAGE>

Description

SPECIFICATION Improvements in or relating to scanners This invention relates to signal detectors, and in particular to periodic burst detectors for scan signals such as signals for the display of a representation of a scanned region.

Scan signal displays (for example SONAR and RADAR displays) may be used for the location of objects within a scanned region. For example SONAR may be used for the location of objects lost on the sea bed. Where signal returns from a transmitter at the scanner are likely to be inadequate for location an alternative arrangement in which a low power transmitter is attached to an object which is to be subsequently located may be employed. In this arrangement the scanner is used passively to perform a scan of the region which the display is observed for a representation of the transmitter signal.

In order to provide long life at an adequate power level to maximise the chance of locating a lost object the transmitter may transmit a signal burst periodically. Unfortunately such signals are particularly difficultto observe on a display, requiring great experience and concentration on the part of the observer.

The task is particularly onerous since the desired signal may be accompanied by many unwanted signals which will add to system noise to produce a confused display which may mask the wanted signals. Observation may not be possible if some signal bursts are not received due to noise or screening, making the picture incomplete. Thus even with an experienced and conscientious observer, the presence of wanted signals may be missed and an object remain lost despite having passed through the scanned region.

According to the present invention a periodic burst detector for a scan signal includes means for sampling scan signal elements during each scan, means for comparing samples of each element with a reference level to indicate burst reception whenever a predetermined number of consecutive samples of the same element exceed the reference level, and means for determining the time between indications to provide an output if burst reception is periodic.

Preferably the scan signal is continuously compared with the reference level by a comparator, which is arranged to control an accumulator which is incremented during each signal element if that element exceeds the reference level, the accumulated value being stored as the basis for the next count relating to that element. A burst reception indication may be provided whenever the accumulated value exceeds the predetermined number. To avoid false indications the accumulator is preferably decremented whenever a scan signal element is below the reference level.

In a preferred embodiment of the present invention the accumulator is provided by a counter and the storage of accumulated values relating to each element by a shift register. During sampling of a scan signal element the accumulated value relating to that element is loaded to the counter which is incremented or decremented as hereinbefore described, the new value being returned to the shift register for subsequent loading when that element is next sampled.

Preferably the means for determining the time between indications includes a processor operating under program control in accordance with an algorithm arranged to determined the time between all indications. Where indications occur periodically a detection output is provided.

A periodic burst signal detector in accordance with the present invention may advantageously be arranged to receive a scan signal from a monitor providing a display of the scan signal for observation. It will be appreciated that in this case each element of the scan signal represents a different bear- ing of the scan representation. The detection output is preferably arranged to provide a signal to an observer that a periodic burst signal has been detected together with a display indicating the element in which the periodic burst has been detected, so that attention may be directed to the relevant bearing.In order that features and advantages of the present invention may be fully appreciated an embodiment will now be described with reference to the accompanying diagrammatic drawings of which: Fig 1 represents signal wave-forms of: a. a submerged ultrasonic beacon and b. a scanning sonar output Fig 2 represents a signal discriminator in accordance with present invention Fig 3 shows a flow chart representing processor operation.

A conventional ultrasonic beacon is assumed attached to a submerged object which is to be located. The beacon is pulsed and transmits a high frequency signal burst 10 (Fig 1 (a)) periodically. A conventional SONAR receiver is mounted on the searching vessel. The receiver has a plurality of sensing heads which form a phased array. The response of the receiver is made directional by altering the relative phase of incoming signals from each element and the resultant beam pattern may be scanned over a sector by sequentially incrementing the relative phase between sector limits, in accordance with known SONAR principles. The sector may be scanned periodically in the same direction.

The SONAR receiver output is a scan signal 12 on which an output pulse 11 occurs whenever the beam traverses the ultrasonic beacon during transmission.

For a scan rate exceeding signal burst duration pulses will be produced at the same relative position during a scan period S on consecutive scans, and the pattern repeated at the burst repetition period, provided the ultrasonic beacon remains within the SONAR receiver range limits.

Where the scan is completed in Ss and the signal burst duration is Bs, repeated every Ps then an output pulse 11 would occur on N consecutive scans The drawing(s) originally filed were informal and the print here reproduced is taken from a later filed formal copy.

and the pattern being repeated every I scans where P S = ; and I = S In accordance with known SONAR principles the scan signal 12 may be used as a video input signal to a raster scanning monitor scanned in synchronism with the receiver beam pattern to provide a visual representation ofthe received signal. As previously described this representation may be observed in an attempt to identify a pattern of N pulses displayed at the same position on consecutive scans, and repeated every I scans but presents a formidable task.

Apart from the effort of concentration the attempt is likely to be hampered by the presence of system noise on the scan signal, in particular a. uncorrelated noise such as circuit noise and received acoustic noise acting to clutter the display b. correlated noise which is synchronized to the scan and likely to lead to false detection c. random electrical interference such as voltage spikes acting to confuse the display.

In accordance with the present invention the scan signal is received at an input 20 (Fig 2). The signal is buffered by a unity gain amplifier 21 and compared to a reference level by a comparator 22 to provide a two level signal on line 23. It will be realized that noise below the threshold level of comparator 22 is rejected. The signal on line 23 passes to a sampling logic circuit 24 wherein an element of the signal is sampled. If the signal element is above the threshold a pulse-received signal is produced at output 25 causing a counter 26 to increment by one. If the signal element is below the threshold (no pulse) then the control signal at output 25 causes counter 26 to decrement by one. This procedure is repeated n times per scan, so that the scan signal is divided into n elements each scan cycle.After each sample has been taken the contents of the counter which is represented by a binary word present at parallel output 27 is loaded into an n stage shift register 28. This word is rippled through the shift register 28 after each scan signal sample is taken and appears at a parallel output 29 of shift register 28 after n samples have been taken. The current binary word at output 29 is loaded to the counter 26 before each sample is taken. This binary word represents information about previous samples of the element about to be sampled. It an element is above the threshold for K consecutive scans, then the counter 26 will incre mentto a maximum value of K.A zero detector 30 inhibits the control signal produced by sampling logic 24 if zero is loaded to the counter 26 so that an erroneous count is not recorded by an attempt to decrement the counter from zero.

If the contents of the counter reaches N then it is likely that an ultrasonic beacon burst is present. The counter output is examined by a comparator 31 with respect to a reference input B, which is set to a binary word representing N on input line 32, to pro duce an output on line 33 if a count of N is reached. If correlated noise is present this will also occur at the same point in consecutive scan and could lead to a count of N being reached falsely. Such correlated noise is rejected by loading the output of the comparatorto a latch 34. If N has been reached, an out put will be present at latch 34. This output is inhibited by gate 35 unless the count subsequently falls below N to enable the gate via an output on line 36.If the count of N were falsely recorded then the count would not subsequently fall due to the continuing presence of correlated noise. An upper limit is set on K by counter 26 overflowing to zero, whereupon further decrements are prevented by zero detector 30.

Clock signals Z., Z2, Z3 (Fig 1 b) are derived from the line scan signal of the raster scanning display via a shaping circuit 46 and a multiplier 37. Signal Z2, which clocks the sampling logic circuit 24 controls the sampling rate and is adjusted according to the number of samples per scan, n. This clock signal is also fed to a recirculating counter 38 which has n stages to provide a record of the element currently being sampled on output lines 39.

It will now be realized that the features of the embodiment thus far described provide a signal on output line 40 whenever a burst signal is detected, together with a binary number on lines 39 indicating in which sample element the burst has been detected.

In accordance with the present invention means are provided for determining the time interval between outputs on line 40 and to compare these with the burst repetition frequency and indicate the detection of a beacon if a burst is detected every I scans.

This is achieved by interfacing output lines 40 and 39 to a processor. The output on line 40 is used to interrupt processor operation and the processor arranged to accumulate the times between interrupts. The processor then searches for an interval of I between any two interrupts, and then successive intervals of I, as determined by reading the binary word on output lines 39. If such successive intervals are present, the processor provides an indication that a beacon has been detected. This process is not restricted to the same element to take account of any long term movement, such as drift.

In operation the indication may be used to indicate the presence of a beacon to an observer, who then has his attention to directed to the conventional SONAR display for observation in the normal way. In this case to provide additional assistance to an observer, the processor is arranged as follows.

The interval between two interrupts is determined and accepted if it lies within the range I t x%, where x may be chosen according to the type of beacon involved and search being carried out. When such an interval is identified the interrupt intervals are further determined and examined for consecutive intervals substantially the same as that first detected. If three such intervals are present the processor provides an indication that the detection of a beacon is 'POSSIBLE'. If four such intervals are present a 'PROBABLE' indication is provided. Once detection is 'PROBABLE' then alternate bursts may be missed, as it is unlikely that all bursts from an ultrasonic beacon will be received due, for example, to the effect of ship roll.The processor is arranged to calculate an approximate bearing from the sample element number on lines 39 and to provide a display of bearing whenever detection is 'POSSIBLE' or 'PROBABLE'.

The indications are provided by lighting one of three light-emitting-diodes (leds) labelled 'NOTHING FOUND', 'POSSIBLE' and 'PROBABLE' respectively under processor control as appropriate. In the case of 'POSSIBLE' or 'PROBABLE' and audible warning is also provided. The bearing indication is provided by lighting one of a number of leds arranged to indicate 5 steps. When no signal is present on line 39, or a 'POSSIBLE' or'PROBABLE' detection has been lost, the 'NOTHING FOUND' led is lit, and the bearing display extinguished.

The processor is arranged to operate under program control in accordance with a predetermined program, which may be represented in the form of a flow chart (Fig. 3). The scope of the program may advantageously be extended to include setting the reference of com parator 31 to control the number of pulses required to establish burst detection and to setting the gain at level control circuitry 40 to control sensitivity, which may be increased if no signals are detected. The comparator reference value may be adjusted once a burst has been detected to increase confidence that a beacon has been reliably detected.

The value of N at input B of comparator 31 may also be set under program control. N may be set to take account of the likelihood of missing pulses, or of counts due to noise in the prevailing conditions.

An embodiment of the present invention has been constructed for use with ultrasonic beacons transmitting a 270 KHz signal for 3 ms at intervals of 500 ms. The SONAR receiver used employed a 30 sensing element array capable of scanning a 30 sector in 130,as, thus N = 16 and I = 3800. It was found that good performance was achieved by setting n = 40 (ie having a 40 stage shift register). A microcomputer was employed as a processor, the signal was received over a 10 sector, making the pulse duration 4 ws, which is too short for present microconputers operating in interrupt mode.However with other SONAR sets it may be possible to configure circuitry analogous to that of Fig 1 within a processor under program control, and such a signal discriminator is within the scope of the present invention. Alternatively the signal present indication could be provided by circuitry operating in the same way as hereinbefore described having the signal on line 40 as its input.

It will be appreciated by those skilled in the art that the present invention may be applied to many different regimes of SONAR apparatus. For example ability to ignore missing signals after a 'PROBABLE' indication will overcome the problem of line blanking during transmission in 'SONAR' sets which can be used actively. 'SONAR' sets which alternately scan to and fro may be considered to produce one long scan covering each sector twice by changing the direction of count of position counter 38 concurrently with the change of scan direction.

Claims (15)

1. A periodic burst detector for a scan signal including means for sampling scan signal elements during each scan, means for comparing samples of each element with a reference level to indicate burst reception whenever a predetermined number of consecutive samples of the same element exceed the reference level, and means for determining the time between indications to provide an output when burst reception is periodic.

2. A periodic burst detector as claimed in claim 1 and wherein an accumulator is incremented each time a signal element exceeds the reference level, the accumulated value being stored as a basis for the next count relating to that element and forcompari- son with the predetermined number.

3. A periodic burst detector as claimed in claim 2 and wherein an indication is provided when the accumulated value exceeds the predetermined number.

4. A periodic burst detector as claimed in claim 2 and wherein the accumulator is decremented each time a signal element is below the reference level.

5. A periodic burst detector as claimed in claim 2 or claim 3 and wherein the stored value is loaded to the accumulator when the element to which it relates is sampled.

6. A periodic burst detector as claimed in any preceding claim and wherein the means for determining the time between indications includes a processor operating under program control in accordance with an algorithm arranged to determine the time between all indications.

7. A periodic burst detector as claimed in claim 6 and wherein an indication provides an interrupt to processor operation.

8. A periodic burst detector as claimed in any preceding claim and arranged to receive a scan signal from a monitor providing a display of the scan signal in which each element of the scan signal represents a different bearing an including means for providing a display indicating the element in which a periodic burst has been detected.

9. A periodic burst detector as claimed in any preceding claim in which a first output is provided when a first number of periodic indications is detected, and a second output when a greater number of periodic indications is detected.

10. A periodic burst detector as claimed in claim 9 and wherein, when the second output is provided, alternate indications may be missed.

11. A periodic burst detector as claimed in any preceding claim and wherein the reference level may be externally set.

12. A periodic burst detector as claimed in any preceding claim and wherein the predetermined number of consecutive samples required for an indication may be externally set.

13. A periodic burst detector substantially as hereindescribed with reference to the accompanying drawings.

14. A location system for the location of an ultrasonic beacon including a periodic burst detector as claimed in any preceding claim.

15. A location system as claimed in claim 14 including a SONAR receiver.