SYSTEM FOR LOCATING A SOUND SOURCE IN A WATER AREA

Abstract: According to the system for monitoring a water area with regards to presence and localization of unknown objects, which transmit sound, in particular submarines three or more listening devices (A1, A2, A3) are thrown out in an area, where indication about an unknown object has been obtained. The listening devices are provided with hydrophones and radio transmitters for transmission of received sound by modula­ tion on a carrier. The devices cooperate with stationary radio stations (M1, M2, M3), where the sound waves are demod­ ulated. Time correlation between the sound waves from an object received by three selected listening devices gives information about the position of the object relative to these selected listening devices. If the position of the listening devices is unknown, for example if the devices are floating free, this position can be determined by comparing the mutual time position for a particular radio signal transmitted from each listening device when received in three stationary radio stations with known positions. The listening devices may suitably also be provided with receivers for receiving radio control signals, for example relating to order for activa­ tion, selection of radio channel, etc.

Description: A method and a system for monitoring a sea area. The invention relates to a method for locating a sound source in a water area by means of hydrophone listening, wherein at least three listening devices are provided each comprising a hydrophone and a radio transmitter with associated antenna and wherein there is further provided one radio master station to which signals indicative of sounds detected by the listening devices are transmitted and which station determines the geographical position of the sound source by correlating the said signals. A method and system for locating a sound source by means of hydrophone listening, which for example is described in U.S. Letters Patent No. 3,723,960, utilizes three hydrophone transducers connected by a radio link to a master station. The signals received by the master station are correlated in order to identify a common sound source. To the times of arrival of a discrete sound emanated by the sound source are compared for determining the geographical position of the source. A problem with this system is that the coordinates of. the hydrophone transducers must be known to some precision beforehand because, as the position of the sound source is determined with respect to the hydrophone, the precision with which the location of the source can be determined is dependent upon it. The object of the invention is to achieve a monitoring of a sea area for discovery and localization of a sound source within that area, which monitoring shall be possible to be used in each sea and which, as compared with known methods, has an improved "endurance" in the meaning that it can be maintained during an extended time period with limited work of qualified persons and with use of simplified and therefore less expensive listening means. This is achieved by means of a method as described in the opening paragraph, which according to the invention is characterized thereby, that there are provided at least two relay stations at geographically known positions, the method comprising the steps of: - the listening devices transmitting; alternatively, sig nals indicative of the identity of the listening device and of the detected sound of the sound source, said sig nals being received by the master station and the relay stations; - the relay stations relaying the said received signals to the master station; - the master station processing the part of the said re ceived signals indicative of the identity of the listen ing devices to establish the geographical position of said listening devices; ; - the master station determining the geographical posi tion of the sound source with respect to the - in the previous step determined - geographical positions of the said listening devices b correlating the part of the said received signals indicative of the detected sound of the sound source. By use of the invention a monitoring can rapid ly be established in each sea area along the cost with out use of expensive stationary equipments. Upon indi cation of unknown objects within a sea area one or more listening devices are placed within the area, whereafter position determination of the sound source takes place by radio transmission from some selected listening devices, which are close to the unknown ob ject. After placing the listening devices in their po sitions and establishment of required stationary radio stations the monitoring work can in its whole be effect ed at any one of the radio stations or at a special eva luation center. The monitoring in accordance with the principles of the invention therefore can be effected with a high degree of "endurance". When using a large number of listening devices these devices only need to have a limited range, whereby the listening devices fur therefore can be of a simple and cheap construction. In order to get a sea area under observation rapidly the listening devices can be placed in their positions by means of airplanes or helicopters. It is then suitable to let the listening devices assume arbitrary positions not knoçn beforehand, for example floating freely, in which case according to another feature of the invention the position of each listening device is determined by use of the difference of travel time for radio waves to at least three radio stations having known positions. A system for locating a sound source in a water by means of hydrophone listening comprising a plurality of at least three listening devices each provided with a hydrophone and a radio transmitter with associated antenna, the system further comprising a radio master station provided with correlator means for determing the position of a sound source by correlating the signals received from the listening devices according to the invention characterized thereby, that the system comprises at least two relay stations at geographically known positions for relaying the signals received from the listening devices to the master station, that the listening devices comprise switching means for alternatively applying to the transmitter a signal indicative of the identify of the listening device and a signal indicative of the detected sound of the sound source and the master station comprising processing means responsive to the part of the received signal of indicative of the identify of the listening devices to establish the geographical position of said listening devices. It is a further object of the invention to control the listening devices for activation or deactivation of the transmitter part, so that the listening device begins to transmit or interrupts transmission of sound information, respectively. Furthermore the control can be related to order the actual channel to be used for the transmission. Hereby each selected listening device can be allotted its own channel. In a system where the listening devices are placed in arbitrary, not known positions, for example floating freely in the sea area, the system is according to the invention characterized in that the listening devices each comprise a radio receiver, that the master station comprises transmitting means for the transmission of control information to the listening devices and that the listening devices each comprise activation control means for controlling the selective activation of the listening device in accordance with the control information. The control or characteristic signal can be the received sound information but due to its low frequency character this signal is less suitable for localization of the listening devices. In a preferred embodiment the characteristic signal is a special signal, which is transmitted intermittently with constant time intervals during short interruptions in the sound modulated carrier. In order to simultaneously identify the different listening devices the special signal may suitably contain an identification information, so called ID-information, which is individual for each listening device. The invention is illustrated in the accompanying drawings, in which Fig. 1 shows a schematic survev of a monitoring system operating in accordance with the principles of the invention comprising a large number of freely floating listening devices and three stationary radio stations, of which one is master station and the two remaining relay stations, Fig. 2 shows a block diagram for an embodiment of a listening device in a system according to Fig. 1, Fig. 3 shows a simplified diagram of the signal processing part in the master station, which relates to determination of the position of a sound source relative to three selected listening devices, and Fig. 4 shows a simplified diagram of the signal processing part in the master station, which relates to determination of the position of a selected listening device. The monitoring system according to Fig. 1 comprises a number of geographically distributed listening devices Ai, A2 ..... An, together covering a sea area to be monitored as regards presence of possible sound sources. Such a sound source is shown at U. The distance between the listening devices is adapted to the circumstances and may for example in an archipelago type of area amounting to about 5 kilometers. The listening devices are assumed to be supported by buoys floating freely in the water. Each listening device comprises a hydrophone and a radio transmitter-receiver with associated antenna. The listening devices are individualized by a special identification address (ID address). An embodiment of a listening device will be described in connection with Fig. 2. According to Fig. 1 the monitoring system furthermore comprises at least three radio stations M1, M2 and M3, of which Mi is master station and M2 and M3 are relay stations. The radio stations may be stationary equipments but alternatively mobile in the meaning that they can be placed temporarily at different places in connection with a sea area to be monitored. During each measurement the radio stations have fixed and known positions. The master station comprises transmitter/receiver for transmission of control information to the listening devices and for reception of information signals from these devices and from the relay stations. In the master station there is furthermore signal processing means for carrying out certain signal processing in real time. Via a radio link or in another manner the master station can be in communication with an evaluation center EC, where final signal processing and position determination takes place. The relay stations M2, M3 also comprise transmitter/receiver and are adapted to receive certain signals from the listening devices and to re-transmit them to the master station in different radio channels. The position determination is in principle effected in the following manner. It is assumed that only the listening devices A7, A2 and A3, which are in the best position for localization of the sound source U, are active. These listening devices transmit simultaneously and each in its radio channel the detected sound on radio carrier to the master station. In the master station the radio waves are demodulated and the obtained sound waves are correlated two and two. Each correlation results in a time difference, which represents a given hyperbola. The sound source is then situated in the crosspoint between the obtained two hyperbolas (in some cases theoretically to cross-points are obtained, of which, however, one can be excluded). At this position determination the difference between the travel time of the different radio waves is not taken into account, because the travel time for the sound waves in the water is appreciably longer than the travel time of the radio waves. At regular intervals each listening device interrupts its transmission of sound waves and transmits instead its ID-address in a particular radio channel. These ID-addresses are relayed further by the relay stations M2 and M3 in new radio channels. It is then assumed that at a given time moment only one listening device, for example Al, is transmitting its ID-address. The master station then obtains this ID-address directly in one radio channel and the same address via M2 in another channel and via M3 in a third channel. As the stations NI, M2 and M3 have known positions also the travel times between M2 and M1 (= . 1) and between N3 and M1 (= % 2) are known. By correlation the ID-signal obtained directly from Al with that obtained via A12 with compensation for the known travel time ¯ 1 a time difference representing a hyperbola will be obtained. In corresponding manner by correlating the ID-signal obtained directly from Al with that obtained via M3 with compensation for the known travel time ' 2 a time difference representing a second hyperbola is obtained. In the cross-point between these hyperbolae Al is then situated. The listening devices A2 and A3 are localized in the same manner during those intervals when they are transmitting ID-signal. Now, when the position of the sound source U relative to the listening devices Al, A2, A3 and the position of each one of these devices is determined, then full information about the absolute position of the sound source U is available. In order to elucidate the function there follows a table showing the different signals appearing in the system, the given band widths and modulation types being valid for an embodiment described as an example in the following. Signal No. 1. Purpose : To transmit the sound of the object to listening buoys. Travel path: Object to at least three selected buoys. Basic type : Sound waves in water. Spectrum : 10 Hz or less up to 1000 Hz. Modulation : Missing. No carrier. Signal No. 1. Purpose : To enable position determination of the ob ject relative to the selected buoys. Travel path: From the buoys to a radio receiver on land. Basic type : Radio waves. Modulated carrier (for example 300 MHz). Spectrum : Of the order 10 kHz. elodulatloll : Sub-carrier, one for each buoy modulated with signal No. 1 on a main carrier. Siiial No. 3 Purpose : To enable position determination of buoy. Travel path: From actual buoy to at least three radio re ceivers on land. Basic type : Radio waves. Modulated carrier (for example 300 MHz). Spectrum : For example 1-10 MHz. Modulation : Pulse modulated key signal. Note : One of the land stations can be master, the signals of the two other receivers being re transmit ted to this master. Sirmal No. 4. Purpose : Buoy-selective commando; activation and chan nel selection. Travel path: From one land station to a buoy (one at a time). Basic type : Radio waves. Modulated carrier (300 NHz). Spectrum : Dependent on required commando speed; for example 10 kHz in case of reasinable comman do speed. Modulation : Pulse modulated key signal. Signal No. 5. Purpose : Area selective activation. Travel path: From three land stations to all buoys within a region specified by the signal. Basic type : Radio waves. Modulated carrier (for example 300 MHz). Spectrum : Dependent on desired accuracy at the area-, selection; (for example 1-10 MHz). Modulation : Pulse modulation. It is evident from the above table that only the signals Nos. 3 and 5 will have a broad band; all other signals have a small spectrum (= band width). The signal No. 5 relating to area-selective activation of the buoys, i.e. activation of all buoys within a selected area, has been included for completeness sake in the above table but will not be discussed further in this specification. As regards signal No. 5 reference is made to Swedish patent application No. 8301019-9, in which an example of such an area selective activation is described ed. Fig. 2 shows schematically the build-up of an embodiment of a listening device in the shape of a buoy adapted to float free in a sea area to be supervised. The buoy consists of an antenna 10 which is kept floating in the water surface by means of a floating body 10a and a body 11, the connection cable of which has such a length and the specific gravity of which is such that it is floating at a suitable depth below the water surface. At the bottom the buoy is provided with a hydrophone 12 for detecting sound waves or pressure waves in the water. The body 11 contains the electric circuits included in the listening device, i.e. comprising a transmitter/receiver unit 13 connected to the antenna. This unit is in the drawing illustrated as containing a receiver 14 and two separate transmitters 15 and 16. The transmitter 15 is adapted to transfer identification information to the master station and has an input connected to a memory 17 via a switch 18 and a scanner 19. The memory contains an ID-number which is unique for each listening device, which information is detected and converted to serial form by the scanner 19. The transmitter 16 is adapted to transfer detected sound and has an input connected to the hydrophone 12 via a switch 20 and signal converter 21. The switches 18 and 20 are switched in synchronism between a first position I, in which the switch 20 is closed and the switch 18 is open, and a second position II, in which the switch 18 is closed and 90 is open. Activation of the switches is effected from a clock 22 via a drive stage 23. The time control can be such that the switches normally are in the position I and only for a short time with constant intervals are set to the position II, for example assumes the position II during 10 ms each tenth second. The clock can run somewhat differently in the different buoys in order to prevent that several devices are transmitting ID-information simultaneously during any longer time period. The information transfer via the transmitters 15, 16 is effected by modulation of a HF carrier, the transmitter 16 being adapted to transmit in several different channels, while the transmitter 15 has its own channel. The channel used in 16 can be selected bv means of a control signal at a control input 24. In reality it is not necessary to have two separate transmitters but a common transmitter can be used, in which case 16 is represented by a given channel in this transmitter. The channel separation can be aehieved thereby that each channel has its own carrier or its own sub carrier on a common HFcarrier. The receiver 14, which is also connected to the antenna 10, is adapted to receive control information and has an output connected to a register 5 via a detector 26. The control information can be shaped as a pulse train of HF pulses representing a digital message, the register 25 then can be shaped as a shift register to which this message is fed and stored. As an example of how such a message can be composed Fig. 2 shows a register 25 consisting of a number of sections. A first section 27 stores a digit sequence representing the ID-number for the listening device to be controlled. This section of the shift register is connected to a comparison device 31, which at a second input receivers information from the ID-memory 17. A second section 28 stores a digit representing an activation command and a section 29 stores a digit representing a de-activation command. Finally a section 30 stores a number representing the actual channel to be used. The output of the comparison device 31 is connected to one input of an AND-gate 32 having a second input connected to the section 28 of the shift register 25. The output of the comparison device 31 also leads to one input of an AND- gate 33 having a second input connected to the section 29 of the register 25. The output of the AND-gate 32 leads to a set control input of a controllable switch 34 and the output of thr AND- gate 33 leads to the reset control input of the same switch. When signal is obtained from the AND-gate 32 the switch 34 will be closed for connecting a current source 35 to a bloc 36 representing those elements in the transmitter part, which require drive voltage. The transmitter then will be activated. In case of signal from the AND-gate 33 the switch 34 will be opened and the transmitter part is deactivated. Those elements in the receiver part, which require drive voltage, are always connected to the current source and the receiver is continuously activated. The laste section 30 of the shift register 25 is connected to the control input of the transmitter 16 via a converter 37 adapted to convert the channel digit number in the register 25 to a suitable control voltage for the transmitter. The operation is as follows. When a listening device is activated the master station transmits a pulse train, the first part of which reaching the section 27 contains the ID-number of the actual listening device and the second part of which reaching the section 28 represents the activation command and finally a pulse train reaching the section 30 representing the channel to be selected. The transmitter is activated and the sound detected by the hydrophone, if any, is transmitted in the selected channel. Each tenth second the transmission of sound information is interrupted and instead the ID information is transmitted from the memory 17 in the special channel. During this ID transmission position determination of the actual listening device takes place. This continues until the master station again transmits the ID number of the actual device together with a signal which ends-up in the section 29 of the register 25, whereby the transmitter is de-activated. Fig. 3 shows schematically the signal processing part of the receiver of the master station Ml, which is concerned with determination of the position of a sound source relative to three selected listening devices. If the channels are frequency channels, i.e. a special carrier or sub-carrier for each channel, the receiver part consists in principle of a number of bandpass filters each tuned to its frequency, in case of sub-carrier preceded by a filter and mixing stage for suppression of the main carrier, Fig. 3 showing the receiver channels for three selected listening devices represented by the band pass filters Fl1 F2 and F3 with associated detectors D1, D2 and D3. The detectors are each connected to an output 01, 02 and 03, where the low frequency modulation signals in the respective channel appear. At the outputs 01, 02 and 03 thus the same sound information appears but in different time positions dependent on different travel times for sound in the water to the respective listening device (then the differences in travel time for the radio waves in the air is not taken into account). The signal processing is in principle effected such that the sound signals are correlated two and two for determining their mutual time difference. Thus, in fig. 3 the signals at the outputs 01 -and 02 are coupled via a switching network SW to a first correlator K1, while the signals at the outputs 07 and 03 via the switching network SW are led to a second correlator. The correlator KI produces the time difference T1 and the correlator K2 produces the time difference T9. The time differences Tl and T2 each represent a hyperbola and the sound source U is situated in the cross-point between these hyperbolae. Fig. 4 illustrates schematically how the position of the different listening devices can be determined by showing the components in the master station required for localization of the first listening device Al (Fig.1). The localization of the listening devices is as mentioned effected during the short interrupts in the sound transmission, when ID information is transmitted in a special channel. It is assumed that the device Al is the only one to transmit ID information at a certain time moment. This information is transmitted as a pulse train in the given channel, i.e. on a given channel frequency, and is retransmitted by the selected relay stations M2 and M3 in two other channels. In order to separate these signals the master station contains three bandpass filters, a first filter F4 tuned to the channel frequency used by the listening device Al (and also by remaining listening devices) for transmission of its ID information, and two filters F5 and F6 tuned to those channel frequencies, to which the said frequency is converted in the relay station M2 and M3, respectively. The filters F4, F5 and F6 are each connected via its detector D4, D5, D6 to an output 04, 05, 06. Thus, at these outputs one and the same ID information will appear, namely in the regarded example the ID-address for the listening device Al, but in different time positions. The signal processing for determining the position of the device Al is in principle effected such that the ID signals at the said outputs are compared two and two after correction for the known travel times from M2 to M1 and from M3 to Ni, respectively, for determining their mutual time difference when they arrived to the respective station M1, M2 and M3. Thus, the signal at the first output 04 is coupled to a first input on a comparison device J1 via a time delay circuit B1 which delays the signal the known travel tirne L. 1 from M2 to M1, while the signal at 05 is led directly to a second input on the comparison device J1. In corresponding manner the signal at the output 04 is coupled to a first input on a second comparison device J2 via a time delay circuit B2 which delays the signal the known travel time ' 2 from M3 to Ml, while the signal at the output o6 is coupled directly to a second input on J2. The comparison device J1 delivers an output signal repre senting a time difference T3, while J2 delivers a signal representing a time difference T4. The time differences T3 and T4 each represent a hyperbola and A7 is situated in the cross-point between these hyperbolac. The time differences T3 and T4 or possibly the calculated position are stored together with the ID-number of the device Al. At different succeeding time moments the corresponding information then will arrive from A2 and A3, which are localized in the same manner and with the same circuits. Each tenth second the ID-number from A1 arrives again and the stored information about the position of the device is continuously updated. The operation of the whole system is as follows. After placing the listening devices in their positions in a sea area containing a possible sound source and establishment of the stationary stations M7, M2 and A3, a general information collecting phase is first started aiming at to determine if there are reliable indications of a sound source within the supervised area and, if so, its approximate position. This information collecting phase may for example involve that the listening devices are called and activated in successive order. As soon as a listening device is activated it will transmit the detected sound and its ID-number, whereby the position of the station can be determined. The ID-number, position and a coarse information about detected sound is stored. o, if it turns out that a number of adjacent listening devices detect a sound exceeding a certain level and of similar character only these listening devices are activated and measuring of the position of the sound source is effected in described manner. In order to optimize the information transmission in relation to required band width the sound signals can be digitized and channel separation be achieved by means of time multiples. Fig. 5 shows a simplified block diagram for a listening device operating according to this principle. The listening devices (buoys) are assumed to co-operate with a number of stationary radio stations one of which, the master, comprising an accurate clock and transmitting synchronization or adjusting pulse to all buoys and activation control order to certain selected buoys. Each buoy has a receiver 100 and a transmitter 101. The receiver 100 is connected to an antenna 104 via two switches 102, 103 normally assuming the shown position. The transmitter 101 can be connected briefly to the antenna 104 via the same switches. To the output of the receiver 100 are connected three detectors, an ID-detector 105, a channel detector 106 and a synchronization detector 107. The synchronization detector 107 controls a clock 108, the output signal of which in combination with the output signal from the channel detector 106 being adapted to control a channel selection circuit 109. A memory 110 is continuously loaded with samples of the sound signal detected by a hydrophone 111, which samples are generated by means of an amplifier 112 and an A/D converter 113. The information in the memory 110 is led via a reading device 114 t the input of the transmitter 101. The memory 110 is zeroed by means of the signal from the synchronization detector 107, while the reading device 114 is activated from the channel selection circuit 109. This circuit also controls the switch 103, while the switch 102 is controlled by means of signal from the ID-detector 105. The operation is as follows. With regular time intervals, for example of the magnitude of 0,1 - 10 seconds, synchronization or adjusting pulse will -arrive for zeroing the clocks in all buoys. These clocks therefore can have moderste accuracy and be of cheap construction. With the assumed estended time intervals the difference in the travel time for the synchronization pulse to different buoys will ne negligable and the clocks can be assumed to run in synchronism. At the same time the memory is zeroed, which memory has suc a capacity that it can store all samples representing the sound signal in the whole time interval between two sync pulses. This time interval between two sync pulses is divided into a number of sub-intervals, for example ten, each representing a channel and for example having a length of 0,01 - 1 seconds. To each sub-interval is associated a channel number 1, 2 . . . 10. When the master wants a certain buoy to transmit its detected sound it first transmits the ID-number of the buoy immediately followed by a signal representing order about activation. The ID detector 105 in the actual buoy reacts and sets the switch 102 to the opposite position as compared with the shown one. The buoy is now activated for transmission. Immediately thereafter the master transmits a pulse se pence representing the channel number to allot to the buoy. This pulse sequence is detected by the channel detector 106 controlling the channel selection circuit 109 so that this circuit 109 delivers a signal a number of subintervals after a sync pulse, which number is equal to the channel number. The signal from the circuit 109 sets the stitch 103 to the opposite position as compared with the shown one and initiates a reading device 114. When the reading device 114 receives a control signal the me mory 110 is scanned with high speed and the sampled sound signal is transmitted in compressed form via the antenna 104 by modulation on a radio carrier in the actual time channel. The same thing is then repeated for another buoy in another channel. In the master station and the remaining stationary stations the relationship channel/ID-number is stored. Therefore the stored sequencies representing the sound signals detected by the different buoys can be shifted in time th account being taken of the time positions of the different channels and correlation being effected in the manner described previously. If desired further shifting can be effected for making correction of time errors arising due to the different travel time of the sync pulse to different buoys. Instead of selective individual activation, as in the described embodiments, it is alternatively possible to activate the listening devices group-wise, for example in dependence on their geographical position, as described in the Swedish patent application no. 8301019-9. If stationary listening devices are present in vicinity of the area to be monitored it is also possible to use these devices in combination with one or several of the free-floating buoys provided with radio transmitters.

Claims: 1. A method for locating a sound source in a water area by means of hydrophone listening, wherein at least three listening devices are provided each comprising a hydrophone and a radio transmitter with associated antenna and wherein there is further provided one radio master station to which signals indicative of sounds detected by the listening devices are transmitted and which station determines the geographical position of the sound source by correlating the said signals, characterized in that there are provided at least two relay stations at geographically known positions, the method comprising the steps of: : - the listening devices transmitting, alternatively, sig nals indicative of the identity of the listening device and of the detected sound of the sound source, said sig nals being received by the master station and the relay stations - the relay stations relaying the said received signals to the master station; - the master station processing the part of the said re ceived signals indicative of the identity of the listen ing devices to establish the geographical position of said listening devices; - the master station determining the geographical posi tion of the sound source with respect to the - in the previous step determined - geographical positions of the said listening devices by correlating the part of the said received signals indicative of the detected sound of the sound source. 2. A method for locating a sound source as claimed in Claim 1, characterized in that the listening devices each comprise a radio receiver that the radio station transmits control information to the listening devices, said control information, upon reception by the listening device, being indicative of the selective activation of the transmitter of that listening device. 3. A system for locating a sound source in a water area by means of hydrophone listening comprising a plurality of at least three listening devices each provided with a hydrophone and a radio transmitter with associated antenna, the system further comprising a radio master station provided with correlator means for determining the position of a sound source by correlating the signals received from the listening devices characterized in that the system comprises at least two relay stations at geographically known positions for relaying the signals received from the listening devices to the master station, that the listening devices comprise switching means for alternatively applying to the transmitter a signal indicative of the identify of the listening device and a signal indicative of the detected sound of the sound source and the master station comprising processing means responsive to the part of the received signal of indicative of the identify of the listening devices to establish the geographical position of said listening devices. 4. A system for locating a sound source in a water area by means of hydrophone listening as claimed in Claim 3, characterized in that the listening devices each comprise a radio receiver, that the master station comprises transmitting means for the transmission of control information to the listening devices and that the listening devices each comprise activation control means for controlling the selective activation of the listening device in accordance with the control information.

Inventors: GUSTAFSON, BENGT GUNNAR
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN, PHILIPS NORDEN AB
Application year: 1984
Main classification: G01S5
IPC codes: G01S5