UNDERWATER COMMUNICATION

Abstract: In an underwater digital communication system using acoustic signals, e.g. between a transducer station 12 and an underwater weapon system 10, successive digits of a message in serial form are sent using, for each digit, a selected sonic or ultrasonic frequency which differs from the frequency used for the preceding digit. Preferably three or four channels, the frequency selected for each digit being chosen according to whether the digit is a 0 or 1 and also differing from the frequency used for the preceding digit. By this technique, checking may readily be carried out to determine possible errors due to refraction effects in prop­ agation through the water.

Description: "UNDERWATER COMMUNICATION" This invention relates to underwater communication system. Acoustic waves of sonic or ultrasonic frequency can be transmitted through water for long distances. Refraction effects occur due to changes in the properties of the water for example due to temperature variation and, as a result of this, it commonly occurs that transmission from one point to another may occur over two or more different paths. If these paths have different lengths, or different velocities of propagation, then there is a time delay between the arrival of the signals over the different paths. The present invention is concerned more particularly with a digital communication system. With a simple train of digital signals, where the presence and absence of a signal indicates 1 and 0 or vice versa, it is necessary at the receiving end to determine both the presence and the absence of a transmitted pulse. Errors can occur due to pulses being repeated by delayed transmission over a second path. Refraction effects may also cause failure to receive a digital pulse. According to one aspect of the present invention, in a method of transmitting digital data through water as acoustic signals, successive digits of a message in serial form are sent using, for each digit, a selected sonic or ultrasonic frequency which differs from the frequency used for the preceding or succeeding digit. This method permits of the loss of a transmitted signal or duplication of reception of a signal to be detected at a distant receiver. In one form o f the invention, a method of transmitting digital data through water as acoustic signals comprises the steps of separating and feeding successive digits of a serial input at the transmitter each into a selected one of either three or four channels such that each digit is fed into a different channel from its preceding digit and such that, after a digit has been fed into one channel, the next successive digit is fed into one or other of two further channels separate from the channel into which the preceding digit is fed, the further channel being selected in accordance with whether the digit is a 1 or 0 or whether the digit is the same or different from the preceding digit, and then transmitting the signals in the separate channels as acoustic pulses on different sonic or ultrasonic frequencies, and, at the receiving end, receiving the signals of the different sonic or ultrasonic frequency to obtain the digital pulses. At the receiver, the pulses received in succession from the various different channels would usually be recombined into serial form. The successive digits should appear on different channel outputs when the signals are received and a logic system may be provided to check this so enabling the loss of an input signal or the duplication of reception of a single signal to be detected. In one method, at the transmitting station, the successive digits of information are separated and fed into three channels such that successive digits are fed into different channels and such that, where two successive digits are the same, the last digit of said two equal digits is fed into a channel different from those for the previous two digits. This arrangement requires only three sonic or ultrasonic transmission frequencies through the water. Preferably however a four-channel system is employed with the l's and 0's of odd digits fed into one or other of a first pair of channels according to whether the digit to be recorded is a 1 or a 0, and with the 1's or 0's of even digits fed into one or other of a second pair of channels according to whether the digit to be recorded is a 1 or 0. Such an arrangement requires the use of four sonic or ultrasonic transmission frequencies. Two particular difficulties in communicating in an ocean environment are:1. Low signal-to-noise ratio of a received signal due to the high ambient noise generated at the signal frequencies. This ambient noise is principally caused by water motion, marine life, passing ships and other man-made noise sources. 2. The echo component caused by the signal bouncing off reflective components on the sea bed and surface reverberation. The non-synchronised nature of 4 Signature Encoding helps in both these two cases; integrating to the fullest extent, the signal can be recognised against the ambient noise. The data-rate can be slowed down to accommodate the echo component. Should the echo component be so severe that any one signal completely merges with the signal following, causing a completely confused signalling situation, the signalling can be made to be discontinuous so that reverberating signals can die before the reception of the next transmitted bit. For a system to work in this manner, the reception device must be free of any time constraint; a very high integrity of error-detection becomes significant and the use of "ARQ" is important. The invention furthermore includes within its sccpe apparatus for transmitting digital data through water from a transmitting station to a receiving station wherein the transmitter includes means for separating and feeding digits of input serial form each into a selected one of three or four channels, such that each digit is fed into a different channel from its preceding digit and suth that, after a digit has been fed into one channel, the next successive digit is fed into one or other of two further channels separate from the channel into which the preceding digit is fed, the further channel being selected in accordance with whether the digit is a 1 or C or whether the digit is the same or different from the preceding digit, means for radiating the digits from the various channels as pulses of different sonic or ultrasonic frequencies for the various channels and, at a receiving station means for receiving the sonic or ultrasonic frequencies on the various channels and demodulating the received signals to obtain digital pulses. In receiving the signal information, error detection means may be provided for checking that digits are received from the appropriate channels. For example in a fourchannel system one digit would be received from either the first or second channel and the next digit from either the third or fourth channel. It is readily possible to provide simple logical checks on the accuracy of the information in this way. Such checking system may be arranged, after receiving a pulse on one channel, to disregard any further pulses on that channel until the appropriate reception has occurred on other channels. It is thus readily possible to correct for the effects of transmission over multiple paths for a one-way or a two-way communication link between moving and/or fixed locations under water. In many circumstances, it is convenient to have part of a communication link utilising acoustic waves in water and part of the link making use of radio signals; in this case a transducer device, carried on a buoy or other floating or fixed object or on a dunking buoy, may be provided to receive radio signals and to convert them into appropriate acoustic signals or vice versa. It will be appreciated that the encoding of digital data into three or four channels for transmission using acoustic waves or the combination of signals received on three or four different frequency acoustic waves into serial form might be effected at the location of the transducer or may in some cases more conveniently be effected elsewhere such as for example at the remote end of the radio portion of the link. The radio-to-sonar or sonar-to-radio link may comprise a simple modulation translator e.g. radio-to-sonar or sonar-to-radio translator. Because of the differing data rates possible for the sonar and radio links, it may be preferred however to employ a data store and interface for forwarding the data. In any of these arrangements, the use of the three or four channels as described above for acoustic transmission through water enables a very great improvement to be obtained in the underwater communication despite instabilities in the transmission path, the occurrence of multiple transmission paths and interference due to transients. The data is preferably transmitted in blocks of predetermined length, i.e. having a predetermined number of possible digits. Block or inter-block markers may be employed to bracket the data stream or streams. This enables a receiver, by checking these markers, to recognise message blocks and to ignore noise when no data is being transmitted. The following is a description of one embodiment of the invention, reference being made to the accompanying drawings in which: Figure 1 is a diagram illustrating a communication link between a control station and an underwater weapon; Figure 2 illustrates part of an acoustic transmitting system for transmitting serial digital data by acoustic waves through water; and Figure 3 illustrates part of a receiving system for receiving signals transmitted by the transmitter of Figure 2. Referring to Figure 1 there is shown diagrammatically an underwater weapon 10 which might typically be located on or buried within the sea bed and which is controlled via a communication link including an underwater path 11 between the weapon and a transducer station 12 typically located on a buoy. From the transducer station 12 communication is effected over a radio link between that station and a control station 14 which may be a fixed land based station or may be a mobile station for example a ship or aircraft. The communication link may be a two-way link or a one-way link depending on the requirements of the weapon system. The present invention is concerned more particularly with the underwater link 11 but it will be appreciated that certain of the coding or decoding operations might be effected at the control station 14 rather than at the transducer station 12 and, in this case, the appropriate signals would have to be transmitted over the radio link as well as over the acoustic link through the sea. In the following description referring to Figures 2 and 3, it will be assumed however that digital data in serial form at the transmitting station is to be transmitted over an acoustic link to a receiving station and there converted back into digital data in serial form. Referring to Figure 2, the input data on a line 20 is fed to a first switching device 21 which operates to feed alternate units of the digital information, as short duration pulses, to two sensing units 22, 23 so that odd digits (i.e. the first, third, fifth etc.) in the serial input are fed to the sensing unit 22 whilst even digits (i.e. the second, fourth etc.) are fed to the sensing unit 23. The sensing units 22, 23 are responsive to the digital values of the input digit and the sensing unit 22 feeds 0's as pulses to modulate a first acoustic signal generator 24 generating signals of sonic or ultrasonic frequency suitable for transmission along the underwater path 11. The sensing unit 22 feeds 1's as pulses to modulate a second acoustic signal generator 25 operating under different frequency. Likewise the sensing unit 23 feeds 0's as pulses to modulate a third acoustic signal generator 26 and l's as pulses to modulate a fourth acoustic signal generator 27. The four signal generators 24 to 27 thus provide respectively signals of four different frequencies which are fed to transducers 28 to 31 respectively to radiate the pulse modulated acoustic waves through the water. In the simple form of apparatus described above, the transmitter has four separate channels and these can be tuned as appropriate to the various different frequencies. It will be appreciated that, by using switchably tuned devices, it would become possible to use common components for the four channels. At the receiving station shown in Figure 3 the various pulses on different acoustic frequencies will arrive in sequence and are received by a receiver and demodulating unit 50 which provides output pulses in sequence which are fed to a staticiser 51. This may conveniently be a register of the kind described in U.K.Patent Specification No.852873. The output from this staticiser is fed to an operating device or data responsive means 52 making use of the received digital data. The output from the staticiser 51 is fed also to a checking unit 54. In one form of checking unit, the output pulses corresponding to the first and second channels, i.e. the odd digits, are fed together to a counter which counter has, after each digit, to be reset before it can count the next digit. The resetting signal is obtained from the digits on the third and fourth channels. It will thus be seen that for example if data is transmitted in groups of a predetermined number of digits, the counter will only count the required number of digits if every odd digit is received and if an even digit is received after each odd digit. This checking thus gives a very high degree of protection against spurious pulses which might arise in the acoustic transmission path and against any momentary or prolonged failure to receive signals, e.g. due to instability of the transmission path. It also provides protection against errors due to a single pulse being received two or more times due to multiple path transmission. In an acoustic transmission system, this possibility of multiple path transmission is of particular significance. The output of the logic unit 54 may be fed hack to the staticiser 51. In a simple arrangement, provision is made so that, on receipt of a signal on the first or second channel, input to the staticiser from either of those channels is inhibited until a signal is received on the third or fourth channel and vice versa. For logic checking used where the information is transmitted in groups containing a predetermined number of bits of digital information, the output from the logic unit 54 may be fed to the record or display unit 53, as shotm at 56, to inhibit the display or recording of incorrect information or to identify failure to receive correct information. In the foregoing, the digital data has been transmitted using a four channel system. It is alternatively possible to encode the information for use on a three-channel system. One convenient way of so encoding the information operates according to the following rule. If two successive data bits are the same, then the last bit of said two bits is fed into a different channel from the previous two bits and successive bits are fed into different channels. Thus, if the channels are A, B and C, if the first bit is a 1 it is fed for example into channel A. If the second bit is a 0 it is fed to channel B. Hence alternating 1's and 0's are encoded ABABAB..... If two successive bits have the same value the last of these two successive bits is fed to channel C. Thus 10100 becomes ABABC. The C channel has now become the '0' channel so 10100101 becomes ABABCACA. Similarly if the number 101001011 is encoded, the output of the three-channel encoder will be ABABCACAB, the B channel now becoming the 1 channel because of the successive l's. However, if 1010010111 is encoded the output of the encoder is ABABCACABC rather than ABABCACABA as might have been expected because when two successive equal value bits occur the last of said bits must be fed into a different channel to the channels into which the two preceding bits were fed. As previously mentioned, it is convenient to employ block or inter-block markers to enable the receiving apparatus to bracket a message block in a data stream or data streams. This enables the receiving apparatus to ignore noise when no data is being transmitted. In an encoding method which is independent of time, one can wait forever if there is a break in transmission. The receiving technique is enhanced if the period of the message is known without taking account of time. This can be readily achieved by a unique signal defining the start of the message and a unique signal definition of the end of message. By cascading different messages with this unique signal between each, an independent integrity check can be made on each message separately. This unique signal can be made, for example, from the components of the four frequency used to encode the message as described above by using a combination such that it cannot be mistaken as part of the message itself, and the message component cannot in turn be mistaken for the beginning or end of block marker. This is achieved as follows: In the arrangement described above, there are four combinations of data, which if present during the message period would generate errors. These are: An odd "O" followed by an odd "1" An odd "l" followed by an odd "O" An even "0" fo llowed by an even "1" An even 1 followed by an even "0" These combinations of signals can be used for interblock markers in the following way. The beginning of the message can be defined as even zero followed by an even one. The message then follows starting with its odd component for example an odd zero and ending with an even component for example an even one. The end of message can be defined as an odd zero followed by an odd one. If part of the message start marker is missing then no part of the message can reproduce or be mistaken for the start sequence and even if it did the integrity check placed on the number of characters known to be in the message would sh ow the message to be in error. Likewise if part of the message due to noise were to reproduce the end of message marker or if part of the message is missing before the end of block marker is detected then there will be an error. Thus a block marker cannot be mistaken for part of the message and no part of message mistaken as a block marker, with the remotest possibility of a mistake occurring due to malfunction of the detection equipment, it would not matter as it would have the effect of producing an error in the error detection logic.

Claims: CLAIMS: 1. A method of transmitting digital data through water as acoustic signals, wherein successive digits of a message in serial form are sent using, for each digit, a selected sonic or ultrasonic frequency which differs from the frequency used for the preceding or succeeding digit. 2. A method of transmitting digital data through water as acoustic signals comprising the steps of separating and feeding successive digits of a serial input, at the transmitter, each into a selected one of either three or four channels such that each digit is fed into a different channel from its preceding digit and such that, after a digit has been fed into one channel, the next successive digit is fed into one or other of two further channels separate from the channel into which the preceding digit is fed, the further channel being selected in accordance with whether the digit is a 1 or 0 or whether the digit is the same or different from the preceding digit, and then transmitting the signals in the separate channels as acoustic pulses on different sonic or ultrasonic frequencies, and, at the receiving end, receiving the signals of the different sonic or ultrasonic frequency to obtain the digital pulses. 3. A method as claimed in claim 2 wherein, at the receiver, the pulses received in succession from the various different channels are combined into serial form. 4 A method as claimed in either claim 2 or claim 3 wherein the receiver includes a logic system to detect the loss of an input signal or the duplication of a signal. 5. A method as claimed in any of the preceding claims wherein three sonic or ultrasonic frequencies are employed and wherein, at the transmitting station, the successive digits of information are separated and fed into three channels such that successive digits are fed into different channels and such that, where two successive digits are the same, the last digit of said two equal digits is fed into a channel different from those for the previous two digits. 6. A method as claimed in any of claims 2 to 4 wherein four sonic or ultrasonic frequencies are employed and wherein the 11s and 0's of odd digits are fed into one or other of a first pair of channels according to whether the digit to be recorded is a 1 or a 0, and with the l's or 0's of even digits fed into one or other of a second pair of channels according to whether the digit to be recorded is a 1 or 0. 7. A method as claimed in any of the preceding claims wherein block or inter-block markers are transmitted to indicate the start or start and end of a message block containing a predetermined number of message data digits. 8. Apparatus for transmitting digital data through water from a transmitting station to a receiving station wherein the transmitter includes means for separating and feeding digits of input serial form each into a selected one of three or four channels, such that each digit is fed into a different channel from its preceding digit and such that, after a digit has been fed into one channel, the next successive digit is fed into one or other of two further channels separate from the channel into which the preceding digit is fed, the further channel being selected in accordance with whether the digit is a 1 or 0 or whether the digit is the same or different from the preceding digit, means for radiating the digits from the various channels as pulses of different sonic or ultrasonic frequencies for the various channels and, at the receiving station, means for receiving the sonic or ultrasonic frequencies on the various channels and demodulating the received signals to obtain digital pulses. 9. Apparatus as claimed in claim 8 wherein the pulses received in succession from the various different channels are combined into serial form. 10. Apparatus as claimed in either claim 8 or claim 9 wherein error detection means are provided checking that successive digits are received from the appropriate channels. 11. Apparatus as claimed in any of claims 8 to 10 wherein four channels are employed and wherein logic means are provided for checking that one digit is received from either the first or second channel and the next digit from either the third or fourth channel. 12. Apparatus as claimed in claim 11 wherein said logic means are arranged, after receiving a pulse on one channel, to disregard any further pulses on that channel until the appropriate reception has occurred on other channels.

Inventors: PARKER, BERNHARD DOLLMAN
Applicants: UNDERWATER STORAGE LIMITED
Application year: 1984
Main classification: H04L27
IPC codes: H04L27