GB2162668A - Underwater communication - Google Patents

Abstract

Underwater frequency-agile data transfer by acoustic transmissions between two stations may be corrupted by delayed transmissions over indirect paths. Such paths include the sea bed, the water surface and refraction paths due to changes in density. The delay between reception of a direct path signal and the first indirect path signal provides a free time in which there will be no corruption. By sequential coded transmissions between the stations, each triggering the next, the free time is determined by both stations and also the direct path transit time. Intelligence transmissions can then proceed in 'free-time' data blocks the frequency being changed after each such period.

Description

SPECIFICATION Method and apparatus for operating a communication link This invention relates to a method and apparatus for operating a communication link and particularly a communication link that incorporates both direct and indirect paths between two transponders. The extra transmission time necessary for transmission over an indirect path can cause simultaneous reception of the direct and delayed signals and consequent signal corruption.

A particular example of this problem occurs with communication between underwater stations by means of pulsed acoustic carrier frequencies, and particularly where the water is relatively shallow, say of the order of 30 metres. In such circumstances, as illustrated in the accompanying Figure 1, communication between two underwater stations A and B is vulnerable to the effects of multi-path propagation, in which the acoustic signal may arrive not only via the direct route AB but also by one or more longer, indirect paths such A x B after reflection at X, where X is a point on an acoustically reflecting body or boundary (here, the sea floor). It is also possible for delayed signals to arrive via refraction processes due to inhomgeneities in the water.

The arrival of the delayed, indirect, signal or signals, can, dependent on amplitude, cause serious corruption of the data being received via the direct route.

An object of the present invention is therefore to provide a method and apparatus for the operation of a communication link subject to signal corruption by simultaneous reception of direct and indirect path signals.

In accordance with one aspect of the present invention, in a method of operating a communication link that incorporates both direct and indirect communication paths between two transponders such as to cause signal corruption on simultaneous reception over both paths, one transponder transmits a timemarked signal which is received by the other transponder by the direct path and, after a time lapse, by an indirect path, the other transponder determining the time lapse and limiting its transmissions of intelligence information to periods not exceeding the duration of said time lapse without a change to a noncorrupting transmission.

The other transponder preferably transmits a time-marked signal in immediate response to the first reception of a time-marked signal over said direct path thus enabling the one transponder to determine said time lapse. The time- marked signals are preferably coded to avoid ambiguities in recognition of a received time-marked signal.

The time-marked signals may comprise a sequence of pulses of different acoustic frequencies, the frequency sequence of which may constitute a code.

The transmission of intelligence information may be effected at different carrier frequencies in successive said periods so that indirect path signals are never received at the same carrier frequency as direct path signals and selection of the direct path signal is thereby enabled.

According to another aspect of the invention, an underwater data link comprises two transponders for transmitting data by means of acoustic signals, each transponder having means for limiting signal transmissions to periods of such limited duration that a receiving transponder can be disabled in respect of signal transmissions at the current carrier frequency after reception of signals at that frequency for such a period, to prevent corruption of intelligence transmission by reception of signals transmitted over an indirect path after reception of the same signal by a direct path, said means for limiting signal transmission periods having means for determining said periods by any method such as aforesaid.

A method and apparatus for operating a communication link in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a diagrammatic view of two submersed communicating transponders; Figure 2 is a signal/timing diagram illustrating the signalling sequence; and Figure 3 is a block schematic diagram of one transponder.

Referring to the drawings, transponders A & B are each able to transmit and receive pulsed acoustic carrier frequency signals. The carrier frequency and bit rate would typically be 100 kHz and 10,000 baud.

Different carrier frequencies may be employed for binary data within an intelligence transmission, pulses of one frequency providing 'l's and of another providing 'O's.

The transponders A & B might be typically several hundred meters apart in a water depth of perhaps 30 meters. In such circumstances the indirect paths AXB and AYB shown would be comparable with the direct path length AB.

It can then be very difficult to decipher the intended data in the presence of corruption by delayed "image" signals, but this corruption can be avoided, even in a multipath environment, if the data is transmitted only in a short block of duration less than, Tb seconds, where Tb is the difference in acoustic transit time between the shortest indirect path and the direct path. Obviously Tb is determined by the geometry and boundary conditions of the scenario. After transmitting the first short block of data, and if the stations A and B have more data to send, they may shift successively to different operating (carrier) frequencies, sending at each frequency only a short length of data block, again of duration less than Tb seconds.After a sufficient interval, dependent on scenario, reverberation will have decayed on each of these frequencies and the same frequencies can be used again.

There is, however, difficulty in knowing in any scenario what duration of data block, less than Tb, may be sent before the frequency must be changed.

Referring now to Figure 2, this shows two vertical lines representing the transponder stations A & B, the downward direction representing the lapse of time.

In the present method of operation, an initial transponder mode is used between the stations A & B, as a diagnostic procedure for each station to discover the acoustic transit time T of the first multi path signal. Thus the maximum corruption-free duration of data block must be less than Tb = T - Td.

An example of an appropriate diagnostic procedure is as follows: (i) Station A, at its local time zero (t,),, sends a short pre-arranged coded signal, for example, Pl, P2, P2, (indicated as P,23) comprising a sequence of three one millisecond pulses at one millisecond intervals and at respective frequencies f., f2 and f2. This constitutes a 'time-marked' signal and provides a timing reference.

(ii) On identification of this signal, station B at its local time zero (tub) immediately transmits a similar time-marked signal having a pre-arranged response code P2, P2, P, (shown as P32,) and then continues to 'listen' for arrival of the first "image" of the signal P123, transmitted on an indirect path and which arrives at time (tub),. Station B now knows that (tb)1 is the maximum data block size in the conditions of the prevailing scenario.

(iii) In turn, station A receives the code signal P22, from B at local time (ta)1 and immediately transmits a further pre-arranged code P212.

Station A continues to listen for the arrival of the first indirect return from B, at time (t,)2. Station A then knows: (a) the acoustic transit time A to B, i.e., (t,), 2 also, (b) the shortest difference between direct and indirect transit times, i.e., (tall - (ta)2 Finally, (iv) At time (Tb)2 Station B receives the code signal P3,2 transmitted from A.

Station B then also knows: (a) the acoustic transit time A to B, i.e., 2 = I-fi 2 2 and from (ii) above, (b) the shortest difference between direct and indirect transit time, i.e.

(tub)1 Thus stations know the difference between acoustic transit times of the indirect and direct paths and thus the maximum duration of data block which may be sent without corruption.

The invention provides an improvement to frequency agile underwater data transfer systems operating in a multipath environment, by the introduction of an initial diagnostic transponder mode which allows the stations to measure the maximum duration of data block which may be sent, free of "image" echo corruption.

It may be seen that in its broadest aspect the invention provides for one station to determine its own 'free time', or maximum data block duration, for its own subsequent transmissions, but that in a preferred form the invention provides for each station to interrogate the other and provide the free time and transit time information for both stations, prior to a two-way communication.

Having established the 'free-time' by the above diagnostic procedure, each station can then transmit intelligence data blocks of slightly shorter period than the 'free time' (to provide a margin of clearance from multipath transmissions) before changing the carrier frequency and again transmitting for the same foreshortened free period. This situation continues until reverberations at the original carrier frequency have decayed sufficiently, at which time the cycle of carrier frequencies can be repeated.

On reception, each transponder will switch its receiver carrier frequency at intervals equal to the transmission periods. Alternatively each receiver may 'listen' for the (known) next carrier frequency while operation on the current frequency, and switch immediately on recognition.

As an alternative to each transponder measuring the time between receipt of direct and indirect transmissions, one transponder may transmit the time- marker signal for free-time determination by the other by may itself measure the duration of transmission from the other transponder in order to determine the duration of the free-time.

It will be clear that any method of time-marking an acoustic signal could be used in the above diagnostic mode. However, the chosen method, of successive pulses of different frequencies has a significant advantage in providing an unabiguous time-marker.

Claims (8)

1. A method of operating a communication link that incorporates both direct and indirect communication paths between two transponders, such as to cause signal corruption on simultaneous reception over both said paths, in which one transponder transmits atime-marked signal which is received by the other transponder by the direct path, and, after a time lapse, by an indirect path, said other transponder determining said time lapse and limiting its transmissions of intelligence information to periods not exceeding the duration of said time lapse without a change to a non-corrupting transmission.

2. A method according to Claim 1, wherein said other transponder transmits a time-marked signal in immediate response to the first reception of a time-marked signal over said direct path thus enabling said one transponder to determine said time lapse.

3. A method according to Claim 2, wherein said time-marked signals are coded to avoid ambiguity in recognition of a received time-marked signal.

4. A method according to any preceding claim wherein a said time-marked signal comprises a sequence of pulses of different acoustic frequencies.

5. A method according to Claim 3 wherein said time- marked signals comprise a sequence of pulses of different frequencies, the frequency sequence constituting a code.

6. A method according to any preceding claim, wherein the transmission of intelligence information is effected at different carrier frequencies in successive said periods so that indirect path signals are never received at the same carrier frequency as direct path signals and selection of the direct path signal is thereby enabled.

7. An underwater data link comprising two transponders for transmitting data by means of acoustic signals, each transponder having means for limiting signal transmissions to periods of such limited duration that a receiving transponder can be disabled in respect of signal transmissions at the current carrier frequency after receipt of signals at that frequency for such a period, to prevent corruption of intelligence transmission by reception of signals transmitted over an indirect path after receipt of the same signal by a direct path, said means for limiting signal transmission periods having means for determining said periods by a method in accordance with any preceding claim.

8. A method of operating an underwater data link, substantially as hereinbefore described with reference to the accompanying drawings.