Communication with an underwater vehicle

A method of communicating with an underwater vehicle comprising a propulsion system for propelling the vehicle through the water. A series of data sets are encoded and transmitted to the underwater vehicle in a series of signal bursts, and decoded at the underwater vehicle. The propulsion system is operated in a series of thrust pulses separated by drift periods such that the propulsion system operates at a relatively high rate during the thrust pulses and at a relatively low (or zero) rate during the drift periods. The drift periods are timed such that each signal burst arrives at the underwater vehicle during a drift period and not during a thrust pulse. The method may be performed with a single vehicle or a plurality of underwater vehicles. The encoded data signals are broadcast simultaneously to the underwater vehicles in the series of signal bursts.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. nationalization under 35 U.S.C. §371 of International Application No. PCT/GB2013/050492, filed Feb. 28, 2013, which claims priority to United Kingdom Patent Application No. 1203671.1, filed Mar. 2, 2012. The disclosures set forth in the referenced applications are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a method of communicating with one or more underwater vehicles, a method of operating one or more underwater vehicles, and apparatus for performing such methods.

BACKGROUND OF THE INVENTION

A known method and apparatus for communicating with an underwater vehicle is described in U.S. Pat. No. 5,119,341. A plurality of buoys determine their positions based on Global Positioning System (GPS) navigation satellites and emit acoustic underwater data messages which contains this position. An underwater vehicle receives the messages and determines its position therefrom. Spread spectrum encoding is used to allow a single beacon carrier frequency for all buoys. Alternatively separate and locally-unique beacon carrier frequencies can be assigned to each buoy.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a method of communicating with an underwater vehicle, the underwater vehicle comprising a propulsion system for propelling the vehicle through the water, the method comprising:a. encoding a series of data sets to produce a series of encoded data signals;b. transmitting the encoded data signals to the underwater vehicle in a series of signal bursts;c. operating the propulsion system in a series of thrust pulses separated by drift periods such that the propulsion system operates at a relatively high rate during the thrust pulses and at a relatively low (or zero) rate during the drift periods;d. timing the drift periods of the propulsion system such that each signal burst arrives at the underwater vehicle during a drift period and not during a thrust pulse; ande. decoding the signal bursts at the underwater vehicle to obtain the series of data sets.

A further aspect of the invention provides an underwater communication system comprising: a transmitter programmed to perform steps a) and b) above; and one or more underwater vehicles each comprising a propulsion system for propelling the vehicle through the water, and a control and processing system programmed to perform steps c), d) and e) above.

The method may be performed with a single vehicle, or more preferably with a plurality of underwater vehicles wherein the encoded data signals are broadcast simultaneously to the underwater vehicles, typically from a single common transmitter, in the series of signal bursts.

Data may be transmitted to the vehicle(s) by a single transmitter only. However, more preferably the encoded data signals are transmitted to the underwater vehicle in a series of signal bursts by a first transmitter at a first location, and the method further comprises:a. encoding a second series of data sets to produce a second series of encoded data signals;b. transmitting the second series of encoded data signals to the underwater vehicle in a second series of signal bursts by a second transmitter at a second location which is remote from the first location;c. timing the drift periods of the propulsion system such that each signal burst in the second series arrives at the underwater vehicle during a drift period and not during a thrust pulse; andd. decoding the second series of signal bursts at the underwater vehicle to obtain the second series of data sets.

Typically the first and second series of signal bursts start at substantially the same time.

Preferably the vehicle comprises an annular hull with a duct, wherein water flows through the duct and generates lift during the thrust pulses and during the drift periods.

A further aspect of the invention provides a method of operating an underwater vehicle, the underwater vehicle comprising an annular hull with a duct; and a propulsion system for propelling the vehicle through the water, the method comprising:a. operating the propulsion system in a series of thrust pulses separated by drift periods such that the propulsion system operates at a relatively high rate during the thrust pulses and at a relatively low (or zero) rate during the drift periods, wherein water flows through the duct and generates lift during the thrust pulses and during the drift periods;b. receiving a series of signal bursts at the vehicle;c. timing the drift periods of the propulsion system such that each signal burst arrives at the underwater vehicle during a drift period and not during a thrust pulse; andd. decoding the signal bursts received at the underwater vehicle to obtain a series of data sets encoded within them.

This method may be performed by a single annular vehicle or by a plurality of underwater vehicles.

A further aspect of the invention provides an underwater vehicle comprising an annular hull with a duct; a propulsion system for propelling the vehicle through the water; and a control and processing system programmed to perform the method described in the further aspect of the invention described above. Typically the annular hull comprises an outer skin defining an outer profile of the hull and an inner skin defining the duct. The inner and outer skins typically meet at a leading edge of the hull and at a trailing edge of the hull.

Typically the control and processing system is housed at least partially within the hull between the inner and outer skins.

Typically the vehicle further comprises an antenna for receiving the signal pulses, wherein the antenna is flush with the inner and outer skins, or housed between the inner and outer skins.

Typically the control and processing system comprises a clock which can be set to provide a clock signal which enables the control and processing system to time the drift periods such that each signal burst arrives at the underwater vehicle during a drift period and not during a thrust pulse.

A further aspect of the invention provides a method of operating a plurality of underwater vehicles to receive a series of data sets which have been broadcast to them, each underwater vehicle comprising a propulsion system for propelling the vehicle through the water, the method comprising for each vehicle:a. operating the propulsion system in a series of thrust pulses separated by drift periods such that the propulsion system operates at a relatively high rate during the thrust pulses and at a relatively low (or zero) rate during the drift periods;b. receiving a series of signal bursts at the vehicle;c. timing the drift periods of the propulsion system such that each signal burst arrives at the underwater vehicle during a drift period and not during a thrust pulse; andd. decoding the signal bursts received at the underwater vehicle to obtain the series of data sets encoded within them.

A further aspect of the invention provides a plurality of underwater vehicles, each comprising a propulsion system for propelling the vehicle through the water, and a control and processing system programmed to operate the vehicle by the method described in the preceding paragraph.

The following comments apply to all aspects of the invention.

The signal bursts may comprise acoustic signal bursts, or they may comprise electromagnetic signal bursts. Typically the (or each) vehicle comprises a receiver such as an acoustic or electromagnetic antenna for receiving the signal pulses.

Where multiple vehicles are provided then the propulsion systems of the vehicles may be operated substantially synchronously such that the drift periods of all of the vehicles start and finish at substantially the same time. Alternatively the propulsion systems may be operated asynchronously such that the drift periods of at least a first one of the vehicles start and/or finish at different times to at least a second one of the vehicles.

The drift periods may be fixed at the beginning of a mission and remain constant for that mission. Alternatively the method may further comprise measuring a parameter for the (or each) vehicle; and varying the timing of the drift periods accordingly.

The timing of the drift periods may be varied asynchronously such that the drift periods of at least a first one of the vehicles are varied differently to the drift periods of at least a second one of the vehicles.

In one embodiment the method further comprises estimating a time of arrival of the signal bursts at the (or each) vehicle; and varying the timing of the drift periods accordingly, wherein a delay in the estimated time of arrival causes a delay in a start and/or finish time of the drift periods. For instance the time of arrival may be estimated by measuring the time of arrival of a pulse train in a previous cycle relative to a known transmission time for that pulse train.

In one embodiment the method further comprises measuring a proximity of the (or each) vehicle to other vehicles; and varying the timing of the drift periods accordingly, wherein increased proximity causes an increase in the length of the drift periods.

In one embodiment the method further comprises measuring a direction of motion of the (or each) vehicle; and varying the timing of the drift periods accordingly. For instance motion away from a transmitter of the signal bursts may cause a delay in a start and/or finish time of the drift periods.

The method may further comprise measuring a speed of the (or each) vehicle; and varying the lengths of the drift periods accordingly. For instance an increase in speed may cause the length of the drift periods to increase.

In one embodiment the average duration of the thrust pulses is less than the average duration of the quiet periods for the (or each) vehicle—for instance less than 50% of the average duration of the quiet periods for the (or each) vehicle. In another embodiment the average duration of the thrust pulses is greater than the average duration of the quiet periods for the (or each) vehicle.

The propulsion system may generate a small amount of thrust during the drift periods, but more preferably the (or each) propulsion system generates substantially zero thrust during the quiet periods.

Typically the series of signal bursts are transmitted by a transmitter with a transmit clock which is used to determine the timings of the series of signal bursts. Preferably the method further comprises synchronizing a receive clock on the (or each) vehicle with the transmit clock; and using the receive clock to determine the timings of the drift periods.

Each data set may consist of a single item of data, or a plurality of items of data. In a preferred embodiment each data set contains the location coordinates of the transmitter of the data. The data may be encoded in a number of ways, but most preferably it is encoded by pulse position modulation. Thus in a preferred embodiment the data is used to determine the position of the (or each) vehicle by the following process:a) determining the positions of three or more transmitters;b) transmitting from each transmitter at least four pulses (the four pulses together constituting a single “signal burst” as mentioned in the first aspect of the invention) wherein a time difference between each pulse and a previous one of the pulses is proportional to a respective co-ordinate of the position of the transmitter;c) receiving the pulses at the underwater vehicle;d) decoding the pulses received at the underwater vehicle by measuring the delays between them, thereby determining the co-ordinates of the transmitters;e) determining the range of each transmitter relative to the underwater vehicle; andf) determining the position of the underwater vehicle in accordance with the co-ordinates determined in step d) and the ranges determined in step e), for instance by multi-lateration.

DETAILED DESCRIPTION OF EMBODIMENT(S)

FIG. 1shows an underwater communication system. Three transmitter buoys1a-care deployed on the surface of the water. Each buoy has a Global Positioning System (GPS) antenna2, a processor3and an acoustic antenna4.

The GPS antenna2receives GPS data signals10from a GPS satellite11and from a Differential GPS (DGPS) reference station12on a surface vessel13. The processor process the GPS data signals10to determine the position of the buoy1in a known manner.

FIG. 2is a schematic diagram illustrating the method steps performed by the processors3. The position of the buoy1a-cis first determined in GPS coordinates (latitude, longitude and altitude) and stored as position data20. This data20is then transformed at step21into a local coordinate system having an origin22(again, defined in terms of GPS coordinates) to give a grid position23. This process is illustrated inFIG. 3which shows an origin22, and a cube24with orthogonal X, Y and Z axes meeting at the origin. Any position within the cube can be defined by three grid coordinates x, y, z relative to the origin22.

The processor3is programmed to cause the acoustic transmitter4to transmit a chirp pulse position modulated acoustic pulse train25which encodes the xyz position of the buoy1as shown inFIG. 4.

This pulse train25is encoded from the grid position data23at step26in accordance with reference chirp data27and survey grid property data28. The reference chirp data27defines for each a buoy a start frequency F1, a finish frequency F2, and a monotonic function which defines how the chirp frequency changes from F1to F2with respect to time (for instance the frequency might change at a constant rate between F1and F2). The survey grid property data28defines the size of the cube24in meters (for instance 4096 m by 4096 m by 4096 m), the resolution required (for instance 0.25 m) and the maximum time between adjacent pulses in the pulse sequence (for instance 0.1 s).

The pulse train25shown inFIG. 4comprises four low-to-high-frequency chirps30-33and a single high-to-low-frequency chirp34. The low-to-high-frequency chirps30-33have a frequency which increases at a constant rate between a first low frequency F1at the beginning of the pulse and a second high frequency F2at the end of the pulse. The low-to-high-frequency chirps30-33start at times t0, tx, ty, and tz, respectively. The high-to-low-frequency chirp34has a frequency which decreases at a constant rate between a first high frequency at the beginning of the pulse and a low high frequency at the end of the pulse. The chirps30,34are used to signal the start of the pulse sequence.

The chirps in the pulse train ofFIG. 4have a frequency which changes at a constant rate. In an alternative pulse train (not shown) the chirps may instead have a period which changes at a constant rate.

The time difference (Δt) between each acoustic pulse and a previous one of the acoustic pulses is encoded at step26to be directly proportional to a respective co-ordinate (x,y,z) of the position of the buoy1a-cin accordance with the equation:
Δt=co-ordinate(x,y,z)×(k)
where k is a co-efficient of proportionality which in this case is 4096/0.1 m/s. In other words:
t0−tx=Xco-ordinate in meters×(0.1/4096)
tx−ty=Yco-ordinate in meters×(0.1/4096)
ty−tz=Zco-ordinate in meters×(0.1/4096)

The chirps from the buoys1a-care frequency-division-multiplexed as shown inFIG. 5. In this example the first buoy1atransmits from F1ato F2a, the second buoy1btransmits from F1bto F2b, and the third buoy1ctransmits from F1cto F2c. The chirps occupy non-adjacent and non-overlapping frequency bands so that F1a<F2a<F1b<F2b<F1c<F2c. The three pulse trains are then de-multiplexed at the underwater vehicles based on their frequency by a process of cross-correlation as described below. By way of example the frequency F1amay be of the order of 10 kHz and the frequency F2cmight be of the order of 15 kHz.

Optionally each chirp from each buoy may also occupy a different frequency band as shown inFIG. 6. In this example the chirps from the buoy1aoccupy four non-adjacent and non-overlapping frequency bands, where F1a0<F2a0<F1ax<F2ax<F1ay<F2ay<F1az<F2az. The chirps from the other two vehicles are also similarly distributed within their respective frequency band. The individual chirps are then de-multiplexed at the underwater vehicles based on their frequency by a process of cross-correlation. This process also induces pulse compression at the receiver, which improves the resolution in time of the pulse arrival at the receiver.

In another example the pulse trains and/or individual chirps may be code-division-multiplexed (for instance by being mixed between up chirps and down chirps, or coded in some other way, perhaps by frequency hopping encoding) then de-multiplexed at the underwater vehicles based on their code.

The underwater vehicles40a,beach have an acoustic antenna44for receiving the acoustic pulses30-34, and a processor45. The processor45measures the delays between the pulses30-33, thereby determining the X, Y and Z co-ordinates of the buoys1a-c. The process for doing this is shown inFIG. 7.

First the received acoustic signal data is received and stored at step50.FIG. 8shows the received signal data at41by way of example. Next this data is cross-correlated in step51with the reference chirp data27to generate cross-correlated signal data52. The vehicles40and the buoys1have synchronised clocks so the vehicles know the time t0at which the buoys have transmitted the first pulse. At step53a time-variable gain is applied to the cross-correlated signal, the gain increasing constantly with respect to time after t0. Once the first peak in the cross-correlated signal52has been detected at step55then the gain value56at that time is recorded and applied for subsequent parts of the cross-correlated signal data52at step57. This time varying gain accounts for the fact that if the vehicle is far away from a buoy then the received signal will be weaker and delayed by a greater time than the received signal for a vehicle which is closer to the buoy1. The graphs42a-cinFIG. 8show the cross-correlated data for the three buoys1a-cafter gain has been applied as described above.

In step58the four peaks in each of the signals42a-care determined by detecting when the signals have exceeded a predetermined threshold. Peaks60a-c,61a-c,62a-cand63a-care shown inFIG. 8for the signals42a-crespectively along with the threshold43. It can be seen that these all have a roughly equal amplitude.

Next the cross-correlated data is interpolated at step59to generate sub-sampled peak data70. The process of interpolation is illustrated inFIG. 9. Signal71shows the analogue input data generated by a transducer and amplifier on the vehicle. An analog to digital converter samples the signal71at various points shown by dots inFIG. 9. The amplitude at the peak72is calculated by interpolating between the sampled data values on each side of the peak.

Returning toFIG. 7, the sub-sampled peak data70is then filtered and processed at step75by rejecting any echoes (for instance echo76shown inFIG. 8), and rejecting any peaks where the amplitude of the peak is too high relative to a previous peak, relative to some average peak value, or relative to a predetermined expected range of amplitude values.

Another output of step75is a ray travel time77which gives the time of receipt of the first peak60a-crelative to the known time t0at which the first pulse was transmitted by the buoys1a-c. Another output of step75is a set of filtered sub-sampled peak data which is decoded at step78in accordance with the grid property data28to determine the position79of the buoy. In other words the filtered sub-sampled peak data is decoded as follows:
t0−tx×(4096/0.1)=Xco-ordinate in meters
tx−ty×(4096/0.1)=Yco-ordinate in meters
ty−tz×(4096/0.1)=Zco-ordinate in meters

FIG. 10shows how the data77,79is used by each vehicle40a-cto determine its position. In step80a raytracer algorithm determines a radial distance81in accordance with the ray travel time77, a stored set of sound velocity profile data82, and the vehicle depth83measured by a pressure sensor onboard the vehicle. This ray tracer algorithm80accounts for the fact that the sound waves will not travel in a straight line from the buoy to the vehicle due to the increase in pressure with depth.

The vehicle now has the radial distance (or range)81and position79of each one of the three buoys1a-c. This data is than analyzed by a trilateration algorithm at step84to calculate the position86of the vehicle. An input to the trilateration algorithm is the velocity87of the vehicle (as measured by onboard algorithms which may interpret the data from devices such as accelerometers and/or as calculated based on previous position measurements). This takes into account the fact that the vehicle may have moved between receiving the first pulse and the last pulse, so the output86of the algorithm84is the position of the vehicle at the time that the last pulse was received.

Any errors in the measurements of the delays Δt between the pulses only translate into small errors in the X, Y or Z co-ordinates because of the proportionality between the delays Δt and the co-ordinate values X, Y and Z. Therefore if there is a gradual decrease of signal-to-noise ratio then the accuracy of the position estimate also degrades gradually.

The use of pulse position modulation also provides a low computation overhead in decoding and encoding.

The use of chirp pulses gives high processing gain due to their high bandwidth (processing gain being proportional to bandwidth multiplied by the period of the signal).

Although only two vehicles40a,bare shown inFIG. 1for purposes of simplicity, a large fleet of such vehicles may be provided (potentially 100 or more) for instance for the purpose of accurately distributing a grid of seismic sensors over a wide area of the seabed. The use of pulse position modulation for encoding the acoustic transmissions ensures that there is a relatively large time difference Δt between the pulses from a given buoy1. This relatively large time difference provides time for any delayed versions of the original pulse, due to multipath effects, to be sufficiently attenuated so as not to cause interference with the current pulse. Thus the likelihood of inter-symbol interference is reduced compared with other encoding methods, such as frequency shift keying, which transmit each symbol consecutively. With such encoding methods it is not possible to increase the time between symbol transmissions without dramatically reducing the data rate of the communication channel.

One of the vehicles40ais shown in detail inFIG. 11. The vehicle has an annular hull100with a duct101; and a propulsion system for propelling the vehicle through the water comprising a pair of rotary propellers105housed within the duct on opposite sides of the central axis of the duct. The hull has an outer skin100adefining the outer profile of the hull and an inner skin100bdefining the duct101. The inner and outer skins meet at a leading edge and a trailing edge of the hull100. The skins100aand100bare circular when viewed in cross-section at right angles to the central axis of the duct. Each propeller105is mounted on a thrust motor107and within a shroud105b. Each motor107is pivotally mounted so the propeller/motor unit can be independently rotated up and down (relative to the orientation ofFIG. 11) to vary its angle of thrust relative to the central axis of the duct. The shroud and propeller of one of the propulsion units is not visible inFIG. 1, but it is identical to the shroud105band propeller105which are shown.

FIG. 12ais a block diagram showing the main functional elements of the vehicle. An acoustic antenna44(also shown inFIG. 1) receives the acoustic signal pulses which are conditioned and analog-to-digital converted by a unit106aand input to the processor45(also shown inFIG. 1) along with clock signals from a time reference unit106dand acceleration signals from accelerometers106e. Although the antenna44is shown inFIG. 1protruding from the hull of the vehicle for purposes of illustration, preferably the antenna44is conformal with the hull100as shown inFIGS. 12b-12d. The hull100has a port and starboard nose109a,109bat one end, a lower tail109cat the other end and an upper tail at which the antenna44is mounted.FIG. 12bis a rear view of the vehicle with the propulsion units omitted, andFIG. 12dis a section through the antenna4. As shown inFIGS. 12band 12dthe antenna44is flush with the skins100a,100b, and as shown inFIG. 12dthe rear edge of the antenna44is curved so as to form a curved trailing edge conforming with the hydrofoil section provided by the skins100a,100b. The skins100a,100bdo not cover the antenna44so acoustic signals are not impeded. A signal wire44aconnects the antenna44with the electronics elements106a,45,106d,106ewhich are housed entirely within the hull100between the inner and outer skins100a,100b.

The processor45operates as described above to determine the position of the vehicle. The processor45decodes the signal bursts to obtain the series of data sets encoded within them and determine the vehicle position. The processor45also controls the angle of thrust of the propellers via actuator motors108. The processor45also controls the operation of the thrust motors107and is programmed to implement a sprint and drift control process as described below with reference toFIGS. 13-15.

FIG. 13shows three vehicles40a-candFIG. 14is a timing diagram showing a synchronous sprint and drift method of operating the vehicles40a-c. As described above, the buoys1a-cencode a series of data sets (each data set containing the X, Y and Z coordinates of the buoy at a given point in time), each data set being coded as a respective pulse train25as described above. These pulse trains25are then broadcast to the underwater vehicles, each pulse train25being initiated by a transmit clock pulse110shown inFIG. 14generated by a transmit clock on the buoy. The cycle repeats regularly every 7 seconds (a second transmit clock pulse111being shown inFIG. 14). If the position of the buoy changes between cycles then the pulse train for the next cycle will also change—otherwise the pulse trains will not change.FIG. 14shows three pulse trains TX1-3broadcast by buoys1a-crespectively.

The receive clocks106don the vehicles40a-care synchronized with the transmit clocks on the buoys1a-c, so they also generate receive clock pulses (not shown) at exactly the same time as the TX clock pulses110,111etc.

Vehicle40areceives the pulse trains TX1-3from the three buoys at different times, and these are shown as three receive pulse trains120a-120c. The time between the beginning of the first pulse train and the end of the last pulse train is illustrated by a receive pulse envelope Vehicle1RX.

Vehicle40balso receives the pulse trains at different times, and these are shown as three receive pulse trains121a-121c. The time between the beginning of the first pulse train and the end of the last pulse train is illustrated by a receive pulse envelope Vehicle2RX.

Vehicle40calso receives the pulse trains at different times, and these are shown as three receive pulse trains122a-c. The time between the beginning of the first pulse train and the end of the last pulse train is illustrated by a receive pulse envelope Vehicle3RX.

The thrust motors107of the vehicles are operated synchronously by their respective processors45in a series of thrust pulses125separated by drift periods126. The propellers105rotate at a relatively high rate during the thrust pulses125and at a relatively low (or zero) rate during the drift periods126. Each drift period126has a fixed length of 5 seconds (starting at or shortly after the clock pulse110) and each thrust pulse125has a fixed length of 2 seconds. The cycle then repeats regularly and indefinitely—a clock pulse111for the next cycle being shown inFIG. 14.

As can be seen inFIG. 14, the drift periods126of the vehicles40a-care timed relative to the receive clock pulse on the vehicle to ensure that that each pulse train arrives at the underwater vehicle during a drift period126and not during a thrust pulse125—with no part of any of the pulse trains arriving during a thrust pulse125.

The annular shape of the vehicle's hull ensures that water flows through the duct101and generates lift during the thrust pulses and during the drift periods. The high lift to drag ratio of the vehicle assists in maintenance of vehicle speed over ground during the drift periods.

In the example ofFIG. 14the propulsion systems of the vehicles are operated substantially synchronously such that the drift periods of all of the vehicles start and finish at substantially the same time. The duration of the thrust pulses125is much less than the duration of the quiet periods126for each vehicle (in this example the duration of the thrust pulses125is 40% of the duration of the drift periods126).

In an alternative example shown inFIG. 15the timings of the drift periods of the vehicles are varied independently and asynchronously.

Vehicle40ais the closest to the buoys1a-c, so it receives the acoustic signals first. Its drift period126ais timed to start just before the beginning of the first pulse train120aand finish just after the end of the last pulse train120c.

The next closest vehicle is vehicle40b, and its drift period126bis timed to start just before the beginning of the first pulse train121aand finish just after the end of the last pulse train121c.

The furthest vehicle is vehicle40c, and its drift period126cis timed to start just before the beginning of the first pulse train122aand finish just after the end of the last pulse train122c.

The advantage of the asynchronous method ofFIG. 15is that the length of the drift periods can be reduced compared toFIG. 14, so in this example the lengths of the drift periods126a-care slightly shorter than the lengths of the sprint periods125a-c(summed over a 7 second cycle).

The timings of the drift periods126a-ccan be varied in a number of ways.

Firstly, the timing can be varied by estimating a time of arrival of the pulse train from each buoy and varying the timing of the drift periods accordingly—later estimated time of arrival causing a delay in a start and/or finish time of the drift periods126a-c. The time of arrival may be estimated for instance by measuring and recording the time of arrival of the pulse train in the previous cycle from each buoy (relative to t0for that cycle). Optionally the estimate can be adjusted to account for any expected change caused by movement of the vehicle since the last cycle—for instance if the vehicle is moving towards the buoy then the drift period is advanced in the next cycle, and vice versa if the vehicle is moving away from the buoy. Optionally the estimate can be adjusted in accordance with both the speed and the direction of the motion of the vehicle—for instance if the vehicle is moving quickly towards the buoy then the drift period will be advanced more in the next cycle than if it is moving slowly towards the buoy.

The timing can also be varied by measuring a proximity of each vehicle to other vehicles, and varying the timing of the drift periods accordingly—increased proximity causing an increase in the length of the drift periods. This ensures that a vehicle does not generate noise which interferes with neighboring vehicles which are close by.