Communications – electrical: acoustic wave systems and devices – Distance or direction finding – With time interval measuring means
Reexamination Certificate
2001-01-02
2003-03-11
Pihulic, Daniel T. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Distance or direction finding
With time interval measuring means
Reexamination Certificate
active
06532192
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a subsea positioning system and apparatus suitable for use in a hydrographic survey system.
Hydrographic surveys on subsea conditions require the accurate positioning of underwater bodies to be determined—the body may be a ‘towfish’ (for sidescan, sub-bottom profiler, swathe bathymetry or magnetometer measurements, for example) or a towed cable or ‘streamer’ (for seismic investigations), an ROV (remotely operated vehicle), manned submersible, or an object placed on the seabed (cables placed on the seabed for seismic investigations, LBL beacons, the data recorders of downed aircraft). If the position of a sensor is not accurately known, its measurements may be of little use; if the exact position of an aircraft's ‘black box’ cannot be determined, then it cannot be recovered.
The conventional way of determining the position of a body is to use a sonar beacon attached to the body, which allows a surface vessel to determine the position of the beacon (and thus the body) relative to the vessel. In the case of the surface vessel, the adoption of satellite positioning using the global positioning system (GPS) and differential GPS, has largely overcome the problem associated in identifying position of the towing vessel on the surface. As the surface vessel can be positioned to within 10 meters or less using GPS, Differential GPS or RTK (real-time kinematic) GPS, the accuracy of the underwater body's position relative to the surface vessel will determine the overall accuracy with which the object's geodetic position can be determined. In the example of the towfish the sensor produces data which allows the position of a seabed feature relative to the towfish to, be determined with high accuracy (within centimeters). Using the two relative positions—the towfish relative to the vessel, and the seabed feature relative to the towfish—the feature position can be converted into an absolute geodetic position (i.e. a position on a nautical chart).
As the radio signals used for GPS do not propagate through water then the problems in positioning accurately an object or towfish become significantly more complex. It is known therefore to use an ultra short baseline (USBL) sonar system and method of determining the object or towfish position. Other known systems are long baseline sonar system (LBL) and short basic line (SBL) and all such systems work on a very similar principle.
USBL uses sonar signals in the frequency range of from 7 to 70 kHz to determine the position of a beacon or towfish. A “ping” signal is emitted from a transducer on the beacon or towfish and received by three or more receivers or hydrophones suspended in the water below the towing vessel on the surface. By calculating the difference in arrival time of the signal at each receiver it is possible to determine by triangulation the azimuth and elevation of the towfish relative to the towing vessel (a typical sonar survey conf iguration is shown in FIG.
1
and is described further below). Using the information that the beacon or towfish transponder's ping signal is generated as a result of receiving a ping from the towing vessel, or as a result of an electrical signal sent down the tow cable (if one is used), it is possible to work out the delay between the ping signal being sent from the beacon or towfish and its reception at the towing vessel's receivers. This time difference can be converted into a distance using the known velocity of sound in water.
By way of example a conventional sonar survey system and configuration is shown in FIG.
1
and is described below. The system, generally indicated by reference number
1
, comprises a subsea towfish
2
towed via a tow cable
4
by a towing vessel
6
on the surface of the sea
8
. The tow fish comprises—an acoustic beacon
10
, which is either integral with or attached to the towfish, and is capable of operating at frequencies in the range of 7-70 kHz. Two different methods can be used to cause the towfish to generate a ping: transponder mode, in which the ping is generated as the result of receiving a ping from the towing vessel; and responder mode, where the ping is generated as a result of an electrical signal sent down the tow cable.
The towing vessel—has a receiver group of hydrophones
12
(usually three or five), built into a single receiving unit which is pole mounted (either using a through-hull or overside mount), so that the hydrophones are in undisturbed water below the vessel's hull. The signals from the receiver unit are fed to a processing unit within the vessel which performs all the signal processing necessary to determine the range and angle to the towfish beacon and formats this data so that it can be output to other navigational equipment. The receiver unit is capable of generating a ping so that the towfish beacon can operate in transponder mode.
In a USBL system and in order to be able to calculate the angular measurements, the ping from the beacon on a towfish, for example, is received by the towing vessel in an array of three or more hydrophones, mounted such that they can determine the azimuth and elevation of the towfish by triangulation (e.g. three hydrophones in an equilateral triangle); this is done by placing the hydrophones within half a wavelength of each other, allowing the angle to the towfish to be determined by measuring the difference in the path length travelled by the ping to reach each hydrophone (calculated from the difference in arrival time, or phase of the ping at each hydrophone); this can readily be converted into an angle relative to the receiver plane.
In all acoustic positioning systems (USBL, SBL, LBL, etc.) the range of the towfish from the towing vessel is determined by measuring the time taken for the ping to travel from the towfish to the vessel (as the speed of sound in water is known, the propagation time can easily be converted into a distance).
SBL and LBL systems establish the angular measurements of the beacon by using multiple receivers, usually placed on the seabed at the edges of the working area (so that the beacon is within the boundary of the area defined by the receivers), which receive the ping from the beacon and transmit the time of arrival of that ping to the surface vessel. The equipment on the surface vessel calculates a 3D position for the beacon using the propagation time of the ping from the beacon to each of the subsea receivers; the receiver on the surface vessel may also be used to obtain a range to the beacon and thus help determine its position.
For both transponder and responder mode, the number of range measurements which can be made in a given period is limited by the range of the towfish from the vessel because it is dependent upon the propagation time of the ping through the water. In transponder mode the maximum number of range measurements which can be made per second is 1500/(2×range in meters)—typically one or two per second. In responder mode the maximum is 1500/(range in meters), allowing twice the number of ranges to be obtained as for a transponder.
Conventional underwater positioning systems such as LBL, SBL and USBL have limitations which arise from the low update rate due to the long time intervals between “pings” and an inability to discriminate between the signal coming directly from the beacon or towfish and one which has bounced off the sea floor, the sea surface or any other sea structure such as rock or a ship wreck etc.—“multipath” signals. The low update rate and the inability to discriminate reduce the accuracy in positioning a beacon or towfish at any given time.
The determination of an object or a towfish position relative to the vessel on the surface by acoustic positioning is almost always the “weakest link” in the positioning chain, and is the source of most of the inaccuracies in feature positioning in surveys.
A cheaper and more accurate subsea positioning equipment would be desirable due to the expense, complexity and inaccuracy of known acoustic positioning systems.
SUM
Coda Technologies Ltd.
Kunitz Norman N.
Pihulic Daniel T.
Venable
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