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US PATENT Office.... Underwater "GPS"

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US PATENT Office.... Underwater "GPS"

Postby U-5075 » Tue Mar 13, 2007 12:44 pm

FROM NewScientistTech
15:24 12 March 2007
NewScientist.com news service
Barry Fox
Underwater GPS
GPS does not work underwater-the radio signals on which it depends cannot pass through water. So submariners have yet to benefit from the revolution in navigation that it allows.

Now engineers working for the US Office of Naval Research think they have found a simple way to let submarines and divers get an accurate GPS fix.

A base station is tethered to the sea bed at known depth and known GPS location. A submersible anywhere in the area sends out a sonar request pulse to which the base station replies with a signal which gives its GPS position and depth as well as the bearing angle from which the submersible’s request arrived.

The submersible then uses its own depth, which is easily measured, plus the round trip pulse time and the bearing angle sent by the base, to calculate its own position. Simple.


From the US Patent Office.

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United States Patent Application 20070025185
Kind Code A1
Green; Maurice D. ; et al. February 1, 2007

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Underwater geopositioning methods and apparatus


Abstract
A method and apparatus for determining the geophysical position of an autonomous underwater system utilizing underwater acoustic modems that exchange broadband underwater acoustic signals. The method of the invention includes the steps of initiating an exchange of broadband acoustic signals between the autonomous system of unknown geophysical position and a base system of known geophysical position wherein the depths of both systems is known. A bearing calculation is made on one of the signals transmitted between the systems, preferably through the use of an array of hydrophones placed closely together at predetermined locations on either the autonomous or base system. Also, the range between the two systems is determined by measuring the time of travel of at least one signal. By the acoustic transmission and sharing of information, as needed, about the known depths of the systems, the known geophysical position of the base system, and the range between the systems, sufficient data is gathered at one or both systems and used to determine the geophysical position of the autonomous system.


Claims

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1. A method for determining the geophysical position of an autonomous underwater system of uncertain geophysical position, said method comprising the steps of: monitoring the depth of the autonomous system; initiating an exchange of broadband acoustic signals between an acoustic modem of said autonomous system and a base unit having an acoustic modem of known geophysical position to share between them information relating to their respective depths, known geophysical position of the base unit, and transit time of the exchanged broadband acoustic signals; determining the bearing and range between the autonomous system and the base unit; and calculating the geophysical position of the autonomous system based on the bearing, depths of the autonomous system and base unit, and known geophysical position of the base unit.

2. A method for determining the geophysical position of a remote unit that includes an underwater acoustic modem, said method comprising the steps of: broadcasting a broadband acoustic signal from the underwater acoustic modem of the remote unit in the vicinity of a nearby base unit whose geophysical position and depth are known, said signal including a request for bearing information and the geophysical position of the base unit, wherein a time stamp is stored on the remote unit in reference to the acoustic signal's transmission and wherein information about the base unit's responsiveness to underwater signals in known to the remote apparatus; receiving the broadband acoustic signal with an underwater acoustic modem on the base unit, determining the bearing from the base unit to the remote unit and transmitting a return broadband acoustic signal containing the bearing, depth, and geophysical position of the base unit; and receiving the return signal at the modem of the remote unit, marking the time of receipt of the return signal and calculating the geophysical position of the remote unit based on the information contained in the return signal and the elapsed time of travel of the acoustic signals.

3. The method of claim 2 wherein the bearing of the acoustic signal from said remote unit to said base unit is obtained by having multiple acoustic transponders on said base unit placed at predetermined locations relative to each other and wherein the phase and timing differences of acoustic signals received by said multiple acoustic transponders is used to triangulate the signal's direction of travel.

4. An apparatus for determining the geophysical position of an autonomous underwater acoustic modem system of uncertain geophysical position, said apparatus comprising: a base acoustic modem system whose geophysical position and depth are known; an underwater acoustic multi-transceiver array connected to either the autonomous underwater acoustic modem system or base acoustic modem system so that the directional bearing between the systems can be calculated by analyzing a signal received at said multi-transceiver array; a depth monitoring device attached to said autonomous underwater acoustic modem system; at least one timing device connected to said autonomous underwater acoustic modem system or base acoustic modem system so that the range between the systems can be calculated; and a processing device connected to either the autonomous underwater acoustic modem system or base acoustic modem system programmed to calculate the geophysical position of the autonomous underwater system from the known depths of the systems, the bearing between the systems, and range between the systems.

5. The apparatus of claim 4 wherein said multi-transceiver array is attached to the base acoustic modem system; wherein said at least one timing device is connected to the autonomous underwater acoustic modem system; and wherein said processing device is attached to said autonomous system and programmed to calculate the range between the base and autonomous systems by timing the travel of the outgoing and incoming signals.
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Postby Gerwalk » Wed Mar 14, 2007 9:26 am

Not sure about the details but a couple of years ago at the Edgerton center at MIT they told me about this type of systems for ROVs and UUVs. They told me that one of the problems is that propagation of sonar waves in the water could lead to errors. This can be minimized using more than one base station, IIRC the stations transmit a pulse continuosly and in that case the submarine doesn't need to reveal it's position.
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