As mentioned above, traditional acoustic solutions for AUV navigation present some installation, calibration constraints and operational limitations. Their performances may be over estimated and in some cases not fully satisfying, and then non-acoustic solutions will be considered here.
The traditional non-acoustic approach, is a set of INS on board combining with a GPS receiver, which is also a dominant approach for AUVs. Due to the accumulated error from INS, the AUV must periodically come to the surface to calibrate the position with the help of GPS. In the case of a team of multiple AUVs, at least one AUV, providing accurate navigation information for others, have to come to the surface for position calibration, which would deteriorate the whole strategy of the team coordination and formation, besides the extra energy consumed to come to the surface and high cost of INS set in each AUVs. Since there some drawbacks of the non-acoustic INS combined with GPS navigation approach for coordinated underwater vehicles, we should seek alternative solutions. Unfortunately, it seems there is no way to directly utilize GPS for underwater navigation, as the electromagnetic signals do not penetrate below the sea surface making the GPS unsuitable for directly underwater navigation. However, more recently, several new ideas about underwater "reproducing" GPS have been proposed in order to improve the accuracy of underwater positioning and navigation, making such system easily used. The ideas of "reproducing" the GPS in the underwater environment which getting the merits of both non-acoustic and acoustic approaches, can be classified in three different groups summarized as follows.
The first type is so-called "false" underwater GPS. A GPS receiver mounted on a buoy is towed on the surface by the underwater targets such as underwater vehicles. A cable or fiber is used to send the GPS position to the underwater target. This technique does not give the true position of the target but the false position even in few tens of meters around the surface buoy, so that it is named as false underwater GPS.
The second type is a "direct" underwater GPS solution. In 1992, Youngberg inspired a direct transposition of GPS signal to underwater world. Acoustic waves but not radio electric signals, directly go from surface buoys replacing satellites to the underwater mobiles (receivers). Then, the underwater platform receives these acoustic messages from the buoys equipped GPS receptors and computes its own position locally. This solution has been presented by M. Youngberg of the US-AIR FORCE and patented (US Patent N°: 5.119.341). The third type is very similar to the second type solution, but it is a "reverse" underwater GPS solution. This method has been recently investigated by Thomas (Thomas, 1998) and is available commercially: the so-called GPS Intelligent Buoy (GIB) system, developed by ACSA in 1999. This system is designed to track the position of an underwater target equipped with an acoustic emitter, by measuring the times of arrival of the acoustic signals at a set of surface buoys equipped with submerged hydrophones and GPS receivers. As fig. 2 illustrated, it is a standard GIB buoy system (http://www.underwater-gps.com). The minimum number of buoys deployed is 2 if there are only 2 unknown parameters X and Y, as the depth (Z) of the target could be known using a telemetry channel. Technology of Time, Frequency or pseudo-random code diversity could be employed when tracking multiple underwater targets.
Fig. 2. GIB standard buoys
As we can see, in the direct type 2 solution, the information fusion is done on board the vehicle which uses downwards acoustic flow of data. In the reverse type 3 solution, the fusion is done on the remote control station which uses upwards acoustic flow of data.
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