Position fixing

Whatever the method used to obtain a biological sample from the sea-bed, it is important to know exactly where the sample came from. Most modern research vessels are equipped with Decca Navigator or Loran C systems which utilize

Figure 3.34 Towed camera sled used by A.J. Southward at MBA, Plymouth.

Reproduced from Gage and Tyler (1991) from photographs and drawings in Southward et al. (1976).

Figure 3.34 Towed camera sled used by A.J. Southward at MBA, Plymouth.

Reproduced from Gage and Tyler (1991) from photographs and drawings in Southward et al. (1976).

medium-frequency radio direction-finding transmitters. Satellite navigation systems utilizing transit satellites are also in common use.

The global positioning system (GPS) is a recent development from the USA that provides precise positioning to within only a few metres. The system works using an array of up to 24 satellites at least 4 of which will always be above the horizon and therefore within range of the ship. Small, hand-held units are now available so that the system can even be operated from small inflatable boats. However, the available accuracy of the system for most users is only about 50-100 m because the system has been degraded by modifying the signals as a national security precaution. Authorized users such as the US military can be given full accuracy by decoding.

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