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USBL tracking systems calculate beacon depth most reliably when the beacon is directly below the transceiver.
High elevation tracking
As the elevation angle of the beacon with respect to the transceiver increases from zero (directly below the transceiver), the accuracy of the computed depth solution will degrade. This is an effect caused by the fact that the most accurate part of the USBL measurement is the range, and as the elevation increases, gradually less and less of the range measurement provides information about depth.
Above elevation angles of about 30°, the depth of a beacon derived from pressure sensors will generally be more accurate than the beacon’s depth as computed using acoustic range and direction observations.
The use of direct as opposed to derived depth observations is referred to as ‘depth aiding’. Figure 1 below shows the regions where depth aiding is appropriate for use.
For targets being tracked at high elevation the effect of refraction will significantly alter the USBL derived depth. At high elevations the acute angle that an acoustic signal arrives at a density boundary increases the effect of refraction. This results in a USBL derived position being calculated from the refracted acoustic path as shown in Figure 2. Sound velocity profiles can be used to mitigate the effect of refraction however in shallow water where tidal fluctuations and local environmental conditions can change within a short time frame it may not be possible to update the sound velocity profile frequently enough. Depth aiding can be used to reduce the effect caused by refraction providing a more accurate USBL derived position.
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