Page 443 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition
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436 CHAPTER 16 Acoustic Positioning
FIGURE 16.8
Comparative positioning system arrangements for (a) USBL, (b) SBL, (c) LBL and LUSBL.
At the shorter transducer spacing distances (i.e., less than 30 ft), SBL produces a higher error level than does LBL due to the greater impact of multipath and other range measurement errors. Also, any errors in transducer spacing measurement, heading indicator errors, GPS inaccuracies, and vessel instability (i.e., due to vessel motion) are propagated through to position inaccuracies. SBL is not normally considered survey quality.
16.6.3 Ultrashort baseline
Ultrashort baseline (USBL, also termed “super short baseline,” or SSBL, by European operators) principles (Figure 16.10) are very similar to SBL principles (in which an array of acoustic transdu- cers is deployed on the surface vessel) except that the transducers are all built into a single trans- ceiver assembly—or the array of transducers is replaced by an array of transducer elements in a single transceiver assembly.
The distances or ranges are measured as they are in an SBL system but the time differences have decreased. Systems using sinusoidal signals measure the “time phase” of the signal in each element with respect to a reference in the receiver. The “time-phase differences” between trans- ducer elements are computed by subtraction and then the system is equivalent to an SBL system.
(Courtesy of Sonardyne.)
