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27.3 Vessel Offsets
The GPS position is the position of the antenna. However, at the instant the position is taken, the vessel will have a given heading and may also have a pitch and roll angle. These are observed with gyros and accelerometers and the offsets to any point of interest can then be converted from the ships reference where X, Y and Z are starboard, forward and above keel draft to a shift in real world coordinates in East, North and Height above sea level. For this to be done accurately, the offset distances need to be correctly tape measured or taken from accurate drawings and the vessel motion reference unit (MRU) has to be accurate in pitch roll and heading. For long offsets this can add another metre or two of uncertainty depending on the vessel size.
Figure 120: GPS considerations on vessels.
It is important that the marine survey report includes a section on expected accuracy of offset calculations based on the instruments and measurement methods used. This information should be submitted to the SFP and be understood by the SFP. A simple sign convention error on offsets can translate into a very significant subsea positional error.
27.4 Acoustic Positioning
The speed of sound in water is about 1500 m/s. Acoustic positioning systems measure the time taken for an acoustic pulse to travel from an emitter to a receiver and convert it to distance. A so called long baseline acoustic array is a set of acoustic transponders (AT) dropped to the seabed that can measure distances between themselves or distances from any mobile transponder such as on an Remote Operated Vehicle (ROV) to navigate relative to the array. Ultra Short Baseline Acoustics use multiple (at least three) high accuracy receivers to measure, not just the distance, but can also use the phase difference of the incoming signals to determine the horizontal and vertical angle to the source.
In order to establish the position of any permanent acoustic transponders on the seabed, two techniques are commonly us d. h s ‘ ox ng n’ nd .
Figure 121: Baseline Acoustic Array.
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