SonTek
ADP Software Manual Version 6.42 (November 1, 2000)
13
Positioning Data
The accuracy of the positioning data is an essential key to the success of moving boat data
collection. A good quality GPS receiver with accurate differential corrections should be used.
The accuracy of the positioning data directly determines the accuracy of the absolute velocity
profiles. For example, if the positioning data has an uncertainty of ±2 m and an averaging
interval of 60 seconds is entered, the uncertainty of the vessel speed for each profile is about
±6 cm/s. This uncertainty is reflected directly in the absolute current speed.
Compass Data
The quality of heading, pitch, and roll data is equally critical. The importance of accurate heading
is magnified because of the vessel motion. For example, if data are being collected from a boat
moving at 5 knots, a heading error of 2° translates into a cross-track velocity error of (5 knots *
sin(2°) = 8 cm/s). Before starting data collection, perform a compass calibration with the ADP
mounted in the exact location and orientation that it will be in during data collection.
Additionally, limit vessel speed to reduce the effect of any heading errors.
Magnetic Variation
The value for magnetic variation entered in the setup menu rotates ADP velocity data from
magnetic East-North-Up coordinates to geographic East-North-Up. This is required since GPS
data yield vessel speed in geographic coordinates. The accuracy of magnetic variation therefore
affects the accuracy of absolute velocity data, with the same sensitivity as heading data discussed
above. In order to obtain up to date magnetic variation (declination) for your specific location use
utility MAGDECL provided with the ADP software package.
Operating Parameters: Cell Size and Averaging Interval
In moving boat applications, you will typically use a larger cell size than stationary applications.
There are several reasons for this: higher vertical resolution is not meaningful because of motion
of the vessel (heave, pitch, roll, and translation), larger cell sizes achieve lower instrument noise
in a short period of time, and the maximum profiling range of the system is typically important.
Recommended cell sizes for the different ADP frequencies are 0.5 m, 1.0 m, 1.0 m, 2.0 m, 4.0 m,
and 4.0 m for the 3000 kHz, 1500 kHz, 1000 kHz, 750 kHz, 500 kHz, and 250 kHz, respectively.
Moving boat velocity data typically require relatively large averaging times (5-10 minutes) to
reduce the effects of boat motion. However, for flexible post processing we recommend
recording data with a shorter averaging interval (1 minute) and performing the additional
averaging in post processing. This allows you to precisely define different segments of the data
(i.e. on station versus in motion) and gives the greatest flexibility in data analysis.
General Data Processing Concerns
As mentioned before, the user should be aware of the increased complexity of moving boat data
collection. SonTek is willing to provide support, guidance, and software to assist in collecting
and analyzing moving vessel ADP data. However, the user will also need to invest time in
understanding the different aspects of moving boat data collection and in developing the tools
needed for effective data analysis.
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