YSI SonTek RiverSurveyor, System Manual

Page: 60 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
48
3.10.3. What does Velocity Reference mean?
Velocity reference refers to the method used to remove vessel motion (i.e., speed) from the water
velocity measured by the ADP. There are three velocity references available:
•
Bottom-track (available on all RiverSurveyor systems; §
3.3
)
•
GPS (available only if connected to your system; §
3.4
)
•
ADP (available on all RiverSurveyor systems)
3.10.4. How will selecting a different Velocity Reference change my results?
When the ADP measures the velocity of the water beneath it, the ADP is actually measuring the
velocity of the water
relative to itself. If the ADP is stationary, it measures water velocity rela-
tive to both itself and the earth. But what happens if the ADP is moving (i.e., on a vessel)?
With a moving vessel/ADP, the water velocity relative to the ADP is the vector sum of ADP ve-
locity over the ground and water velocity over ground. As such, we must subtract the velocity
vector of the ADP to compute the speed of the water over ground (i.e., true water speed).
There are three velocity reference options in RiverSurveyor :
•
Bottom-track – When this option is selected, the vessel velocity computed using bottom-
track is removed from the water velocity measured by the ADP. This is the default velocity
reference for all bottom-track systems. All RiverSurveyor systems have bottom-track.
•
GPS – When this option is selected, the GPS-computed vessel velocity is removed from the
water velocity measured by the ADP. GPS capabilities are a RiverSurveyor option.
•
ADP – This option will show the raw velocities measured by the ADP without removing the
vessel velocity. This option is typically used for diagnostics when there may be some mag-
netic or other interference to the system. All RiverSurveyor systems have this option.
By default, bottom-track will be selected as the velocity reference. However, for systems without
bottom-track, GPS will be chosen.
Typically, bottom-track is the best option in situations when there are no adverse affects on the
bottom-track measurements (§
3.3
). Bottom-track generally provides higher accuracy measure-
ments of vessel speed and direction than GPS. GPS is typically used in situations where using
bottom tracking is not possible or is inaccurate (such as locations that exhibit
moving bed ).
The benefit of using RiverSurveyor is that the velocity reference can be changed at any time dur-
ing post-processing, or when the system is not collecting data, by clicking the appropriate veloc-
ity reference icon on the
Processing
toolbar:
•
– Click
Bottom-Track Velocity Reference
or press
B
.
•
– Click
GPS Velocity Reference
or press
G
.
•
– Click
ADP Velocity Reference
.
3.10.5. How do Track Reference and Velocity Reference relate to one another?
In most cases, it is simpler to use the same track and velocity reference for measurements. It is
easier to visualize and check your results. However, there are situations when this may not al-
ways be the best option. Bottom-track is generally the more accurate method of removing vessel
velocity from ADP measured velocity; however, the track reference choice is not as simple.