YSI SonTek RiverSurveyor, System Manual

Page: 36 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
24
3.3. Bottom-Track
The following topics are described in this section.
•
What is bottom-track?
•
How does bottom-track work?
•
How does bottom-track work with RiverSurveyor?
•
When and where should I use bottom-track with RiverSurveyor?
•
What are the limitations of bottom-track?
•
Bottom-Track Terminology
3.3.1. What is bottom-track?
The ADP uses the bottom-track feature to measure the speed of a vessel (e.g., a boat, the River-
Cat system) relative to the river bottom. The vessel speed is then subtracted from the measured
water velocity to give the absolute water current profile independent of vessel motion.
•
Bottom-track allows the ADP to measure both its velocity (speed and direction) over the
Earth, and the depth of the water beneath the system.
•
Bottom-track data is used to remove vessel motion from measured water velocity to deter-
mine the “true” water speed and direction. That is, when you mount the ADP to a vessel
(e.g., a boat, the RiverCat system), bottom-track measures the velocity and direction of
ADP/vessel movement over ground.
•
True water speed and direction is used with simultaneous water depth measurements to
measure discharge in the RiverSurveyor program.
•
Accounting for vessel motion is essential when making water velocity measurements from a
moving vessel. Bottom-track and GPS data are two ways to account for vessel motion.
•
The bottom-track feature comes standard with RiverSurveyor systems.
3.3.2. How does bottom-track work?
As the ADP profiles (§
3.2.2
), it transmits a series of short pulses to measure the relative water
speed. In a RiverSurveyor system (or a standard ADP system with the bottom-track option en-
abled), the ADP also transmits a series of
long pulses that are designed to estimate the velocity
of the ADP over ground (i.e., vessel speed).
The Doppler shift of the reflected acoustic energy from these long pulses (i.e., bottom-track
pings) off the riverbed floor is used to determine the ADP/vessel velocity. Bottom-track pings
are transmitted once per second (for each transducer beam). The resulting measurements are av-
eraged at the end of each
profiling interval to determine the average water depth, vessel speed,
and vessel direction during this time.
Let us consider the performance of the bottom-track measurements. As mentioned, a bottom-
tracking ADP determines the velocity relative to the bottom (which is assumed to be identical to
the vessel velocity) once each second, and then averages the raw estimates over the user-selected
averaging interval . Because the bottom velocity is derived from solid object reflections (i.e., the
river bed), natural variability (i.e., standard deviation) of the bottom-track velocity measure-
ments is lower by an order of magnitude when compared to the ADP’s water-track data. As such,
the precision of bottom-velocity measurements is always better than that of water-velocity data.
Based on this comparison, bottom tracking can be considered to introduce no significant errors
to water current measurements.