YSI SonTek RiverSurveyor, System Manual

Page: 42 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
30
3.4.3. How does DGPS work?
DGPS works the same way as GPS, but
with the addition of a stationary receiver or
base station (differential beacon) to correct
the GPS signal and provide sub-meter ac-
curacy (
Figure 3-12
). Both the stationary
receiver and the moving receiver (mounted
on your vessel) are relatively close to one
another (compared to the distance to the
satellites). The stationary and moving re-
ceivers are therefore assumed to be under similar atmospheric conditions, so the uncertainty with
the positional calculation is reduced. The stationary receiver can then provide correctional in-
formation to the moving receiver (and any other receivers in the area).
A second DGPS correction technique enables the use of geostationary satellites to create a “vir-
tual base station” that is used to generate GPS corrections. The use of this technique requires a
DGPS receiver designed to accept satellite-based corrections and a yearly subscription purchased
from one of a few companies capable of this technology.
One such company is Om). OmniSTAR provides
yearly subscriptions and additional technical literature describing the satellite virtual base-station
DGPS technology.
3.4.4. How does GPS work with RiverSurveyor?
GPS can be used to provide navigational information to the RiverSurveyor program, such as ac-
curate position and time. Using GPS data, RiverSurveyor calculates the speed and direction of
travel of the vessel, and marks the water current profile data from the ADP with an exact loca-
tion. During data post-processing, this has the added benefit of allowing you to analyze specific
sites of interest. The use of GPS with
RiverSurveyor makes it ideal for bathymetric surveying.
When measuring velocities from a moving vessel, the ADP is measuring the total vector sum of
the water current velocity and the vessel velocity. RiverSurveyor already computes the vessel ve-
locity from the GPS data. It is therefore a simple calculation to subtract the vessel velocity from
the measured total velocity, leaving the resultant (true) water current velocity.
The bottom-track feature in RiverSurveyor ADPs (§
3.3
) is another source of positional data for
current profiles. Data analyzed in RiverSurveyor can be referenced to either GPS or bottom-track
for display of the vessel track (known as the track reference and set on
Processing
toolbar;
Table
7-2
). The track reference will automatically default to bottom-track if it is present, but the user
can modify this setting.
GPS data to the RiverSurveyor is most commonly sent through an external GPS receiver unit po-
sitioned above the ADP and attached to a second serial port on your computer. RiverCat systems
can be equipped with an optional DGPS receiver (Trimble
Ag GPS 114) that transmits data di-
rectly to the ADP electronics. In this situation, the GPS data is included in the ADP’s data
stream, and only position, speed, and directional data will be available for display. This elimi-
nates the requirement for additional base and remote station radios (requiring dedicated com-
puter serial ports).
Figure 3-12. Differential GPS (DGPS) infrastructure