YSI SonTek RiverSurveyor, System Manual

Page: 41 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
29
3.4. Global Positioning Systems (GPS and DGPS)
The following topics are covered in this section:
•
What are GPS and DGPS?
•
How does GPS work?
•
How does DGPS work?
•
How does GPS work with RiverSurveyor?
•
Setting up GPS with RiverSurveyor
•
When and where should I use GPS with RiverSurveyor?
•
What are the limitations of GPS?
•
GPS Terminology
3.4.1. What are GPS and DGPS?
Vessel navigation and positioning at sea or on land has never been a precise or easy task. It is for
this reason that the U.S. Department of Defense developed the Global Positioning System (GPS)
to provide accurate positional information to receiver units anywhere on earth. Recent develop-
ments and widespread commercial and public use of GPS has lead to very affordable systems
that are located in virtually every modern vessel and a growing percentage of land vehicles.
When you purchase a GPS system, you are actually buying a GPS receiver unit. This unit re-
ceives multiple signals from orbiting satellites that tell you with a high degree of accuracy where
the receiver is located on the earth’s surface. The receiver unit can then send the GPS informa-
tion (e.g., time, position, and quality-related data) through a serial connection to a computer or
other device.
Differential GPS (abbreviated as DGPS) is similar to GPS except that it is a more accurate and
expensive system. Depending on your data quality requirements, the accuracy provided by a
DGPS system may be needed when using this method to measure river discharge (see §
3.4.7
–
GPS and DGPS Accuracy
).
Note : Throughout this manual and the RiverSurveyor program, we use the abbrevia-
tion “GPS” to mean both types of global positioning systems – GPS and DGPS. Where
necessary, we differentiate between the terms (i.e., §
3.4.2
, §
3.4.3,
§
3.4.7
). To achieve
the accuracy requirements for discharge measurements, DGPS must be used.
3.4.2. How does GPS work?
GPS uses a series of 24 satellites in predictable orbits around the planet. The orbits of these sat-
ellites have been designed so that 6 satellites should be “visible” to receiver units at any time or
location on the Earth’s surface. The satellites also use high-precision atomic clocks to provide
time to an accuracy of 0.000000003 seconds.
Each receiver unit calculates the distance to each of the visible satellites by determining the
amount of time it takes to receive the satellite’s signal. The receiver then computes its location in
three dimensions based on the intersection of these distances (similar to how triangulation works
in two dimensions). The three-dimensional output parameters from this computation are longi-
tude, latitude, and altitude. There is a degree of uncertainty with this calculation that depends on
factors such as atmospheric conditions (e.g., rain, clouds, fog), multipath error (§
3.4.7
), and re-
ceiver accuracy. This uncertainty can be minimized using DGPS. The reported altitude compo-
nent contains the highest degree of uncertainty.