YSI SonTek RiverSurveyor, System Manual

Page: 43 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
31
GPS data is transmitted in a data stream through the serial port. This data stream is divided into a
series of sentences or
strings , each with an individual prefix used to describe the string’s format.
The most commonly used GPS string is the “GGA” string. This string contains the time, posi-
tion, and fix-related data. An example of a GGA string is:
$GPGGA,175944,3253.1308,N,11710.5321,W,1,06,1.5,102.0,M,-32.9,M,,*70
RiverSurveyor can accept the following GPS data strings in NMEA 0183 format for determining
vessel position – GGA, GXP.
3.4.5. When and where should I use GPS with RiverSurveyor?
Bottom-track (§
3.3
) will give you positional and vessel speed/direction information. So why do
you ever need to use GPS? There are several functional advantages and limitations associated
with GPS and bottom-track.
•
Certain environmental conditions can limit or prohibit the use of bottom-track for positional
data (§
3.3.5
).
•
GPS can be combined with bottom-track to give a higher degree of confidence in the vessel
position data. Keep in mind that GPS and bottom-track are two totally independent methods
of measuring position – one will not correct the other.
•
Data received from the GPS receiver can be combined with bottom-track data (§
3.3
) and/or
echo sounder data (§
3.5
). Bottom-track provides both position and depth information, while
echo sounders provide only depth data. So to get accurate depth data into RiverSurveyor (for
applications such as bathymetric profiling or discharge computation) it is necessary to use
either bottom-track or an echo sounder in combination with GPS.
•
Bottom-track is a relative position system (e.g., my current position is 5 m North of my po-
sition 10 seconds ago). GPS is an absolute position system (e.g., my current position is X
2
Latitude and Y
2
Longitude, and my last position was X
1
Latitude and X
1
Longitude). There-
fore, as a significant distance is traversed (typically more than 1000 m), the uncertainty as-
sociated with positional information from bottom-track increases. The uncertainty from
GPS will remain the same. So when traversing shorter distances (0.1 to 1000 m), it is pref-
erable to use bottom-track because of the lower uncertainty; for longer distances (e.g., over
1000 m), GPS can minimize the positional uncertainty. For distances between these limits,
either positional source should be adequate, and a combination of the two can be helpful.
•
GPS is not influenced by sources of magnetic interference.
•
Unstable “moving bed” conditions may limit the use of bottom-track (§
3.3.5
).
•
Exceeding the bottom-track depth limit (§
3.3.5
) may limit the use of bottom-track, whereas
GPS will not be affected by water depth.
Based on these issues, GPS is ideally suited for long transects, survey sites that have a moving
bed, or sites where the riverbed exceeds the maximum bottom-track range.
3.4.6. Setting up GPS with RiverSurveyor
Before establishing communication with your GPS receiver through RiverSurveyor , consider the
following. Where necessary, refer to your GPS documentation for more information.
•
When mounting your GPS antenna, try to mount it directly above the ADP system, or as
close to it as possible.