Geoscience Reference
In-Depth Information
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APA—Auto Pilot A sentence
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APB—Auto Pilot B sentence
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BOD—Bearing Origin to Destination
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BWC—Bearing using Great Circle route
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DTM—Datum being used
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GGA—Fix information
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GLL—Lat/Lon data
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GSA—Overall Satellite data
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GSV—Detailed Satellite data
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MSK—Send control for a beacon receiver
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MSS—Beacon receiver status information
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RMA—Recommended Loran data
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RMB—Recommended navigation data for GPS
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RMC—Recommended minimum data for GPS
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RTE—Route message
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VTG—Vector track and Speed over the Ground
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WCV—Waypoint closure velocity (Velocity Made Good)
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WPL—Waypoint information
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XTC—Cross track error
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XTE—Measured cross track error
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ZDA—Date and Time
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ZTG—Zulu (UTC) time and time to go (to destination)
The hardware interface for GPS units is designed to meet the NMEA requirements. They are
also compatible with most computer serial ports using RS232 protocols; however, strictly speak-
ing, the NMEA standard is not RS232, but rather EIA-422. The interface speed can generally be
adjusted (set to 9600 or higher), but the NMEA standard is 4800 baud with 8 bits of data, no parity,
and one stop bit (www.gpsinformation.org/dale/nmea.htm).
9.7.3 n
e t w o R k
-b
a s e d
R
e a l
-t
i M e
k
i n e M a t i c
gPs (Rtk gPs)
Another approach, gaining popularity in a number of countries, is to support the users through the
local networks of Continuously Operating Reference Stations (CORS) that normally serve a range
of applications, especially those requiring high accuracy in postprocessing or in real time (although
the real-time support is still limited). Government agencies, such as NGS, Department of Transpor-
tation (DOT), or international organizations, such as IGS, deploy and operate these networks. All
users typically have free access to the archived data that can be used as a reference (base data) in
carrier phase or pseudorange data processing in relative mode. Alternatively, network-based posi-
tioning using carrier-phase observations with a single user receiver can be accomplished with the
local specialized networks, which can estimate and transmit carrier phase correction (see, e.g., Can-
non et al., 2001; Chen, 2000; Dai et al., 2001; Kim and Langley, 2000, Raquet, 1998; Rizos, 2002a;
Vollath et al., 2000; Wanninger, 2002).
The long-range RTK technique (defined for 100 to 300 km or longer station separation) is the
most challenging GPS data reduction method. As the base-rover separation increases, many dis-
tance-dependent biases, such as atmospheric or orbital errors, may become significant even in the
relative mode, which complicates the ambiguity resolution process. This, in turn, may seriously cor-
rupt the positioning results, unless these effects are properly accounted for. In general, the success
of precise GPS positioning over long baselines depends on the ability to resolve the integer phase
ambiguities when short observation time spans are required, which is especially relevant to RTK
applications. The distance over which carrier-phase ambiguity can be resolved may be significantly
