B&G Network GPSplus - Manual (Page 62)

63
GPS PLUS DIFFERENTIAL INFORMATION
The GPSPLUS system is capable of accepting a Differential
GPS input conforming to the RTCM SC 104 V2.0 format.
Refer to the installation sheets for wiring details.
Once the GPS has obtained a fix, and the Differential
receiver has locked onto the beacon, the GPSPLUS display
will show the following:
NOTE: The above is not a new page. The value of HDOP is
replaced with GPS DIFF .
The basic GPS system available to the normal user
contains a deliberate inaccuracy, called Selective
Availability (SA). This limits the accuracy of the GPS
position to an average of 100 metres. To improve the
accuracy a system called Differential GPS (DGPS) was
designed to run with the normal GPS system.
The basic principle of DGPS is that a known position
(Lighthouse, Coastguard Station, etc.) has a normal GPS
receiver installed. A computer monitors the position from the
GPS and compares this to its known position. The error
between the GPS position and the known position is
calculated then transmitted over a conventional radio link to
the vessel. The GPS on board the vessel recalculates the
position taking into account the errors. A separate receiver
and antenna are needed to receive the Differential beacon
signal.