Differential and RTK Operations 45
Differential and RTK
Real-time differential positioning involves a reference (base) station receiver computing
the satellite range corrections and transmitting them to the remote stations. The reference
station transmits the corrections in real time to the remote receivers via a telemetry link.
Remote receivers apply the corrections to their measured ranges, using the corrected
ranges to compute their position.
RTK (Real-time kinematic) positioning can be used in lieu of real-time differential
positioning. RTK uses the carrier signal in addition to the code signal and is much more
accurate. Although messages transmitted and calculations performed vary, RTK is
essentially a special form of differential positioning. A base station receiver is required to
transmit RTK data to remote receivers. The remote receivers use the RTK data to compute
a corrected position.
As stand-alone, the GG Surveyor can compute a position to around 15 meters. Differential
GPS achieves sub-meter precision at a remote receiver, and RTK positioning achieves
centimeter accuracy at a remote receiver.
A communication link must exist between the base and remote receivers. The
communication link can be a radio link, telephone line, cellular phone, communications
satellite link, or any other medium that can transfer digital data.
Base Stations
Setting Up a Differential Base Station
You must have the Base option [B] installed on the receiver.
Send the commands listed in Table 5.1 to the receiver to generate RTCM differential
corrections using message types 1 and 31.
Table 5.1: Differential Base Station Commands
Command Description
$PASHS,RST Reset the receiver to factory defaults
$PASHS,PEM,4 Set the Base differential mask to four degrees
$PASHS,POS,ddmm.mmm,d,dddmm.mmm,d,saaaaa.aa Enter the phase center of the antenna
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