Instruction manual

Differential GPS Page C-1
Reliance Fundamentals
Differential
C
Differential GPS
This appendix presents a brief overview of real-time differential, including code
differential and carrier phase differential, sources of error, differential data messages.
Differential remote/rover and base operations are available as receiver options. The RZ
sensor is capable of both code-based differential and carrier phase differential.
Fundamentals of Differential GPS
The fundamental concept for any differential GPS system is to utilize the GPS
measurement data from a stationary GPS receiver at a known site (base station or
reference station) to correct the errors in the measurement data of a GPS receiver at an
unknown site (remote station or rover station), thus achieving highly accurate relative
position. The GPS differential data transferred from the base station to the rover station
can be either pseudo-range measurements, carrier phase measurements, or a combination
of both. Differential data can be in either raw data format or in a specific correction
format.
The most common real-time differential GPS system uses a standard pseudo-range
correction format called RTCM, sometimes referred to as code-based differential. The
pseudo-range differential GPS can achieve accuracy ranging from several meters to less
than a meter. To achieve centimeter accuracy, the carrier phase data must be used as well.
A differential GPS system which uses the pseudo-range and carrier phase measurement
data is called Carrier Phase Differential (CPD) GPS or often called real-time kinematic
(RTK).
A communication link must exist between the base/reference and remote/rover 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.
Code Based Differential
In code based differential, the base receiver determines the range correction by subtracting
the measured range from the true range, computed by using the accurate position entered
in the receiver. This accurate position must have been previously surveyed using GPS or
some other technique. The remote receivers subtract the received corrections from their
measured ranges and use the corrected ranges for position computation.