GRX1 Chapter 1
Calculating Differential Positions
DGPS, or Differential GPS, is a relative positioning technique where the measurements from two
or more remote receivers are combined and processed using sophisticated algorithms to
calculate the receivers' relative coordinates with high accuracy. DGPS accommodates various
implementation techniques that can be classified according to the following criteria:
The type of GNSS measurements used, either code-phase differential measurements or
carrier-phase differential measurements
If real-time or post-mission results required. Real-time applications can be further divided
according to the source of differential data and communication link used.
With DGPS in its most traditional approach, one receiver is placed at a known, surveyed location
and is referred to as the reference receiver or base station. Another receiver is placed at an
unknown location and is referred to as the remote receiver or rover. The reference station
collects the code-phase and carrier-phase measurements from each GNSS satellite in view.
For real-time applications, these measurements and the reference station coordinates are
then built up to the industry standard RTCM - or various proprietary standards established
for transmitting differential data - and broadcast to the remote receiver ( s ) using a data
communication link. The remote receiver applies the transmitted measurement
information to its observed measurements of the same satellites.
For post-mission applications, the simultaneous measurements from reference and rover
stations are normally re-corded to the receiver's internal memory ( not sent over
communication link ). Later, the data are downloaded to computer, combined, and
processed. Using this technique, the spatially correlated errors - such as satellite orbital
errors, ionospheric errors, and tropospheric errors - can be significantly reduced, thus
improving the position solution accuracy.
A number of differential positioning implementations exist, including post-processing surveying,
real-time kinematic surveying, maritime radio beacons, geostationary satellites ( as with the
OmniSTAR service ), and satellite based augmentation systems ( WAAS, EGNOS, MSAS ).
The real-time kinematic (RTK) method is the most precise method of real-time surveying. RTK
requires at least two receivers collecting navigation data and communication data link between
the receivers. One of the receivers is usually at a known location ( Base ) and the other is at
an unknown location ( Rover ). The Base receiver collects carrier phase measurements,
generates RTK corrections, and sends this data to the Rover receiver. The Rover processes this
transmitted data with its own carrier phase observations to compute its relative position with
high accuracy, achieving an RTK accuracy of up to
1.0
cm horizontal and
2.0
cm vertical.
GRX1 Chapter 1