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Introduction
GMS-2 Operator’s Manual
1-4
Calculating Differential Positions
DGPS, or Differential GPS, typically uses the measurements from
two or more remote receivers to calculate the difference (corrections)
between measurements, thus providing more accurate position
solutions.
With DGPS, 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 range
measurements from each GPS satellite in view and forms the
differences (corrections) between the calculated distance to the
satellites and the measured pseudo-ranges to the satellites.
These corrections are then built up to the industry standard (RTCM or
various proprietary standards) established for transmitting differential
corrections and broadcast to the remote receiver(s) using a data
communication link. The remote receiver applies the transmitted
DGPS corrections to its range measurements of the same satellites.
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
of the GPS.
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 the wide area augmentation system (WAAS) service.
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 cm
horizontal and 1.5 cm vertical.
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