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Description and Operation
Principles of Operation
Calculating Differential Positions
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1.3. 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 recorded 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 measuring, real-
time kinematic measuring, maritime radio beacons, geostationary satellites, and satellite based
augmentation systems (WAAS, EGNOS, MSAS). The real-time kinematic (RTK) method is the most
precise method of real-time measuring. 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.