Introduction
Principles of Operation
Calculating Absolute Positions
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1.1.2. Calculating Absolute Positions
When calculating an absolute position, a stationary or moving receiver determines its three-
dimensional position with respect to the origin of an Earth-Center Earth-Fixed coordinate system.
To calculate this position, the receiver measures the distance (called pseudoranges) between it and
at least four satellites. The measured pseudoranges are corrected for clock differences (receiver
and satellites) and signal propagation delays due to atmospheric effects. The positions of the
satellites are computed from the ephemeris data transmitted to the receiver in navigation
messages. When using a single satellite system, the minimum number of satellites needed to
compute a position is four. In a mixed satellite scenario (GPS, GLONASS, Galileo), the receiver
must lock onto five or more satellites to account for the different time scales used in these systems
and to obtain an absolute position.
1.1.3. 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, measured 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.
