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ephemerides (satellite positions as a function of time), to ensure
the satellites transmit data properly.
• User – The community and military that use GNSS receivers and
the corresponding satellites to calculate positions.
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
pseudo-ranges) between it and at least four satellites. The measured
pseudo-ranges 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 at least five
satellites to obtain an absolute position.
To provide fault tolerance using only GPS or only GLONASS, the
receiver must lock onto a fifth satellite. Six satellites will provide fault
tolerance in mixed scenarios.
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
