User's Manual

Principles of Operation
P/N 7010-0816
1-3
Despite numerous technical differences in the implementation of
these systems, satellite positioning systems have three essential
components:
• Space – GPS, GLONASS, and GALILEO satellites orbit
approximately 12,000 nautical miles above Earth and are
equipped with a clock and radio. These satellites broadcast
ranging signals and various digital information (ephemerides,
almanacs, time and frequency corrections, and so forth).
• Control – Ground stations located around the Earth that monitor
the satellites and upload data, including clock corrections and
new ephemerides (satellite positions as a function of time), to
ensure the satellites transmit data properly.
• User – The community and military that use GNSS receivers 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 five or more satellites to account for the
different time scales used in these systems and to obtain an absolute
position.
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.