Introduction
GRS-1 Operator’s Manual
1-4
stage. For information on the status of this system, visit the
Galileo Industries website (http://www.galileo-industries.net).
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 digital
information (ephemerides, almanacs, time&frequency
corrections, etc.).
• 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 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.
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