User's Manual

P/N 7010-0752

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