Viavi 87339, Operation Manual

Page: 131 / 198

Principals of Operation
6 -14
6.1.4.E Clock Bias
To solve the problem of clock bias, consider a receiver is placed on a straight line
beneath two satellites. As the position of all GPS satellites is known via information
contained in the almanac and ephemeris, the distance between any two satellites (S), is
known. By measuring the travel times from each satellite, it is possible to exactly
establish the distance (D) despite having an imprecise receiver clock, using the following
formula:
D = ( Δ t1 – Δ t2) x c + S
2
Fig. 6-17 GPS Pseudo Range
In this example two pseudo ranges were employed to determine a position in one
dimensional space. To calculate a position in two dimensional space, three pseudo
ranges are required, Latitude, Longitude and Δ t.
To calculate a position in three dimensional space, four pseudo ranges are required;
Latitude, Longitude, Altitude and Δ t . The number of pseudo ranges (GPS satellites), must
exceed the number of unknown dimensions by a value of one.
D
S (known distance between satellites)
∆ t
2
∆ t
1