1. All GPS satellites have synchronized atomic clocks as time keepers.
2. The coordinates of all satellites, acting as moving control stations, are known precisely with the help of system control.
3. Satellite coordinates and time signals are transmitted to ground receiver.
4. These signals reach the ground delayed by distance traveled.
5. Making use of simple resection principle and the range information to each satellites, the receiver computes its coordinates

During GPS based positioning, the following steps are followed (Enge and Misra, 1999):

• Basic navigation point position can be calculated like a resection in which satellites are the orbiting control stations .
• Range vectors are measured to each of the satellites using a time dependent code based on the times of transmission and receipt of the signals.
• Since these times are biased by a common amount due to offset between the satellite and receiver clocks ; they are called pseudoranges .
• Pseudorange measurements from four satellites are needed to estimate the user position and the corresponding receiver clock bias.

The above principle is explained in Figure 6.1 where 4 known GPS satellite coordinates are shown as (Xi, Yi, Zi). The unknown coordinates of GPS receiver u=(Ux, Uy, Uz) are calculated by solving 4 range (or pseudorange P) equations. A minimum of four equations are needed to solve for four unknowns- three unknown position coordinates (Ux, Uy, Uz) and to account for the fact that atomic clocks onboard GPS satellites and quartz clocks in GPS receivers are not synchronized. This unknown time variable is called receiver time offset or bias (dTu).