Navigation Set Clock Error

Figure 1-12.—Satellite ranging. Navigation Set Clock Error GPS navigation sets determine distance to a satellite by accurately measuring the time difference between satellite signal transmission and when the navigation set receives this signal. This difference in time is directly proportional to the distance between the satellite and the receiver. Therefore, the same time reference must be used by both the receiver and the satellite. The clock in the GPS receiver in not nearly as accurate as the atomic clock in the satellite. This causes the receiver and satellite clocks to be slightly out of sync, which in turn causes the time measurements to be inaccurate. The error is further compounded by the distance calculation, so the position of the navigation set cannot be accurately determined. The navigation set compensates for these errors by using the distance measurement from a fourth satellite to calculate the clock error common to all four satellites. The navigation set then removes the clock error from the distance measurements, and then determines the correct navigation set position. Signal Delay and Multipath Reception Two types of atmospheric delay can affect the accuracy of navigation set signal measurements. The first is tropospheric delay. Tropospheric delay can be accurately predicted; the prediction is included in the almanac data. The second type of delay is caused when the satellite signal passes through the ionosphere. This type of signal delay is caused by the ionosphere being thicker in some areas and by satellite signals received from nearer the horizon having to pass through more of the ionosphere than those received from directly overhead. Ionospheric delay will phase shift the lower satellite transmission frequency, L2-RF, more than the higher frequency, L1-RF. The navigation set measures ionospheric delay by measuring the phase shift between these two signals and then uses this computation to compensate for the ionospheric delay. Multipath reception is caused by a satellite signal reflecting off of one or more objects. This causes the 1-16