Satellite clock offset relative to GPS system time and ephemeris errors: GPS system time is maintained by the GPS master control station (MCS) through a set of highly accurate Caesium clocks; the clock offset of satellites is measured daily and transmitted to each satellite by MCS for retransmission to receivers which apply the necessary correction algorithmically; satellites themselves incorporate highly stable atomic clocks onboard.
There is always a certain degree of inaccuracy in estimating satellite ephemeris. Certain components in ephemeris errors cannot be isolated from satellite offset errors and are therefore combined with the satellite clock error in the error budget (see Table 10.3).
*User clock offset from GPS system time: As mentioned in the preceding text, user offset can be removed by solving the range equation.
Error due to propagation delays: Errors in range are caused by delays introduced by ray bending and velocity reduction while traversing the ionosphere. The delay is approximately inversely proportional to the square of the frequency and the correction can be derived by comparison at two frequencies, 1,227.6 and 1,575.42 MHz. The correction is derived at the MCS and downloaded to users via the satellites. Tropospheric delays are independent of frequency and can be readily estimated at the receiver by applying an elevation angle dependent correction.
*Group delay errors are caused by processing delay on satellites; their value is estimated in ground tests and transmitted to users along with other corrections.
*Multipath errors are caused by signals arriving at the receiver from different paths.
*Receiver noise and resolution degradation are caused by hardware and software limitations in the receiver. Receiver motion can introduce addi. tional errors. Use of well designed receivers and filtering algorithms such as the Kalman filter reduces the impact of the error.
