Sensitivity of GPS acquisition to initial data uncertainties

The effects of errors in the initial position and time on the estimated Doppler frequency shift of the received signal due to the motion of the satellite relative to the receiver are assessed. It is noted that the Doppler estimate is most sensitive to both position and time errors when the satellite is directly overhead, requiring the common practice of acquiring the highest satellite first to be reexamined. Numerical results for position and time sensitivities are tabulated for the entire spectrum of receiver/satellite orientations. It is inferred that the time error can be reduced to an insignificant level by the use of a battery-powered real-time clock; the Doppler errors due to position errors can be dealt with by introducing an appropriate bandwidth in the receiver's acquisition algorithm.