The Global Positioning System: A Shared National Asset (1995)
Chapter: Reduced Satellite Clock Errors Through Use of Improved Clocks
During autonomous ranging operation, each satellite will form an ensemble from the 14 satellites in view and will compare its offset relative to that ensemble. Further reduction of the clock error could be achieved if the clocks from all 24 satellites were used to create a single ensemble clock, as opposed to the current plan of letting each satellite form its own 14-satellite ensemble. For an ensemble of 14 clocks, the clock error is expected to be 1.1 meters (1σ) after a 4-hour period, as compared with an error of 0.9 meters (1σ) for a 24-satellite ensemble. This is discussed in greater detail in Appendix M.
The major advantage, however, of using a single, 24-satellite clock ensemble is not improved accuracy. Rather, it would allow quartz oscillators to be used on some satellites instead of atomic clocks, which are heavier, more expensive, require higher power, and have lower reliability than quartz clocks.1 Since clock offset measurements are made frequently during autonomous ranging operation, the requirements on satellite oscillator stability are greatly reduced.2 Therefore, quartz clocks could replace atomic clocks on at least some of the GPS satellites.3 In addition, since atomic clocks require yearly maintenance, use of quartz clocks on some satellites also would reduce the ground control station workload.4 Finally, the formation of an all-satellite ensemble clock may permit a failed clock in any one satellite to be detected and replaced more quickly and reliably.
In order to utilize an all-ensemble of all the 24 Block IIR GPS satellite clocks, the satellite software must be reprogrammed, and supporting ground software must be developed. In addition, further effort is needed to determine the optimal number and combination of quartz and atomic clocks.
Reduced Satellite Clock Errors Through Use of Improved Clocks
As discussed above, an ensemble reference clock can be used to reduce clock errors, relax requirements for clock stability, and eliminate the need for atomic clocks on some satellites. In order to improve the accuracy and the instantaneous frequency offset further, more accurate atomic clocks must be used on the satellites that will be carrying atomic
