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TECHNICAL ENHANCEMENTS FOR FUTURE CONSIDERATION 124 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 (1Ï) after a 4-hour period, as compared with an error of 0.9 meters (1Ï) 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 1 According to Martin Marietta Astro Space Division of Lockheed-Martin, atomic clocks have been used in the past on GPS spacecraft and have provided a mixed heritage of superb stability and long life in some cases but unexplained premature degradation and failure in others. Each Block IIR satellite will carry two rubidium clocks and one cesium clock. The total cost of all three clocks represents approximately 3 percent of the price of the GPS spacecraft. 2 With two-way time transfer measurements between satellites made every 15 minutes (900 seconds), the predictions need only to be good over this time period. Note that the clock error is the product of clock stability and prediction time. It is the reduction in prediction time from 1 day to 15 minutes that reduces the clock stability requirement by two orders of magnitude and, thus, enables the potential use of quartz oscillators. 3 Since quartz clocks and atomic clocks have different frequency accuracies, their offsets would be weighted when determining a single ensemble time from all 24 satellites, that is, more weight would be given to the atomic clocks in the ensemble. 4 Maintenance on the GPS clocks requires that each satellite is out of service 1 day per year.