The Global Positioning System: A Shared National Asset (1995)
Chapter: Use of a 24-Satellite Ensemble Clock
4
Technical Enhancements for Future Consideration
In the previous chapter, several GPS upgrades were proposed than could provide a stand-alone position accuracy approaching 5 meters (2 drms). Even though such an accuracy would satisfy many user requirements, as discussed in Chapter 2, better accuracy would still be required for many applications, such as Category II and Category III aviation landings, surveying and mapping, all-weather aircraft carrier landings, and some scientific applications.
Since satellite block changes are likely to occur at intervals of 5-10 years, there are a limited number of opportunities to take advantage of worthwhile technical advances and to refine the specifications based on new applications. Because of the anticipated worldwide dependance on the system, the committee believes that it would be shortsighted not to consider significant future improvements that would make GPS more generally useful and forestall the possible development of competing systems.
Below, several options for further GPS improvement are considered. Although the NRC committee determined that the supporting analyses for these options were not carried to the point where specific recommendations could be fully endorsed, the committee believes that the options presented have particular merit and should be seriously considered for future incorporation. Thus, these options are presented as suggestions for consideration rather than as recommendations. First, technical enhancements to improve the overall performance of the GPS for all users are presented; these are followed by enhancements that will benefit specific GPS user groups.
GPS Improvements To Improve Overall Performance
Use of a 24-Satellite Ensemble Clock
Currently, clock offset corrections are determined on the ground and then sent to the individual satellites once a day as they pass over a GPS monitoring station. The Block IIR satellites will have the capability to determine their clock offsets autonomously relative to a space-based ensemble clock and exchange clock information with other satellites via crosslinks every 15 minutes. As a result, satellites can obtain clock information more often than once per day, which should result in a reduced clock error.
