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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 108 operation of the Block IIR satellites. Also, if the recommendations of this report are implemented, a simulation facility would enable prompt and effective testing of the proposed modifications prior to their incorporation in the operational system. An Air Force estimate for the cost of an operational control segment simulator is $14.4 million.47 A simulator for the space and ground segment should be provided as soon as possible to test software and train personnel. Operational Control Segment Software. The current OCS system software was written several years ago. The hardware has since been upgraded, and over the years some software revisions have been made. However, the various upgrades have been written in different programming languages. This has produced a system that is lacking in modularity and is both difficult and expensive to maintain and upgrade. Because of this, an increasingly large percentage of the OCS budget is used to make relatively small changes to the system. Since the original software was designed, significant improvements have been made in software development and management technology. Today, a system can be designed and implemented that would have improved reliability, longevity, and ease of enhancements through modular software engineering practice. Given the current state of the OCS software, the DOD's planned changes, and the recommendations contained in this report, the most economical and effective solution to this problem is to develop a new OCS software suite using current technology and methods. There is an option in the 1995 OCS upgrade procurement to either upgrade the existing software or to replace it with improved software that is easier to maintain and upgrade, but the choice is left up to the winning contractor. The operational control segment software should be updated using modern software engineering methods in order to permit easy and cost-effective updating of the system and to enhance system integrity. This should be specified in the 1995 OCS upgrade request for proposal. Planned Block IIR Operation Currently, each Block II/IIA satellite is updated once a day from the OCS with clock and ephemeris corrections generated by the MCS's Kalman Filter. As a result of military requirements, each Block IIR satellite will have a Kalman Filter on board and will be able to autonomously determine clock and ephemeris corrections independent of the OCS.48 By 47 Memorandum from Col. Bruce M. Roang to the NRC committee, 23 December 1994. 48 Block IIR satellites have a military requirement to maintain a specified position accuracy for up to 180 days without clock and ephemeris updates from the MCS. This mode of operation is called autonomous navigation, or autonav. Autonav is accomplished by making inter-satellite pseudorange measurements using UHF crosslinks and on-board processing to determine each satellite's ephemeris and clock offset.
