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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 106 a. This analysis has been made for a single-frequency C/A-code receiver aided by a barometric altimeter (required for aviation supplemental navigation use of GPS) with a visibility mask angle of 5 degrees. b. The probability of having 21 satellites operating is assumed to be 98 percent. c. The probability of having 24 satellites operating is assumed to be only 70 percent. However, the values in this table reflect the fact that if 24 satellites are fully operational, an incremental improvement in availability exists. d. Although these values would intuitively be lower than the 1 nautical miles terminal area protection limit value, availability improves for the 0.3 nautical miles non-precision protection limit because the barometric altimeter inputs improve in this phase of flight. Overall System Improvements Improved Monitor Station Receivers. The receivers currently used at the monitor stations are outdated compared with currently available commercial receivers. The receivers at the monitor stations can track only 11 satellites at a time, and the tracking schedules cannot easily be revised or priority given as to which 11 satellites to track.43 This results in a tracking gap of 3 to 4 hours per satellite per day. In addition, these receivers do not take full advantage of high-precision carrier phase data, which could be used to reduce multipath error contributions to the monitor station observables. Since some of the monitor sites suffer from very poor multipath environments, reduction of multipath errors is important. The main deficiency with current receivers, is that they can track only the Y-code and not the C/A-code, which is currently used by both civilians and the military. If there is a problem with the C/A-code, the MCS usually finds out only when C/A-code users call in to complain. By upgrading the monitor stations with a high-quality, all-in-view receiver with C/A code, Y-code, L1, L2, (and L4) observables, OCS performance would be improved as follows: (1) the integrity of the C/A-code could be monitored, which would allow faster detection and correction of a problem by the OCS; (2) the high-precision carrier phase data could be used to reduce multipath error to the monitor station observables, thereby improving overall GPS accuracy; and (3) all satellites in view could be monitored, which would eliminate existing individual satellite tracking gaps of 3 to 4 hours per day and allow prioritized monitoring of any failing satellite signals. Improvements to the monitor station facilities would require both software and hardware upgrades. Currently, the Air Force plans to award a $5 million contract to replace the monitor station receivers via a competitive bid in the summer of 1995. However, computer and software modifications required to take advantage of the improved receivers will not be upgraded at the same time. There also is a requirement in the 1995 OCS contract to replace the monitor station computers in order to take advantage of the new receivers, but there appears to be little coordination between the two procurements and little attention paid to the interfaces needed to optimize the system. The cost of replacing 43 As many as 14 satellites can be in view of a monitor station at one time.
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 107 the monitor station computers and the software could not be obtained at the time of this report, since the contract had not been awarded. 44 Procurements for the replacement of the monitor station receivers, computers, and software should be carefully coordinated. The new receivers should be capable of tracking all satellites in view and providing C/ A-code, Y-code, and L1, and L carrier observables to the OCS. Upgradability to track a new L4 signal also should be considered. OCS software also should be made capable of processing this additional data. Backup Master Control Station. In view of the rapidly expanding use of GPS for both the military and civilians, it is critically important that the GPS be capable of continuous operation in all foreseeable contingencies. Currently, a considerable degree of redundancy exists in the space segment. However, very little if any redundancy exists in the operational control segment. Presently, a backup MCS is in place at the current OCS contractor's facility, but there are no firm, long-term plans to maintain such a facility. It is possible that the eventual implementation of the Block IIR autonomous navigation operation capability could remove some of the urgency for a backup system, but even so, such a capability will not be operational until near the year 2000 or later and will not completely eliminate the need for a backup MCS.45 Air Force representatives have estimated that a backup MCS will cost around $14.4 million.46 Firm plans should be made to ensure the continuous availability of a backup master control station. Operational Control Segment Simulator. Presently, there is no dedicated capability to test and prove out system hardware and software modifications or to train personnel in any new operational procedures resulting from the changes. Instead, the operational control segment and the space segment currently are used for testing and training purposes. This procedure not only imposes some degree of risk on the operational system and interferes with operational performance. Tests and training activities could be effectively performed in a facility that functionally simulates the operational system. This is a particularly critical issue in the near future because of the planned OCS upgrades and the deployment and 44 Information provided by Capt. Earl Pilloud, Chief, GPS Control Segment, Air Force Space Command, 23 February 1995. 45 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 (ultra high frequency) crosslinks and on-board processing to determine each satellite's ephemeris and clock offset. 46 Memorandum from Col. Bruce M. Roang to the NRC committee, 23 December 1994.
