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APPENDIX C 153 however, have been designed for improved performance and reliability, including the solar arrays, the gyroscopes, the batteries, and the nuclear-detonation detection system payload. In addition, the navigation payload on board the Block IIR satellites carries one cesium and two rubidium clocks, rather than the two rubidium and two cesium clocks present on the Block II/IIA spacecraft.15 The Block IIR satellites also have two important operational capabilities not available from the Block II/IIA satellites. First, each subsystem and payload has been designed to allow on-orbit software reprogramming, allowing for much greater operational flexibility and upgrading, and second, the satellites can maintain specified positioning accuracy without contact from the operational control segment (OCS) on the ground for up to 180 days. This mode of operation, known as autonomous navigation or autonav, is accomplished by relaying positioning information between satellites using ultrahigh frequency (UHF) inter-satellite links. 16 The draft request-for-proposal (RFP) for the next generation of satellites beyond the Block IIR design, known as the Block IIF, is currently scheduled to be released in the spring of 1995, and the final version is currently scheduled for release in the summer. The first launch is anticipated in 2001.17 Operational Control Segment The GPS operational control segment (OCS) consists of the master control station (MCS), located at Falcon Air Force Base in Colorado Springs, Colorado; remote monitor stations, located in Hawaii, Diego Garcia, Ascension Island, and Kwajalein; and uplink antennas located at three of the four remote monitor stations and at the MCS.18 The four remote monitor stations contribute to satellite control by tracking each GPS satellite in orbit, monitoring its navigation signal, and relaying this information to the MCS. These four stations are able to track and monitor the whereabouts of each GPS satellite 20 to 21 hours per day. Land-based and space-based communications are used to connect the remote monitoring stations with the MCS. The MCS is responsible for overall satellite command and control, which includes maintaining the exact orbits of each satellite and determining any timing errors that may be present in the highly accurate atomic clocks aboard each satellite. Errors in a satellite's orbital position or in a satellite's timing are determined by analyzing the same signal and 15 Detailed information about the GPS Block IIR rubidium frequency standards can be found in: William J. Riley, ''Rubidium Atomic Frequency Standards for GPS Block IIR," in Proceedings of ION GPS-92 5th International Meeting of the Satellite Division of the Institute of Navigation (16-18 September 1992). 16 Because Block IIR satellites will be launched on need to replace failing Block II/IIA satellites, it is impossible to determine exactly when autonav capability will become operational. 17 According to the GPS Joint Program Office, current plans for the Block IIF contract include 6 short-term, and 45 long- term, "sustainment" satellites. 18 A backup MCS also exists at Loral Federal Systems in Gaithersburg, Maryland.
