National Academies Press: OpenBook

The Global Positioning System: A Shared National Asset (1995)

Chapter: Recommended Upgrades to the Operational Control Segment

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Suggested Citation:"Recommended Upgrades to the Operational Control Segment." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
Page 98

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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 98 PERFORMANCE IMPROVEMENTS TO THE GPS OPERATIONAL CONTROL SEGMENT AND SATELLITE CONSTELLATION Current Status of the Operational Control Segment and Planned Upgrades The current operational control segment (OCS) consists of a master control station (MCS) plus four additional monitor sites that collect GPS Y-code measurements from a maximum of 11 satellites each.29 All but one of these sites are capable of sending uploads to the GPS satellites. There are plans to award an OCS consolidated contract in July 1995 to provide maintenance until the year 2000, to make improvements to the existing software architecture and user interfaces, and to support the deployment and operation of the Block IIR satellites. There is an option in the contract to replace the operational control software, but the winning contractor can choose to upgrade existing software rather than replace it. In February 1996, the Air Force plans to award another OCS contract, which will be effective beginning in the year 2000. This contractor will assume responsibility for the operational control segment, the Block IIR constellation, and the development and deployment of the Block IIF satellites. However, neither of these contracts address critical upgrades that would enhance the operation of the OCS and thus enhance the performance of GPS. The NRC committee recommends changes below that will enhance the overall GPS operation and improve performance. Most of these changes focus on the OCS and can be implemented immediately. Some improvements, however, focus on the operation of the Block IIR constellation and cannot be introduced until several Block IIR satellites are in orbit. Recommended Upgrades to the Operational Control Segment In addition to other operational functions, such as satellite health monitoring and routine maintenance, the GPS control segment is responsible for determining the ephemeris 30 and clock parameters and uploading them to the satellites. A partitioned Kalman Filter31 at the master control station estimates the orbits and clock errors for each 29 Information provided by Air Force Space Command, 1 December 1994. 30 Ephemeris is defined as a satellite's position as a function of time. 31 A Kalman Filter incorporates both observations and mathematical models of the system dynamics to produce an estimate of the current state of a system. By using knowledge of how the system state can change over time, the Kalman Filter allows the contributions of individual measurement errors to be averaged. In the MCS filter, the system state includes satellite orbital parameters, clock parameters, and numerous other elements.

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The Global Positioning System (GPS) is a satellite-based navigation system that was originally designed for the U.S. military. However, the number of civilian GPS users now exceeds the military users, and many commercial markets have emerged. This book identifies technical improvements that would enhance military, civilian, and commercial use of the GPS. Several technical improvements are recommended that could be made to enhance the overall system performance.

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