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Suggested Citation:"User Equipment." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
Page 154

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APPENDIX C 154 navigation message from each satellite that is used by GPS receiver equipment. Using a Kalman Filter, computers at the MCS process the data collected at all the monitor stations in order to estimate these errors.19 Updated orbits and clock corrections are relayed once a day to each satellite by the four ground antennas. The day-to-day operations at the MCS are carried out by personnel belonging to the 2nd Space Operations Wing of Air Force Space Command. Routine maintenance is also conducted by the Air Force and its contractors. Remote monitoring stations are largely automated, but a small number of contract personnel do monitor and maintain each station's equipment. Average annual personnel and maintenance cost for the MCS, the four remote monitoring stations, and all their associated equipment is approximately $30 million.20 User Equipment GPS user equipment varies widely in cost and complexity, depending on the receiver design and application. Receiver sets, which currently vary in price from approximately $400 or less to $30,000, can range from simple one-channel devices that only track one satellite at a time and provide only basic positioning information, to complex multi-channel units that track all satellites in view and perform a variety of functions. Most GPS receivers, however, consist of the same three basic components: (1) the antenna, which receives the GPS radio signal and in some cases provides anti-jamming capabilities; (2) the receiver-processor unit, which converts the radio signal to a useable navigation solution; and (3) a control/display unit, which displays the positioning information and provides an interface for receiver control. The subsections of a typical GPS receiver-processor unit include the front-end section, the digital signal processor, and the microprocessor. The front-end section translates the frequency of a GPS signal arriving at the antenna into lower or intermediate frequency (IF) and converts the signal from analog to digital. This more manageable signal is then passed to the digital signal processor, which "tunes in" to these signals using tracking loops that compare incoming signal data to internally generated models of the satellite signals. GPS receivers normally track more than one signal at a time using multiple channels, but also can track multiple signals using either a single channel sequenced between satellite signals or a multiplexing channel. Once the digital signal processor is successfully tracking a set of GPS signals, the ranging data it extracts is passed to the microprocessor, where 19 A Kalman Filter incorporates both observations and mathematical models of the system dynamics to produce an optimal 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. 20 U.S. Department of Defense and U.S. Department of Transportation, The Global Positioning System: Management and Operation of a Dual Use System -A Report to the Secretaries of Defense and Transportation

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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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