National Academies Press: OpenBook

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

Chapter: Findings and Recommendations

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Suggested Citation:"Findings and Recommendations." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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Page 97

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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 97 Findings and Recommendations The NRC committee determined that the addition of a new, L-band signal, L4, offers civilian users much improved precision in many reception environments as well as preserving selective denial options for the military. The NRC committee anticipates that domestic suppliers of commercial GPS receivers, who also are the suppliers of dual-frequency military receivers, would enjoy some advantage over foreign competitors in providing dual-frequency civilian receivers. The NRC committee believes that the L4 signal could be added to several Block IIR spacecraft using the existing volume and power on the Block IIR spacecraft. If it is assumed that the L4 signal transmits at a radiated power similar to the L1 or L2 signals, then approximately 180 watts of DC power is required.25 The exact amount of power however, will depend on the specific frequency selected for L4. Since the current Block IIR L-band (L1, L2, and L3) navigation payloads and harnesses weigh around 160 kilograms (353 lbs), the L4 signal generation system is expected to weigh approximately one-fourth to one-fifth that amount.26 Based on information provided to the NRC committee through various presentations, it is believed that the sufficient power for an additional frequency can be made available on the Block IIR spacecraft by utilizing the currently unused Reserve Auxiliary Payload power margin, and by re-definition and re-allocation of other existing margins. In order to add a new signal, several Block IIR hardware modifications are required, including the addition of a frequency synthesizer, modulator/intermediate power amplifier, a high-power amplifier, and a payload processor.27 The NRC committee believes that adequate space for this additional hardware currently exists on the Block IIR spacecraft. Based on cost information for the current Block IIR L-band navigation package, the committee believes that the addition of another, unencrypted L-band signal would cost approximately $1.3 million per Block IIR satellite.28 Immediate steps should be taken to obtain authorization to use an L-band frequency for an additional GPS signal, and the new signal should be added to GPS Block IIR satellites at the earliest opportunity. 25 Information provided by Martin Marietta Astro Space Division of Lockheed-Martin, 6 February 1995. 26 Information provided by Martin Marietta Astro Space Division of Lockheed-Martin, 12 April 1995. 27 Information provided by Martin Marietta Astro Space Division of Lockheed-Martin, 6 February 1995 and by ITT Corporation, 13 March 1995. 28 It is estimated that the non-recurring design and development costs for each of the existing Block IIR L-band signals are $11 million, and the unit price for each existing L-band signal is around $500,000 per satellite. It is estimated that the cost for each L4 signal payload processor would be $100,000, and the non-recurring costs for deliverable test equipment would be $3 million. Information provided by ITT Corporation, 13 March 1995.

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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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