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TECHNICAL ENHANCEMENTS FOR FUTURE CONSIDERATION 125 clocks. A candidate future reference clock is the hydrogen maser. For terrestrial use, oscillators based on hydrogen masers have become the standard because they provide the best combination of low-phase noise, acceptable short- and long-term drift, reliability, and cost. 5 Hydrogen masers have been developed for space use, but none have been flown to date. It is possible that hydrogen masers could possibly be incorporated on Block IIF spacecraft, and the feasibility of doing so should be examined. If it appears viable, a research and development program should be initiated to develop a suitable space-qualified hydrogen maser oscillator suitable for GPS spacecraft. Satellite-Based Integrity Monitoring Perhaps the most innovative and promising method of signal integrity monitoring is through space-based monitoring, rather than ground-based monitoring. This capability known as Satellite Autonomous Integrity Monitoring or SAIM, would require the instrumentation of GPS satellites to monitor transmitted L-band signals from each other for accuracy and usability. If an anomalous signal is detected, neighboring satellites could inform the faulty satellite through the crosslinks. The faulty satellite could then autonomously begin broadcasting a code that could not be tracked by users' receivers. At the same time, the faulty satellite could inform the master control station (through the crosslinks) that there is a problem. With SAIM, the response time for commanding the faulty satellite to transmit a nonuseable code to the users after detection of a signal anomaly would be less than 1 second. Such a response time would meet many of the current integrity requirements, including those of the most stringent aviation applications. In order to fully implement SAIM, however, extensive satellite modifications are necessary.6 For example, a new crosslink design concept is required that is based on a CDMA (Code Division Multiple Access) protocol rather than the current Time Division Multiple Access (TDMA) protocol. This new crosslink would transmit the same navigation message observed by users to each neighboring satellite, which could then detect anomalies in the message. Since the required design modifications could be significant, fully operational SAIM would probably have to be incorporated in the Block IIF satellite design rather than the Block IIR satellites already under construction. There is, however, a less extensive modification that could be incorporated in the Block IIR satellites to provide significant interim improvements in integrity monitoring. This modification would consist of the installation of a radio frequency field probe in the antenna near-field regions of the Block IIR satellite, which would monitor the integrity of its own satellite's L-band transmissions. Since the Block IIR crosslink transponder data unit is currently designed to transmit data every 36 seconds, the integrity information derived from 5 Hydrogen masers are used for very long-baseline interferometry, which is used by Earth scientists to monitor tectonic deformations and Earth orientation. 6 Based on information submitted to the committee by Martin Marietta Astro Space Division of Lockheed-Martin, which was reviewed by the Block IIR payload supplier, ITT, 24 January 1995.