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APPENDIX C 167 augmentations. RAIM algorithms rely on redundant GPS satellite measurements as a means of detecting unreliable satellites or position solutions. All RAIM approaches look for inconsistencies in either the raw measurements or in the position solutions derived from these measurements. RAIM techniques are generally most effective when six or more satellites are in view of the receiver. This means that RAIM alone is not always the best way to improve GPS integrity, and other solutions are often required.38 Combined Use of GPS and GLONASS GLONASS is often discussed as a potential means of augmenting the basic capabilities of GPS by providing additional ranging signals to a user, and integrated GPS/GLONASS receivers are available from a limited number of suppliers. GLONASS, or Global Navigation Satellite System, which is operated and managed by the military of the former Soviet Union, consists of three segments just as GPS does. The GLONASS space segment also is designed to consist of 24 satellites, but these satellites are to be arranged in three 64.8Âº orbital planes 19,100 kilometers (11,870 miles) above the Earth, rather than six planes. The full GLONASS constellation is currently scheduled to be completed in 1995.39 GLONASS differs most from GPS in the way that the user segment differentiates one satellite from another. Instead of each satellite transmitting a unique PRN code as GPS satellites do, GLONASS satellites all transmit the same PRN code on different channels or frequencies.40 All of these frequencies, however, are in the L-band spectrum near eitherthe GPS L1 or L2 signal, which simplifies the task of designing integrated receivers. There are still two additional differences between the two systems that must be taken into consideration by combined receiver designers. First, GPS and GLONASS use different time standards for system synchronization. GPS utilizes UTC (Coordinated Universal Time) maintained by the U.S. Naval Observatory (UTC[USNO]), whereas GLONASS uses the UTC standard kept in the former Soviet Union (UTC[SU]. Discrepancies between these two time scales can reach tens of microseconds, which is significant for systems that keep time with better than 1 microsecond accuracy. Secondly, GPS and GLONASS use different coordinate systems. GLONASS positioning is based on the Soviet Geodetic System (SGS 85), while GPS uses the World Geodetic System (WGS 84) for position determination. Discrepancies between these coordinate systems exist, and must be corrected by combined receivers. 38 More detailed information about RAIM can be found in: R. Grover Brown, "A Basic GPS RAIM Scheme and a Note on the Equivalence of Three Raim Methods," Navigation: The Journal of the Institute of Navigation 39, no. 3 (1992). 39 Further information about the status of the GLONASS program is available from the National Air Intelligence Center, Wright-Patterson Air Force Base, Ohio, which routinely monitors GLONASS developments. 40 This technique is known as FDMA (Frequency Division Multiple Access).