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APPENDIX C 169 determines its position by computing lines of position based on radio pulse transmissions from three stations within a chain.43 As with GPS/INS integration, the addition of another navigation system provides redundancy. If GPS signal reception is poor due to a lack of satellites in view or due to signal interference, an integrated system can maintain a specified level of navigation performance using only Loran-C. The system integrity and availability of a GPS/Loran-C system is also improved over GPS alone. A study focused on integrity and availability requirements for aviation non-precision approaches has shown that RAIM performance is significantly improved by the presence of Loran-C signals, and availability improves from 99 percent for a GPS receiver with RAIM and a barometric altimeter to 99.7 percent for a GPS/Loran-C receiver with RAIM.44 The integration of Loran-C with DGPS has also been proposed as a potential means of improving both integrity and accuracy. Integrity information and differential corrections could potentially be broadcast on Loran- C signals from existing ground-based transmitter stations to GPS/Loran-C receivers. If this proposal proves to be technically feasible, the entire continental United States and Western Europe could potentially be provided with DGPS capability using Loran-C signals.45 PERMANENT DIFFERENTIAL GPS AUGMENTATIONS It is impossible to estimate the number of temporary DGPS networks in use around the world at any given time because of the ease with which they can be established, utilized, and then removed. GPS users such as surveyors and resource monitors may go through this process several times in one day. It is possible, however, to describe some of the permanent DGPS services that are currently operating or are under development by the U.S. government, state and local governments, foreign governments, and the private sector. 43 More information about Loran-C can be found in the Federal Radionavigation Plan. 44 The availability of Loran-C alone for non-precision approaches is approximately 99.75 percent. Source: James V. Carroll, "Availability Performance Comparisons of Combined Loran-C/GPS and Stand-alone GPS Approach Navigation Systems." in Proceedings of the IEEE Position Location and Navigation Symposium (Las Vegas Nevada, April 1994), pp. 77-83. 45 Lambert J. Beekhuis and Hein J. Anderson, "EuroFix and the Effect of Cross Rate Interference," in Proceedings of ION- GPS 93: 6th International Technical Meeting of the Satellite Division of the Institute of Navigation (Salt Lake City, Utah, September 1993), pp. 721-729.