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Suggested Citation:"Receiver Errors." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
Page 161

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APPENDIX C 161 can correct approximately 50 percent of the total ionospheric delay. 30 The model parameters are transmitted in the navigation message and are updated infrequently. High performance C/A-code receivers often perform codeless or cross-correlation tracking of the L2 signal to permit them to derive ionospheric correction parameters. These techniques suffer from substantial signal-to-noise ratio losses and do not work well in high-blockage or high-dynamic situations. Tropospheric delay cannot be eliminated through the use of two frequencies, but both C/A-code, and Y- code receivers can eliminate most of this error using software modelling.31 Clock and Ephemeris Error As shown in Figure C-7, the atomic clocks on board each GPS satellite are designed to provide highly accurate timing specifications. Even a small amount of inaccuracy, however, combined with the fact that the estimated orbital positions, or ephemeris, of each satellite are also not exact, can cause a certain amount of error in a receiver's position solution. Multipath Errors Multipath errors occur when incoming GPS signals bounce off a reflective surface such as a building or a body of water before reaching a user's receiver. For highly specialized receivers that are able to eliminate other error sources, pseudorange and/or carrier-phase multipath is frequently a dominant error source. Receiver Errors GPS receivers themselves introduce several sources of error to the measurement of satellite ranges. Thermal noise produced by the environment and the various components within a receiver cause small random errors. Received signal to noise ratio, quantization of the analog to digital converter, and the type of tracking loop used by a receiver are also determining factors in the noise level. Typical receiver errors can be as little as 1 centimeter or as large as several meters. This error is quite random in nature and is often reduced by averaging or smoothing over a short period of time. 30 Space Vehicle Nav System and NTS PRN Navigation Assembly/User System Segment and Monitor Station, Interface Control Document MH08-00002-400, Revision F, 25 July 1977. 31 For a typical C/A-code receiver, the remaining tropospheric ranging error amounts to approximately 0.7 meters (la). Higher quality C/A-code receivers, and Y-code receivers eliminate all but 0.2 meters of this error.

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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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