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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.