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APPENDIX C 160 Table C-1 GPS Positioning Errorsa Error Source Range Error Magnitude (meters, one sigma) SPS PPS Selective Availability 24.0 0.0 Atmospheric Delay 7.0 0.7 Clock and Ephemeris 3.6 3.6 Multipath 1.2 1.8 Receiver Noise 0.6 0.6 Total User Equivalent Range Error (UERE)b 253 4.1 Typical Horizontal DOP (HDOP)c 2.0 2.0 Total Stand-Alone Horizontal Accuracy, 2 drmsd 101.2 16.4 a. The error budget figures included in this table are conservative estimates for a typical stand alone C/Acode receiver using standard correlation techniques, and a typical dual frequency Y-code receiver. This information was provided by the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration (NASA), Pasadena, CA. Notes related to each component of this error budget, and the assumptions made to derive its value, are provided with Table 3-1 in Chapter 3. b. The total UERE is determined by adding the squares of the individual error magnitudes and taking the square root of the total. c. Dilution of precision (DOP) is discussed below, and HDOP is mathematically defined in Appendix D. d. The 2 drms horizontal positioning error is equal to 2 times UERE times HDOP. This mathematical relationship is further defined in Appendix D. Atmospheric Error Atmospheric error is caused by the delay of the GPS signal as it passes through the Earth's atmosphere. Part of this delay is due to the troposphere and part is due to the ionosphere. Because the ionospheric effect is dispersive and is a function of frequency, dual-frequency GPS receivers can calibrate this effect by measuring the differential delay and/or phase advance between the L1 and L2 frequencies, thus eliminating a great deal of the atmospheric error. Civil users do not have direct access to dual frequency observations but have several means for reducing the ionospheric error contribution. For stand-alone navigation most C/A-code receivers apply an ionospheric correction, known as the Klobuchar Model, which
