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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 68 CURRENT GPS PERFORMANCE Accuracy As can be seen from Table 3-1, the contributors to civilian SPS signal accuracy errors are SA (Selective Availability), the atmospheric error, the clock and ephemeris errors, the receiver noise error, and the multipath error. For the military PPS (Precise Positioning Service) signal, the largest error contributors are the clock and ephemeris errors, the receiver noise, and multipath errors, since the PPS signal is not degraded by SA. The ionospheric error for the PPS signal is small relative to that for the SPS signal since the military has access to both the L1 and L2 frequencies and can correct for the ionospheric error. Table 3-1 Observed GPS Positioning Errors with Typical SPS and PPS Receiversa Error Source Typical Range Error Magnitude (meters, 1Ï) SPS with II/IIA satellites PPS with II/IIA satellites Selective Availabilityb 24.0 0.0 Atmospheric Error Ionosphericc 7.0 0.01 Troposphericd 0.7 0.7 Clock and Ephemeris Errore 3.6 3.6 Receiver Noisef 1.5 0.6 Multipathg 1.2 1.8 Total User Equivalent Range Error (UERE) 25.3 4.1 Typical Horizontal DOP (HDOP)h 2.0 2.0 Total Stand-Alone Horizontal Accuracy, 2 drmsi 101.2 16.4 a. It is assumed here that a ''typical" SPS and PPS receiver has a four-satellite position solution. b. J. F. Zumberge and W. I. Bertiger, "Ephemeris and Clock Navigation Message Accuracy in the Global Positioning System," Vol. I, Chap. 16. Edited by B. W. Parkinson, J. J. Spilker, P. Axelrad, and P. Enge (To be published by AIAA, in press 1995). This error is manifested as increased clock and ephemeris errors when SA is on. c. For the SPS signal, the ionospheric content is quite variable, with large diurnal variations, and large variations over the 11-year solar cycle. Depending on the Total Electron Content (TEC), a delay at L, ranging from less than 1 meter to 70 meters can result. A typical SPS receiver has an algorithm that can remove about 50 percent of the ionospheric error, leading to an error ranging from less than 1 meter to 35 meters. For the above table, an error of 7 meters was used, which is typical for a daytime mid-latitude ionospheric error near the maximum of the 11-year solar cycle, after correction by the standard algorithm. Because the ionospheric error is not independent between satellites, it should not strictly be considered a range error to be multiplied by HDOP (Horizontal Dilution of Precision). When the ionospheric content is uniform above the receiver, such as during the pre-sunrise morning, it contributes little to horizontal error, but maps into errors in the vertical position and receiver clock. When there are significant gradients in the ionospheric content, however, such as exist at local dawn and dusk, errors are induced into the horizontal position. Therefore, the use of 7 meters for a range error, which is multiplied by HDOP, is a somewhat conservative choice. For the PPS signal the ionospheric error is removed by a linear combination of the L1 and L2 observables. This correction leaves residual ionospheric error of 1 centimeter or less.
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 69 d. For a typical SPS or PPS receiver, software models correct for all but around 0.7 meters (la) of the tropospheric error. The tropospheric error is even more highly correlated than the ionospheric error, due to its uniform distribution. The errors introduced by the troposphere normally map into the vertical position and receiver clock errors. As for the ionospheric error, the multiplication of this error by HDOP to obtain the horizontal error is a conservative calculation. e. This value is based on observed data as noted in "Ephemeris and Clock Navigation Message Accuracy in the Global Positioning System." (See note a above). The combined clock and ephemeris error does not contain SA epsilon error in the broadcast ephemeris nor the SA dither error in the broadcast time. f. For a SPS receiver, the receiver noise for independent 1-second measurements can actually range from around 0.25 to 2.0 meters, depending on its design. For a PPS receiver, the single-frequency pseudorange noise error is less because the ten times faster Y-code chip rate overcomes the 3 dB to 6 dB signal-to-noise ratio penalty relative to the C/A code. In forming the linear combination required to removed the ionospheric error, Y-code corrected = 2.55(Y)L1-1.55(Y)L2, the noise error of the Y-code is effectively multiplied by the root sum square of 2.55 and 155, which is approximately 3. (A single-frequency PPS receiver like the Plugger would have a receiver range error smaller by a factor of three, but at the cost of retaining a 7-meter error due to the ionosphere). The PPS receiver noise error can range from 0.1 to 0.8 meters (1s), for independent 1-second measurements. g. For a SPS receiver, multipath can typically range from 0.4 to 5 meters (1s), depending on the antenna, antenna surroundings, and receiver design. For a PPS receiver, the single-frequency multipath error is somewhat less, typically by a factor of 0.5, because of the faster chip rate. In forming the linear combination required to remove the ionospheric error, Y-code corrected = 2.55(Y)L1-1.55(Y)L2, the Y-code multipath error is effectively multiplied by the root sum square of 2.55 and 1.55, which is approximately 3. This explains why the PPS multipath error exceeds the SPS multipath error. (A single-frequency PPS receiver like the Plugger would have a multipath error smaller by a factor of three, but at the cost of retaining a 7-meter error due to the ionosphere). The PPS multipath error can range from 0.3 to 2 meters. h. HDOP can vary depending on the geometry of the satellites. For a typical SPS or PPS receiver, the geometric strength of a four- satellite solution is limited, so a conservative HDOP of 2.0 was used. i. These values are based on observation and differ from the accuracy values specified by the DOD (Department of Defense), shown in Figure C-7, Appendix C. Specific technical modifications to GPS to reduce the errors discussed above and improve the accuracy for both the military and civilian communities are discussed in detail below. As explained in the table notes above, the exact numbers in the tables can vary. If all of the recommendations are implemented, the committee believes that the stand-alone horizontal GPS accuracy will approach 5 meters (2 drms). Greater stand-alone accuracy could take the place of differential GPS systems for some users who require accuracies of a few meters (2 drms). For example, greater standalone GPS accuracy would allow many vehicle positioning and navigation requirements to be met without the use of DGPS. To use a military example, precision weapons, such as missiles and smart bombs that have been equipped with GPS, presently require expensive
