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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 91 Additional Benefits of L4 The existence of an unencrypted L4 signal greatly reduces a civilian receiver's probability of RF interference by providing a second frequency, which can be used in the event that L1 is subject to interference. The wide-band L4 signal also would aid in commercially important emerging markets where reception is less than ideal, since GPS must operate in applications subject to strong and intermittent multipath and signal blockage. The success or failure of GPS in those applications depends upon quick recovery of accurate pseudorange measurements once the signal is restored. From the military perspective, the addition of the L signal retains A-S on both L1 and L2 and is quite flexible with respect to selective denial of civilian service. Of all the frequencies mentioned above, 1237.83 MHz would be the most difficult to jam because it is the closest to L2. However, based on an analysis described in Appendix J, this frequency could be selectively jammed without affecting the use of the Y-code on L2. In order to selectively deny civilian service, broadband jamming of L1 and L4 could be used. Note that even if no navigation message is broadcast on the L4 signal, it should be jammed because the last ephemeris information could be used in combination with L4 ranging data to locate a target. It also should be noted that broadband jamming of both L1 and L4 would eliminate the capability for dual-frequency ionospheric corrections. This would reduce PPS accuracy and force the U.S. military to rely on other methods of obtaining ionospheric corrections. As discussed later in this chapter, ionospheric correction models broadcast on the navigation message remove only about 50 percent of the ionospheric error. However, by using receivers with the capability to store the last known ionospheric correction and updating that information with a process called Differential Ranging Versus Integrated Doppler (DRVID), ionospheric corrections can be improved further over the 50 percent correction obtained in the L2 broadcast models. Reduction of Receiver Noise and Multipath Errors As shown in Table 3-5, when using a typical SPS receiver, the receiver noise and multipath actually increase when another frequency is added because of the noise and multipath from the additional frequency. As a result, the beneficial effects of adding another frequency to reduce the ionospheric error are diminished. If more advanced receivers are used, reductions in the receiver noise and multipath errors can be achieved, and the HDOP can be reduced to around 1.5.24 The error reductions achieved by using a more advanced receiver results in stand-alone SPS performance ranging from 11.3 meters to 13.1 meters (2 24 The characteristics of a more advanced, dual-frequency SPS and PPS receivers (as compared to the typical receiver described previously) include: (1) use of more satellite signals in the solution (typically six to eight satellites), (2) lower noise amplifier, (3) better tropospheric model, (4) on-board multipath processing capability and low-multipath antenna, and (5) lower C/A-code measurement noise due to narrow correlator spacing. For an all-in-view receiver and a elevation mask angle of 5 degrees, an HDOP of 1.5 is predicted 95 percent of the time. Source: Analysis completed by Mr. Tom Hsiao of the MITRE Corporation, 15 February 1995.
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 92 drms), depending on the L4 signal bandwidth and frequency, as shown in Table 3-6. These accuracies would satisfy the accuracy requirements for aviation traffic alert/collision avoidance systems (TCAS). The PPS performance would improve to 11.1 meters (2 drms) or 4.6 meters (CEP), as shown in Table 3-7. With accuracy levels of 11.3 to 13.1 meters (2 drms), GPS availability also is enhanced, and RAIM is improved as well. For example, for a stand-alone horizontal accuracy of 100 meters, the availability of four satellites would increase from the previous value of 99.94 percent to approximately 99.96 percent. RAIM availability, which is dependent on the presence of six useable satellite signals, is shown in Table 3-8. Although not shown in Tables 3-6 or 3-7, even further improvements to the receiver noise and multipath errors can be made through use of the most advanced receivers that have improved receiver signal processing, are integrated with auxiliary sensors, and have multi-element antenna arrays.
Table 3-5 Elimination of Ionospheric Error by the Addition of Another Frequency. Error Source Typical Range Error Magnitude (meters, 1Ï) SPS With II/IIA SPS Improved (no SA, additional narrow L-band signal) SPS Improved (no SA additional wide L- Satellites band 1237.83 1258.29 1841.40 1258.29 1841.40 Narrow-band, C/A- Narrow-band, C/A- Narrow-band, C/A- Wide-band, P-type Wide-band P-type type code type code type code code Selective Availability 24.0 0.0 0.0 0.0 0.0 0.0 Atmospheric Error Ionospheric 7.0 0.01 0.01 0.01 0.01 0.01 Tropospheric 0.7 0.7 0.7 0.7 0.7 0.7 Clock and Ephemeris Error 3.6 3.6 3.6 3.6 3.6 3.6 Receiver Noise 1.5 4.6 4.9 6.9 2.7 5.6 Multipath 1.2 3.7 3.9 5.6 2.7 4.8 Total User Equivalent 25.3 6.9 7.3 9.6 5.3 8.2 Range Error (UERE) Typical Horizontal DOP 2.0 2.0 2.0 2.0 2.0 2.0 (HDOP) Total Stand-Alone 101.2 27.8 29.0 38.5 21.2 32.9 PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION Horizontal Accuracy (2 drms) 93
Table 3-6 Effect of Reduced Ionospheric Error by the Addition of Another Frequency and Additional Improvements with Using a More Advanced SPS Receivera Error Source Typical Range Error Magnitude (meters, 1Ï) SPS With II/IIA SPS Improved (no SA, additional narrow L-band signal) SPS Improved (no SA, additional wide L- Satellites band signal 1237.83 1258.29 1841.40 1258.29 1841.40 Narrow-band, C/A- Narrow-band, C/A- Narrow-band, C/A- Wide-band, P-type Wide-band P-type type code type code type code code code Selective Availability 24.0 0.0 0.0 0.0 0.0 0.0 Atmospheric Error Ionospheric b 7.0 0.01 0.01 0.01 0.01 0.01 Troposhericc 0.7 0.2 0.2 0.2 0.2 0.2 Clock and Ephemeris 3.6 3.6 3.6 3.6 3.6 3.6 Error Receiver Noised 1.5 0.6 0.7 0.9 0.5 0.8 Multipathe 1.2 1.5 1.6 2.3 1.0 1.9 Total User Equivalent 25.3 3.9 4.0 4.3 3.8 4.2 Range Error (UERE) Typical Horizontal DOP 1.5 1.5 1.5 1.5 1.5 1.5 (HDOP)f PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION Total Stand-Alone 76.0 11.9 12.0 13.1 11.3 12.5 Horizontal Accuracy (2 drms) 94
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 95 a. The characteristics of a more advanced, dual-frequency SPS receiver (as compared to the typical receiver described previously) include: (1) use of more satellite signals in the solution (typically six to eight satellites), (2) lower noise amplifier, (3) better tropospheric model, (4) on-board multipath processing capability and low multipath antenna, and (5) lower C/A-code measurement noise due to narrow correlator spacing. b. With the addition of an unencrypted, coded signal, the SPS ionospheric error is removed by a linear combination of the L and L 1 4 observables. This correction leaves residual ionospheric error of 1 centimeter or less. c. For improved receivers, software models correct for all but around 0.2 meters (1Ï) of the tropospheric error. d. For an improved SPS receiver, the receiver noise for independent 1-second measurements can be as low as 0.2 m for the narrow-band signal, and 0.1 meter for the wide-band signal. These are the single-frequency errors and must be increased to account for the linear combination used to calibrate ionospheric errors. For example, the narrow-band error must be multiplied by a factor of 3.1 when 1237.83 MHz and 1575.42 MHz (L1) frequencies are used. e. For an SPS receiver with a low-multipath antenna and on-board multipath reduction processing, the multipath can be as low as 05 meters (1Ï) for the narrow-band signal, and 0.2 meters (1Ï) for the wide-band signal. These errors are very dependent on the number of reflective objects near the antenna. These are the single-frequency errors and must be increased to account for the linear combination used to calibrate ionospheric errors. For example, the narrow-band error must be multiplied by a factor of 3.1 when 1237.83 MHz and 1575.42 MHz (L1) frequencies are used. f. For an all-in-view receiver and a elevation mask angle of 5 degrees, an HDOP of 1.5 or less was predicted 95 percent of the time. Source: Analysis completed by Mr. Tom Hsiao, the MITRE Corporation, 15 February 1995. Table 3-7 Effect of Using a More Advanced PPS Receiver on Stand-Alone Accuracya Error Source Typical Range Error Magnitude (meters, 1Ï) PPS with Typical Receiver PPS with Advanced Receiver Selective Availability 0.0 0.0 Atmospheric Error Ionosphericb 0.01 0.01 Troposphericc 0.7 0.2 Clock and Ephemeris Error 3.6 3.6 Receiver Noised 0.6 0.3 Multipathe 1.8 0.6 Total User Equivalent Range Error (UERE) 4.1 3.7 Typical Horizontal DOP (HDOP)f 2.0 1.5 Total Stand-Alone Horizontal Accuracy, 2 drms 16.4 11.1 a.The characteristics of a more advanced, dual-frequency PPS receiver (as compared to the typical receiver described previously) include: (1) use of more satellite signals in the solution (typically six to eight satellites), (2) lower noise amplifier, (3) better tropospheric model, and (4) on-board multipath processing capability and low-multipath antenna.
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 96 b. For a PPS receiver, the ionospheric error is removed by a linear combination of the L and L observables. This correction leaves 1 2 residual ionospheric error of 1 centimeter or less. c. For improved PPS receivers, software models correct for all but around 0.2 meters (lÏ) of the tropospheric error. d. For an improved PPS receiver, the receiver noise for independent 1-second measurements can be as low as 0.1 meters (1Ï). These are the single-frequency errors and must be increased to account for the linear combination used to calibrate ionospheric errors. The single- frequency error of 0.1 meters must be multiplied by a factor of 3 when the standard L2= 1227.6 MHz and L2= 1575.42 MHz frequencies are used. e. For an improved PPS receiver with a low-multipath antenna and on-board multipath reduction processing, the multipath can be as low as 0.2 meters (1Ï). These errors are very dependent on the amount of reflective objects near the antenna. These are single-frequency errors and must be increased to account for the linear combination used to calibrate ionospheric errors. For example, the single-frequency error of 0.2 m must be multiplied by a factor of 3 when the standard L2 = 1227.6 MHz and L1 = 1575.42 MHz frequencies are used. f. For an all-in-view receiver and a elevation mask angle of 5 degrees, an HDOP of 1.5 or less was predicted 95 percent of the time. Source: Analysis completed by Mr. Tom Hsiao, the MITRE Corporation, 15 February 1995. Table 3-8 Effect of SA Removal and Dual-Frequency Capability on RAIM Availability for Aviation Applicationsa Aviation Application Availability With SA Set to Zero Availability With SA Turned to Zero and L4 Added Phase of Flight Protection Limit 21 Satellitesb 24 Satellitesc 21 Satellites 24 Satellites En Route 2.0 nautical miles 96.34% 99.98% 96.80% 100.00% Terminal Area 1.0 nautical miles 94.39% 99.95% 95.19% 99.98% Non-precision 03 nautical miles 91.10% 100.00%d 93.12% 100.00%d Approach a. This analysis has been made for a single frequency C/A-code receiver aided by a barometric altimeter (required for aviation supplemental navigation use of GPS) with a visibility mask angle of 5 degrees. b. The probability of having 21 satellites operating is assumed to be 98 percent. c. The probability of having 24 satellites operating is assumed to be only 70 percent. However, the values in this table reflect the fact that if 24 satellites are fully operational, an incremental improvement in availability exists. d. Although these values would intuitively be lower than the 1 nautical mile terminal area protection limit value, availability improves for the 03 nautical mile non-precision protection limit because the barometric altimeter inputs provide extra information in this phase of flight.
