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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 79 Impact of SA on Civil Users Turning SA to zero, or completely eliminating SA, would have an immediate positive impact on civil GPS users. The following benefits can be expected: â¢ improved stand-alone navigation, positioning, and timing accuracy; â¢ improved availability for any given positioning accuracy; â¢ enhanced ability to perform RAIM; â¢ reduced data rate requirements for DGPS corrections; â¢ enable system modifications that further improve accuracy; and â¢ improved WAAS. Each of these benefits is discussed further below. Increased Stand-Alone Navigation, Positioning, and Timing Accuracy. The stand-alone accuracy for SPS users would immediately increase from 100 meters (2 drms) to around 30 meters (2 drms) if SA were turned to zero, as shown in Table 3-3.8 For many users currently employing DGPS techniques, such as emergency response vehicles, accident data collection, and vehicle command and control, stand-alone horizontal accuracy of approximately 30 meters (2 drms) is sufficient. Currently, DGPS-equipped receivers cost substantially more (several hundred dollars) than a stand-alone receiver. Savings would result from the elimination of the need for a DGPS receiver and electronics to insert the messages to the GPS receiver. Savings also will result from elimination of the user fee imposed by private DGPS providers. 8 Recent measurements with SA off have ranged from 5 meters to 10 meters (2 drms). However, the accuracy without SA greatly depends on the condition of the ionosphere at the time of observation and user equipment capabilities.
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 80 Table 3-3 The Effect of Eliminating SA on GPS SPS Stand-Alone Horizontal Accuracya Error Source Typical Range Error Magnitude (meters, 1Ï) SPS with SA (II/IIA Satellites) SPS with No SA (II/IIA Satellites) Selective Availability 24.0 0.0 Atmospheric Delay Ionospheric 7.0 7.0 Tropospheric 0.7 0.7 Clock and Ephemeris Error 3.6 3.6 Receiver Noise 1.5 1.5 Multipath 1.2 1.2 Total User Equivalent Range Error (UERE) 25.3 8.1 Typical Horizontal DOP (HDOP) 2.0 2.0 Total Stand-Alone Horizontal Accuracy, 2 drms 101.2: 32.5 a All footnotes to Table 3-1 also apply to Table 3-4. Improved Availability. As explained earlier in this chapter, GPS availability is directly related to accuracy. When the stand-alone horizontal accuracy of the system improves to around 30 meters (2 drms), the availability of any accuracy greater than 30 meters will increase. For example, the average observed availability of the 100- meter (2 drms) SPS for a receiver located in Chicago, Illinois is currently 99.2 percent. For the same 100-meter accuracy level with SA removed, the availability would increase to approximately 99.94 percent.9 Enhanced Integrity Monitoring. The ability of a receiver to detect invalid GPS pseudorange measurements autonomously also would be greatly enhanced if SA were turned to zero. RAIM is generally possible if six or more satellites are visible and are providing pseudorange accuracies that allow the easy detection of an inaccurate signal. With SA set at its current level, each satellite range may be in error by 25 meters (ls) or more, as shown in Table 3-3. This makes it difficult to distinguish a failure. Without SA, pseudorange accuracy improves to almost 8 meters (la), dramatically improving the ability to isolate specific satellite faults, as well as signal tracking problems within the receiver itself. An analysis of the impact on RAIM with the elimination of SA was conducted for this study by the MITRE Corporation. The improved RAIM capability has been quantified in terms of 9 Based on analysis conducted by the MITRE Corporation for the Memorandum from the MITRE Corporation to the NRC committee, 7 February 1995. For more details, see footnote 1 earlier in this chapter.
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 81 the availability of six useable satellites for three phases of aircraft flight. These results are shown in Table 3-4 and discussed further in Appendix F. Table 3-4 Effect of SA Removal on RAIM Availability for Aviation Applicationsa Aviation Application Availability With SA at its Current Availability With SA Turned to Zero Level Phase of Flight Protection Limit 21 Satellitesb 24 Satellitesc 21 Satellites 24 Satellites En Route 2.0 nautical miles 93.16% 99.89% 96.34% 99.98% Terminal Area 1.0 nautical miles 89.96% 94.39% 94.39% 99.95% Non-Precision 03 nautical miles 80.89% 98.88%d 91.10% 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 0.3 nautical mile non-precision protection limit because the barometric altimeter inputs provide extra information in this phase of flight. Reduced Data Rate Requirements for DGPS Corrections. In addition to reduced receiver costs and DGPS provider fees, a stand-alone horizontal positioning accuracy of approximately 30 meters (2 drms) would allow users to avoid the complexity and expense of receiving differential corrections or post-processing their data. Users requiring accuracies from around 1 meter to 30 meters could still use DGPS, but at a much reduced update rate.10 10 The required update rates are derived below, assuming 0.2 meters is allotted to the clock portion of the differential correction for SA at its present nominal level and for SA turned to zero. In addition, this analysis is only valid assuming that precise range-rate information is provided in the navigation message. The result is that the update rate is about two orders of magnitude lower when SA is turned to zero. This advantage would be less for lower accuracy requirements. Other requirements may force higher update rates for specific differential users. Example with SA at current level: The lÂ¬ SA range acceleration is 0.004 m/s2 from Table 3-2. In order to calculate the update rate required for differential corrections, set 0.2 m = 0.5(a)(t2), where a = 0.004 m/s2. Solving for t results in a required update period of t = 10 seconds.