Receiver autonomous integrity monitoring (RAIM) is a method for ensuring the integrity of GPS through the use of redundant satellites. For a GPS position solution the pseudoranges of at least four satellites are required. If more than four satellites are in view, the resulting redundancy may be used for integrity by determining the consistency among all of the pseudorange measurements. Thus, in principle, if five satellites are in view, it may be possible to detect the presence of a large position error but not to identify which satellite's pseudorange is erroneous. If six or more satellites are in view, it may be possible to identify a faulty satellite that is causing a large position error. However, the ability of RAIM to perform the detection and identification functions depends upon the relative geometry between the satellites and the user's location, and upon the nominal pseudorange errors as expressed as a standard deviation. At any place and time the geometry is fixed. Therefore, RAIM improvement would be possible if the standard deviation of pseudorange errors could be significantly decreased. This significant decrease can be obtained by setting selective availability to zero. The effect on RAIM due to setting selective availability to zero can be measured in terms of availability and RAIM outage duration.

RAIM algorithm requirements involve alarm rate, probability of missed detection, and position protection level. False alarm rate must be controlled; otherwise RAIM would be a nuisance. Of course, the missed-detection probability must be low to provide protection when large errors occur. The position protection level means that there will be an extremely low probability that the user's position error will exceed this level without a warning. Table 1 contains a summary of the RAIM algorithm requirements used in the following analysis. These requirements are presently used by the FAA in its evaluation of RAIM.

Four RAIM augmentations were investigated. They are:

1.  Redundant pseudoranges
2.  Redundant pseudoranges + altimeter input
3.  Redundant pseudoranges + accurate clock
4.  Combination of all above

The altimeter input provides another range source. An accurate crystal or small atomic clock is calibrated when RAIM is available, and used if RAIM becomes unavailable.

Effects of eliminating Selective Availability (SA) on RAIM are considered only for the en route and nonprecision approach phases of flight. The effects are not considered for precision approach because the required accuracy for that phase of flight is too high to meet even with elimination of SA.

For all of the above RAIM augmentations, availability and outage durations were calculated for routes between major city pairs for en route navigation and at representative terminal areas for nonprecision approach. These are listed in Table 2. Then their average availabilities were tabulated. Separate tabulations were made for

Front Matter (R1-R20)

Executive Summary (1-12)

1 Introduction (13-18)

2 GPS Applications and Requirements (19-66)

3 Performance Improvements to the Existing GPS Configuration (67-122)

4 Technical Enhancements for Future Consideration (123-134)

Appendix A: Study Participants (135-138)

Appendix B: Abbreviated Committee Biographies (139-144)

Appendix C: Overview of the Global Positioning System . . . (145-176)

Appendix D: Accuracy Definitions and Mathematical Relationships (177-178)

Appendix E: Report From Mr. Michael Dyment, Booz, Allen & Hamilton (179-200)

Appendix F: Report From Dr. Young Lee, The MITRE Corporation (201-212)

Appendix G: Increased Bandwidth Performance Analysis (213-214)

Appendix H: Signal Structure Options (215-220)

Appendix I: Report from Mr. Melvin Barmat, Jansky/Barmat... (221-248)

Appendix J: Selective Denial of Civilian GPS Signals by the Military (249-252)

Appendix K: Direct Y-Code Acquisition (253-254)

Appendix L: Enhanced Signal Structures for the Military (255-262)

Appendix M: Accuracy of a 14-Satellite Ensemble Versu a 24-Satellite Ensemble (263-264)