Use of GPS as a supplemental navigation system only requires the former; use of GPS as a primary means of navigation requires both the former and the latter. A supplemental navigation system requires a primary navigation system to be part of the avionics so that in the event of loss of the supplemental system, the pilot can use the primary navigation system. A primary navigation system can operate on its own. Today GPS can be used as a supplemental means of navigation. In the future when GPS is used as a primary means of navigation, RAIM (or some external system) would have to provide the identification function.

Table 3 contains the results for the RAIM detection function when SA is present (pseudorange standard deviation = 33 m) and when SA is absent (pseudorange standard deviation = 4.3 m for dual frequency users and 8.3 m for single frequency users). While the GPS satellite constellation with all 24 satellites operating represents the best case for GPS satellite availability, the probability that all 24 satellites will be operating is estimated to be only about 70 percent. On the hand, DOD guarantees at least 21 satellites to be available with 98 percent probability, and thus the 21 satellite constellation represents a realistic case to address for a primary system.

The results of Table 3 indicate significant improvement when selective availability is set at zero. Since the FAA requires only barometric altimeter input to RAIM for supplemental navigation, the availability improvement from about 90 to 99 percent for supplemental nonprecision approach is very significant when a typical set of 21 satellites are operating.

The results of Table 4 again indicate significant improvement when selective availability is set at zero. The improvement of availability of RAIM identification function for a nonprecision approach is from about 94 to over 99 percent when a typical set of 21 satellites are operating. This is a significant improvement.

The results indicate that if selective availability is set to zero, RAIM availability and outage durations will be significantly improved. As shown in the Air Navigation Requirements Table in NRC report, the required availability for the FAA's Wide Area Augmentation System is 99.999 percent for GPS to be used as a primary navigation system in the en route and nonprecision approach phases of flight. The results of the above analysis indicate that this level of availability cannot be achieved by RAIM alone even when selective availability is set to zero unless perhaps access to dual frequency is available, and the constellation contains at least 24 satellites (Table 4).