will reduce service coverage.  In the event that the actual receiver differs significantly

from the "model" receiver used to develop the standards, an independent assessment

of service coverage may be advisable by means of computer simulations.

6.1.5.4  Integrity Checking

The service reliability concept defined here is closely related to the NATO concept of

integrity. Consequently, user equipment that employs integrity checking algorithms may

be able to detect the majority of "service failures" and continue to maintain a valid

position solution by choosing a set of satellites which excludes the one experiencing

the service failure.  Various integrity monitoring algorithms have been developed by the

civil aviation community which are well documented in open technical literature, and

most receiver manufacturers are familiar with them.  Most of these algorithms are

based on the principle of a consistency check using additional range measurements

and developing multiple solutions for comparison purposes (aiding measurements can

be included). However, when such algorithms are employed, a minimum of five

measurements are usually required, rather than the four required for a minimum

position solution.  Therefore, the overall system availability is likely to be determined by

the availability of the integrity decision, rather than the availability of the navigation

solution.  Fortunately, the availability of an integrity decision based on PPS

measurements is extremely high, since PPS is not subject to SA "noise" which can

make SPS integrity decisions more difficult.  Table 6 9 gives some results for the

availability of an integrity decision from a recent study of a PPS integrity algorithm for

military aviation which included pressure altimeter aiding.  The results are based on a

five degree mask angle and a 556 metre position error threshold, suitable to protect the

accuracy required for a nonprecision approach. The probability of detecting a service

failure for this algorithm is 0.999, which when multiplied by the probability of occurrence

of a service failure yields an overall level of integrity in excess of 0.99999.

Table 6 9.  Availability of the Integrity Decision

Number of

Best Global

Average Global

Worst Global

Satellites

Availability

Availability

Availability

24

N/A

100.000%

N/A

23

99.998%

99.985%

99.965%

22

99.993%

99.866%

99.391%

21

99.94 %

99.37 %

97.55 %

Again, an assessment of the mission requirements is warranted to determine the

integrity threshold, probability of residual "service failures", and duration of integrity

"outages" that can be tolerated.  For example, an application that involves safety of life

may require that a position solution be declared invalid unless a positive confirmation

of integrity is achieved. In contrast, a weapons delivery system might allow the position

solution to be valid unless a  negative assertion of integrity  is determined, with the

residual loss of integrity considered

6 14