6.1.4.2  GPS Service Failure Characteristics

A GPS service failure is defined as an excursion of unpredictable magnitude of the

horizontal position solution due to a control segment or satellite fault which is unrelated

to the normal predictable long term PPS horizontal accuracy distribution.  A GPS

service failure is characterized by a large single satellite range error which is unrelated

to the normal long term PPS range error distribution.

The characteristics of a service failure and the factors which affect service reliability are

listed below.  Each is discussed in more detail in the following sections.

Ranging signal failure frequency

Failure duration

Failure magnitude and behavior

Distribution of user population around the globe

Probability that the failed satellite is used in the position solution

Effect that the failure has on the position solution, given the failed satellite s

contribution to solution geometry and the receiver s response to the failure

condition.

6.1.4.3  Failure Frequency Estimate

The GPS satellite positioning service failure history over the past several years

indicates a very low service failure rate (excluding Block I satellites).  However, when a

service failure does occur, it can result in extremely large position and/or velocity

errors.  This behavior will typically persist until action is taken to remedy the problem.

Based upon an historical assessment of Block II satellite and Control Segment failure

characteristics, GPS should experience no more than three major service failures per

year (excluding Block I satellites).  This failure rate estimate is conservative  

expectations are on the order of one per year, based upon projected navigation

payload component reliabilities and the assumption that action will be taken to switch

redundancy configurations if early indications of an imminent failure are detected.  An

allocation of three per year allows for a possible increase in service failures as the

Block II satellites reach the end of their operational life expectancy.

6.1.4.4  Failure Duration Estimate

The duration of a failure is a function of the following factors:

Control Segment monitor station coverage

Control Segment monitor station, communications and Master Control Station

availability

Master Control Station failure detection efficiency and timeline

Timeline for correcting the problem or terminating the failed satellite s service.

6 9