Performance of Stand-Alone GPS - Understanding GPS: Principles and Applications - page 360

Table 7.16 Maximum Duration of RAIM/FDE Outages over CONUS
with a 5º Mask Angle and Three Geostationary Satellites with SA Off

Nonprecision
Approach RAIM/FDE Function En Route Terminal

Fault detection 0 minutes 0 minutes 0 minutes

Fault detection and exclusion 5 minutes 15 minutes 65 minutes

Fault detection and exclusion
with baro aiding

0 minutes 5 minutes 35 minutes

immediately detected by the ground subsystem, such that the broadcast data are
affected and there is an induced position error in the airborne subsystem. The differ-
ential corrections and integrity parameters for each monitored satellite are broad-
cast to the aircraft via a VHF datalink. GBAS systems are discussed in detail in
Chapter 8.

7.6 Continuity

Continuity, as defined in [37], is “the probability that the specified system perfor-
mance will be maintained for the duration of a phase of operation, presuming that
the system was available at the beginning of that phase of operation.” The level of
continuity provided by GPS thus varies with the specific performance requirements
for any given application. For example, the level of continuity of GPS for a
low-accuracy time-transfer application will be much higher than the level of GPS
continuity for an aircraft nonprecision approach. The former application only
requires a single visible GPS satellite, whereas the latter requires at least five visible
satellites with good geometry to support RAIM.
Some useful information regarding the continuity of the GPS satellites, based
upon observed performance from January 1994 to July 2000, is provided in [17].
During this timespan, on average, each in-orbit GPS satellite ceased functioning 2.7
times per year and was out of service for a total downtime of 58 hours. The majority
of these instances (referred to as downing events ) were related to scheduled mainte-
nance—accounting for 1.9 downing events per year and an average total downtime
of 18.7 hours. The remaining 0.9 1 downing events per year per satellite were
unscheduled and accounted for a total average downtime of 39.3 hours. Causes of
unscheduled outages include failures of one or more satellite subsystems that
resulted in a loss of service.
For many applications, only unscheduled downing events are of concern. Sched-
uled maintenance activities are generally announced well in advance and can often
be planned around. For such applications, the probability that any given GPS satel-
lite will fail over a 1-hour time interval is approximately 0.0001. This value is com-
puted by dividing the average of 0.9 unscheduled downing events per year by the
number of hours in a year, 8,760.

1. Note that the component values provided in [17] of 0.9 and 1.9 do not add to the total of 2.7, also in [17].

This is presumably due to rounding errors.