Performance of Stand-Alone GPS - Understanding GPS: Principles and Applications - page 353

( )

− 1

T T

AHWHHW ≡

( )

− 1

T T

SIHHWHHW ≡−

n

W −

1

=

σ

2

0 ⋅⋅

⋅⋅

0

1

0 σ

2

0

2

⋅⋅ ⋅⋅ ⋅⋅ ⋅⋅

00 ⋅⋅ σ n

2

2 2 2 2

σ=σ

2

+ σ + σ + σ + σ (7.73)

i i ,URA i,uire i,tropo i,mp i,revr

where the error components are user range accuracy (clock and ephemeris error),
user ionospheric range error, tropospheric error, multipath, and receiver noise.
The HPL is formed by the same method as nonweighted RAIM.

HPL = Slope max × normalized pbias = Slope max × λ

Availability of RAIM
Availability of RAIM is determined by comparing the HPL to the maximum alert
limit for the intended operation. RAIM was developed and primarily has been used
to support aviation applications. Therefore, the focus of the availability analysis in
this section will be on aviation applications. The horizontal alert limits for various
phases of flight are shown in Table 7.6.
If the HPL is below the alert limit, RAIM is said to be available for that phase of
flight. Since the HPL is dependent on the satellite geometry, it must be computed for
each location and point in time. Since RAIM requires a minimum of five visible sat-
ellites in order to perform fault detection and a minimum of six for fault detection
and exclusion, RAIM and FDE will have a lower availability than the navigation
function. An analysis of the nominal 24-satellite constellation has been performed
to evaluate the availability of RAIM [47-51].
Although a 7.5º mask angle is specified in FAA TSO C129, a 5º mask angle is
specified for FAA TSO C146 receivers, and most receivers use a 5º mask angle or
lower. A 5º mask angle is applied to this analysis, and availability is evaluated over a
worldwide grid of points at 5-minute samples over a 24-hour period.

Table 7.6 GPS Integrity Performance
Requirements
Phase of Flight Horizontal Alert Limit

En route 2 nmi

Terminal 1 nmi

NPA 0.3 nmi

Source : [45].