Viavi 87339, Operation Manual

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Principals of Operation
6 -23
To overcome this, RAIM was developed and is a mandatory feature of all aviation-grade
receivers. RAIM uses combinations of satellites to determine the receiver position.
Should a large discrepancy between position solutions occur, a RAIM alert is created
rendering the GPS navigator unreliable. Different phases of flight use different values of
“integrity alarm limits” prior to issuing a RAIM alert.
The ability of a receiver to perform RAIM computations is dependent upon the number of
satellites in view, their geometry and the mask angle which is dependent upon the ability
of the antenna to track satellites near the horizon and any local terrain. Whereas GPS
needs a minimum of four satellites to produce a three-dimensional position, a minimum of
five satellites are required for RAIM. For this reason, RAIM may not be available in
circumstances of poor satellite coverage or poor satellite geometry.
Table 6-2 RAIM Alarms
Phase of Flight Alarm Limit Time to Alarm
Enroute (oceanic, domestic, random
and JV routes)
2.0 nm 30 sec
Terminal 1.0 nm 10 sec
RNAV Approach Non Precision 0.3 nm 10 sec