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RAIM RAIM stands for Receiver Autonomous Integrity
Monitor. It is a way a GPS receiver can verify when
the signals it is receiving from one or more of the
satellites are useable. It does this by using more
satellites than are required for a position solution. In
essence, the receiver compares the pseudo-ranges, or
calculated distance measurements, from the satellites
against what it expects. If this comparison yields too
big a value, based on the current RAIM alarm limit,
an Alarm is generated and you get a message telling
you that a RAIM alarm has occurred.
It is important to understand that RAIM provides
integrity, not accuracy. Accuracy is the ability of the
GPS to determine a correct position with good
satellite signals, enough satellites in view, and an
acceptable geometric position of the available
satellites. Integrity is the ability of the GPS system to
detect when its position may not be accurate enough
for IFR operations due to bad or false satellite
information.
GPS systems limited to VFR use must meet the exact
same accuracy requirements as those authorized for
IFR use. Systems authorized for IFR use must also
have an integrity monitor such as RAIM.
How Is RAIM Used?
TSO C129 calls for three different levels of RAIM
protection: En Route, Terminal, and Approach. Plus,
the receiver has the ability to predict RAIM
availability at any given location and time.
Why Predict Availability?
Because RAIM requires that more satellites be used
than are required for a position solution, RAIM may
not always be available. The TSO requires that RAIM
be available with a 0.3 nm. alarm limit, from a
distance of 2 nm inbound to the Final Approach Fix
(FAF) until the Missed Approach Point (MAP) is
reached. A means must be provided to allow the pilot
9-29
Approach Basics (GX50/60)