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RAIM
Operations
RAIM
What Is Raim?
RAIM stands for Receiver Autonomous Integrity Monitor. It is a way a
GPS receiver can detect when the signals it is receiving from one or
more of the satellites are wrong. 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 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 C129a 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 to predict whether RAIM will be available at the
estimated time of arrival at the destination. Your Apollo NMS
provides you with a RAIM prediction page. This page allows you to
determine if RAIM should be available at any location and time. If it
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