Approach Basics (GX50/60)

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

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