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RAIM errors: when can I proceed with and when must I abort an approach

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Instrument Rating

I have a question about what to do in the event of a RAIM error that appears after I have crossed the FAF.  

In the current FAR-AIM, page 58, 1-1-26, we read: “If a RAIM failure occurs after the FAWP, the receiver is allowed to continue operating without an annunciation for up to 5 minutes to allow completion of the approach (see receiver operating manual). If the RAIM flag/status annunciation appears after the FAWP, the missed approach should be executed immediately.”
 
It appears that there are situations of RAIM failures/errors occurring past the FAWP where I am able to proceed, and other RAIM failures/errors where I must abort.  How do I perceive and determine which are which so I take the appropriate actions.  Is the sole difference whether I receive an alert / annunciation?  If there is no alert or annunciation, how am I to perceive that a failure or error has occurred?  

Thanks in advance for your help.  

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1 Answers



  1. John D Collins on Jun 22, 2021

    RAIM is an algorithm that provides for the integrity or trustworthiness of the GPS position. It requires more satellites than are needed to provide a position in time and space. So there will be times when the RAIM algorithm can’t work because of too few satellites with sufficiently good geometry to do the algorithm. This is what is predicted before flight with a RAIM prediction, that is that the algorithm will be able to determine if the GPS position can be trusted throughout the flight.

    So there are two distinct scenarios, RAIM can work or not work. If RAIM is working, it can indicate the GPS position can be trusted or not. So a RAIM failure is a failure to be able to compute if the position can be trusted or not, whereas a RAIM flag/status indication means that the GPS position can’t be trusted. As long as the RAIM calculation can provide for a valid RAIM calculation at the FAWP, you can continue another 5 minutes even if RAIM itself can no longer be calculated. However, if the result of the RAIM calculation determines the position is not at all trustworthy, the GPS position is flagged and is not usable.

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