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RAIM loss before/after FAF

Asked by: 7150 views Instrument Rating

In the AIM it states the procedures if RAIM is lost prior or after the FAF, which are pretty much the same- go missed. However my question is, how do you execute the missed approach procedure since the majority of GPS approaches (at least that I have encountered) use GPS for the missed approach procedure, and are not navigable via alternate means? The AIM itself states the after you loose RAIM, GPS should be advisory only- so you cannot really fly the missed procedure using GPS as primary means (even though it is probably the only means). I hope I was clear on my question, and thank you for an answer.

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  1. John D Collins on Jun 26, 2016

    RAIM is an algorithm that computes a residual statistic measured in distance or meters. In order for a GPS approach final approach segment to be flown, RAIM must support a result that bounds the actual position to be within +/- 0.3 NM. The missed approach only requires RAIM to bound the actual position to be within +/- 1.0 NM, so RAIM may not be satisfactory for the final approach segment, but able to support navigation on the missed approach segment.

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