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Why after passing the FAWP a GPS approach is allowed to operate for 5 minutes if there is a satellite problem? Why would the GPS receiver not immediately alert the pilot that there is satellite problem?


In other words, the GPS receiver may indicate all is normal but in actuality there is a satellite problem. If I am flying a GPS approach, I want to know immediately if there is a satellite problem. Excerpt from AIM below.

******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.**********

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  1. Best Answer

    John D. Collins on Mar 26, 2013


    That is a good question and I am not sure I can answer it fully. RAIM (Receiver Autonomous Integrity Monitoring) is based on redundant calculations by the receiver. It takes 4 satellites to determine a position in time and space. Normally more than 4 satellites are received simultaneously, so a position can be determined with different groupings of 4 satellites. These various groupings will produce slightly different positions and can be used as a cross check of the accuracy of the position. RAIM uses these cross check differences to estimate the potential position uncertainty. Since the positions of the satellites can be predicted, it is possible to determine if the RAIM calculations may be performed at a specific position and time. There is good geometry and bad geometry of the satellites that will affect the usability of the cross check. The RAIM prediction takes this into account and will indicate if the geometry is not suitable for a good cross check. This is the RAIM check we do prior to flight.

    There is a distinction between the availability of the RAIM function and the calculation of a RAIM result. The availability of RAIM is dependent on the configuration and geometry of the satellites at a point in time and space. RAIM availability means that the RAIM calculation may be performed. If the RAIM calculation is performed, it may be a good or bad result. If the result is above the alarm limit, the alarm will be generated and you will have to abort the approach. If RAIM is unavailable, it means that the RAIM calculation can’t be performed and it doesn’t mean that the position is in error, it is just that the error can’t be determined. If RAIM can’t be performed, then the integrity of the position accuracy is in question. That is why at the point on the approach 2 NM from the FAF, the receiver must be able to predict that the RAIM calculation function will be available at the FAF and the MAP or the approach may not proceed. If however, in spite of the prediction that RAIM could be calculated, it can’t be after passing the FAF, the failure of the RAIM calculation capability (not the failure of a calculation) may be delayed up to 5 minutes prior to generating an alarm. On the other hand, if the RAIM calculation can be performed, and the result is unsatisfactory, then there is a 10 second time to alarm requirement and the approach must be aborted.

    In other words, as I read the TSO requirements for a GPS, there are two situations that will generate an alarm:

    1) The lack of availability of the RAIM calculation function
    2) A bad result from a RAIM calculation

    The latter has a 10 second time to alert. The former must be continuously available prior to the FAF and predicted to be available to be at the FAF and the MAP. If it is lost after the FAF, a 5 minute time to alarm is permitted.

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  2. Dan Chitty on Mar 27, 2013

    Thank you John for the feedback. Your explanation is very logical.

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