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Lose RAIM but we have WAAS, can you shoot the GPS approach?

Asked by: 5612 views Commercial Pilot, FAA Regulations, Flight Instructor, Instrument Rating

My student got the questions from checker “if you loss RAIM but WAAS, can you shoot the approach?”
And the answer she wanted was No.
But I wonder, doesn’t WAAS provide accuracy and integrity check? Isn’t that why with WAAS that meets tso c145/146, you don’t need to do RAIM check, as long as WAAS is available, shouldn’t you have accurate differential data for you to continue the approach
in fact, if your gps receiver has WAAS, its using the waas massage from the GEO, it’s not even using raim/fde, correct?
I’ve been searching for answers, but couldn’t find a definitive one anywhere, I’d appreciate it if someone could help clarify this, thank you

2 Answers

  1. Best Answer


    John D Collins on Mar 06, 2020

    If the GPS receiver is TSO C145/146 and you are inside a working SBAS (aka WAAS) service volume, it does not use RAIM for integrity purposes, so you should not get a RAIM warning. As you indicated, navigation integrity data is provided by the WAAS system itself and is broadcast via a GEO satellite.

    You stated ” shouldn’t you have accurate differential data for you to continue the approach
    in fact, if your gps receiver has WAAS, its using the waas massage from the GEO, it’s not even using raim/fde, correct?”. The statement is partially correct except that the receipt of the WAAS integrity data does not mean that the position solution has adequate integrity to continue the approach, it means that the integrity data, “adequate or not” for the intended operation is available to the receiver to make the decision if you can rely on the corrected position to satisfy the approach integrity requirements. If the integrity data is below the required values for the approach, the receiver will annunciate INTEG or something to that effect, indicating that the position data can’t be relied on to conduct the procedure. So for example, the integrity is checked 60 seconds before the FAF for meeting an LPV criteria and if it does not, a downgrade of the approach is to LNAV and if the integrity does not even meet that criteria, the approach is abandoned.

    Integrity is the concept of trustworthiness of the navigation position information. RAIM is one method that uses substitution of additional satellites and calculating a containment radius. RAIM availability is based on geometry and how many satellites are in view. When RAIM is available, RAIM calculations may be performed. The result of the calculation is the containment radius and the containment radius needs to be within certain bounds for a given operation. If you can’t use RAIM because of too few satellites in view, there is no way to determine the containment radius. It does not mean the position is not accurate, just there is no way to determine if it is accurate or not. The availability of RAIM should not be confused with a good or bad integrity result using RAIM.

    WAAS uses monitoring stations and determines accuracy of each GPS satellite at the monitoring station locations. Correction data and integrity data is up linked to a GEO satellite and relayed to the WAAS receiver. The integrity data can be good or bad, but as long as the WAAS signal is being received, the receiver has a means of determining the integrity.

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  2. gggggn on Mar 11, 2020

    When I flew last time, ‘Integ’ massage pop up on G430. And 3D differential Nav was not available too.
    So does that mean WAAS failed and gps automatically reverted to RAIM,
    And RAIM has been also failed?

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