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WAAS/GPS

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Instrument Rating

Flying a C172SP with G1000, WAAS. No RAIM check required on the ground. But you are supposed to check for WAAS NOTAMS. The G1000 verifies WAAS integrity prior to the FAF. You then get an indication of which minimums you can use: LPV, LNAV/VNAV, LNAV. So if WAAS NOTAMED prior to departure, questions:

(1) Can you still plan a GPS approach, as if you never had WAAS?

(2) Do you have to do a RAIM check on the ground? Will the G1000 let you?

(3) Must you now have an alternate means of navigation appropriate to the flight?

(4) Do you have to pick an alternate (if required) that has an IAP other than GPS?

Thanks.

1 Answers



  1. John D. Collins on Dec 16, 2011

    Ted,

     

    It depends on the NOTAM. If “Unreliable” is used in the NOTAM, the level of service that is unreliable will be identified such as LPV or LNAV/VNAV. In this case, the WAAS system is still in operation, but the expected integrity may not be adequate to permit vertically guided approaches.  In this case, you should base your plans on the possibility that you may not be able to use vertical guidance.  Just because you can’t use vertical guidance doesn’t mean that WAAS is unavailable and as long as WAAS is available, it is used instead of RAIM to determine integrity.

     

    If instead, the NOTAM states WAAS Unavailable, this means the entire system is not available or an entire area is unavailable and it can’t be used for integrity.  In this case, the default is to RAIM and the same rules apply to a WAAS GPS that would apply to a non WAAS C129a GPS, that is you would need to do a RAIM check prior to flight and would need to pick an alternate that has an approach that does not depend on GPS and matches the equipment you have on board.  This is an extremely rare situation, as most of the WAAS service volume has redundancy, for example  there are three WAAS satellites, multiple up link stations and processing facilities.  I am aware of one instance where WAAS was totally unavailable for a two hour period where the system crashed during a software update.  WAAS unavailable could also occur on the north west portion of Alaska as there is a single WAAS satellite that provides WAAS to this area and it could be down for maintenance or due to an unplanned outage that would remove WAAS coverage from this area.

     

    1) Yes you can plan an approach as if you never had WAAS and in fact if the NOTAM is just WAAS Unreliable, you can still perform a vertically guided approach if the integrity is adequate at the time of the approach and don’t have to use RAIM.  VPL (Vertical Protection Limit) is the WAAS integrity parameter that determines if a vertically guided approach may be flown,  For a LPV DH 250 feet or higher, this must be 50 meters or lower, the same is true for LNAV/VNAV or LNAV+V.  For a LPV with a DH below 250 feet, the VPL must be 35 meters or less.  Unfortunately these are not displayesd on the G1000, but another somewhat related parameter, VFOM (Vertical Figure of Merit) is displayed and is a good indicator of whether or not you will have adequate integrity for vertical guidance.  Any value below 60 feet will indicate that you are likely to obtain vertical guidance for a LPV with a DH of 250 feet or higher and a value below 40 feet works for a LPV with a DH below 250 feet.

    2) The G1000 has a RAIM check on the satellite page.  With WAAS Unavailable, you need to do a RAIM check prior to flight.  With WAAS Unreliable, it is not necessary.

    3) In the case of WAAS Unavailable, yes. In the case of WAAS Unreliable, no.

    4) In the case of WAAS Unavailable, yes, and in the case of WAAS Unreliable, no.

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