Inflight RAIM Prediction requirement?

[RogerRamjet01]

I have a question for the more experienced IFR RNAV (GNSS) users out there.

IS there a firm, legal requirement to perform an inflight RAIM prediction?

During my instrument rating I was taught to obtain a RAIM prediction using the reciever (TSO 129) once within 30nm and prior to commencing track / descent for the RNAV approach. But I cannot for the life of me find the documented basis for this.

I realise there is a requirement to assess RAIM / FDE for flightplanning purposes, and that this wil impact selection of alternates if you have TSO 145 / 146 equipment in the aircraft (and that if you have TSO129 equipment then you cannot use the GPS to satisfy alternate requirements anyway).

However after hunting through the relevant CAAPs and CAOs, as far as INFLIGHT RAIM availability / prediction is concerned, I can only find statements to the effect that RAIM must be "available" prior to descent below LSALT, and that a missed approach must be commenced if a RAIM warning appears.

CAAP 179a(1) mentions that the equipment automatically performs a prediction for the approach, and will not enter approach mode until RAIM is to predicted to be available. That document (section 8) also specifically mentions the limitations of reciever based predictions compared to the ones you find attached to the Met briefings (I am guessing because the latter include GPS notams issued since the last GPS almanac issue).

So wouldnt it be more accurate in any case to do your RAIM / FDE prediction using the Airservices data preflight?

Please note I am not casting in doubt the wisdom of doing a double check RAIM prediction prior to an RNAV approach - I am just wondering if there is a requirement for it in law.

Any comments / thoughts appreciated.

[27/09]

The receiver is performing the RAIM function all the time.

The prediction function just predicts whether or not RAIM will be available at a particular point at a particular time.

The prediction function in the GPS uses the GPS's onboard almanac to do the prediction and does not take into account any space vehicles that may have been taken out of service. The most accurate RAIM prediction is one that uses up to date data from the US Department of Defence. So, yes it is more accurate to do a RAIM/FDE prediction using the Air Services service.

The FDE programme that I am familiar with requires you to enter, which if any, space vehicles are out of service before it does the prediction. The data on the space vehicles can be got from the US coastguard site. GPS Constellation Active Nanu Status (http://www.navcen.uscg.gov/navinfo/Gps/ActiveNanu.aspx)

I don't see the the point of doing a RAIM prediction just prior to the approach. I've never heard of this requirement. Why would there be any change in the prediction from the time you did the prediction prior to your departure.

[SCE to Aux]

There is no need to perform an in-flight RAIM prediction. From memory, once an approach is armed, the GPS navigator will preform a RAIM prediction at about three miles from the Final Approach Fix. The navigator will not go into approach mode unless RAIM is predicted to be available (at the 0.3 NM Horizontal Integrity Limit) for the remainder of the approach. Loss of RAIM annunciation will be inhibited for 5 minutes?, however, RAIM alerts will continue to be available.

[PlankBlender]

Agree with the previous posters re. practical application (e.g. Garmin 430/530 will show any RAIM warning where it's really hard to miss), but for instrument tests and renewals, there's a PASS/FAIL item on the form for the RNAV and GPS arrival approaches that reads:

- GPS integrity checked

I am being taught just to flick over to the satellite page and ensure there's at least reception from four satellites. Takes less than five seconds all in, but something that's easy to miss like the NDB IDENT ON, and it's an automatic FAIL if you miss it, just like going under the minima :eek:

[27/09]

I am being taught just to flick over to the satellite page and ensure there's at least reception from four satellites. Takes less than five seconds all in, but something that's easy to miss like the NDB IDENT ON, and it's an automatic FAIL if you miss it, just like going under the minima

If the GPS provides a RAIM warning whats the point of doing the integrity check that you mention? It's a bit like monitoring the audio on a VOR to ensure it's working, the flag on the CDI tells you when it's not working.

If the unit does not provide any sort of warning then I can see the sense in doing it, but I don't know of any GPS that doesn't provide a warning

I've never heard of it, and I fly GPS approaches regularly.