How does RAIM (Receiver Autonomous Integrity Monitor) outages affect the ability to do GPS / RNAV approaches?

I see in the EGKK AIS daily predictions of RAIM outages- but what is the practical effect? Can you still do GPS approaches in these periods?

Just wondering
T'bug

I am surprised there is not some sort of AIC on this - the quick answer is no, you can't shoot the approach during a RAIM outage - hence the prediction service so that you can plan around it.

For the same reason that you cannot start an ILS approach when the localizer is OTS, or a VOR approach when the VOR is OTS, you canot start/complete a GPS approach when RAIM is not available.

The notam that RAIM might not be available is a guide for you to plan around these times.

Pretty simple, no?

I'm a 744 driver and curently my outfit let us make the initial approach using RNAV - Clearance to do Final Approach is forthcoming. I'm sure RAIM will be covered then.

Uping the stakes:---

If your approach navigation is solely by GPS - I am not surprised that a RAIM outage prevents an approach

If your navigation kit is Dual FMS with DME / DME & IRS updating does that give the redundancy to compensate for RAIM being unavailable?

I know folks are / have been doing these approaches down at LGW - what is actually happening on the day when you are due to make an approach during a forecast RAIM outage?

T'bug

ICAO has (fairly) recently gone to great lengths to specify standards in the titling of RNAV approaches. Thus, a RNAV (GNSS) approach is not intended to support RNAV (DME/DME) or RNAV (VOR/DME). It follows then that you need to check the chart title and, if still not clear from that, check any notes on the chart.

All RAIM does, as much as I can remember, is maintain a list of received satellite signals and then rotate processes the list. If the signal from one particular satellite is incorrect the constant cross comparison highlights it and deselects that satellite from the list.

RAIM prediction just lets you know when there will not be enough strong signals to populate the redunancy list.

In the event of there not being enough signals to populate the list then the accuracy of the received satellite information cannot be accurately determined and a RAIM failure is generated.

The upshot of all this technical moolah is that the accuracy of the GPS cannot be positively determined to be good enough for the GPS approach and therefore the approach is not allowed to be flown. Reasons given is that the loss of one or more GPS signals could lead to gross inaccuracies within the lateral nav of the aircraft.

I seem to remember that some airports, Munich for example, have a GPS ground station providing additional info, can't quite rmember what they are called though.

Cheers, lookout and fly accurately!

A pseudolite. Is this the project? http://www.eurocontrol.fr/projects/s...per_ION_98.pdf

RAIM is essentially peculiar to stand-alone GPS receivers, and may or may not include a FDE (fault detection & exclusion) facility. In an integrated avionics suite with certificated GPS the RAIM function is most likely to be performed by an "equivalent " software process. This will involve input from other navigation sensors.

Whether or not this allows for procedures to be flown during predicted RAIM outages I cannot say. That would be covered by national rules, and/or op specs I would imagine.

Here is a little snippit from an AFM I am familliar with:-

When a GPS Only Approach is planned (GPS only in title or GPS
required by operational rules), prior to dispatch, the crew is required to verify that the predictive RAIM at the destination ETA is within the approach criteria. This information (RAIM AVAILABLE), is displayed on the PREDICTIVE RAIM page on the MCDU.

I would suggest if you have any doubts you should see what it says in your AFM.

When I did my Air-Law exams, you were not allowed to fly IFR with GPS as the sole aid, unless the system was RAIM capable, and not indicating a failure. In Europe that is.