How to Be a RAIM-Maker

Insights into GPS flight planning

November 1, 2003

GPS is a powerful tool but you need to do some planning in order to get the most out of it, and that means understanding an important aeronautical decision-making (ADM) tool called RAIM prediction. It's built right into your panel-mount GPS.

This is not a rehash of procedures in your user's manual. Your time is too valuable. Instead, let's look at what RAIM is, some precautions associated with performing a RAIM prediction, what the results mean in the context of proper flight planning, and potential RAIM alerts during a GPS approach. While most of the content is generic to all panel-mount GPS units, there are a few specific references to the Garmin GNS 430.

There is an old saying about pilots: "A superior pilot uses his superior judgment to avoid situations requiring his superior skill." Pilots using GPS should learn and practice the RAIM prediction as an aid to ADM.

What is RAIM?

RAIM stands for receiver autonomous integrity monitoring and is a behind-the-scenes navigational reliability check that automatically runs when the GPS is navigating anywhere. Since at least four GPS satellites are required to determine a unique aircraft position, the RAIM feature addresses the question of whether enough satellites are available and whether their signals are accurate at any one time. Alerts can tell us when GPS reliability is compromised. RAIM criteria are stricter for an approach than for en route navigation.

RAIM information is critical when deciding whether to conduct or continue a GPS approach. Wouldn't it be nice to know, well in advance, if RAIM will be available at your destination? You can find out. There are three ways to obtain advance information on RAIM availability:

  • Flight service stations (FSS). Ask the flight service briefer for GPS notams when filing the flight plan. The briefer can tell you about predicted satellite outages affecting your flight. Just remember to ask for this specific product because, even if the briefer knows you are GPS-equipped, GPS notams are not normally offered during a briefing.
  • DUATS. GPS notams are available when retrieving other FAA notams during an outlook or standard briefing. Here is an actual GPS notam, typical of what you'd see on-screen, and its translation into plain English: GPS 11/023 GPS PRN 23 OTS WEF 0212031815-0212041815. Translation: GPS satellite number 23 will be out of service from December 03, 2002, at 1815Z until December 04, 2002, at 1815Z. Incidentally, PRN in the notam stands for pseudo-random noise, referring to the GPS signal transmission format.
  • RAIM prediction. With the RAIM prediction feature on your GPS, you can find out the same information in the cockpit at any time. With that in mind, there are two times when it makes sense to do the RAIM prediction: prior to leaving the chocks and approximately one hour before reaching your destination while there is sufficient fuel to consider many other options. At both times, cockpit workload is relatively low. Your GPS user's manual explains the procedures.

Before performing any RAIM prediction, check to make sure the database is current. An expired database is a show-stopper for IFR anyway. You'll also want to check the time offset. If the time is incorrect, the results of any subsequent RAIM prediction will be meaningless.

Now proceed to the RAIM prediction function. The result will be valid for plus or minus 15 minutes from the selected arrival time.

What if you are not sure of your estimated time of arrival? No problem. Select a likely range of ETAs 30 minutes apart and run a RAIM prediction for each ETA in the range.

Evaluating the results

The usual result of a RAIM prediction, and the one you want, is "RAIM available" or words to that effect. If "RAIM not available" (or a similar message) is displayed, there are a few options to consider:

  • It may be better to advance, delay, reroute, or cancel the flight if GPS is to be the primary means of navigation.
  • It may be necessary to use other navigation sources in lieu of GPS.
  • It may be more advantageous to proceed VFR.

Here is a very important point: If you are filing GPS to your destination and an alternate is required, the alternate must have other than a GPS approach published, and both you and your aircraft must be capable of performing it. The reason this rule exists is clear. Suppose your destination only has a GPS approach, and you are preparing to fly it in instrument meteorological conditions. True, the GPS automatically runs a RAIM check continuously, but the results are only usable at the present moment. By the time you have begun the approach, it's too late to exercise many of the options mentioned above, options you would have had if you had made a RAIM prediction at a time when workload was low. Now suppose the RAIM check fails during the approach. You go missed and decide to proceed to your alternate. There's a good chance that the RAIM outage affecting your destination will also affect your alternate. That's why you need a backup means of getting into the alternate if it will be necessary to fly an instrument approach there, too.

RAIM alerts during an approach

Remember to include the GPS display in your scan. On the Garmin GNS 430, the two possible RAIM alerts are "INTEG" and "WARN." These alerts appear as a small yellow field with black letters in the lower left corner of the screen, depending on the detected fault.

INTEG means satellite coverage is insufficient, but some good data is getting through. The GPS will still provide navigational information, but this is to be considered secondary to other sources.

WARN means satellite coverage may be sufficient, but navigational errors exceed built-in protection limits (0.3 nm for the approach). All GPS navigation data will be disabled.

The few real-world RAIM alerts that I have experienced in more than 1,500 hours of daily GNS 430 use have all been short-lived, with the GPS resuming normal displays and functions after only a few seconds. I would therefore wait a moment to verify the condition before taking alternative measures. The alert may soon clear itself out, and then it would be permissible to continue the approach.

In either case, if you observe one of these messages for more than, say, 15 to 30 seconds, or are unsure of GPS reliability, abandon the approach, switch to VLOC mode, and use other navigational sources such as a VOR.

Here is an interesting fact: GPS is designed so that if RAIM integrity is lost after passing the final approach waypoint, no RAIM alert will be displayed, allowing you up to 5 minutes after the fault is detected to complete the approach.

Robert Jex is a CFI and ATP currently instructing at a large flight school in east-central Florida.