My instructor has been teaching me to run a RAIM check for each RNAV approach we do. For some flights this can be 4 or more RAIM checks for the various airports/approaches and adds significantly to the workload.

I started doing some research and I think I've gathered a few things. First RAIM applies over large areas, not singular GPS points (makes sense), the FAA provides a tool to determine RAIM availability from the ground as part of preflight, and last that a RAIM check while briefing the approach seems Like it might not be necessary since the GPS is required to run a RAIM check 2 miles prior to the FAF. Additionally since the GPS has no way to know about unexpected NOTAM announced outages the 430 RAIM precalculation is a bit less reliable.

Is it a requirement to run the manual RAIM check in flight or is it more appropriate to use the FAA tools and determine it as part of preflight preparation?

  • 3
    $\begingroup$ It can be confusing to read the AIM and FAA publications if you aren’t aware of the difference between the formal definition of GPS and the colloquial definition. Equipment certified under TSO−C145() or TSO−C146() are referred to as augmented GPS, WAAS-capable GPS, or WAAS. Prior to WAAS availability, GPS systems were certified under TSO-C129() or TSO-C196(). The pre-WAAS systems were certified as supplementary navigation and require a RAIM check. WAAS systems do not require a RAIM check. $\endgroup$
    – JScarry
    Apr 24, 2017 at 13:51
  • $\begingroup$ You should check the FAA site in your pre-flight. If you check the FAA site for the RAIM status at your destination, you will know if you can plan on an RNAV approach. You won’t be able to fly an RNAV approach unless the unit passes the RAIM test when you are there. $\endgroup$
    – JScarry
    Apr 24, 2017 at 13:53
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    $\begingroup$ From what I remember (I may be wrong), the 430 indicates on the screen during an approach if the RAIM check was successful. I don't remember ever having to check it manually, my instructor just had me confirm the indication was there before continuing past the FAF. If the RAIM indicator wasn't there, we went missed. $\endgroup$
    – Pondlife
    Apr 24, 2017 at 14:04

2 Answers 2


The FAA website predicts pre-flight whether RAIM will be available

The RAIM algorithm will tell you during your flight if GPS is working correctly.

The pre-flight FAA website check is not a substitute for the use of the RAIM function during flight. It will only show whether the RAIM function is available during flight.

RAIM stands for Receiver Autonomous Integrity Monitor. It is a part of aviation graded GPS receivers that checks consistency of all used GPS satellite signals in order to raise an alert if the GPS system is not working correctly.

For RAIM to work, there must be at least 5 usable satellites in view of the GPS receiver. For GPS without RAIM, at least 4 satellites are needed. If there are only 4 satellites visible, or more than 5 but with an unfavourable geometry, RAIM cannot work and therefore the GPS receiver cannot be certain that the produce position fix is correct.

The FAA website predicts the availability of RAIM; based on the constellation of GPS and the health status of its satellites, the route and the time of flight it is possible to calculate whether RAIM is available for your flight. This can be done before you take-off.

If there is no RAIM available during part of your flight, you can't rely on GPS during that part. The GPS may still work fine, but there is no way to check its integrity. A single fault in a GPS satellite or, more likely, a corruption in the downlinked GPS satellite almanac will cause the position fix to be off. You must use alternative navigation means if GPS RAIM is not available.

The FAA website does not say anything about the actual quality of the GPS system during your flight. This is what RAIM does. It continuously monitors all satellite signals for consistency and flags any inconsistencies. There are various categories of RAIM such as Fault Detection and Fault Detection & Exclusion but all RAIM algorithms will give you a containment radius.

The containment radius is the maximum size that a position error can grow due to a fault in the GPS system without the RAIM algorithm being able to detect it. The value of the containment radius depends on the geometry of the satellites.

The requirements for the value of the containment radius depend on the flight phase. For En-Route mode the RAIM containment radius value should be under 2 NM, terminal mode under 1 NM, and approach mode 0.3 NM.

  • $\begingroup$ This reminded me to update the GPS data on my phone. With luck I'll get more accurate readings on my run tracking! :) $\endgroup$
    – FreeMan
    Apr 24, 2017 at 12:54
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    $\begingroup$ @DeltaLima Good description of RAIM, but the issue here is the operation of the installed system (tag indicates GNS430 which is an older non-WAAS design). IIRC, installation of those units required a RAIM annunciator which would provide RAIM failure alerts with no pilot action. The answer should be in the Aircraft Flight Manual Supplement (AFMS) for the installed GPS. An AFMS is required as part of the STC approving the installation of the GPS. $\endgroup$
    – Gerry
    Apr 24, 2017 at 13:33
  • $\begingroup$ @Gerry - The original asker may not have wanted to create a tag for gns430w since the two devices are so similar. $\endgroup$
    – Steve V.
    Apr 24, 2017 at 16:01
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    $\begingroup$ @SteveV. Similar, yes; but they provide different capabilities and the answer will depend on the actual unit installed. Also, just because the unit has a capability doesn't mean it was installed in a manner that can use it. How you use an installed piece of equipment (an appliance in FAA certification terms) is controlled by the AFMS which becomes part of the aircraft's flight manual under the STC. The best answer should be to 'read the manual'. $\endgroup$
    – Gerry
    Apr 24, 2017 at 16:18
  • $\begingroup$ This answer could be improved by referencing the applicable requirements for IFR certified GPS installations conforming to the various TSOs. For example WAAS installations do not need a pre-flight RAIM check, wheras non-WAAS receivers generally need a pre-flight RAIM check. See AC 90-108. $\endgroup$
    – J W
    Apr 24, 2017 at 21:44

Answer lies on AIM 5-1-16 "rnav and RNP operations" paragraph f:

f. During the pre−flight planning phase RAIM prediction must be performed if TSO−C129() equipment is used to solely satisfy the RNAV and RNP requirement [...] Operators may satisfy the predictive RAIM requirement through any one of the following methods: [...] 2. Operators may use the FAA en route and terminal RAIM prediction website: www.raimprediction.net; 3. Operators may contact a Flight Service Station (not DUATS) 3. Operators may use a third party interface, incorporating FAA/VOLPE RAIM prediction data [...]

Conclusion: once you check the route of flight through SAPT on FAA en route and terminal ram prediction website, it is good for all those RNAV IAP approaches!

While flying from Phoenix, Westwind school. Similar question came to me.. Where all of instrument student and most instructor does check Raim availability immediately after Engine starts and turn on the MFD on G1000 C172S nav III.

This will do raim prediction (under TSO C129 equipment's internal capacity) for that location (because it's default search location is p.pos- present position) and +/- 15 minutes. But if you have not inserted RNAV Approach in flight and have not activated the approach, it is not going to calculate the ETA/ETE to the target airport. Similarly, if you have just loaded the RNAV Approach on flight plan and not have activated, then it is not going to select the destination airport (or missed approach waypoint) for RAIM prediction. Contrary to you having to check up to 4 RNAV approach destination airport, we've been checking only the ramp of departure airport...

Remarks: All the IFR-certified GPS unit will do RAIM check 2 NM prior to reaching FAF. So during flight we do not need to. As long as we have performed prediction throughout the route. We will know whether to avoid that airport's RNAV approach, just in case prediction says 5+min of RAIM service might not be available for some part of leg.


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