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3 Answers

When is RAIM required?

Asked by: 20176 views , ,
Aircraft Systems, FAA Regulations

I cannot find this simple answer.... must be looking in the wrong places....

I know that RAIM is required when conducting a GPS Instrument Approach Procedure, but is it required just to even depart IFR, knowing that you will be using your GPS to navigate in the terminal and enroute phases of flight too (AKA - any flight other than the IAP)?

A more simple way to ask this question.... When exactly is RAIM required?  :)

The AIM is giving me ALMOST answers... but I am not totally satisfied with what I am finding yet.




3 Answers

  1. Bill Trussell on Mar 01, 2012

    Check out the GPS article in this FAA newsletter to start with:
    RAIM prediction is what is of primary concern for Non-WAAS equipped GPSs.   If you are WAAS equipped then you are good to go without doing prediction.

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  2. John D. Collins on Mar 01, 2012

    RAIM is used by a non WAAS GPS to determine the integrity of the navigation solution. Integrity is the function that informs the pilot when the navigational guidance can or cannot be depended on. RAIM prediction is calculated based on the geometry of the satellites; some geometries don’t lend themselves to determining a good position solutions while others do. RAIM can be continuously calculated by the receiver by determining a position with four satellites and then recalculating using a fifth satellite to substitute for one of the other satellites. This process is accomplished by the fifth satellite replacing one of the other satellites at a time, and comparing the five resulting position differences. If all of the positions are close to one another, than the RAIM value is small.  The greater the divergence in positions, the larger the value of the calculated RAIM, and the less certain one is of the actual calculated position. If this uncertainty reaches a threshold value, the pilot is notified that RAIM is not available. 


    The RAIM threshold values are based on what mode the GPS is currently using for navigation, enroute, terminal, or approach.  The RAIM value required for GPS approach is established as +/- .3 NM, enroute it is +/- 1 NM, and when enroute it is +/- 2 NM.  Obviously the most stringent requirement is needed for approach and with the current constellation of 31 active GPS satellites; it is close to being available approaching 100 percent of the time. The likelihood of RAIM not being available for enroute and terminals navigation is much higher and for all practical purposes is 100 percent. Some GPS receivers have a more advanced version of RAIM that includes Fault Detection and Exclusion (FDE) capability.  With FDE, additional satellites may be used to determine if using a particular satellite in the navigation solution always generates a higher value of RAIM, in which case the errant satellite can be excluded from the position calculations.


    Non WAAS GPS is only approved for supplemental navigation and other navigation equipment are required suitable to the route being flown (normally this consists of VOR, ILS, ADF, and or DME equipment.  If RAIM is unavailable when enroute or in the terminal mode, then the pilot must verify their position by the other navigation equipment and if required, revert to the other equipment.  In the case of loss of RAIM on an approach, the GPS will not permit the aircraft tp enter approach sensitivity at 2 NM prior to the FAF and the pilot should abandon the approach.  If a RAIM failure is indicated after reaching the FAF, the pilot should discontinue the approach and execute the missed approach procedure, although the RAIM failure will not be indicated for up to five minutes after it is initially detected after passing the FAF.


    It is a good idea to check RAIM as part of the preflight planning for a flight using a non WAAS GPS, but I have not found where it is required. If you are aware of RAIM outages for a given airport at a particular planned arrival time, you can change the departure time to one that RAIM is anticipated to be available, or you can adjust your planned route to use ground based navigation aids.

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  3. Peter King on Mar 06, 2012

    Unfortunately, the guidance is sprinkled all over the place.  You can reference:

    AIM 1-1-19
    AC 90-94 (cancelled but grandfathered by AC 90-105)
    AC 90-100A
    AC 90-105

    The simple answer is that you must have RAIM whenever you are flying an RNAV or RNP route with a non-WAAS IFR GPS (TSO C-129) or with a WAAS IFR GPS (TSO C-145A/146A) whenever WAAS is unavailable.  In addition, you must now perform a RAIM PREDICTION as part of your preflight planning (i.e. before takeoff) if you anticipate flying any RNAV or RNP route.
    RNAV or RNP routes  include:

    GPS Approaches
    RNAV (GPS)
    RNP (GPS) approaches
    RNAV Routes (T-Routes and Q-routes)
    Victor airways using GNSS MEAs

    Fortunately, most GPS units automatically provide the correct level of RAIM along those routes, but some older GPS units need to be forced into terminal (1.0nm RAIM) mode for departures.
    When you must perform RAIM PREDICTION is a separate question (and probably what you were originally asking).  If you are planning to use any RNAV routes, you must now perform a RAIM prediction as part of your preflight planning.
    14 CFR 91.103 “Preflight Action”, which says:
    Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight.
    That’s a pretty wide net that I would argue RAIM prediction falls into.
    In addition, AC 90-100A Section 10 “U.S. RNAV FLIGHT CREW REQUIREMENTS”, Paragraph a. “Preflight planning”, Subparagraph (5) says:
    If TSO-C129 equipment is used to solely satisfy the RNAV requirement, GPS RAIM availability must be confirmed for the intended route of flight (route and time) using current GPS satellite information.
    AC 90-100A goes on to say how you can perform the pre-flight RAIM prediction.  A predicted RAIM outage of more than five minutes is a no-go.  GO READ THIS PART OF THIS AC IN DETAIL UNTIL YOU UNDERSTAND IT.
    AC 90-105 has a similar requirement that a RAIM prediction must be performed as part of your preflight planning. 
    I find the easiest way to perform a RAIM prediction is http://www.raimprediction.net.
    You should also carefully read the Airplane Flight Manual Supplement for your GPS.  Often, additional operations are approved, including using your IFR GPS for supplemental enroute navigation.  The AFM Supplement may have different RAIM requirements for those operations.  For example, I know of a few systems that allow for a RAIM failure enroute as long as you cross-check your position against ground-based systems periodically.

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