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FAA Regulations, General Aviation

What is the difference between WAAS and RAIM?  If you have a WAAS GPS, is RAIM required?


Don Owens


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4 Answers

  1. Lucas on Jul 16, 2012

    WAAS uses three additional satellites in geostationary orbit some 22,000 miles above the earth’s surface—at that altitude they take 24 hours to orbit and don’t appear to move in the sky. WAAS transponders on the satellites broadcast signals that tell a WAAS-equipped navigator which of the regular GPS satellites can be believed and which should be ignored; they also add differential corrections to the GPS signal based on information from ground stations. The result is supposed to be so reliable that the FAA doesn’t require any other kind of navigation equipment to file IFR, and it increases accuracy to a few feet in both horizontal and vertical directions, without any requirement for a separate barometric input. Navigators built this way meet Technical Standard Order (TSO)-C146a.
    Now WAAS can also be unavailable (you might be out of the coverage area or it fails altogether) and that is when RAIM prediction comes into play.
    So If you have a TSO C146 WAAS receiver there is no requirement for a RAIM check unless again it fails.

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  2. John D. Collins on Jul 16, 2012

    The WAAS system monitors the GPS satellites with approximately 35 ground stations and calculates correction data as well as integrity data.  The correction data includes correcting for the position of the GPS satellite, any atomic clock errors (one billionth of a second is a foot of position) and for determining the delay of the signal through the ionosphere.


     Integrity is the concept that warns the pilot if the position information isn’t suitable for use for the current mode, such as enroute, terminal, or approach. In the WAAS system, two integrity values are calculated by the system: HPL (Horizontal Protection Limit) and VPL (Vertical Protection Limit). These provide bounds for the horizontal and vertical navigation solution and are calculated to the 99.99999% certainty level. These values along with the GPS satellite correction information are forwarded to a WAAS GPS thru the geostationary satellites that Lucas described.  As long as HPL and VPL are available from the WAAS satellites, they are used by the WAAS GPS instead of RAIM.


    RAIM (Receiver Autonomous Integrity Monitoring) is also used to establish if the GPS receiver can depend on the position. RAIM only deals with the horizontal position and not the vertical. RAIM works by using 4 satellites to obtain a position. If a fifth satellite is available, RAIM can be calculated by the GPS receiver by substituting the fifth satellite in place of each of the other satellites, replacing one at a time. This provides 5 positions. The closer all the positions are to one another, the more you can depend on the position.  This can also be done with additional satellites and can be augmented by the barometric altitude as an input to the algorithm. Sometimes, the geometry of the satellites don’t allow for determining a good position or the number of satellites in view can cause this as well.  RAIM is used by the standard GPS and WAAS HPL/VPL is used by a WAAS GPS. If for some reason, the WAAS signal can’t be received, the WAAS GPS defaults to a non WAAS mode of operation and RAIM is used. But for the vast majority of the time, WAAS is available and it is used instead of RAIM. 

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  3. Luke on Nov 01, 2017

    Can I get a source for that? Not doubting you, just interested in reading more. Thanks!

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  4. John D Collins on Nov 02, 2017

    There are many available sources for standard GPS and WAAS. One is the AIM, sections 1-1-17 for GPS and 1-1-18 for WAAS. There will also be information in terms of limitations that you will find in an AFMS for a WAAS GPS receiver.

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