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RAIM vs WAAS again

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Aircraft Systems

Despite ever thing I have been able to read on RAIM and WAAS, I am still confused on their interaction or exclusion in a GPS receiver.

RAIM has to built into a GPS receiver as part of it's software during manufacture.  I understand how it works (mostly) in a non-WAAS receiver.  What I don't understand and can't find is if a GPS receiver has the extra software/hardware to receive and process WAAS data to verify satellite accuracy, does is not have RAIM capability?  Is the RAIM software disabled or not installed in that model?   If it is installed, does it still operate/coexist with WAAS functions?

I know if the receiver has WAAS capability, RAIM is not required, but does that mean RAIM is disabled?   I went through the Garmin 430W giant manual, but cant find anything. 

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1 Answers



  1. John D Collins on Feb 28, 2020

    In the case of RAIM, calculations are performed in the receiver to determine the integrity of the GPS solution. With WAAS, the integrity data is transmitted by the WAAS signal to the receiver. Of course, one must be inside the WAAS (or SBAS) service volume to receive and process the integrity data. When a WAAS GPS receiver is outside the service volume of the SBAS satellites, it does not receive the integrity data and reverts to using RAIM. RAIM would also be used if WAAS service failed or was unavailable.

    A 3D position for GPS needs 4 satellites to be received simultaneously. The basic RAIM algorithm uses an additional fifth satellite.The fifth satellite is used to substitute for each of the other 4 satellites, one at a time and determine a total of 5 positions. The spread in the individual solutions is used to determine the integrity of the position solution, that is a containment radius in which the actual position has a high probability of being inside. An enhancement to the RAIM algorithm is to use the baro pressure altitude (baro aiding) as one satellite position, reducing the number of satellites needed to calculate the containment radius to four satellites. Most GPS systems use baro aiding to improved the availability of being able to calculate a RAIM solution. Modern GPS systems also use FDE (Fault Detection and Exclusion) which allows an erroneous GPS satellite to be excluded from the position determination.

    One last point, is the distinction between being able to calculate RAIM or RAIM availability verses a calculation using the RAIM function and determining the containment radius is not suitable for a given operation, example approach. So a RAIM prediction only predicts if the geometry is suitable at a place and point in time to determine RAIM. It does not mean that the accuracy of the GPS solution at that place/time is not accurate, just that there is not independent method of the GPS can use to determine the accuracy or containment radius.

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