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WAAS vs RAIM (Cont’d)

Asked by: 593 views Aircraft Systems, Commercial Pilot, FAA Regulations, General Aviation

Thanks Johns D. Collins for taking time on my last post.

As you mentioned:-

"4 healthy satellites are required to calculate RAIM with Baro Aiding. Most GPS systems use Baro Aiding." 

I understand that the LOI only matters with lateral navigation, however from the FAA doc. , I believe WAAS also calculates what RAIM does which is HPL plus VPL.

Now my question was, why do we need at least 5 satellite not 4 satellite if we have WAAS capability, simply because WAAS does calculate HPL +VPL ?

Looks little bit grey but if you could provide with some of the credible sources along with idea, that would be ideal.


Thank you.


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1 Answers

  1. John D Collins on Aug 18, 2017

    Conner, my reference is RTCA DO 229D. This is a pay walled document that can be purchased from RTCA for about $370. Section 2 is used as the technical specification for TSO C145/146 WAAS receivers.

    RAIM and WAAS integrity are determined differently. VPL has nothing to do with LOI as it is only used for vertical navigation and comes into play only on an approach. If the VPL is above the limit value for the approach, the approach is downgraded to LNAV without any vertical guidance.

    RAIM is the acronym for Receiver AUTONOMOUS Integrity Monitoring. I capped the key word autonomous. This means that the receiver itself is able to determine an integrity value by performing calculations, it is self contained, autonomous. It does this by calculating lateral positions using 4 satellites. A fifth satellite is used to calculate four more calculations, by substituting for one of the other 4 satellites. So you end up with 5 calculated positions. These positions are used to determine the residuals from a common position. That sets the distance that contains all of the positions. The result must be within tolerances or you get the LOI. Geometry of the satellites plays an important role in the calculation because, not all combinations of satellite positions will produce the same uncertainty in the result. It is like the error in determining a location based on the intersection of two VORs. The best result will be obtained if the position of the VOR radials are 90 degrees from each other. If your position is on a line directly between the VORs, then the uncertainty is the entire length of the distance between the VORs. So there are cases where RAIM can’t be calculated due to geometry and this condition is known as not having RAIM (area of predicted RAIM Outage) or IOW, the calculation can’t be performed. This is different than the case where RAIM can be calculated, but is not suitable for for a given use, for example an approach. The RAIM areas that the geometry will be unsatisfactory to provide a usable calculation can be predicted by knowing where the satellites are, where they will be in the future, and where the geometry will not be satisfactory to determine a RAIM calculation.

    The RAIM algorithm can be improve upon by using the aircraft pressure altitude, as the vertical position, although less accurate than the lateral position, can substitute for one of the satellites. This is called Baro Aiding and probably all IFR GPS installations use this. With Baro Aiding, only four satellites are needed to determine a value of RAIM.

    With WAAS, the calculations are determined by the WAAS system as each ground station that measures its position via the GPS satellites is at a know location, and the error can be determined one satellite at a time. This integrity data is uplinked to the WAAS satellites and is broadcast as part of the WAAS correction messages. So the WAAS system itself is determining the integrity information via explicit measurement and passing it on to the WAAS receiver to determine the HPL.

    In general, inside the WAAS area of coverage, the WAAS integrity data is used and RAIM is not. That is why one does not need to check RAIM availability for a flight with a WAAS receiver, unless they are flying outside the service volume of WAAS or there is a WAAS system wide outage via a NOTAM.

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