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My question stems from this other question: Why is training regarding the loss of RAIM given so much more emphasis than training regarding the loss of SBAS?

Two answers say losing RAIM puts a question mark on the integrity of the GPS position. And losing SBAS only affects LPV approaches.

Yet from this other question, Why weren't SA-On GPS units updated to become SA-Aware?

According to Advisory Circular (AC) 90.114A CHG1, now and past 2024, SBAS-equipped planes do not need to do a preflight prediction check.

Why skip the important RAIM prediction step?


RAIM = Receiver autonomous integrity monitoring
SBAS = Satellite-based Augmentation System
LPV = Localizer performance with vertical guidance

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The slide refers to ADS-B integrity, which requires a different integrity level than RNP. The ADS-B preflight check they refer to is a different than the check for RNP.

The FAA requires an integrity bound of 0.2 NM or less for ADS-B. For RNP2 (en-route) and integrity bound of 4 NM is sufficient. RNP-approach requires and integrity bound of 0.6 NM.

The probability of not achieving the integrity level required for ADS-B is significant for TSO C129 (SA ON) installations, much lower for TSO C196 (SA Aware) installations and insignificant for TSO C145/146 (SBAS) installations.

The better performance of thw SBAS installations is because SBAS does a few things to improve integrity:

  1. it is an extra GPS satellite at a fixed (geostationary) position.
  2. it supplies parameters for an error correction model that improve the accuracy of GPS signals received by the GPS receiver
  3. it does its own integrity monitoring of the whole GPS system and reports any issues with GPS within 6 seconds to the WAAS receivers.
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