Originally Posted by
pilotnik
OK, I was thinking about that too, but consider this: If RAIM is not available then the computer is not able to detect a bad signal from satellite. So if a bad signal cannot be detected the ANP will not reflect the actual performance at all. RNP approaches are ONLY allowed based on GNSS sensors, so it seems to me that without RAIM, the ANP reading is useless. Isn't that right?
No, it is not right. RAIM stands for Receiver *Autonomous* Integrity Monitoring. In plain English, that means the GPS receiver's ability to determine *by itself* the integrity of the GPS signals. The operative concept is *by itself* (that's what "autonomous" in the title means.) There are other means of verifying integrity and accuracy of GPS signals whcih are not "Receiver Autonomous". Systems of augmentation such as SBAS, WAAS, ground based augmentation, etc provide alternate means of determining the integrity of the GPS signals, so ensuring position integrity is not dependent on "RAIM". A NOTAM for lack of RAIM coverage does not make it illegal for all operators to fly an RNP approach. RNP only means "Required Navigation Performance" and there are different ways to achieve the required navigation performance for that approach. Without you having a detailed knowledge of the specific navigation equipment installed in the aircraft in question, and a knowledge of what was displayed on their cockpit navigation displays at the time, you have no way of knowing whether they were legal to fly the RNP approach. However, chances are pretty good that the were legal to fly the approach.