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Reading @Antzi's question about GLONASS and GALILEO led me to the NTSB's Current GPS SPS Navigation Service Display or SPS PDOP (dilution of precision) map for GPS.

Each time the map updates (every few minutes) the contours change, and there are little "redline islands" that jump around. Below I captured a screen shot that includes the legend plus a bit of a "redline island".

The legend says:

GPS Service Contour

RAIM Service Outage: RNP 1 (Red Line): RNP 3 (Green LIne): EnRoute (Yellow Line)

Color Scale is Position Dilution of Precision (PDOP)

Question: What is a RAIM service outage, what is RNP, what does EnRoute mean, and why do these redlines jump around so quickly? Are these generally closed contours, such that whatever they indicate is true for the entire area inside the contour, or is it true only along the line itself?

enter image description here

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RAIM=receiver autonomous integrity monitoring. Basically it is an algorithm incorporated in the receiver to eliminate anomalous satellites from the position computation. Obviously to do that there should be more degrees of freedom than unknowns, so at least 5 satellites should be in sight.

RNP=required navigation performance. It is the minimum accuracy level that an aircraft has to be able to have for its position estimation to enter a required airspace. In particular Enroute is a RNP for cruise flight operations (the admissible PDOP are the yellow ones).

Those quickly red contours, I would bet that they are peaks of PDOP, to put it in other words, regions where maybe the number of available satellites is less than four at that instant or where the satellites geometry with respect to the user is very poor. The GPS constellation is designed in such a way that those situations should be rare, but not impossible.

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  • $\begingroup$ thanks for the concise yet complete answer, it all makes sense now! $\endgroup$ – uhoh May 21 at 10:53

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