Meaning of ADS-B NUCp values and accuracy of MLAT


#1

What are the actual distance accuracies corresponding to the NUCp values reported by ADS-B? I know that 0 means very poor and higher numbers are better, but I can’t find a definition of what the different values actually mean. The actual specs are all behind the RTCA paywall and information I find online is confusing…

Related question, how accurate are the positions from MLAT? They report NUCp of 0 and the MLAT flight tracks seem to wander around a bit so they don’t seem to be very accurate.

Thanks! Kent


#2

Yeah, you need Doc 9871. There are some draft versions floating around that are not paywalled.

Actual mlat accuracy as implemented by FA is usually around 0.5km but can be substantially worse depending on the geometry and what the aircraft is doing (it tracks with a constant-velocity Kalman filter, so it does not handle turns very well). So we could probably claim NUCp 2 or 3 … but there’s not much point and we certainly don’t want anyone relying on it!