Actual Nav Performance - FMC calculation
 

Hi, hope someone can help me out and answer this for me.

How does an FMC work out the its ANP? My knowledge is based exclusively on B737, and I am mainly thinking in terms of classic versions, although I guess other aircraft/avionics manufacturers are similar.

Is it purely theoretical eg. does it think, I am getting information from, say, 3 DME stations therefore statistically speaking the FMC position is 95% within a box, say, 0.3 nm across? And if it looses radio updating completely does it just gradually widen the box over time based on theoretical performance of the IRS?

Is it more practical eg. does it compare the data from the radio sources and create a space within which its position almost certainly lies (rather like those nav exams I did a long time ago)? Will it reject obviously wildly incorrect data?

Does it measure the distance between the radio positions, the IRS positions and the FMC position and use this to create a performance figure?

Thanks in anticipation.

See if this helps.

boeing.com/commercial/aeromagazine/aero_16/navigation_algorithms.html