FlightPath...
I don't think we understand each other. I completely understand everything you've said about RAIM, WAAS, how GPS works, etc.
What I meant when I said I didn't understand this stuff is below.
It appears that your Garmin is saying the same as I stated for EPE. In your example, that "The accuracy of the aircraft’s GPS fix is calculated using Estimated Position Uncertainty (EPU), Dilution of Precision (DOP), and horizontal and vertical figures of merit (HFOM and VFOM). EPU is a statistical error indication and not an actual error measurement.
Note Accuracy, ie EPE is calc'd USING....it also states EPU is NOT the actual error measurement.
The quote says that the accuracy is calculated using EPU and HFOM... so EPU and HFOM shouldn't be the same, correct? Well take a look at this:
http://adsb.tc.faa.gov/WG6_Meetings/...20briefing.PDF
It says, "HFOM is also known as EPU". This is what I mean when I say I don't understand this stuff! I'm getting conflicting information. I'm hoping you can see what I mean here!
According to this (
Aero 12 - Required Navigation Performance), EPU is the same as ANP.
Here is a definition of EPU:
http://www.honeywell.com/sites/servl...B-7FF07F662003
Another definition of EPU:
http://www.mitre.org/work/tech_paper...93/09_4093.pdf
OK, disregarding what I wrote above, this is the way I see it now: HFOM is basically the calculated accuracy of the GPS position with a 95% confidence level in the calculation. DOP takes into account satellite ranging errors and geometry. EPU is the 95% confidence that you lie within a circle, with a radius in NM equal to the EPU, centered on the estimated GPS position. So, take that estimated GPS position (one pin-point position) and apply the 95% confidence in the accuracy (HFOM) and then take into account the geometry and satellite ranging errors (DOP) and after all that you will get the EPU. There are more components involved but right now that's the way I see those terms related to each other.
But nothing has shown me this relationship clearly so I don't know if my view is correct. It's not a matter of understanding something complicated here, it has to do with getting clear and accurate definitions and some explanation that ties them all together! This is not rocket science.
EDIT: Read Note 3 under 3.2.11 --
http://adsb.tc.faa.gov/WG6_Meetings/..._MASPS--V2.pdf
Read slide 17 and 20 --
http://rms.ion.org/wp-content/upload...g-20091210.pdf