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
HFOM, Horizontal Figure of Merit, is for GPS the 95% horizontal position accuracy bound, also known as EPU (Estimated Position Uncertainty).
For GPS, HFOM is calculated by:
HFOM = 2 * UERE * HDOP
With
UERE = User Equivalent Range Error (1 sigma value)
HDOP = Horizontal Dillution of Precision, a factor depending on satellite geometry.
The 2 is in there because, in a normal distribution, 2 sigma deviation from the mean covers 95%.
The UERE is an estimation of the Pseudo Ranging accuracy, i.e. an estimation of how accurate the distance between the GPS receiver and a satellite can be measured.
It is a combination of signal in space errors (e.g. satellite clock noise, ionospheric/tropospheric delay, ephemeris data error) and user equipment errors (e.g. user clock noise, multipath, radio interference). UERE is given as the (estimated) standard deviation of the range error.
Until the year 2000 the UERE was dominated by satellite clock noise that was added by the US military on purpose to the civil GPS signal to lower the accuracy in order to prevent enemy troops to use GPS to their advantage. This is called Selective Availability (SA).
In the year 2000 SA was disabled permanently so from then on the Pseudo Range errors was much smaller.
Old generation GPS receivers have the assumption that SA is turned on still hard coded into their system. Therefore these receivers largely over estimate the UERE and as a result the HFOM that they report is nowhere near an accurate estimation of their position uncertainty. Newer GPS receivers are aware of the SA state so their HFOM is, under the same circumstances, much smaller. But even the HFOM of the SA aware receivers is very conservative.
ATCast