On Dec 14, 10:58 am, "Wyatt Emmerich" wrote:
My Garmin 530W has something called HFOM and VFOM and the numbers are
usually higher than a couple of feet, which you describe as the practicable
accuracy of a WAAS.

Can you explain this for me?

FOM = Figure of Merit

Not sure if Garmin is truly using HFOM/VFOM as defined; if so, these
are bounds on the expected accuracy values *assuming there's no
satellite failure*.

HPL/VPL (Protection Level) are part of the Integrity function being
provided by the receiver. They are sometimes referred to as
"containment" values. These are based on the receiver being able to
detect (FD in the case of TSO129) and exclude (FDE in the case of
TSO145) a faulty measurement (ranging errors).


Regards,
Jon