Italia,
Okay, perhaps this may help. The unit or system, is giving you a position based on many factors. Many of these terms relate to sets, subsets, version, and couplings.
My explanation was given for a commercial application, that basically has every single couple. By couples I mean, multiple IRU's, antennae, and capability.
As another poster already mentioned, the Garmin is stand alone, and provides the navigation for the aircraft. On a commercial aircraft, the FMS does, coupled with many other instruments.
When the unit is coupled with the IRU, the HFOM used by the FMS comes from that couple, combined with GPS/MMR/etc.
I assume that your Garmin is a stand alone unit, therefore there are no couples that become part of the calculations. Different manufacturers look at the calculations differently, and make certain assumptions, based on capability.
Read slide 17 and 20
Note on 18 that RAIM algorithm depends on the manufacturer...
This is exactly why ADSB is in its current 'state' of development.
They have spent hundreds of millions to study and define ADSB, but it doesnt match, or needs to be translated from the terminmology and calculations used on the ac.
ADSB out!!, these guys crack me up!
What I find humourous, that in the same document, they mention rollout beginning in 2008, most in 2010, and the rest by 2013...then later in the document, they say that FAA will define the strategy for ADSB by 2012, with full implementation by 2020! (This doc was from 2009) Now the deadline is to first tier ADSB by 2020, with ADSB out by 2030....
Put this right along side of the microwave landing systems, byt the time it gets sorted out, the technology is hopelessly outdated and wont be used...
plus, dont confuse the ADSB broadcast terminology with any known reference to the real world...