this is an interesting document... that details how some of the different systems work, depending on manufacturer..
Review result of ADS-B Working Group Meeting
On Airbus aircraft, the HIL (Horizontal Integrity Limit) parameter is provided by the GPS (MMR – Multi Mode Receiver- or GPSSU –GPS on Airbus aircraft)
through the IRS (Inertial Reference System) bus.
Indeed, current transponders are not able to receive information directly from the GPS system, so the IRS is used as a “mail box” for GPS parameters (position, velocity, and HIL).
Honeywell:
“The answer to your question is that we follow Arinc 718A. Thus, for calculating the NUCp value used in BDS 05 and 06, if HIL label 130 is unavailable, then the transponder will use HFOM label 274.”
Honeywell and Collins:
Rockwell Collins also answered to Airbus query by indicating that the transponder TPR-901 822-
1338-021 uses HIL (label 130) as an initial source for computation of NUCp, and HFOM (label 274) if HIL is not available. Honeywell and Collins transponders have the same behaviour concerning the encoding of NUCp.
HIL is the primary source, HFOM is used if HIL is not available. This is compliant with Arinc 718A and OACI, annex 10, amendment 77.
ACSS:
“The XS-950 -10005A transponder sets the "Type Code" based strictly on the received value of HIL (Label 130). For airborne position extended squitters (BDS 0,5), if Label 130 is not received, the "Type Code" will be set to 18. This "TypeCode" = 18 corresponds to a NUCp= 0.
For surface position extended squitters (BDS 0,6), if Label 130 is not received, the "Type Code" will be set to 8. This "Type Code" = 8 corresponds to a NUCp = 6
On a side note, even though Honeywell provides the GBAS system, their MMR cannot receive the signal, only the Collins MMR can.