We contacted the FAA and requested a detailed track log of all parameters for this aircraft. Note that we are unsure of the accuracy of this log, but it probably provides useful information.
As shown in the compliance report and confirmed in the detailed track log, the only missing elements in the output from this transponder are pressure altitude, so a weak output seems very unlikely. For example, there is no drop-out of the GPS data supplied by the GPS receiver module internal to the GTX 345.
As Shadetree mentioned, the GTX 345 is receiving pressure altitude from the D180 and it appears to be correct and always passes the Kinematics checks.
Unfortunately, the supplied pressure altitude data appears to drop out 5% of the time, which not only shows up to the ground stations as missing data in the squitter, but this missing data also affects the Navigation Integrity Category (NIC) which is the calculated integrity containment radius around an aircraft's reported position. The NIC drops to zero and stays there when the pressure altitude supplied to the transponder is not continuous and reliable.
The same D180 was previously being used successfully with a GTX 327, but since that transponder only responds to interrogations and does not continuously squitter like the GTX 345 ADS-B Out transponder, this condition, if present, went undetected.
Just to be clear, we have no idea of the cause and this might be some kind of installation issue. If it cannot be solved, we have the option of installing the small pressure sensor option to the back of the GTX 345 and removing the link to the D180.
Thanks,
Steve