Well, I emailed the PAPR for manual interpretation and here's the FAA's response:
The good news is the reports are easy to read because if there is red ANYWHERE that means the ADS-B system is not meeting performance standards (failure). However, there are exceptions and one is fringe of coverage due low altitude flying/mountains. In your case, it appears the red is due to low altitude (below 2000 ft) and lack of ADS-B coverage. I suggest you check the system again after a few more flights. It appears your system is functioning correctly.
His assessment is correct in that the flight started at low altitude (but, I've had 2 "perfect" PAPR's following the same flight profile). So, I guess one can't assume too much and has to request a manual evaluation anytime there is a failure to find out if it is significant or not! I certainly appreciate the FAA's prompt and personal response!
Zenith STOL 750
N750A @ TN66
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