Farmstripflyer wrote:I have something strange going on with my transponder and looking for a bit of advice. Panel mount altimeter and Dynon D100 set to correct QNH and both confirm altitude along with Garmin nav and Skydemon. However, Trig TT21 reads 100’ below and ground based radar show me 200’ above aircraft readings! Trig had recent software update prior to installing ADSB using Pilotaware as GPS source. I am planning to send Trig back to factory for checks but could this have anything to do with the Pilot Aware or any other thoughts?
There are several elements here.
1. Your transponder shows a flight level on its display, and outputs that value to ground radar - nothing to do with static sources, in terms of what the radar receives. However ATC will correct for the difference between the relevant QNH and 1013.2, so the altitude reported by ground radar will differ from the FL shown on the transponder when QNH does not equal 1013.2 . Are you allowing for this in your 300' difference between TT21 and ground based radar?
2. Your transponder's accuracy - I believe 200' is allowed , others may have a better figure.
3. When those have been taken into account, then there comes into question the suitability of the static source or sources for altimeter, transponder etc, plus the accuracy of the individual instruments!.