Transponder derived altitude.
Working Farnborough Radar last week, the controller asked me a few times to double check my altitude and QNH set. I was cruising at 2,700ft, but the controller informed me that I was indicating 2,400ft. The aircraft (PA28) had dual altimeters, both set to the QNH of 1025 and were both indicating 2,700; the GTX330 transponder was showing FL023.
1025-1013=12
x30 = 360
2,700-360 = 2,340
FL023 would appear to be about right. My understanding is that the transponder sends ALT info based on the aircraft's pressure altitude and then the ATC equipment converts this information based on the reported QNH. Where could this discrepancy have arisen from, could there be a fault with the equipment on the aircraft?