I've had a transponder go u/s on me and it has flipped between zero and something in the ionosphere. This was reported to me by ATC as there was no indication in the aircraft.
The FL data field (jn my radar data) has two check "bits". The highest bit indicated that the data is valid (i.e. within valid FL range) and the lowest bit indicates positive or negative FL. I'm confused by the need for the two bits. If a negative FL indicates a fault then why is there a validity bit
