@nitpicker330 You may be right but in my experience, the ALT doesn't drop to zero before the signal is lost. The signal transmitted is digital, i.e. it sends a value. I'm not sure however on the degree of error correction used in the signal for decoding or determining a valid transmission. And I might point out my experience is with planes going out of range, not those having some sort of problem which results is the signal stopping.
Also Flightradar & similar services do not use RADAR...they use a digital signal transmitted by the plane which contains the GPS location, speed & altitude plus the Callsign details. The digital data is decoded to reveal the GPS location of the plane as determined by the GPS on the plane. In my opinion this is very accurate...as accurate as a GPS can be...+- 10m horizontal?