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As usual, the media in this case are being unhelpful.
Civilian radar is passive, it cannot see a plane, it only sees the transponder (when its turned on and functioning).
The transponder broadcasts information to local ATC networks.(like your phone does to your network)
1. Position
2. Altitude
3. Bearing
4. Condition. ( the pilots normally set this to 1000 )
If there is a problem, either pilot can type in 7500 (Hijack); 7600 (catastrophic failure) or 7700 (mayday).
Military radar, on the other hand, is active (doesn't matter if the transponder is working or not) it "sees" the aircraft. However its range is much shorter..<200 miles in good conditions.
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