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originally posted by: roadgravel
a reply to: baburak
The planes in Europe were not off primary radar, the transponders were not being seen.
originally posted by: datasdream
Simple explanation. The transponder boxes send out an ID. If the ID is scrambled or the signal level is blocked by a higher power signal on the same frequency then it can't be seen. Most likely done by design. Since it was concentrated in a single area it might be someone playing around. It would be interesting to find out if they have a way to triangulate an interference source.