my findings with the BU-303 models are that they respond REALLY well to a metal surface underneath, hence the quite strong magnet inbuilt, which helps its inbuilt "active" antenna.
also, if an aircraft flys overhead, this can cause a signal loss thanks too the planes "rad-out" system, which sends radio waves to the earth, that bounce back to the aircraft in order to calculate its own alt. etc etc.
note - not all aircraft have these, just major planes like Boeing, Airbus etc.
also - this radar unit in aircraft is extremely powerful, if it is fully charged (IE in operation) and you walk directly under it while an aircraft is using the radar you will no doubt get cancer, and prolly microwave your insides
this can cause the BU-303 models to drop out, due too all the interferance.
major aircraft use this when in flight, IE after climbing to altitude or before decent, so if you have aircraft flying overhead, dont expect the same performance from your BU-303 model...
just my observations.