Originally Posted by
Bittell Lakes
I can remember watching a DanAir 727 take off and head south on that radar. After a while, it disappeared, then reappeared. The controller said it had probably just accelerated through the radars (something?) speed, at which the radar couldn't see the traffic. The antenna was huge, about a quarter moon shape, painted red. About 10 rpm, I guess. The other, higher frequency radar was prone to show weather and was only used for SRAs or in case of unavailability of the main radar.
Probably 'blind' speed.
Difficult to explain but it's to do with the moving target indicator (MTI) which normally suppresses all non-moving objects. On 'simple' radars where the radar pulses are sent out at a regular rate, at some time the reflected pulse from the aircraft equals the emission of the next pulse, the MTI thinks the blip hasn't moved and cancels it.
On more recent radars (the S232 was designed in the early '50s) the pulse recurrence frequency (PRF) is staggered. On the 10 cm Plessey AR1 of early '60s vintage, the PRF 'stagger' was was in the ratio of 1.2.3 and the first blind speed was in excess of 1,000kts.