On Thu, 13 Sep 2001, Hire, Ejay wrote:
Most ATC towers do not have true radar. I.e. the ability to detect flying objects above altitude x by bouncing radio waves off of the object and computing the time vs. Doppler shift vs. inclination to determine altitude/heading/speed.
In modern (non-military) atc systems, this info is relayed by the transponder to atc.
If true, this is a weird practice. In Sweden, there are several civilian radars that are true radars with transponder receivers mounted on them as a compliment. Having served in the Swedish Airforce and actually having access to information provided by both civilian and military radars, my experience is that as long as you're flying fairly high (ie several thousand feet) even the civilian radars are going to see you fairly far away. Yes, your transponder is visable long before that, but if you turn it off you'll still be visable. We received information from both the transponder and actual civilian radar on our screens and most of the time the civilian traffic including small props were visable as both transponder position/hight and radar echo. What might be the case is that the true radar echo information is not relayed to the individual ATC because even the civilian high altitude radar gets cluttered by false echos. The military low-altitude costal radars are the ones really cluttered. Man, if it was like in the movies with a beep each time the radar swept an echo, all military radar officers would be deaf :) -- Mikael Abrahamsson email: swmike@swm.pp.se