|
Edited on Thu Feb-12-04 01:34 PM by Devils Advocate NZ
Many people don't seem to have ANY idea what they are talking about, yet act like THEY are the knowledgable and logical ones, like this person:
It may now perhaps be reasonable to surmise that something in Washington was an intended target but when all that anybody knew was that that a plane had vanished from the ATC screens, how the hell were they to know what was intended or where it was headed to anyway? For the time being how were they to know that Flight 77 had not already crashed?
If the author of the above understood how radar ATC worked, he would know that his statement is wrong. So here is the truth.
A radar works by transmitting high frequency (microwave) radio waves into the sky. When an aircraft flies into range of a radar, its skin reflects this radio energy back towards the radar which receives it after a short delay. Where the beam is pointing tells you what direction the return is coming from, and the time taken for the signal to go out and back to the radar tells you how far away the aircraft is.
However, this information (position, and after several "paints" speed and heading) is not adequate to use for ATC purposes, and having a special military style "tracking" radar is too expensive and would require too many radar sites. So to overcome this limitation, aircraft carry a radar "transponder". This transponder detects the incoming radar waves, and sends out its own signal carrying information such as altitude and flight number.
So the radar not only receives the reflected waves that it transmitted, it also receives a data stream from the transponder. Combined, the two sources of information (skin paint and transponder signal) tell ATC everything they need to know to direct the flight according to its flight plan and other traffic.
So, when the transponder was switched off on Flt 77, the aircraft did NOT disappear from the radar scopes. In fact not even STEALTH aircraft truly disappear from radar - their signal is merely reduced to a level where the radar controlling computer filters it out believing it to be "clutter" (returns from things such as birds and clouds).
What happened that day was that the extra information provided by the transponder of altitude etc disappeared from the scope, but the normal aircraft return did not. The controllers knew where it was, and where it was heading at all times. However, they also knew that something was very wrong on Flt 77 (and the other flights where the transponder signal was turned off). No commercial aircaft is supposed to fly without its transponder on, and failing a reply from the pilots as to why the transponder had been turned off, ATC would have declared an emergency.
On edit: I forgot to say what "radar" actually means - it is in fact a word made out of the acronym RDAR, which stands for Radio Direction And Ranging.
|