Sept. 11th, anyone heard an explanation for this -
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post #1 of 4 Old 09-14-2001, 09:37 AM Thread Starter
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Sept. 11th, anyone heard an explanation for this

Hey there,

I was wondering if any of you have heard an explanation for this?

How did four planes get so far off course without anyone knowing about it? Don't they track these planes by radar? I heard they turned off the transponder but what does the radar show when they do that?

Any body heard anything about this?


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post #2 of 4 Old 09-14-2001, 12:24 PM
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My understanding is that the transpoder is one of the easier things you learn in flight school, although I don't know of why anyone would want to turn it off ever (someone have an answer?). I heard they did track these on radar but the closest planes were National Guard F16's and since they were not on alert, it would have taken them too long to reach the planes to do any good. And, since this has never happened before they may not have been quick to shoot down a passenger jet for obvious reasons.

I don't even feel like Jeepin right now, friends. Seems so insignificant in the face of this.

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post #3 of 4 Old 09-14-2001, 10:11 PM
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Air traffic control radar is passive radar. For an airplane to appear on radar, it's transponder (xpndr) must be turned on. Each plane, while in contact with ATC, must have their transponder set to a four-digit code assigned by the tower: 0000-9999. In an emergency (if I remember ground school correctly), the pilot is supposed to "squawk" (or dial-in) xpndr code 7777. This will appear as an aircraft in distress on the radar controller's screen.

But what happens when the xpnder is turned off? The plane vanishes from ATC radar. ATC radar, as a passive system, requires that a return signal be transmitted from each aircraft being tracked.

If an aircraft MUST be tracked, and it's transponder has failed or been disabled, active radar must be used. Active radar "paints" it's sweep arc with radio waves and "receives" radio reflections and doppler frequency shift instead of relying on a return signal from the target aircraft. Weapon target acquisition radar and weather radar are also types of active radar.

As you might guess, Active radar has much shorter range and requires a lot more power to operate than passive radar. Plus, ATC does not have active radar systems at their disposal. To the best of my knowledge, only the military has active radar emplacements.

Hope this helped, and didn't cause too much confusion...
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post #4 of 4 Old 09-17-2001, 04:41 PM
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From: jeepsonly
I don't even feel like Jeepin right now, friends. Seems so insignificant in the face of this.
Even here in Canada there was a round of shock and amazement at what happened.

This weekend was the AURS cleanup days where around 200 people showed up to clean up one of our local playgrounds. The start of the day was given to rememberance of the incident in NewYork and Washington. Wheeling was good but many of us had our thoughts on friends and relatives that were in NewYork at that time. Our prayers were with those who lost their loved ones.

The time with nature was a big help for me .. and being able to see that no matter what happens, there will be those to care about what is going on in the world and those who will try to make a difference for good.

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