Transponders are turned off for a variety of reasons. For one, they clutter the radar around airports from planes on the ground, but not taxiing, landing, or taking off. The FAA tells pilots when to turn transponders on or off. They can only be on when the FAA assigns a code to them, or the plane is squawking 1200 (VFR). Often, while in flight, the FAA will ask a pilot to "Recycle Transponder", or turn it on and off to reset a possible fault condition. Because the transponder ID codes are re-used many times during a day, the FAA has to assign the code they want squawked. They do this so the same codes are not active within a particular radar set's range. This system dates back many decades and relies on a 4-digit (octal coded) spacing between a pair of response pulses to ID the plane. It is impossibly antique, but so is the rest of the FAA's stuff. Airplanes are the last AM communication users (except CB) on the face of the earth. Edward Gisske, P.E. Gisske Engineering 608-523-1900 gisske@offex.com ----- Original Message ----- From: "Mike Hord" To: Sent: Thursday, June 17, 2004 4:17 PM Subject: [OT:] Aircraft transponders > One of the things that I'm hearing in NPR coverage of the > 9/11 commision's findings is that the aircraft in question were > lost because the hijackers turned off the transponders. > > I'm now certain that any question I can ask can be answered > by someone on the PICLIST, so I'll ask this now: > > Why on Earth is there an off switch on the transponder? > What could EVER make turning the transponder off a good > idea? > > Side point: how difficult would it be to set up several > antennas to pick up aircraft transponders, triangulate their > position, and project that info onto a map? I think it would > be fun to monitor local air traffic. Is this even legal? I know > that's the kind of thing that may not go over well these days. > > Mike H. > > _________________________________________________________________ > Is your PC infected? Get a FREE online computer virus scan from McAfee. > Security. http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963 > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.