>-----Original Message----- >From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU] >On Behalf Of Mike Hord >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? All aircraft avionics (to my knowledge have an "off" button. Some aircraft have multiple transponders - thus you only have one on at a time. It's possible. You turn off the transponder when you're on the ground, unless you're squawking a ground code to keep from cluttering the radar returns of flying aircraft. All kinds of reasons. I'm a bit out of my depth here, as all the aircraft I fly don't have transponders, but there's always a need to turn them off at some point. If nothing else, they're a big power-hog, so it would be necessary to turn them off if/when the power fails and you're running off batteries, that is, assuming you're flying in Class E or other uncontrolled airspace. >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. Heck, just buy a cheap TCAS system (<$10,000 IIRC) and mount it on your roof. AFAIK, it's legal to operate one of the outside an aircraft. Only problem is the limited range - only good to a couple of miles. - Robert -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.