Post by thread:[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.

>-----Original Message----- >From: pic microcontroller discussion list [spam_OUTPICLISTTakeThisOuTMITVMA.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.

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 .....gisskeKILLspam@spam@offex.com {Original Message removed}

A transponder re-transmits a radar pulse with digital encoding the most common in light aircraft is a serial data frame and 12 user selected bits for an ident number. Some idents have special meanings emergency, radio failure, default visual flight rules for example. Air traffic control assign ident numbers to help track flight progress of specific airplanes . Other information like altitude is encoded in commercial A/C transponders. In general all electrical equipment in airplane can be turned off if for no other reason to deal with fire in electronics issues. (The breakers may be the only way some transponders can be turned off) The signals from a transponder are not very powerful. The radar antenna that receives them have a lot of gain. I don't think it would be that easy to triangulate on that signal. w.. Mike Hord wrote: {Quote hidden}> > 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.

I had a newly installed transponder start leaking smoke & stink in the cabin one time. I used the off switch. In the mid to late 1970's, ATC discovered they could filter out the weather and most all of the traffic that they were seeing on their screens by turning down the gain on their primary radars and dealing only with the transponder returns. Separation of IFR traffic is their concern. The pilots now deals with non transponder equipped traffic and weather conditions. Before that time the ATC folks were very good at helping with weather avoidance and vfr traffic. I am very skeptical about any claims that there is more traffic now than used to be. General aviation appears way down to me. There used to be a proximity warning device manufactured by Genave (I think) that received and indicated the general direction of transponder returns from nearby aircraft. It was unfortunate enough to come to market at a time when the Fed's were mandating ELT's, encoding altimeters and new transponders for every one. John Ferrell http://DixieNC.US {Original Message removed}

Hi Mike: Many years ago I worked for a company supplying microwave components for an aircraft transponder tracking system. The transponders in aircraft are triggered by a signal near 1 GHz, not by the search radar, although both the transponder and search radar antennas are mechanically connected and point in the same direction. The system had two antennas, one a directional antenna pointed toward the airport and picked up the transponder triger signals and knowing the bearing to the airport and the rotation rate of the search radar was the method of knowing the bearing of the search radar at the airport. The other omni antenna picked up the transponder returns from the aircraft which now can be matched with known bearing, and using signal strengh and the aircraft altitude from the data message, the distance to the aircraft can be estimeted. The result is a Plan Position Indicator just like in the control center showing all the aircraft. Of course a purpose built receiver could pickup the transponders with just an omni antenna, but then you would not know where in the sky the plane was located, unless you combined that info with ACRS data or some other information fusion technique. Have Fun, Brooke Clarke, N6GCE http://www.PRC68.com {Quote hidden}>Date: Thu, 17 Jun 2004 16:17:32 -0500 >From: Mike Hord <gaidinmdKILLspamHOTMAIL.COM> >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. > > > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Transponders have switches so they can turned on off before engine start or shutdown. They can malfunction, and ATC may ask you to turn it off do position reports verbally. They also draw a lot of current, in the event of an electrical failure and when running on batteries only the pilot has to shut down non-essential equipment. When flying in formation, only the lead aircraft has his transponder on, etc. Some of the reasons why they have switches. I have had instances where cycling the power on a transponder has fixed a problem with ATC picking me up on their scopes. Eventually the analog radar/transponder interraction as we know it now will go the way of the buggy whip. The FAA has been working on GPS-based digital transponders with data links. See http://www.alaska.faa.gov/capstone/faq.htm for details. Regards, Bob Richard Graziano <.....rgrazia1KILLspam.....ROCHESTER.RR.COM> wrote .. > The power switches were put on the transponders originally for test and > calibration but that was pre-crazy world. > > {Original Message removed}

It's not a technical solution; this site <http://www.passur.com/sites.htm> provides real-time (or, perhaps, near real-time) tracking of air traffic at select airports. It is quite interesting to watch the patterns. I was recently involved in a project at the FAA's Air Traffic Control System Command Center (ATCSCC) in Herndon, VA. They use a similar web-based application with additional information available about each aircraft. -- Alan At 04:17 PM 6/17/2004, Mike Hord wrote: >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. ======================================================================= Alan Schnittman | Brainchild Evolution, Inc | EraseMEschnittspam_OUTTakeThisOuTmindspring.com ----------------------------------------------------------------------- prototype design & development | computer interface embedded control | analog & digital circuits | software development ----------------------------------------------------------------------- -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body