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'[OT:] Aircraft transponders'
2004\06\17@171930 by Mike Hord

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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.

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2004\06\17@172518 by Robert B.

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I'd imagine they need to be able to be power cycled in the event of failure,
like pretty much every other electronic device.  Also its undoubtedly
protected through a fused circuit to prevent a fire in event of malfunction,
which makes a convenient way to "turn it off".


{Original Message removed}

2004\06\17@173140 by Robert Ussery

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

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2004\06\17@173558 by Edward Gisske

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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
.....gisskeKILLspamspam@spam@offex.com

{Original Message removed}

2004\06\17@174349 by Walter Banks

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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}

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2004\06\17@195959 by John Ferrell

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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}

2004\06\17@205543 by Brooke Clarke

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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}

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2004\06\17@230036 by Richard Graziano

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The power switches were put on the transponders originally for test and
calibration but that was pre-crazy world.

{Original Message removed}

2004\06\18@115301 by Bob Japundza

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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 <.....rgrazia1KILLspamspam.....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}

2004\06\18@172047 by Alan Schnittman

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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.



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