WHAT DOES A TRANSPONDER DO.........AND HOW DOES IT WORK?

WHAT YOU NEED TO KNOW, AND PERHAPS A LITTLE MORE

All mode A/C panel mounted airborne transponders are basically the same regardless of who, where and when they were built.  Designed during World War II as a method of identifying "friend or foe" the typical general aviation transponder has changed little is nearly 60 years.  Current transponder technology with the exception of MODE S (which will be discussed below) has changed little.  It was "state of the art" 35 years ago, and still is.  What has changed are the components.  Tubes gave way to transistors.  Later integrated circuits were used.  Newer units employ microprocessors.  Some units use a tube, transistors, chips and a microprocessor, a true hybrid!

Transponder technology is the fundamentally oldest of all the avionic technologies in current use today because the system is virtually unchanged.  Many transponders built over 40 years ago still see regular service. Surprisingly there are an abundance of "old dog" transponders out there that will "outlive" many of the newer models that haven't even been built yet.  This is not conjecture. Even the newer MODE S transponders don't seem to have as bright a future as once thought.

Your transponder is basically a transceiver device that sends information to ground based radar station automatically each time it is interrogated by that station. It also enhances the image of your aircraft the radar operator is observing on his screen.

Imagine for a moment you are a radar operator tracking 2  non-transponder equipped aircraft, one 5 miles away and the other 50.  Both aircraft show up on the radar screen as primary targets, or radio waves bouncing off the skin of the aircraft. To accurately view either aircraft you must set the gain (receiver sensitivity) of your screen accordingly.  To pin-point the guy 5 miles away you reduce the gain until your target is a fine point on the screen, however the 50 mile target is gone.  So you increase the gain and as you do that crisp little dot that was the 5 mile target becomes a hazy large ball that grows bigger as you continue to turn the knob.  Eventually the 50 mile target comes into focus, with considerable interference from closer target.

Transponders solve the inherent physical limitations of radar, and they do it well.  Essentially what happens is each time the radar antenna is pointed at you, your transponder will receive an INTERROGATION signal.  Your transponder will REPLY with a signal of its own.  This REPLY also includes additional data such as squawk codes, altitude reporting and ident.  This allows ATC to set their gain at unity so they track you and everyone else. Other ATC based features include easier hand offs, improved flight following and collision avoidance.

Who's better off?  You, because your fellow pilots have transponders. Or your fellow pilots, because you have a transponder? Most of us prefer to have someone out there watching over us all.

As a pilot you are already aware of the benefits of using a transponder in the modern ATC environment.  But you may not be familiar with the following:

There is a new system out there, operational along the East Coast.  It's steadily growing and is worth keeping an eye on

At this point it is may be unwise to invest in MODE S equipment.  We don't recommend them, nor do we repair them because we don't see a future in a 20 year old system that is quickly becoming obsolete.

  TO FIND OUT WHAT THE FUTURE HOLDS