Hello all. I'm trying to learn a bit more about transponders, the different types there are, how they work, what the controller sees on his radar screen etc. Does anyone know where I can find such informaiton?I realize this is way off topic and for that I apologize. However, I decided to post here to find a suitable answe given that there are quite a few knowledgeable PMDG fans posting here whom I've come to greatly respect!Best Regards,Victor Limahttp://www.precisionmanuals.com/images/forum/800driver.jpg

Typical Air Traffic control radars cannot track an aircraft without use of a transponder. ATC radar can 'paint' an aircraft but cannot distinguish it from a flock of birds or a weather ballon, etc, without use of a transponder. The ATC lingo for a radar target that is not being tracked is called a 'primary' target, whilst an aircraft with a transponder broadcasting the assigned ATC code is a 'secondary', or tracked target. Tracked radar targets display heading, groundspeed, and altitude (if the transponder is Mode C equipped) information on the controller's display. Primary, untracked targets simply show a generic return on the radar display.ATC radar broadcasts an interrogation signal which the aircraft's transponder 'replies' to when it is recieved, providing secondary tracking information to the controller's display. Rough examples of the above:Primary Target (no transponder) only might look like this on a controller's display: Secondary (tracked target with transponder on assigned code) would look something like this: ........ AA1171 350/479The first example (primary target only) displays no heading, altitude or groundspeed information. The forward slash is a typical symbol used by modern ATC radar to represent targets. The second example shows little dots like a trail, indicating track/heading information (in this example the tracked aircaft is moving from the right on the screen to the left). The data block to the right of the tracked aircraft indicates that the target is American Airlines flight 1171 and it's at 35,000 ft, with a groundspeed of 479 knots. The signal processing architecture of typical ATC radars is very limited without supplemental transponder information on a target. High precision military radars of course are a whole different story :)

I've had to try to fly without transponder (most GA planes only have one) only to have most controllers refuse service (I was VFR). The newest most capable transponders uses a different frequency band, Mode-S, and broadcast the aircraft's latlon position along with squawk code and Mode c altitude. This provides a capability for aircraft to "see" each other if properly equiped without any additional ground-based equipment. The FAA is a decade behind in implementing a ground based system that will play along with this scheme, I think it is implemented along the busy East coast corridor. Check out the Garmin web site at http://www.garmin.com/aviation/adsb.html for a good description of ADS-B