Why Does...

It seem like most folks refer to their transponder as a radar (or their transponder reply as a radar return)? Sorry for the rant, but they are two different animals entirely. A radar sends out a pulse of RF which reflects off of a target, returns to the receiver, is processed (speed, range, etc...) and is displayed. Nothing required but a target out in the airspace (or, on the ocean, or the ground, depending). A transponder, on the other hand, is analogous to your radio. An interrogator sends out a message...(who are you? What are you doing? What's your speed?...etc...) and your transponder is kind enough to reply with the answers...just as if you were talking to ATC. The beauty is when the radar returns and the transponder replys come together and agree. When they don't agree, or in some cases when they completely disagree, that's when folks smarter than I start to ask questions. For anyone who would like to see this, look me up when you're in the Keys...be glad to give you a tour of a facility and show ya some hardware.
Ok, rant over. You may now return to your regularly scheduled programming...

Not all that different

While the transponder provides additional information (altitude, assigned code if any) the basic location functions are still radar-like. e.g., azmuthal position is determined by the resolution of the rotating interogation signal, and distance is determined by the speed of light out and back (with the standard transponder delay accounted for).

The FAA also refers to it as "secondary radar."

BobTurner said:

While the transponder provides additional information (altitude, assigned code if any) the basic location functions are still radar-like. e.g., azmuthal position is determined by the resolution of the rotating interogation signal, and distance is determined by the speed of light out and back (with the standard transponder delay accounted for).

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Have to disagree. While a radar return is simply a piece of RF that has shifted (indicating speed if it's a doppler radar) and has a time difference (indicating range)...and, I'm being simplistic here, a transponder (otherwise known as an IFF system) actually interrogates your aircraft. If your aircraft chooses to not respond, then there is no return. If it does choose to respond it will do so dependent upon what equipment you have installed onboard (it may respond mode S, mode 3, mode C, mode 2, or an interleavig of the modes). A transponder is in no way a radar system. Reference Merriam-Webster's definition of radar :

: a device or system consisting usually of a synchronized radio transmitter and receiver that emits radio waves and processes their reflections for display and is used especially for detecting and locating objects (as aircraft) or surface features (as of a planet)

A transponder has no reflection. The signal is received by your installed equipment and replied to. The FAA can call it what it wants, but it isn't a radar.

Ron,

Are you saying that the "interrogator" signal is different than the radar pulse hitting the aircraft?

Bevan

If it's not radar...

Then why does ATC get a "blip" if the aircraft has no transponder or the transponder is turned off?

RADAR

From Wikipedia:

"The term RADAR was coined in 1940 by the United States Navy as an acronym for RAdio Detection And Ranging."

I think that what ATC does with my transponder certainly is radio detection and ranging. Some of it use reflected radio waves and some interrogates the transponder and it replys. But both cases are RAdio Detection And Ranging.

Air Traffic Control Beacon Interrogator normally is co-mounted with the primary radar antenna an turns at the same rate, 6 RPM for en-route and 12 RPM for airport surveillance radars. The interrogator can interrogate your transponder several times as it sweeps by. The interrogator requests the mode your transponder will send - mode 2, mode 3A, mode 3C, mode S or Mode IV. Mode IV is the military Identification Friend or Foe (IFF) mode and provides encrypted interrogation and reply.

The radar processing system at the radar site normally process both the primary and secondary (beacon) returns and combine them into a single radar return. If the primary correlates with the secondary return (within beam width and range gates) the reply data is included with the single radar plot. This will be the mode code and altitude for a mode C equipped return.

The Air Traffic Control System at the Center or Airport receives plots from a single or multiple sources and provides the tracking logic to project the aircraft path and associate the mode code with a flight plan for presentation on the screen.

I see nothing wrong with calling the Beacon return a radar return. There is an interrogation and an active return vs. a passive return.

ATC will only get a blip on the screen from the primary radar but will not get anything from the transponder if it is off or on standby.

Mel said:

Then why does ATC get a "blip" if the aircraft has no transponder or the transponder is turned off?

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Because ATC has Radar too. That blip is the radar return, and has nothing to do with the transponder.

Bevan said:

Ron,

Are you saying that the "interrogator" signal is different than the radar pulse hitting the aircraft?

Bevan

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Indeed I am. They are two completely separate systems.

Mel said:

Then why does ATC get a "blip" if the aircraft has no transponder or the transponder is turned off?

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Because the RADAR return is reflecting off of your aircraft. It's a different transmitted bit of RF than the interrogator signal.

I know I'm getting picky here, but they are truly two completely different things. Beacon (IFF) is beacon...Radar is radar. With beacon, you could, very slowly, provide the same information over your radio to a human in the system if there were never such a thing as a transponder (it is, at it's core, a two way radio receiving questions and responding with answers). You can't, however, catch a pulse of RF and throw it back.

Oh, and 94kb...great response. You've said alot better than I.

OK. I understand what you are saying.

I worked on IFF systems in the Air Force and I do understand the system.

But I DO think you are getting picky with the terminology.

RonH said:

Indeed I am. They are two completely separate systems.

With beacon, you could, very slowly, provide the same information over your radio to a human in the system if there were never such a thing as a transponder (it is, at it's core, a two way radio receiving questions and responding with answers). You can't, however, catch a pulse of RF and throw it back.

Oh, and 94kb...great response. You've said alot better than I.

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Not exactly. The human would say, "I'm out here". ATC says, "where?". Human says, "I don't know. Just out here somewhere". The transponder, otoh, only replies when the highly directional interogation signal is pointed at it, so atc can figure the bearing. And ATC figures the range from the delay time between pulse out and pulse back, just as it does with primary radar. I would say that "catching some RF and throwing it back (with squawk code and altitude encoded), amplified, is a very good if not overly simple description.