Radar is based on the principle of transmitting radio waves and measuring the length of time-of-flight, direction, and frequency shift of the returning reflections or echoes.

In aviation, we call this "primary radar" and we use the term "secondary radar" to denote the ATC/transponder system.

Secondary radar uses a transmitter that sends pulses that the transponder picks up and then sends coded pulses of its own back. But these are not based on reflections off the airframe.

Why then call the ATC/transponder system "secondary radar" when the principle is very different from what we might call "real" (aka primary) radar? Or does the "secondary" here meaning "in addition to" rather than "another form of"?

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    $\begingroup$ It still uses radio waves to detect aircraft and find the distance to them, right? $\endgroup$
    – cpast
    Mar 6, 2015 at 0:00
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    $\begingroup$ Yeah, but by that definition the ATC contacting the plane by radio and verbally asking the pilot for a GPS position readout would also be radar. Which is essentially what SSR is, but automated (and not as detailed). $\endgroup$
    – RoboKaren
    Mar 6, 2015 at 0:02
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    $\begingroup$ SSR doesn't ask the plane for its position, though; it computes distance and azimuth from timing, just like primary radar. $\endgroup$
    – cpast
    Mar 6, 2015 at 17:57
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    $\begingroup$ @cpast is correct. Only the altitude and 'squawk' code are encoded in the response (in Mode C, at least... a bit more goes into Mode S responses, IIRC.) In addition to timing, though, actual antenna response differences are measured to help determine bearing more precisely than could be done with timing alone. The timing could be faked (by delaying the response,) but faking antenna response differences would be really hard to pull off reliably. $\endgroup$
    – reirab
    Mar 7, 2015 at 8:01
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    $\begingroup$ Without going through the history, better terminology would be the primary is a passive system, relying on a reflected return from the aircraft. The secondary system is an active system, which broadcasts a reply in response to an interrogation. Azimuth and time of "flight" are used to determine the location of the transponder responding to the interrogation. Newer secondary modes do not require an interrogator and self-report location. Probably not really secondary radar, but the function is similar. All three methods are independent. $\endgroup$
    – mongo
    Apr 16, 2017 at 2:57

2 Answers 2


The terminology is a combination of history and "trustworthiness" (for lack of a better term).

Historically, primary radar was invented first. So it makes sense that this would be called "primary" and another scheme invented later would be called "secondary".

ATC systems like to ensure that the data they are working with is trustworthy. Primary radar systems are based only on the reflected radar signals. If you get a reflection, you know there is something there, whether or not it has a transponder (or indeed, whether or not it is an aircraft). This is a certainly a key point in military radar systems, and ATC radar was derived directly from military radar.

With secondary radar (transponder) operation, there is no guarantee that the transponder isn't lying (whether deliberately or unintentionally). A faulty transponder could certainly report false locations to the ATC system. Or, you can probably imagine scenarios where a false location might be deliberately returned.

ATC systems take both the primary and secondary location information for an aircraft, and correlate them to represent one aircraft in the sky. They don't report the exact same physical location, so there is a degree of fuzziness in the system to account for that. If the two reported locations drift too far apart, you get an uncorrelated track. Sometimes, if two aircraft cross each other's track in certain ways, the ATC system can get confused and associate the wrong primary radar blip with the wrong transponder report. The controllers have procedures to deal with and correct these kinds of problems.

So, primary radar is primary because it (a) came first, and (b) is the most trustworthy source of aircraft location information. Secondary "radar" (which, as you point out, is not really radar) is called such because it is integrated within the primary radar system.

More generally, the various technologies for locating and identifying aircraft in airspace are collectively called "surveillance". Examples of ATC surveillance are primary radar, transponders, multilateration, ACARS position reports, and other technologies.

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    $\begingroup$ Well, "not really radar" depends on exactly how you define radar. Secondary radar is RAdio Detection And Ranging. It does not, however, use reflections, like primary radar. It relies on the transponder to tell the correct altitude and timing of the response (along with whether or not there is a response) to determine distance and bearing. $\endgroup$
    – reirab
    Mar 7, 2015 at 7:52

The reason why it is called "secondary" radar is because it is in addition to the primary existing radar system, the surveillance system.

In other words even with no transponders the radar system would still exist because large airports and regional zones have air surveillance radars that monitor airspace and show contacts. These contacts include everything that reflects radar energy; not just planes, but birds, even clouds of dust, etc. The is the primary radar system.

The transponders leverage this existing system by transmitting signals these radars can receive and interpret. This allows us to put a bright dot on the screen (and other information like the code) because we know it is an aircraft and it is not, for example, a bird. Because this identification and localization system only works with transponders it is considered a secondary system. Since the primary radar always works, even if the aircraft does not have a transponder, it is the primary system. It is also primary in the sense that originally there were only surveillance radars; the interrogating radars that receive transponder signals were developed afterwards.

To further illustrate this, refer to this image of a typical ATCRBS antenna:

ASR-9 Radar

The larger curved antenna is the PRIMARY antenna, it detects aircraft (and everything else, birds, etc) uncooperatively. The smaller, ladder-like antenna on top of the primary is the SECONDARY antenna. This is what is used to detect the transponder's signal. The information from the secondary is then (traditionally) overlaid on the primary's display (although nowadays they are often digitally combined).

  • $\begingroup$ I'm not sure what you are trying to say, nor that I agree with you. There is almost no commonality between primary and secondary RADAR. Switch off the transponder and you will disappear on secondary RADAR. There is no transmitted/reflected pulse. It does not "repurpose" anything. It is a completely different concept and certainly not "built on top" of a primary system. $\endgroup$
    – Simon
    Mar 6, 2015 at 22:32
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    $\begingroup$ @Simon I have changed my language a little bit to make it clearer and added more explanation with a photo. The secondary is on "top" of the primary in the sense that the data from the secondary is overlaid on the display of the primary, and because the PSR system was the original system and the SSR was added onto it later. $\endgroup$ Mar 6, 2015 at 23:10
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    $\begingroup$ @Simon Also the SSR antenna is literally physically on top of the PSR, as you can see in the photo. $\endgroup$ Mar 6, 2015 at 23:16
  • $\begingroup$ @Simon Secondary radar still determines position the same basic way as primary radar, just with a transmitter reply instead of a reflection. $\endgroup$
    – cpast
    Mar 7, 2015 at 0:01
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    $\begingroup$ @cpast Yes of course, but in a very different manner. My comments were to the original answer which seemed to imply that SSR was some kind of extension to primary RADAR. Tylers' edits have done a good job of clarifying that. $\endgroup$
    – Simon
    Mar 7, 2015 at 10:16

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