ATC technologies are evolving to include other components than primary and secondary surveillance radars, but I believe these two pieces of equipment are still the backbone of ATC. Primary radar (PSR, 2.8 GHz) works usually on higher frequencies than secondary radar (SSR, 1.030 GHz).

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Top: M10SR SSR | Bottom: Morava PSR | Right: Sky Search-3000 PSR+SSR co-mounted

Primary radar uses the natural echo reflected by the target to determine its angular position and range, while secondary radar uses the cooperative answer provided by the target transponder. The answer is equivalent to a natural echo, but is delayed by a known time, and is more powerful. It can also contain additional information such as level.

I read that SSR is more accurate in position determination than PSR. Indeed SSR doesn't work without the support of the airborne transponder, and the target can disappear from SSR display when the transponder has failed or has been disconnected.

My questions:

  • In appearance SSR + transponder can provide information to ATC without the need for a primary radar support. Is it true, if not why?

  • SSR can technically receive a natural echo on 1.030 GHz before the SSR reply which is sent on 1.090 GHz, is this signal used?

  • Do ATC systems usually display PSR data for controllers continuously or only on demand (e.g. when a target transponder doesn't answer SSR interrogator)?


3 Answers 3


SSR can work independently and this is quite common as well. Primary radars cost a lot to maintain, and as more and more planes are equipped with transponders, primary radars are being shut down. Many areas now have secondary radar coverage but no primary radar coverage.

SSR only relies on replies sent as a response to its own interrogations. This works well in theory, but becomes a problem in practise in areas where many SSR units transmit on the same frequency resulting in many overlapping replies. This results in FRUIT and garbling.

For areas where both primary and secondary radar data is used, the data is combined to create individual radar plots that are shown to the controller. The controller typically does not have to worry about the exact source of data, and will not worry about switching on or off individual sources (at least not during normal operations). Modern ATC units receive radar data from many different sources, including both SSR and PSR stations, and process and combine this data to a single representation. This happens by using technology such as ARTAS. On the controller radar screen, aircraft plots will look different depending on their data sources. For example, a PSR-only target may be shown as a circle, an SSR-only target as a diamond and a SSR+PSR target as a square. The exact specifications vary from system to system.

  • $\begingroup$ So, nowadays, in a lot of areas, an airplane whose transponder fails will become invisible to radar entirely, and ATC will have no way of keeping other airplanes from colliding with it... yeah, eliminating PSR sounds like such a great idea. $\endgroup$
    – Vikki
    Commented Dec 19, 2018 at 22:13

PSR is a separate system to SSR. The accuracy of SSR means that some ATC systems are switching off their PSR except for operation in the terminal area. And for those only limiting them to what is flying within a certain radius of the radar head.

The reason for this is mainly cost. The cost of running a PSR is huge. The reason though for keeping them on in the terminal area is the advantage of a aircraft without a working transponder can still be seen and separated from others.

The natural echo of an SSR system is not used. What is sent back to the SSR head is a data package. In that package is all the information about the aircraft's identification (4-digit octal code) and altitude. Some SSR transponders provide a lot more detail(aka auto-pilot settings and position, mode-S) and some older ones just identification code. The echo, if used, would only give a position that would take up more processing power process with no additional benefits to the ATC system. The return of the code is what is used for position calculations.

All surveillance data is displayed continuously. The radar tracks may be joined together to create a combined symbol or changed to a completely new symbol. Some systems have the ability to degrade the system resources at a local level but no controller I know would ever deliberately do that.

In my area of operations we get aircraft that we see them on PSR, SSR, ADS-B, and ADS-C all at the same time. We never turn any of them off we just use them all.

Sorry about my original answer I put some Mode-S information in a way that assumed it was all PSR equipment, what I said was incorrect and has been corrected. Thanks J. Hougaard for picking up my mistake.

  • $\begingroup$ I think you are confusing SSR with mode S. You claim that an SSR reply contains information about altitude and position, but this is not necessarily true. The most simple form of SSR is mode A, which only transmits a 4 digit transponder code. The more common one, mode C, transmits two things: transponder code and altitude. The position is still calculated by the ground unit, just like a PSR. $\endgroup$ Commented Apr 16, 2017 at 8:13
  • $\begingroup$ @J.Hougaard Thanks, I have done a basic correction. I will re read my technical manuals next time before I answer a question on information that I don't use daily. Lesson Learned $\endgroup$
    – Bullfrog
    Commented Apr 17, 2017 at 1:22

I've finally discovered exactly how SSR/MSSR calculates position, and I'm still utterly confused how this is not clearly stated all over the internet and in actual teaching material:

The trick lies in that the SSR/MSSR beam azimuth is very narrow, only a few degrees. The system calculates a transponder's distance from the radar through timing the interrogation-response-time, and since it does this 30ish times (MSSR uses far less pulses) every time the beam hits the aircraft it will be quite accurate. It now knows that the aircraft is at a given distance from the radar inside a corridor that is very narrow, i.e. it knows its position, just as you can figure out your position from a VOR/DME using a radial and distance.

The beam is a cone of course, so the further from a radar you are the larger the margin of error. Still, a MSSR can be as accurate as 1-2nm at a distance of 60nm. Adding in data from several radar sites allows systems to triangulate and provide even better accuracy on the situation display.

  • 3
    $\begingroup$ What unit in nm? nano-meter? $\endgroup$
    – CrossRoads
    Commented Jun 3, 2018 at 21:27
  • $\begingroup$ Nautical miles (The radar sends EM waves is in the 2-30cm wavelength region, nanometers only come into the picture at the induvidual pulses back and forth from the radar) $\endgroup$
    – Eseem
    Commented Jun 5, 2018 at 22:12
  • $\begingroup$ "The trick lies in that the SSR/MSSR beam azimuth is very narrow": Azimuth isn't determined by the direction of the signal but derived from the wavefront direction by interferometry. The antenna is divided in two halves to receive the reply from two close but separate locations, phases are compared to sense the wavefront plane. This way azimuth resolution can be 0.7° with a typical 2 or 3° beam width at 3dB (wider beam ↔ increased range for the same rpm). $\endgroup$
    – mins
    Commented May 20, 2020 at 14:34

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