Does a secondary surveillance radar need the support of a primary surveillance radar? 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).

  

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)?
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.