Information about radar procedures

[hamil]

Hello everyone,
After that accident in South America I would like to learn a little more how a radar system works and what ATC uses as procedures... Can someone give me some explanations?
- Is there any difference in terms of radar identification (resolution, for example) between a terminal radar (APP) and an enroute radar if an airplane is flying with its transponder switched off? Is it possible (if there is no altimetry) to have correct numbers in terms of heading and speed?
- Can an ATC controller identify a potential risk of collision between two targets but one has its transponder off? Does the radar system work precisely or sends any alert (visual or aural) if two airplanes are getting closer in this case? If not, in general, what are the recommended procedures: radar vectors? essential traffic information for both airplanes only?
Thanks,
Rgds
hamil

[Il Duce]

hamil! So many questions in one posting! Most ATC agencies use a combination of primary and secondary radars. The primary radar provides the controller's displays with a return or "blip" from the aircraft concerned (or, for that matter, any other solid object which is within radar coverage). The secondary radar works only when the aircraft is fitted with a serviceable transponder and the pilot elects to switch it on. This provides a "label" alongside the primary return and helps to identify the aircraft along with other information depending on the type of transponder, the radar display etc. This information may include the aircraft's flight level/altitude, destination, speed etc. Some radar systems are set up to alert controllers of potential conflictions between aircraft - a great deal are not. In the majority of cases it is up to the controllers to spot the conflictions and deal with them appropriately. This is made easier by the information provided by the secondary radar but, as I mentioned before, without a serviceable transponder, an unidentified aircraft will be simply a "blip" from which you may be able to discern direction of travel and a relative speed but little else. Hope that goes some way to answering your questions. The methods of using the radars vary between civil and military establishments - some keep the aircraft apart without the use of radar at all.

[Pierre Argh]

I would like to learn a little more how a radar system works

Firstly there is no such thing as a single radar system, so any answer must be definition general. Il Duce has picked up most of you points, but I would add the following

Primary Radar, without SSR, is simply the display of the reflected radar signal on the Controller's screen. This information may or may not be processed. Without processing, the display is raw, i.e. no speed or heading information, however ATCOs are trained to interpret blips and visually assess relative movement (closure, conflictions etc) between such raw returns and to maintain track identity. As traffic intensity increases in any given airspace, this task becomes much harder; hence the use of SSR or procedural techniques are, generally, used/necessary as a safety back-up.

As to the Controllers actions, this depends on type of service being provided and the classification of the airspace within which the aircraft are flying... for example, within Controlled Airspace in the UK it is quite acceptable for a Controller to ignore a "primary only" return, unless they have specific information that the return is within their airspace, as the regulations to enter Controlled Airspace mandate the carriage of SSR... in otherwords, if the conflictor doesn't show SSR then it cannot be in regulated airspace? But outside Controlled Airspace, a Controller providing a radar service (Radar Advisory) is obliged to pass advise to the pilot to resolve a confliction whether the conflictor is showing SSR or not. (pse accept for brevity, this is a gross precis of the rules but I hope remains accurate. I also note you refer to an incident in SA, my knowledge of procedures outside the UK is limited... but this should give you some basic principles?)