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.