|3. Everything / Maths, Science & Technology / Physics / Radar|
Primary and Secondary Radar
The History of Radar | Radar History: Isle of Wight Radar During The Second World War | Radar: The Basic Principle
Radar development may have been driven by military needs, but it is now used widely in civilian application for the detection of ships and aircraft. This Entry aims to give an insight into the basic concepts of the surveillance radar systems used by ships and for air-traffic control.
There are two types of radar used for the detection of aircraft and ships: primary and secondary.
Primary radar is a pulsed beam of ultrahigh frequency radio waves 'shone' in a circle from a rotating aerial. If the radar beam 'illuminates' an object, some of the energy is reflected back. How much an object reflects depends on its size, shape and material. Metal aeroplanes and ships reflect well, while non-metallic ships and small boats often have metal radar reflectors mounted high up to improve their chances of been seen. But, at best, only a small amount of the out-going energy is reflected back. Thus the out-going pulse has to be strong, typically several megawatts. Of course, some military aircraft are designed and constructed to be non-reflective - the so-called stealth aircraft.
Thus the direction of the 'target' can be determined from the direction the aerial was facing when the target was illuminated. To find out how far in that direction the target lays, it is necessary to measure the time between the out-going pulse and the return. This is done almost instantaneously and a blob or 'trace' appears on the radar screen at a position representing the direction and range of the target.
If the target is a boat or ship, this is probably all the information you need. Indeed many ships and small boats are equipped with just such radar.
If, however, the target is an aircraft and you are an air-traffic controller, you will want to know a whole lot more. Enter secondary radar or, to give it its full title, Secondary Surveillance Radar (SSR).
The big difference with SSR is that it doesn't rely on reflections. Aircraft are equipped with a transponder. This transponder transmits a 'reply' when it receives a radar 'interrogation' signal. The interrogation signal is completely separate from any primary signal. As the reply is not just a reflection much less power is needed, typically around 1kW for interrogation pulses, slightly less for replies. Range and direction can be determined from the SSR signal in much the same way as with primary radar, measuring the time between sending the interrogation and receiving the reply, making allowance for the turn-round delay in the transponder. The advantage of SSR is that all sorts of information can be encoded into the Transponder's reply.
Transponders are linked directly to the aircraft's altimeter. The reply contains the aircraft's height, and thus the controller's screen displays the height against the trace. The transponder can also provide identification information, so the controller knows which trace is which aircraft. The pilot can select codes to relay a variety of information including, but not limited to, various emergencies such as hijacks. The same basic system is used by the military - called IFF (Identification Friend or Foe) - to identify 'friendly' aircraft.
Radar beams travel in straight lines. Even in flat terrain the curvature of the earth provides a radar shadow - the greater the range, the higher a target must be to allow detection. For example, at a range of 250 miles an aircraft would have to be over 30,000ft1. Thus it is possible to 'fly under the radar'. The problem is, the closer the aircraft gets, the lower it has to fly. Eventually it can no longer hide under the horizon and can only avoid detection if there are radar shadows from hills and mountains etc. By flying dangerously close to the ground, it is possible, in hilly terrain, to get relatively close and still avoid detection, until you fly into that hilly terrain.
The latest SSR systems use a system call mode 'S'. It is fitted on larger aircraft and allows interrogation to be 'addressed' to specific aircraft. The system remembers where individual aircraft are and interrogates specific aircraft one at a time, and only in the part of the sky where they are known to be. This reduces unwanted replies and general radio frequency pollution. It also allows much more information to be exchanged. Mode S transponders are also an essential part of the airborne collision avoidance system.
Other Entries in This Project
People have been talking about this Guide Entry. Here are the most recent Conversations:
Most of the content on this site is created by h2g2's Researchers, who are members of the public. The views expressed are theirs and unless specifically stated are not those of the BBC. The BBC is not responsible for the content of any external sites referenced. In the event that you consider anything on this page to be in breach of the site's House Rules, please click here to alert our Moderation Team. For any other comments, please start a Conversation below.