How does a radar work?

You're up in a plane, cruising at altitude. All systems are normal. But above the clouds, everything looks pretty much the same. How do we know where we are, where we're going and what's ahead of us? More importantly, how do we avoid collisions? The answer is radar, which uses radio waves to detect the presence, direction, distance, and speed of distant objects. Speaking with Chris Smith, Ramsey Faragher of the University of Cambridge Computer Laboratory has his very own radar to demonstrate how this craft-y system works...

Ramsey - A radar system creates a very carefully crafted pulse of electromagnetic radiation, that's very powerful but very short in time, and it pings this pulse out and then sits there quietly and waits to hear for the ping to bounce off things and come back. The only reason this works is because the speed of light, while very, very fast, is thankfully not infinite. So, you can time how long it takes for an echo to come back to you and then easily work out how far away something is. But radars have a big challenge. They spray this radiation out into the world and it will bounce off trees, buildings, wind turbines, any old object, hopefully, the odd aircraft if that's what it's supposed to be detecting. But a radar has to deal with all of these reflections coming back and pick what it is interested in. And so, there's a few things that radars can do in order to make sure it's just tracking the aircraft. We can demonstrate one of these here in the studio - one of these methods of tracking just what you're interested in. So, I've got a little radar with me that's about the size of a mobile phone. It has two little antennas that are about the size of your thumb. One of those antennas does the transmitting and the other one does the receiving. It's going to spray radiation out into the studio and it'll bounce around all over the place. But we've done something very clever so that it only tracks Phil, your producer, and we'll demonstrate that in a moment.

Chris - I'm slightly concerned that it's tracking him at the level of his crotch. Is there any damage that could be done to a person from exposure to radar energy?

Ramsey - Not in these power levels, no, that's okay.

Chris - He may experience a warming sensation. Okay, so you're going to ask him to start in one position and then move. What are we going to be hearing on the radio?

Ramsey - So, I've rigged up this laptop so that when the radar pulse is sent, the laptop makes this beeping sound...

[beep]

Chris - So, that's the pulse being sent out in all directions around the room.

Ramsey - Yup. So, as soon as we hear that, we can imagine that we need to listen for a pulse to return. If we time how long it takes, we'll work out how far away things are. And so that it's easy to hear when the return pulses come back, it sounds slightly different. So, this is the sound that the laptop will make when it receives the pulse back again...

[boop]

So, it's slightly lower in pitch. So, if I start the system going, you'll hear the beep and then the boop - the high pitch and the low pitch, and you'll hear them quite close together because Phil is starting very close to the radar.

Chris - He's about a foot away from your radar, isn't he?

Ramsey - Yeah.

Chris - So, start it off then. And if Philip could start walking backwards across the room away from the radar source...

Ramsey - So now, there's a clear gap between the two and if Phil hurries back over, the gap would be very small again.

Chris - So, it's using the time it takes the reflections to come back and it knows how fast light travels and therefore, it can work out how long the light has been traveling to go to Philip and back to the receiver and therefore, it works out what the distance is. One problem I can spot with this though is it doesn't know whether he is that distance away in a horizontal across the room or almost on top of the radar device but vertically above it. So, it doesn't actually have any idea of what sort of angle he's at relative to it, does it?

Ramsey - That's right. So, the radar is very, very good at measuring velocity. It's okay at measuring range but it's not very good at measuring angles. So yes, there is no measurements really that I was making there that could tell me the angle around to the sides or up and down that Philip was to the radar.

Chris - Is that why air traffic has corridors where they know they put aeroplanes in certain altitude blocks and then they know a pulse coming back from there must correspond to a ping back from that angle, that corridor?

Ramsey - Yes. So, a radar that's used for a traffic control typically rotates in the horizontal plane so it kind of sprays out the radiation along the compass bearings, if you like. And that's how it tries to determine your heading, but the altitude you're at, that's very hard for the radars to distinguish. What they do to get around that problem is that a transponder in the aircraft which is a radio receiver that emits information as soon as it hears that it's hit by the radio, the radar pulse, those transponders, they broadcast the identity of the aircraft and they broadcast its altitude, and sometimes some other information as well.

Chris - Do they fire off when the see the radar pulse come in then? So, it's listening for the pulse to arrive and it says, "Aha! Someone's pinging my aeroplane" and it then sends back a signal going, "Here, I am!"  It announces itself.

Ramsey - Yeah, that's right. So, as soon as the radar pulse comes back, it comes back with this extra radio information, that's transmitted on a different frequency from the aircraft, that contains identity, height and often some other information too.

Chris - Are there any ways that we can track aircraft in addition to using radar, because we've had a number of high profile cases recently of aircraft disappearing and then being either impossible to locate or there being a significant delay before they were found. And then everyone goes hunting for black boxes which we'll find out about later. Is there any better way of keeping tabs on where planes are in the sky?

Ramsey - So, there is a big push at the moment to replace what we call primary radar systems which is literally spraying electromagnetic radiation out from the airport. Replacing those primary systems with secondary systems and one of those secondary systems is the use of GPS on board the aircraft. So, the aircraft simply record their GPS information and then broadcast it over a radio channel. This technology is called ADSB. It's being rolled out on large aircraft and even small aircraft like gliders will carry ADSB transmitted now as well. So, that has drastically reduced the cost associated with tracking aircraft. It means that if a system fails, it fails just on that aircraft and just that aircraft needs for a system to be replaced where of course, if the primary radar went down at Heathrow then it would drastically affect hundreds of aircraft. So, it's a great way of reducing the costs and spreading the risk of a problem.