How does radar pertain to civil aviation? There has been no more significant development in the maintenance of safe skies since radio, in my humble opinion. 'See and be seen' worked for a while, but commercial aviation has long since passed the point past where it ceased to be practical. Much of the preceding writeups are relevant but I'd like to add some bits and bobs of my own.

Primary Radar

This is how we started out. Your basic Radio Detection And Ranging. An antenna that transmits measured pulses and listens for echoes from objects in their path. There are Continuous Wave and Pulse-Doppler radars that eschew or combine facets of pulse-based radar, but the civil radar is generally of the pulse type.

Primary Radar—or Primary Surveillance Radar (PSR), to use the full title—produces the basic radar picture that an air traffic controller sees, though in today's world it's a frighteningly basic picture. If you've ever seen a photo or video of ATC radar, it might seem a confusing picture, with clusters of numbers huddling around dots swarming around the screen. Trust me: it's a lot more frightening without those numbers. And that's the information that primary radar gives you. A bunch of blips.

It's a basic system of echo-location that radar uses to determine the position of stuff nearby. But a couple of factors determine certain of its capabilities. As pointed out above there are several components in a radar system, one of which is the transmitter. This transmits pulses of R/F energy for a specific interval. This interval is called the 'pulse length'; for instance, five microseconds. The time interval between pulses is called the 'pulse recurrence interval'.

The reason these things affect the detection capabilities of the radar is that it generally cannot transmit and receive at the same time. It transmits for a time, then listens for echoes. Because pulse echoes are so much fainter than the pulses themselves, the listening equipment greatly amplifies them before they are processed. If this processing equipment were subjected to a full-power pulse it would fry, so it is isolated from the radar receiver while the pulse is being transmitted.

So, while a radar system is transmitting a pulse, it cannot "hear" anything. Let's use our earlier figure of five microseconds for the pulse length. Radio waves propagate at the speed of light (186,000 miles/sec), and in five microseconds would travel just under a mile before the antenna stops transmitting. If the pulse hits anything less than half a mile away, the echo will reach the antenna before it has finished transmitting. Pulse length, therefore, affects the minimum range of primary radar systems. The longer it is, the larger the 'blind' area surrounding the antenna.

What of the pulse recurrence interval (PRI)? This affects maximum range, for the simple reason that after transmitting the pulse, you have to allow some time for the echoes to reach you before you start transmitting again. If you have already started transmitting pulse two when the echoes from pulse one arrive, you won't hear them. A short PRI will give you relatively fast updates on short-range echoes, because the system doesn't 'wait' very long between pulses, but you won't be able to see anything reliably past a certain range. Conversely, long PRIs allow the radar to see further, at the expense of refresh times, which will be particularly long for stuff far away. As always, it's a trade-off.

Presumably there was a time when PSR was everything? Yes, indeed. To initially identify aircraft—which would all, of course, appear as identical blips—controllers had to instruct them either to make a turn (which would subsequently be observed on the controller's radar screen), or to report their position in relation to a specific geographical location (which would be rendered in the appropriate position on the screen). Once identified, controllers had nothing more than their working memory informing them which blip was which aircraft. This still happens on the odd occasion, when the more up-to-date tools fail, and it usually ain't pretty.

Secondary Radar

Forgive my slight regurgitation of other writeups in this section.

Secondary Radar—Secondary Surveillance Radar (SSR)—is an 'active' system, where PSR is purely passive. SSR is a two-part system, using equipment on the ground and installed on aircraft. The part on the aircraft is called the 'transponder' and it's from here that most numbers you'd see on a controller's screen originate.

Like PSR, SSR transmits a measured pulse and then listens. But rather than echoes, it receives replies. When the transponder on an aircraft detects an SSR pulse, or 'interrogation', it transmits a 'reply'. The fact that this is a transmission rather than an echo means that secondary radar has at least double the range of PSR; increasing the PRF of a primary radar system does increase range to a point, but beyond that it becomes difficult to pick out echoes from background noise.

The data that SSR replies may contain depends on the capabilities of the transponder sending it. There are several categories of data, called 'modes':

Mode A

Mode A, or 'mode alpha,' is the most basic kind of information available from a transponder. A pilot-selectable code can be set in the cockpit as an identity code of sorts for the aircraft. The radar system correlates the code with the relevant primary radar echo, and the code appears next to it on the radar display. The code is four digits long, each of which is represented by a 3-bit binary code, giving a range of 0-7 for each digit.

There are plenty of simply hilarious controller jokes involving setting an '8' in a transponder code.

Some of you are probably thinking "that isn't many codes," and you're right. 84 isn't much, and it has proven quite limiting. As it is, each ATC unit has an agreed—usually small—range of codes that it can assign to aircraft, so that there is as little potential for confusion as possible. When the aircraft enters the airspace of another ATC unit, it will probably be instructed to change its Mode A code to one of theirs, or to the 'neutral' 7000 code which basically means "I'm doing my own thing, no-one is controlling me." That way other controllers, in areas of overlapping radar coverage, will be able to see who, if anyone, is controlling the aircraft adjacent to their sector.

Regular, scheduled flights frequently have the same Mode A code assigned to them every time, and that Mode A code follows them for the duration of the flight rather than changing every time the aircraft enters another sector. A system called 'code/callsign conversion' allows suitably-equipped radar displays to show the callsign of the aircraft in the place of its Mode A code. This is very handy.

There are also a number of special-purpose codes. Here's a few of them:

0033 - dropping parachutists in the next five minutes
0036 - helicopter making a power line inspection
7003 - Red Arrows making a transit or air display
7500 - hijack or other unlawful interference with flight
7600 - radio failure
7700 - general emergency (engine failure, etc). Usually, if an aircraft selects this code, its callsign flashes SOS! in a different colour.

Plenty of RAF bases have their own ranges of Mode A codes too, as do many UK police forces.

Mode A codes are colloquially referred to as 'squawk' codes, which a few of you may be familiar with. It's actually slightly inaccurate, as 'squawk' is a secondary feature of a transponder. A controller may ask a pilot to "squawk ident", in response to which the pilot should press the 'IDENT' button on their transponder, after which a flashing circle will appear around the relevant blip on the controller's display for a few seconds. It's a handy way of picking out an aircraft if you lose its identity in background clutter. It's also a handy cover if you forget what you were going to say:

Me:     American 393...
AAL393: American 393?
Me:     American 393, uh... squawk ident.
AAL393: Identing now, American 393.
Me:     *clears throat*

Yes, you give the "squawk ident" instruction to an aircraft you're controlling if you lose its identity somehow (you are no longer sure which blip represents it on the screen), and need to confirm it. You don't say it to an unknown aircraft, so I confess I rolled my eyes when that army dude at the beginning of Transformers told Blackout to "squawk ident" after he intruded their airspace.

Anyway, moving on...

Mode C

I've gone into this a bit in another writeup so I'll be brief here - the Mode C, or 'mode Charlie' data from a transponder gives information on the aircraft's level, and this again appears next to the relevant blip on the radar display. Not wanting to go into the peculiarities of altitudes, heights and flight levels, the Mode C data is always provided as a flight level. If the aircraft is operating on an altitude, this will automatically be recalculated at the controller's end so that it is correctly displayed on the radar screen.

Mode C data is accurate to within 200 feet of the aircraft's actual altitude, and this is factored into controllers' decision-making. If one aircraft is in level flight and another is climbing or descending through that level, they are not considered vertically separated until the climbing/descending aircraft has exceeded the minimum separation by 200ft and is still going in the correct direction. If both aircraft are climbing or descending the rule applies to both. The 200ft rule increases to 400ft if either aircraft is supersonic, regardless of whether they are in level flight or not.

Since SSR data is broadcast indiscriminately, it can be picked up by any suitably-equipped receiver in range. Mode C is an essential part of TCAS (Traffic Alert and Collision Avoidance System), and is used by that system to gauge what, if any, collision threat is posed to an aircraft by those around it. TCAS uses several small antennas on the fuselage of the aircraft to triangulate the source of each SSR signal and build up a picture of the sky around it.

Mode S

This is the relatively recent one. It provides a huge leap in information potential, but has yet to be completely integrated into ATC systems. It has all the capabilities of Mode A and Mode C, but adds a raft of further information about the aircraft, including a unique 24-bit code which is set by the manufacturer and kept by an aircraft for life, eventually replacing Mode A altogether in theory.

Information transmitted by Mode S transponders includes heading, climb/descent rate, roll angle, speed and most interestingly, the selected altitude and heading. This is what the pilot has dialed into the autopilot (typically the autopilot flies the aircraft other than takeoff and the final stages of landing, and the pilot just dials into the autopilot what they want the aircraft to do), and is awesome because pilots have a long history of acknowledging one instruction and dialing something completely different into the autopilot.