Radar began to be widely adopted by air traffic controllers in the 1950s and is still the mainstay of most air traffic control systems around the world today.
There are two types of radar: primary and secondary. Primary radar sends out electromagnetic waves that are bounced off any object in their path — in this case, an airplane — and does not rely on the plane's transponder having to send any signals back .
"This primary radar can see everything no matter if the transponder is on or off, but the primary radar can't identify the object. It can just see a point on the screen," says Mikael Robertsson, co-founder of Flightradar24.com, a flight-tracking website based out of Sweden that gets about six million visitors a week.
Primary radar is generally used more for military air defence than civil aviation, which relies on secondary radar.
Air traffic controllers who manage commercial air traffic rely on secondary radar, which also sends out electromagnetic waves, but when the plane picks them up, its transponder sends back a signal identifying the plane.
This signal comes in the form of a unique four-digit code, called a squawk, that the pilot has entered and that corresponds to that specific flight.
No other plane within a certain distance of that flight will have the same squawk, but a plane might be assigned a different squawk partway through its journey if it is flying a long distance and enters airspace where there is another plane with the same code, says Sid McGuirk, a former air traffic controller with the Federal Aviation Administration in the U.S. who teaches air traffic management at Embry-Riddle Aeronautical University in Florida.
For a plane to be detected by secondary radar, there needs to be a radar station within about 300 kilometres, and since these stations need to be on land, radar coverage is limited over large bodies of water and is also affected by geography, the curvature of the Earth and a plane's altitude.
In places such as North America and Europe, there are enough radar stations spread across the land mass that coverage overlaps, and little territory is left "off the radar."
In the case of Flight MH370, the plane was still in range of ground-based radar stations when its transponder stopped transmitting over the Gulf of Thailand, rendering it invisible to secondary radar. It is now thought, however, that it remained visible on military radar, which reportedly saw it change course and head west toward the Malacca Strait, rather than continuing east in the direction of Vietnam — although the Malaysian military later denied this.
Patrick Smith of the website AskthePilot.com points out that pilots have the ability to turn off transponders.
"In the interest of safety — namely, fire and electrical system protection — it’s important to have the ability to isolate a piece of equipment," he says on his site. "Also, transponders will occasionally malfunction and transmit erroneous or incomplete data, at which point a crew will recycle the device — switching it off, then on — or swap to another unit."
A third type of flight surveillance system is known as ADS-B, or automatic dependent surveillance-broadcast. ADS-B relies on radio waves being emitted by another type of transponder, which is usually attached to the bottom of the plane and controlled from the cockpit. The ADS-B transponder sends out radio waves containing all kinds of information about the airplane, including GPS information about the plane's location relayed by navigational satellites, but also the flight number, speed and vertical velocity, which indicates whether the plane is climbing.
Anybody can pick up these radio waves using a cheap receiver similar to that used in car radios, says Robertsson.
"With ADS-B, you get much more data at lower cost [than secondary radar]," he says.
But even though the technology is about eight years old and most major plane manufacturers already outfit their planes with ADS-B transponders, it is not yet the norm in air traffic control.
"Australia was the first country that started to use ADS-B (for the whole country) in December last year," Robertsson says. "Any change in the aviation industry takes a very long time."
ADS-B is also used in regions of the U.S. such as Alaska and the Gulf of Mexico as well as in parts of the Middle East and will eventually become the global standard. The FAA is expected to adopt the system by about 2020.
ADS-B receivers in Malaysia tracked flight
Robertsson's Flightradar24 organization has 3,200 ADS-B receivers deployed around the world, most hosted by volunteers, and two of them on the east coast of Malaysia detected Flight MH370.
"It was quite far away from our receivers, so in that area, the coverage is limited to about 30,000 feet, and this aircraft was flying at 35,000 feet, so it was within our coverage — until it disappeared," Robertsson says.
The altitude at which ADS-B can detect planes varies by geography and the location of receivers — in some parts of Europe, where Flightradar24 has more receivers, it can be as low as 500 feet, Robertsson says.
Flightradar24 also has receivers on the east coast of Malaysia as well as in Vietnam and Indonesia, but none of those picked up Flight MH370. The receivers on Malaysia's west coast didn't pick up the plan, which indicates that if the plane did, indeed, head in that direction as the military has suggested, its transponder was switched off or damaged somehow.
Robertsson says that at this stage, it's impossible to know whether the Malaysia Airlines plane would have been visible for longer if there had been more ADS-B receivers in the region.
"If the aircraft dropped below [30,000 feet] and if there would have been more receivers in that area, we would have seen what happened, but if the transponder was turned off, then it wouldn't help to have more receivers, so it depends on what happened in this case," he says.
Robertsson's organization began as a hobby six years ago and has grown into an unofficial global network that often provides up-to-date flight information to airlines and airports (but not air traffic controllers) and sells flight-tracking apps for smartphones and tablets (about four million to date, says Robertsson).
"Today, it's really hard for an airline to get information about their own fleet," says Robertsson. "Maybe they get an update 10 minutes before that the aircraft is one hour delayed."
Aside from being tracked through radar and ADS-B, planes also stay in contact with air traffic controllers and ground stations using radio communication and a system known as ACARS, which functions somewhat like text messaging.
Pilots can talk to controllers using radio signals transmitted over ultra high frequencies (UHF) or very high frequencies (VHF) — sometimes these communications are relayed through private third parties.
When airlines want to alert pilots to something, they can also use the Aircraft Communications Addressing and Reporting System, or ACARS, which relays simple short text messages through radio signals and satellites.
"For example, in 9/11, all of the air carriers — through ACARS — sent a message telling their flight crews to secure their cockpits," says McGuirk.
Flight MH370 was reportedly outfitted with the capability to communicate via ACARS.
Pilots rely on GPS, but that doesn't help the controllers on the ground track a plane's movement. Pilots can get information on their location from navigational satellites in the same way your car's GPS does,
"Your video map happens to show streets and highways; their video map shows airways and land masses and airports, and it has a pretty sophisticated database, so it will give them a visual of where they are," McGuirk says.