![]() The ADS-B transponder sends out radio waves containing all kinds of information about the airplane, including GPS data 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.Īnybody can pick up these radio waves using a cheap receiver similar to that used in car radios, says Robertsson. 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. (Courtesy of )A third type of flight surveillance system is known as ADS-B, or automatic dependent surveillance-broadcast. The ADS-B system of tracking planes uses signals sent from GPS satellites and plane transponders to relay information such as a plane's location, speed and flight number to radio receivers on the ground. Planes like the Boeing 777 usually have two secondary-radar transponders on board, with one serving as back-up. "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." "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. It's still not clear how that flight's transponder became disabled, but pilot Patrick Smith of the website points out that pilots need to have the ability to turn off transponders. 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. who teaches air traffic management at Embry-Riddle Aeronautical University in Florida. "Unless you have that overlap and beyond that 200-mile range, you don't have radar coverage," said Sid McGuirk, a former air traffic controller with the Federal Aviation Administration in the U.S. 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," but that is not the case in the middle of the Atlantic Ocean, for example. The code is assigned to the plane by air traffic control and entered into the transponder by the pilot.įor 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. This signal contains a unique four-digit code, called a squawk, that corresponds to that specific flight. Secondary radarĪir 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 and giving its altitude, speed and bearing. It's unknown if any military primary radar picked up Flight QZ8501. It's believed the radar saw the flight change course and head west toward the Andaman Sea. While the transponder on Flight MH370 stopped transmitting, the Boeing 77 should have remained visible to any military primary radar that was scanning the area at the time. Primary radar is generally used more for military air defence than civil aviation, which relies on secondary radar. ![]() It can just see a point on the screen," says Mikael Robertsson, co-founder of, a flight-tracking website based out of Sweden that gets about six million visitors a week. "This primary radar can see everything no matter if the transponder is on or off, but the primary radar can't identify the object. Passport trafficking concerns raised in wake of missing flight.Malaysia Airlines Flight MH370 search marked by 'confusion'.Key developments in Malaysia Airlines Flight MH370 search.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. ![]() There are two types of radar: primary and secondary. Radar was first widely adopted by air traffic controllers in the 1950s and is still the mainstay of most air traffic control systems around the world today.
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