Malaysia Airlines: What is a transponder?
March 12, 2014 -- Updated 1404 GMT (2204 HKT)
(CNN) -- You keep hearing experts talking about the missing Malaysia Airlines Flight 370 and the fact that its transponder went from on to off.
What are aviation transponders? Here's a quick primer:
Q. What information does a transponder send, and who does it send it to?
A transponder is a radio transmitter in the cockpit that works with ground radar. When the transponder receives a signal from a more sophisticated ground "secondary" radar, it returns a squawk code with the aircraft's position, its altitude and its call sign. It is constantly being radar pinged, helping air traffic controllers on the ground determine the airplane's speed and direction, too.
Take a virtual look inside a Boeing 777
Rather: Put aside conspiracy theories
Search widens for missing Flight 370
Q. What does "squawk" mean?
It's a four-digit identifying code that the pilot enters into a transponder for each flight. It helps air traffic control recognize each plane.
Q. Why are transponders necessary?
Primary radar is more basic, effective only at seeing the radar reflection of objects. It paints targets, displaying them with a blip on a screen. The bigger the object, the bigger the reflection. Transponders enhance the reflected signal and provide the additional information for air traffic control.
Q. Is there a way to use it to indicate a problem on board the aircraft?
There are codes for different emergencies. For a hijacking, it's 7500. For communications failure, 7600. Emergencies are 7700.
Q. How do you turn off a transponder?
There is a switch that you would move from "ON" or "SBY" (standby) or "ALT" (altitude). You could also pull the circuit breaker for the transponder in the cockpit.
Q. Why would you turn off a transponder during a normal flight?
There could be several reasons. One reason could be when airplanes get close to each other (perhaps they are approaching an airport). Air traffic controllers may then request pilots to turn the transponders off or to standby. Also, if the transponder is sending faulty information, the pilot might want to turn it off. Planes are still visible on primary radar until they get below the radar's coverage ability.
Timeline of the flight
How does a jet go missing?
Today's five most popular stories
Part of complete coverage on
June 26, 2014 -- Updated 2211 GMT (0611 HKT)
The search for MH370 is moving to an area farther south in the Indian Ocean, said the Australian Deputy Prime Minister.
June 25, 2014 -- Updated 0033 GMT (0833 HKT)
Erin Burnett speaks to Miles O'Brien about the latest in the search for Malaysia Airlines Flight 370.
June 18, 2014 -- Updated 1653 GMT (0053 HKT)
Ten experts say that the search for MH370 should move hundreds of miles away from the previous search area.
June 17, 2014 -- Updated 1322 GMT (2122 HKT)
His wife never came home from her flight on MH370, and now K.S. Narendran is left to imagine the worst of possible truths without knowing.
June 16, 2014 -- Updated 1951 GMT (0351 HKT)
Families are desperate for results as the search for MH370 reaches a grim milestone. Anna Coren reports from Beijing.
June 9, 2014 -- Updated 1455 GMT (2255 HKT)
Relatives of passengers are launching an effort to raise $5 million for investigations and a "whistle blower" reward.
June 9, 2014 -- Updated 0731 GMT (1531 HKT)
Making sure another plane is never "lost" again is the immediate priority for the airline industry.
May 30, 2014 -- Updated 1536 GMT (2336 HKT)
Was the sound of Malaysian Airlines Flight MH370 striking the water captured by ocean devices used to listen for signs of nuclear blasts?
May 28, 2014 -- Updated 2229 GMT (0629 HKT)
What was believed to be the best hope of finding the missing plane is now being called a false hope. Rene Marsh explains.
May 28, 2014 -- Updated 2105 GMT (0505 HKT)
Involved parties, including the manufacturer Boeing, are bracing for a long public relations siege.
May 29, 2014 -- Updated 1134 GMT (1934 HKT)
Official: The four acoustic pings at the center of the search for Flight 370 are no longer believed to have come from the plane's black boxes.
May 27, 2014 -- Updated 1421 GMT (2221 HKT)
There is one fundamental question which continues to swirl: Has Inmarsat got its numbers right?
May 27, 2014 -- Updated 1213 GMT (2013 HKT)
Data from communications between satellites and missing Malaysian Airlines Flight 370 was released
May 27, 2014 -- Updated 0742 GMT (1542 HKT)
Family members of the people aboard missing plane want independent investigators to review the newly released satellite data.
May 21, 2014 -- Updated 1147 GMT (1947 HKT)
CNN's Richard Quest explains what kind of information should be contained in the Inmarsat data from Flight MH370.
May 27, 2014 -- Updated 0046 GMT (0846 HKT)
The underwater search for the missing Malaysia Airlines plane will effectively be put on hold this week, and may not resume until August at the earliest.
May 19, 2014 -- Updated 1304 GMT (2104 HKT)
Movie-makers in Cannes have announced they're making a thriller based on the disappearance of Malaysian flight MH370.
May 6, 2014 -- Updated 1925 GMT (0325 HKT)
The search for the missing Boeing 777 has gone on for eight weeks now. CNN's David Molko looks back at this difficult, emotional assignment.