Radar, radio and sonar can all be used to locate missing planes. Now, a crowdsourcing initiative is allowing regular people to comb satellite images in the search for flight MH370, which went missing last Saturday.
What's known is this: Contact was lost with Malaysia Airlines Flight 370 about an hour after takeoff from Kuala Lumpur en route to Beijing. But the details of the case are shrouded in mystery: Why were two passengers traveling with stolen passports onboard? Did the plane turn back, as announced at one point? And what of reports that passengers' mobile phones have still been ringing?
The riddle of flight MH370 hinges on its apparent disappearance in mid-air. But how can an object weighing several tons and measuring about 60 meters across by 64 meters long (200 by 210 feet) simply vanish? Experts believe it's simply a matter of time before some traces of the plane are found. And so the search continues, using various tools - including a crowdsourcing initiative that has ordinary people combing through satellite imagery.
Holes in radar coverage
Radar is the primary tool for tracking passenger flights. Radar works through ground stations which emit radio waves that bounce off objects. The return signal allows the location of the object to be determined.
"We don't have so-called primary radar over water," said Jörg Handwerg, a pilot who works with the German pilots' association Cockpit. A lack of base stations on the open sea means there are holes in radar coverage, he told DW.
Handwerg added that airplanes are equipped with transponders that automatically emit signals indicating their position. But the reach of such signals is limited to about 370 kilometers (230 miles). Without a station to receive such signals, "They simply float into nothingness," Handwerg said. But he added, "when [a craft] gets closer to the coast, it reappears" on radar screens.
The lack of radar coverage over the open ocean is contributing to difficulties in locating flight MH370. The possibility that ground radar tracked the flight off an island in the Strait of Malacca led to expansion of the search area to include this wide ocean zone.
Radio a further tool
But all leisure and business travelers don't just become invisible as soon as they reach cruising altitude over the ocean: Regular old radio helps planes keep in touch. "Radio allows us to stay in contact nearly all over the world," Handwerg said.
Pilots are in regular radio contact with air traffic control, sharing their current position along a predetermined flight path, Handwerg explained.
This makes it even stranger that the last-reported radio transmission from the MH370 cockpit was "Alright, good night," just before the craft crossed into Vietnamese airspace, as reported in Beijing on Wednesday. Most flights that run into problems are able to send a distress signal before their radio system goes down.
The so-called black box is often key in determining what happened to a flight and can also help in locating a downed plane. This device, included on every commercial plane, records flight information and is also equipped with a radio transmitter that should emit a signal if the plane crashes.
But, as Handwerg pointed out, this is not very useful if the plane crashes into water, as, due to water's density, "The signal doesn't reach very far."
Searching with sonar
Sonar is the underwater version of radar - but instead of sending radio waves through the air, sonar sends sound waves into water, using the reflected sounds to establish the location of an object.
Sonar technology was employed in the search for Air France flight 447, which crashed over the Atlantic off the coast of Brazil in June 2009. In that case, bodies of passengers and pieces of the wreck were found on the ocean surface within five days - but not until April 2011 was the main section of the plane located four kilometers (2.5 miles) underwater.
Scientist Peter Herzig, who developed sonar crafts which are meant to map the oceans, explained how sonar can help identify the density of an object that's located.
"Based on the reflection of the sonar waves, we can determine whether it's a hard or soft reflector," Herzig said. A hard reflector could be the metal from a plane.
The black box is also supposed to send a sonar "ping" in case of a crash into water. However, this can't be heard over long distances. Although the government of Malaysia has two submarines, it has not yet employed them to help find a possible wreck. It was reported that fishermen are using sonar to search for the missing Malaysia Airlines flight.
Crowdsourcing satellite initiative
Another high-tech means for trying to locate the missing flight is a crowdsourcing initiative based on satellite imagery. Colorado-based satellite network company DigitalGlobe is providing satellite imagery of the search area to its daughter company Tomnod, a humanitarian platform that cuts the images into small chunks and allows volunteers to search for and tag any out-of-the-ordinary items on the ocean surface.
DigitalGlobe ocean images currently being analyzed date from March 9 and cover more than 2,600 square kilometres (1,000 square miles). The Tomnod platform makes sure that more than one volunteer reviews each image, although users can search through and tag as many images as they want.
After passing through the first filter of the crowd, a Tomnod algorithm identifies significant images to pass along to professional imagery analysts, which - if deemed important - will then be communicated to search and rescue efforts, explained Patrick Meier, a humanitarian technology specialist who has collaborated on the Tomnod project.
"It's adding some data quality control mechanisms within the wisdom of the crowd," Meier told DW.
A similar Tomnod crowdsourcing initiative was used to estimate the number of internally displaced in Somalia in 2011, in order to assist the United Nations in its refugee relief efforts.
Tomnod administrators say that with more than 100,000 page views every minute, they have been struggling to keep the site running. And they've not seen any breakthroughs yet. "It's really quite extraordinary that no one has seen a sign of this aircraft," Meier said.