How Things Work: Using the Doppler effect to find missing MH370 flight

Today, smartphones can be used to triangulate a person’s position within a few meters from almost anywhere on the Earth. Despite having this advanced technology, the search for Malaysia Airlines flight MH370, which vanished on March 8, 2014, is still ongoing, and there have been no traces of the plane. Much of the world has been shocked at how a Boeing 777-200 jetliner carrying 239 people on board can simply disappear without a trace.

After the plane’s communications were disabled, the only data from the plane were a series of pings to Inmarsat 4-F1, a communications satellite that sits perfectly stationary above the equator at the Indian Ocean.

Unfortunately, three satellites are required to determine an object’s location on the earth. A single satellite can only measure distance based on the time it takes for radio waves to go from the object to the satellite. A sphere can be generated with this distance, as the radius and the object can be anywhere on this sphere. By having two satellites and two distance measurements, the object is confined to anywhere on the intersection of the two spheres. A third sphere can be used to find the intersection of all three and, thus, the location of the object.

Based on the single Inmarsat data, searchers identified two possible paths for the plane on the one sphere they generated. The Guardian reported that engineers working for Inmarsat used a clever approach that involves the Doppler effect to determine the plane’s final path.

The Doppler effect is when sound, light, or radio waves are emitted from a source and the observer receives a changed frequency of waves. If the source is moving toward the listener, the distance between successive waves decreases so they bunch together, increasing the frequency. Conversely, if the source is moving away from the listener, the distance increases between waves and frequency received drops.

Analysts identified slight changes in the frequency of the radio waves that the satellite received from the plane and deduced that the plane was traveling south away from the satellite at the equator. The plane’s last ping was then deduced to be about 1,500 miles away from the coast of Australia.

Now that the general location of the plane is known, the task of searching through the southern Indian Ocean remains. Search teams are listening for a black box flight recorded, which sends pings for about 30 days a distance of two miles. They must identify a ping that lasts 9.3 milliseconds, a tenth of the time of a blink of the human eye that is repeated every 1.08 seconds in a vast ocean that is filled with many other noises.

CNN reported that a Chinese and Australian search vessel heard pings at the right frequency on April 5 but have not heard them since. An official close to the search is now reporting that the batteries on the black box are most likely dead.

After one month of searching, MH370 has eluded the whole world, seeming to simply disappear into thin air.