Satellite Geolocation and Flight 370

Posted: April 8, 2014 by Marner in Uncategorized

The other day I provided a link to a story about investigators using satellite-based geolocation to narrow the search area. It was interesting to me because it is tied to my background in electronic warfare. The article didn’t go into any detail on the process, but I’ll explain it here for those fellow geeks who are interested.

Inmarsat, a satellite communications (SATCOM) company, provides transportation SATCOM services for ships and aircraft. This is accomplished via a transponder installed in the vessel that send a short message to one of the Inmarsat satellites every hour. The transponder is activated when the trip begins and deactivated when it ends. The amount of information contained in one of these messages, or pings, varies depending on what level of service the vessel has subscribed to. The pings can provide location, velocity, altitude, and direction of travel, or they can contain no data other than the vessel’s terminal identification number. In the case of MH370, it looks like they subscribed to the latter level of service.

Inmarsat’s satellites are geosynchronous, meaning they remain at one spot in relation to the surface of the Earth by orbiting at around 22,300 miles altitude. That altitude matches the satellite’s orbital speed with the rotation of the Earth, keeping the satellite over one particular spot at all times. The first principle Inmarsat used in narrowing the search area was called time difference of arrival (TDOA), also known a differential time offset (DTO). The signal coming from the plane’s transponder is electromagnetic energy, which travels at the speed of light, approximately 186,000 miles per second. Inmarsat measured how long it took for the transponder signal to travel from the plane to their satellites and from that they were able to determine how far the plane was from the satellites. The difference in the time that each satellite received the signal tells how much farther the plane was from one satellite versus the other satellite. The maximum altitude the plane was capable of attaining generated a wide arc running north-south, or longitudinal. That still left a massive area to search.

To narrow it down even more, they used another process called frequency difference of arrival (FDOA), also known as differential frequency offset (DFO). This uses the Doppler Effect to find intersecting east-west, or latitudinal, arcs. While the speed at which electromagnetic waves travel is a constant (the speed of light), the frequency isn’t. The Doppler Effect is when a wave becomes compressed or elongated when it arrives at a receiver. Sound waves and electromagnetic waves work the same way in this regard. When something is moving away from a receiver, the wave will be stretched, resulting in a slightly lower frequency. Inversely, if the object is moving toward the receiver, the wave will be compressed and the frequency will be slightly higher. Using the shift in frequency, Inmarsat was able to determine the intersecting latitudinal arcs. To do so, they had to look at signals sent previously by other aircraft (reference signals) along the last longitudinal arc to make comparisons and narrow the size of the latitudinal arcs. This brought it down to two areas: one in the north over land and one in the south over the Indian Ocean. The northern route was ruled out because of the lack of radar or visual sightings of the aircraft, leaving the current search area as the last likely location of the aircraft. The southern area was so large because Inmarsat lacked a sufficient number of reference emitters to narrow it down any further than they did and they had no way of determining how far or what direction the plane flew after the last ping.

So, at a very broad level, that is how the search area got restricted to a specific area in the Indian Ocean. It wasn’t magic or guessing, as some pundits on the news channel have suggested, it was simple physics. It is a proven process that has been used for a long time to geolocate emitters interfering with satellite communications which Inmarsat used for the first time to try to locate a lost aircraft. Rather than beat up on Inmarsat for taking so long to narrow the search area, as I’ve seen some pundits do, we should be applauding them for the hard work they did in adapting this process to help find the plane and the victims.


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