Getting Location from GPS, the Dummies Version

Triangulation with GPS

Ask a kid a difficult question sometime – we mean a real kid, maybe six or seven instead of an eye-rolling adolescent – and you'll either get a hilariously unreal answer or a simple "I don't know." Ask some of eHow.com's freelancers the same question and, even if the real answer should be "I don't know," you just might get that hilariously unreal answer. Take, for instance, the question "How Is a Person's Precise Location Determined by GPS?"¹ eHowian accountant Aaron Charles took that one on, but instead of admitting he didn't know, he published a typical DotD answer.

The answer, as anyone who's done a little research into the question, is an extension of simple triangulation: GPS receivers compute the distance from the current point on the ground to one of the satellites orbiting the Earth. If you envision that distance as the radius of a circle, the "target" can be anywhere on a line defining the perimeter of that circle. Calculate the distance to a second satellite, and the target point must be at either of the two spots where the two circles' perimeters intersect (like a Venn diagram). With three signals, you theoretically have a precise spot; the more satellites your receiver can see, the more precise the location; down to about 10 meters.

That's not how Charles explained it, though. The answer, he said is, "...'trilateration,' which, according to PC Magazine, 'uses the geometry of triangles, the known locations of two or more reference points, and the distances from the unknown point to those known locations.' With this dynamic at work, the GPS receiver calculates the time it took for the signal from each satellite to reach the receiver, which indicates how distant each satellite is." OK, Aaron, you were getting there, sort of ; though "the geometry of triangles" isn't actually true. What's next? Well, he said that,

"...Using such data from three satellites pinpoints an exact location, meaning the latitude and longitude of a given GPS unit -- and person. A GPS device then translates that data into visual images on a GPS screen and map."

Umm, yes, Aaron; but the OQ wanted to know how the "data from three satellites pinpoints an exact location"; you still hadn't answered that. Maybe there's more... but no: Aaron went on to explain "multilateration"... sort of. In other words, although Charles threw enough big words and techy-sounding phrases at the screen to fool eHow's content editor, he still didn't answer the question! That's all we needed to name the boy our Dumbass of the Day.

¹ Leaf Group has removed this non-answer, but its original (ehow.com/info_12273134_persons-precise-location-determined-gps.html ) can still be seen with the archive.org Wayback Machine.

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