Last time I wrote about relativity and how it affects Global Positioning System satellites. GPS works by measuring the amount of time it takes for a radio pulse to travel from the satellite to the receiver and using the elapsed time to determine the distance to the satellite. It’s like saying you drove at 80 miles per hour for one hour and then concluding that you went 80 miles. The satellites send out radio transmissions that give the name of the satellite and the time the transmission was sent, so the receiver will always know how long the radio transmission was traveling and its speed – the speed of light. At first glance, it’s an easy problem. But even if you’re traveling one hour at 80 miles per hour, if your timing is off by a minute, you’ll misjudge the distance by over a mile.
Radio pulses travel at 669.6 million miles per hour, which means the timing has to be impeccable if you want to gauge the distance correctly. Clocks on GPS satellites are precise to within billionths of a second and each is almost perfectly synchronized. At this level of precision you have to consider relativity if you want to synchronize your clocks.
Once you’ve actually done this, and once your GPS receiver is setting its clock based on those satellites, you can figure out your position.
Knowing the distance to one satellite isn’t helpful by itself. There is a collection of points that are the same distance away. If you were to plot them on a graph, you would end up with a sphere. If you know your distance from two different satellites, then your graph shows two intersecting spheres, and you know that you must be somewhere at one of the points where they touch. As you add more satellites, the number of these points becomes fewer and fewer. By the time you have four satellites, there is only one point where all the spheres are touching, so you know your position exactly.
Well, not exactly. After accounting for the accuracy of the clocks and the problems caused by relativity, your GPS receiver can tell you where you are to within 16 meters. Ground stations have been set up at precise latitudes and longitudes to help receivers correct for this error. Since the stations know their true position, they can tell how accurate the satellites’ information is and send it to GPS receivers to help correct them. Ground stations like these increase the accuracy of GPS to within three to eight meters.
That’s pretty amazing. And it’s fine if you’re hiking, boating, or driving. Generally it’s less space than you’ll need to turn around. But, believe it or not, there are people who use GPS for even more precise measurements. A lot of them call themselves geologists.
One of the most fundamental ideas in modern geology is plate tectonics, the idea that continents are like enormous, interlocking rafts drifting on a sea of molten rock. They collide and get sucked under one another. The process is responsible for causing earthquakes, raising mountains and building chains of volcanic islands. It’s an extraordinarily powerful phenomenon and it happens at an agonizingly slow rate. Continents drift about 10 millimeters per year, but a few are hauling ass to the tune of 130 millimeters annually. Here’s where a difference of eight meters can mean a lot.
Luckily, many errors in GPS measurements are predictably high or low at certain times. There are several institutions around the world dedicated to analyzing and correcting GPS readings to obtain maximum accuracy. By working with these researchers, making custom ground station corrections and averaging GPS readings over extended periods of time, geologists have been able to obtain millimeter-precision using GPS and track the movements of the continents.
But the scientific theory of continental drift has been around since 1912, when Alfred Lothar Wegener, a German researcher, introduced it. Wegener spent the rest of his life trying to back up his theory, finally freezing to death in 1930 on an expedition across the Greenland ice cap.
Continental drift eventually matured into the modern theory of plate tectonics. Truly accurate measurements were nearly impossible until the moon landings beginning in 1969, but that’s a topic for another column…
Josh Braun is the Daily Nexus science/environment editor.