How does GPS work?
If you’re like most people, you think it works something like this: there are satellites in orbit around the Earth. Your phone or other GPS device sends a signal to the nearest one of the satellites. Some math happens and the satellite responds with your location.
This is wrong. It’s wrong for reasons that should be obvious. Despite that, everyone believes some version of this, as near as I can tell.
This came up during the current high school mock trial season, in which my son is a mock prosecutor. In a mock trial season, all the schools in California study the same fictional case, and then the prosecution of one school meets the defense from another school in a “scrimmage” conducted like a jury trial. This year’s fictional case is a murder, and the trial begins with a defense motion to suppress some evidence: namely, GPS location data from the defendant’s car (which shows the defendant apparently stalking the victim in the days before the murder). The question argued by the kids, and that the judge must decide, is this: if a person’s car is continually transmitting its location to a third-party service provider (think Google) and the police search that third party’s records, does this infringe upon the person’s Fourth Amendment rights protecting against unreasonable searches?
I’ve sat in on several practices and scrimmages. The discussion of this motion centers on something called the Third Party Doctrine, which says that if you voluntarily give your information to a third party, you cannot reasonably expect that information to remain private, and the government can obtain that information without violating your Fourth Amendment rights. So what’s “voluntary” and what’s “giving” and what’s a “third party”? Drilling into these questions is where the universal misunderstanding of GPS often comes up. If your GPS device is already giving its location to a satellite (the debate goes), how is that different from giving it to a company that provides driving directions?
I’ve heard this now from the students arguing the case, and from their teacher, and from the volunteer attorneys coaching the team, and even from the Superior Court judge who presided over their first tournament meet yesterday. It’s disturbing not only because of the technological illiteracy it reveals, but also because it shows how accepting we’ve become of the idea that our private data is simply out of our control.
In fact a GPS device never sends anything to the satellites in orbit. The satellites are broadcast-only, like a radio station, which has no idea when you tune into it, or a clock tower, which doesn’t respond with the current time only when you ask for it. They are artificial stars that are always “visible” to the devices that know how to see them.
Each satellite continually broadcasts its own position in space, plus the current time according to its super-accurate atomic clock. Your GPS device receives this signal from several different satellites at once. Because of the speed-of-light delay, the signals from different satellites take different amounts of time to reach you. So though the satellite-A signal might say “it’s six o’clock and 33.227 seconds,” the satellite-B signal reaching you at the same instant might say “it’s six o’clock and 33.221 seconds,” which tells your GPS device that you’re closer to satellite B than to satellite A and by how much.1 With a couple more satellites’ signals it’s possible for your device to triangulate its position on Earth with high accuracy.
Why do people mistakenly believe that GPS satellites answer location queries from devices on Earth? In large part because of the way our smartphones work. They depend heavily on outsourcing work to computing resources in “the cloud,” continually sending requests and receiving responses, and we’ve grown accustomed to things working this way.
Why should it be obvious that, in the case of GPS, this is wrong? For one thing, our personal electronics have worked this way for not very long. We’ve forgotten that, before smartphones, standalone GPS receivers were sold as exactly that: receivers. Back then (just a decade or so ago) I don’t think anyone believed GPS devices ever sent signals anywhere, or in any other way leaked information about our whereabouts. With a court order, the police could seize your GPS receiver and inspect its memory of where it had been, but that information lived nowhere else, and it was largely outside anyone’s imagination that it even could.
Another reason this should be obvious: your smartphone is small. It has a small little battery and a small little antenna inside. They’re strong enough to send signals to the nearest wifi station, which is usually located within a few dozen feet, or the nearest cell tower, which is within a few dozen miles, but not to GPS satellites, which are over twelve thousand miles away.
A final reason this should be obvious: there are very many GPS devices making very many location queries every minute of the day. Responding to that many requests in a centralized location would take massive computing resources, the kind that Google and Amazon and Facebook have built multiple gigantic data centers to handle. We can’t put gigantic data centers in space. The stuff we can put in space has to run on solar power and be light and simple as possible. It has to require no maintenance.
Now, to be fair, when you use a service like Google Maps to get driving directions, you do send your location to Google, which is then able to compute the best route for wherever it is you’re going. So the misconception isn’t total. But the location you send to Google came in the first place from old-fashioned GPS triangulation that, in itself, never needs to send anything anywhere. (Note that you can use Google Maps in “offline mode,” where maps are downloaded to your device before you start your trip, and while you’re en route, Google’s servers never get involved. Your device has everything it needs to show you your location and the route you should take. Not so long ago this was how all GPS devices worked!)
What does it say that so many of us believe the wrong thing about how GPS works, and are happy to use it anyway? It suggests to me one of two things: either we’re inattentive to encroachments on our privacy, the basis of our liberty; or we are attentive, we just put a low price on that privacy, trading it away for the convenience our smartphones offer. I’m not sure which is worse. I am sure that earlier generations would not have been nearly so willing to use technology that they understood so poorly.
- In this example, you’re 0.006 light-seconds closer to satellite B, which is about 1,118 miles. [↩]