Just how accurate is GPS, exactly?

Tell me if this sounds familiar to you. You’ve lost your phone but you know it’s somewhere in the house. So, you go to the web and activate the “find my phone” feature. Pretty much every phone has one now. It shows you on a map just where your phone is. Not just in your house, it shows you what room. That’s pretty impressive performance, right?

GPS accuracy isn’t actually that good.

According to this dated-looking government web site, GPS is really only accurate to about 16 feet. So, what that says is there’s no way that the display on the web could tell you where your phone is. Not only that, it also couldn’t tell you if you’re in the correct lane when you’re driving, and you have seen your cardo that little trick too.

If you read through this web site you’ll also find out that GPS used to be really, really inaccurate but around 2000 they stopped intentionally messing it up, which enabled the GPS industry to really take off. Before that it really was just a military technology.

If GPS isn’t truly that good, how come it seems like it is?

Dual frequency GNSS?

If you do some research on this subject, you’ll read about so-called “dual frequency GNSS” systems that are supposedly coming. Dual-frequency systems have been used by the military for at least a decade and they combine the readings from two sets of GPS signals for location measurement that is supposedly accurate to under 10 centimeters, about 4 inches.

The problem here is that as I write this there aren’t really any dual-frequency GPS phones. It’s not clear if car GPS systems have this technology but most likely they do not.

Dual-frequency GNSS looks like it’s going to be a great technology but most likely you’ll see it in 2-3 years. In the meantime we still have to solve the mystery of how your phone can be so accurate without it.

In the car: gyroscopes can help

Car GPS systems are actually becoming less rare as people prefer to use their phone. Systems like Android Auto and CarPlay let you use the most up-to-date maps on your car’s screen, and other folks with older cars can just, well, stare at their phones as much as they safely can.

Car systems tend to be more expensive than phones. In addition to being really well built, so that they can keep working in a car for a long time, they also have to work at those times when GPS signals are weak. This was much more of a concern with early-generation systems, but many car GPS systems today still have backup gyroscopes for when GPS systems fail.

A gyroscope is a spinning object that due to its rotation, is able to maintain its position in space. You can use a gyroscope to determine exactly where you are with a very high level of precision. Modern gyroscope/GPS systems use GPS to get a close reading on your location, and then use gyroscopes to help be more precise.

GPS systems are good guessers

It turns out that a lot of GPS’s displayed information is actually a good guess. GPS systems for cars know that you’re more likely to be driving on a road, so if the data says you’re 4 feet off the road it’s probably wrong. This is why you sometimes see your GPS “jump” when you get off the highway. It can’t quite tell you’ve exited, until there’s enough momentum that it makes sense to the system.

This kind of guessing helps GPS systems in cars to be more precise, but what about the example from the beginning of this article? When you go to find your phone, how is it so precise?

aGPS: Assisted GPS

All phones use aGPS, which isn’t really GPS at all. aGPS, or Assisted GPS, uses cell towers in addition to satellites. Because your phone is often in communication with three different towers at least, it becomes possible to know your location in space just by measuring the time it takes for the signal to get to different cell towers. This can be an incredibly precise measurement, or if the signals are weak it can be incredibly imprecise.

You know when you’re using Google Maps and your location is in the middle of a blue circle? That big blue circle shows all the places where you could be. Your phone uses aGPS, plus a dose of good old fashioned guessing to make that blue circle smaller and smaller until it is fairly sure that it can tell you where you left your phone.

Which, by the way, is between the couch cushions. I could have told you that at the beginning of the article but I wanted you to read the whole thing.