GPS Hidden Messages - Computerphile

We're going to be looking at some work I've been doing on GPS, um global positioning system. And where you know about this is obviously your smartphone or your your satnav. And the way that that works is you're able to measure the distance between you and the satellites. And if you know how far four

of these satellites are away from you, can work out your position on the globe. The way that GPS works is it has this signal that helps you with positioning.

But it's also got some data that's encoded on top of it. It's in very slowly, only 500 bits per second. So, about a thousand times as fast as the sort of slowest modems that people have grown up with.

But it's enough to send data that tells you where each satellite is at each point in time. And that's essential because all you know is the distance.

So, if you want to know where you are, you know need to know where they are. But it's got some other stuff in there.

Um some of that we know about, so it talks about the satellite health. Um so, it'll tell you whether to pay attention to it. It also tells you about what type of satellite it is. Tells you a bit about the atmosphere, so you can do some corrections for that. But there was one that stood out to me, which it just

said, "At the discretion of operating command." So, it says text message, but it doesn't say anything else about what's in it.

And this is not normally something that is going to be made available on your smartphone or even standard GPS chips.

Uh but if you have a software-defined receiver, um then you can get it. So, um I had got those from some other projects. I got a GPS antenna, and then downloaded the right software to get the GPS data, and I saw these messages that from a first glance look random.

And that is actually quite unusual. You don't really get random data in nature. Either someone makes it random or is encrypted data. But either way, I thought it was interesting. And a lot of the work I've done on other projects like the Tor project are all about trying to make sense of encrypted data

and particular traffic analysis. So, using the patterns in that encrypted data to work out what it means, what it tells you about the system. So, how often it changes, um how large is it, where it's coming from, all this sort of metadata can actually tell you more information than you can get from just

the content. The GPS terminology is frames. And um each frame is divided up into a subframe. And then some subframes are divided up into pages. So, this um particular data that we're interested in is a present on on one of these pages.