PCPlus 310: How your phone betrays your location

I’d have to say this article is perhaps the most satisfying of all the articles I’ve written for PC Plus. For a start, it did not even start out as an article but as some research I did to help my wife with a case (she’s a prosecutor). Some of the cases she prosecutes involve computers, and some involve some kind of techy knowledge she has to understand in order to present it to a jury. Seeing as I’m a kind of captive techy guy at home: I get asked questions about hashes, about deleted files, about browser caches, and all sorts of other things. And then there was this case…

PC Plus logoThis particular case involved being able to pin down the defendant to a particular place in Denver at a particular time. All she had was a printout from the defendant’s cell phone company showing numbers called, calls received, and which cell phone tower acted as the connection to the phone network. Not only that but there was an indication of the azimuth the cell phone was located. What the heck was an azimuth? And why was it important in this particular case? You’ll have to read the article to find out: it all turned into quite a pleasing detective story.

The case also involved something else: she managed to get a list of cell phone towers in Denver with their latitude and longitude. Enter some fun programming: I had to find the set of towers nearest the place in question (say, within a 5 mile radius, which involved converting lat/long pairs into the number of miles from a point) and show that the cell phone tower being used was in all cases the closest one. So, I had to plot them on Google Maps, which involved creating a little bit of code to convert lat/long pairs into the JavaScript objects expected by Google and then feed them into the API. You can see the resulting map in the article.

Since I wrote the article, it’s been used to educate other prosecutors in other cases. I’ve had some great feedback on it from other readers, too.

This article first appeared in issue 310, Summer 2011. TechRadar.com also published it online at the end of August.

You can read the PDF here.

(I write a monthly column for PCPlus, a computer news-views-n-reviews magazine in the UK (actually there are thirteen issues a year — there's an Xmas issue as well — so it's a bit more than monthly). The column is called Theory Workshop and appears in the Make It section of the magazine. When I signed up, my editor and the magazine were gracious enough to allow me to reprint the articles here after say a year or so.)

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2 Responses

#1 Scott Bussinger said...
18-Aug-12 11:28 PM

Fun project! But I'm curious -- if they had been making a call while driving down the freeway they'd have connected up with a bunch of different towers over the duration of the call, right? It sounds like your call data only gave one tower/azimuth per call. So what tower did they give you, the tower where the call started, the tower where the call ended, or some other criteria?

julian m bucknall avatar
#2 julian m bucknall said...
19-Aug-12 7:09 AM

Scott: you're right. If they had been traveling down the freeway, the calls they'd have made or received would have been flagged to different cell phone towers. The data D. got from the network provider only showed the cell tower + azimuth at the time the call was made or received (so, if they'd been driving along the data she had wouldn't have shown that travel for a single call). As it happens, the defendant was making and receiving a series of short calls over a period of 15-20 minutes (I no longer have access to the data, of course, so I'm relying on memory but I think it was 5 or 6 calls). During that time the phone (and hence defendant) didn't move. Since the defendant was saying he wasn't at such-and-such place at the time, yet his phone was in the area, that was all the evidence D. needed to bring it up in court.

Now, it could be that the network provider does maintain a list of towers you connect to during a single call, but she didn't have that data. We were lucky in the sense that the defendant had many calls during a shortish period of time, all of which were pretty much using the same tower and azimuth.

Cheers, Julian

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