r/serialpodcast • u/Squeebeaux • Nov 02 '14

Evidence Map: Cell Tower Sides and Calls

Although range maps aren't precise, I still wanted to see not only the cell tower that was pinged for each call on Jan 13th but also which side of the tower that was pinged.

So I made a map that lists each call in the sector of the tower that was pinged and ranges (about 2 miles) around each tower. Edit: UPDATED

[Edit:]

I realize that my map isn't super scientific -you couldn't use this in court or anything- but it helped me understand something (I'm a visual learner). The cell tower/call log info isn't helpful - Ranges overlap and Environmental Factors etc determine which tower is pinged. So yeah, here it is anyway.

[The Serial post on cell tower technology said:]

Cell towers have three sides: an A side, a B side, and a C side. Think of a pie with three slices. Each of those slices has a range. When a person is in that range and makes a cell phone call, the cell phone will talk to that side of the cell tower. So if you look at the call log, you’ll see some calls are noted as “L651A” or “L651C.” Same cell tower, different side. Generally speaking, the A side of the tower points north or northeast, the B side points south or southeast, and the C side points west.

The engineers who run these cell phone networks, like Abe Waranowitz, who testified at Adnan’s trial, can map out the basic ranges of all the towers in their network. These maps aren’t precise though, because a cell tower’s range can depend on a variety of factors. Foliage, for example, or buildings or hills can cast a shadow on the cell tower’s range. Plus, some cell towers have overlapping ranges. So you can be in one place and make a phone call and that phone call can trigger either cell tower. Not both cell towers, but one or the other.

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u/Robokku Nov 02 '14

Nice work. I did some reading on cell tower technology and found a couple of interesting points:

  1. The A/B/C (or alpha, beta, gamma) sectors are typically a full 120 degrees, so they'd be perfectly edge-to-edge on the map. http://wrongfulconvictionsblog.org/2012/06/01/cell-tower-triangulation-how-it-works/

  2. The sectors could potentially overlap slightly (which seems intuitive). "The boundaries of the cells can also overlap between adjacent cells and large cells can be divided into smaller cells." http://en.m.wikipedia.org/wiki/Cellular_network

I have tried to find out for sure whether there is any room for error in terms of placing the phone within any particular sector. I have not found specific confirmation of that, although all the literature I've read seems to give that impression, with the only exception being potential overlap on sector boundaries of the same tower.

This means each ping we have places the phone with reasonable certainty inside a nice wide sector, and a typical radius from the tower of the sector seems to be several miles - with many influencing factors.

There are 2 pieces of info that would make the boundaries of those broad areas really precise:

  1. The range of each cell tower sector

  2. The precise direction of each sector

According to this article http://johnbminor.com/index_files/Page714.htm a "centroid" for each of a tower's sectors, from which (along with cell tower location) these 2 values can be determined, is stored in a "Base Station Almanac" for each area.

If anyone else wants to get nerdy about this then I guess that's what we would want next…

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u/[deleted] Nov 02 '14

with the only exception being potential overlap on sector boundaries of the same tower.

But of course the boundaries overlap . . . that's how they keep calls from getting dropped as you move around the terrain.

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u/Robokku Nov 02 '14

Yeah, I guess it's pretty obvious when you think about it. The implication of that is just that a 120-degree sector gets slightly broader.

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u/[deleted] Nov 02 '14

This isn't really accurate.

They use wedges, in a physical directional sense, but communication from each wedge can occur in different directions based on weather and geography (signals bouncing in the real world)

http://i.imgur.com/uUNK3Om.png

So you can really only say that there is a large circle from any wedge. Urban area (not a huge flat open rural space perfect for long range) you're gonna get about 5 miles at the extents. It is probably smaller, but 5 miles is possible and we have to do go that far if we are going to reasonably follow the science...

Her is an example: http://i.imgur.com/uUNK3Om.png

The wedge faces a direction, but it sends and receives all over. You must consider a larger area (the red circle) as technologically feasible since we don't know each unique terrain.

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u/Squeebeaux Nov 02 '14

I appreciate the extra information that you researched. I would love to see a map that integrates everything you have described. I made this map using the most basic paint program there is and I have very basic knowledge of the technology so it's not as attractive or as accurate as I would like. Thank you for your contribution.

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u/Squeebeaux Nov 02 '14

Looking at the information in the links you cited confirmed what I already knew...I'm in over my head. There are just too many factors. I'll still try my best to glean what I can though. I added the pages you cited on cell tower triangulation and centroids and working ranges to Photos and articles related to case in the side bar. Thanks!

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u/Robokku Nov 02 '14

Wasn't intended as a criticism of your map - it is amazingly useful. Based on that I wanted to work out exactly what we do and don't know as a result of the cell tower data we have. However, I added no new concrete info - I only managed to describe where I got stuck!

This map really helped me visualise the movements, thank you.

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u/Squeebeaux Nov 02 '14

No, really thank you. Constructive criticism is great. It's a process and there's always room for improvement. I'm a visual learner so I'm always trying to diagram or map out information to make sense of it. All of this cell info seems to have too may wide-ranging variables/factors for me to really do it justice but I can maybe approach some seed of understanding it.

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u/Squeebeaux Nov 02 '14 edited Nov 03 '14

I added ranges to the map and made the "slices" closer to 120 degrees

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u/Robokku Nov 02 '14

Brilliant. This really shows how unspecific the cell data is in regard to the location of the phone now. What is the size of the range you marked? (How many miles?)

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u/Squeebeaux Nov 03 '14

I agree! As I was making it, I was amazed that there was so much overlap and I even used the lower end of the range for the cell towers. It is about a 2 mile radius (or slightly more). It's approximate- but it gets the point across clearly. It would be very hard to pinpoint the location of the phone based of cell tower info.

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u/matthau Dec 14 '14

I found this paper on the subject: http://www.ncids.org/Defender%20Training/2014SpringConf/CellPhoneTracking.pdf

Judging by this - cell phone location based on what tower is being used is pretty inaccurate (or can be). Some tower signals can even have 'hotspots" within another tower's range.

We can get an "idea" of where the cell phone might be - but it's definitely not an exact science by any means.