Cell Phone Evidence and the Serial Podcast

As a cell phone expert who is called to testify in criminal cases about cell phone evidence, I’ve gotten dozens of calls about the new Podcast “Serial” hosted by Investigative Journalist, Sarah Koenig. Her series investigates the murder of Hae Min Lee, an 18 year old student at Woodlawn High School in Baltimore and the conviction of her ex-boyfriend Adnan Musud Syed for the crime. Much of the evidence in the trial revolves around cell phone evidence.

Is cell phone evidence reliable? Is it being used improperly? Is it misunderstood? Yes, Yes and Yes. Here’s some facts that might make this more clear.

Fact – Cell tower evidence generally can only locate you to an area that is the size of 323 football fields.

The fact is that a lot of unqualified people are using cell phone evidence in trials and people are being wrongly convicted and wrongly acquitted. I’m often involved in circumstantial cases that rely solely on cell phone evidence. Let’s look at the facts.

1. The Basics – Cell Phones are Radios – Just like your car radio, a cell phone is a radio. It picks up a signal from the closest cell tower with the strongest signal. That’s what keeps you connected to the phone network. Cell towers and Radio station towers are similar. When you drive away from the tower that broadcasts your favorite radio station, the signal gets weaker and eventually you can’t hear your show anymore, but now you are picking up the radio station in the next town. If you keep driving through that town, you’ll lose that radio station and pick up the next radio station on your route. Radio stations don’t really overlap, so you always know which one you are connected to. Cell Towers are the same. As you move around you disconnect and connect to the closest tower with the best signal. The cell towers don’t overlap much. Generally you are always connected to the closest tower. Unlike radio stations that can cover an entire state, cell towers only cover a small area. Sometimes only one block, but most times they cover about 2 miles around them. So there are a lot of cell towers out there.

2. Evidence – When you make a call or receive a call, the phone company writes down the date and time of the call so they can bill you, and they also record which cell tower you used. This is the evidence that can be used in criminal case. If you are attached to a cell tower at a certain time, we know you are in the vicinity of that cell tower and not anywhere else. As correctly stated in the podcast, ‘being connect to one cell tower, excludes you from being anywhere else.’ Cell phone evidence only is recorded when you make or receive a call. If you aren’t using your phone, the phone company doesn’t keep a permanent record of where you are. Only the data that is used for billing is kept (and only for a short time.)

3. How Accurate is Cell Phone Data? – Cell phone records define the cell tower you were using when you made or received a phone call. We can identify where you were during a call because we know the cell tower and we know where that cell tower is located. If you are connected to a call tower in Baltimore, Maryland, we know you are not in Denver, Colorado. That may be very useful in a trial to either side. But most cases involve distances that are quite a bit smaller in area. So that begs the question; “What’s the size of the area that a cell phone tower covers?”

A cell tower, like a radio station tower, covers an area around itself. A radio station may be heard for fifty miles in every direction. A small radio station may only cover ten mile in every direction. A cell tower generally covers about 2 mile in every direction. If your cell phone records show that you are connected to a cell tower that’s located at Woodlawn High School in Baltimore, we know that you are no further than two miles away from that cell tower. If you were further away, you’d connect to a different cell tower during the call; say the tower that’s in the Best Buy parking lot in Baltimore.

Let’s do the math. Remember your high school math? The area of a circle is equal to πr². You learned about pi-r-squared in high school and thought you would never need it, didn’t you? You get to use this formula now. pi-r-squared gives you the area of a circle. So a cell tower that extends out 2 miles covers an area of 12 1/2 square miles. (Area = 3.14 x 2 x 2)

OK, that’s a pretty big area. How does that help? This is the point. Cell phone evidence can be tremendously helpful but there are limits. Because a phone is connected to a tower during the call, I can place you within an area of 12 1/2 square miles. That may be useful if I’m trying to exclude you from being at a crime scene and the crime scene is covered by a different cell tower.

But what if you are attached to a cell tower during a phone call that also covers the crime scene? This weak connection is often used in court. No expert can placed you at a crime scene based on the fact that you made a phone call from a cell tower that covers the crime scene. No expert can exclude you from a crime scene either based on this evidence. But both prosecutors and defense lawyers attempt to do this.

Jurors are often lulled into thinking that because a user was on a call that is connected to a cell tower can place the user at a specific address. That’s just not possible to say. A cell tower that covers a specific address that includes the crime scene also covers a lot of other addresses. Here’s what I tell jurors about the size of the area that a cell tower covers a two mile radius:

This cell tower covers an area that is 12 square miles.
That’s equal to more than 8,000 acres.
That’s equal to 968 NFL Football fields.
No expert can tell you where within that area a phone is located.

Think about this? If I want to say that someone was located at their Uncle House in Baltimore that’s on one acre of land, I’d have a 1 in 8,000 chance of being right based on cell tower evidence.

If I’m trying to say two people were together because they both were attached to the same cell tower at the same time, it would be a big stretch too. Cell phone evidence doesn’t let me state that the two people could have been in an area of the size of ONE football field. Think of how big that area is. Two people on one football field may not even be in sight of each other. Or they may be standing together. The evidence doesn’t allow an expert to say. But two phones attached to a cell tower isn’t an area of one football field; this area is 968 football fields.

Yes, you can get better location for a cell phone. When you call 911, the dispatchers can identify your location to within 140 feet (most of the time). That’s still about a half of a football field. Yes, we can “Ping” a phone to find a lost person and get pretty close. We can do this in real time and it saves lives. But evidence that’s brought into trials is “historical cell phone records”. These are records that have been stored and only shows which cell tower you used. (And sometimes which side of the cell tower you are using.)

Is cell phone evidence reliable? Yes, the records are reliable. (Did you ever try to fight a charge on your phone bill?)

Is it being use improperly? Yes, too many well meaning people with just a vague understanding of cell phones are able to draw attractive and convincing Google maps with circles and arrows on it that show inaccurate conclusions. They are creating inaccurate evidence in court for both sides.

Is cell phone evidence misunderstood? Yes, cell phone evidence is remarkable useful but it’s not DNA. If that’s understood the questions that jurors have in trials about the reliability of cell phone evidence can be put to rest and help find the truth.

Sarah Koenig podcast “Serial” is just one story. Dozens of people have been affected by improper use of phone evidence.

>>>>>>>>>>>>>

Ben Levitan is one of the most sought after legal expert witnesses in multiple telephone and data technological areas.

A proven authority, Levitan’s career has spanned more than 25 years as an engineer in the field of cellular communication and includes 27 patents in cellular technology innovations.

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6 Responses to Cell Phone Evidence and the Serial Podcast

  1. Sue says:

    Hi Ben, you say that the average cell phone tower coverage area is about 2 miles in every direction. What is the largest cell phone tower coverage area and what is the smallest one that you’ve ever personally come across?

    I hope you don’t mind me sneaking in another question. How likely is it that a tower would cover an area with no inhabitants so within that 12 square miles there are no inhabited areas at all.

    Thanks

    • benlevitan says:

      Remember in high school math class you learned how to figure out the area of circle using the formula “Pi R squared”? You thought you would never use it in real life? This is where you get to use it. You are right. The area that a cell tower covers vary based on a lot of factors. Ideally, a cell phone company would like to be able to put up one cell tower (say in Coventry) that would cover all of the UK. Cell towers are expensive, so the fewer the better.

      The reality is that a cell tower can only handle a limited number of phone calls. Let’s say a cell tower is only capable of handling 100 phone calls. If more than 100 people are in the coverage area of a cell tower and want to make a phone call, they will find that the system is busy and not be able to have service until other are finished with their calls.

      This limitation means that the cell phone companies have to look at the area and determine how many people are likely to want to make mobile calls in a certain area and then decide how many cell towers they need and how big of an area each cell tower can cover to be sure everybody in that area can always make and receive calls.

      In downtown London, a cell tower likely covers about 1 city block. It’s likely that there are so many people who want to make calls that a cell tower that tries to cover any area bigger too many people would not be able to get service. However in the country where there are few people, it’s possible that there are not 100 people who want to use phone service within 40 km (25 miles) so the coverage can be much greater.

      So to finally answer your question – The typical maximum range of a cell tower is about 25 miles. That would cover 1,963 square miles. The minimum size is another story. A cell tower on a pole or a building can be set to cover a small area such as a city plaza but this isn’t cost effective. What is happening is that we are now putting out “small cells”. These are cell towers that are the size of a loaf of bread. They can be placed on lamp post, inside malls and offices. They are low cost, low maintenance and provide perfect cell phone coverage for a small area. As such they also provide a very accurate indication of phone location.

      These types of cell towers are becoming very popular. These small cells will be used in office building, in shopping malls and in subways and other spaces to add capacity and provide service. They solve another problem. A problem exists with emergency calling in the USA. If you call 911, the operator automatically receives your location so that help can be sent to you without you even knowing your own location. This works well and usually can pinpoint your location to 50 meters (140 feet). The problem is that if you are in a high rise building, the operator does not know which floor of the building you are located. Small cells will solve this problem. For this reason, the government may require this type of cell tower be implemented in tall buildings.

      And the smallest cell I’ve scene is sitting in my house. It’s probably good from the living room to the kitchen. This “home hotspot” will also become well known in the next few years.

      • Sue says:

        Thanks for the comprehensive answer 🙂

        Was that small cell technology around in 1999? I’m thinking of the Serial podcast in particular with the suggestion that one cell tower only covered Leakin Park and possibly the road around the outside, a small area with no inhabitants.

        Sue…and yes from the UK 😉

  2. benlevitan says:

    These small cells are very new. In 1999, AT&T’s network was 2nd Generation (2G) technology. It launched 3G in 2004. I would expect that only part of one cell tower covered Leakin Park.

    • Ben Scott says:

      Hi Ben, on the Karas On Crime podcast you said that AT&T upgraded the network from 2g to 3g between the time of the murder and the test drive that they did 10 months after the murder. Is this right as above you say they upgraded in 2004?

      • benlevitan says:

        Cell phone companies are constantly upgrading and maintaining their networks. You know that since the 1990’s there have been tremendous growth in the cell phone use and in the technology. (I spent much of that time in industry committees that defined and designed what 2G, 3G, 4G and LTE included. Each is a new and improved version of the standard cell phone network with specific new features.) The AT&T network was involved in this constant upgrade, but just as importantly, there there was a tremendous growth in subscribers and cell phone use. That means more cell towers and a different “configuration”.

        All that said a “Drive Test” done 10 months after the crime is pretty useless. The FBI has a department that does “drive tests” in the area of crimes scenes to try to determine what cell tower a person would be connected to if they were at the scene of a crime. If they do these drive test immediately after the crime, the results are valid. If they wait 10 months or two years, the data is pretty worthless.

        I hope that answers your question. Please feel free to follow up.

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