CRIMINAL DEFENSE TECH TIDBIT – Challenging Historical Cell Site Analysis

Challenging Historical Cell Site Analysis

If you’ve encountered a map generated by a law enforcement official depicting cell tower coverage with a 120-degree wedge, it’s crucial to challenge its validity. Such wedges do not accurately represent coverage; they merely illustrate the direction of an antenna.

Around 13 years ago, the FBI established the Cellular Analysis Survey Team (CAST) to utilize cellular data for tracking live phones on networks. However, the principles and methodologies employed by CAST differ significantly from those used by mobile network operators. The training for CAST is an intensive, graduate-level program condensed into a few weeks, delivered by college professors who specialize in cellular engineering. Notably, Garrett Discovery Experts are the only defense experts in the U.S. who have completed the same extensive training as FBI CAST personnel, receiving instruction in the same facility, from the same professors, and using the same materials. Currently, only 94 FBI agents hold CAST certification after attending this rigorous course.

In response to requests from local and state law enforcement for assistance with cases involving cellular data, the FBI created a two-day “Basic Historical Cell Site Analysis” course. This training focuses primarily on using software to plot dots and wedges on maps, dedicating just 30 minutes to “Basic Cellular Theory,” which covers the fundamental components of cellular networks. Many officers who attend this course lack engineering backgrounds on forensic training, making their courtroom testimony questionable. The FBI provides two key guides to participants: the “April 2023 – FBI Cellular Analysis and Geo-Location Field Resource Guide” and the “Basic Historical Cell Site Analysis Student Workbook.” The FBI provides the following disclaimer:

Studying this manual and attending basic training does NOT constitute an individual as certified to testify.
Cell site analysis is a great investigative tool; however testifying in court regarding the records is difficult and requires significant training.
Prior to testifying, CAST agents undergo over 250 hours of training before they can testify.

There is a significant disconnect in the community: the methodologies taught in the two-day course are flawed and differ from those presented in comprehensive CAST program. Essentially, participants of the two-day course are being taught incorrect theories, akin to learning that the world is flat while the multi week CAST attendees are instructed the world is round. This misinformation has permeated the defense expert community, often dominated by former law enforcement officers who have only completed the brief training.

Although one could elaborate extensively on the issues stemming from these flawed methodologies, let’s focus on a single crucial aspect: the shape used to represent cell tower coverage. Transcripts of FBI testimony often reveal the true nature of radio frequency coverage. “I seeit gives you back lobe, side lobe, main lobe, beam width,” to which agents affirm. However, the prevailing law enforcement practice of estimating coverage with a 120-degree wedge is fundamentally flawed. In reality, no drive tests conducted by governmental bodies would yield such results disproving their own theories.

Cellular antennas radio frequency coverage consists of main lobes, side lobes, and rear lobes. As illustrated using Radio Planner – a tool employed by cellular companies for network planning – the depiction of coverage is not as simplistic as a 120-degree wedge. On the left is a real radio frequency plot overlayed with what the government would do based on their theory, which is to place a 120-degree sector (shown in red lines). On the right is from the FBI’s training manual. Based on what you see below, if the real coverage was used the defendant could be as far as Ferndale which is the opposite direction the antenna is pointing.

Where Did This Go Wrong?

You might wonder how law enforcement arrived at these incorrect methodologies. The issue lies in the distinction between “best coverage” and “total coverage.” Cellular operators design networks to optimize best coverage, while law enforcement often misrepresents this as total coverage. The shaded area depicted by law enforcement was meant to show potential best coverage, but is often testified to as total coverage.

The Defense Must Act

It’s imperative for the defense community to challenge these practices. Our work must focus on total coverage, not just the best coverage depicted by law enforcement. When determining where a defendant may have been in relation to a cellular tower, we need to asses total coverage, which for a 4G tower can extend up to 28 miles, depending on terrain and tower height.

Below is a radio frequency map showing the total coverage of a tower in Miami affixed to the side of a tall building. This diagram was created using Blazar, an industry approved Radio Frequency planning tool. Blazar is proven to be within 3% of real-world coverage and is supported in the cellular industry as one of the defacto tools alongside Atoll. Law enforcement depicted coverage for this same antenna as shown by the red lines. If we are to assume law enforcements estimations are correct, which they are not, this would be fraud upon the court.

The blue area is best coverage, green is good coverage, yellow is less than good and orange can still hold a call, but is most likely the area your phone would be searching for another tower to connect to.

You may question the likelihood of maintaining a connection to the tower while traveling through the less favorable coverage zones. This concept, known as Cellular Hysteresis, indicates that the mobile phone will remain connected to a tower even as they pass other viable towers until the signal gets weak and the phone searches for a new tower. In almost all cases, in urban developments, the phone will pass 2-20 towers before searching for a new cell antenna.

Recently, I had the opportunity to draft comprehensive deposition questions for an FBI CAST agent in Florida. This deposition revealed critical insights into the discrepancies in methodologies.

For a deeper understanding, you can access the FBI CAST deposition here: Download FBI CAST Deposition

Garrett Discovery, an NACDL Affinity partner, boasts a team of experts in digital forensics-including computer, mobile, cell site, audio, video, and electronic medical record forensics. With over 500 testimonies across 49 states and a commitment to over 250 indigent cases yearly, we employ real cellular engineers who hold numerous patents in the field. Additionally, Garrett Discovery operates 702 Cellular Academy, where we provide training on cell site analysis.

By addressing these methodogical flaws, we can ensure that justice is served accurately and effectively in our legal system.