Cell Tower Data Analysis

Get a second opinion.

Outdoor coverage estimate for a 2G DCS1800 test cell in the SOMA district of San Francisco. Most of the coverage on this map is on rooftops, not at street level. This coverage estimate was verified with measurements, which were found to be consistent with signal levels on this map. Image by the author. Obviously, this is not a nice, neat circle.

Many of the calls I get from attorneys are requests for the analysis of cellphone records to determine the location of a phone at some given time. The analysis can resolve questions of fact, but lazy thinking and easy answers are giving cell tower data a bad name.

HOW IT WORKS

When done correctly, the location process work like this:

1. A cellular network is organized into “cells”. A cell is usually a pie-slice-shaped area, or “sector”, on one side of a tower site. A tower site typically has several cells, pointing in different directions.
2. Call records will usually give the tower location and general direction of the cell where a where a call started. They may also give the tower location and sector direction of the cell where a call ended. (This information may also be available for SMS and even for data sessions, depending on how promptly the request is made. In criminal cases, other types of data may also be available, like registration events, handovers, and timing advance, if they were requested ahead of time.)
3. When the logged event happened (the call started, for example), the phone must have been in the covered area of the given cell. This must be true because this is the definition of “coverage”.