Hot Topic in Forensics: The FBI Cellular Analysis Survey Team (CAST)

The strong demand in criminal cases for historical cellular location information has created opportunities for law enforcement to extend the limits of what is possible. The FBI Cellular Analysis Survey Team (CAST) is promoting a new methodology that it claims greatly improves the precision of historical cell sector analysis.

Figure 1. Historical cell sector analysis has a range of error that extends throughout the entire sector.

The FBI CAST says it can dramatically enhance location accuracy using the historical cell site sector information plus timing information inherent in a cellular network. Because historical GPS or other precise location data is not typically collected and stored on a cell phone or by the network, the reliability and accuracy of the FBI CAST enhanced cell site analysis based on cell sector plus timing and power information is unknown.

To be clear, the FBI CAST enhanced cell site analysis is not E911 location information that is calculated in real time from a burst of GPS data sent by the cell phone or a hybrid of available GPS data and network data collected from three or more cell towers. CAST relies on historical signal data that a network measures and calculates for purposes other than providing its customers with precise location information.

Figure 2. A timing band based on network measurements of round trip time for a signal.

Traditional historical cell sector analysis is not precise. It provides a cell site sector as the location for the target phone. As shown in figure 1, accuracy is limited to the radius of the serving sector, which is typically measured in miles. CAST says it can increase location precision with a “timing band” primarily based on the time it takes a signal to travel from a cell tower to a cell phone and back. A typical timing band involving one cell tower is illustrated in figure 2. The cellular industry, however, considers the accuracy of time banding to be poor, and highly variable over different environments.

When multiple timing bands intersect, the FBI CAST claims it can provide a pinpoint location as shown in figure 3 based on RTT data. The cellular industry considers the accuracy of RTT location data to be low, primarily because signals travel multiple paths. Even when CAST lacks network timing information, it maintains that it can accurately create a cell phone footprint

Figure 3. A timing band based on network measurements of round trip time for a signal.

within a cell sector based on an after-the-fact survey of signal power collected during a drive-test. The cellular industry does not recognize this method to determine location. In any of its various forms, the CAST enhanced historical cell site analysis delivers on its promise of more precise location information. The FBI cannot, however, demonstrate its reliability or accuracy.

A public defender trial team recently made this very point when it challenged the FBI CAST claim that it could reliably map the historical location of a target cell phone  within a cell sector based on a drive-test conducted ten months after-the-fact. An expert in historical cell site analysis testified for the defense. The prosecution called two witnesses: a FBI Special Agent and a network engineer. The circuit court for Anne Arundel County (Silkworth, J.) excluded the FBI CAST report under Frye-Reed. The circuit court rejected as unscientific the FBI’s attempt to map the historical “signal footprint” of voice calls within a sector primarily based on a survey of signal measurements collected during a drive test ten months after-the-fact. This is an important first win.

An admissibility challenge involves two basic questions. First, does the network reliably collect and report the underlying data, in particular the signal timing and power measurements. Second, is the enhanced historical cell sector analysis a reliable method to determine accurate location information for a target cell phone at a point in the past? A closely related third question is what scope of expertise required to establish the reliability of the data collected and the methods used to interpret that data. The answers to these questions involve a lot of physics.

To illustrate the complexity of the data, considers the error in RTT time measurements of a signal caused by multi path. Think of a pond with wooden stakes placed around the shoreline. These stakes represent cell towers. A stone is dropped in the middle of the pond. It creates ripples that travel from the center towards the stakes at a certain rate of speed that can be directly measured. But when there is an obstruction—say a log, branch, or leaf, floating in the pond—the ripples must go around or over it. A direct measurement of speed suddenly becomes much more challenging. There are now many smaller ripples that that eventually reach the stakes. Which ripple should be measure that most accurately can be used to estimate distance the ripple has traveled? Further complicating the measurement is are the reflections of ripples that bounce off the states and travel to other stakes. How can the interference of these ripples on the time measurements be taken into account?

The FBI CAST has very simplistic answers to these complex questions that amount to a “trust me because I say so” opinion about reliability. A Special Agent is unqualified to assess the reliability of the data collection or interpretation. And the shortcut of “because the network functions it must be reliable” does not answer the question about reliability and accuracy of location services. While signal timing and power information is inherent to network operations, the precision of that data is determined by business necessity. It follows that network data collected for one purpose is not always reliable for another purpose. For example, the network must measure signal time to sync voices, but the degree of precision needed to prevent pauses during a two-way simultaneous radio communication falls short of the precision needed of signal time measurements to accurately calculate location. In fact, the contrary is more likely true because a cellular carrier does not want to collect historical location information about its customers because of privacy concerns. A prospective cellular customer is more likely to choose a network that does not collect and store her historical location information.

The tremendous demand for the FBI CAST enhancement of cell sector information with signal timing or power information will create many more opportunities to challenge its admissibility.

Contact RaquinMercer to consult with Steve Mercer about a challenge to cell phone location evidence in your case.