Recent cases have shown that the government uses vague
descriptions of cell site coverage areas. These usually consist of
an arbitrary radius from the cell site in a circular pattern or in a
directional or sectorized arc pattern. The prosecution’s expert witness stated that “a cell tower would
generally have a coverage radius of about one to one-and-a-half
miles.”4 He also said that urban areas could have smaller coverage
areas, but he did not say how much smaller. Further, it was stated that
cellular companies are installing small cells that may cover an area of
10 meters.
As in the following cases, the radius is typically arbitrary and not
scientifically determined. The prosecution will typically create a
coverage area that is its “best case” and encompasses the crime scene,
thus “proving” that the defendant was in the area of the crime scene. Hundreds of small cells could fit in a circle with a radius of one and
a half miles. It is very clear that identifying the cellular coverage of
a cell cannot be done arbitrarily. A much more scientific approach is
needed.
When the prosecution provides this type of map as evidence, it is
important for the defense to obtain the CDR and CSLI information
directly from the wireless operators and hire its own expert to create
coverage plots that are based on a more scientific approach. People v. Brim 5
Case Analysis
United States v. Carpenter 1
While all the press
in this case is about
Fourth Amendment
rights, a cellular network
design expert will
investigate the actual
testimony in regard to
the determination of the
cell site coverage area.
In Carpenter, the U.S.
Supreme Court held
that police must usually
get a warrant to access
Figure 3: Cell site with 120 degree sector
historical CSLI.
and 2-mile radius
At trial in Timothy Carpenter’s case, FBI agent Christopher Hess
said that in an area like Detroit the cell sector’s signal could reach
“typically anywhere from a half-mile to two miles.” 2 The government
assumed a three-sector cell each with a 120-degree angle for each
sector with a radius of two miles. The image would look something
like Figure 3, with the arrow pointing to the proposed coverage area.
This is not the exact location of the cell site in the case, but a sector
with a two-mile radius is quite large, and in reality, there could be a
dozen or more cells within this arc. Also, the actual coverage would
look nothing like the arc shown in this figure.
Thus, again it is clear that the government over estimates and
simplifies the coverage area. By using a more scientific approach,
the defense team can mitigate or even eliminate the prosecution’s
argument about the phone being within the coverage area at the time
of the crime.
United States v. Davis 3
The defendant in United States v. Davis was charged with
committing several armed robberies. MetroPCS supplied 67 days of
cell site records for the phone in question, as well as the relevant cell
site locations.
In Ronald Brim’s case,
the prosecution’s expert
used a one-mile radius
for each cell site. This was
the “outside reach” or best
case for the prosecution.
Los Angeles is very dense
and nearly all of its cell
sites are sectorized. Thus,
this representation of
the cell site coverage was Figure 4: Omnidirectional cell with
highly slanted toward the arbitrary one-mile radius
prosecution.
In all these cases and many more, prosecutors used arbitrary methods
of calculating the coverage of a cell site and used the “best case”
scenario for them to place the phone at the crime scene. Daubert
requires that “scientific methodology” be used. This is an opportunity
for defense attorneys to contest the prosecution’s methods and
to use an expert witness that will bring more proven methods of
determining cell site coverage.
Expert Witness
The defense team brings in an expert witness to analyze the mobile
and network data, make a determination as to the location of the
phone at the time of the incident, and dispute the prosecution’s
claims concerning the location of the mobile phone. This must be
based on scientific evidence that satisfies the Daubert6 or Frye7 rules
depending on the jurisdiction in which the case arises. To meet the
Frye standard, scientific evidence presented to the court must be
interpreted by the court as "generally accepted" by a meaningful
segment of the associated scientific community. The Daubert standard
supersedes Frye and provides a set of guidelines to assure that data
is relevant and comes from “scientific knowledge.” The data must be
presented by an expert witness complying with Rule 702, which is as
follows:
A witness who is qualified as an expert by knowledge, skill,
experience, training, or education may testify in the form of an
opinion or otherwise if:
(a) the expert’s scientific, technical, or other specialized knowledge
will help the trier of fact to understand the evidence or to
determine a fact in issue;
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