Document ID: ./input/supremecourt_opinions/opinions/17pdf/16-402_h315.pdf
Page Number: 31

4 

CARPENTER v. UNITED STATES 

KENNEDY, J., dissenting 

circular geographic area around the site.  It has three (or
sometimes  six)  separate  antennas  pointing  in  different 
directions.  Each  provides  cell  service  for  a  different  120-
degree (or 60-degree) sector of the cell site’s circular cover-
age area.  So a cell phone activated on the north side of a 
cell  site  will  connect  to  a  different  antenna  than  a  cell 
phone on the south side. 

Cell  phone  service  providers  create  records  each  time  a
cell  phone  connects  to  an  antenna  at  a  cell  site.    For  a 
phone  call,  for  example,  the  provider  records  the  date, 
time, and duration of the call; the phone numbers making
and  receiving  the  call;  and,  most  relevant  here,  the  cell
site used to make the call, as well as the specific antenna
that made the connection.  The cell-site and antenna data 
points, together with the date and time of connection, are 
known as cell-site location information, or cell-site records. 
By  linking  an  individual’s  cell  phone  to  a  particular  120- 
or  60-degree  sector  of  a  cell  site’s  coverage  area  at  a  par-
ticular time, cell-site records reveal the general location of 
the cell phone user.

The location information revealed by cell-site records is 
imprecise,  because  an  individual  cell-site  sector  usually 
covers a large geographic area.  The FBI agent who offered
expert  testimony  about  the  cell-site  records  at  issue  here 
testified  that  a  cell  site  in  a  city  reaches  between  a  half 
mile  and  two  miles  in  all  directions.  That  means  a  60-
degree  sector  covers  between  approximately  one-eighth 
and two square miles (and a 120-degree sector twice that 
area).  To put that in perspective, in urban areas cell-site 
records often would reveal the location of a cell phone user
within  an  area  covering  between  around  a  dozen  and 
several  hundred  city  blocks.    In  rural  areas  cell-site  rec-
ords can be up to 40 times more imprecise.  By contrast, a
Global  Positioning  System  (GPS)  can  reveal  an  individ- 
ual’s location within around 15 feet. 

Major cell phone service providers keep cell-site records