was intended for outdoor use; the weak signals transmitted from the 24 space vehicles (SVs) that constitute the GPS space segment are difficult to detect indoors and blocked by tall buildings. 16 GPS is also designed to work with autonomous receivers; GPS signals are modulated to provide the receiving unit with the locations and orbits of the SVs, information needed to compute the receiver’s location.d The locations and orbits are provided on the same carrier used for (civilian) distance estimation. In order to avoid interference, the data rate for these transmissions is slow—only 50bps— so a receiver takes up to 12. 5 minutes to obtain all the information it needs to perform a location fix. Networks often assist cellphones by providing this information over much-faster cellular links, 9 but cellphone manufacturers are apparently looking to other means for quick, accurate location fixes for their subscribers.

This brings us to the April 2011 ker-
fuffle over Apple’s and Google’s use of
cellphones to identify Wi-Fi and cell-
tower locations. In testimony before
the U.S. Congress’s Judiciary Com-
mittee’s Subcommittee on Privacy,
Technology and the Law, Guy Tribble,
Apple’s vice president for software
technology, confirmed what analysts
of the consolidated.db file had already
determined: Apple iPhones record
the MAC address and signal strengthe
for detected access points, then time-
stamp and geo-tag that data. The geo-
tag consists of a GPS/cell-tower-derived
location estimate of the iPhone that
has detected the access point. For de-
tected cell sites, the cell-tower ID and-
signal strength are combined with the
detecting iPhone’s location estimate.

e Signal strength is converted into a “horizontal accuracy number”; Apple does not collect the user-assigned name for the network.

of the measurements. 5 Following creation of a map of the locations of cell sites and access points, a position fix for a cellphone can be computed through trilateration using received signal-strength measurements.

Trilateration is similar to what is performed by GPS receivers, with the added benefit that the distances are much shorter and the access points and cell towers are not moving. Overall, one would expect the resulting location estimates to be at least as good as a GPS fix in urban and residential areas and could be of sufficiently fine granularity as to be able to resolve an individual address.

The presence of consolidated.db in
iPhones (a database of time-stamped
GPS fixes for the cellphone) gives the
appearance that Apple is tracking
iPhone users, but Tribble said the “data
is extracted from the database, encrypt-
ed, and transmitted—anonymously—
to Apple over a Wi-Fi connection every
12 hours (or later if the device does not
have Wi-Fi access at that time).”
The extent the data is anonymous
is questionable without further detail.
The author generated the figure here
using the consolidated.db database on
his iPhone and the iPhone Tracker ap-
plication developed by Pete Warden.f
His well-traveled path from Ithaca, NY,
to Washington, D.C. (National Science
Foundation and Defense Advanced Re-
search Projects Agency) and onward to
his parent’s house in Virginia Beach,
VA, is apparent for all to see. It would
take little effort to associate this trace
with the author. As the Netflix example
covered later suggests, there is more to
anonymization than stripping a loca-
tion trace of its associated phone num-
ber and user-account ID.

 

Personality of Place The iPhone location trace says a lot about the author, including his predilection for visiting Washington, D.C., New York, and his parents. What would a more fine-grain set of tracking data, like that potentially being

 

A cellphone’s travels; data from consolidated.db in the author’s iPhone.

f In an FAQ at http://petewarden.github.com/ iPhoneTracker/\#5, Warden noted that the data is actually more accurate than the maps generated by the tool; Warden inserted the intentional dithering to reduce the privacy risk created by the tool while still making apparent the problem with consolidated.db.