SCIENCE OBSERVER
Dial L for Location
David Schneider
In 1993, an 18-year-old woman named Jennifer Koon was violently
abducted at a shopping mall in Rochester, New York. She managed to
call 911 from her hijacked car, but with no way to determine the
geographic position of her cellular phone, police were unable to
intervene during the 20 minutes that elapsed before she was shot to
death by her attacker. Such tragedies helped spur the Federal
Communications Commission to order wireless carriers to install
equipment for locating the source of cellular phone calls to 911, a
process that began in 1996. The last decade has seen considerable
maturation of the relevant technology, including the ability of cell
phones to use the satellite-based Global Positioning System. Still,
the nation's cellular carriers are not likely to meet the FCC
deadline for making such capabilities nearly universal by the end of 2005.
To understand why not demands a short review of regulatory history.
The initial scheme for dealing with 911 calls from cell
phones—Phase I in the FCC's plan—required wireless
carriers only to provide the call-back number of the phone making
the emergency call and the location of the cell tower with which it
is communicating. Phase II mandated that cell-phone companies be
able to locate a caller's position much more precisely, using one of
two basic strategies. They can adopt what the FCC calls "a
network solution," which demands a significant upgrade to the
wireless infrastructure but typically allows subscribers to use
standard phones. Or carriers can implement "a handset
solution," which necessitates less modification to the network
but requires that each user's phone contain special position-sensing
hardware. According to the FCC, a network-based system must be
capable of locating a caller to within 100 meters at least
two-thirds of the time, whereas a handset-based fix must just as
often be accurate to within 50 meters.
Many different approaches are possible. (A nice overview of these
methods can be found, for example, in the March 7, 2002 issue of
EDN magazine, found at http://www.edn.com/contents/images/198901.pdf.)
The simplest, a network-based scheme, is called "time
difference of arrival." It compares when the transmission from
a phone reaches various towers. The difference in arrival times
between two towers is enough to establish that the caller is
positioned somewhere on a specific hyperbola. Two sets of
differences (determined from three towers) fully locate the caller
at the intersection of two hyperbolic curves.
Another network-based solution, called "angle of arrival,"
requires only two towers, but special antennas must be set up to
determine the direction from which the caller's signals emanate.
This information then allows the position of the source to be
determined by simple triangulation.
Remarkably enough, a third network-based method exists that can, at
least in principle, operate with just a single, standard cell tower.
The technique is based on the fact that a phone's transmissions
reach the receiving antenna not just once but many times. The first
signal to arrive is the one that follows a direct path from phone to
tower. Then come a series of signals that have reflected off
buildings, power poles and the like. The peculiar mix of repetitions
and delays received at the tower depends on the set of paths that
the signals follow, which in turn depend on the location of the
phone. Thus one can ascertain the caller's position by comparing the
pattern of signals received with a library of "multipath
fingerprints" and selecting the closest match.
Competing with such network-based approaches are the systems that
rely on special-purpose hardware installed in each user's phone.
One, known as "enhanced observed time difference," relies
on synchronized signals transmitted by each tower. The phone uses
those signals to determine its distance from various towers,
information that it relays back to the network for calculating a
position. Another handset-based solution is to outfit each phone so
that it can sense signals from satellites of the Global Positioning
System. The handsets differ from full-blown GPS receivers in that
they alone cannot calculate positions, because some of the
computational burden is shifted to the cellular network—hence
the name "assisted GPS."
Whereas some carriers have adopted network-based solutions, Verizon
Wireless and the newly merged Sprint Nextel Corporation (which
together serve more than 40 percent of U.S. cell-phone users) use
assisted GPS. Following FCC requirements, these companies must
ensure that 95 percent of their phones are GPS-enabled by the end of
this year. According to documents submitted to the commission, the
management of Verizon concludes that "although it will come
close to the 95% penetration level, it may not reach that milestone
by December 31, 2005." Sprint's statements to the FCC are even
more vague about what levels it expects to achieve. And Nextel is
clearly running behind, with only half of its phones GPS-ready as of
August 2005. As it explained in a recent report to the FCC, the
company suffered a blow last year when it discovered a
"software glitch which effectively dropped Nextel's A-GPS
handset base to zero." It seems that a bug in the software used
in all of its phones rendered them unable to process GPS signals
received after July 18, 2004. Nextel has thus had to mount a massive
campaign to get its customers to have their phones reprogrammed.
But even after all the wireless carriers get their networks and
their customers' phones properly equipped, substantial difficulties
will remain. For one, the FCC requirements pertain only to what
positional information the phone companies must be able to
provide—they don't mandate that the public-safety officials
fielding calls to 911 have the equipment necessary to receive that
information and to act on it. So even if location services on your
phone and wireless network are all up and running, it'll be hit or
miss as to whether the dispatcher on the other end of your emergency
call can determine your position and send help to the right spot.
(The same might be true even after businesses begin monitoring
positional data from cell phones to hawk location-based services, a
worrisome possibility that privacy advocates are now warning about.)
One reason is that the money for upgrades to their 911 systems is
hard for some communities to come by. Although many states charge
cellular-phone users a fee for improvements to the wireless
911 system, these funds are sometimes used for other things. For
example, New York diverts most of these monies to the state's
general fund, for state police or, more recently, to various
"homeland security" programs. Some New Yorkers are,
however, working to straighten out this rather warped system. In
particular, Assemblyman David R. Koon has battled against Governor
George Pataki in an effort that get those funds spent on upgrades
that would allow firefighters, police and paramedics to locate a
cell phone being used to summon emergency aid. Koon is motivated by
more than the usual ambitions of a politician: It was his daughter
who died in 1993 when no one could locate the source of her wireless
911 call.