| This article first appeared in the August 1998 issue of Monitoring Times. |
By the time you finish reading this column more than 500 emergency calls will have been made using a cellular or PCS phone. With more than 60 million wireless subscribers out there, any accident or injury is often reported by passers-by before those involved can get to a payphone. Some automobile manufacturers are even offering cellular telephone packages that can automatically dial 911 in the event of an accident.
In most areas of the country, when a 911 call is made from a traditional wired phone the number and service address are delivered to a Public Safety Answering Point (PSAP), usually staffed by county or municipal emergency operators. Using database information from the phone company, the service address and telephone number appear on a screen in front of an operator, giving him or her the address where the call came from and a call-back number in case the connection is cut off. As you can imagine, getting the number and location of a 911 caller who is using a cellular telephone is much more complicated.
Locating a cellular telephone
As covered in the December 1996 column of PCS Front Line, the fixed portion of cellular network consists of a number of cell sites connected to a mobile telephone switching office (MTSO). Calls from a mobile phone are transmitted by radio to the nearest cell site, where the conversation is passed to the MTSO and on out to the public switched telephone network (PSTN). When a cellular subscriber makes a call to 911, the MTSO must decide how to handle the call. Delivering the call like any other call won't work, since the service address and telephone number that would show up at the PSAP would be for the MTSO, where the landline is connected, not the caller. Also, because cellular systems cover a wide geographic area, the PSAP that serves the MTSO might not be the same one that serves the caller's location.
To remedy these problems the Federal Communications Commission has issued a series of rules to get 911 calls to the right place with the right information. Phase I, which went into effect in April of this year, requires wireless service providers to identify the cell site that a 911 call is coming in over and to deliver the mobile phone number of the caller. In Phase II, scheduled to begin in 2001, service providers must deliver to the PSAP the latitude and longitude of the caller, accurate to at least 410 feet two-thirds of the time. A number of different location strategies have been suggested to meet the Phase II requirements.
Signal Strength
Traditional cellular systems have special receivers at each cell site that measure the signal strength of each mobile telephone operating in their coverage area. When the signal strength falls below a certain level, the system begins a "hand-off" procedure to have a (presumably) closer cell site serve the mobile. By examining the signal strength of a particular mobile a guess can be made as to how far away it is. Handing off the phone to two or more cell sites, each of which makes a guess about distance, can help narrow down the general direction. However, in real-world conditions these methods are not very accurate. Received signal strength is affected by a number of factors besides distance, including blockage from buildings, foliage from trees, and even the orientation of the caller with respect to the cell site. While it's good enough to transfer calls from site to site, signal strength alone will not meet the accuracy requirements of the FCC.
Time Difference of Arrival
Angle of Arrival
We all learned in school that time and distance are related by the formula
distance = speed X time,
so that if we know
how fast something is moving and how long it takes we can determine the
distance it traveled. Time Difference of Arrival (TDOA) schemes use this
basic idea to locate the position of a cellular telephone. Three widely
separated cell sites use a synchronized and extremely accurate clock to
record the exact time a signal arrives. By making educated guesses as to
when the signal might have left the cell phone some distance estimates
can be calculated for each cell site. If we represent these estimates
as circles centered on each of the sites, there will be an area where all
three overlap. By adjusting our starting time guess forward and backward
we can narrow the size of that overlap and pinpoint the caller's location.
Besides a very accurate clock, this method requires a significant amount
of communication between the cell sites and some computer equipment to
perform the necessary computations.
Radio signals are traveling electromagnetic waves. If we receive the
same signal at two different antennas, the waves may be slightly offset
from each other. For example, one antenna may be receiving the peak
of a wave at the same time another antenna is receiving the trough.
This offset is more commonly called a phase difference, and can be
determined with a properly equipped receiver. In fact, some amateur
radio "fox hunting" equipment uses this technique. In Angle of
Arrival (AOA) location systems each cell site is equipped with a special
antenna array consisting of several individual antennas spaced less than
one wavelength apart. Using the phase differences between antennas,
a bearing from the site to the caller can be computed. Two of these
bearings from two different cell sites will (hopefully) cross at the
location of the cellular telephone. On the minus side, AOA equipment
is sensitive to multipath, losing directional accuracy when the signal
bounces off buildings and other reflective objects before arriving at
the antenna array. Like TDOA, it also requires coordinating multiple
cell sites and some sophisticated computing equipment.
Global Positioning System
The United States Air Force operates a fleet of orbiting spacecraft that continuously transmit navigation information. This Global Positioning System uses 24 satellites to provide very accurate "position solutions" to users equipped with a proper receiver. Advances in miniaturization and large production runs have lowered the size and cost of a GPS receiver to the point where it is feasible to install one inside a cellular telephone. GPS does have a number of limitations, however. Besides the additional weight, cost, and battery drain of a GPS receiver, a position solution requires a clear line of sight to at least three satellites. Any shadowing that blocks the reception of GPS signals would render the system inoperative. The downtown area of large cities, for instance, would create problems due to the urban canyon effect of skyscrapers and other tall buildings. In addition, GPS receivers require a certain amount of time to produce a position solution, anywhere from 30 seconds to several minutes from being powered on. This kind of delay may be unacceptable in certain emergency situations.
Multipath Fingerprinting
One idea that's still in the proof-of-concept stage is multipath fingerprinting, sometimes called raytracing. In most location schemes multipath is, at best, a nuisance. Radio signals bounce off billboards, buildings, and other objects, scattering the signal before arriving at the cell site. Multipath fingerprinting counts on this scattering to uniquely identify a location by comparing the received signals to a database of surveyed patterns. If the scattering pattern just received matches a pattern that was previously measured, the assumption is that the caller is located at the same place the stored measurement was taken. This process requires a lot of work beforehand and the matching process can take a long time, but the idea is interesting nonetheless.
Big Brother
Almost everyone agrees that locating a phone during an emergency call is a good thing. What is not so good is that the same technologies that find you for 911 can also find you while you're making other calls, and even when your phone is on but not in use. The implications of this are still being worked out, as civil libertarians argue for strict privacy and law enforcement officials demand access to all location information. Implementation of the 1994 Communications Assistance for Law Enforcement Act (CALEA), which will make wired and wireless communication networks "wiretap-friendly," is a subject for another column, but it appears now the civil libertarians are fighting a losing battle.
Delivering 911
In survey after survey subscribers have reported that safety was the primary reason they signed up for cellular service. Many subscribers have never used their phone, merely keeping it nearby in case they ever need to make an emergency call. This has been a significant source of revenue for service providers, who found themselves on the losing end of a related FCC ruling issued last December.
In the same proceeding that established the Phase I and Phase II 911 caller location requirements, the FCC also ruled that wireless service providers must complete 911 calls to the correct PSAP whether the phone is under a subscription plan or not.
Prior to the ruling many wireless service providers were routinely blocking 911 calls from phones that were not currently subscribed or from roamers whose carriers did not have a local roaming agreement. Other callers were transferred to authorization centers, where they were asked to provide a credit card number or other form of payment. These validation procedures often delayed the arrival of emergency personnel to the scene, and in many cases prevented help from arriving at all. In outlawing such delays and denials of service, the FCC noted that one of their mandates under the Communications Act is "promoting safety of life and property through the use of wire and radio communication."
Working secondhand and older-generation cellular telephones are often available for a few dollars at garage sales, flea markets, and thrift stores. Under FCC rules, these phones are perfectly suited and completely legal for placing 911 calls in times of emergency. These might make nice gifts for spouses or older children who may need to make an emergency call, but who can't afford a cellular subscription.
That's all for this month. As always, more information is available on my website at http://www.decode.com, and I welcome electronic mail at dan@decode.com. Also, a plethora of cellular and PCS information is available in my new book Inside Mobile Telephone Systems, available from Index Publishing at (800) 546-6707. Until next time, happy monitoring!