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| Cellular Tracking More Indepth			      |
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					\  by: Tradeser	     /				  
   "Learn how the PTSN works, don't	 \   tollphree.com  /	 
	rip it off much."		  \________________/

		  

After releasing my text on Methods Used To Track Cellular 
Phones in F41th 12, I was kinda dissapointed on how I didn't
go more indepth on how each tracking method works. This led
me on to a venture to find more information about cellular
tracking. For all of you that don't know, my summer job 
before college was working for a wireless provider in my 
area. This gave me access to a large database of information,
yet getting it would prove hard. After making numerous calls
to our wireless fraud depatment, some information on Measure
Angle Of Arrival and Measuring Time Difference Of Arrival was
faxed to my work place. I have typed up what they sent me. 


[.Snip.]

Measure Angle Of Arrival

Another technique that uses existing mobile device signals to
estimate location is known as angle of arrival or direction 
finding. Angle of arrival has been well developed among 
military and government organizations since it operates with
no modification to mobile devices. The technique requires a 
complex antenna array  an arrangement of several antennae 
in a precise, fixed pattern  at cell site locations. Antenna
arrays, in principle, work together to determine the angle 
(relative to the cell site) from which a mobile signal 
originated. When angles of arrival are computed for several 
cell sites, the mobile unit’s location can be estimated based 
on the point of intersection of projected lines drawn out 
from the cell sites at the angle from which the signal 
originated. An angle of arrival system can perform well 
when tracking a continuous transmission, such as a voice 
transmission. The system must follow each voice channel 
assignment as a caller moves from cell to cell, and the call
is handed off from channel to channel. This can be difficult
if the angle of arrival antennae are not positioned to 
interpret the in-band voice channel signaling. It is more 
difficult to use angle of arrival to compute the location 
of a mobile unit emitting brief (one-tenth of one second) 
reverse control transmissions. It may also be difficult to 
use angle of arrival based on digital voice channel 
transmissions and data transmissions because of the brevity 
of the signals and the channel sharing that exists. Another 
significant drawback to angle of arrival systems is the 
logistical and aesthetic dilemma of adding antenna arrays 
(that can number from four to 12, depending on the angular 
resolution required) to cell sites at a time when communities 
are enacting increasingly harsh zoning regulations. Also the 
accuracy of angle of arrival systems is reduced as a mobile 
unit moves away from a cell site. To illustrate this problem, 
think of cutting a triangular pizza slice. The transmitter is 
moving away from the tip of the slice. As it moves farther away, 
the ambiguity (or width) of the pizza slice becomes greater even 
though the angle of the cut remains constant. Angle of arrival 
also is extremely sensitive to wide angle reflections (known 
as multipath reflections) that occur when a mobile device’s 
transmissions reflect off natural and man-made structures, 
particularly buildings and mountains. These reflections can 
have power stronger than the direct path taken by the signal 
from the mobile unit to the antenna array. These multipath 
reflections can trick an antenna array into calculating an 
incorrect angle. For this reason, continuous reverse voice 
channel transmissions are more desirable than brief reverse 
control channel transmissions for angle of arrival systems 
because continuous reverse voice transmissions give angle of 
arrival systems time to attempt to resolve ambiguities due to 
multipath reflections. Angle of arrival technology may lend 
itself to the future use of smart antennae, which are compact 
antenna arrays that shape the cell site transmitter and receiver 
signals into a beam that focuses on the mobile unit and ignores 
other signals. Although potentially expensive, these antennae 
could improve call quality.


[.Snip.]

Measuring Time Difference Of Arrival

Time difference of arrival is another well known technique used 
for determining the locations of mobile devices. Time 
difference of arrival has been used since radar systems were 
first invented over 50 years ago and is used with GPS technology 
today. It is well suited to estimating location of all wireless 
devices because it works with both brief transmissions, such as 
the reverse control channel, and with longer transmissions, such 
as the reverse voice channel. These systems work on the basis of 
a highly precise timing of a mobile unit’s signal as the 
transmission is received at various cell sites. From the precise 
timing, appropriate triangulation can be performed to estimate 
position, as well as speed and direction of travel. In contrast 
to angle of arrival systems, the distance from the transmitter 
to any cell site is not a factor in accurate timing, and 
therefore does not degrade accuracy. Also, in contrast to both 
measured power and angle of arrival systems, time difference of 
arrival systems do not require that the signal be received at any 
appreciable power level (i.e. relative to the background noise 
level in the wireless band).

For many applications, time difference of arrival offers many 
positive benefits. The system requires no modifications to 
existing mobile devices, regardless of modulation protocol. Thus 
all of the existing 22 million8 analog cellular telephones could 
be supported. As with other locating techniques, time difference 
of arrival systems typically require the addition of new equipment 
at cell sites though existing antennae can be used in many cases. 
Where existing antennae are not available, simple whip antennae 
(i.e. the type of cellular antennae on most car windows) can be 
used.

Because antenna requirements are simple and unobtrusive, time 
difference of arrival receivers can be put in many locations, 
including areas without cell sites. This might be advantageous if
one wanted to improve location estimates in a particular area and 
a regular cell site was not required or desired. This simpler 
configuration has the added benefit of lower implementation cost 
compared to angle of arrival systems.

Though time difference of arrival systems are also affected by the 
same multipath reflections that impair angle of arrival systems, 
they are affected to a lesser degree because of the superior 
timing resolution and frequency resolution characteristics of the 
technique. It is generally considered easier to measure time 
precisely than to measure angle precisely.

The lower implementation cost of time difference of arrival systems 
permits receiver installation at more cell sites, which leads to 
a statistical averaging of the multipath reflections,9 especially 
required in urban environments where there is a greater density of 
man-made structures.

Like angle of arrival, time difference of arrival systems are best 
suited for applications requiring the location information at a 
central site. Unmodified mobile devices are currently not capable 
of displaying position, but the central site can forward the 
information to a data receiver.

This wide range of available technology choices each offers at least 
one technical fit for each desired application whether it be for an 
E-911 system, Billing by Location, fraud detection and prosecution, 
System Planning and Design or the hundreds of new applications that 
will become available in the next decade.