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------------------------------------------------------------- | Cellular Tracking More Indepth | ------------------------------------------------------------- \ 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.