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Why Zone-Based Real-time Location Systems Are Superior
For indoor applications, zone-based approaches provide far more reliable results than triangulation-based methods, and at a cheaper cost.
Although manufacturers of such systems try to average their results over several readings, and calibrate the readers upon installation using special installation software, the results remain inaccurate and unstable. A window opening, or a person passing by, can move the result by several meters, despite all of these attempts to overcome the problems inherent to this method.
Some companies utilize Wi-Fi tags and make triangulation calculations according to the RSSI received by different Wi-Fi access points. Such systems employ RSSI triangulation methods, using network access points to read and measure the signal strength.
The main argument in favor of these systems is the supposed use of the existing Wi-Fi network. In real terms, however, the number of additional network points and their respective costs that are necessary in order to reach an accuracy of +/-3 meters (9.8 feet) in indoor environments, with partition, seriously challenge these sales arguments.
Furthermore, numerous articles written by experts note the challenges inherent to deploying Wi-Fi based technology (for examples, see The Impact of Next Gen Wi-Fi Technology on Healthcare and Wi-Fi Location-Based Services—Design and Deployment Considerations).
Time Difference of Arrival (TDOA)
The second approach to localization by triangulation is to compare each signal's time of arrival, rather than their intensities. The underlying principle is simple: If a tag's distance varies between the different readers, the closest reader will receive the signal before the second closest, and so forth. The difference in the time of receiving the signals divided by the speed of light should be an indication of the difference in distances between the tag and the three interrogators, thereby allowing us to calculate the tag's position.
For indoor applications, the expected time differences will be in the order of magnitude of a few nanoseconds. Consequently, the readers must utilize expensive internal crystal clocks, and the local-area network (LAN) must be fully synchronized at the nanosecond level. These requirements inevitably make the system expensive. Furthermore, for indoor applications, more than three readers are necessary in order to obtain reasonably accurate results, even in an area measuring only 20 meters by 20 meters (66 feet by 66 feet)—typically, 10 to 12 rooms, including passages.
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