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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.
By Abraham Blonder
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.

Wi-Fi-Based Systems
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.

USER COMMENTS

Glenn Tamir 2011-08-25 01:44:20 PM
The important question however is , does the accuracy meet t It may be possible to achieve incredible levels of accuracy with certain active RFID systems. However, the question that often does NOT get asked is, what is the need of the customer and the application? Does it really matter if one system can show which floor tile an IV pump is sitting on if all the customer needs to know id where it is located in a general area??? It's not about hitting the bullseye but meeting the needs of the customer and the application.
Glenn Tamir 2011-08-26 12:58:08 AM
I Completely agree with this article In this very well-researched article, it is clear that the Second-Generation Zoning method is the least expensive and most reliable system for healthcare applications and other indoor environments. LogiTag Systems, Inc., which has been specializing in RFID technology since 2004, has designed it RTLS system called "LogiTrack" based on this very technology. Thank you for validating that the direction LogiTag has been pursuing is the right course to take. It is reassuring to know this and bodes well for our future as we enter the US Market.
Jack Vandenberghe 2011-10-05 11:12:34 AM
What about for a warehouse? I like this artilce and am just wondering how RTLS could be applied not to single rooms, but to a large warehouse area that is not partioned into rooms?

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