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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.
In order to compensate for the differences between the electronics of two readers, some companies use complex tuning procedures as part of their project implementation, while others employ patented double-pass measurements that make the system even more expensive. Some systems use ultra-wideband (UWB) technology in order to improve detection range, by utilizing short pulses over a broader spectral range, generally using higher frequencies (3.1 GHz to 10 GHz). All of these technologies suffer from multi-pass beams that reduce their localization accuracy.
TDOA-based systems are definitely more accurate than those based on RSSI. Their accuracy varies between 5 percent and 10 percent of the distance between the readers, depending on the respective systems and environments involved. In practical terms, in an indoor environment with partitions (such as offices or hospitals), four synchronized readers are required to cover approximately 15 rooms. The location accuracy with such systems, for indoor applications with partitions, varies between +/-2 meters (6.6 feet) and +/-3 meters (9.8 feet).
Since the triangulation results are expressed in x,y coordinates, ignoring the partitions, a localization accuracy of +/-2 meters or +/-3 meters can easily lead to localizing a person or object in the incorrect room.
Zoning methods can be used when the localization requirement is for a zone (room-level accuracy). Initially, zoning methods utilized a reader within each zone (room). Each reader's gain is tuned to receive only signals from tags within said zone. When used in open space, however, this method is not very accurate, as it is impossible to delimit the RF zones by virtual lines. Reflection and tag orientation can easily lead to locating the tag in an adjacent zone. Such errors, however, can be reduced by comparing the respective RSSI levels, though they can not be completely eliminated.
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