Real time location system
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Real Time Locating Systems (RTLS), incorrectly named Real Time Location Systems, are used to track and identify the location of objects in real time using simple, inexpensive nodes (badges/tags) attached to or embedded in objects and devices (readers) that receive the wireless signals from these tags to determine their locations. RTLS typically refers to systems that provide passive (automatic) collection of location information.
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[edit] RTLS performance measurement terminology
- Granularity - the degree to which reported location information is accurate
- Currency - the degree to which reported location information is timely
- Stability - the degree to which reported location information is repeatable over time
For an RTLS to function, the location of tagged items must be determined either by a central processor or by an embedded mobile computing facility. Locating is generally accomplished in one of the following ways[verification needed]:
- 1. ID Signals from nodes are identifiable to a single reader in a sensory network thus indicating the coincidence of reader and nodes.
- 2. ID Signals from nodes are picked up by a multiplicity of readers in a sensory network and a position is estimated using one or more locating algorithms (see Methodologies Used in Real Time Location Systems listed below)
- 3. Location Signals from signposts with identifiers are transmitted to the moving nodes and are then relayed, usually via a second wireless channel, to a location processor.
- 4. Mobile nodes communicate with each other and perform metering distances. For details see Real Time Locating Systems.
Examples one (1) and three (3) have much of the same characteristics. They typically require that a node be assigned at a time to a single reader/signpost. Separation from overlapping readers/signposts is roughly provided by RSSI or Physical Space Division (walls/floors/ceilings). Readers/signposts are often associated with highly stable location boundaries (i.e. a room or room division). In these examples, locations are listed as "Current Location" or "Last Known Location."
Example two (2) requires that distances between nodes in the sensory network be determined in order to precisely locate a node. In this instance, the determination of the location is called Localization. The location is calculated through Trilateration or Multilateration from the determined distance between the nodes or through Triangulation from the determined angles between nodes. The determination of distances is called Ranging.
[edit] Types of Technologies Used in Real Time Locating Systems
Types of technologies used in Real Time Locating Systems include:
- Active Radio Frequency Identification (Active RFID)
- Active Radio Frequency Identification - Infrared Hybrid (Active RFID-IR)
- Infrared (IR)
- Optical Locating[verification needed]
- Low-frequency Signpost Identification
- Semi-Active Radio Frequency Identification (semi-active RFID)
- Radio Beacon
- Ultrasound Identification (USID)[verification needed]
- Ultrasonic Ranging
- Ultrawide Band (UWB)
- Wireless Local Area Network (WLAN, Wi-Fi)[verification needed]
[edit] Methodologies Used in Real Time Locating Systems
Methodologies used in Real Time Locating Systems include:
- Angle of Arrival (AoA)
- Line of Sight (LoS)
- Time of Arrival (ToA)
- Time Difference of Arrival (TDoA)
- Received Channel Power Indicator (RCPI)
- Received Signal Strength Indication (RSSI)
- Time of Flight (ToF)
- Two Way Ranging (TWR)[verification needed]
- Symmetrical Double Sided - Two Way Ranging (SDS-TWR)
- Near-field electromagnetic ranging (NFER)
