Techniques for ranging radio beacons have been around for some time. So have the Geometrical methods for locating objects in space based on the ranging measurements. These are the foundations upon which platforms such as GNSS have been built. While platforms such as GNSS provide us with unparalleled visibility into location at outdoor environments they do not address the need for localization at indoor environments where the GNSS signals are too weak. For this reason, much effort has been invested by researchers in the academy and industry to find an optimal solution to the problem of locating objects indoor.
There are various techniques that can be used for ranging a Tag in an RTLS platform. Some techniques rely on the Received Signal Strength Indicator (RSSI) in the receiver to estimate the distance from the transmitter. These techniques rely on the a-priori knowledge of the signal power at the transmitter antenna. Other techniques involve time measurements such as Time of Arrival (ToA) which is a technique used in systems where transmitter and receiver have a perfectly synchronized time reference.
The figure above illustrates an additional technique: Two Way Ranging (TWR). With this technique both the Tag and the Anchor can independently estimate the range between them if they can measure the Time of Flight (ToF) of a signal between the two nodes:
The figure shows how a Tag starts the process by sending a “Poll” message and recording the exact time of transmission as measured by its own clock reference (TSP). The Anchor then records the exact time of arrival of this signal as TRP (using its own reference clock), followed by the transmission of a “Response” signal at time TSR. When the Tag receives the “Response” it records its time of arrival (TRR) and immediately responds with a “Final” signal at TSF. The ToF can be estimate by the Anchor using the following simple relationship:
RTLS systems have been around for over a decade thanks to the ubiquity of wireless communication standards such as WiFi and Bluetooth. Thanks to the recent (2002) authorization by the FCC for unlicensed use of Ultra Wideband Impulse Radio (UWB-IR) technology this unique technology, which is uniquely applicable to ranging applications, is also available for use. Ranging methods such as Time-of-Flight, Angle-of-Arrival and Received Signal Strength Indication are a few of several ranging techniques that have emerged. And not only Radio Frequency techniques have been utilized for ranging. Additional techniques in use are based on Infrared and Ultrasound signalling. The emergence of the above technologies as well as others have been a fertile ground for many innovative products for indoor localization.
There are a host of technologies used for RTLS: WiFi, Bluetooth, UWB-IR, RFID to name a few. Each has it’s own merits and problems and has its “sweet spot” for different applications. The following “spider” diagram illustrates the differences between the technologies with respect to different parameters.
The good news is that we are only at the beginning of the Internet of Things (IoT) revolution which will fuel unprecedented technological and business innovation of which the result would be our ability to seamlessly track the real-time location and monitor other sensory data from almost any object of value around us.