Location systems in real time

Real-time location systems (RTLS) are used to track and identify objects in real time using 'nodes' or 'tags' attached to, or embedded in, the objects tracked and 'readers' that receive and process the wireless signals from these tags to determine their location.

RTLS systems may perform passive or active (automatic) collection of location information. A closely related term is intelligent positioning system (IPS) that continuously determines the position of an object in real time in a given physical space.

Many wireless technologies are used to establish the communication between tags and readers. Most popular ones include Wi-Fi, GPS, infrared, Bluetooth and active and passive RFID systems.

Table 1 summarises the differences of these technologies, their prominent features and the performance that determines their suitability to different applications.

There is no 'one size fits all' solution when it comes to the use of wireless technologies.

	RFID - passive	RFID - active	Bluetooth	Wi-Fi	GPS
Power usage	None	Low to medium	Medium	High	Medium
Data rate	Low	Low to medium	Medium to high	High	Not applicable
Coverage	Low	Medium	High	High	Very high (outdoor)
HW costs	Tags - low Readers - med to high	Medium	Medium	High	High
Security	Limited	Medium	High	High	Not applicable
Major advantage	Low price of tags, no battery required in tag	Low price of reader	Mainstream technology with high uptake	Mainstream technology with high uptake	Long-range coverage
Major disadvantage	Short range (need multiple readers or reference tags required for RTLS)	Tag does require battery (life expected to be up to 1-3 years)	Power consumption (battery life typically 1-2 weeks)	Low accuracy, very high power consumption (battery life typically 1-2 days)	Does not work inside buildings

By positioning the readers at fixed locations in a given environment, the location of the mobile tags is determined by analysing the various aspects of the communications between readers and tags.

Most widely used techniques used for these calculations are distance/angle estimation, position computation and localisation algorithms. The choice of a method depends on the application requirements.

With distance/angle computation, a variety of input data is used, either alone or in combination, in estimating the distance/angle between the tag and the reader.

Angle of arrival (AoA) uses directive antennas or an array of antennas. The method can be effectively applied in combination with RSSI and TDOA distance estimation techniques to reduce the error in position estimation algorithms.

Time (difference) of arrival, TDOA, is the distance between the tag and the reader and is directly proportional to the time taken by the signal to travel between the two. This method is less used due to the precision required with synchronisation of clocks of the devices within the system; however, when implemented correctly it is a more accurate method.

Received signal strength indicator, RSSI, is where the received signal strength is inversely proportional to the square of the distance. This is one of the most commonly implemented techniques, due to its practicality, low cost and availability.

A comparison of these techniques is provided in the following table.

TECHNIQUES	ANGLE OF ARRIVAL	TDOA	RSSI
ANTENNA TYPE	Directional antenna (monopole), multimode antenna.	Any based on requirement.	Monopole, bidirectional, multimode.
ALGORITHMS	Triangulation, along with TDOA applied to array of antenna.	Triangulation/Trilateration. Difference in arrived signal time gives estimated distance.	Triangulation. Received signal strength (RSS) proportional to square of distance.
ACCURACY	Potentially high. Determined by the position of the angles, and the robustness of installation.	Potentially very high dependent on clock synchronisation.	Variable as received signal strength changes with changing energy of transmitted signals.
ADVANTAGES	When used with TDOA, can be an accurate measurement of the position of asset.	Efficiently used by the GPS satellites, US Federal Communication Commission.	Simplest of all implementation techniques. Low cost.
DISADVANTAGES	Costly depending on size of antenna array. Often used with TDOA, this makes cost very high.	Very costly to make use of expensive electronic quartz clocks to maintain synchronisation.	Lower level of accuracy.

Once the readers have individually derived the distance between them and the tag, the next step is to compute the position of the tag within the area. Again, several computation methods are available though the most used include:

Trilateration/multi-trilateration - The tag distance from each reader has been calculated using one of a mix of the methods discussed. A circle is drawn from each reader with the radius equal to the distance of the tag from the respective reader.

If there are three readers, then the three imaginary circles drawn will meet at one or more points. The locality in which there are the most intersections provides the location of the tag.

Triangulation - With the use of angle of arrival, each reader in the network knows the angle between the reader and the tag. The distance between readers is predefined. Hence, using trigonometric identities, the other readers reference their angles to the readers, to calculate the position of the tag.

Probabilistic approaches - With trilateration, this method is based on the use of readers and reference tags and creates a matrix of possible locations which is refined as more data is acquired from other readers about the tag. This method has high computational requirements.

For a large range of applications trilateration using the RSSI algorithm provides sufficient accuracy and provides lowest total cost of ownership. For applications requiring greater accuracy, additional techniques can be applied.

For the end user, this is the most important part of the RTLS as this is the part of the system that displays the location of the tag on a map (2D or 3D).

Clarinox is a company that provides a flexible, robust RTLS software solution that can be rapidly adapted to multiple technologies and used with its own or third-party hardware to meet the needs of the application.

The software solution encompasses middleware, API, XML and web-based user interface.