Localisation and positioning are probabilistic

Any system for determining position or location is probabilistic in that the position it computes is based on noisy measurements and uncertainty. There is no such thing as absolute certainty or guarantee of the output. Therefore any application utilising the output of the system should take this probabilistic nature into account. All too often applications have not done this adequately leading to a level of scepticism amongst users, and in some cases mistrust of the systems.

Locating objects requires a system

In most ‘professional’ uses of locating systems it is necessary to place a level of trust in the system and to have confidence that it is correct. For these applications it is often useful to quantify the required system performance:

• What is meant by a location or position? Is it occupancy of a ‘zone’ such as presence in a room, a parking bay or a particular stretch of road? Is the position static or described by a sequence of positions (trajectory)?

• What is the duration of the location ‘event’? What defines the start and end times of the event? How long do we have (realistically) to generate a location output?

• What is the acceptable level of false positive reports? A false positive is when the system reports that an object is in a particular location (for example room or parking bay) but it isn’t.

• What is a tolerable level of failed location reports? This is when the system is unable (or unwilling) to output an object’s location. This is generally a less severe error condition in which the user is told that the system does not know where the object is and has to resort to backup or alternative methods for obtaining the position.

RFID and RTLS

RFID

RFID (Radio Frequency IDentification) systems are systems in which the identity of a tag is read by a reader or interrogator. This is accomplished when the tag is in the vicinity of the reader’s antenna. The reader transmits a radio signal which is picked up by the tag which responds by sending back its identity. Tags may be passive (no battery) or active (containing a battery) and typically range in size from millimetres to centimetres. Passive tags and smaller form-factor tags generally have short reading ranges (centimetres), but active tags may have much longer reading ranges (metres up to several tens of metres). Tags can also be writable allowing the system via the reader to store information in the tag.

Generally RFID tags operate at 13 MHz or in the 840–960 MHz bands (depending on local radio regulations applying in the country of operation). Tags may be very low cost, but readers are relatively high complexity and are correspondingly more expensive.

There are applications in which the use of RFID is perfectly adequate for the purpose of locating or positioning objects. Readers may be placed at strategic ‘pinch points’ throughout the region of interest. Whenever tags move through the pinch points the reader identifies the tag and a software application keeps track of the zone in which each tag is located. These systems are particularly advantageous when there are very large numbers of tags, but relatively few zones (pinch points).