What is the best wireless technology for real time indoor position tracking?

If I want to track a wireless transmitter within a 20ft x 20ft room with an accuracy within 1ft, what is the best wireless technology to use (WiFi, UHF, ultrasonic)? It should also be a best mix of accuracy and lowest cost. The orientation of the transmitter should have no impact on accuracy. Please provide successful example projects.What is the best wireless technology for real time indoor position tracking?
Position fixes rely on trigonometery, so there needs to be range and direction, range and range or direction and direction. More than two gives a better fix.



I don't think there are any off the shelf systems that achieve the goals you state, but I can see some potential for various systems.



The first link is a paper about using WiFi for room level tracking. The device in this case monitors the signal level from several wireless access points (which it can identify). This is a range-range system, based on an estimate of the range from the received signal strength. Accuracy is not likely to be as good as you seek, but it depends on a lot of conditions. The received signal strength of several access points is used to compute range, and the ranges (and locations) are used to calculate the location of the device being tracked. The problem is that the RSSI is not greatly predictable. The expected accuracy with several reasonably located wireless access points is of the order of 2m. By mapping the signal in the room (calibrating the received levels) this can be improved to maybe 0.5m, but that will deteriorate due to movement of people and objects in the room. The second link is the paper on Microsoft "RADAR" which gives other details on such a system.



This approach is applicable to other radio systems like blue tooth. A system could be made using UHF transmitters and receivers. There is no reason it would work any better. The device being tracked could be a transmitter, with receivers at strategic locations, or a receiver with transmitters at strategic locations.



A form of active transponder can improve the ranging dramatically. This uses an FM subcarrier, and the range is determined by measuring the round trip phase of the subcarrier. This can be done by looping the subcarrier back to the master device through the remote device (a transponder). Two way communication is needed. The frequency stability of the subcarrier is important. Assuming the subcarrier phase can be measured to 1 degree (in reality this is easily achieved), and the subcarrier is 10MHz (30m wavelength) the resolution is 0.08m or better. The origins of this are in the third link (see tellurometer block diagram). The idea can be modernised by using a digital sequence (pseudo random code) instead. Unfortunately this system needs two transponders to get range-range. Hope that helps. The same idea can be applied to infra red LEDs but the range is limited to the distance of a few meters. In this case the subcarrier is a pulse rate modulated sine wave shape. Orientation may be an issue though. This may be just right for you.



The ultra sound idea has the potential for sufficient accuracy, but orientation is an issue. The simple ultrasound tape measures from hardware stores (or equivalents with serial ports) could be adapted to make a range-range system.