1. Technical Field
The invention relates to a position determination system for determining the position of one or more mobile units with respect to one or more fixed position base units.
2. Background Information
Indoor real-time location systems (RTLS) can operate with different levels of accuracy depending on the system infrastructure. In general, a real time location system can provide a 3 dimensional position (e.g. x, y and z coordinates) using for example triangulation on ultrasound or radio frequency (RF) signal amplitudes (e.g. from a Wi-Fi infrastructure). However, when deployed in a single floor, it is not normally possible to determine an accurate vertical (z) coordinate. An accurate vertical coordinate can be obtained by installing significant additional infrastructure. For example, with additional infrastructure, three dimensional coordinates can be determined to centimeter accuracy. However this additional infrastructure can be very costly.
In some scenarios there is high value in fixing the position along the vertical axis. For example in a home care or hospital setting it is essential to quickly establish if a patient has collapsed or fallen on the ground. It is sufficient to know which room the patient is in when lying on the floor, i.e. the horizontal (x, y) plane does not need to be determined to high accuracy, while the vertical (z) axis needs to differentiate between the patient lying on the floor and sitting in a chair. This is contrary to the accuracy of a normal indoor real time location system which provides high accuracy in the horizontal plane while providing low accuracy in the vertical axis.
Accelerometers may appear appealing at first sight, but such an inertial system also requires attitude estimation (using gyros) and becomes rapidly complex. An inertial system would also need to be on all the time in order to integrate the acceleration to achieve velocity and position and would thus draw too much power for a battery powered tag.