The mobile phone as a platform for assisting the independent living ...

correlation between distance and signal strength is not sufficiently high because of avariety of radio artifacts, as well implementation features such as automatic powermanagement by the hardware based on signal strength. By very carefully controlling thetransmit power management features, it may be possible to obtain a mean absolutepositioning accuracy of 1.2 m.Fig. 6: Trilateration example 6Multilateration-based localization is based on measuring the time intervals between thetransmission of a pulse from the traveller and its and reception at multiple receivers.Such measurements require features not supported in the low-cost commodity devices.Further, even if such features are available, the Bluetooth specification permits a clockjitter of 10 microseconds, which translates into a measurement error of roughly threekilometres, making it unsuitable for most localization applications. However,multilateration is an attractive alternative when using slower signals, such as sound.Time-of-flight applies by recording the time it takes for a signal to propagate from atransmitter to a receiver at a known velocity. Since this method involves timemeasurement, a specific clock resolution must be attainable by the system according tothe timing technology used. Ultrasound based systems do not require a very goodresolution as the waves travel at the speed of sound so the differences to be measured areusually in the milliseconds range. On the other hand, systems using RF signals such asWLAN and Bluetooth would need to have a much higher clock resolution since radiowaves travel at the speed of light so the time difference is usually in the nanosecondrange which is 6 times less in the order of magnitude than the ultrasound basedtechnologies.6 Image courtesy of Wikimedia Commons, http://commons.wikimedia.org/wiki/File:Trilateration.png26