  We demonstrated a simple technique for monitoring range variation with
millimeter-precision between two remote sites using off-the-shelf wireless
communication modules. The need for the flexible positioning of wireless
devices is significantly increasing as more devices are being connected and new
services are being developed that require devices to collaborate with one
another. We showed that one can monitor the distance variation by analyzing the
propagation delay of the wireless communication signal between devices. We
previously reported a technique for synchronizing clocks with picosecond
precision by monitoring the time variation of two rubidium clocks located at
remote sites. Precise measurement of the propagation time variation was
necessary for precise synchronization of the clocks, and we used this
information to estimate the distance with high precision. In a localized
situation, our technique makes it easy to implement a millimeter-precision
measurement system. Furthermore, it is less complex in terms of system design
and can be a low-cost alternative to existing systems that require precise
position measurement. We envision that this demonstrated protocol will be
implemented in wireless communication chips and microprocessing units.
