  This work performs the design, real-time hardware realization, and
experimental evaluation of a positioning system by ultra-violet (UV)
communication under photon-level signal detection. The positioning is based on
time-difference of arrival (TDOA) principle. Time division-based transmission
of synchronization sequence from three transmitters with known positions is
applied. We investigate the positioning error via decomposing it into two
parts, the transmitter-side timing error and the receiver-side synchronization
error. The theoretical average error matches well with the simulation results,
which indicates that theoretical fitting can provide reliable guidance and
prediction for hardware experiments. We also conduct real-time hardware
realization of the TDOA-based positioning system using Field Programmable Gate
Array (FPGA), which is experimentally evaluated via outdoor experiments.
Experimental results match well with the theoretical and simulation results.
