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Cellphone users can now be pinpointed with accuracy of a few metres. All
networks can locate a handset to within a few hundred metres by seeing which
base station it is using, and some cellphones have GPS signals that allow them
to be located far more precisely. Now Qualcomm of San Diego has developed a
system that doesn’t need GPS. All phones emit a timing signal, which nearby base
stations retransmit. By measuring the time lag between signals, any GSM
cellphone can work out its position by triangulation and send this information
back to the base station.