  Despite the prevalence of GPS services, they still suffer from intermittent
positioning with poor accuracy in partially shadowed regions like urban
canyons, flyover shadows, and factories' indoor areas. Existing wisdom relies
on hardware modifications of GPS receivers or power-hungry infrastructures
requiring continuous plug-in power supply which is hard to provide in outdoor
regions and some factories. This paper fills the gap with GPSMirror, the first
GPS-strengthening system that works for unmodified smartphones with the
assistance of newly-designed GPS backscatter tags. The key enabling techniques
in GPSMirror include: (i) a careful hardware design with microwatt-level power
consumption that pushes the limit of backscatter sensitivity to re-radiate
extremely weak GPS signals with enough coverage approaching the regulation
limit; and (ii) a novel GPS positioning algorithm achieving meter-level
accuracy in shadowed regions as well as expanding locatable regions under
inadequate satellites where conventional algorithms fail. We build a prototype
of the GPSMirror tags and conduct comprehensive experiments to evaluate them.
Our results show that a GPSMirror tag can provide coverage up to 27.7 m.
GPSMirror achieves median positioning accuracy of 3.7 m indoors and 4.6 m in
urban canyon environments, respectively.
