  Accurate localization of a large number of objects over a wide area is one of
the keys to the pervasive interaction with the Internet of Things. This paper
presents Hawkeye, a new mmWave backscatter that, for the first time, offers
over (i) hundred-scale simultaneous 3D localization at (ii) subcentimeter
accuracy for over an (iii) hectometer distance. Hawkeye generally applies to
indoors and outdoors as well as under mobility. Hawkeye tag's Van Atta Array
design with retro-reflectivity in both elevation and azimuth planes offers 3D
localization and effectively suppresses the multipath. Hawkeye localization
algorithm is a lightweight signal processing compatible with the commodity FMCW
radar. It uniquely leverages the interplay between the tag signal and clutter,
and leverages the spetral leakage for fine-grained positioning. Prototype
evaluations in corridor, lecture room, and soccer field reveal 6.7 mm median
accuracy at 160 m range, and simultaneously localizes 100 tags in only 33.2 ms.
Hawkeye is reliable under temperature change with significant oscillator
frequency offset.
