The Internet of Radar Things

One radar knows only how far away you are: a whole ring of places you might be. A second radar shrinks the ring to two candidates, but a ghost remains. A third pins you down, in an empty world.

The world is not empty. To a millimeter wave, a metal panel, a whiteboard, or a window is a mirror: energy bounces off it specularly and arrives late, as if from a second target standing behind the surface. Turn multipath on and watch the dashed returns agree on a phantom: a mirror-image target that even three honest radars will lock, because naive trilateration cannot tell a reflection from a body.

The mirror cuts both ways. A flat surface glints: a lone radar sees the panel only at one specular point, the white notch straight across from it, which is why walls all but vanish from single-radar point clouds. Let the radars coordinate, with each one listening to the others' echoes, and every pair contributes another specular point: the network images the red stretch of the panel that no radar could see alone.

Both tricks, rejecting the phantom and imaging the surface, take more than geometry: coherent phase across devices, Doppler consistency, polarization. Off-the-shelf radars share no cables, no clock, and no common oscillator, so making them cooperate at that level, turning cheap, independent radars into one coherent aperture, is the core problem of this thrust.