Millimeter-wave radar

Millimeter-wave radar sends out radio waves only a few millimeters long and listens for the echo. From the echo it works out how far away an object is and how fast it is moving.

Robotaxis use it because radio waves get through rain, fog and snow that blind cameras.

The trade-off is detail: radar sees a few scattered points, not a sharp picture, so it can tell that something is there long before it can tell what it is.