Phased arrays in time and space: A review

Phased arrays can be formed by separating elements in time as well as space. In this configuration, surface or target velocities may be observed, rather than the radar backscatter at each position in an image or area. We have used time series data acquisitions using imaging radars to map and measure mm-scale velocities at m-scale resolutions over areas tens of km in size. Our applications are mainly for geophysical use, where our main results to date are in mapping natural hazards such as earthquakes and volcanos, groundwater management systems, and ice motion in the polar regions. In all of these the fundamental observable is velocity of the surface, at a precision of mm to cm/yr. The data we use are acquired by spaceborne imaging radar systems, typically by a single satellite operating in a repeat orbit. Repeat times for the systems range from 11 to 46 days, so in many cases we analyze data acquired over months to years, an appropriate time scale for most of the processes we study.