Software defined radio has been an enabling technology for geophysical radar remote sensing in the last few decades. Software defined radio has made it easier than ever to implement complicated radar signal processing algorithms with minimal engineering effort. We will discuss several case studies including: passive radar, spread spectrum radar, MIMO radar, and synthetic aperture radar. We will discuss how software defined radars can be used to study meteors, the Earth’s ionosphere, space debris, and the Moon. While some of the applications require high power and large aperture, some of them can be implemented with a single antenna and a few milliwatts of transmit power. We will discuss radars ranging from inexpensive RTL dongle based passive radars to using the Arecibo Observatory radar to study the Earth’s ionosphere.