This paper describes an ADS-B based collision avoidance radar that allows small autonomous UAVs to detect cooperative and non-cooperative targets in air space co-located with manned air. The targeted host platforms are small UAVs with 3.3 m or shorter wing spans. This radar system is effective against all air traffic, with or without transponder-based collision avoidance systems such as TCAS or ADS-B. It is even effective for stationary structures such as high-rise buildings and towers. It is based on the standard ADS-B signal, so no additional spectrum is needed. This system can leverage many commercial off-the-shelf components in existing transponders. Furthermore, it does not require modifications to the existing air traffic control system in the Continental United States or worldwide. Therefore, it should be easier to get the FAA's approval. The development and manufacturing costs are relatively inexpensive.