Large phased‐array radars can play a very important part in arms control. They can be used to determine the number of RVs being deployed, the type of targeting of the RVs (the same or different targets), the shape of the deployed objects, and possibly the weight and yields of the deployed RVs. They can provide this information at night as well as during the day and during rain and cloud covered conditions. The radar can be on the ground, on a ship, in an airplane, or space‐borne. Airborne and space‐borne radars can provide high resolution map images of the ground for reconnaissance, of anti‐ballistic missile (ABM) ground radar installations, missile launch sites, and tactical targets such as trucks and tanks. The large ground based radars can have microwave carrier frequencies or be at HF (high frequency). For a ground‐based HF radar the signal is reflected off the ionosphere so as to provide over‐the‐horizon (OTH) viewing of targets. OTH radars can potentially be used to monitor stealth targets and missile traffic.