The AN/SPG-51 is a ship-based, fire control, pulse-doppler radar which has been developed and produced by Raytheon for the US Navy to guide the RIM-66 Standard missiles. The AN/SPG-51 operates in the frequency range between 4 GHz and 10 GHz with a constant performance of 22.5 KW. It is used for targeting, tracking, and guiding surface-to-air missile on Virginia-class cruisers, California-class cruisers, and Kidd-class destroyers. The French Cassard class frigates also employs this radar. Raytheon has produced several versions: 1) AN/SPG-51A, which was the first model introduced in the 1950s; 2) AN/SPG-51B, which is a ground-based variant; 3) AN/SPG-51C, which features full automatic target tracking; 4) AN/SPG-51D, which is a tracking/illumination radar for the Standard missiles, used on Virginia-class cruisers; 5) AN/SPG-51E, which has also the capacity to guide the RIM-8 missiles.