RADAR (RAdio Detecting And Ranging) was first employed during World War Two to locate enemy ships and planes. Since that time radar has evolved into the navigation aid primarily used to avoid collisions. Today it's often referred to as ARPA or automatic radar plotting aid and is a combination of radar and computer technologies merged to augment watch standing. There are two basic marine radar frequencies commonly known as "X" and "S" band. "X" band, because of its higher frequency, 10 GHz provides a higher resolution and a crisper image while "S" band, at 3 GHz is less affected by rain and fog. In most situations larger vessels are fitted with both "X" and "S" band radars while smaller vessels will only have an "X" band. Vessels in excess of 300 gross tons are required to have two operational marine radars and one of those radars must be an ARPA. An industry misconception is that if a vessel is fitted with three radars that all three must be operational. This is not true; only two radars, one being an ARPA, are required.