RADAR (Radio Detection And Ranging) was initially installed on US sub's during the latter part of 1941 when the USS Plunger field tested the first version during an actual war patrol. It was a rather primitive and unreliable setup called the SD. Having a limited range (6 - 10 miles), it was non-directional and primarily useful in detecting enemy aircraft.

A major technological improvement came about in 1942 with the development of the SJ RADAR. Designed as a surface search system, it provided exact range and bearing  which provided the navy with the opportunity to redefine many of the tactics employed by the skippers (The most important and beneficial being the night surface attacks).  Further advancement in RADAR technology came about in 1943 when the PPI (Plan Position Indicator) or "improved SJ" system was introduced.

Normally, when one commonly thinks of a RADAR screen, the improved SJ comes to mind with its' large round luminescent screen and glowing radial arm methodically sweeping the dial in a circular motion. The PPI was capable of translating the RADAR pulse information into a picture, with all bearings relative to the submarine which was identified by being  at the center of the screen. When the RADAR detected an object, the signal would be bounced back and the information was shown in the form of a momentary "blip" on the screen. The blip would continue to be visible as long as the RADAR was in the proper range to maintain contact.

ONI reporting on enemy RADAR systems was somewhat spotty. A major concern of the United States submarine force was that prior to the start of the war there was information that the Japanese had developed a reliable and accurate land based radio direction finding (RDF) system in the Pacific. In fact, Japan was well ahead of the United States in that category. The Japanese were detecting and pinpointing US naval vessels well before Pearl Harbor using their RDF. While the Imperial Japanese Navy's ability to detect RADAR signals was well known, it was unclear whether individual  Japanese warships possessed a RADAR system of their own. In fact, the IJN's RADAR appears to have been limited strictly to aircraft carriers and battleships.