SJ radar

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SJ radar was a 10-centimeter S-band radar used on United States submarines in World War II. It was a surface-search radar with limited ability to detect aircraft. It gave direction and distance to surface ships and low-flying planes, and could also be used for simple submarine-to-submarine communications with a telegraph key, providing secure point-to-point signaling in a wolf pack.

Technical details:
- Wavelength: 10 cm (S-band)
- Pulse repetition frequency: about 1,350–1,650 pps
- Beamwidth: 7.5° × 13°
- Pulse width: 1 microsecond

Introduction and service:
- Prototypes appeared December 1941; common by mid-1942; all submarines had it by early 1943.
- It was the second submarine radar, an improvement over the earlier SD model.
- A variant of the SG radar used on surface ships, adapted to fit through a hatch and sit close to the curved hull.

Legacy:
- Working examples exist on museum submarines USS Cobia (SS-245) and USS Cod (SS-224).