SJ Surface Search Radar

Photograph of SJ surface search radar antenna

National Archives #19-N-40140


Wavelength 10 cm
Pulse Width 1 microsecond
Pulse Repetition Frequency     
1350 to 1650 Hz
Power 50 kW
Range 12 nautical miles (22 km) on battleship
8 nautical miles (15 km) on destroyer
A scope (early production)
PPI scope
Accuracy 15 yards/0.3 degrees
14 meters/0.3 degrees
Resolution 50 yards/5 degrees
46 meters/5 degrees
Production: 300 built by 1945-2. All U.S. submarines were fitted with SJ by early 1943.

SJ was a submarine radar. The first few were equipped with A-displays, but the remainder had PPI displays. Early versions were usable only when surfaced, but the ST modification converted SJ to a periscope radar that could provide accurate range information to a submerged submarine.

This radar was treasured by submariners. Though designed for surface search, it was capable of detecting bombers at a useful range.

IFF equipment began to be added to SJ in late 1943. This was crucial in allowing submarines to perform lifeguard duty without excessive danger of friendly fire incidents.


Alden (1979)

Friedman (1981)

Morison (1950)

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