SD radar

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SD radar was an early submarine radar used by the United States during World War II. Developed by the Naval Research Laboratory around 1940–41, it was built by RCA Victor and first introduced in 1941. More than 400 units were made.

What it did
- Installed on submarines to warn of incoming enemy aircraft.
- Used an omnidirectional antenna, so it could detect aircraft but not provide exact bearing to them.
- Rotated to scan around the submarine but didn’t give precise direction to targets.
- Functioned mainly as a proximity detector rather than a targeting tool.
- Because the radar signal could reveal the submarine’s location, it was used sparingly.

How well it worked
- Detection depended on aircraft size and altitude.
- Large planes at about 1,000 feet could be seen from roughly 12–20 miles away.
- Small planes at about 1,000 feet could be detected from about 8–15 miles.
- Planes flying lower than 1,000 feet were harder to detect or unseen.

Technical basics (approximate)
- Frequency: about 114 MHz
- Pulse width: 8.5 microseconds
- Rotation: about 4 times per minute (360° scan)
- Beamwidth: about 40° (horizontal)
- Range: about 20 miles
- Altitude coverage: up to about 1,000 feet
- Power: around 100 kW
- PRF: 60 Hz

Limitations
- The signal could reveal the submarine’s position to enemies, so the radar was used carefully and not continuously.