Chain Home Low

Chain Home Low (CHL) was the name of a British early warning radar system operated by the RAF during World War II. The name refers to CHL's ability to detect aircraft flying at altitudes below the capabilities of the original Chain Home radars, where most CHL radars were co-located. CHL could reliably detect aircraft flying as low as 500 feet (150 m). The official name was AMES Type 2, referring to the Air Ministry Experimental Station at Bawdsey Manor where it was developed, but this name was almost never used in practice.

CHL traces its origins to early experiments with Airborne Interception radar systems in 1936. These were developed as a short-range radar that would be used to close the gap between Chain Home's (CH) approximate 5 miles (8.0 km) accuracy and the visual range of a night fighter pilot at about 1,000 yards (0.91 km). Developed by a second team at Bawdsey Manor led by "Taffy" Bowen, the new radar had to operate at much smaller wavelengths in order to limit the antenna sizes to something that could be practically fit on an airplane. After considerable experimentation, the team settled on a set working at 1.5 meter wavelength, about 193 MHz in the VHF band.

In early experiments with the new set, the team found that detection of other aircraft was problematic due to their target's relatively small size, but especially the very strong returns from the ground. The latter caused a very strong signal that appeared to be at a range equal to the aircraft's current altitude, and everything beyond that was invisible in the clutter. This meant that a typical night bombing run by German aircraft at 15,000 feet (4,600 m) altitude would only become visible at that range, far less than the desired minimum of 5 miles (about 26,400 feet).

These same experiments demonstrated an unexpected side-effect. As the aircraft flew around over Bawdsey, which is located on the coast of the English Channel, the team found strong constant returns that they later realized were the cranes at the Harwich Docks, miles away. Other smaller returns were quickly identified as boats in the Channel. These were being detected at ranges far beyond the maximum range against aircraft, in spite of the antennas not being designed for this role.

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Wikipedia