Chain Home Low (CHL) was the name of a British early warning radar system operated by the RAF during World War II.

Robert Watson Watt seized several dozen of the Coastal Defense (CD) systems that were in final construction and installed them at CH stations and key locations along the seashore to fill this critical gap in the coverage.

CHL remained an important part of the Chain network for the rest of the war, and was retained in the post-war era until it was replaced during the ROTOR upgrades by the AMES Type 80.

Developed by a team at Bawdsey Manor led by "Taffy" Bowen, the new radar had to operate at much shorter wavelengths in order to limit the antenna sizes to something that could be practically fit on an aeroplane.

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 resulting 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 (26,000 ft).

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 realised were the cranes at the Harwich docks, miles away.

On 3 September the team's test aircraft, Avro Anson K6260, detected several Royal Navy ships in the Channel, and the next day repeated this performance in spite of almost completely overcast skies.

"[3] The British Army was actually the first to consider radar, when Alan Butement and P. E. Pollard submitted a paper in 1931 suggesting using pulses of radio signal to measure the distance to ships.

In October 1936 a liaison team led by Edward Paris and Albert Beaumont Wood was set up at Bawdsey, officially known as the Military Applications Section, but universally referred to as the "Army Cell".

They revisited their original concept to develop Coast Defence radar (CD), allowing the Army's coastal artillery to aim their guns at night or in fog.

This was due to the minimum angle of the CH being about 1.5 degrees above the horizon, which meant aircraft were below the radar's sight until they approached within a few miles.

In keeping with its rapid introduction, CHL was a relatively simple manually-directed system that required the operator to hunt for targets by swinging the antenna back and forth looking for returns.

This major embarrassment was due largely to supremely effective jamming on the part of the Germans, who managed to render the CH and CHL radars covering the coast entirely ineffective without the operators even noticing.

[citation needed] To address this, the RAF began development of the AMES Type 11, a truck-mounted CHL system operating at 500–600 MHz.