| Country of origin | UK |
|---|---|
| Introduced | Mk. I late 1939 Mk. I* early 1941 |
| No. built | 410 |
| Type | AA direction |
| Frequency | 54.5 to 85.7 MHz |
| PRF | 1.5 kHz |
| Pulsewidth | 3 μs |
| Azimuth | ±20° from current bearing |
| Precision | 50 m in range |
| Power | 50 kW peak |
| Other Names | Radar, Anti-Aircraft No. 1, Mk. 1 |
 GL Mk. II transmitter van | |
| Country of origin | UK |
|---|---|
| Introduced | late 1941 |
| No. built | 1,679 |
| Type | AA direction |
| Frequency | 54.5 to 85.7 MHz |
| PRF | 1 to 2.5 kHz |
| Pulsewidth | 1 to 1.2 μs |
| Range | 50,000 yd (46 km) detection 30,000 yd (27 km) tracking 14,000 yd (13,000 m) gun direction |
| Azimuth | ±20° from current bearing |
| Elevation | 15–45° |
| Precision | 50 m (55 yd) in range, under 0.5° directionally |
| Power | 150 kW peak |
| Other Names | Radar, Anti-Aircraft No. 1, Mk. 2, SON-2 |
Radar, Gun Laying, Mark I, or GL Mk. I for short, was a pre-World War II radar system developed by the British Army to provide range information to associated anti-aircraft artillery. There were two upgrades to the same basic system, GL/EF (Elevation Finder) and GL Mk. II, both of which added the ability to accurately determine bearing and elevation. The name refers to the radar's ability to direct the guns onto a target, known as gun laying.
The first GL set was an elementary design developed from 1936 onward. Based on the early Chain Home radar's electronics, GL used separate transmitters and receivers located in wooden cabins mounted on gun carriages, each with its own antennas that had to be rotated to point at the target. The transmitted signal was quite wide, in a fan shape about 120 degrees across. This made it useful only for measuring slant range information; target bearing accuracy was approximately 20 degrees, and it could not provide any elevation information. Several were deployed with the British Expeditionary Force and at least one was captured by German forces during the Dunkirk evacuation. The subsequent German evaluation led them to believe that British radar was much less advanced than German radar.
Plans to address these shortcomings were underway even as the first Mk. I units were reaching service in 1939, but these Mk. II units would not be available until 1940 at the earliest. An expedient solution was the GL/EF attachment, which provided bearing and elevation measurements accurate to about a degree. With these improvements, the number of rounds needed to destroy an aircraft fell to 4,100, a tenfold improvement over early-war results. About 410 of the Mk. I and slightly modified Mk. I* units had been produced when production moved to the Mk. II, which had enough accuracy to directly guide the guns. Higher accuracy and simpler operation lowered the rounds-per-kill to 2,750 with Mk. II. After the invasion of the Soviet Union in 1941, about 200 Mk. II units were supplied to the Soviets who used them under the name SON-2. By the end of the production run, 1,679 Mk. IIs had been produced.
The introduction of the cavity magnetron in 1940 led to a new design effort using highly directional parabolic antennas to allow accurate ranging and bearing measurements from much smaller antennas. These GL Mk. III radar units were produced in the UK as the Mk. IIIB and a locally designed model from Canada as the Mk. IIIC. Mk. II remained in service in secondary roles as Mk. III's replaced them at the front. Both of these were replaced by the superior SCR-584 starting in 1944.