| Country of origin | UK |
|---|---|
| Introduced | Mk. I late 1939 Mk. I* early 1941 |
| 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 |
| Other Names | Radar, Anti-Aircraft No. 1, Mk. 1 |
 GL Mk. II transmitter van | |
| Country of origin | UK |
|---|---|
| Introduced | late 1941 |
| 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 |
| Other Names | Radar, Anti-Aircraft No. 1, Mk. 2, SON-2 |
Radar, Gun Laying, Mark I, or GL Mk. I for short, was an early 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 first GL set was an elementary design developed from 1935 onward. Based on Chain Home, 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 antenna produced a signal that was semi-directional and was only capable of providing accurate slant range information; target bearing accuracy was approximately 20 degrees, and it could not provide target elevation information. A number were deployed with the British Expeditionary Force and at least one was captured by German forces during the Dunkirk evacuation. Their evaluation led them to believe that British radar was much less advanced than German radar.
Plans to introduce the Mk. II with accurate bearing and elevation were underway from the start, but these would not be available until 1940. An expedient solution was the GL/EF attachment, providing 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 ten-fold 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. 1,679 Mk. IIs were ultimately produced.
The introduction of the cavity magnetron in 1940 led to a new design effort using highly directional parabolic antennas to allow both ranging and accurate bearing measurements while being much more compact. These GL Mk. III radar units were produced in the UK as the Mk. IIIB (for British), 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 were generally replaced starting in 1944 by the superior SCR-584.