| Country of origin | UK |
|---|---|
| Manufacturer | EKCO |
| Introduced | 1955 (testing) |
| Frequency | X band |
| RPM | 1,000 |
| Range | 25 mi (40 km) |
| Diameter | 18 in (460 mm) |
| Azimuth | 45º |
| Power | 200 kW |
| Other Names | Green Willow |
| Related | Red Steer |
Radar, Aircraft Interception, Mark 20, or AI.20 for short, also known by its rainbow codename Green Willow, was a prototype aircraft interception (AI) radar developed by EKCO for the English Electric Lightning interceptor aircraft. It was ordered as a backup system in case the more advanced AIRPASS radar from Ferranti failed to develop, but when AIRPASS entered testing in 1955, AI.20 was dropped. The system was used, almost unchanged, as the basis for the Red Steer tail warning radar used in the Handley Page Victor and Avro Vulcan strategic bombers.