Early versions of the Lightning were equipped with the Ferranti-developed AI.23 monopulse radar, which was contained right at the front of the fuselage within an inlet cone at the centre of the engine intake. Radar information was displayed on an early head-up display and managed by onboard computers. The AI.23, an immediate predecessor of the AI.24 Foxhunter, supported several operational modes, which included autonomous search, automatic target tracking, and ranging for all weapons; the pilot attack sight provided gyroscopically-derived lead angle and backup stadiametric ranging for gun firing. The radar and gunsight were collectively designated the AIRPASS: Airborne Interception Radar and Pilot Attack Sight System. The radar was successively upgraded with the introduction of more capable Lightning variants, such as to provide guidance for the Red Top missile.