Air-search radar of some kind... NOT for night fighters, not the right shape...
Aha! Automatic Gun Laying Turret, "Village Inn" used by Halifax/Lancaster bombers!
Cool. Thought it deserved a bit more detail from the wiki page:
he Automatic Gun-Laying Turret (AGLT) was a British radar-aimed FN121 turret fitted to some Lancaster and Halifax bombers in 1944. The AGLT system was devised to allow a target to be tracked and fired-on in total darkness, the target's range being accurately computed as well as allowing for lead and bullet drop. The system was referred to by the codename Village Inn during development.
The system, known as TR3548, was devised by a team led by Dr Philip Dee and designed under the aegis of chief designer Dr Alan Hodgkin, after receiving a request from the Air Ministry for such a system in early 1943. Village Inn was evaluated and tested by the Telecommunications Research Establishment (TRE) at RAF Defford using the Lancaster Mark I ND712 Lancaster Mark III JB705 and Mk II LL736 and LL737 and subsequently put into production.
The system consisted of a transmitter/receiver unit mounted in the navigator's compartment, operating through a conical scanning parabolic aerial attached to the standard Fraser-Nash turret. It worked on a wavelength of 9.1 cm (3 GHz) with a pulse repetition frequency of 660 hertz. The magnetron used was the CV.186 of approx 35kW. The electronics sent a signal back to the turret, where it was displayed on a cathode ray tube (CRT) display screen positioned adjacent to the gun sight, the image of which was projected on to the Mark IIC gyro gunsight via a semi-transparent mirror.
Initially, ranging information was provided only at the transmitter situated in the navigator's compartment and was read off to the gunner over the intercom, the gunner using foot pedals to set the target range on the sight. In production equipment the process was made automatic, the range information being fed electronically directly into the gunsight, with the navigator's "running commentary" only being retained for the benefit of the rest of the crew. The gunner simply maneuvered his guns to place the "blip" in the centre of the gunsight's reticle, and opened fire when the range was appropriate. Windage, bullet drop and other factors were already calculated by the gunsight.
http://en.wikipedia.org/wiki/Automatic_Gun-Laying_Turret
