The No 4 MK1 Zone Position Indicator (ZPI) radar was the early warning component of the Canadian GL III(c) radar system.
It provided the approximate range and bearing of incoming targets to the APF (accurate position finder) No3 Mk1 to the left.
The ZPI would track all aircraft within range. Data for the most threatening aircraft was passed to the APF radar, which would then follow the target accurately enough to allow the anti-aircraft guns to engage. The ZPI would continue to track other incoming aircraft and “hand them off” to the APF as each engagement was completed.
Range: 45.72 miles (73.54 kilometres)
Detachment: (GL III system) Two radar trailers, power generator, two towing vehicles and 30 gunners