Image: GL Mk. IIIc ZPI being raised

Description: Crew raise the antenna boom of the Zone Position Indicator (ZPI), a medium-range radar system that was used for early warning and passed information to a nearby Accurate Position Finder (APF) for target tracking. In the field, both radars would be deployed within tens of meters of each other, powered by a common generator unit set up between them. Once deployed, the horizontal boom on the top of the mast would rotate to scan for targets in all directions. Together, the ZPI and APF formed the Gun Laying radar, Mark IIIc, designed and built in Canada, the country's first locally designed radar system. The APF was based on the cavity magnetron, which had been demonstrated to National Research Council members in late 1940, before the UK designers had delivered their own similar design. Unlike the UK, the Canadians had no medium-range mobile radar systems, so they designed the ZPI to fill that need. Unlike the APF, the ZPI was based on simpler conventional tube-based electronics, operating on the common 1.5 metre band that was widely used by UK radars. The electronics design was adapted from the Air-Surface-Vessel radar design that had been put into production in Toronto for the US Navy and Coast Guard. Since the ZPI and APF worked on very different frequencies, they could be used in close concert withougt causing interference.
Title: GL Mk. IIIc ZPI being raised
Credit: http://www.rcsigs.ca/index.php/Radiolocation_(Radar)_Unit_GL_III(c)
Author: Unknown Canadian Army photographer
Usage Terms: Public domain
License: Public domain
Attribution Required?: No