Ground Zero Indicator

It consisted of four horizontally mounted cardinal compass point pinhole cameras within a metal drum.

[1] The GZI consisted of four horizontally mounted cardinal compass point pinhole cameras within a white enamelled metal drum, each 'camera' contained a sheet of photosensitive paper mounted within a clear plastic cassette on which were printed horizontal and vertical calibration lines delineating compass bearing and elevation above the horizon.

Differing notched cutouts at the bottom of the cassettes ensured they could not be mounted at the incorrect cardinal point.

Sixty seconds after any reading on the Bomb Power Indicator an observer exited the post and changed the GZI cassettes.

Observers trained blindfolded so that they could exit the post up the vertical steel ladder and complete a GZI cassette change quickly and even in the pitch dark of a winter's night.

Ground Zero Indicator, showing the interior graticules (and the insert at bottom left shows the instrument mounted with its cover in place)