Page 5 - Royal Observer Corps

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. The bright flash from a nuclear explosion would burn a mark on
one or two of the papers within the drum. The position of the burn spot enabled the
bearing and height of the burst to be estimated. With triangulation between
neighbouring posts, these readings would give an accurate height and position.

The altitude of the explosion was important because a ground or near ground burst
would produce radioactive fallout, whereas an air burst would produce only short
distance and short lived initial radiations (but no fallout). Once combined with the
peak-overpressure readings from post Bomb Power Indicator readings the power of
the burst in megatons could also be calculated by the Triangulation Team in the
group control building, using a hand held plastic calculator device.

Operations

Normally stored below ground the drums were mounted at the start of exercises or at
Transition to War on a special above ground mount set in the concrete of the
entrance hatch. Concentric lugs on the mount ensured that the drum was mounted
correctly even in the dark.

The light sensitive photographic paper was unfixed chemically and had to be
protected from daylight by being carried in a protective pouch strapped to the
observer's chest. The cassettes were always lodged in a specific order so they could
be changed in the pitch black with practice. Differing notched cut-outs at the bottom
of the cassettes ensured they could not be mounted at the incorrect cardinal point.

Daylight inevitably darkened the unfixed papers so routine changes were made at
mid-day during winter operations and twice daily, at mid-day and sunset, during
summer months. The sun burned a distinctive sun trail across the papers when there
was no cloud cover.
Sixty seconds after any reading on the Bomb Power Indicator an observer exited the
post and changed the GZI cassettes. The exposed papers were returned
underground for assessment. The pre-exposed papers had a graticule grid exposed
on the papers that showed degrees of bearing. The fireball from any nuclear burst
within range would have burned a mark on the paper. The spot size and bearing
would be reported to the group control together with an indication that the spot is
touching or clear of the horizon, essential for indicating an air or ground burst.