English: Post Instrument, Mk. IIC from 1936. From the collection of the RAF Museum.

The Post Instrument was used by the Royal Observer Corps to automatically convert azimuth and altitude measurements of an aircraft into a map grid coordinate that was then reported to their Observer Center by telephone.

Operators sight along the black arm to point the instrument at the target aircraft. The sights include open iron sights on the top of the arm, and a crosshairs arrangement inside the framework for more accurate measurements once the instrument was correctly positioned.

The vertical angle is controlled by the black knob roughly centered in the image, which turned a gear riding on the rack along the back of the vertical square rod. The markings facing the camera are altitudes, which are read through the "window" cut just above the maker's mark (Pullin of London). As the knob is turned, the assembly it is part of moves up and down. This is connected to the black sighting arm through a small roller, seen just to the left of the knob, which rides within the slot in the arm. A pointer (black) and small scale (sliver/grey) connected to the rear pivot allows the altitude angle to be read.

The square vertical rod is mounted to the light-blue coloured slider that runs fore and aft along the three horizontal brass rods. The slider is positioned using a knurled knob that would normally be located directly below the altitude knob, but is missing in this example (it would be mounted to the square rusted post, just left of mid-frame).

The entire system is mounted to a roughly T-shaped framework tipped with three wheels. The wheels rode on the ring around the map, while a center post in the map kept it from sliding off the map. A pointer directly behind the front wheel (right) could be used to read off angles, but is missing in this example.

To use the Post Instrument, the Observer would first dial in an estimated altitude using the black knob. This would cause the vertical angle of the sights to be set to an initial value. They would then swing the entire instrument in azimuth to find the target aircraft, which would normally be seen above or below the pre-set sight line. They would then turn the second knob (missing) which also causes the sights to move up and down, until the aircraft was centered in the sights. Turning that knob also caused the entire sliding section to move fore and aft, moving the brass pointer. Once the sight was pointed at the target, the Teller (telephone operator) would read off the indicated grid coordinate by looking for the grid location under the pointer.

One problem with with Post Instrument is that its accuracy is entirely dependant on the estimated altitude, which the instrument initially had no way of measuring. This was addressed in 1940 with the introduction of the Micklethwait Height Corrector, which has been added to this example.

The Corrector consisted of a second horizontal slider connected to the left side of the main device, not directly visible in this example, along with an indicator connected to the sighting system through the vertical arm screwed to the front of the sight (light blue, angled off vertical). As the sight was raised or lowered the arm pulled on a long horizontal indicator arm, which is only just visible at the left where it connects to the framework between the wheels (light blue, just off horizontal). The Corrector was read by comparing the position of the horizontal arm against a separate vertical scale, which can be seen extending upward just to the left of the spirit level.

To use the Corrector, the Observer would track an aircraft as normal until is passed over another plotting point, marked on their maps. This example is from a post called "V3", which can be seen marked in the center of the map. The position for V2 is visible on the map closer to the camera, and another post marking can be seen just below the attachment point for the horizontal knob. As soon as the instrument was aligned in azimuth with the marking for one of these posts, say V2, the operator would stop following the target in azimuth and instead turn another knob (not visible here) until the blue pointer lay on the V2 mark. The horizontal arm of the Corrector now lay over a correction, say "+1500", which was reported to the Observer Center and added or subtracted from the original altitude estimate. The Observer Center would then indicate the corrected altitude to all of the posts.

In this case, the difference in position between the two indicator pointers tells us the Observer had to slide the Corrector towards the front of the instrument on the right. This means that the target was spotted at a lower angle than initially estimated, which indicates it was flying at a lower altitude than estimated. The correction would then be sent to the Center, and everyone in the area would reset their Correctors to zero and change the altitude setting to the corrected measurement.

Just to the left of the Corrector's pointer is a circle drawn on the map marked SOUND CIRCLE. This was used to take sound-only measurements, at night or in bad weather. In this case the Observer would first set the range knob (missing) until the brass pointer was directly over the sound circle. They would then swing in azimuth and turn the altitude knob until the sights were aligned with the approximate location of the sound. The azimuth angle and the altitude indicated on the vertical post were then reported. At the Center, an operator would use triangulation from several reports to attempt to determine the location of the target, and its altitude by lookup the measured angle on a chart. In effect, the angle was being reported as the altitude as if the aircraft were at 5 miles range, which the operator had to correct to the actual range.

The two brass clips on the Corrector, one just above the central pivot, appear to be holders for a flashlight for night use. Other images show a similar lamp held on a gooseneck fitting attached to the front (right) pointed in the same direction. The map appears to indicate this post reported to "Group Denby". Also note that angles are measured in 1/600ths of a circle, a short-lived angular measurement system that was replaced post-war by angular mils.