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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Post Instrument | 1/3 | https://en.wikipedia.org/wiki/Post_Instrument | reference | science, encyclopedia | 2026-05-05T09:43:29.277739+00:00 | kb-cron |
The Post Plotting Instrument, or simply Post Instrument and sometimes the Observer Instrument, was the standard optical sighting system used by the UK's Royal Observer Corps (ROC) to determine the location of aircraft. It was used during the period from the mid-1930s into the early 1950s, and was one of the main sources of daytime tracking information during World War II. There were two versions of the Post Instrument, a pre-war model using a pantograph, and a wartime version of somewhat more sophistication. Both required the operator to estimate the altitude of the aircraft and enter that into the device, then point a mechanical indicator, or sight, at the aircraft. The motion of the sight moved an indicator on a small Ordnance Survey National Grid map. The grid location indicated by the pointer was then telephoned to central control rooms, where several such reports were combined to produce a more accurate location estimate. Later models added the Micklethwait Height Corrector, which allowed the posts to measure altitude with some accuracy and thus improve the quality of the measurements. The ROC also developed a methodology that allowed the Post Instrument to be used to produce measurements purely by sound, but it is unclear how often this was used.
== Background == Prior to the introduction of radar, optical tracking systems of widely varying complexity were commonly used to spot and report aircraft positions. The Post Instrument was intended to be at the simple end of the scale, an inexpensive and easy to use instrument to make rough but rapid measurements of the locations of aircraft. Post Instruments were installed at hundreds of observation posts across the UK, typically in small groups of three or four posts about 3 to 5 miles (4.8 to 8.0 km) apart. This spacing allowed the operators to cross check each other's altitude measurements. Each post was normally manned by two or three operators, one operating the Post Instrument, another using the telephone to report the locations to a plotting center, and the third, if present, operating as a lookout and helper.
== Pre-war model == The original Post Instrument was mounted on a metal rod extending vertically from the centre of a circular table. A small section of a map showing the surrounding area was attached to the tabletop. The instrument itself consisted of an open rectangle of metal bars, with the long axis horizontal. Hinges at the connection points between the bars allowed the bars to be rotated to form various parallelograms. Similar hinges were located at the midpoints of the long horizontal bars of the rectangle. These midpoint pivots connected to the vertical bar on the table. The result was a pantograph that allowed the long horizontal bars to be rotated into the vertical to point upward at an aircraft, sighting along the upper bar. A final piece was a separate vertical bar connected to the two horizontals and pivoted in the same way so that it remained pointing vertically as the horizontal bars were rotated. This bar was able to be moved along the horizontal bars, fore and aft, which was used to adjust the estimated altitude. To use the system, the operator would first estimate the altitude of the target aircraft and then move the smaller pointer to that altitude as measured against a scale on the upper horizontal arm. They would then rotate the apparatus around the vertical shaft so the target aircraft lay along the line of the upper bar and then rotate the bar vertically until it pointed at the aircraft. The vertical pointer now pointed to a grid location on the map, which could be read off to the reporting centre.
== Wartime model ==
The original model worked but was somewhat difficult and time consuming to use. Just prior to the war a new version was introduced that was easier to use. Officially known as the Observer Instrument, Mark 2, the first examples were built by R.B. Pullin & Co., starting in 1934. The vertical rod of the original version was replaced by a horizontal framework, roughly T-shaped, that was suspended above the table on three wheels running on a metal track around the rim of the map. This provided a much more robust framework for holding the sighting system, and rotated much more smoothly. A pointer behind the front wheel made it easy to read off the bearing, when required. Travelling along the framework horizontally fore and aft was a sliding mechanism that held the sights. This formed the altitude adjustment that would be set prior to sighting. The map pointer was connected to the bottom of the slider. The sights, in the form of an open-framework tube containing a crosshairs, was mounted to the horizontal slider on a vertical square tube. Sightings could be taken either through the crosshairs or along open sights on the top of the tube. A geared rack running down the back of the tube held the sights at a selected angle, and the angle was adjusted by rotating a geared knob on the right side of the sights. As the sights were rotated upwards, they forced the horizontal slider to the rear, moving the pointer over the map.