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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Post Instrument | 2/3 | https://en.wikipedia.org/wiki/Post_Instrument | reference | science, encyclopedia | 2026-05-05T09:43:29.277739+00:00 | kb-cron |
== Micklethwait Height Corrector == Wartime models were modified in 1940 with the Micklethwait Height Corrector, named for its inventor, Eric Walter Eustace Micklethwait. Micklethwait was an observer at the Gower Street Post on the roof of a building at University of London, near Euston Square tube station. Formerly a patent clerk, he devised the Corrector and quickly patented it. The Corrector consisted of a second map pointer on a second horizontal slider, with a crank that moved the horizontal slider fore and aft. A second arm suspended from the main sight tube was pushed up and down as the horizontal portion slid. This arm was measured against a short vertical bar marked with altitude corrections. The system indicated only corrections, not the actual altitude. Two or more posts had to work in concert to use the system, using two measured angles and simple trigonometry to solve the altitude. The simplest measurement took place when an aircraft flew directly over one of the posts. Other posts that could see the same aircraft would continue to track the target as normal, set to whatever altitude they had initially estimated. When the first post called that the aircraft was directly overhead, the other posts would crank the Corrector until its pointer lay over a marking for the other post printed on the map. For instance, if the original estimate was 10,000 feet (3,000 m) and the aircraft was actually at 11,000 feet (3,400 m), the operator at a second post would set his instrument to 10,000 and continue to measure as normal until they heard the first post call "aircraft overhead". At this point they would stop measuring the aircraft and instead crank the Corrector until its pointer lay over the marking on the map indicating the location of the first post. By comparing the position of the bar suspended from the sighting tube to the Corrector's vertical scale, they would see it indicate +1000. This correction was called to the ROC center who then forwarded it to all the posts in the area to update their altitude settings to 11,000 feet. When the target did not pass directly over a post the calculation was somewhat more complex. In this case two posts would measure the location of the aircraft at the same time, and then one would call the aircraft's measured grid reference to the other. The second would then place a ruler on the map lying along the line from their measured location on the map to the one called in from the other post. They would then crank the Corrector until its pointer lay directly above the nearest point on the ruler, and the correction could then be read off as normal. An alternate procedure involved the use of an operator at the observer center. Gathering indicated grid locations from several sites set to the same arbitrary altitude, they would triangulate the grid location of the target and pass this information back to the posts. The operators at the posts would then crank the Corrector until its pointer lay over the calculated grid location, at which point the correction could be read.
== Sound measures == The Post Instrument was introduced in an era when sound locating was still common, and some techniques for measuring the angle by sound were developed. This basically consisted of moving the horizontal slider until the indicator pointer was over the "sound line", a circle on the map representing a 5 miles (8.0 km) distance around the post. The operator would then rotate the sights horizontally and vertically to try to point the sights in the direction they estimated the sound to be coming from. Instead of using the map, the operator instead called the horizontal and vertical angles to their operations room. The horizontal angle could be read off a scale around the outer edge of the map, but the vertical angle was instead measured by dropping the last three zeros of the altitude measurement, so if the sights were over the 14,000 foot marker, they would call in "angle 14". In the operations room, a plotter trained in sound measurements would take the angle measurements from multiple stations and determine the location by plotting the angles on a map and looking for the intersections. They would then calculate the distance from one or more of the posts and calculate the altitude using the formula altitude = angle x calculated distance ÷ 5. For instance, if they found that the aircraft was 4 miles (6.4 km) from a particular post that indicated the angle was 14, then the altitude would be 14 x 4 / 5 = 56/5 = about 11,000 feet. Plotters were equipped with pre-calculated tables to make these calculations quickly.
== Reporting system ==