Editing Chain Home (section)

===Distance and bearing measurement===
[[File:Chain Home screen shot -NEDAD.2013.047.058A.jpg|thumb|right|Chain Home display showing several target ''blips'' between 15 and 30 miles distant from the station. The marker at the top of the screen was used to send the range to the fruit machine.]]
[[File:WAAF radar operator Denise Miley plotting aircraft on a cathode ray tube in the Receiver Room at Bawdsey 'Chain Home' station, May 1945. CH15332.jpg|thumb|right|The operator display of the CH system was a complex affair. The large knob on the left is the goniometer control with the ''sense'' button that made the antenna more directional.]]

Determining the location in space of a given blip was a complex multi-step process. First the operator would select a set of receiver antennas using the motorized switch, feeding signals to the receiver system. The antennas were connected together in pairs, forming two directional antennas, sensitive primarily along the X and Y axes respectively, Y being the line of shoot. The operator would then "swing the gonio", or "hunt", back and forth until the selected blip reached its minimum deflection on this display (or maximum, at 90 degrees off). The operator would measure the distance against the scale, and then tell the plotter the range and bearing of the selected target. The operator would then select a different blip on the display and repeat the process. For targets at different altitudes, the operator might have to try different antennas to maximize the signal.{{sfn|Neale|1985|p=75}}

On the receipt of a set of [[polar coordinates]] from the radar operator, the plotter's task was to convert these to X and Y locations on a map. They were provided with large maps of their operational area printed on lightweight paper so they could be stored for future reference. A rotating straightedge with the centrepoint at the radar's location on the map was fixed on top, so when the operator called an angle the plotter would rotate the straightedge to that angle, look along it to pick off the range, and plot a point. The range called from the operator is the line-of-sight range, or ''[[slant range]]'', not the over-ground distance from the station. To calculate the actual location over the ground, the altitude also had to be measured (see below) and then calculated using simple [[trigonometry]]. A variety of calculators and aids were used to help in this calculation step.

As the plotter worked, the targets would be updated over time, causing a series of marks, or ''plots'', to appear that indicated the targets' direction of motion, or ''track''. ''Track-tellers'' standing around the map would then relay this information via telephone to the filter room at [[RAF Bentley Priory]], where a dedicated telephone operator relayed that information to plotters on a much larger map. In this way the reports from multiple stations were re-created into a single overall view.<ref>{{cite web |url=http://www.raf.mod.uk/history/fightercontrolsystem.cfm |title=The RAF Fighter Control System |publisher=RAF |date=6 December 2012 |access-date=10 February 2013 |archive-url=https://web.archive.org/web/20130118225830/http://www.raf.mod.uk/history/fightercontrolsystem.cfm |archive-date=18 January 2013 |url-status=dead}}</ref>

Due to differences in reception patterns between stations, as well as differences in received signals from different directions even at a single station, the reported locations varied from the target's real location by a varying amount. The same target as reported from two different stations could appear in very different locations on the filter room's plot. It was the job of the filter room to recognize these were actually the same plot, and re-combine them into a single track. From then on each track was identified by a number, which would be used for all future communications. When first reported the tracks were given an "X" prefix, and then "H" for Hostile or "F" for friendly once identified.{{sfn|Neale|1985|p=81}}{{efn|Other codes may have been used as well, this is not intended to be an exhaustive list.}} This data was then sent down the telephone network to the Group and Section headquarters where the plots were again re-created for local control over the fighters.

The data also went sideways to other defence units such as [[Royal Navy]], Army anti-aircraft gun sites, and RAF [[barrage balloon]] operations. There was also comprehensive liaison with the civil authorities, principally [[Air Raid Precautions]].