Concept

Mortars, using indirect fire, became a major threat to infantry in World War II. It was found that mortar bombs in flight could be detected and tracked by radar. US and UK anti-aircraft radars were used and specialised mortar locating radars appeared at the end of the war, and were used in Korea with varying degrees of success. Hostile mortars had to be accurately located before they could be attacked with indirect fire from guns or mortars. Since hostile mortars moved frequently to avoid return fire it was essential to have a means of locating them to a few tens of metres of accuracy and to be able to respond quickly when they are located.

Previous radar systems used parabolic reflectors or similar systems to produce a narrow beam of radio energy rather like a flashlight beam. This beam was then swung around the sky by moving the entire reflector, with returns, or blips, appearing on the displays when an object was caught in the beam. For tracking mortar shells this was a particularly difficult task, requiring the operator to have the antenna pointed in roughly the right location by estimates based on previous rounds, and then following the shell through its trajectory. Finding was made a bit easier if the beam cone had a large angle, the problem with this was that it reduced the accuracy of location.

The key advance in tracking mortar shells was the Foster scanner, a type of radar antenna. Instead of producing a beam of radio energy, the Foster scanner produced a fan (pie-slice shape). In the case of the Green Archer, the scanner was built in a manner to produce a beam that was less than 1° wide, but rapidly scanned across a 40° wide band in front of the radar. Any object in the scanner’s view would appear on the display each time the beam crossed its horizontal bearing. To measure the vertical angle, some other system was required.

Green Archer solved this problem by quickly moving the antenna between two set vertical angles. The scanner was first set so it scanned back and forth near the horizon line. When a mortar shell was seen on the display, the operator used a grease pencil to mark its location. He then pressed a button that quickly raised the scanner so it was pointed at a higher vertical angle. This happened rapidly enough that the bomb would take some time to reach this higher altitude, at which point it would appear on the display again and this second location would also be marked. The operator them placed cursors over the marks and input the plot to the radar’s analogue computer.

These two plots, the time between them and the angle between the two beam positions gave two points on a parabolic curve. Such a curve is defined by two points and is a good approximation of a mortar bomb trajectory. Using these, the azimuth of the radar beam centre and the radar’s coordinates, the mortar position coordinates were calculated. These could be adjusted to reflect the actual height of the ground.