Page 32 - Summer2020

ARTILLERY LOCATION IN WWI
second and a maximum firing range of 11 kilometers. It could be fired at a rate of 1 to 2 rounds per minute. These guns could be positioned at larger distances from the line of fighting and behind obstacles such as forests or hills.
WWI began as a war of mobility, with seven German armies advancing rapidly through Belgium and into France in August 1914. Their progress was impeded by the French, with the help of the British Expedition- ary Force (BEF), before they were able to invade Paris. For the next four years, the Allied and German armies defended lines of demarcation that roughly ran from the North Sea near the border between France and Belgium to the corner between France, Germany, and Switzerland in what became known as trench warfare. Machine guns kept troops in their trenches and the fronts stationary so that the most effective way to inflict injury on the enemy was to lob shells at them from behind one’s own line.
Methods of Artillery Location
All sides in WWI recognized the need for locating artil- lery fairly quickly. Three basic methods were used: aerial location, flash ranging, and sound ranging. Aerial location was performed by using both observation balloons and airplanes. Aerial photography made great strides during this period and provided valuable intelligence for locat- ing enemy positions and for map making. However, these advances came with risks because as airplanes developed, so did antiaircraft artillery. Also, aerial surveillance was not effective in conditions of fog or rain, which were not uncommon. Observation balloons bobbed around in the atmosphere, making it difficult to measure bearings accu- rately. In addition, each side would hide or camouflage its batteries to avoid detection or set out decoys.
Flash ranging, developed alongside sound ranging, was another method for locating artillery. Three to four observation posts were ideally surveyed 1,400-1,800 meters from the front lines along a 3- to 4-kilometer base. Efforts were made to conceal these posts from the enemy, but concealment was not always possible. There had to be a sufficient distance between each observer for accurate triangulation. The observer had a survey instru- ment at his position, similar to a transit or a theodolite, and a phone line hardwired to the switchboard at “Flash Central.” The observer would press a key or button that was part of the phone set when he saw the flash from an enemy gun. If multiple observers pressed the key at
the same time, the operator at Central could be confi- dent they were observing the same flash. The observers measured the bearings from their position to the flash of the observed gun and reported them to Central through the telephone. On a map board at Central, the observ- ers’ positions had been accurately plotted. The operator at Central stretched catgut strings from the observers’ positions on the board along the corresponding bearings so that the intersection of the strings from the different observation posts provided the location of the gun or the battery. Captain (later Colonel) Harold Hemming, a Canadian gunnery officer posted to Ranging and Survey, is credited with the development and advancement of flash ranging. Flash and sound ranging were developed at the same time and complemented each other (Bragg et al., 1971; Mitchell, 2012).
Beginnings of Sound Ranging
When the war started, Charles Nordmann was a profes- sor of astronomy at the Paris Observatory in Meudon, France. He was serving at the front in the French Army in 1914 when he conceived the idea of sound ranging and obtained permission to test it. He sought the assistance of Lucien Bull, who had been developing galvanometers for electrocardiography at the Institute of Marey in Paris (Van der Kloot, 2005; Mitchell, 2012).
Nordmann determined that a gun could be located by measuring the time differences of arrival of the sound from a gun to different observation positions. Nord- mann and Bull conducted sound ranging experiments to test this idea in late November 1914. Two guns were located in St. Cloud, a village located on the west side of Paris. In one approach, human observers, “tappers,” would press a key similar to the ones used by the flash- ranging observers when they heard the guns fire. They also used stopwatches to record the time from the flash of the muzzle to the time they heard the sound. In a sepa- rate approach, four microphones placed along a baseline of 4,500 meters recorded the signals. The tests proved successful, and the guns were located with an error of 40 meters in range and 20 meters in bearing. The human observers were within 0.05 second of the microphones. The “tappers” were deployed to Arras, Belgium, near the British line, to establish sound-ranging (SR) sections. Because the method was subjective and lacked precision, the French continued to develop and improve systems using microphones (MacLeod, 2000).
32 Acoustics Today • Summer 2020