FM-3-09 Fire Support and Field Artillery Operations Download
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Denied, Degraded and Disrupted Operations 30 April 2020 FM 3-09 C-7 MEASURING AN AZIMUTH WITH THE M2 COMPASS C-32. To read the azimuth scale by reflection, hold the compass in both hands at eye level with arms braced against body and with the rear sight nearest your eyes. Place the cover at an angle of approximately 45° to the face of the compass so that the scale reflection can be viewed in the mirror. Level the instrument by viewing the circular level in the mirror. Sight on the desired object and read the azimuth indicated on the reflected azimuth scale by the south-seeking (black) end of the compass needle. For more information on this procedure see ATP 3-09.50 C-33. The M2 compass is an alternate instrument for orienting or laying howitzers. The unmounted magnetic compass is a multipurpose instrument used to obtain angle of site and azimuth readings. C-34. This method of lay is used during a deliberate occupation when an aiming circle is not available. The M2 compass is used to lay one howitzer. Once laid for direction, that howitzer is used to reciprocally lay the remaining howitzers. C-35. The steps for laying with the M2 compass are as follows: Place the M2 compass on a stable object (such as a collimator cover, and aiming circle tripod, or a mattock handle) away from magnetic attractions. Measure the azimuth to the pantel of the howitzer to be laid. Determine the deflection to the howitzer by subtracting the azimuth of fire from the measured azimuth. C-36. This method of lay has advantages and disadvantages that must be considered before it is used. The main advantage to using this method is that all howitzers will be oriented parallel. Disadvantages are the compass is graduated every 20 mils and can be read to an accuracy of 10 mils. The accuracy depends a great deal on the ability of the instrument operator. Note: A mil is a unit of measure for angles that is based on the angle subtended by l/6400 of the circumference of a circle (TC 3-09.81). The mil is used because of its accuracy and the mil relation formula, which is based on the assumption that an angle of one mil will subtend an arc of one meter at a distance of 1,000 meters. The graphic representation of a mil is a lower case letter "m" with a virgule (/) through it. MEASURING AN AZIMUTH USING AIMING POINT-DEFLECTION METHOD C-37. The aiming point-deflection method of lay is used during an emergency fire mission when the need to expedite the mission is more critical than the need for first-round accuracy. A distant aiming point is required and must be at least 1,500 meters from the position. All howitzers must be able to pick up a line of sight to the distant aiming point. A compass, or a map and protractor are the only equipment needed. For more information on this procedures see ATP 3-09.50. MEASURING AN AZIMUTH USING COLLIMATOR AND AIMING POSTS C-38. The collimator is the primary aiming point for the howitzer. The emplacement distance for the collimator will vary because of terrain encountered, but is normally between 4 and 15 meters from the howitzer. Displacement is corrected by matching the numbers in the panoramic telescope with the corresponding numbers in the collimator. If the collimator is not emplaced within the distances stated above, the graduations visible in the collimator will not align properly and the sight picture will be out of focus. Therefore, it will be impossible to correct for displacement. C-39. The aiming posts are a secondary aiming point for the howitzer. The aiming posts are emplaced approximately 50 and 100 meters from the gun for the M119-series and M109-series howitzer, or 75 and 150 meters for the M777-series howitzer. The increased spacing of the M777-series howitzer's aiming posts is due to the pivot point of that weapon system. When firing out of traverse missions, the gunner can lose sight of the aiming posts at the shorter distances. Increasing the distance of the aiming posts addresses the problem. For more information on measuring an azimuth using the collimator and aiming posts see ATP 3-09.50.