July-August - Air Defense Artillery School

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Improvised Depression Position By Captain David J. Caldwell, Coast <strong>Artillery</strong> Corps EDITOR'S NOTE: Tile method of obtaillillg apprCY.I:imate range as describ~ ill tile folloll'illg article lIatHrally illl'olres certaill assllmptiolls. Ullder some atmospheric mid tide cOllditiollS, large errors ma)' be expect~. The method herein described may be used as an emergency range finding system to serve when the normal range finding equipment of a battery is not being manned, or may be used as an expedient when other range finding equipment is not available. Frequent use may be made of this device in equipping observation posts with a simple yet dependable range finder, thus enhancing the value of intelligence reported. The system is not adapted to stations of low elevation, 150 feet above sea level being the practical minimum. Tests bv the writer over a six months period at a number of different installations, varying in height from 220 feet to 850 feet, have proven its accuracy. Use is made of either a 1910 Al or an M1918 azimuth instrument. The 1910 A 1 instrument, being of heavier construction and more stable, is preferable. The azimuth instrument must have a very stable mounting so that it will continuously remain level and be free from vibration. Its height above sea level must be accurately determined. The operation in brief is as follows: The instrument is focused on the target and the target water-lined with the horizontal cross wire. (The 1910 Al instrument has no horizontal cross wire; however, the tops of the horizontal scale graduations may be used as such). Care being taken not to change the depression angle at which the target was water-lined, the instrument is refocused on a previously calibrated range pole located in the vicinity of instrument, turning the instrument in azimuth if necessary, and the range to the target read on the scale of the range pole with the horizontal cross wire as an index. Preparation and Calibration of the Range Pole: This range finding instrument makes use of the law of similar triangles. In figure 1, I represents the instrument at a height b above sea level E. T represents the target, R the horizontal range to the target and h the combined vertical effect of curvature and refraction; r is the horizontal distance (fixed) from the instrument to the range pole and h' is the vertical distance on the range pole from a point level b E h r ---------..., eveI I :h' I R Fig. I n(in inches) = ( b+h )12 r R Where: b and h are in feel, Rand r in yards. T LEVEL HORIZON RANGE POLE Fig.2 er with the instrument when the telescope is horizontal to the point on the range pole where the line of sight from I toj the water-lined target cuts the range pole. From the law of similar triangles we have the following relations: r R , .. (b + 11I 12 r h' b + h or h (m mches) = R • where r is expressed in yards, b in feet, II in feet and R in yards. . The value of h for anyone particular range may be OIr tained from Table I, Appendix VI, Fi\l 4-10, or may be obtained approximately by substituting in the formula II inI feet equals .185 x R2 where R is expressed in thousands of] yards. pose. Either method is sufficiently accurate for this pur- Various values formula and the of R iue chosen for substitution in th~ corresponding value of h' determined 1'0 each range. The range pole is then calibrated using th values of h' so determined and labeling with the correspond. ing range. The distance to the horizon for the particula instrument height involved (this is the case when b equal h) should be determined by use of the table or formula f vertical effect of curvature and refraction, and the value 0 h' determined so that a "range to horizon" mark may placed on the range pole. After the range pole has been calibrated it is placed a the correct horizontal distance from the instrument (t distance used in the calculation), and adjusted vcrticall until, with the instrument water-lined on the horizon. t line of sight through the instrument intersects the range po 200
-19-13 I?\lPRO\'ISED DEPRESSION POSITION FINDER 53 Jt the graduation placed thereon for distance to the horizon. The range pole is then fastened in place and will read Jccurate ranges as long as its location is not changed and the base of the azimuth instrument is kept le\'el. Rallge 67700 30000 28000 26000 5000 4000 3000 ILLUSTRATIVE EXAMPLE r = 27.2 vds. b = 847 ft. range to horizon-67700 yds. b + h h' 1794.0 8.66" 1013.2 11.02" 991.8 11.56" 971.8 12.20" 851.6 850.0 848.7 55.60" 69.42" 92.39" NOTE: Values for intenllediate ranges should be determined or marked in by interpolation where the distance is small. General Hints: (1) The azimuth instrument must be kept le\"el. (2) The azimuth instrument must be adjusted so that vertical parallax is eliminated with the instrument focused on either the target or the range pole. This can be accomplished with the ~1l91O Al instrument by adjusting the eye piece by trial and error until there is no parallax with the instrument focused on the target or on the range pole (the eye piece adjustment remains fixed and the change of focus obtained by using the focusing nut. See figure 17, page 42, Fi\l 4-15). \Vith the 1918 instrument, parallax may be eliminated when viewing the range pole by holding a small disc, into which has been drilled a small aperture, over the eye piece. (3) In choosing the distance the range pole is to be placed from the instrument, several factors must be considered. Frequently the distance will be governed by available towers, poles, trees or buildings in the vicinity upon which to fasten the pole. In general, the distance r should be determined for the particular height of instrument involved that will result in h' equal to between 90 and 120 inches for a range of 3000 yards, and a location chosen in that neighborhood.
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July-August - Air Defense Artillery School

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