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CHAPTER 4

LAYING THE BATTERY, MEASURING, AND REPORTING



Section I

THE AIMING CIRCLE


4-1. DESCRIPTION OF THE AIMING CIRCLE

The M2A2 aiming circle is the primary means of orienting the cannon battery or laying weapons on the azimuth of fire. This section presents information that should be the starting point for any training program on the aiming circle. The aiming circle components are discussed below.

a. Telescope. The telescope is a four-power, fixed focus optical instrument with a reticle pattern like the one shown in Figure 4-1.

Note: Some aiming circles may not be equipped with the P-2 reticle pattern (national stock number [NSN] 1240-01-152-8516) which is used to perform the POIARIS 2 method of hasty survey. modification can be made at direct support maintenance. Verify the expiration date in the lower right hand corner of the P-2 reticle pattern. If it has expired, turn in the aiming circle to direct support (DS) maintenance for repair.

b. Reflector. The reflector (Figure 4-2) is a plastic signal post mounted on top of the telescope. It is used as an aiming point for other instruments sighting on the aiming circle.

c. Elevation Knob. The elevation knob (Figure 4-3) is used to raise and lower the telescope line of sight. It is also used to measure vertical angles. (See paragraph 4-26.)

(1) Elevation scale. Each graduation on the elevation scale (Figure 4-3) represents 100 mils. The scale is numbered at 100-mil intervals. The black numbers 0 to 1100 indicate elevation (+). The red numbers 0 to 400 indicate depression (-).

(2) Elevation micrometer scale. Each graduation on the elevation micrometer scale (Figure 4-3) represents 1 mil. The scale is numbered at 10-mil intervals. The black numbers 0 to 100 indicate elevation (+). The red numbers 0 to 100 indicate depression (-).

d. Magnetic Compass. The magnetic compass is located in the main housing (Figure 4-3). For rough centering, the magnetic needle may be seen through the windows on top of the body assembly. A small glass magnifier (Figure 4-3) and reticle with three vertical lines are at one end of the recess. These aid in aligning the end of the magnetic needle.

e. Leveling Vials. There are two tubular leveling vials and one circular leveling vial (Figure 4-2) on the aiming circle. One tubular leveling vial (on the left side of the elbow telescope) is used in leveling the telescope so that the operator can measure vertical angles. The other tubular leveling vial (located on the left side of the main housing) or the circular leveling vial (fisheye bubble) is used in leveling the aiming circle for measuring horizontal angles. When the tubular leveling vials are not in use, the protective covers should be closed to prevent damage.

f. Azimuth and Azimuth Micrometer Scales.

(1) The azimuth scale (Figure 4-3) is located below the magnetic compass housing. It is graduated in 100-mil increments from 0 to 6,400 mils and is numbered every 200 mils. The portion of the azimuth scale from 3,200 mils through 6,400 mils has a second 0-3200 scale numbered in red from the black 32 through the large black 0 on the azimuth scale.

Note: Also located on the azimuth scale is an imaginary line called the 0-3200 line. This line starts at the black 0 and goes through the black 32 on this scale. This is the line we orient on the azimuth of fire during the laying process.

(2) On the azimuth scale, the red numbers 0 to 3200 indicate azimuth. The lower row of graduations parallels the 3,200- to 6,400-mil upper row of graduations. This permits the aiming circle to be used with other instruments that have scales labeled from 0 to 3,200 mils. The red numbers are used only in the following instances:

(a) You are reading red numbers to an M12-series sight, and you are measuring a deflection.

(b) You are checking the lay of one aiming circle with a second circle. The operator of only one of the two circles will see red. He can read red to preclude having to add or subtract 3200 to or from the reading.

(3) The azimuth micrometer (Figure 4-3) is located on the azimuth knob on the lower right side of the magnetic needle housing. It is graduated in 1-mil increments from 0 to 100 mils and is numbered every 10 mils. Azimuth micrometer can be read to an accuracy of 0.5 mils.

g. Upper (Recording) Motion. The upper motion allows the operator to place values on the azimuth scale and azimuth micrometer by means of the azimuth knob (Figure 4-3). The values are read on the azimuth scale index, which is located below the magnetic needle magnifier. The upper motion of the instrument has both a fast motion and a slow motion. Pulling back on the azimuth knob enables fast motion. Rotation of the azimuth knob produces slow motion. Horizontal angles are read in two parts--the thousands and hundreds of mils are read from the azimuth scale, and the tens and units of mils are read from the azimuth micrometer.

h. Lower (Nonrecording) Motion. The lower motion is controlled by the orienting knobs (Figure 4-2). It is used to orient the 0-3200 line of the aiming circle without changing the values on the upper motion. Lateral movement of one orienting knob enables fast movement of the lower motion of the aiming circle. The two orienting knobs should be used at the same time for slow movement of the lower motion. Caps are provided for covering the orienting knobs to prevent unintentional use of the lower motion.

i. Leveling Screws. The three leveling screws (Figures 4-2 and 4-3) are used to level the aiming circle. These screws are on a spring plate located below the orienting knobs and above the baseplate assembly.

j. Base Plate Assembly. The base plate assembly (Figure 4-3) serves as the base of the instrument when it is mounted on the tripod and also serves as the base of the carrying case. It is a flat circular plate to which the instrument is attached by means of the spring plate. An instrument-fixing screw is threaded into a socket on the underside of the base plate assembly to attach the instrument to the tripod. The socket is kept clean and free of obstructions by a spring-loaded cover that remains closed when the instrument is not attached to the tripod. The base plate is fitted with a rubber gasket that forms a watertight seal when the cover is latched to the baseplate.

k. Notation Pad. A rectangular notation pad (Figure 4-2) on the baseplate is used for recording the declination constant, date of declination, and initials of the person performing the declination.

l. Filter. The filter (Figure 4-3) is a lens, which is placed over the eyepiece for protection against the sun's rays. It is stored on the side of the telescope body. It is held in place by a spring-loaded ball.

m. Compass Needle Locking Lever. When the locking lever (Figure 4-2) is in a vertical position, the needle is locked. When the lever is turned either right or left to the horizontal position, the needle is unlocked. To preclude damage to the magnetic needle, this lever should be returned to the locked position gently.

CAUTION

The needle must be locked when not in use.

4-2. SETTING UP THE AIMING CIRCLE

a. To setup the aiming circle (Figure 4-4), do the following actions:

(1) Unstrap the legs of the tripod, loosen the leg clamp thumbscrews, and extend the legs to the desired length. Tighten the leg clamp thumbscrews.

(2) Place the tripod over the point to be occupied. One tripod leg should be pointing in the approximate direction of sighting, and the leg with the night-light mount should be to the operator's left. Attach the plumb bob to the hook under the instrument-fixing screw assembly. The plumb bob should be within a 1-inch radius of the station marker.

(3) Firmly embed the tripod legs. Make sure the tripod head is approximately level when the legs are embedded. Then remove the tripod head cover.

(4) Pull back the spring-loaded cover on the base of the baseplate and place the aiming circle on the tripod. Loosely screw the instrument-fixing screw assembly into the base plate.

(5) Center the plumb bob over the orienting station by moving the base plate of the aiming circle.

(6) Tighten the instrument-fixing screw into the baseplate of the aiming circle.

CAUTION

Be careful not to exert excessive pressure when you are tightening the instrument-fixing screw. The slotted arm may bend and damage the tripod head.

(7) Remove the aiming circle head cover, and hang it on the tripod head cover or a leg clamp thumbscrew to prevent damage.

(8) Install the night lighting device accessory case if necessary.

b. Some common malpractices are as follows:

  • Not clearing the area of magnetic attractions such as weapons, steel helmets, and eyeglasses.
  • Failure to set up the tripod so that one leg points in the direction of the sighting. This puts one more tripod leg in the instrument operator's way as he moves around and increases the chance that he will kick a leg and knock the aiming circle off level.

4-3. LEVELING THE AIMING CIRCLE

There are two methods of leveling the aiming circle for normal use. Either the circular leveling vial or the tubular leveling vial (Figure 4-5) can be used.

a. The preferred method is to level the aiming circle by using the circular leveling vial (fisheye bubble) as follows:

(1) Loosen the leveling screws approximately halfway.

(2) Rotate the head of the aiming circle until the circular leveling vial is over the leveling screw adjacent to the notation pad.

(3) Using the thumb and forefinger of each hand, turn the other two leveling screws in opposite directions. The bubble will move in the same direction as the left thumb.

Note: This is known as the left thumb rule.

(4) When the bubble moves on line with the fisheye, center the bubble by using only the third leveling screw. Rotate the head over each of the other two screws. If more than half the bubble moves out of the center ring, relevel the instrument. If the bubble cannot be centered, use the technique discussed in b below. Then turn the instrument in for repair as soon as possible.

b. Level the aiming circle by using the tubular leveling vial as follows:

(1) Loosen the three leveling screws (approximately halfway) to permit the instrument to be leveled. Rotate the instrument until the axis of the tubular leveling vial is parallel to any two of the three leveling screws. Center the bubble by using these two leveling screws. Grasp a screw between the thumb and forefinger of each hand. Turn the screws simultaneously so that your thumbs move either toward each other or away from each other. This movement tightens one screw as it loosens the other. The bubble always moves in the same direction as the left thumb.

(2) Rotate the instrument 1,600 mils, and center the bubble by turning the third leveling screw.

(3) Rotate the instrument back to the first position, and relevel the bubble if necessary.

(4) Repeat these steps until the bubble remains centered in both positions.

(5) Rotate the instrument 3,200 mils from the first position. If the bubble remains centered in this position, rotate the instrument 3,200 mils from the second position. If the bubble remains centered in this position, rotate the instrument throughout 6,400 mils. If the bubble remains centered, the instrument is level. If the bubble does not remain centered when the instrument is rotated 3,200 mils from the first position, the leveling vial is out of adjustment. To compensate, using the same leveling screws that were used to place the instrument in the first position, move the bubble halfway back to the center of the leveling vial. Rotate the instrument 3,200 mils from the second position; and using the other leveling screw, move the bubble halfway back to the center of the level vial. The instrument is now level, and the bubble should come to rest in its vial at the same off-center position (within one graduation) regardless of the direction in which the instrument is pointed. If the leveling vial is out of adjustment, the instrument should be turned in for repair at the first opportunity.

(6) The plumb bob normally remains attached to the aiming circle until the firing unit is laid.

4-4. TAKING DOWN THE AIMING CIRCLE

Take down the aiming circle as follows:

a. Elevate the telescope to about 300 mils.

b. Ensure that the magnetic needle is locked.

c. Cover the tubular leveling vials. Be sure the M51 instrument light is turned off and secured in its case.

d. Ensure the caps of the orienting knobs are closed.

e. Place the azimuth knob over the notation pad.

f. Turn the leveling screws counterclockwise until the screws are to their lower stops. Then loosen each leveling screw knob one-quarter turn.

g. Place the carrying case cover over the aiming circle, and latch the cover locks.

h. Unscrew the instrument-fixing screw, and remove the instrument from the tripod.

i. Replace the tripod head cover.

j. Retract and collapse the tripod legs, and tighten the thumbscrews.

k. Strap the tripod legs together.

4-5. DECLINATING THE AIMING CIRCLE

a. The aiming circle must be declinated when any of the following situations exist:

  • After an electrical storm.
  • Anytime the instrument has received a severe shock; for example, if it is dropped from the bed of a truck to the ground. The magnetic needle is a delicately balanced mechanism, and any shock may cause a significant change in the declination constant.
  • Anytime the aiming circle is moved outside a 25-mile radius from the area in which it was last declinated. Because of local magnetic attractions, any move of the aiming circle may result in an appreciable change in the relationship of grid north and magnetic north as measured by the instrument.
  • A minimum of once every 30 days to determine if any changes in the declination have occurred because of the annual shift of magnetic north or because of accidents involving the instrument that may not have been reported. If a radical change is observed, the instrument should be declinated again within a few days to determine if the observed change is a real change in the characteristics of the instrument.
  • When the instrument is first received.
  • Anytime the instrument is returned from ordnance repair.

b. The aiming circle must be declinated in an area free from magnetic attractions. Azimuths must be known to two or more azimuth marks, preferably in opposite directions. These azimuth marks should be a minimum distance of 300 meters, preferably 1,000 meters.

c. Declinate the aiming circle as follows:

(1) Set up the aiming circle, and level it.

(2) With the upper (recording) motion, set the known azimuth to the azimuth marker.

(3) With the lower nonrecording motion, sight on the azimuth marker that corresponds to the azimuth set with the upper motion. (See Figure 4-6.)

Note: At this time, the 0-3200 line will be aligned with grid north.

(4) Release the magnetic needle. With the upper motion, float and center the magnetic needle. (See Figure 4-7)

(5) Read the declination constant directly from the azimuth scales (to the nearest 0.5 mil).

(6) Using a second azimuth, repeat the above steps. (If a second azimuth marker is not available, use the first marker again.)

(7) Compare the two declination constants determined. If they agree within 2 mils, determine the mean. Express it to the nearest whole mil by using artillery expression. On the notation pad, record the mean (four-digit number), the date, and the initials of the individual performing the declination.

Note: If the two values differ by more than 2 mils, repeat the entire process.

d. A declination station may be established by simultaneous observation, hasty astro, observation of Polaris, or circumpolar observation. (See Chapter 5.)

e. Declination can be performed by scaling a grid azimuth to two distant points. The following procedures are used:

(1) Place the aiming circle over the selected point and level it.

(2) Select two distant points on a map. Scale the direction to each from the occupied point.

(3) Using the direction scaled from the map, declinate the aiming circle by the procedures previously discussed.

(4) Compare the two values determined. They must agree within 10 mils.

(5) If the values determined agree within 10 mils, determine the mean; record it on the notation pad. If the values do not agree within 10 mils, repeat the entire procedure.

Note: A declination constant determined by simultaneous observation or from a map should be verified as soon as possible.

4-6. CARE AND HANDLING OF THE AIMING CIRCLE

a. Protect the aiming circle from shock.

b. Keep the instrument clean and dry.

c. Clean the lens with an optical lens cleaning brush and lens tissue only.

d. Keep the magnetic needle locked anytime it is not in use.

e. Keep the aiming circle head cover over the aiming circle head.

f. Cover all tubular leveling vials.

g. Rotate the azimuth knob until it is over the notation pad before trying to replace the head cover.


Section II

PRECISION LIGHTWEIGHT GPS RECEIVER (PLGR)


4-7. GLOBAL POSITIONING SYSTEM DESCRIPTION

Global positioning system (GPS) is a space-based navigation system which provides worldwide, continuous, all weather, three-dimensional position information. The GPS system consists of the following three primary components:

  • The satellite constellation. Ensures worldwide coverage with a minimum of four satellites within electronic line of sight to any point on the earth.
  • A master control station and five monitoring stations. These provide ground-based support for the satellites.
  • The GPS receivers. These receivers provide the operator with navigational and location data.

Currently, the primary receiver being issued to ground forces is the AN/PSN-11, precision lightweight GPS receiver (PLGR). The PLGR can provide location information precise enough for use by cannon artillery. For detailed information on the GPS/PLGR operation, maintenance, and additional equipment, refer to TM 11-5825-291-13. Major components of the PLGR are shown in Figure 4-8.

4-8. FA SURVEY APPLICATIONS

The PLGR can be used to determine the grid location of the orienting station. Individual howitzer locations can also be determined, but individual PLGR-derived positions are not on common survey control. The following actions must be taken to ensure accuracy when using the PLGR as an artillery positioning device:

a. Verify PLGR Setup.

(1) Crypto. The proper crypto keys must be loaded before the PLGR will function using the precise positioning system (PPS). The PPS must be used to achieve the necessary accuracy and to avoid enemy electronic warfare measures. If the crypto variable is not loaded, the PLGR cannot be used to establish artillery positions.

(2) Datum. Map datum should be the same as the operational datum being used by all other maneuver, fire support and target acquisition units. If the same datum is not used, significant position errors are possible.

(3) Coordinates. The UTM coordinate format is preferred since it is the standard used by survey and most fire control systems.

(4) Elevation. Mean sea level is the preferred selection since most military maps use it as the basis for the elevation scale.

(5) Units of measurement. Meters is the preferred selection since most military maps refer to distance and elevation in meters.

(6) Almanac data. Almanac data must be 1 day old. If almanac data are not 1 day old, there are possible satellite vehicle or timing errors not noted by the PLGR.

b. Figure of Merit (FOM). FOM is an accuracy estimation of the data displayed by the PLGR which ranges from one through nine. FOM 1 is the best accuracy estimation displayed by the system, and FOM 9 is the worst. For artillery positioning, only coordinates determined with a FOM 1 will be considered for use.

c. Mode of Operation. The PLGR offers the choices of FIX, CONTINUOUS, or AVERAGING as modes of operation. The AVERAGING mode yields the most accurate data and is preferred when determining a position for indirect fire weapons.

d. Verify Position. Position verification, to check for unacceptable errors, must always be done prior to firing. The following are possible methods of verifying a PLGR-derived grid coordinate:

(1) Always use two persons to check the PLGR data.

(2) Use two different PLGRs to independently determine the position data.

(3) Use resection or graphic resection if identifiable points are visible.

(4) Conduct a map spot of the location. A map spot is the minimum acceptable verification and should be used in conjunction with the other available means.

WARNING

Azimuth determined with the PLGR is for navigation only The PLGR azimuth is not accurate enough for use in establishing directional control and should never be used for that purpose. Tests show that the PLGR-determined azimuth may be in error by as much as 200 mils.

4-9. GPS LIMITATIONS AND CONSIDERATIONS

a. GPS receivers rely on electronic line of sight with the satellites. The PLGR must be able to acquire at least four satellites before a three dimensional position can be determined. Dense foliage, buildings, mountains, and canyons will mask GPS signal and cause the receiver to fail. All GPS receivers automatically try to track satellites to a position as low as 10 above the level horizon. Each receiver has a function which displays the direction and vertical angle to each of the satellites being tracked. Use this display to determine if signal masking is what is causing the system to fail. When a satellite signal is masked, the operator can either reorient the PLGR antenna or move to another location to improve signal reception.

b. Multipath distortion (reflected signals) may occur if the receivers antenna is tilted away from a satellite. This causes a reflected signal to be received which has more power than the direct signal. Coordinates determined under these conditions can be off by as much as several hundred meters. To correct this problem, reorient the receiver antenna to a position that eliminates the distortion.

c. As with all aspects of artillery operations, personnel must be properly trained in the safe operation of equipment. The PLGR will provide accurate location data only if it is operated in strict accordance with the procedures outlined in the technical manual. Battalions and batteries must ensure that training and testing on GPS operations are part of their safety certification program. Specifically, all battery level leadership must be able to identify, and properly correct when needed, the setup functions of the PLGR. These personnel must also be proficient in map reading and hasty survey techniques needed to conduct an independent verification check of the PLGR-derived position.


SECTION III

GUN LAYING AND POSITIONING SYSTEM


4-10. DESCRIPTION OF THE GUN LAYING AND POSITIONING SYSTEM

The gun laying and positioning system (GLPS) supplements the M2A2 aiming circle, and will be the primary instrument used to orient howitzers in cannon units not equiped with Paladins. When used in conjunction with the PLGR, the GLPS will determine grid location, establish directional control, and allow the operator to transfer directional control to the individual howitzers using standard laying commands. Additionally, the GLPS eye-safe laser range finder eliminates the need to measure subtense to determine the distance to the howitzer.

The system is man-portable and tripod-mounted. It can be emplaced and used much the same as the M2A2 aiming circle, but without the need for external survey support. Technical data on GLPS capabilities are located in Table 4-1, and the major components of the system are shown in Figure 4-9. Battery leadership must be proficient in the setup and orientation procedures for the GLPS as listed in the equipment operator's manual. An independent check of the GLPS orientation must be made before using it to lay the howitzers.


Section IV

THE M2 COMPASS


4-11. DESCRIPTION OF THE M2 COMPASS

The M2 compass (Figure 4-10) is the alternate instrument for orienting, or laying cannons. The unmounted magnetic compass is a multipurpose instrument used to obtain angle of site and azimuth readings. The components of the M2 compass are discussed below.

a. Azimuth Scale. The azimuth scale is numbered every 200 mils from 0 to 6400. The scale is graduated every 20 mils and can be read to an accuracy of 10 mils.

b. Sights. The compass has front and rear leaf sights and a mirror in the cover for sighting and reading angles.

c. Levels. The compass is equipped with a circular level for leveling the instrument before the azimuth values are read. A tubular level is used with the elevation scale to measure angles of site.

d. Angle-of-Site Mechanism. Rotation of the level lever causes the elevation level and the elevation scale index to rotate as a unit. The index clamps against the bottom piece to keep the mechanism from moving unless it is actuated by the level lever.

e. Magnetic Needle and Lifting Mechanism. The magnetic needle (the white end of the needle) shows a magnetic north direction for orienting purposes. The needle is delicately balanced and jewel-mounted, on a pivot, to rotate freely. The magnetic needle reading is taken when the bubble is centered in the circular level. The lifting mechanism includes a needle-lifting (locking) pin and a needle lifting lever. The lower end of the pin engages the lever. The upper end projects slightly above the body of the compass to engage the cover when it is closed. Thus, it automatically lifts the needle from its pivot and holds it firmly against the glass window.

f. Azimuth Scale Adjuster Assembly. The azimuth scale adjuster assembly rotates the azimuth scale to introduce the declination constant. Two teeth at the adjuster engage teeth on the underside of the azimuth scale. Thus, turning the adjuster with a screwdriver rotates the azimuth scale approximately 1,800 mils. The scale is read against a fixed index under the rear sight hinge.

4-12. DECLINATING THE M2 COMPASS

a. The procedure for declinating the M2 compass from a surveyed declination station free from magnetic attractions is as follows:

(1) Set the M2 compass on an aiming circle tripod over the orienting station, and center the circular level.

(2) Sight in on the known, surveyed azimuth marker.

(3) Using the azimuth adjuster scale, rotate the azimuth scale until it indicates the same as the known surveyed azimuth.

(4) Recheck sight picture and azimuth to the known point. Once the sight picture is correct, and the azimuth reading is the same as the surveyed data, the M2 is declinated.

b. The procedure for field-expedient declination of the M2 compass is as follows:

(1) Using the azimuth adjuster scale, set off the grid-magnetic (G-M) angle (shown on the bottom of all military maps).

(2) Once the G-M angle has been set off on the azimuth scale, the M2 compass is declinated.

4-13. MEASURING AN AZIMUTH AND SITE TO CREST WITH THE M2 COMPASS

a. The procedure to measure an azimuth follows:

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 (Figure 4-11) 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.

b. The procedure to measure site to crest follows:

Hold the compass on edge with both hands at eye level with arms braced against the body and with the rear sight nearest your eyes. Place the cover at approximately 45 to the face of the compass (Figure 4-11) so that the elevation scale reflection can be seen in the mirror. Sight on the crest of the highest object in the sector of fire. Center the elevation scale tubular level with the lever on the back of the compass and by viewing the elevation scale in the mirror. Read the elevation in mils on the elevation scale. Measure the sight to crest two additional times, and record the average.

CAUTION

When measuring an azimuth, be sure no magnetic materials are near the compass.

4-14. CARE AND HANDLING OF THE M2 COMPASS

The M2 compass will not stand rough handling or abuse. Keep the compass in the carrying case (with the mirror side facing in) protected from dust and moisture.


Section V

LAYING


4-15. PRINCIPLES OF LAYING

a. Reciprocal laying is a procedure by which the 0-3200 line of one instrument and the 0-3200 line of another instrument are laid parallel. To fully understand the principle behind reciprocal laying (Figure 4-12) you must first understand several concepts and definitions.

(1) As was mentioned previously, the aiming circle is the primary means by which the cannon battery orients weapons on the azimuth of fire. The aiming circle is used to measure horizontal clockwise angles from the line of fire to the line of sight to a given aiming point. In reciprocal laying, the aiming point for the aiming circle is the howitzer panoramic telescope. The pantel is used to measure horizontal clockwise angles from the line of fire or the rearward extension of the line of fire to the line of sight to a given aiming point. In reciprocal laying, the aiming point for the howitzer pantel is the aiming circle.

(2) The line of fire, as it relates to the principle of reciprocal laying, is any line parallel to the azimuth of fire. It is first established by the aiming circle in order that the aiming circle can be used to orient the howitzer on the azimuth of fire.

(3) The rearward extension of the line of fire is the exact opposite of the line of fire.

(4) A deflection is defined as a horizontal clockwise angle measured from the line of fire or the rearward extension line of fire to the line of sight of a given aiming point with the vertex of the angle at the pantel. That is to say, the angular measurements taken when reciprocally laying with the aiming circle and the howitzer pantel will always be deflections.

b. The principle of reciprocal laying is based on the following geometric theorem: given two lines cut by a common transversal whose alternate interior angles are equal, then those two lines are said to be parallel. The parallel lines are the 0-3200 lines of the aiming circle and the line of fire and rearward extension of the line of fire of the howitzer. The common transversal is the line of sight established between the aiming circle and the pantel. The alternate interior angles are the deflections as read from the instruments.

4-16. PROCEDURES FOR LAYING

a. When the 0-3200 line of the aiming circle is parallel to the azimuth of fire, the instrument operator uses the upper motion to sight on the lens of the pantel and reads the deflection on the azimuth and azimuth micrometer scales. He announces the deflection to the gunner on the howitzer.

Note: If the weapons are equipped with M12-series sights, deflection cannot exceed 3,200 mils. Therefore, the red numbers on the aiming circle are used if the black numbers exceed 3,200 mils.

b. The gunner sets the announced deflection on the pantel. He orders the howitzer to be shifted, and/or traverses the tube, until the line of sight through the pantel is again on the reflector of the aiming circle (2-step deflection method).

Note: Because the pantel is not directly over the pivot point of the tube, the pantel displaces horizontally. When the telescope has been sighted on the aiming circle, the gunner reports READY FOR RECHECK.

c. The instrument operator again sights on the lens of the pantel and reads and announces the deflection. This procedure is repeated until the gunner reports a difference of 0 mils between successive deflections. The piece has then been laid.

d. If all pieces are not able to see the aiming circle, one piece can lay others (reciprocal laying). The commands for laying reciprocally from another howitzer are the same as those given in paragraph 4-19. If weapons are equipped with the M100-series panoramic telescopes, the readings between any two weapons will be 3,200 mils apart. To prevent confusion, the gunner on the laying piece adds or subtracts 3,200 mils to or from his reading before announcing it to another howitzer. If the howitzer to which the laying gunner is referring is on his left, the gunner must add 3200. If it is on his right, he must subtract 3200 (left, add; right, subtract [LARS] rule). See Figure 4-13.

e. The methods by which the battery or platoon can be laid are as follows:

(1) Orienting angle. This method requires the use of an M2A2 aiming circle and a known azimuth established by survey.

(2) Grid azimuth. This method requires the use of a declinated M2A2 aiming circle.

(3) M2 compass. This method requires the use of a declinated M2 compass.

(4) Aiming point-deflection. This method requires that an aiming point at least 1,500 meters from the battery or platoon position be visible.

(5) Howitzer back-lay. This method requires the use of a declinated M2 compass and M2A2 aiming circle.

f. The deflection to each weapon should be recorded by the instrument operator for use by the FDC.

Note: The chief of section supervises the laying process and verifies all data after the gunner lays the howitzer.

4-17. LAYING BY ORIENTING ANGLE

a. When occupying a position, it is preferable that all fire support assets have survey data available to them. Common survey enables units to mass fires more accurately. Survey data will not always be available; therefore, all efforts should be made to establish directional control as early as possible.

b. If survey is available, the following, as a minimum, will be provided (Figure 4-14):

  • The orienting station (OS), with grid and altitude to the station.
  • The azimuth of the orienting line (OL).
  • The description of the end of the orienting line (EOL).

c. Steps in orienting angle (OA) method are as follows:

(1) Using the plumb bob, position the aiming circle over the orienting station.

(2) Compute the orienting angle (Figure 4-15) by subtracting the azimuth of fire from the azimuth of the orienting line (add 6,400 mils, if necessary).

(3) With the upper motion, set the orienting angle.

(4) Using the lower motion, sight on the EOL. The 0-3200 line is now parallel to the azimuth of fire.

(5) Using the upper motion, sight on the howitzer pantel.

d. A quick check of survey data prior to laying the battery is recommended, time permitting and METT-T dependent. This check is done in a minimal amount of time and with the same circle which is set up on the OS. Conduct this check in the following manner:

(1) Confirm the location on the OS tag by map spot, GPS, or the most accurate means available.

(2) On the upper motion, set off the declination constant.

(3) On the lower motion, float and center the needle.

(4) On the upper motion, sight in on the EOL. The instrument reading should be the azimuth to the EOL 10 mils.

EXAMPLE

Laying by Orienting Angle:

The battey is being laid on azimuth 0600. The azimuth to the orienting line is 2000. Do the following:

  • Subtract azimuth of fire (0600) from the azimuth of the orienting line (2000). This gives you the orienting angle 1400. (2000 - 0600 = 1400).
  • Set up the aiming circle over the orienting station.
  • Roughly orient the 0-3200 line along the azimuth of fire. Using the upper motion, set off 1400.
  • Using the lower motion, sight on the end of the orienting line. The line of sight now coincides with the orienting line. Make certain that the index remains at 1400.
  • Now the 0-3200 line of the aiming circle has been established in the desired direction; that is, on the azimuth of fire. The next step is to lay the platoon by using the upper motion. This makes the tubes of the howitzers parallel to the 0-3200 line of the instrument. The howitzers are oriented on the azimuth of fire.

e. Survey may not be available. To establish directional control and lay units by the orienting angle method, the orienting line must be established. Without survey, the orienting line can be established by hasty survey techniques. (See Chapter 5). Once the orienting line is established, the steps shown in c above apply. The OS grid location and altitude may be obtained by GPS, map spot or by hasty traverse.

4-18. LAYING BY GRID AZIMUTH

a. If surveyed data are not available and hasty survey is not possible, the next best method of orienting the aiming circle is to use the magnetic needle.

Note: Laying by grid azimuth involves the use of the magnetic needle of the aiming circle. The instrument must be set up where it is free from magnetic attractions. The minimum distances are as follows:

Power lines and electronic equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 meters

Railroad tracks, artillery, tanks, and vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 meters

Barbed wire, personal weapons, and small metallic objects . . . . . . . . . . . . . . . . . 10 meters

b. To orient the aiming circle on a grid azimuth by using the magnetic needle, complete the following steps:

(1) Determine the instrument reading (IR) to be placed on the instrument by subtracting the azimuth of fire from the declination constant (DC) of the aiming circle (add 6,400 mils to the declination constant, if necessary).

(2) Place this value on the upper motion.

(3) Using the lower motion, center the magnetic needle. (See Figure 4-7.) The 0-3200 line is now parallel to the azimuth of fire.

c. The next step is to lay the platoon reciprocally by using the upper motion and sighting on the howitzer pantel.

EXAMPLE

Laying by Grid Azimuth:

The platoon is being laid on azimuth 3900. The DC of the aiming circle is 0400 mils. (See Figure 4-16.)

  • Subtract the azimuth of fire from the declination constant. The larger number (3,900 mils) cannot be subtracted from the smaller number (0400 mils). So 6,400 mils must be added to the smaller number. Thus, 6400 + 0400 = 6800 mils; 6800 - 3900 = 2900. The horizontal, clockwise angle from the desired direction of fire to magnetic north (aiming point) is 2,900 mils.
  • Roughly orient the 0-3200 line along the azimuth of fire.
  • Using the upper motion, set 2900 (black numbers).
  • Center the magnetic needle by using the lower motion. This action places the 0-3200 line of the aiming circle on grid azimuth 3900. Make certain that the index remains at 2900.
  • After establishing the 0-3200 line of the aiming circle on grid azimuth 3900, lay the battery reciprocally by using the upper motion. This procedure places each howitzer tube on grid azimuth 3900.

4-19. COMMANDS

The following commands are used in laying a platoon with an aiming circle. The deflections announced are examples only. For brevity, only the exchange between the instrument operator and the gunner of number 3 is given.

Instrument operator:

PLATOON ADJUST, AIMING POINT THIS INSTRUMENT.

(All gunners identify the aiming point. Normally, pieces are laid in the order of their readiness.)

Gunner of number 3:

NUMBER 3, AIMING POINT IDENTIFIED.

(Using the upper motion, the instrument operator turns the head of the aiming circle until the line of sight is on the pantel of the number 3. He then reads the azimuth and azimuth micrometer scales.)

Instrument operator:

NUMBER 3, DEFLECTION 3091.

Gunner of number 3:

NUMBER 3, DEFLECTION 3091.

(The gunner, using the two-step deflection method, sets the deflection on his sights and traverses the weapon until he is sighted back on the aiming circle.)

Gunner of number 3:

NUMBER 3, READY FOR RECHECK.

(The instrument operator again turns the head of the aiming circle until the line of sight is on the pantel of the number 3 and announces the deflection.)

Instrument operator:

NUMBER 3, DEFLECTION 3093.

Gunner of number 3:

NUMBER 3, DEFLECTION 3093, 2 MILS.

(This indicates a difference of 2 mils from the previous deflection of 3091, which the gunner had placed on the pantel. The gunner sets 3093 on the pantel and traverses the tube until he is sighted on the aiming circle. Meanwhile, the instrument operator may or may not proceed with the laying of the other pieces. When the gunner of any piece announces a difference of 10 mils or less, the instrument operator normally continues to lay that particular piece until it is laid to zero mils.)

The gunner then announces:

NUMBER 3, READY FOR RECHECK

Instrument operator:

NUMBER 3, DEFLECTION 3093.

Gunner of number 3:

NUMBER 3, DEFLECTION 3093, 0 MILS.

(When the deflection announced by the instrument operator and the deflection on the pantel are identical, the howitzer is laid.)

Instrument operator:

NUMBER 3 IS LAID.

4-20. LAYING BY ALTERNATE METHODS

Based on an ever-changing tactical environment, situations could arise that would make it impractical or impossible to use the primary methods of lay (orienting angle and grid azimuth). In such cases, the firing element must be able to keep supporting the maneuver forces. Alternate methods of lay that give the unit this capability are discussed in paragraphs 4-21, 4-22, and 4-23.

4-21. M2 COMPASS METHOD

a. Normally, 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.

b. The steps for laying with the M2 compass areas follows:

(1) 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.

Note: Laying by the M2 compass method involves the use of the magnetic needle of the compass. The compass must be set up where it is free from magnetic attractions. The minimum distances are as follows:

High tension power lines . . . . . . . . . . . . . . . . . . . . . . . 55 meters
Field gun, truck, tank . . . . . . . . . . . . . . . . . . . . . . . . . 10 meters
Telegraph, telephone, or barbed wire . . . . . . . . . . . . . 10 meters
Machine gun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  2 meters
Steel helmet or rifle . . . . . . . . . . . . . . . . . . . . . . . . . .  0.5 meter

(2) Measure the azimuth to the pantel of the howitzer to be laid.

(3) Determine the deflection to the howitzer by subtracting the azimuth of fire from the measured azimuth (memory aid SAM).

Note: Memory aids for laying are in tables 4-2 through 4-5.

(4) Lay the howitzer by using the command NUMBER (so-and-so) ADJUST, AIMING POINT THIS INSTRUMENT, DEFLECTION (so much).

(The gunner, using the two-step deflection method, sets off the announced deflection on the pantel and traverses the tube to pick up a proper sight picture on the compass. He then says NUMBER [so-and-so], AIMING POINT IDENTIFIED, DEFLECTION [so much]. This howitzer is now oriented on the azimuth of fire and is laid.)

(5) To lay the remaining howitzers, command: NUMBER (so-and-so) IS LAID, PLATOON ADJUST, AIMING POINT NUMBER (so-and-so).

(The gunner on number [so-and-so] would then reciprocally lay the remaining howitzers in the platoon as outlined in paragraph 4-16d, using the commands in paragraph 4-19.)

c. This method of lay has advantages and disadvantages that must be considered before it is used.

(1) Advantages. The main advantage to using this method is that all howitzers will be oriented parallel.

(2) Disadvantages. 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.

EXAMPLE

Laying by M2 Compass:

The advance party is preparing a position for occupation on an azimuth of 4550. Because of combat loss, no aiming circle is available. The platoon leader has determined that line of sight will exist between number 4 and the remaining howitzers. The gunnery sergeant is told to lay number 4 upon occupation using the M2 compass method of lay. The GSG takes a mattock handle from the advance party vehicle and moves 100 meters to the left front of number 4 position. Upon arrival of number 4, the GSG (using the mattock handles as a stable platform for the compass) measures an azimuth to the pantel of number 4, He subtracts the azimuth of fire from the measured azimuth (adding 6,400 mils to the measured azimuth if necessary).

Measured Azimuth                              0730
                                                       + 6400
                                                          7130
                                                       - 4550
                                                          2580
Azimuth of Fire                                - 4550
Deflection                                          2580

The gunnery sergeant commands NUMBER 4 ADJUST, AIMING POINT THIS INSTRUMENT, DEFLECTION 2580.

The gunner on number 4 uses the two step deflection method to pick up line of sight on the compass and announces NUMBER 4 AIMING POINT IDENTIFIED, DEFLECTION 2580.

The GSG commands NUMBER 4 IS LAID, PLATOON ADJUST, AIMING POINT NUMBER 4.

4-22. AIMING POINT-DEFLECTION METHOD

a. Normally, 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 (DAP) is required and must be at least 1,500 meters from the position. (See paragraph D-4.) Also, all howitzers must be able to pick up a line of sight to the DAP. A compass, or a map and protractor are the only equipment needed.

Note: The aiming point-deflection method can be used for a deliberate occupation if an aiming circle or an M2 compass is not available. If this method is used for a deliberate occupation, it is desirable to lay only one howitzer by using the DAP. Once that howitzer is laid, the gunner reciprocally lays the remainder of the firing element to ensure all howitzers are oriented parallel.

b. The steps for laying by the aiming point-deflection method are as follows:

(1) Determine the azimuth from the center of the battery or platoon to the DAP (scale from the map or measure with a declinated compass, or use the BCS procedures in paragraph F-11.)

(2) Derive the back-azimuth of fire by adding or subtracting 3,200 mils to or from the azimuth of fire.

(3) Compute a deflection by subtracting the back-azimuth of fire from the azimuth to the aiming point (memory aid: AP-BAF).

(4) To lay the platoon, command PLATOON ADJUST, AIMING POINT (description), (location), DEFLECTION (so much).

Each gunner, using the two-step deflection method, picks up a line of sight to the DAP, and announces NUMBER (so-and-so) AIMING POINT IDENTIFIED, DEFLECTION (so much).

(5) When all sections have reported, command THE PLATOON IS LAID.

Note: During an emergency mission, the X0 or FDO will normally announce the initial fire commands (charge and quadrant) to the adjusting piece. The remaining fire commands will be determined and issued by the FDC.

c. This method has advantages and disadvantages which must be considered before its use.

(1) Advantages.

(a) It is a very rapid method of lay. It may be used immediately upon occupation and only one command is required to lay the entire firing element.

(b) Minimal preparation of the position is required.

(c) The DAP can be used as an aiming point.

(2) Disadvantages.

(a) A DAP must be available.

(b) Line of sight to the DAP may be obscured by smoke, fog, darkness, and so forth.

(c) Howitzers will not be laid parallel when oriented with a common deflection to an aiming point, except when that aiming point is on the left or right flank. If the DAP is to the tint of the position, the sheaf will converge. If the DAP is to the rear of the position, the sheaf will diverge. See Appendix D for further discussion of aiming points.

EXAMPLE

Laying by Aiming Point-Deflection Method:

The XO receives an emergency fire mission while en route to a new position and signals the convoy. The emergency data base is imput into a BUCS and the initial firing data are determined. As the firing element approaches the position, the XO notices that there is a water tower located about 4 km to the left flank of the position. Using his map, he scales an azimuth to the water tower.

The XO pulls into position and aligns his vehicle in the general direction of the azimuth of fire. While the howitzers are moving into positon, the XO determines the deflection to announce. (He uses the memory aid AP-BAF).

Azimuth of Fire                                0150
+3200                                           +3200
Back azimuth of fire                         3350

Azimuth to the aiming point              4925
Minus back azimuth of fire             -3350
Deflection to DAP                          1575

After all howitzers are in position and the deflection has been determined, theXO moves to the front of the position and announces BATTERY ADJUST, AIMING POINT WATER TOWER, LEFT FLANK, DEFLECTION 1575. Each gunner performs the two-step deflection method, picking up a line of sight to the DAP, and announces NUMBER (so-and-so) AIMING POINT IDENTIFIED, DEFLECTION 1575.

When all sections have reported, the XO commands THE BATTERY IS LAID.

The XO announces fire commands to the adjusting piece to fire the initial round in the mission. The FDC announces the remaining commands by voice.

4-23. HOWITZER BACK-LAY METHOD

a. Normally, the howitzer back-lay method of lay will be used during an emergency fire mission. It should only be used if the grid azimuth and aiming point-deflection methods are not possible. An M2 compass and an aiming circle are the only equipment needle

b. The steps for the howitzer back-lay method are as follows:

(1) Determine the azimuth of fire by using the BCS or LCU hip shoot procedures, or scale it from a map after plotting the target and the proposed firing position.

(2) Mark the azimuth of fire with stakes and tape.

(3) Ensure adjusting piece positions are alongside (SP) or over (towed) the marked azimuth, all other howitzers are on line. Ensure the aiming circle is emplaced in the left rear of the position area.

(4) FDC announces firing data to the adjusting piece, and the first round is fired.

(5) The instrument operator on the aiming circle commands NUMBER (so-and-so) (adjusting piece), IS LAID, NUMBER (so-and-so) (adjusting piece), REFER, AIMING POINT THIS INSTRUMENT.

(6) The adjusting piece gunner announces NUMBER (so-and-so) (adjusting piece), AIMING POINT IDENTIFIED, DEFLECTION (so much).

(7) The instrument operator sets the referred deflection on the upper motion of the aiming circle. With the lower motion, he sights on the pantel of the adjusting piece. The 0-3200 line on the aiming circle is now parallel to the howitzer tube after firing the first round. He then lays the remaining howitzers using the upper motion by commanding PLATOON ADJUST, AIMING POINT THIS INSTRUMENT.

Note: Normally only one deflection will be read to each howitzer, and the piece is laid.

(8) Internal battery communications (small-unit transceiver, runners, or wire, whichever is the most expedient) is established.

(9) The remaining battery/platoon vehicles establish a perimeter defense to the rear of the gun line.

c. This method has advantages and disadvantages which must be considered before its use.

(1) Advantages

(a) Firing capability with the adjusting piece is immediate.

(b) Minimal preparation of position is required.

(2) Disadvantages

(a) Laying process is not as timely as other methods.

(b) If boresight errors exist at adjusting piece, these will be passed onto the remaining howitzers.

EXAMPLE

Laying by Howitzer Back-Lay Method:

The firing platoon receives an emergency fire mission while en route to a new position and signals the convoy. The BCS operator inputs the target grid into a BCS. He determines the azimuth of fire and the initial firing data using the BCS emergency fire mission procedures. He gives this information to the FDO and the platoon leader. Upon occupation of the position, the platoon leader aligns his vehicle in the general direction of fire. The platoon leader then marks the azimuth of fire for the adjusting piece by using a strip of engineer tape approximately the same length as the weapon system. This tape should have a wooden stake on one end and a nose plug on the other end. To mark the azimuth, the platoon leader throws the nose plug in the direction of the azimuth of fire. Then, using the M2 compass, he aligns the tape on the azimuth of fire by moving the stake end of the tape. At the same time the platoon leader is marking the azimuth, these other actions are taking place:

  • The platoon sergeant takes the aiming circle to the left flank of the firing position. He ensures he has moved far enough to the left rear of the weapons so that he is beyond the point where the left flank howitzer will be positioned. He then sets up the aiming circle, levels it, and is prepared to accept a referred deflection from the adjusting piece.
  • The chief computer positions the FDC vehicle so that the rear of the vehicle is facing the gun line. This allows the FDC to observe the state of readiness of the firing sections and announce voice fire commands.
  • The platoon leader quickly guides the adjusting piece into position where the engineer tape is aligned along the azimuth of fire. Self-propelled weapons drive alongside the line, while towed howitzers are brought in from the direction of fire and centered over the tape.
  • The adjusting piece is laid on the target as soon as it is aligned on the engineer tape. The FDC has provided firing data (charge and quadrant) to the platoon leader. These are announced to the adjusting piece, and the first round is fired. The gunner establishes an aiming point.
  • As the adjusting piece is preparing to fire the first round, all other howitzers are pulling on line. Positioning is critical. Each chief of section must position his howitzer so that his gunner can seethe aiming circle and where the line of sight is not impaired for any other howitzer.
  • Immediately after the adjusting piece has fired the initial round of adjustment, the platoon sergeant will command NUMBER (so-and-so) (adjusting piece) REFER, AIMING POINT THIS INSTRUMENT.

  • Upon hearing the above comment, the gunner--

    - Using the azimuth knob, without moving the tube, rotates the head of the pantel and sights on the instrument (or object).

    - Checks the sight mount bubbles, centering them as necessary, and rechecks the sight picture.

    - Reads the deflection appearing on the azimuth counter (M100-series pantels), azimuth and azimuth micrometer scales (M12-series pantels), or azimuth main and micrometer scales (L7A2 pantel).

    - Announces NUMBER (so-and-so), AIMING POINT IDENTIFIED, DEFLECTION (SO much).

Note: When referring to the second circle to verify initial lay, the gunner must ensure that the tube is on the azimuth of fire before he uses the above procedures.

  • The platoon sergeant sets the referred deflection on the upper motion of the aiming circle and with the lower motion, sights on the pantel of the adjusting piece. The platoon sergeant is now ready to lay the remaining pieces of the platoon.
  • Each gunner identifies the aiming point and receives his deflection. The deflection is set off, the tube traversed until his line of sight is back on the aiming circle, and the piece is laid. In the interest of time, there normally are no rechecks. If the initial target location was good and if the platoon leader accurately laid out the azimuth tape, the second volley may well be fire for effect.
  • An aiming point is established for each weapon.
  • The platoon sergeant should remain at the aiming circle during the firing of the mission in case a weapon must be re-laid or the lay of the platoon needs to be refined.
  • The platoon leader, using his M2 compass, should visually check the gun line to ensure that there is no gross error in the lay of the weapons.

4-24. ESTABLISHING AIMING POINTS

a. During deliberate and hasty occupations, after the battery has been laid, the crew of each piece sets up the collimator and/or emplaces aiming posts before the tube is moved. The deflection at which the aiming points are established is recorded by each gunner. The direction in which the battery is initially laid and the corresponding common deflection are used as references from which the FDC can derive firing deflections for future targets.

Note: For further discussion on aiming points, see paragraph D-4.

b. The collimator is the primary aiming point and is placed 4 to 15 meters from the sight of the weapon. When the collimator is emplaced, 3 numbers (5, 0, 5) and 11 graduations will be visible in the reticle. If possible, place the collimator under cover to the left front or left rear of the weapon. See Figure 4-17 for the proper sight picture.

c. When aiming posts are used, the far aiming post should be placed 100 meters from the howitzer and the near aiming post should be placed 50 meters from the howitzer. If the situation or terrain limits placing the far aiming post 100 meters from the howitzer, place it out as far as possible and place the near aiming post halfway between the howitzer and the far aiming post.

d. As soon as the platoon is laid and the aiming point(s) is (are) emplaced, the platoon leader should have the gunners of all pieces refer to a distant aiming point if one is available. A DAP must be at least 1,500 meters away. This DAP can be used as the primary aiming point if something happens to the collimator or aiming posts.

e. Close-in aiming points are moveable, established by battery personnel, and can be seen at night. The DAP has the advantage of being readily available upon occupation of a firing position.

f. During an emergency occupation, it is recommended that a distant aiming point be selected. If a DAP is not available, then several other options can be used. Two examples are as follows:

  • Set up the collimator.
  • Set up an aiming post at a point halfway between the aiming circle and the pantel.

4-25. SAFETY AND VERIFYING THE LAY OF THE PLATOON

a. Safety and verification of tasks by leaders are disciplines that exist in the field artillery, regardless of whether operations are performed in combat or in peacetime. For every task that is performed, there is another person in a leadership position (section chief, platoon sergeant, platoon leader or XO, FDO, or BC) who verifies the accuracy of the action performed, This system of double checks is inherent in all operations and is not to be considered a limiting factor in providing timely fire support. Therefore, commanders must ensure that their units have a system of independent safety checks. These checks ensure that all cannon battery and platoon operations (for example, FDC mission processing and orienting howitzers for direction) affecting firing is checked by someone other than the person who performs the action. Though most independent checks take place before missions are received, performing independent checks is a continuous process and must be rigidly enforced to ensure fires are timely, accurate, and safe. These checks may include, but are not limited to, the following:

  • Verification of entered azimuth of lay (LCU or BCS) by the FDO and the platoon leader.
  • Verification of target grid.
  • Verification of battery or platoon lay.
  • Verification of AFCS data (Paladin units only).
  • Verification of weapon location.
  • Verification of met data.

b. During a deliberate occupation, the lay of a unit is verified immediately following the verification or conduct of boresighting. It is the "S" step in TLABSPAP. The unit SOPs will specify the method and sequence of verifying lay during an emergency occupation.

c. After the platoon is laid, the platoon leader verifies the lay by use of another M2A2 aiming circle referred to as the verification circle. To verify lay of the platoon, the following steps are taken:

(1) The platoon leader or designated safety officer sets up and orients an M2A2 aiming circle by using a method other than that used by the lay circle. However, in combat situations, the BC may authorize the orientation of the verification circle using the same method as the lay circle, METT-T dependent. This aiming circle must be located where it can be seen by all howitzers and should not be any closer than 10 meters to the lay circle.

(2) The verification circle operator picks up a line of sight on the lay circle. Then he commands, LAY CIRCLE REFER, AIMING POINT THIS INSTRUMENT. The lay circle operator will sight his instrument onto the verification circle by use of the recording motion.

(3) If the lay circle and the verification circle deflections are within 10 mils or as specified by local range regulations, the instrument operator on the verification circle places the deflection read by the lay circle on the upper motion of the verification circle. With the lower motion, he sights back on the lay circle. This serves to align the 0-3200 line of the verification circle parallel to the 0-3200 line of the lay circle.

Note: When an aiming circle is used to verify another aiming circle for direction, the readings between the two circle will be 3,200 mils apart (Figure 4-18). This is because both circles measure horizontal clockwise angles from the line of fire. To prevent confusion remember that, if you see red, read red.

(4) The instrument operator on the verification circle commands PLATOON REFER, AIMING POINT THIS INSTRUMENT. All gunners refer and announce the deflection to the verification circle. If the deflection referred by a howitzer is within that tolerance given in the local range regulations, the operator on the verification circle announces that the howitzer is safe. Once all howitzers are safe, the operator announces THE PLATOON IS SAFE.

Note: When referring to the verification circle to verify lay, the gunner must ensure that the tube is on the azimuth of fire before he uses the above procedures.

d. The platoon leader should walk the gun line and visually check the tubes to ensure they are parallel. An M2 compass should also be used to ensure the tubes are on the azimuth of fire.

4-26. MEASURING VERTICAL ANGLES

a. Normally, vertical angles (VAs) from the M2A2 aiming circle to the howitzers are measured during advance party operations. The VA and distance from the orienting station to each howitzer are required by FDC to determine piece locations in BCS or LCU. The aiming circle can also be used to determine sight to crest at each howitzer during advance party operations.

b. The VA to a point is measured from the horizontal plane passing through the horizontal axis of the instrument (M2A2 aiming circle). It is expressed as plus or minus, depending on whether the point is above (plus) or below (minus) the horizontal plane. It is measured to the height of instrument which is about chest high on the average individual at the gun position. The steps for measuring a VA are as follows:

(1) Using the circular leveling vial, ensure the aiming circle is properly set up and leveled.

(2) Using the elevation knob, center the upper tubular leveling vial. This yields a correction factor to be applied to all measured vertical angles.

(3) Read and record the corrections from the elevation scale. If black numbers are used, the correction factor is plus; if red numbers are used, the correction factor is minus.

(4) While measuring the subtense (used to determine distance), elevate or depress the telescope to place the horizontal cross hair of the aiming circle on the chest of the gun guide. The subtense method is described in paragraph 5-10a.

Note: If there was not enough time to measure VAs during advance party operations, the instrument operator can align the horizontal cross hair at chest level of a cannoneer during occupation. The key is to measure the VA at a height that approximates instrument height.

(5) Read and record the value on the elevation and elevation micrometer scales to the nearest 1 mil.

(6) Subtract the correction (corr) factor from the reading obtained in the preceding step. The result is the vertical angle for that howitzer.

4-27. MEMORY AIDS

Tables 4-2 through 4-5 show memory aids for use in training personnel to lay the battery and/or platoon.


Section VI

MEASURING AND REPORTING DATA


4-28. ACCURACY

a. The accuracy of lay is directly related to the method used to orient the howitzers on the azimuth of fire and the alignment of the fire control equipment.

b. Measuring and reporting data provides us with a method of correction for errors in the lay. The FDC may require a check of weapon direction be made. Normally this check is made after a registration or after survey control is established. It serves to--

  • Check the data fired.
  • Check the accuracy of lay.

c. Weapon direction may be verified by

  • Reporting the correct deflection.
  • Measuring the azimuth.
  • Measuring the orienting angle.

d. Reporting will reveal any sloppy procedures being used by the gun crews, such as failure to level the bubbles and improper sight picture. Measuring provides a check on the accuracy of the lay. These checks must be made before END OF MISSION is received at the firing weapon.

4-29. BACKWARD AZIMUTH RULE

a. The backward azimuth rule is a mathematical relationship used to apply equal changes in angles used in the laying process. This rule establishes the relationship between three elements as follows:

An increase in deflection causes an equal decrease in azimuth and an equal increase in orienting angle. A decrease in deflection causes an equal increase in azimuth and an equal decrease in orienting angle.

b. Using this rule and assuming that there are no errors in the lay of the weapons, weapon direction may be verified.

4-30. REPORTING THE CORRECT DEFLECTION

Upon completion of a mission, and before announcing END OF MISSION the FDC may request that the executive officer or platoon leader report the correct deflection. To report, the XO or platoon leader--

  • Goes to the weapon.
  • Checks the level of the bubbles (centers if necessary).
  • Checks for the correct sight picture (corrects if necessary).
  • Reads the deflection from the pantel.

CAUTION

When making corrections to the sight picture or leveling the bubbles, the tube must not be moved.

  • Reports the deflection read from the pantel to the FDC as CORRECT DEFLECTION (so much).

EXAMPLE

Your platoon (M109A3, 155-mm SP) is laid on azimuth 5000 (orienting angle 0600), with a common deflection of 3200. After firing a registration, FDC requests that the platoon leader report the azimuth or orienting angle. The platoon leader completes the required check sand reports CORRECT DEFLECTION 3250. He then does the following:

  • Determines the change between the common deflection and the correct deflection. (Common deflection = 3200; correct deflection = 3250; increase = 50 mils.)
  • Applies the difference according to the backwards azimuth rule to determine the azimuth fired. (Deflection = + 50 mils; azimuth = .50; mils; 5000 - 50 = 4950.)
  • Reports CORRECT AZIMUTH 4950.

4-31. MEASURING THE AZIMUTH OF THE LINE OF FIRE

a. If a gun fires out of safe, the platoon leader or XO may wish to determine the azimuth at which the round was fired. This is the most common reason for measuring the azimuth of the line of fire. Another reason is that a unit may use a less desirable method of lay and then improve this method. An example would be a unit that lays by the grid azimuth method because there is no survey control. When survey closes, the unit will want to know if the azimuth of fire it has been firing on is different from the originally intended azimuth of fire. There are two methods of measuring the azimuth of the line of fire. The difference depends upon whether or not survey control is available.

b. If the line of fire is being measured following a firing incident, the crew of the howitzer in question will not move the tube but will simply refer to the aiming circle. If, on the other hand, the platoon leader wishes to measure the azimuth of fire, he must first ensure that the tube of the howitzer being measured is at lay deflection.

c. These steps are performed first, regardless of whether or not survey control is available:

(1) The aiming circle operator--

  • Orients the 0-3200 line generally parallel to the tube of the weapon.
  • Commands NUMBER (so-and-so) REFER, AIMING POINT THIS INSTRUMENT.

Note: Memory aids for measuring and reporting are in tables 4-6 through 4-8.

(2) The gunner measures the deflection to the aiming circle without moving his tube and announces NUMBER (so-and-so), AIMING POINT IDENTIFIED, DEFLECTION (so much).

(3) The aiming circle operator--

  • Sets the announced deflection off on the upper (recording) motion of the aiming circle.

Note: If the weapon has an M12-series sight, the referred deflection may have to be set on the aiming circle by using the red numbers. A general rule is that if a weapon with an M12-series sight is left and forward of the aiming circle, red numbers are used when reading the aiming circle to lay the howitzer.

  • With the lower (nonrecording) motion, sights in on the panoramic telescope.

Note: The 0-3200 line of the aiming circle is now parallel to the tube of the firing weapon.

(4) The final step at the aiming circle depends on whether survey control is available. If it is not available, follow the steps in subparagraph d below. If it is available, follow those in subparagraph e below.

d. The following is the final step if survey control is not available:

(1) The platoon leader, with the upper (recording) motion, floats and centers the magnetic needle.

Note: Since the magnetic needle is being used, the aiming circle must be declinated and set up away from magnetic attractions.

(2) The value now on the upper motion is the instrument reading. Subtract the instrument reading from the declination constant. The difference is the azimuth of the line of fire (az of the LOF). For a memory aid, see Table 4-6.

EXAMPLE

Your M198 platoon conducted an emergency occupation using the howitzer backlay method to lay on an azimuth of fire of 0900. You were instructed to stay in position and continue answering calls for fire. The platoon is at end of mission, and all howitzers have returned to their lay deflections. You go to the aiming circle and measure an instrument reading of 5750. The aiming circle has a declination constant of 0300. Your final step is to solve for the azimuth of the line of fire.

DC(+ 6400 if needed) - IR = az of the LOF
(0300 + 6400) - 5750 = 0950

Your platoon howitzers were initially laid on 0950, but your BCS or LCU currently has an azimuth of fire entry of 0900. You have two options at this point:

  • Leave the platoon laid on 0950, and have the FDC correct the computer entries for azimuth of fire and piece locations.
  • Re-lay the platoon on the originally intended azimuth of 0900.

Note: The first option requires less time and effort and is thus normally preferred if the tubes were laid parallel. However, if the unit was laid by an alternate method of lay where the tubes are not truly parallel, then re-laying would be preferred.

e. If survey control is available, the following is the final step:

(1) The platoon leader with the upper (recording) motion, sights in on the end of the orienting line.

(2) The value now on the upper motion is the orienting angle. Subtract the orienting angle from the azimuth of the orienting line. The difference is the azimuth of the line of fire. For a memory aid, see Table 4-7.

EXAMPLE

Your battalion FDC has instructed you to continue firing from your present position. A survey team is on its way to put you on common survey. In preparation, you place an OS marker directly below the aiming circle plumb bob. You also establish an EOL at least 100 meters away. Upon closing, the survey team provides you with a grid to your OS and an azimuth to the EOL of 5,363 mils. With the howitzers at their lay deflections, you now measure the OA of 4400. Your final step is to solve for the azimuth of the line of fire.

OL (+ 6400 if needed) - OA = az of the LOF
5363 - 4400 = 0963

Your platoon howitzers have been laid on 0963 all along. You have two options at this point:

  • Leave the platoon laid on 0963, and have the FDC correct its computer entries for azimuth of fire and piece locations.
  • Re-lay the platoon on the originally intended azimuth.

Note: The first option requires less time and effort and is thus normally preferred if the tubes were laid parallel. However, if the unit was laid by an alternate method of lay where the tubes are not truly parallel, then re-laying would be preferred.

4-32. CORRECTING BORESIGHT ERROR

a. If the battery or platoon is to deliver accurate fire, the boresight of the weapon must be correct. Boresighting is the process of ensuring that the optical axis of the weapon sights are parallel to the cannon tube. The primary methods of boresighting are the distant aiming point, test target, and standard angle.

(1) If a howitzer is not in boresight, the tube is disoriented in relation to the amount of boresight error. If the sight is off to the right, the tube is disoriented by that amount to the left. If the sight is off to the left, the tube reflects that error to the right.

(2) Once a weapon is properly boresighted, the deflection to an aiming point (aiming circle or collimator) is reestablished. The angle (deflection) between aiming point and pantel has not changed from when the weapon was laid. The entire angle has simply rotated by the amount of boresight error. Since the angle has not changed, the deflection recorded to the aiming point is set off on the pantel and the tube is traversed onto the aiming point. The relationship made at the time of lay is now reestablished. It is improper, after correcting for boresight error, to move (or fine tune) the aiming point to the tube in relation to the deflection (numbers) originally established at the time of lay.

b. Emergency occupation of a firing position may require firing before boresight is verified and any error is corrected. In such cases, the howitzers must verify boresight as soon as possible. If this verification discloses an error (the 0-3200 line of the pantel and the howitzer tube are not parallel) the platoon leader takes corrective actions after measuring the error and reporting it to the FDC.

c. When a deflection is read from the pantel or when an azimuth is measured, the deflection or azimuth determined is that of the 0-3200 line of the pantel as read from the azimuth counter. When the FDC requests the platoon leader to MEASURE THE AZIMUTH, the azimuth requested is that of the howitzer tube. If the howitzer is out of boresight, the data derived is inaccurate by the amount of the error.

Note: If the howitzer in question was used to lay the aiming circle or the rest of the howitzers, the other howitzers are out of lay by the amount of error found. The platoon leader should take corrective action to orient them on the correct azimuth of lay (after all howitzers have verified foresight).

d. Boresight is verified as discussed below.

(1) Howitzers with an alignment device.

(a) The azimuth counter (not the reset counter) and the appropriate alignment device are used when verifying boresight. The numerical error of boresight can be determined from the azimuth counter. The amount of error is the difference between the required deflection for the alignment device (according to the -10 manual) and the deflection read (on the azimuth counter) once the vertical hairline has been aligned.

(b) The backwards azimuth rule will be used to determine the correct azimuth of the howitzer tube.

(c) Once the error has been measured and reported the platoon leader directs the following actions to be taken to correct the error.

  • Boresight the weapon.
  • Correct the azimuth of lay and piece location in the FDC or re-lay the piece.

(2) Howitzers without an alignment device.

(a) The azimuth scale (nonslipping) is used during boresighting. The tube is pointed at the aiming point when a DAP or test target is used. If the sight is pointed to the right of the aiming point the tube is pointed to the left of the 0-3200 line of sight. Before this correction is made, any azimuth measured will be greater than the azimuth of the tube and any orienting angle will be smaller than the true orienting angle. (The opposite relationship also exists.)

(b) The amount of error is determined by referring the sight to the aiming point and reading the deflection. This deflection is compared with the deflection used for boresighting and the difference is the amount of error in boresight. The XO applies the appropriate correction for the boresight error to the measured azimuth or orienting angle prior to reporting to the FDC. He corrects the lay of the pieces as required.

4-33. TRAINING

Training in the proper methods of determining and reporting data should be part of training personnel to lay the platoon. Tables 4-6 through 4-8 will help personnel remember procedural steps. These procedures are performed with the aiming circle and the howitzer pantel.




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