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CHAPTER 8

GYROSCOPIC AZIMUTH

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This chapter discusses the skills needed to operate the azimuth gyro (SIAGL [Lear]). It also discusses the conversion of the azimuth determined by gyroscope (true azimuth) to grid azimuth.

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Section I

SIAGL (LEAR)

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This section is a guide for survey personnel to develop the skills needed to operate the azimuth gyro. The azimuth gyro available for use is the SIAGL (Lear). The SIAGL is a portable gyrocompass used to determine a true north direction at any selected station, latitude permitting. The SIAGL can determine direction without lengthy computations or existing control in almost any weather condition. Accuracy of the SIAGL is comparable to that of astro observation. All persons involved in FA survey operations must understand the functions of this instrument and be able to determine a direction with it.

8-1. PRINCIPLES OF OPERATION

A gyroscope acts as a north-indicating device. When leveled, the gyroscope detects the earth's rotation and orients itself to north. Once the gyroscope locks in on north, the true azimuth to any point can be determined by reading the horizontal scales. Since the effects of the earth's rotation vary from place to place, the gyroscope will be less accurate at points farther north or south of the equator.

8-2. COMPONENTS AND ACCURACY

a. The SIAGL can determine true north with high accuracy without the help of celestial or landmark sightings. The SIAGL consists of the following:

• Gyroscopic reference unit (GRU) with an integral theodolite and tripod assembly.
• Electronic control unit (ECU).
• Transit case, which houses the reference unit.
• Alternating current-direct current (AC-DC) converter.
• Auxiliary equipment.

The complete SIAGL, minus the wind screen, is contained in a transport case. The instrument can determine and indicate true north within about 20 minutes after power is applied.

b. The accuracy of the SIAGL depends on the latitude of the location. The accuracy at a specific location can be computed by the formula:

Table 8-1 lists the accuracies that can be expected at various latitudes from 0° to 75°.

c. The real value of the SIAGL is to establish initial directional control for cannon battalion surveys.

d. The LATITUDE select switch on the ECU maintains instrument operating time at different latitudes of operation. The switch is adjusted with a screwdriver and has eight positions. Adjust the LATITUDE switch to correspond to the closest latitude of operation. Table 8-2 lists the switch positions and their corresponding latitudes.

e. The TEST METER on the ECU provides a visual check of the operational status of the instrument in both testing and operating modes. Satisfactory operation of the circuits associated with TEST SELECT switch positions is indicated when the meter pointer moves to the green area (right of center). The meter and the TEST SELECT switch are used with the PRESS TO TEST switch, which must be pressed to obtain meter readings during self-test operations. When the gyrocompass activates, the meter pointer indicates the relationship of the pendulum and follow-up assembly.

f. For the specific functions of other GRU controls (Figure 8-1) and ECU controls (Figure 8-2), refer to TM 5-6675-250-10.

8-3. INITIAL SETUP PROCEDURES

The following steps are performed when unloading and unpacking the equipment.

a. Remove the GRU from the transit case as discussed below.

(1) Press the pressure relief valve.

(2) Unfasten the latches securing the upper and lower sections of the case together. Remove the upper section.

(3) Unfasten the clamps securing the GRU in the case. Remove the unit from the case.

(4) Inspect equipment for damage and for loose or missing parts.

CAUTION Avoid unnecessary exposure of the equipment to dust, soil, or other abrasive materials.

b. Loosen the clamps on each leg of the tripod, and extend each leg to within 1/2 inch of full extension. Tighten the leg clamps. Extend the spades 1/4 to 3/8 inch by rotating the leveling screws.

8-4. FINAL SETUP PROCEDURES

With the equipment unpacked and the tripod extended (Figure 8-3), complete setup procedures as discussed below.

a. Course-level the instrument by releasing one of the tripod leg clamps. Adjust the leg to obtain a level indication on the tripod circular level. Repeat the procedure for each of the other two tripod legs.

b. To center the unit over a fixed reference point, extend the plumb pointer located on the bottom of the GRU housing. Place the reference unit on the ground so the pointer is near the fixed point. Press the leg spades into the ground. Check the plumb pointer from two positions 1,600 mils apart.

CAUTION When ground wind speeds are more than 20 miles per hour, use of the wind shelter is mandatory to obtain azimuth accuracies listed in Table 8-1.

c. Release the horizontal lock on the theodolite. Rotate the alidade to position the objective end of the telescope over the NORTH mark on the reference unit; tighten the horizontal lock. The telescope must be level (white index marks lined up) for the compass to operate properly. Release the three hold-down clamps. Depress the plunger on the magnetic compass. Rotate the GRU in the tripod until the two compass needle images are in coincidence. Check to ensure the plumb pointer is still over the fixed reference point. Adjust the plumb as necessary, tighten the hold-down clamps, and recheck the compass.

d. Connect the interconnect cable between the ECU and the GRU. Connect the power cable between the INPUT POWER connector on the ECU and a 24-volt DC power source. Auxiliary cables and the AC-DC converter are stored in the transport case for use as needed. The AC-DC converter allows operation from a 115-volt, 60- or 400-Hz power source. Remove the converter from the transport case by unfastening the screw assembly. Cap and chain assemblies protect the two receptacles on the converter. One receptacle connects the converter to the power source. The second receptacle connects the converter to the control unit.

CAUTION Observe the cable markings and connectors. The cables may be connected backward if not aligned properly.

e. For fine leveling, rotate the theodolite alidade so the long axis of the plate level is in the same plane as any two of the tripod legs. Adjust the fine level control to place the bubble in the center of the level vial. Rotate the alidade 1,600 mils, and adjust the remaining leg. Continue the process until the alidade can be rotated 6,400 mils without displacing the bubble more than one division.

8-5. HORIZONTAL CIRCLE ALIGNMENT PROCEDURE

Align the theodolite horizontal circle to the reference mirror as discussed below.

a. Place the MODE SELECT switch to THEO ILLUM.

b. Adjust the reticle focus to get a sharp, clear image of the reticle through the telescope eyepiece.

c. Turn the telescope focus counterclockwise until it stops; then turn it clockwise two turns.

d. Rotate the telescope to place the mirror azimuth (rounded to the nearest 5 mils) on the horizontal scales.

e. Depress the telescope until the vertical scales read about 2,400 mils. The green cross or a portion thereof will be visible in the telescope.

f. Adjust the focus to obtain a sharp, clear image, and center the green cross in the reticle. (See Figure 8-4.)

g. Adjust the microscope focus control and the THEO BRT control until the scales appear in clear focus.

h. Adjust the micrometer control to position the fixed index mark (Figure 8-5) of the horizontal (H) scale to the exact center of the nearest double line.

i. If the vernier scale does not appear under the fixed index, adjust the micrometer control to position the fixed index mark of the horizontal scale to the center of the nearest double line.

j. Record the H-scale reading (Figure 8-5) as discussed below.

(1) If the fixed index is over a three-digit number, record the number and add a 0 as the fourth digit.

(2) If the fixed index is over a number 5, record the preceding three-digit number and add a 5 as the fourth digit.

k. Record the vernier scale reading.

l. Obtain the direct horizontal reading by adding the recorded readings of the horizontal and vernier scales. (See Figure 8-5.)

m. Obtain reverse readings as discussed below.

(1) Release the horizontal and vertical locks on the telescope, and plunge the telescope.

(2) Subtract 3,200 mils from the mirror azimuth. Place the first four digits of the answer on the horizontal scales.

(3) Depress the telescope until the vertical scales read about 4,000 mils.

(4) Repeat steps in h through l above to obtain the reverse mirror reading.

n. Obtain the mean mirror reading as discussed below.

(1) Apply 3,200 mils to the reverse reading.

(2) Add the direct and reverse readings.

(3) Divide the sum obtained in (2) above by two to obtain the mean mirror reading.

o. If the mirror reading determined in n above is within 0.04 mil of the value displayed on the mirror azimuth plate just below the reference mirror window, make no adjustment. If the error is greater than 0.04 mil, use the adjustment procedures below.

(1) Subtract the last four digits of the reverse reading from the last four digits of the direct reading. (Disregard the first two digits.)

(2) Divide this value by two to determine the collimation error.

(3) Algebraically add the collimation error to the value on the mirror azimuth plate.

(4) With the telescope in the direct position, center the image (green cross) accurately in the reticle pattern as in a through f above. Using the micrometer, set the value determined in (3) above on the horizontal and vernier scales. Using the horizontal circle adjusting tool, center the index between the nearest double line. See Figure 8-1 for location of the horizontal circle setting control.

(5) Take an additional direct and reverse reading. Compare these with the mirror azimuth plate, and repeat adjustment until the desired reading is obtained.

8-6. SELF-TEST

Place the telescope in the direct position. Perform the self-test as discussed below.

a. With the MODE SELECT switch set to SELF TEST and the PWR ON indicator lit, place the TEST SELECT switch to SOURCE VOLTAGE.

b. Press the PRESS TO TEST switch. Verify that the TEST METER pointer falls between 22 and 33 volts DC on the meter scale.

Note. You must press the PRESS TO TEST switch for each of the test positions.

c. Place the TEST SELECT switch to REG AC. Verify that the TEST METER pointer is in the green band of the meter scale.

d. Repeat step c with the TEST SELECT switch in the REG DC and BIAS positions.

e. Place the TEST SELECT switch to the SERVO CW position. When looking down on the reference unit, verify that the theodolite rotates in a clockwise direction and the TEST METER pointer is in the green band of the meter scale.

f. Place the TEST SELECT switch to the SERVO CCW position. When looking down on the reference unit, verify that the theodolite rotates in a counterclockwise direction and that the TEST METER pointer is in the green band of the meter scale.

g. Place the TEST SELECT switch to either SERVO CW or SERVO CCW until the yellow index mark is in the center of the yellow servo operating band.

h. Place the TEST SELECT switch to BRAKE. Verify that the BRAKE ON indicator lights and the TEST METER pointer is in the green band.

i. Place the TEST SELECT switch to GYRO. Verify that the TEST METER pointer centers in the yellow area of the test meter scale (0).

8-7. OPERATION OF THE INSTRUMENTS

Note. For daytime operation, the PANEL ILLUM control must be in the DAY position and the theodolite illumination must be at least two-thirds full illumination.

a. Place the ECU MODE SELECT switch to BIAS (audible click).

b. Rotate the CAGE-UNCAGE knob on the GRU clockwise until the UNCAGED indicator lights (audible click). If the GRU will not uncage, check the fine level of the instrument.

c. Observe the TEST METER pointer swing left and right at an approximate rate of one reversal every 5 seconds. When the pointer comes to a stop, verify that it is at 0. If necessary, unlock the outer ring on the BIAS control by turning it counterclockwise, and turn the inner control knob in the appropriate direction to move the needle to 0. When the pointer is at 0, turn the outer ring clockwise to lock the BIAS control in position.

d. Rotate the CAGE-UNCAGE knob on the GRU counterclockwise until the UNCAGED indicator goes out (audible click).

e. Place the MODE SELECT switch to GC (gyrocompass). Verify that the GYRO SYNC indicator lights within 2 minutes.

f. Rotate the CAGE-UNCAGE knob clockwise until the UNCAGED indicator lights.

CAUTION During the initial wait for the READ AZIMUTH light, ensure the yellow index mark does not approach either end of the yellow servo operating band. Failure to keep the index mark near the center of the operating band will cause extensive damage to the instrument. If the yellow index mark does approach either end of the servo operating band, place the CAGE-UNCAGE knob in the CAGE position. Rotate the GRU in the tripod 60° in the direction that the index mark was moving. Relevel and rebias the instrument, then proceed with the gyrocompass operation.

g. Verify the READ AZIMUTH indicator on the ECU lights about 15 minutes after uncaging the gyro.

h. After the READ AZIMUTH indicator light illuminates, take a direct and reverse reading on the azimuth mark with the theodolite and record the readings. This constitutes one set of readings. With the telescope in the direct position, press the RESET button. The READ AZIMUTH indicator light will illuminate in about 45 seconds. Take a second set of readings to the azimuth mark. The mean of each set must agree within 0.3 mil for fifth-order accuracy. If time permits, take another set of readings to check the mean of the first two sets.

i. Stop the gyroscope by the procedures outlined in paragraphs 8-9a(2) and (3).

j. Place the MODE SELECT switch to BIAS. Rotate the CAGE-UNCAGE knob on the GRU clockwise until the UNCAGED indicator light illuminates. Recheck BIAS to ensure that the TEST METER pointer is within the shaded (yellow) band of the meter.

k. If the difference between the two meaned azimuths is greater than 0.3 mil or if the TEST METER pointer is not within the shaded (yellow) band, disregard the two sets of readings. Retake both sets, starting with c above.

8-8. READING THE THEODOLITE

a. Adjust the reticle focus to obtain a sharp, clear image of the reticle seen through the telescope eyepiece.

b. Release the horizontal and vertical locks on the theodolite.

c. Align the telescope on the target, and tighten the horizontal and vertical locks.

d. Adjust the telescope focus until the target appears in clear focus.

e. Adjust the azimuth and elevation controls to center the target image accurately in the reticle pattern.

f. Adjust the microscope focus control and the optical scales THEO ILLUM control on the ECU panel until the H-scale image appears in clear focus.

g. Adjust the micrometer to position the fixed index mark of the H-scale to the center of the nearest double line.

h. If the vernier scale does not appear under the fixed index, adjust the micrometer to position the fixed index mark of the H-scale to the center of the nearest double line.

i. Record the H-scale reading (Figure 8-5) as discussed below.

(1) If the fixed index is over the three-digit number, record the number and add a 0.

(2) If the fixed index is over a number 5, record the preceding three-digit number and add a 5.

j. Obtain the direct azimuth indication by adding the recorded readings of the horizontal and vernier scales.

k. Read the vertical scales (Figure 8-6) in the same manner as the horizontal scales.

l. Mathematically average the last four digits of the direct and reverse readings to determine true azimuth (recorded as shown in Figure 4-15).

m. Convert the true azimuth to grid azimuth by using the BUCS. (Refer to Chapter 8, Section II.

8-9. MARCH-ORDERING EQUIPMENT

a. Take down the instrument as discussed below.

(1) Rotate the CAGE-UNCAGE knob counterclockwise until the UNCAGED indicator light is out.

(2) Place the MODE SELECT switch to BRAKE ON. Verify that the BRAKE ON indicator lights and goes out at the completion of the braking sequence (in about 90 seconds).

(3) Place the MODE SELECT switch to PWR OFF.

(4) Verify that the yellow index mark is in the center of the yellow operating band. If it is not, place the MODE SELECT switch to SELF TEST. Place the TEST SELECT switch to BIAS CW or CCW to center the index mark. Then place the TEST SELECT switch to BRAKE, and return the MODE SELECT switch to PWR OFF.

Note. Center the Index mark in the yellow operating band to be sure the GRU will fit in the transit case.

(5) Dismantle the equipment in reverse order of setup for removal from service or transport to a new work site.

b. Secure the transit case by fastening the upper and lower sections of the case together. Place the transit case in the transport case, and fasten the strap latches.

c. When using vehicle transportation, securely tie the transport case down before moving.

8-10. MAINTENANCE

The SIAGL will operate in all climatic categories. However, because of known design limitations of theodolites, use of the procedures below will permit successful operation.

a. Avoid subjecting the instrument to extreme sudden changes in temperature.

b. Allow the instrument to stabilize at the operating temperature.

c. Select operating sites that can shield or protect the instrument from the full effects of the extreme environment. Make use of available tents or other shelters.

d. At the end of a mission, return the instrument to its transport case. Do not allow the instrument to be exposed in conditions where it cannot be used.

e. Remove moisture and fogging of the optics by storing the instrument in a warm, dry place. (Locally fabricated "hot boxes" and a desiccant can be used.)

f. Adhere to the standard operation, transportation, maintenance, and storage procedures prescribed for the various climatic regions.

g. During operations in winds exceeding 20 miles per hour, use a wind shelter.

h. Ensure that the equipment is clean and dry before storage.

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Section II

CONVERSION OF GYROSCOPIC AZIMUTH TO GRID AZIMUTH

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The azimuth determined by gyroscope is a true azimuth. For use in artillery survey, this azimuth must be converted to grid azimuth. The difference between true azimuth and grid azimuth (Figure 8-7) is called grid convergence.

8-11. DETERMINATION OF CONVERGENCE

a. The sign of grid convergence correction is plus when the observer's station is west of the central meridian and minus when the station is east of the central meridian. (See Figure 8-8.) Grid azimuth is determined by algebraically adding the true azimuth and the value of grid convergence. If the azimuth is determined in the Southern Hemisphere, the signs of convergence are opposite the corrections for the Northern Hemisphere.

b. Convergence is a function of both latitude and longitude. Its value is computed on DA Form 5599-R (Computation--Convergence of True Azimuth to Grid Azimuth (BUCS)) (Figure 8-9) by using the geographic coordinates and the grid zone of the observer. (A reproducible copy ot this form in included in the Blank Forms section of this book.) This form is basically the same as the other forms that you have used. The top six blocks are for administrative information. Below the administrative data, you will find notes that refer to the operation of the BUCS in this program. Under the notes on the left side of the form, you will find specific instructions on the use of this form. To the right of the instructions, you will find the data record where all known, field, and computed data are recorded. Blocks that are marked with a bold arrow are where field data are entered. Four separate computations for convergence can be computed on one form. (See Table 8-3 for instructions on computing DA Form 5599-R.)

8-12. DETERMINATION OF CONVERGENCE WITH NOMOGRAPH

a. Table 6a of FM 6-300 is a grid convergence nomograph. This nomograph permits a graphic determination of grid convergence in mils. The arguments for entering the nomograph are the easting coordinate rounded to the nearest 100 meters and the northing coordinate rounded to the nearest 10,000 meters.

b. The convergence is extracted from the right column of the nomograph as discussed below.

(1) With a straightedge, align the value of the easting coordinate on the left column of the nomograph with the value of the northing coordinate on the center (slanted) column.

(2) Read the amount of the convergence in mils from the right column.

(3) Apply the grid convergence to the true, or astronomic, azimuth to determine the UTM grid azimuth.

c. Grid azimuths determined with the nomograph are satisfactory for cannon battery orienting lines obtained by astro observation or gyroscopic means. The nomograph should not be used to determine convergence when directional control is initiated at a battalion SCP for fifth-order survey.

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*Section III

North-Seeking Gyroscope

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The north-seeking gyroscope (NSG) is a portable gyro compass that determines both true and grid azimuth with an accuracy of ±0.2 mil probable error (PE). (See Figure 8-10.) The FA surveyor can use the NSG to determine an azimuth, whenever required, that is within artillery orientation specifications. The NSG does not require any precise orientation or computations. Detailed information on the operation and maintenance of the NSG and additional equipment can be found in TM 5-6675-333-10 (TO 33D7-9-53-1).

8-13. PRINCIPLE OF OPERATION

The NSG is a precise survey instrument that finds the direction of geodetic north and determines azimuths relative to geodetic north (true north) or grid north. Trained personnel can determine a grid azimuth (after setup) in about 4 minutes. During the setup, there is a requirement to enter UTM coordinates accurate to the nearest 200 meters easting and 1,000 meters northing.

8-14. COMPONENTS

The NSG is housed in three watertight transport cases. Upon receipt of equipment, inspect each component, including the transport cases, for completeness and damage in accordance with (IAW) TM 5-6675-333-10. The instrument can be transported by vehicle, hand carried by using the carrying handles, or backpacked by using the shoulder straps located in each case.

a. SKK3-1 Gyro Compass. The gyro compass is a single box unit that is designed to fit into the gyro tripod. The heart of the gyro compass is a gyro motor suspended on a thin metal tape. Internally the gyro makes two coarse measurements and one (or two) fine measurements near north to determine an azimuth. The keyboard is attached to the gyro compass, and it is the operator's link to the gyrocompass. It has alphanumerical screens for data entry and display of azimuth and system status. The theodolite horizontal scale can be aligned with respect to the gyro mirror that is viewed through the autocollimation attachment. Power is applied to the NSG via the back of the gyro compass.

b. Power Supply.

(1) The SEB42-1 battery is a rechargeable 24-volt NICAD cell battery that is capable of about 35 measurements at 68°F. Use of lighting accessories will decrease battery life. The battery clamps to the rear of the gyro compass and is protected from discharging by a fuse.

(2) The SLG6-1 battery charger is capable of charging two SEB42-1 batteries at the same time, using either AC or DC. Depending on cell voltage and temperature, the charger automatically provides the appropriate current. The charger will accept 10 to 33 v DC or 115 to 230 v AC. At temperatures between 41°F to 122°F, it takes less than 7 hours to completely charge a dead battery, whereas at 14°F, it may take up to 14 hours.

(3) The SEV22-1 power adapter enables the gyro compass to be powered directly from vehicle power (9 v to 33 v). The adapter is protected with a fuse and is connected to the rear of the gyro compass as is the battery.

c. T16SK-1 Theodolite. The theodolite is a modified version of the current T16-84 (upright image). One distinction is the autocollimation box attached to the side, This is used to align the theodolite with the gyro. The scales are read the same as the T16-84 scales. The T16SK-1 comes with accessories such as electric lighting for night operations, target rod (barber pole with tricolor lights), eyepiece filter (sun filter), sun shade, adjustment tools, and rain and dust cover. For a complete list of accessories, see TM 5-6675-333-10.

Note. The telescope must be in the reverse position when determining azimuth. The laser range finder studs will face up when in the reverse position. When no range finder studs are present, the microscope (reading scale) will be on the left side of the telescope when facing the keyboard.

d. GST90-5 Gyro Tripod. The gyro tripod has three leveling screws, a circular level, a clamping screw, and a tilting dish capable of lateral movement of 1 inch for simple positioning of the north-seeking gyroscope. For transport, there is a protective cover for the tilting dish.

8-15. INITIAL SETUP PROCEDURES

The NSG can be set up over an established point (point already in the ground), or a point can be established after setup. Because of the size and the lack of an easy method for plumbing the tripod, it may be easier and faster to establish the point under the plumb bob after first setting up and leveling the NSG. If the point is already established, care must be taken to ensure the instrument is plumbed directly over the point. Setup procedures are in TM 5-6675-333-10.

Note. In a windy environment, setup the NSG as low to the ground as possible. The tripod legs should be extended only enough to allow for leveling procedures.

8-16. SURVEY APPLICATIONS

a. System Settings. When the gyro is turned on, the system will default to the factory settings of Grid (azimuth), NE (UTM coordinates), and 1U (standard measurement mode, which includes self-calibration). To change the azimuth setting, press the MODE key to select GEO (true north) and then press the STEP key. There are two other major settings: local parameters and measurement modes.

(1) Local parameter. The local parameter most commonly used will be NE (UTM coordinates). If the user wants to enter location in geographic coordinates, he selects LC (latitude and meridian correction) from the Control Menu.

(2) Measurement mode. The measurement mode most commonly used will be 1U. The advantage in using the 1U mode is that the operator does not have to decide whether or not to calibrate and does not have to know the zero-calibration procedure.

b. Entering Data. The keyboard is a right fill enter (input) type of keyboard. This means that when data are entered, the last number in a string of numbers is entered first. The decimal point on the keyboard applies only to azimuths.

c. Entering UTM Coordinates. Only four digits are entered when entering UTM easting and northing during the setup. When entering the easting coordinate of 546321.5 (accurate to the nearest 200 meters), the user enters 5463 (546,300 meters) by first entering 3, then 6, then 4, and finally 5. When entering the northing coordinate of 3819765.7 (accurate to the nearest 1,000 meters), enter 3820 (3,820,000 meters) by first entering 0, then 2, then 8, and then 3.

Note. When surveying within 1,000,000 meters north of the equator, the user enters three digits northing and four digits easting.

8-17. DETERMINATION OF E-2 ADDITIVE CONSTANT

The E-2 additive constant is a setting that can be input into the gyro computer to compensate for wear and tear on the instrument. When the equipment is new, the factory setting is 0 mils. Over time, the connections that fit the theodolite to the gyro can become worn. This can be caused by rough use of the equipment or just general wear and tear. For this reason, the gyro and theodolite must remain as a set. If for some reason a different theodolite or gyro is used together, a new E-2 additive constant for the pair should be determined. This procedure is performed under the following conditions:

• Upon initial receipt of equipment.
• When azimuth is suspect.
• After rough handling.
• Annually.
• Upon return from maintenance.

Note. The E-2 constant should be done more often, at least semiannually, if the gyro gets a lot of use.

a. Accuracy. To determine the E-2 additive constant, you need at least one survey line with an azimuth known to fourth-order or higher specifications. Ideally, the length of the survey line should be at least 1,000 meters.

CAUTION PADS does not meet the requirements to establish an azimuth line for this procedure. Determining the E-2 additive constant is a very critical procedure. This will affect all future azimuths. This procedure should be performed under the direct supervision of the survey chief.

b. Procedure for Determining E-2 Additive Constant.

(1) Set up the instrument at one end of the survey line.

(2) Adjust horizontal collimation error to zero. This should be done whenever the NSG is being used. Any collimation error will affect the accuracy of the azimuth that is determined.

(3) Determine gyro compass azimuth, and orient the theodolite.

(4) Sight in on the other end of the survey line, and read the horizontal scales.

(5) Calculate the azimuth correction. This is the additive constant (E-2). If the measured azimuth is more than the known azimuth, the correction is a negative value. If the measured azimuth is less than the known azimuth, the correction is a positive value. (See the example below.)

(6) Repeat the steps in paragraphs (2) through (5) five more times (for a total of six corrections).

(7) Algebraically apply the mean (E-2new) additive constant to the present correction (E-2old). (See the example below.)

c. Procedure for Entering the E-2 Additive Constant.

(1) Press the MODE key until CONTROL appears in the status display; then press the STEP key.

(2) Use the cursor and +/-key to enter the code (59382).

(3) Press the STEP key to read the previously stored additive constant.

(4) Compute new additive constant; then algebraically apply it to the old additive constant as shown above.

(5) Use the cursor and +/-key to enter E-2new, and press the STEP key to store it.

(6) Record date, old E-2 additive constant, new E-2 additive constant, and horizontal collimation error (if any) on a label attached to the gyro and protected by some sort of waterproof material.

Note. If the azimuth correction appears excessive, the accuracy of the azimuth line must be verified. Another option would be to use another verified azimuth line.

8-18. MAINTENANCE

Refer to TM 5-6675-333-10 for preventive maintenance schedule and troubleshooting table.

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