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Chapter 8

SITE


This chapter implements a portion of QSTAG 224.

Site is computed to correct for situations where the target is not at the same altitude as the unit. To understand site, a brief description of the trajectory is necessary.

8-1. Initial Elements of the Trajectory

a. Vertical Interval. The vertical interval is the difference in altitude between the unit or observer and the target or point of burst. (See Figure 8-1.) The VCO determines the vertical interval by subtracting the altitude of the unit or observer from the altitude of the target or point of burst. The vertical interval is determined to the nearest meter and is a signed value.

b. Angle of Site. The angle of site compensates for the vertical interval. The angle of site is the smaller angle in a vertical plane from the base of the trajectory to the straight line joining the origin and the target. The angle of site has a positive value when the target is above the base of the trajectory and a negative value when the target is below the base of the trajectory. The angle of site is determined to the nearest 0.1 mil and is a signed value. It carries the same sign as the VI.

c. Complementary Angle of Site. The complementary angle of site is an angle that is algebraically added to the angle of site to compensate for the nonrigidity of the trajectory. When large angles of site or greater ranges for any one charge are involved, a significant error is introduced because of changes in the shape of the trajectory. If CAS is not added to angle of site in low-angle fire, the trajectory will pass under the target if the target is at an altitude higher than the unit. The trajectory will pass over the target if the target is at an altitude lower than the unit. Complementary angle of site is dependent on the following:

  • Charge.
  • Range.
  • Angle of site.
  • Weapon system.
  • Projectile family.
  • Angle of fire (high or low).

(1) For a given charge and range, there is a specific complementary angle of site for every 1 mil angle of site. This specific value is listed in Table G of the TFT, Columns 12 and 13, in the form of the complementary site factor (comp site factor or CSF). The CSF must be applied to a particular angle of site to determine complementary angle of site. The CSF must be determined by interpolation for the chart range to the nearest 10 meters. Complementary angle of site is computed to the nearest 0.1 mil and is a signed quantity. The sign is the same as the CSF value.

(2) A study of listed values for the CSF reveals that for short ranges the CSF is negligible. As the range increases, the factor increases for any given charge. Thus, at greater ranges, the CSF is significant even for small angles of site. The CSF also varies with the charge for any given range.

d. Site. Site is the algebraic sum of the angle of site and the complementary angle of site. It is determined to the nearest mil and is a signed value.

e. Angle of Elevation. The angle of elevation is the vertical angle between the horizontal and the axis of the bore required for a projectile to achieve a prescribed range under standard conditions.

f. Quadrant Elevation. Quadrant elevation is the algebraic sum of site and the angle of elevation. It is determined to the nearest mil.

8-2. Site in High-Angle Fire

Site has a relatively small effect in high-angle fire because of the large angle of fall. In high-angle missions, a minus site must be used to compensate for a positive vertical interval and a plus site must be used to compensate for a negative vertical interval. Therefore, high-angle site will have the opposite sign of the VI.

8-3. Determination of Altitudes

The altitude of the unit or base piece is normally known by map spot or survey and labeled on the firing chart. To determine the target altitude, the VCO must analyze the call for fire sent by the observer.

a. The observer may report a target location by using the grid coordinate method. This method requires a map of the target area. The easiest way to determine altitude from a map is by reading the contour lines. The VCO plots the grid sent by the observer and extracts the altitude from the map.

b. The observer may report a target location by using polar coordinates. He locates the target in relation to his own location by sending a direction and distance to the target. The observer may also transmit an up or down vertical shift from his location. If the observer transmits a vertical shift, the altitude of the target is determined in relation to the observer by applying the vertical shift to the observer's altitude. If not, the grid is plotted and altitude is determined as in paragraph a.

c. The observer may report a target location with reference to a known point plotted on the firing chart. This method of target location is known as shift from a known point. The vertical shift sent by the observer is applied to the known point altitude to determine the target altitude.

8-4. Determination of Site without a Graphical Site Table

In Table 8-1 are the procedures for determining site without a GST.

8-5. Determination of Site Without a GST, Requiring Interpolation

The example in Table 8-2 uses data for the firing unit location and firing chart from Chapter 6. The following data are given:

Weapon System:                                                           M109A3
Charge:                                                                          4GB
Chart Rg From 1/A to the Tgt (Grid 430 290):            4,340 meters
1/A Altitude:                                                                  1062
Target Altitude (Map Spot):                                         1040

8-6. Determination of Vertical Angle

The vertical angle is the smaller angle in a vertical plane from the horizontal to a straight line joining the observer and target. The angle of site and the vertical angle are essentially the same angles viewed from different perspectives. (See Figure 8-2.) The steps for determining VA are in Table 8-3.

8-7. The Graphical Site Table

a. The computation of site with the TFT is time consuming. The GST was developed to provide a quick and accurate computation of vertical angle, angle of site, and site. The GST can also be used to compute the vertical interval when the site, the charge, and the range are known or when the vertical angle and the distance are known. It can be used to convert yards to meters or meters to yards and to multiply and divide. Each GST is designed for a particular weapon and projectile family, and the computations are valid only for the weapon specified on the GST.

b. The GST consists of three parts: a base, a slide, and a cursor with a manufacturer's hairline. (See Figure 8-3.)

(1) Base. The base is marked by the D scale, which is a logarithmic scale of variable graduations. This scale is used to determine VI, VA, angle of site, and site. The accuracy depends on the values read off the scale. The back of some GSTs have instructions on how to use it.

(2) Slide. The slide is marked with a C scale, gauge points, and site-range scales.

(a) C (range) scale. This scale is identical to the logarithmic D scale, and there are two sides to the slide. The C and D scales, along with the M gauge point, are used for computing vertical interval, vertical angle, and angle of site. Multiplication and division may also be performed by using the C and D scales.

(b) Gauge points. The C scale is marked with two M (meter) and YD (yard) gauge points. The M gauge point multiplies the value opposite the C index by 1.0186, which gives a precise solution to the mil relation formula and is used in all computations ( = 1.0186 W/R). The YD gauge point multiplies the value opposite the M gauge point by 0.9144, which gives an immediate solution to the formula: (YARDS x 0.9144 = METERS).

(c) Site-range scales. These scales are used to compute site when the VI and range are known or to compute the VI when the site and range are known. For each charge indicated, there are two site-range scales. One is black, marked "TAG," and the other is red, marked "TBG." Each side is placed in relation to the M gauge point so that site is read on the D scale opposite the M gauge point when VI on the D scale is divided by range on the site-range scale. The TAG and TBG scales are constructed to include CAS. They differ from each other just as the CSF for a plus angle of site differs from the CSF for a minus angle of site. The TAG scale is used when the VI is plus, and the TBG scale is used when the VI is minus. The value of site is read or placed opposite the M gauge point. When there are no site range scales for a particular charge or the scale does not include the appropriate gun target range, site for that charge must be computed manually.

(d) Range changeover point. On all GSTs for all charges, there is a point on all site-range scales where the scales begin to "double back"; that is, the cursor is moved to the left rather than to the right for an increase in range for a given VI. The range at which each scale reverses direction is called the range changeover point. The location of the changeover point can be shown by plotting site as a function of site in mils and range in meters (Figure 8-4). Recall that site equals the angle of site plus the complementary angle of site. In Figure 8-4, at the lesser ranges (5,000 to 7,000 meters), the angle of site is decreasing at a greater rate than complementary angle of site is increasing; thus, site decreases. At the longer ranges (8,000 to 9,000 meters), the angle of site is decreasing at a lesser rate than the complementary angle of site is increasing; thus, site increases. The site curve shows decreasing values up to a range of about 7,600 meters and then increasing values beyond. The range at which site is at an absolute minimum value is 7,600 meters and is the range changeover point for that charge and projectile.

(e) Cursor. The cursor has a vertical hairline, known as the manufacturer's hairline. It enables the user to place or read a value on the slide opposite another value on the base.

8-8. Average Site

a. A considerable amount of time can be saved in mission processing if average site can be precomputed for the area of operations. As time permits after occupation, the VCO should develop a color-coded average site map (Figure 8-5). The average sites and altitudes would be listed within each color-coded area. Site is computed for vertical interval segments on the basis of ranges and charges to be used most frequently. The error in site will normally be small and is an acceptable tradeoff of accuracy for speed. When a target is plotted on the average site map, the VCO can read and announce site. This technique may not be practical in certain situations, for example, in mountainous terrain or in fast-moving situations. Here the VCO could use the altitude of the nearest preplotted target to compute site.

b. The VCO creates and improves his average site map by using the following steps.

(1) Plotting of contour intervals. The VCO color-codes his map along with selected contour intervals, creating zones with little variation in altitude. VI is based on the mean altitude in each zone. Compute site for each color-coded zone by using the range to the center of the zone and the appropriate charge. This will result in an average site to use for all targets plotted within a color-coded zone.

(2) Refining average site. As time permits, average site values can be refined by computing additional values for variations in range within a color-coded zone. This will determine if there are significant changes in site caused by changes in range. For example, site would be computed for a zone between the 300 and 320 contour intervals by using ranges throughout the zone (that is, 5,000, 6,000, 7,000). If site changes by more than 1 mil, the VCO would announce the refined site.

8-9. Determination of Angle of Site and Vertical Angle With the GST

a. The procedures for computing angle of site and vertical angle are the same. Both are computed by using the C and D scales and are not associated with a particular charge or a particular weapon. In each case, two values are needed: the range (or distance) to the target in meters and the number of meters the target is above or below the howitzer or observer (vertical interval).

b. The diagram in Figure 8-6 is known as the Magic T. It can be used to help determine angle of site, VA, and site when using the GST. The horizontal line in the Magic T represents division, and the vertical line represents multiplication.

c. The following steps in Table 8-4 show how to determine angle of site and VA by using the GST.

8-10. Determination of Site With the GST

Site is computed by using the site-range and D scales. The value determined will be valid for a particular charge, weapon, and projectile family. Two values are needed--the range to the target in meters and the vertical interval. Use the steps in Table 8-5 to determine site with a GST.

8-11. Sample Problems

The examples in Tables 8-6 through 8-9 use data for the firing unit location, known point, and observer (T03) from Chapter 6. The following data are given:

Weapon System:                                     M109A3
Charge:                                                    4GB
Chart Rg From l/A to Known Point 1:    4,960 meters
Distance from T03 to Known Point 1:    1,760 meters
1/A Altitude:                                            1062
T03 Altitude:                                           1127
Known Point 1 Altitude:                          1024

a. Determination of Site (Manual Computation). Table 8-6 shows an example of manually determining site.

b. Determination of Vertical Angle (Manual Computation). Table 8-7 shows an example of manually computing VA.

c. Determination of Angle of Site and Vertical Angle With the GST. Table 8-8 shows an example of determining angle of site and VA with the GST.

NOTE: The values in parentheses pertain to the observer and the determination of VA.

d. Determination of Site With a GST. Table 8-9 shows an example of determining site with a GST.

8-12. High-Angle Site

a. Site is always computed for high-angle fire and added to the determined angle of elevation, which yields high-angle QE. However, site may have a relatively small effect in high-angle fire because of the large angle of fall. Therefore, if the angle of site is small and the FDO directs to ignore it, then site may be ignored.

b. In high-angle fire, an increase in the angle of elevation decreases range. A decrease in the angle of elevation increases range. The complementary site factors, found in Table G of the TFT, are relatively large (greater than 1) and are the opposite sign of the VI and angle of site. Therefore, the site will have the opposite sign of the VI and angle of site.

c. High-angle site is determined by using the CSF (TFT) or the 10-mil site factor from the GFT. Using the GFT is the preferred method. The reading obtained from the 10-mil site factor scale is the actual site for each 10 mils of angle of site. The site is computed by multiplying the angle of site, divided by 10, by the 10-mil site factor. The 10-mil site factor is always negative.

8-13. Determination of High-Angle Site With the TFT

The procedures for computing high-angle site with a TFT (Table 8-10) are the same as low-angle manual computations of site (Table 8-1). A GST can be used to compute the angle of site.

8-14. Determination of High-Angle Site With a High-Angle GFT

The use of the high-angle GFT to determine site is the preferred method. (See Table 8-11.)

8-15. Determination of 10-Mil Site Factor Without a High-Angle GFT

The 10-mil site factor is the value of high-angle site for every 10 mils of angle of site. The 10-mil site factor can be determined manually by solving two equal equations for the 10-mil site factor.

SI = < SI + CAS (FOR LOW AND HIGH ANGLE)
SI = < SI + ( | < SI | X CSF)

FOR POSITIVE ANGLES OF SITE:

HIGH ANGLE SITE = < SI ( 1 + CSF )

FOR NEGATIVE ANGLES OF SITE:

HIGH ANGLE SITE = < SI ( 1 - CSF )

USING THE HIGH ANGLE GFT:

HIGH ANGLE SITE = (< SI / 10) X 10-MIL SI FACTOR

HOW TO DETERMINE 10-MIL SI FACTOR WITHOUT A GFT:

FOR POSITIVE ANGLES OF SITE: 10-MIL SI FACTOR = 10 ( 1 + CSF )
FOR NEGATIVE ANGLES OF SITE: 10-MIL SI FACTOR = 10 ( 1 - CSF )

NOTE: If the 10-mil site factor is not listed on the high-angle GFT, use the last listed value or change charges.

The FDC can compute high-angle site by manually determining the 10-mil site factor for those situations when a high-angle GFT is not available. The 10-mil site factor from the GFT actually reflects the complementary angle of site for a positive VI. Therefore, this method will introduce a slight inaccuracy when estimating for negative VIs.




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