1. Determine the distance to a target by using the "flash-to-bang" method given a situation.
2. Identify the procedures used to determine distance to a target using an observed fire (OF) fan and terrain association.
3. Determine lateral distance to a second point given a point of known distance and an observation.
2301. Distance
Once you have determined the direction, you must determine the distance to the target. Distance is the horizontal space between a reference point and a target or between you and the target.
Determining a distance is normally the most difficult variable in target location. It is also used in the adjustment of fire. The standard unit of measure for a distance is the meter. Distance can be determined by one, or a combination, of these methods: lasers, flash-to-bang, or estimation.
a. Lasers. Lasers are the preferred method for determining observer to target (OT) distance.
When a laser is used, distance can be determined to an accuracy of 10 meters. The MULE and the AN/GVS-5 laser rangefinder provide an observer with the fastest and most accurate means of determining distance.
b. Flash-to-Bang. Because we know that sound travels at a speed of 350 meters per second, we can use sound to estimate distance. To use this method, you must count the number of seconds between the detonation of the round (FLASH) and the sound (BANG) of the impact.
Multiply the number of seconds by 350. The answer is the distance in meters from the observer to the target or round. Use the equation
FLASH-TO-BANG (in seconds) X 350 = DISTANCE
This method has several uses to the observer. It not only determines a distance, it can be used to confirm a distance. It can also be used to determine the distance to the muzzle flash of enemy
(1) A mental estimate is made by use of a known unit of measure. Distance is estimated to the nearest 100 meters by determining the number of known units of measure, such as a football field (100 yards), between you and the target.
(2) You must consider the following effects when estimating a distance:
vObjects appear nearer
v When the background is in contrast to the color of the object vObjects appear more distant
v In poor light or fog
v When only a small part of the object is seen
v When looking over a depression, most of which is visible v When the background is similar in color to the object
(3) When visibility is good, distance can be estimated by using the appearance of tree trunks, their branches, and foliage (using the naked eye) in comparison to map data.
(4) You must always do a terrain/map analysis to assist in estimating distances. When you are looking in a specific direction, the estimation of distance can be enhanced by studying the terrain to associate what you are seeing on the ground with what you see on your map. The use of an observed fire (OF) fan helps you associate map and compass direction.
(5) Altitude is the vertical distance measured from sea level. The standard unit of measure for altitude is the meter. Altitude is determined from a map study using the contour lines and contour interval.
2302. Observed Fire (OF) Fan
The OF fan is a transparent protractor that can be used to show the direction and distance on the map. It helps the observer identify on the map what he sees on the ground. The OF fan has 17 radial lines that are 100 mils apart and cover a total area of 1600 mils. The radial lines represent OT direction. The OT distance is represented by arcs marked on the fan every 500 meters and labeled every 1000 meters beginning at 1000 and extending to 6500. The OF fan is available
a. Orienting the OF fan. To use your OF fan effectively you must first orient your map, locate your position, and mark your location on the map. Then locate a reference point in the direction of your area of operation. Now you are ready to orient your OF fan by performing the following steps:
(1) Place the vertex of the OF fan over your location.
(2) Place the center radial line in the direction of your reference point.
(3) Move the fan slightly until one of the radial lines is parallel to a grid line (regardless of what direction to the reference point may be), one of the radial lines will always point to a cardinal direction.
(4) Attach the OF fan in position on the map by using a piece of tape along one edge of the fan. The fan can then be lifted, if required, to inspect the map without disturbing its orientation.
(5) With a marker pen or grease pencil, number the even directions (every second radial line) dropping the last two zeros.
b. Determine target location. To determine the target location, you must first determine direction using one of the methods previously discussed. Sight along that direction on your OF fan and map until you associate the terrain around the target with the proper contours on the map.
Observe the ticks on your OF fan, or measure with a protractor, and read the distance +/- 100 meters. In figure 2-8, you would determine the distance to the target located at grid 531269 as 3200 meters.
Figure 2-8. Target location using the OF fan.
2303. Determining Lateral Distance
Lateral distance is the distance left or right, from one object to another, perpendicular to your line of sight. Determining lateral distance is important for you in order to adjust fire and for locating targets using the shift from a known point method of target location, which will be discussed in the next lesson. The accuracy of your lateral distance estimation will depend upon how well you determine the distance from you to the target. Once you have established the distance from you to the target, you will use the mil relation formula (W=R x m) to convert the measured angle in mils to a lateral shift in meters. This formula is based on the rule that 1 mil of arc, at a distance of 1000 meters, will equal 1 meter of lateral distance. It's similar to how windage and elevation changes on your rifle create different points of impact at different ranges.
The mil relation formula: W = R x m
W = Width or lateral shift (distance) in meters
R = Range or distance measured in meters (divided by 1000)
Follow the sample problem to use the mil relation formula to determine a lateral distance.
Looking at figure 2-9, follow the step-by-step instructions:
Figure 2-9. Lateral distance determination.
Known: Distance to known point-2500 meters angle in mils-60 mils
Step 1: Find the value of R. Divide the distance to the known point by 1000, 2500/1000 = 2.5. The value of R = 2.5.
Step 2: The measured angle is 60 mils. The value of m = 60.
Step 3: To find W, multiply the value of R by the value of m, 2.5 x 60 = 150. The value of W = 150 meters.
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Exercise: Complete items 1 through 3 by performing the action required. Check your reponses against those listed at the end of this study unit.
1. Which of the methods for determining distance is the most accurate?
a. Estimation c. Flash-to-bang
b. Lasers d. OF fan
2. You have observed an enemy crew served weapon firing. You saw the muzzle flash and counted 4 seconds until you heard the report of the weapon. What is the distance to the enemy?
a. 140 meters c. 1400 meters
b. 1240 meters d. 12400 meters
3. When determining distance to a target using terrain association and the OF fan you read distance to the target to +/- ________ meters.
a. 1 c. 50
b. 10 d. 100
4. You have observed a target to the right of a known point. The distance to the known point is 1500 meters. You measured the angle between the known point and the target to be 50 mils. What is the lateral distance from the known point to the target?
a. 50 meters c. 500 meters
b. 75 meters d. 750 meters
Lesson 4. METHODS OF TARGET LOCATION