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APPENDIX F

FIREFINDER MASK CONSIDERATIONS



This appendix explains how to calculate the track volume for the Firefinder
radars. It is intended for use by the TA radar operators to enable them
to determine whether the radar site will provide enough track volume to
locate firing weapon positions. It also tells what actions they can take
when they calculate insufficient track volume from the position.

DEFINITIONS

The following are definitions of terms associated with mask.

Flat mask is a single mask angle or a default value in the radar when no mask angle is entered into the radar computer. The flat mask default for the AN/TPQ-36 radar is 20 mils. The flat mask default for the AN/TPQ-37 radar is 8 mils.

Mask angle is the vertical angle from the radar to the top of the mask, or screening crest, at a given azimuth. For a given sector of search, the lowest mask angle and the highest mask angle are calculated and entered in the radar computer during its initialization procedures.

Mask variation is the difference between the lowest and highest mask angles.

Vertical scan is another system characteristic of the radar. It identifies the maximum vertical scanning capability of the specific type of radar. Vertical scan for the AN/TPQ-36 radar is approximately 80 mils with all scanning frequencies enabled. Each frequency that is disabled results in a loss of approximately 2.5 mils of vertical scan. (See TM 11-5840-354-10-1 for exact values for loss of vertical scan.) Vertical scan for the AN/TPQ-37 radar is approximately 104 mils. Since this radar uses phase or phase scan rather than the phase or frequency scan used by the Q-36, no vertical scan is lost when some of the frequencies of the Q-37 are disabled.

Figure F-1. Radar Characteristics

TRACK VOLUME

Track volume is determined by two factors--the vertical scan of the radar and the amount of vertical scan that is lost because of the terrain contour, or screening crest, in front of the radar. From any radar position, the altitude of the screening crest along the terrain contour in front of the position will vary in the radar sector of search. This varying screening crest altitude results in varying mask angles along the terrain contour. The variance between the smallest mask angle and the largest mask angle reduces the radar vertical scan.

Sometimes this reduction is enough that the available scan coverage is less than the 50-mil track volume required by the radar to extrapolate firing weapon locations. When the track volume is reduced below 50 mils, the radar operator must compensate either by artificially adjusting the low mask angle or by moving the radar to a new position from which adequate track volume is available.

Whenever possible, an aiming circle or other accurate measuring device should be used to determine mask angles along the terrain contour. The measured mask angles are entered in the computer to depict the terrain contour. Otherwise, the radar will radiate into hill masses that are higher than the flat mask default in the radar computer.

The Firefinder search fence starts at the lowest mask angle entered in the radar computer (or at the flat mask default value if no lowest mask angle is entered) and goes to the highest point of the vertical scan. The first step in calculating track volume for the radar site and search sector is to subtract the low mask angle from the high mask angle. This difference must then be subtracted from the vertical scan of the radar. The result is the track volume for the radar site.

Although the ideal mask variation (see Chapter 3) may be met, the maximum allowable mask variation for each radar can be calculated by subtracting the 50 mils of track volume required for firing weapon location from the vertical scan of the radar.

Figure F-2. Example

Thus, it can be seen that any mask variation exceeding the allowable maximum (that is, 30 mils for the Q-36 and 54 mils for the Q-37) would not allow enough track volume for the radar to determine firing weapon locations. In that event, some action must be taken to regain enough track volume to perform the radar mission.

The radar uses the lowest and highest mask angles entered during initialization to establish mask variation internally. If the mask variation of the position exceeds the maximum allowable for the radar as discussed above, the lowest angle can be adjusted (raised) by the operator to reduce the actual mask variation to the maximum allowable for purposes of performing the radar mission. Of course, since the lowest mask angle determines the lowest part of the search fence, this adjustment will prevent the radar from tracking any rounds that are below the new lowest mask angle. If this is unacceptable, then the radar must be moved to a new position that has a lower mask.

TRACK VOLUME CALCULATION
AND SUBSEQUENT ACTIONS

On the basis of the previous discussion, the following steps should be used to calculate available track volume from a given radar position. Included are further steps to be taken when the calculated track volume from the position is not enough for the radar to perform its mission.

  • Step 1. Determine the lowest mask angle from the position. Use the formula from Chapter 4 (mask angle = change in elevation / range in thousands).

  • Step 2. Determine the highest mask angle. Use the same formula used in Step 1.

  • Step 3. Subtract the lowest mask angle determined in Step 1 from the highest mask angle determined in Step 2 to obtain mask variation.

  • Step 4. Subtract the mask variation determined in Step 3 from the vertical scan for the type of radar involved to obtain total track volume available.

    o If total track volume available is greater than 50 mils, then the position is adequate for performing the mission.

    o If total track volume available is less than or equal to 50 mils, then proceed to Step 5.

  • Step 5. Subtract the mask variation determined in Step 4 from 50 mils to determine how much the mask variation must be decreased to obtain 50 mils track volume.

  • Step 6. To decrease the mask variation, add the value determined in Step 5 to the lowest mask angle and enter the sum in the radar computer as the revised lowest mask angle.

If the radar technician's evaluation of the Step 6 value is that the resulting adjustment of the vertical scan reduces the capability of the radar to perform its mission adequately, then he must coordinate movement of the radar to a new position and recalculate data.

The following example illustrates the calculation of track volume. A solution is provided for each type of Firefinder radar. Remember that mask variances greater than 30 mils for the AN/TPQ-36 radar and 54 mils for the AN/TPQ-37 radar may prevent the radar from obtaining the required track volume to provide firing weapon locations.

Figure F-3a. Example - Situation

Figure F-3b. Example (Continued)

Figure F-3c. Example (Continued)





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