Afghanistan: firing artillery accurately with air force met support

In the summer of 2002, Task Force Panther, 82d Airborne Division, Fort Bragg, North Carolina, received the mission to replace the 101st Airborne (Air Assault) Division in Afghanistan in support of Operation Enduring Freedom. Due to limited airspace and the limited amount of indirect fire assets tasked--a 105-mm battery and two 120-mm mortar platoons for the one FA battalion deploying--the division artillery meteorological (Met) learn remained at Fort Bragg. Left without one of the five requirements for accurate, predicted fire, Charlie Battery, 1st Battalion, 319th Airborne Field Artillery Regiment (C/1-319 AFAR) developed techniques to accurately account for the extreme climate of the combat theater.

Before the battery arrived, A/1-319 AFAR, already in Afghanistan, was reporting range and fuze setting errors during live fire missions while using 120-mm mortars. These errors largely were due to the high temperatures and reduced air density of the Southwest Asian country. C Battery began to see similar firing errors when it calibrated in the desert south of Kandahar. The battalion's Q-36 radar at Kandahar repeatedly reported rounds impacting over the target while the radar was set in the friendly mode.

Due solely to weather conditions, the fire direction center (FDC) faced nearly 500 meters in range errors. With initial assistance from the Air Force's Bagram weather team, we developed procedures to negate these errors by using the Air Force's Interactive Gridded Analysis and Display System (IGRADS).

IGRADS is web-based software that generates 24-hour forecasts of weather conditions up to 50,000 feet above the surface for any given latitude or longitude and is accessed through secure internet protocol relay (SIPR) accounts. IGRADS outputs the data in the format of altitude in feet above ground level (AGL), pressure in millibars, temperature in Celsius, density in grams per cubic centimeter, absolute humidity in grams per cubic meter, wind speed in meters per second and wind direction in degrees. The information can be interpolated and converted into a computer Met message; then with the weighting factors found in FM 6-16, Tables for Artillery Meteorology (Electronic) Ballistic Type 3 and Computer Messages, it can be converted to a ballistic Met message for mortars.

Quantifying the Problem. Although accounting for Met may make little difference at installations where the weather parallels standard conditions much of the year (such as Fort Bragg), the lack of Met was a serious deficiency in the summer heat and high altitudes of Afghanistan. Low air density, a function of high temperatures and low pressures, reduces the drag of a projectile and, therefore, causes positive range errors.

Heightened temperatures also affect the drag of a round because of their effects on the compression waves that form in front of and behind the projectile. This drag effect is not linear; but for most M119A2 firing data, an increase in temperature corresponds to an increase in achieved range. High desert temperatures combined with high altitudes, therefore, can cause significant deviations from standard.

The Army already has had this problem in Southwest Asia where extreme temperatures and low density caused range corrections of up to 4,700 meters (FM6-15 Tactics, Techniques, and Procedures for Field Artillery Meteorology, Page 3-13).

The first Met data the Air Force provided using its IGRADS simulation software showed the disparity between the air density and temperature in our area of operations and those of standard conditions (represented in the battery computer system's standard Met file). See Figure 1. On 10 September, the surface temperature at Bagram that is 1,456 meters above mean sea level (MSL) was 299 degrees Kelvin at 1630 local time. The corresponding temperature in standard Met was 279 degrees Kelvin. This seven percent increase would lead to a 195-meter range error when firing at a distance of 11,000 meters, according to Tabular Firing Table (TFT) 105-AS-4. Additionally, the air density at the surface of the Bagram Met was seven percent lower than the equivalent altitude of standard Met, leading to a 281-meter range error. These errors are even more significant when coupled with the fact that 1-319 AFAR's main mission is to provide close supporting fires to Task Force Panther.

Despite the large differences in temperature and density, pressures only diverged slightly between the Bagram and standard Mets. This similarity is largely due to the fact that Bagram and the location on which standard conditions is based both are about 30 degrees latitude, one of the semi-permanent pressure regions created through the earth's patterns of air circulation.

Accounting for Temperature Changes. Before C/1-319th departed Bagram, the Air Force weather station simulated Met data for Firebase Cobra Strike at Khowst that is at an elevation of 1,140 meters. (See the map in Figure 2.) This Met data gave a good representation of pressures for the firebase, but it was still imperative to account for the changes in temperature that happen within a 24-hour period. The difference between the Khowst Met (taken when the surface temperature was approximately 68 degrees Fahrenheit) and the temperature in the middle of that same day (100 degrees Farenheit) caused a plus-250 meter range error because of temperature's dual effect on drag. It was not possible to track these temperature changes through bi-hourly Met messages, as is the normal procedure. Instead, we had to formulate a new technique.

To isolate temperature changes, pressure was set as being independent of temperature, a decision supported by later analysis of temperature and pressure changes through different 24-hour periods. Due to the complexity of meteorological conditions, no direct relationship between temperature and pressure existed in any of the periods studied.

Analyzing temperature gradients over different periods, we found that surface temperature changes in a given day did not affect temperatures at altitudes beyond 4,000 feet (approximately 1,250 meters) above the surface. Irrelevant of the surface temperature, all temperatures from Line 04 of the computer Met message and above were the same in any 24-hour period. This trend is displayed by the data in Figure 3 on Page 40 from 17 November 2002.

Using this information, we created Met messages for five-degree surface temperature intervals from 55 to 100 degrees Farenheit. We calculated these Met messages by taking the given surface temperature and proportionately reducing it to the temperature at 4,000 feet, based on the temperature gradient of the Air Force Met data. This procedure created 10 Met messages with various surface temperatures but with identical temperatures at 4,000 feet and above (See Figure 4 on Page 40.) The same pressures were used for all 10 Met messages.

Once the Met messages were created, the FDC selected them based on propellant temperature. Because propellant temperatures usually lag behind air temperature, whether the air is getting warmer or cooler, the FDC would select a Met file offset from the average propellant temperature. For example, the 80-degree Met would be used if propellant temperatures were increasing and averaged around 75 degrees Fahrenheit. In this manner, we accounted for temperature and its effect on the projectile drag.

While the temperature gradient of current Met data is the best representation of the temperatures of the area for a given period, the entire temperature gradient can shift over time. Alternatively, we found that the standard temperature decrease of 6.5 degrees Celsius per 1,000-meter increase in altitude mirrors actual graphs of temperatures for Khowst. (See Figure 5.) Therefore, one should consider using the standard temperature change when no recent Met data is available.

A high-burst registration validated Charlie Battery's procedures a few days after we arrived in Khowst. The registration was conducted 7,005 meters to the northeast with two observers using precision lightweight global positioning system receiver (PLGR) grids. After applying Met and the muzzle velocities for the registered lot, the range correction was only two meters with a fuze setting correction of 0.1. There was still a significant deviation correction, but wind data was not known or applied for the registration.

Further support came during a rocket-assisted projectile (RAP) shoot to the northwest using AH-64 Apache attack helicopters for a laser-adjust mission. At a range of more than 13,300 meters and without a registration, the range correction was merely 33 meters.

Army doctrine warns against using meteorological information more than four hours old or more than 10 kilometers from the midpoint of the trajectory in mountainous terrain. By using IGRADS and accounting for surface temperature changes, Charlie Battery fired accurately with weather information that was up to 30 days old.

Some difficulty arose when the battery conducted missions at altitudes significantly different than the Met station. When the battery flew to Shkin, for example, the firing point altitude of approximately 2,200 meters was nearly double that of Firebase Cobra Strike and Cobra's Met station. Using propellant temperatures as a basis for selecting the Met file to use would not work there. Therefore, we had to estimate the temperature at Cobra's Met station as compared to Shkin's surface temperature. To avoid simply guessing the temperature difference between the Met station and the firing point, we used Table D of the tabular firing table to help calculate it.

The drawback to Table D is that it only works for changes in altitudes less than 390 meters. So we extended the data mathematically and applied human logic for altitude changes greater than 390 meters.

Easy Access to IGRADS. Once the Army emplaced a SIPR line into the artillery tactical command post (TAC) at Forward Operating Base (FOB) Salerno, which was a few hundred meters from Firebase Cobra Strike, we could get Met data directly from IGRADS. Not only could we compute current Met messages for the firebase, but we also could produce Met messages for any Met station the mission dictated.

Before departing on a weeklong ground assault convoy that covered more than 400 kilometers southwest of Gardez during Phase Ill of Operation Alamo Sweep, we generated Met messages at two- to four-hour intervals over a 24-hour period for all three of the future firing points. (See Figure 4.) In this manner, the battery no longer had to estimate the temperature difference between a current firing point and the previous Met station or rely on a Met message that was calculated for a Met station much farther away than the advised distance of 10 kilometers.

We prepared the Met messages for the three new position areas by computing a day's average pressure at each zone for each firing point and then using that data for each of the 10 Met messages. We again created Met messages for five-degree temperature changes, this time ranging from 30 to 75 degrees Fahrenheit.

For Firebase Cobra Strike, Met data now could be forecast with IGRADS that fell within the distance and time requirements of FM 6-15. Based on a daily access to the SIPR net, we could account for winds after we converted the wind speeds to knots and wind directions to mils. When it was not feasible to obtain the Met data for the day, we did not account for wind and used the average pressure profile to generate Mets, as explained in Figure 4. Wind changes are often enough to prevent an FDC from using wind data that is even a few hours old.

Ballistic Mets. IGRADS output also can be converted to ballistic Met messages for mortars, a capability that has become more important to the artillery due to recent deployments of artillery batteries armed with 120-mm mortars.

However, a ballistic Met message is not as straightforward as a computer Met message. The air density and temperature values at each line of the Met message represent a weighted average of the conditions from the surface through that line of Met and back to the surface again and are listed as percentages of standard. The ballistic air density for Line 2 is, therefore, a weighted average of the air densities of Line 1 and Line 2.

To convert the Air Force Met data to a ballistic Met message, one must use the weighting factors and standard conditions found in FM 6-16. Figure 6 shows an example computation for ballistic air density for Line 2 of the Bagram Met. Ballistic temperatures are computed in a similar manner (consult FM 6-16, Pages 2-83, 2-104, 2-133).

Future Use of IGRADS. The Air Force's IGRADS has proven a powerful tool to support an artillery or mortar battery left without a supporting Met section. IGRADS allowed C/1-319 AFAR to fire accurately in a rugged climate despite the lack of normal artillery Met support. Range errors were small or nonexistent. We could also have decreased our deviation errors with more consistently available wind data.

The Field Artillery and Army should consider tapping into this system or implementing a similar system. To fully use the software's capability and free an FDC from relying on a spreadsheet or a calculator, we would have to alter the output of the software to match the format of computer and ballistic Met messages. When that happens, the Army will be better poised to rapidly react to small-scale warfare across the globe.

It is likely that another battery will find itself without artillery Met support somewhere in the world during future phases of the War on Terrorism, and it is in America's best interest to set it up for success.

[FIGURE 3 OMITTED]

[FIGURE 5 OMITTED]

Figure 1

Standard Met Compared to Bagram Met. The Met was taken at 1200 Zulu on
10 September 2002.

             Standard Met (Sea Level)
Zone      Midpoint  Pressure  Temperature      Density
No.        Height     (mb)    ([degrees]K)  (gm/[m.sup.3])

 00          0        1013       288.2          1225.0
 01         100       1001       287.5          1213.3
 02         350       0972       285.9          1184.4
 03         750       0926       283.3          1139.2
 04         1250     (0872)     (280.0)        (1084.6)
 05         1750     (0820)     (276.8)        (1032.0)
 06         2250      0771       273.5
 07         2750      0724       270.3

                     Bagram (1456 Meters MSL)
Zone      Pressure        Temperature      Density
No.         (mb)          ([degrees]K)  (gm/[m.sup.3])

 00        (848)             299.2         (986.1)
 01         839              298.5          985.8
 02         815              296.9          977.6
 03         778              293.8          922.1
 04         734              289.8          882.5
 05         692              284.9          844.6
 06         651              281.0
 07         613              276.1

Legend: gm/[m.sup.3] = Grams per Cubic Meter K = Kelvin mb = Millibars
MSL = Mean Sea Level

Figure 4: Example of Computation of Computer Met Messages Using IGRADS Data

Step 1: Access Data From IGRADS Website.

a. Go to http://weather.offut.af.smil.army.mil/igrads.html using a secure internet protocol relay (SIPR) account.

b. Select Afghanistan--5-kilometer map.

c. Select alphanumeric output.

d. Input latitude and longitude in degrees and minutes.

e. Select the date and time for the Met data -available in one-hour intervals for a 24-hour period.

Step 2: Convert Raw Data into Computer Met.

a. Multiply altitude in feet by 0.3048 to obtain altitude in meters.

b. Add 273.15 to degrees in Celsius to obtain degrees in Kelvin.

c. Based on the age of the Met data, either set wind data to zero or convert direction to mils and speed to knots. If the time of the IGRADS Met data matches within a couple of hours of the time the Met message will be applied, then wind data can be considered fairly reliable. Otherwise, it should be set to zero.

d. Interpolate the data to obtain the weather information at computer Met message midpoint altitudes.

    IGRADS Generated Met for          Converted Altitude
 34[degrees]57'69[degrees]17',          and Temperature
         101200 Sep 02
Altitude      Temp              Pressure   Altitude       Temp
(ft AGL)  ([degrees]C)            (mb)      (m AGL)    ([degrees]K)

Surface        26                848.28        0          299.2
  1000         24                819.34      305          297.2
  2000         22                791.07      610          295.2
  3000         19                763.56      914          292.2
  4000         17                736.81     1219          290.2
  5000         14                710.70     1524          287.2

                Final Bagram Met Interpolated at
                 Midpoint Altitudes, Lines 00-04
Altitude  Altitude            Temp           Pressure
(ft AGL)  (m AGL)         ([degrees]K)         (mb)

Surface       0              299.2             848
  1000      100              298.5             839
  2000      350              296.9             815
  3000      750              293.8             778
  4000     1250              289.8             734
  5000

Step 3: Generate Met Messages for Different Surface Temperatures.

a. Using the existing temperature gradient, proportionately converge temperatures at 1,250 meters (or use standard temperature change of -6.5[degrees]C per 1,000,-meter increase).

b. Use the same pressures for all Met messages generated.

     Bagram Met--Surface           Met for Surface
Temperature  of 79[degrees]F        Temperature of
                                     90[degrees]F
Altitude      Temp      Pressure       Altitude      Temp
(m AGL)   ([degrees]K)    (mb)         (m AGL)   ([degrees]K)

   0        (299.2)       848             0        (305.4)
 100         298.5        839           100         304.3
 350         296.9        815           350         301.6
 750         293.8        778           750         296.4
1250        (289.8)       734          1250        (289.8)

          Met for Surface              Met for Surface
          Temperature of         Temperature of 100[degrees]F
           90[degrees]F
Altitude  Pressure       Altitude      Temp      Pressure
(m AGL)     (mb)         (m AGL)   ([degrees]K)    (mb)

   0        848             0        (310.9)       848
 100        839           100         309.4        839
 350        815           350         305.7        815
 750        778           750         298.7        778
1250        734          1250        (289.8)       734

Figure 6

Procedures for Computing Ballistic Air Density

                      Line 1  Line 2

Standard Air Density  1213.3  1184.4  1. 1213.3 x 0.43 + 1184.4 x 0.57 =
(gm/[m.sup.3])                           1,196.8(gm/[m.sup.3]) (Standard
                                         Ballistic Air Density for
                                         Line 2)

Bagram Air Density    985.8   977.6   2. 985.8 x 0.43 + 977.6 x 0.57 =
(gm/[m.sup.3])                           981.1 (gm/[m.sup.3]) (Bagram
                                         Ballistic Air Density for
                                         Line 2)

Weighting Factors      0.43    0.57   3. 981.1/1,196.8 = 82.0% (Bagram
for Line 2                               Air Density for Line 2
                                         Expressed as Percent of
                                         Standard)

First Lieutenant Joshua D. Mitchell is the Fire Direction Officer for C Battery, 1st Battalion, 319th Airborne Field Artillery Regiment, 82d Airborne Division, Fort Bragg, North Carolina, currently deployed to Afghanistan for Operation Enduring Freedom. In his previous assignment with the 82d Division Artillery, he was the Fire Support Officer for B Company, 2d Battalion, 505th Parachute Infantry Regiment. He holds Bachelors Degrees in Biomedical Engineering and Mathematical Sciences from Johns Hopkins University. He was a Distinguished Graduate of his Field Artillery Officer Basic Course and the recipient of the Gunnery Award (FAOBC 06-2000) at Fort Sill, Oklahoma.

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