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Most of the time ballistic testing is concerned with article performance in flight. The crucial weather effect here is atmospheric-caused drag. Drag is produced by two components: air-density and wind. Measurements of these two items are necessary in order to distinguish between genuine atmospheric effects and other drag variables such as round-to-round differences in fin angles. For direct-fire tests (low elevation angles), the dominant effect by far is the crosswind. The relative contribution of crosswind along the flight-path upon horizontal deflection at the target is much greater near the gun than at the target. Special anemometer networks with proper spacing for ballistic tests are maintained by ATC.
Crosswind effect on
target as a function of distance to the target Measuring crosswind (suitable for ballistic corrections in complex terrain, hilly ground, and/or nearby trees and obstructions) is practically impossible with point measurements. ATC is currently working on new instrumentation that may do this accurately even under high wind conditions. Air-density requires representative measurements of temperature, humidity, and pressure. For indirect-fire tests (high elevation angles), both air-density and wind are important. Upper-air as well as surface measurements are required to capture vertical profiles of these parameters. Usually done by launching, balloon-borne throw-away, instrumentation (rawinsondes), these measurements are costly, both in material and labor. To reduce these costs, the ARMY has contracted with National Center for Atmospheric Research (NCAR is the international center for scientific excellence in meteorology supported largely by the National Science Foundation) for a technology transfer that will provide range-sized air-space computer models. These models will dynamically interpolate point readings into four-dimensions for more accurate ballistic calculations. ATC presently utilizes SODAR and WSR88D radar to supplement and minimize rawinsonde observations. Check with ATC meteorologists for interpreting test plan requirements. Sometimes a special approach for measurements are needed. We are here to help you.
In testing for weather vulnerabilities, it is vital to include as many climatic scenarios from the most likely potential theaters of war. Aberdeen Test Center is ideally located for varied seasonal climates. Note on the map that ATC is located in the continental temperate zone which is the same zone that includes the most highly populated areas of the world (see green area).
Effective simulation in Virtual Proving ground (VPG) scenarios depends on real-world data fidelity. On the small-scale, meteorological data sets that contain the measurement type required and their effects (such as humidity, rain, ice, and snow) are very rare because these data were seldom requested prior to the "smart weapon" era. Currently, the prestigious National Center for Atmospheric Research is building range-scale atmospheric models for some Army ranges that will help build these data bases. Unfortunately, however, there seems to be no reliable substitute for free atmospheric testing in a richly-instrumented range. One key element is the climatically-shaped features in the environment, such as vegetation and forests. Note in the following picture the numerous weather effects which all interact with one another.
Climatic chambers can simulate some environmental effects in a small space, but the combined effects are elusive. For example, rounds shot into deep foliage (as in Vietnam's jungles) had to be tested in the actual climatic zone to experience proper "background" effects caused by flora native to that zone. This is even more critical for "smart weapons".
Air density is computed from the equation of state for a gas, and requires pressure, temperature, and humidity measurements. Since air is never dry and has a variable amount of moisture in it, a "fake" temperature called virtual temperature is used. Virtual temperature is the temperature dry air must have to get the same density as moist air. ATC meteorology software routinely computes these values. WARNING: as with all atmospheric parameters, air density is "noisy" and varies significantly in small time and space scales. For most purposes, therefore, you need time and space-averaged measurements, not instantaneous point readings. Consult meteorology personnel for guidance.
Although water vapor constitutes only a small portion of air, it exercises profound effects. Not only does this compound have three physical states with complex heat exchanges between each one (see rain, ice, snow effects), but as a dipole molecule, water affects many areas of the electromagnetic spectrum--the visible, the infrared, and the millimeter-wave portions. Smart weapons tests will require humidity measurements. Units of measurement include relative humidity (RH-ratio of vapor pressure to saturation vapor pressure) and occasionally absolute humidity (mass of water vapor in grams, per volume of air in cubic meters). WARNING: automated sensors often become inaccurate above 90% RH and under 20 % RH; RH in cold weather needs to be computed with respect to ice, not liquid.
With the advent of lasers and similar devices that concentrate light energy, the atmospheric effects of optical turbulence have come into prominence. Optical turbulence is caused by small-scale variations in air-density that cause the stars to "twinkle". It can severely degrade lasers and other coherent light. Special non-standard measurements must be made with instruments called scintillometers. Optical turbulence is affected by winds and stability and will someday be an important part of military weather prediction models in the boundary layer.
Air pressure measured at some arbitrary height is called station pressure. For comparison purposes it is often "reduced" to sea-level using a hypothetical air column of standard characteristics. It does not vary horizontally over several kilometers under most circumstances so it can be measured remotely for tests involving explosive devices. WARNING: automated pressure devices need to be carefully calibrated and monitored due to instrumentation drift.
Testing involving infrared signatures and effects inevitably requires measurements of incoming solar energy. The quantity most often measured is the energy arriving on a unit horizontal surface per unit time. The proper term for this quantity is irradiance. It sometimes is called solar flux although this term is inaccurate because it fails to acknowledge that the solar energy measured is for a unit horizontal surface only, not an unlimited surface. The proper terminology ought to be solar flux density. Another term often used is solar load which refers to the cumulative irradiance over a finite time interval, say 24 hours. Irradiance is measured with radiometers. Sometimes test plans call for "diffuse" irradiance which means the normal irradiance minus that portion due to direct sunlight from the solar disk. Special non-standard instrumentation is required for this work. WARNING: in programming measurements a compromise must be reached between the response time of the instrumentation and the need for measuring rapid changes in irradiance. Increasingly, test plans call for measurement of infrared flux density in addition to solar flux density. This requirement arises because energy is transferred between the visible and infrared portions of the spectrum. These measurements are usually made with special radiometers such as those available at ATC.
Aberdeen Test Center is a four season, temperate climate, near the water testing facility. The Chesapeake Bay environment provides a realistic test and training space that approximates that of many potential engagement areas in the world. ATC target backgrounds--grass, trees, etc.--transition through a relatively wide climatic range. The marine environment also provides a natural corrosive source for test articles.
For an adequate climatic data set, a period of at least five years is generally required. At ATC we use historic general climatology data maintained by the National Climatic Data Center. For specialized climatic data (such as frequency of crosswinds at certain ranges, variation in irradiance, etc.), we expect to complete the minimum five year period at the end of fiscal year 1997 and have decision tables available sometime in 1998. Other DOD ranges have similar climatologies which ought to be consulted for test planning purposes. WARNING: Smart weapons technology will require special climatic data sets that require significant seasonal exposure to prepare. These data must be generated years ahead of their use and are essential for Virtual Proving Ground modeling. Consult with ATC meteorology personnel about your long-range needs.
Test programs involving various kind of smokes will often encounter severe environmental restrictions that escalate test costs from repeated delays. These restrictions are based upon modeled, worse case scenarios. The way to monitor compliance for such restrictions on your test is to determine real-time, not worse-case, meteorological conditions and compute smoke dispersion with an EPA-approved model. ATC utilizes a real-time smoke dispersion model that utilizes the ATC Aberdeen Meteorological Network (AMNET). Our forecasters can advise you on best-scheduling times-of-day for the given conditions of your test. We also can document actual conditions during your test to answer any environmental complaints that might arise afterward. One example of smoke dispersion radically differing in the vertical
Sound pollution is predictable and ATC monitors its effects prior to testing and during all periods of testing. Sound propagation is strongly determined by weather effects. At ATC we daily run a sound propagation model based upon the latest real-time meteorological conditions. By knowing your blast weight (in equivalent TNT pounds), height, and location, we can tell you how far and with what intensity the noise will be heard. If requested, we can include general outlooks of sound propagation in our 24 and 48-hour forecasts.
As fuzing systems become more sophisticated, weather effects become more important in their evaluation. Test plans should include testing in "hostile" weather conditions and avoid the need for modification costs. Fuzes that operate by visual light intensity need to be tested in thick cloudy conditions; infrared fuzes need to be tested in sunrise/sunset conditions as well as rainy weather due to various infrared weather effects. Mechanical fuzes that require airstream access need to be tested in rain, ice, and corrosive environments.
Sometimes tests require close monitoring of heat effects, either for safety of test personnel or for discovering limitations of certain apparel and equipment. To monitor correctly, the various components of heat effect must be measured. What are these components? The human body is designed with a cooling system superior to most animals. It utilizes three mechanisms to fight the heat. Unfortunately, certain weather conditions neutralize all three of these mechanisms at the same time and can cause severe stress. Sweating, the best known of the three sub-systems, relies upon evaporation. Evaporation, however, can be rendered useless by high relative humidity and stagnant air.
Two other, lesser known mechanisms are conduction and radiation. Conduction works efficiently as long as the outside air temperature remains below 98 degrees (F) and the air movement stays over 5 mph. Radiation works efficiently if infrared emissions aren't re-radiated back upon the body. Efficient radiation of heat from the body won't happen unless the air around us is moderately or very dry. Therefore heat is most dangerous to the body's cooling mechanisms when temperature and humidity are high and when wind speeds are low. At ATC these conditions are constantly monitored during the warm season through the use of the Wet-Bulb, Globe Temperature (WBGT). The WBGT number is a surrogate temperature number to express how efficient these three bodily cooling mechanisms are operating. WARNING: you are at risk for heat stress whenever the WBGT is over 85.
High humidity occurs in many potential battlegrounds and failure to consider its effects on man, machine, and the environment has led to a number of costly military mistakes. Besides stressing the human body when accompanied by high temperatures, high humidity degrades cloth, wood, and electronic equipment, and causes corrosion effects. High humidity also causes severe infrared effects, as well as refraction anomalies in atmospheric propagation of radio and radarenergy. Test planning must consider the environmental background that accompanies high humidity and directly affects system performance. An interesting example from the Vietnam period is the case of artillery rounds tested in the desert that failed to penetrate dense jungle growth without pre-maturely detonating. Does your test plan include testing under these real-world conditions? ATC offers a marine environment for Army testing that experiences humidity as high during the summer months as that of areas far to the south.
Low humidity can stress man , machine, and the environment as much as high humidity. Low humidity is difficult for some automated sensors to measure, but ATC Meteorologists can assist you in this area of concern. One effect is static electricity generation around sensitive electronic boards under low humidity conditions. Low humidity permits highly efficient radiative heat transfer in the infrared area, creating vivid thermal imagery for certain objects. Low humidity air overlying moist air can create a severe gradient in the index of refraction that can cause "ducting" of radio and radar transmissions. Army test ranges such as Yuma Proving Ground and White Sands Missile Range provide excellent large areas for low humidity testing.
Ice crystals are extremely complex. Their structure mirrors the
atmospheric
Besides affecting mechanical equipment, ice seriously affects radar reflectivity measurements in those frequencies of major interest to military equipment designers. Not only do ice crystals come in different shapes, but with temperatures hovering around the freezing point, they change form as they melt and then re-freeze. This effect could have been one cause in the recent Bosnia experience where certain mine detectors were reported to have performed poorly. The real world of ice crystals (snow and frost) is very complicated, but new sensor technology available at ATC enables accurate measurements of these specialized effects. Soldiers, using "smart" equipment would be aware of the amount of equipment degradation in black-box performance they can expect under freezing conditions. At ATC, measurements can be made during winter months in all such ice crystal conditions--whether frozen ground, snow, or melted ice/snow.
Not much is currently known about weather effects on biological agents other than the hypothesis that such agents are probably transported in the atmosphere in the same way as pollens and spores. If so, then wind and stabilityare key parameters to study. The critical question, unlike that of chemical weapons, is over how large an area will the atmosphere disperse these agents and leave them potentially active? At ATC we have an array of surface and upper-air instrumentation to monitor such dispersion
Weather effects were such an important part of chemical weapons, that early research in the atmospheric boundary layer was promoted by their development programs. Historically, World War I revealed the shortcomings of using chemical weapons in the open atmosphere. The critical question is how long and where will the atmosphere allow the agents to stay in sufficient concentration to do their job? Both wind and stability must be known to answer this question.
Corrosion is but one effect of the atmosphere on materials. Unfortunately, the pure atmosphere by itself isn't solely responsible for material degradation. It's chemicals carried in the atmosphere together with their interaction with water vapor and other gases that degrades man's materials. Delicate sensor technology is especially vulnerable to these effects. Prolonged exposure to both the free atmosphere as well as, airborne chemicals, is required for real world testing.
Meteorological conditions profoundly affect infrared images, so much so that meteorologists use infrared imagery to monitor the atmosphere from satellites. Weather conditions either increase or decrease target-background contrast in a complicated fashion:
Intervening Atmosphere Results of testing done with one atmospheric situation are not easily transferable to another situation, making unexpected problems in actual battle very likely. To make tests results as transferable as possible, and lower testing costs, be sure your plan includes measurements of: (1) temperature; (2) relative humidity; (3) visual range; (4) target and background temperatures; (5) target range and size; (6) sensor spectral response; (7) spectral emissivities for target and background; (8) spectral transmittance of the atmosphere. Non-standard meteorological instrumentation is required to make some of these measurements as well as a knowledge of weather conditions expected during the period of testing. The best "smart sensors" can be rendered less effective when faced with weather conditions not included in their design and testing. Although it is necessary to keep designs as simple as possible, systems should be tested in various weather conditions to document any system degradation.
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