[MP-3] Night Operations and NVG Considerations

Night Operations. Survivability and success in combat operations are a function of minimizing threat capabilities while maximizing one's own.  One way to maximize our capabilities is through the use of night low-level operations which reduce the probability of detection by visual, electro-optic, or electronic means.  Night operations increase the chance of surprise and decrease the chance of detection and weapons engagement by enemy forces. Night operations place increased emphasis on night imaging devices, which operate in the optical radiation portion of the electromagnetic spectrum.  The use of these devices requires a thorough understanding of the night environment.  This understanding includes the relationship between ambient illumination, the terrain, the night imaging device, and the human eye.  Each of these are critical links in the night vision chain.  This section presents techniques and considerations specifically for night operations.  Special emphasis is placed on NVG operations since the use of NVGs is the safest means of conducting low-level night operations in the H-60.  Detailed mission planning/preparation, alternate contingency planning in the event of mission changes, effective crew resource management, and situational awareness are all crucial elements to successful night operations.

Dark Adaptation.  For the average human, it takes approximately 30 minutes for the retina to be fully dark adapted.  Use of NVGs will lessen dark adaptation.  Crews should remember this after removing their goggles as they will most likely be taxiing unaided and moving around a dark aircraft after shutdown.

Controlling exposure to bright sunlight is important.  It may take your eyes up to five hours to recover from intense sunlight.  If you are out on a bright, sunny day prior to a night flight, sunglasses are recommended.  Additionally, your eyes need oxygen to maintain good night vision and smoking reduces oxygen intake and decreases night vision.

The most critical hours for night flying are morning and evening twilight.  This is because some sunlight is visible during twilight, and the rods and cones in your retina haven't fully adapted.  You're not getting full night vision or full day (color) vision.  Consequently, objects on the ground and in the air won't be easy to see.  Also, be aware that when you have full night vision, your best visual acuity is 20/200 (unaided), and you will have a blind spot in the center field of vision that you will have to "look-around" to see objects clearly.  This blind spot will be reduced with a high moon illumination.

Dim cockpit lights to the lowest practical level.  As the mission continues and your dark adaptation increases, dim the lights further.  Use no more light in the cockpit than is necessary.  H-60 cockpit instruments have back lighting, but the lower console radios are not lit.  Keep all lights dim as possible to minimize reflections on the cockpit windows.  Minimize use of lip/finger lights in threat areas.  Dim other interior lights as much as possible to avoid detection.

 

NVG Planning Considerations.

Mission Planning.  Thorough and detailed mission planning is absolutely critical to successful NVG operations.  As a minimum, aircrews must consider the tactical environment, terrain, illumination, weather, crew capability and length of mission.  Aircrews need to ensure they use the scale map(s) providing as much detail as necessary for mission accomplishment, and that all maps used are updated with the latest CHUM, with emphasis on any obstructions to their planned route of flight.  Additionally, a thorough knowledge and understanding of some very specific terms is necessary to fully appreciate the NVG operations environment.

Illuminance vs. Luminance.  The most common photometric terms used are illuminance (expressed as either lumens, lux, or foot-candles) and luminance (usually expressed as foot-lamberts).

Illuminance.  Illuminance refers to the amount of light which strikes an object or surface.  An example of illuminance is the amount of ambient light which strikes the ground from the moon.

Luminance.  Luminance refers to the amount of light reflected from a surface.  An example of luminance is moonlight which is reflected from certain aspects of the terrain which enables us to see those features.  The moonlight striking the ground is illumination while the light reflected off the terrain which enables us to see the terrain is luminance.  The relationship between illumination and luminance yields a ratio (incident light to reflected light) which is called albedo.  While illumination from the moon may remain constant, luminance from different terrain varies.  This is why the features of a blacktop road are more difficult to see than the features of a light colored concrete road.  In summary, ambient light sources provide illumination or illuminance.  However, what our eyes "see" and night imaging devices detect is the light reflected from objects and terrain or luminance.  A high illuminance or lux means a large amount of light is striking an object or surface.  Whereas a high luminance means a lot of light is being reflected by an object or surface.

Night Sky Illumination.  There are many sources of ambient illumination which combine to light the night sky.  Natural sources include the moon, stars, solar light, and other background illumination. There are also artificial sources, which are sometimes referred to as "cultural lighting".  Some examples of artificial sources include lights from urban areas, automobiles, fires, weapons, searchlights and flares. Remember, illumination includes only light which strikes the object or terrain to be observed.  It does not include the light reflected off the object or terrain; this is luminance.

Environmental Effects.  The environment can have a major impact on the success or failure of NVG missions.  Aircrews must consider available illumination, visual illusions, and weather conditions to be encountered during the planning and execution of any NVG mission.  Any condition of the atmosphere which absorbs, scatters, or refracts the illumination, either before or after it strikes the terrain, will effectively reduce the usable light available.  The exact amount of reduction is difficult to predict because a common factor cannot be applied to each condition of cloud or fog coverage.  Make an estimation of light reduction by considering the basic illumination as a starting point and then knowing the particle size of the atmospheric condition (i.e., clouds or fog) and its density (lack of space between particles). Table 1 provides factors which affect luminance used by night vision goggles.

Table 1. Factors Affecting Luminance.

Atmospheric Transmission      Weather: clouds Fog, Rain, Snow      Obscurants: Dust (Brownout), Snow (Whiteout), Chemicals, Smoke

Terrain      Contrast: Albedo, Texture      Shadows: Moon Angle

Aircraft      Heading, Altitude, Attitude      Cockpit Lighting      Cockpit Structures

Illumination Considerations.  In addition to moon illumination, consider other light sources and their effect on the mission.  Bright lights (direct or reflected) from internal or external sources seriously degrade the NVG effectiveness.  Avoid flying directly toward moonrise/moonset or sunrise/sunset as vision may be severely restricted by the intensity of the light.  Bright lights can create shadows that may hide obstacles. In marginal to poor illumination conditions (effective illumination approaching 5% such as starlight only conditions) aircrews should fly lower and slower.  The low altitude/slow airspeed combinations required to adequately discern landscape and surface obstructions can be prohibitive to conducting safe NVG operations.  Aircrews should also consider how sky conditions affect available illumination.  For instance, if the sky condition is overcast, and you are near a large city or other light source, the amount of reflected light may be more than sufficient for your NVG mission.  On the other hand, you may have 100% moon illumination with overcast skies and the effective illumination may not be sufficient for safe NVG operations.  The bottom line is aircrews must evaluate conditions every night and apply common sense and good judgment prior to conducting NVG operations. 

Terrain Luminance.  Our ability to see terrain features and objects is greatly influenced by the amount of light they reflect.  Natural or artificial illumination reflects off terrain in varying amounts (albedo).  Two characteristics of the illuminated terrain are responsible for our ability or inability to see and distinguish differences in terrain features; contrast and shadows.

Contrast.  Contrast is a measure of the luminance difference between two or more surfaces. Contrast can vary from 100 percent to 0 for surfaces darker than their backgrounds, and from 0 to infinity for surfaces brighter than their backgrounds.  Contrast in the night terrain environment is dependent upon differing albedo values for each type of terrain surface. Albedo is the ratio of light which strikes an object to the amount of light an object reflects. Since the eyes and NVGs use reflected light to see, reflectivity is critical to determining the amount of light available for vision.  In order to get an accurate picture of reflectance in the near infrared, where NVGs are most sensitive, specific IR albedos must be calculated for this portion of the spectrum.  These differences can become very important, especially at lower ambient light levels.  As an example; a flight progressing from fields covered with fresh snow (0.85 albedo) to a stand of coniferous trees (0.13 albedo) on a clear moonless night (.00022 lux) creates very different levels of terrain luminance.  Over the snow, 85 percent of the light is reflected off the terrain, while the coniferous trees reflect only 11-13 percent of the same light.  The effect of contrast is usually more pronounced at lower ambient light levels.  The ability to camouflage military targets is enhanced because of the reduced contrast cues provided by NVGs.

Reflectivity.  Reflectivity will also vary with conditions of the terrain even though terrain type remains constant.  For instance, dry sand is twice as reflective as wet sand.  Overall contrast is usually improved with higher light levels.  As the ambient light increases, more light is reflected.  Shades become more recognizable and an object's overall definition is improved. Objects with a poor reflective surface appear black during low light levels and dark green with NVGs during high light levels.  Objects or terrain features with good reflective quality appear green and become progressively lighter as the ambient light increases.  Some specific examples of the effects of contrast are as follows:

Roads.  The surface of some dirt roads provides excellent contrast with surrounding terrain. Roads that cut through heavily forested areas are easily recognized if visible through foliage. The light color of concrete highways, normally an excellent reflective surface, is easily identified during most conditions.  Asphalt roads, however, are usually difficult to identify because the dark surface absorbs available light.  Although, in desert areas, the resulting contrast can make asphalt roads readily detectable.

Water.  There is usually very little contrast between a land mass and a body of water during low light conditions.  When viewed from the air, lakes and rivers appear dark green in color. As the light level increases water begins to change color, land-water contrast increases, and reflected moonlight is easily detected.  When a surface wind or sea state exists, the ripples on the surface or wave action improve the contrast further aiding in surface identification and providing height and distance cues.

Open Fields.  Contrast is usually very poor in fields covered with vegetation.  Most crops are dark-colored and absorb light.  During harvest or the dormant season, the color of the vegetation changes to a lighter color improving contrast.  A freshly plowed field lacks vegetation.  The coarse texture of the upturned soil absorbs light, reflecting very little.

Forested Areas.  Contrast is usually poor in heavily forested areas.  If the leaves have chlorophyll, which is an excellent near IR reflector, the trees will appear very light.  This is true for all plant material which contains chlorophyll.  Vegetation will appear darker if there is no chlorophyll present. When heavy vegetation provides no contrast, objects and terrain features are concealed.  Excellent contrast does exist, however, between deciduous and coniferous trees as well as between open fields and surrounding forested areas.

Desert or Snow.  Flight over the desert or snow covered fields has good and bad aspects.  It can be good because of the high reflectivity which provides more light for the NVGs to use.  However, because of the usually poor contrast cues, less texture and detail is available to the NVG user.

Mountains.  Mountain ranges can be easily identified because of the dark color of barren mountains against the light color of the desert floor or snow covered field.  Lower rises in terrain between the viewer and the higher ranges can be difficult to identify in low ambient light.

Shadows.  Shadows are created when an object impedes light from striking a surface. Objects which may create shadows include terrain, buildings, clouds, and aircraft.  The amount of terrain obstruction within a shaded area, as perceived with NVGs, is dependent on the amount of ambient illumination and relative position of the moon.  Shadows will appear darker and obscure the terrain within the shadow more on a night with high illumination than a night with low illumination.  This phenomenon is a function of the gain or automatic brightness control of the NVGs.  Flying directly into a low angle moon or setting sun amplifies the problems created by shadows due to the goggles "shutting down" or "washing-out" with a bright light source in their field of view and makes it nearly impossible to detect terrain in front of you.  Shadows created by scattered clouds may give the NVG user certain visual illusions, especially in mountainous terrain.  Shadows cast on the sides of mountains may give the NVG user the perception that the lighted areas are low lying clouds or fog which may give a false perception  of a ridge line or horizon.  Shadows from clouds cannot be predicted during mission planning.

Light Prediction and Measurement.  Ambient light level prediction and measurements are very complex processes requiring an appreciation for the limitations involved.  Computer models currently exist, for example, the Computer Generated Global Light Level Calendar, which has the ability to predict illumination for any given latitude/longitude for the next thirty years with greater accuracy and reliability than previous methods.  As useful as this system is for planning purposes, the predictions only provide limited information concerning ambient illumination based on moon phase, azimuth and elevation.  This is due to the fact that there are many factors which affect the amount of light reaching the intensifier tubes of NVGs.  The prediction process, while useful for baseline planning, does not provide real time enroute light level data.  Variables such as weather, moon angle and terrain shadowing, terrain albedos, and aircraft heading can significantly alter the luminance used by the NVGs.  Light level variations are insidious and their recognition is critical to mission success.  An example of limitations encountered in attempting to predict usable illumination may prove beneficial here.  For a given night, the Light Level Calendar may predict an illumination value of .0093 lux (based on a moon phase of 84%, located 12 degrees above the horizon).  NVG flight would certainly be possible under these conditions.  During flight planning take into account the low moon angle and associated terrain shadowing hazards.  Weather must also be taken into account.  For example, a thick overcast or broken layer, regardless of the predicted illumination levels, may reduce the actual illumination striking the terrain below those levels in which NVGs can effectively operate.

Visual Illusions.  Many of the visual illusions encountered while flying at night unaided are present when using NVGs.  Lack of depth perception is one of the most common problems.  For example, when flying over terrain that is flat or flying over large bodies of water it can be extremely difficult to judge aircraft altitude.  You can easily start a very slow descent and not catch it visually.  

Weather Considerations.  Due to reduced visual cues and inherent depth perception problems, aircrews must exercise caution when conducting NVG operations in areas of inclement weather.  Weather may appear further away than it actually is, and you could inadvertently enter IMC.  Additionally, you may be able to see through very thin fog and not realize you're entering an area of IMC until it's too late. Aircrews should use all available weather forecasting resources to avoid areas of IMC.  Be aware of cues in the flying environment to assist in avoiding inclement weather.  Large halos around ground lights, areas of extreme darkness where there is a known light source and the loss of a visible horizon all indicate possible areas of IMC.  In addition to restrictions to visibility, wind information can also be hard to obtain.  When accurate wind information is not available, base wind determination on forecast winds, on-board systems and any available outside indications.

Effects of Altitude.  Generally, the higher you fly the less visual terrain definition you have and you may lose the ability to pick out distinguishing features that are useful for navigation. If available illumination prevents you from flying at a lower altitude, increase altitude as necessary.

Crew Coordination.  Effective crew coordination is absolutely essential for successful low-level night operations.  Crew members must communicate in a clear and concise manner and provide useful and timely inputs as necessary.  This is especially crucial in light of the following limitations and cautions which must be observed when operating on NVGs.

Brownout/Whiteout conditions may greatly degrade the NVG user's pre-existing visual acuity even further.  On approach and landing, VSDS indicators may be used to detect drift during the final few feet of the approach.

Terrain/other obstacles in the "shadow" of more distant/higher terrain, man-made obstacles, or clouds may not be seen when wearing NVGs. 

Different colored lights cannot be distinguished (i.e., all lights appear to be the same color). Situation permitting, it may be beneficial to glance under the NVGs to identify an unknown light source. NVGs have the capability of "seeing" through light rainshowers, despite the fact unaided visibility may be at zero.  Should the rainshowers intensify to the point where the goggles are no longer effective, the crew should be prepared to execute IMC procedures.

Wearing NVGs for an extended period can cause extreme fatigue.  Eye fatigue can be lessened by periodically removing the NVGs to rest the eyes.  NVG users must guard against mission effectiveness degradation due to prolonged usage.  Commanders and operations officers must  weigh crew experience, qualification, weather conditions, and other environmental factors when required to perform long NVG sorties which are not normally part of the unit's mission or which require an aircrew to fly a maximum crew duty day.

With adequate available outside illumination, NVG vision enhancement is inversely proportional to altitude and airspeed--the lower and slower you fly, the better you see.  In marginal or poor illumination conditions, the low/slow altitude and airspeed combinations required to adequately see may be prohibitive in conducting safe NVG operations at normal cruise airspeeds.

WARNING:  Electric power lines, unlighted towers, poles, antennas, dead trees, and all types of wires are extremely difficult to see while conducting NVG operations.

Aircrews should avoid flying directly toward moonrise/moonset or sunrise/sunset as vision may be severely restricted by the intensity of the light. 

Alternate Light Sources.  Alternate light sources can provide the illumination needed to successfully complete NVG missions.  The IR searchlight, laser pointers and LZ lighting add illumination and increase mission safety.

LZ Lighting.  Artificial light sources can aid the crew in accomplishing NVG terminal operations.  Some examples are ground lighting patterns and external IR lights from other aircraft.  All improve visibility during terminal operations.  Lighting patterns may be established in blowing snow, dust, tall grass, and similar environments by a variety of methods.  Bundles of chemlights and chemlights in plastic water bottles are useful in marking a landing zone.  The crew should make a low pass over the LZ to throw out the marking devices at a prescribed time and interval from both sides of the aircraft. This technique is similar to the NVG water operations pattern and results in an excellent reference for landing or hover.

NOTE:  If communication is established with ground party, ensure they know the rescue helicopter crew will be using light sensitive NVGs to avoid inadvertent blinding of the crew.

Weapons Delivery Effects.  Muzzle blast created by aircraft weapons can seriously degrade NVGs. Aircrew members must coordinate with the pilots when employing the weapons. Firing full forward should be minimized to reduce the possibility of shutting down the pilots NVGs. Additionally, if weapons have to be fired while in a hover, the pilot on the opposite side of the weapon being fired should be on the controls if possible.  If weapons are being employed during a formation flight all members of the formation need to pay particular attention to the effect their firing is having on the other members of the formation.  Aircrew briefings need to be very specific in assigning field of fire for each weapon in the formation.

When the Gau-2B Minigun is being fired, voice communication in the cockpit is impaired significantly.  Consideration should be given to this and briefings should cover what actions will be taken to ensure communications if an emergency or other event requiring immediate communications occurs while the miniguns are being fired.