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False radar returns such as the bright circular return shown below and known as an inversion ring may deceive radar operators and commanders alike. Anomalous propagation of radar energy is a serious concern to the Fleet. Operational commanders must understand the problems, the potential for offensive or defensive advantage, and the role the atmosphere plays.
ncertainty as to what actually took place in the Gulf of Tonkin during the night of 4 August 1964 will always remain. The radar operators on board the USS Turner Joy (DD-951) and the USS Maddox (DD-731) saw echoes that they evaluated as attacking boats. It is evident that considerable confusion existed in the Combat Information Center (CIC) of both ships that night, and it is probable that freak atmospheric conditions caused anomalous propagation of radar energy and were the cause of at least some of the reported surface contacts. These false radar targets ("ghosts”) are a perplexing problem to the operational commander.
At the other extreme, but equally as important, are the radar "holes” that exist because of anomalous propagation. It is conceivable that one of our ships cruising the Mediterranean, eastern Atlantic or eastern Pacific could receive a direct hit from an air-to-surface missile with no prior warning. The ship’s radars and operators could have been functioning perfectly all day and searching the very location from which the attack was launched, yet never have a target on their scopes.
Widely varying detection ranges for the same target can be a clue to the existence of either radar holes or ghosts. Certain phenomena of the earth’s atmosphere are known to make all of these things possible. But how many naval officers are even aware of these radar holes into which they might fall, or of the ghosts into which they might bump with disastrous results?
Anomalous propagation of radar energy—commonly referred to as AP or simply as radar refraction problems—is a serious concern to the Fleet but one that is not likely to be satisfactorily solved in the near future. The important point is that radar operators and operational commanders must understand problems associated with anomalous propagation, the potential that exists for using these conditions to one’s offensive or defensive advantage, and the role the atmosphere plays in causing these conditions.
A number of factors can complicate the radar coverage pattern on a given day. These include such items as equipment, operator, and calibration problems, as well as a host of environmental factors such as reflection, refraction, diffraction, and scattering. Birds can sometimes cause distinct unexpected returns. Over oceanic regions, such phenomena as sea clutter can drown out all other sea surface targets on a windy day; multi- path (receiving of both a direct and sea-reflected ray) can cause gaps in elevation coverage. But one of the most significant phenomena of all when considering anomalous propagation and ghosts is the process of refraction.
Refraction is a very common phenomenon: we have
all seen the shimmering, watery look on a sun-heated highway or the apparent closeness of an object sub merged in a deep swimming pool. In these instances, the visible wavelengths are affected. Equally deceptive is the enhanced TV picture that is sometimes received from relatively distant stations.
On the West Coast, where residents of Ventura and Santa Barbara counties are served by seven VHF and six UHF television channels from the Los Angeles area, the empty channels (6, 8, 10, and 12) can be filled by picture and sound from stations in the San Diego and Tijuana areas, over 125 miles to the southeast' Similar conditions have been noted in Corpus Christi, Texas, where TV viewers occasionally receive programs from distant Dallas and Brownsville on the same channel. In the Baltimore-Philadelphia-New York area, similar conditions were noted as far back as 1950. The cause of such oddities is probably refraction.
As early as World War II, the importance of refraction to the propagation of radar energy was widely recognized. It was observed that one radar would be able to pick up targets far beyond the normal radar horizon while, at other times, a radar could not detect an object which could be clearly seen visually. Possibly the most distant surface targets ever detected by radar were points on the Arabian peninsula seen from 1 1.5-meter radar at Bombay, India, during the dry season—a distance of about 1,500 miles. During the wet monsoon season, the same radar had difficulty seeing surface vessels only 20 miles away.
Refraction is the process in which energy rays are bent, because of changes in their velocity of propagation, caused by the rays’ travel through mediums ^ varying density and composition. Evaluation of a substance’s ability to cause such refractive bending effect is measured in terms of the index of refraction 0< simply the refractive index. In the atmosphere, for radar and radio frequencies, this index is affected by thc molecular properties of water vapor and is also dependent on density which is determined by the air’s temper ature, humidity, and pressure. Thus, the refractive index of air can be obtained from measurement of these parameters. Whenever energy rays enter a substance 0 non-uniform density and composition at small obliflue angles, some refractive effects will occur. Thus radaf and light waves are both bent as they traverse such an atmosphere just as sound waves are similarly ben1 (or refracted) as they pass through the variable densities of bodies of water. The more change in index of refnc‘ tion experienced by a ray of energy and the shallower the angle of incidence, the more it will be bent °r refracted. Normally, the refractive index decreases wid’ height in the atmosphere and, depending on thf amount, wide variations are possible not only in d'c
Don’t Fall in the Radar Hole 57
amount of bending but also in the shape of the radar coverage patterns.
The similarities of radar and sonar problems are striking. As sonarmen and radarmen start on opposite sides of the air-sea interface, each must grapple with the bending and propagation of energy in a fluid of varying densities to try to "see” and identify nearby neighbors and foreign objects. Even within the electromagnetic spectrum, however, refraction effects are not uniform. Optical wavelengths generally will experience less bending than radar wavelengths because the former are less affected by atmospheric moisture. Thus, what can be seen visually may not be seen by radar 2nd vice versa.
The actual radar coverage pattern obtained at a given dme has as much to do with the "geometry” of the situation as it does with the refractive index variations. The angle at which radar energy intersects a refractive layer determines the future course of the beam. Radar energy beamed at angles greater than five degrees is t>ent in a generally normal, gentle manner regardless °f the refractive condition of the atmosphere and produces expected coverage and detection. On the other I'and, radar energy beamed at very small angles to the horizontal (especially near one degree or less) can, in rhe presence of very strong refractive layers, be changed, trapped, or ducted parallel to the earth’s surface Jnd be propagated far beyond the normal radar horizon. Such layers act as natural "wave guides,” channelling an<l constricting radar energy and frequently leading t0 the detection of targets at ranges that would other- v'lse be beyond the radar’s capability.
Thus,
special naval encounters may be plagued or enhanced • both holes and ducts.
These features and considerations are schematically j. °wn in Figure 1. The lines in the figure emanating ^0rri the ship’s radar represent rays of energy at various Cvution look angles. Where lines are close together, tj|Crgy is concentrated. They are not straight because JV have been bent or refracted by the atmosphere. e figure portrays (on a greatly expanded vertical a e) a carrier radar’s inability to detect an incoming Wk' ng T°rce because the aircraft are in a radar hole, '*e at lower look angles, the radar energy propagates beyond the normal radar horizon in the duct t 'fth the incoming aircraft. As illustrated here, at- f's might be detected visually before they could
In between, there may be a wedge of air space refining which is essentially void or at least very defiant of radar illumination. This "radar hole”—is an irrational air space within a region of expected nor- f 1 radar coverage, which, owing to strong refractive irs, is not illuminated by a radar at small look angles, a radar scanning the skies during routine tracking
be seen by the ship’s radar—and, thus, too late. At lower altitudes they could have been detected at great ranges. Neither ship nor attacking forces in the figure is shown as friendly or enemy since the atmosphere applies its myriad of effects upon both friend and foe alike. The side with the greatest knowledge of the atmosphere, and with the ability to use this knowledge in a tactical situation, has an advantage over the other.
In Figure 1, the base of the elevated atmospheric "trapping layer” affecting the ship’s radar occurs fairly close to the water. In many places around the world, such layers are observed to be elevated at much greater heights and with much greater thicknesses. Layers may be several thousand feet high and as much as 1,000 feet thick from base to top. When elevated and thick, these layers and potential ducts would quite obviously have their greatest effect on airborne or mountain radars.
Of special importance to ship radars, in addition to strong, elevated layers, is a feature termed the "evaporative duct.” This is a region approximately 30 to 90 feet thick at its maximum, resting on the surface of large bodies of water in which the refractive index decreases so rapidly with height that low angle radar energy may be substantially trapped. The resultant duct
is caused by the rapid decrease of moisture in the first few tens of feet above the water surface and is directly related to the difference in temperature between the sea surface and the air immediately above, as well as the air’s local humidity and wind characteristics. Unlike the elevated layers there is no distinct "top” to the duct. The evaporative layers can produce greatly extended ranges for shipboard surface search radars whose antennas are situated within the duct and pointed at very small elevation angles. However, since the shallow evaporative duct typically does not extend as high as a ship’s superstructure, ship’s radars may be above this region and unable to take advantage of its potential for extended range as they look down into it. Thus, with a strong evaporative duct, it is possible for a submarine to pick up distant ships which cannot see the submarine because of both the size of the submarine target and the fact that ship radars may be above the duct. If both submarine and ship radars are located within a strong evaporative duct, it might be possible for both of them to detect distant incoming fliers or missiles also located within the duct, just skimming above the water’s surface (assuming a low Sea State).
Unfortunately, the frequency and pattern of occurrence of evaporative ducts is not yet well known. It is possible, for instance, that the presence and intensity of the evaporative duct is related to the larger scale weather patterns which result in the thicker elevated layers. Extensive studies on the evaporative duct are being conducted for the Navy by personnel at the Johns Hopkins University Applied Physics Laboratory (APL) in Silver Spring, Maryland, and at the Naval Electronics Laboratory Center (NELC) in San Diego. Latest evidence points to a rather strong geographical and seasonal dependence around the world making some oceanic regions more susceptible to its effects at certain times of the year and rendering other oceanic regions and other times of the year nearly immune to its effects on a week-by-week basis. At best, it is a very difficult meteorological feature to measure. The duct is best detected indirectly by means of signal strength monitoring and by (theoretical) calculations from surface weather and sea data. Scientists at APL, using wave theory to describe propagation in the presence of an evaporative duct, are developing methods for predicting the actual field strength of radar energy at designated distances from the transmitter for various atmospheric conditions.
The importance of the evaporative duct to the Navy lies not only in its potential ability to propagate energy from surface radars well beyond the normal horizon but also as a possible mechanism for explaining some of the mysterious unverified returns known as ghosts. There is a continuing controversy over whether these
puzzling apparitions can be explained by low, strong elevated ducts, by irregularities in space of the evaporative duct, or by low-flying birds. Hopefully, probing of the evaporative duct by scientists will determine if there is any such ghostly influence, and more important, if it is possible and desirable to re-orient and design ship radar systems to benefit from the duct’s presence.
In addition to the more discrete effects on radar propagation caused by discernable refractive layers and features in the atmosphere, there are a host of other processes and factors which are related to the refraction problem and which can have considerable bearing on radar coverage. For instance, some atmospheric irregu- larities not detectable visually can cause backscattff from refractive interfaces large enough to produce detectable signals on a radar scope. This is often the source of "weather clutter” when no storms or clouds are present and might be used to good advantage fot detection of dangerous clear air turbulence. It is, therefore, another possible explanation of ghosts.
Radar frequency is another one of those separate factors that affects the refractive problem at sea. For example, for thin surface ducts, as the radar frequency is lowered, an increasingly greater change in refractive index between the surface and the duct top is required for trapping to occur.
Occasionally, a process called multipath may cause an added confusion factor to the evaluation of radar coverage and the identification of ghosts. Multipath occurs when there are two signals returned from a target, a direct one and a bounced or sea-reflected one. If the path of a target causes a land mass or island temporarily to come between the target and the radar, the reflected signal may be interrupted leading to drastic changes in radar range, and the target may disappear. This may lead to the erroneous conclusion that the radar had been tracking a ghost when the target was real. But while the importance of these other factors should not be minimized as they relate to the overall refraction and radar coverage problem in the atmosphere, the structuring and layering of the atmosphere itself remains the greatest concern when discussing refraction. It is the factor which often causes today5 radar coverage to appear normal and tomorrow’s radar coverage to seem like a complete mystery.
What type of atmospheric or weather condition5 distinguishes the days having standard refraction fr0111 the days which produce anomalous propagation, radar holes, and trapping? And which ones appear to fav°f ghosts? To answer this, it is necessary to restate that the atmosphere’s ability to refract or bend radar energy is determined by variations of refractive index. Th>s
Don’t Fall in the Radar Hole 59
parameter in the atmosphere is a function of temperature, humidity and pressure (which determine the air’s density) and is especially sensitive to humidity due to water vapor’s molecular properties. Pressure, temperature and humidity are routinely measured in the atmosphere so that the index of refraction can be directly calculated. The refractive index itself is normally a rather cumbersome number so it is usually further expressed in terms of the simpler, more convenient N-unit.
The average vertical decrease of temperature, humidity and pressure observed in the lowest thousand feet of atmosphere over continental regions results in a standard decrease of N of 12 N/units in the first thousand feet. This condition is sufficient to cause standard refraction (Figure 2)—a smooth bending of radar or other electromagnetic energy in the same sense as the curvature of the earth but not at the same rate (actually the radius of curvature is about four-thirds that of the real earth). Moist or cloudy regions may cause an increase of N with height termed sub-refractive. The corresponding radar coverage then is shorter than normal and rays are observed to bend upward away from the earth’s surface.
When atmospheric conditions result in a faster decrease of N than the standard rate, the condition is termed super-refractive and, as the term implies, radar beams are bent more sharply than occurs with standard refraction and radar coverage is extended. Over the oceans the typical decrease of moisture with height causes an N gradient of 18 N/l,000 feet to be the normal state. Thus, conditions there are typically super-refractive according to these definitions.
When the N-gradient equals or exceeds 48 N-units per thousand feet, the refraction equals the earth’s curvature and the extreme case of super-refraction called trapping or ducting may occur for energy propagated st very small angles to the horizontal. As was pointed out earlier, radar holes may appear above the ducts for slightly higher radar look angles.
In order for N to decrease with height at the rate required for trapping, the humidity must decrease very sharply with height and/or the temperature must increase sharply with height. The latter case constitutes what meteorologists term an inversion. Both temperature inversions and abrupt decreases in humidity with height often occur together in stratified layers. These conditions are most common over subtropical bodies of water in summer. In fact, during the months June through October, such strong super-refractive layers are ar> almost everyday occurrence over large portions of the Mediterranean and the Northeast Pacific along the coast of Southern California. This is the dry season tvhen afternoon skies are nearly always fair and winds
are normally light to moderate. In fact, strong super- refractive conditions in general are a distinctly fair- weather phenomenon.
It might seem paradoxical for the most troublesome radar weather to occur in what otherwise appears to be perfect operating weather, but this is not at all inconsistent with atmospheric theory or observation. The reason is that clear weather is frequently associated with the eastern side of vast high pressure regions in the lower and middle atmosphere which produce a subsiding flow of dry air. This downward moving dry air precludes thick convective cloud formations which might lead to rain and inclement weather, and produces a warming and drying out of the air. The net result is a warm, dry air mass overlaying a shallow, cool, and—if over water as off the California coast—relatively moist layer just above the surface (Figure 3). If one progressed upward through these two different air masses, he would find that the moisture dropped rapidly with height and the temperature increased with height through the inversion. At the same time, he would observe a very strong super-refractive gradient. Sometimes, the shallow cool marine layer contains fog or low stratus clouds. Near islands and land areas, warming by the sun and local wind effects evaporate this thin cloud layer, typically resulting in a clear sky by midday. Undetected by human eyes, however, is the residual presence of a very strong super-refractive layer and potential disaster for the unsuspecting force commander in that area. Over much of the Mediterranean and probably regions like the Red Sea, the warmer waters and drier air above apparently preclude the low cloud formation so that the prevailing clear skies give no indication of the presence of refractive layers above the surface.
Atmospheric refractive layers do not remain at the same altitude or the same intensity from month to
60
month, day to day, or even hour to hour. Rather, they are observed to vary in these respects according to the times of day, seasonal changes and local weather patterns such as moving storms and regions of fair weather. Elevated refractive layers are lowest and strongest in fair weather and highest and weakest near storms.
The sensitivity of refractive conditions to weather variables is further illustrated for an evaporative duct which is dependent upon both the overall weather situation and especially relatively small-scale, local wind effects and sea surface temperature conditions. Anything that aids the evaporation of water from the surface such as moderate wind speeds and dry air flowing over a shallow moist layer is favorable to formation and maintenance of an evaporative duct, especially in mild or warm climates and over temperate or warm bodies of water.
Sometimes, the large-scale weather patterns combine with the small-scale evaporative conditions at the sea surface and form a single, exceptionally strong, surface-based, super-refractive layer that is deeper than the normal evaporative duct. This occurs when strong subsiding flows of desert-dried or continental air flows across the coast and out over large bodies of water. Examples of such winds are the sometimes dust laden khamsin winds of the eastern Mediterranean and the Mid-East and the often hot, dry Santa Ana winds of Southern California. In other parts of the world similar dry winds are known by such names as the bora near the Adriatic coast, the mistral along the southern French coast on the Mediterranean and the sirocco which blows across the North African coast. In all cases, very dry air flows out over the water for various distances causing great humidity decreases with height and sometimes sharp temperature inversions in the very low layers (Figure 4.)
These conditions—which produce gaping radar holes for surface-to-air searches and strong ducts and extended range for surface-to-surface searches—can present one of the most confusing radar coverage situations. While these dry winds lead to the most exceptional cases, it may be generally stated that any condition that results in a mass of dry air overlaying a moisture-laden layer results in a strong super-refractive condition, with or without appreciable winds.
In tropical and warm-water regions the absolute moisture content of the air is generally so high that relatively subtle changes in humidity are often sufficient to cause large changes in the N-values over very short distances. These may occur throughout the lower and midlevels of the atmosphere or very close to the surface, thus posing problems to both shipboard and airborne radar systems. It may be that such subtle and randomly spaced humidity and temperature gradients near the ocean surface are partially responsible for the ghosts and anomalous targets occasionally seen by radar.
Over the warm waters in the northern Sea of Japan during cold, clear, low wind or calm nights, rising bubbles of warm moist air called thermals have been blamed as the cause of irregularities on the shape of the moon as observed visually from shipboard. 0n these same nights, the radars were also plagued by ghosts. Similar situations have been observed in the Mediterranean, in the North Sea, and off New England, but the cases that were recorded off the coast of Korea suggest that more than ghosts might have been out there in the dark.
Moreover, some ghostly calls to general quarters can be attributed to birds. It is known that migratory waves of hundreds of birds returning to their roosts in harbofS around the world can sometimes produce alarming radar returns. And, among radar meteorologists, con troversies often occur over whether a given radar retuffl
figure 4 Seaward flow of dry air causing severe refractive effects
ls the result of atmospheric anomalies or birds.
But there is still another important possible explanation of ghosts. According to one theory, held by several Navy scientists, the atmosphere does not really resemble dte simplistic picture we want to assign to it. For ■nstance, it is nearly always assumed in operational applications that elevated inversions and super-refractive Byers are flat and horizontally stratified. Yet, because °f topographic features like islands and mountainous coastlines and numerous gravity wave-like disturbances ln the atmosphere (which are very difficult to detect), the elevated layers are often wave-like and may exhibit height changes of hundreds of feet within distances of a few tens of miles or less. These waves, depending 0n their origin and the direction of the wind flow ahove and below, may move in any direction at various speeds and may exert a greater influence on propagation than the variations caused by large scale weather fea- tUres, especially during stable, persistent periods or seasons.
When layers are low and strong, radar beams from ships’ air-search radars may intersect these layers, some- t'nies producing a distinct weather-caused return. An example of such a return is shown on page 55. This ls a photograph of a radar scope observed by shipboard Personnel during an actual Fleet exercise in the North- Pacific off the California coast. The bright, somewhat circular return at a range of approximately 30 njutical miles is called an inversion ring, because strong
refractive layers are commonly observed to coincide with temperature inversions. It may be the result of either backscatter from a strong super-refractive layer or it may actually be a return from the sea surface caused by a sharp bending and near reflection of energy from the elevated layer. Thus, because of the latter possibility, even though a radar is pointing up at small look angles, it may actually give a surface return of sea clutter due to a strong elevated refractive layer. If such a layer is deformed by waves, as current research indicates, rather than uniformly horizontal, the intersection of a radar beam with the layer could at certain places and times occur at angles critical enough to cause localized bending of the beam downward to the ocean surface. At other times, the intersecting may occur at angles which result in more normal bending of the radar beam. To the radarman watching his scope, bright blips may seem to approach his ship at high speed and, as quickly, disappear. Very subtle changes in the place and angle where the radar beam impinges upon the elevated super-refractive layer as it meanders in the wind flow may cause the returning signal to appear to jump, and may explain the sometimes unrealistically high speeds of these returns. This possible interpretation of ghosts is schematically illustrated in Figure 5. To operational commanders in combat situations, the appearance of false targets closing in is no less alarming than real ones.
With such pronounced effects on radars possible
owing to refractive anomalies and layers in the atmosphere, it is natural to wonder how frequently these atmospheric conditions may be expected to occur. Unfortunately, there is little climatological information of this sort available and there is no known tabulation on the frequency of ghosts. There are, however, significant bits of data on refractive layers and ducting. Past reports by the APL have suggested that surface evaporative ducts occur approximately 80% to 90% of the time over subtropical and tropical oceans, especially in summer. Past reports from the NELC have estimated that surface trapping for X-band radars in the Near East and Southeast Asia because of the evaporative duct may occur 85% to 90% of the time.
According to the APL reports, in colder climates, such as the North Sea, X-band ducting by evaporative ducts has been estimated to occur from 40% of the time in winter to 90% of the time in summer. Only in polar regions are ducts apparently infrequent, probably because of the very low duct heights and quite low values of absolute humidity despite the fact that evaporation rates are frequently high, particularly near the Siberian coastline. Current NELC efforts appear to point in general to a wide geographical and seasonal variation of this oceanic evaporative phenomenon.
As for elevated trapping layers and ducts, apparently these, too, vary widely but can be very persistent in certain places of the world. Pacific Missile Range data taken at Point Mugu in coastal Southern California indicate that elevated trapping layers could occur over 90% of the time in summer and well over 50% of the time in winter in that area. It is likely that similar high percentages will be found to occur over the Mediterranean also.
If refraction can cause such profound effects on radar coverage so much of the time, we ought to try to use it to our advantage. The first step towards applying
refractive information to tactical situations is the recog' nition by operational commanders of refraction as * significant factor in radar propagation. Unfortunately’ a skipper without meteorological training could not in the past always be expected to understand the vagat' ies of the weather and refraction. In some cases a skipper may understandably reason that if his radarS saw 150 miles yesterday, then they damn well better see 150 miles today.
On the other hand, an enthusiastic CIC evaluatot who has been aware from firsthand experience of the effects of refraction and the atmosphere may not unreasonably blame any poor or peculiar radar coverage on "weather”. Unfortunately, in some cases weather might include not only atmospheric refraction, but also cloud or storm returns and even real radar malfunctions or radar operator deficiencies.
With the help of both the meteorologist and the computer a lot of the guesswork can be taken out of explaining unusual observed radar coverage and delineating areas susceptible to continuing or future anomalous radar propagation.
Atmospheric profiles can now be entered into small shipboard computers to calculate the approximate paths and bending of radar rays for various initial elevation angles and altitudes. Such ray traces can provide useful visual pictures similar to those produced at shore stations (Figure 6) of where radar holes and trapping conditions may occur. Some of these computer systems are being evaluated by NELC personnel for possible operational use in the Fleet and show promise. ^ present these systems are capable of assessing whether radar coverage can be expected to be normal, long °f short and if trapping or holes should occur. If so, they can calculate such parameters as the distance to the beginning and end of radar holes at various altitudes Such advisories or "forecasts” of radar propagati°n conditions, though unverified, can provide useful guid
r
ance to the operational commander for his consideration. They can be prepared into short messages called AnaProps (anomalous propagation) and transmitted to all Fleet units.
From there, if these systems prove reliable, it would be up to the commander to consider the implications brought about by the prevailing refractive conditions and make any adjustments necessary to satisfy the immediate situation. He may choose to reposition his
aircraft to fill in holes or take advantage of (or avoid) trapping layers, making use of the fact that it is the radar beam propagated at a very small elevation angle with respect to refractive layers that is most affected by refractive bending. He may order a different spacing of his picket ships when ghosts are encountered to minimize the chance of all ship’s radars seeing the same refractive anomaly and thereby revealing the true nature of the target. By carefully choosing his airspace and his options, the commander has the potential to increase his radar horizon and detect incoming attack forces in sufficient time to launch a successful intercept even though the enemy may be taking advantage of known anomalous refractive conditions (Figure 7).
At the Long Beach Naval Shipyard, engineers are developing a method of evaluating shipboard air-search radars by tracking jet aircraft at various altitudes and at selected distances from the ship. The received signals are evaluated on range-height charts and comparisons are made of apparent radar horizons with what would be expected under normal conditions. The techniques can distinguish between radar malfunctions and anomalous propagation owing to refraction. Meteorological N-profile data and radar hole and ducting computations are used to corroborate the findings and the technique may be able to eliminate urgent or costly radar repairs
in cases where anomalous propagation may shorten the radar range.
Of course, during emergencies and combat situations, there is seldom time available for detailed studies and analyses of atmospheric effects. Besides, the atmospheric anomalies are normally in a steady state of change. Therefore, to be a winner in a particular confrontation, the operational commander may not only have to demonstrate immediate superior weaponry but may also have to demonstrate a quick, skillful application of atmospheric data to his particular radar situation. To do this, he must optimally have meteorological data that is representative of his position in space in as near to real-time as possible. Unfortunately, because the commander is seldom able to attain these real-time benefits, he must use what is available to him.
Much of what we know about detailed atmospheric structure above the surface is based on the radiosonde package. This package is attached to a balloon which is released to ascend through the atmosphere. Temperature, humidity, and pressure sensors measure the atmospheric structure, and the raw data are transmitted to the ground where they are reduced and processed into usable formats. The refractivity or N-values are calculated from the temperature, humidity, and pressure data for each significant level.
The desirability of the balloon system lies in its being cheap and simple to use under most environmental conditions. Unfortunately, this positive aspect is offset by the fact that there is no control over the package once it is released (it travels with the wind) and its humidity sensors have a sluggish response rate. Of critical importance is the fact that because of the ascent rate and the time required for alternate switching between the sensing mechanisms, data points are at least 100 feet apart and the lowest data points characteristically occur at about 200 feet above the launching site. Thus, this method can miss detecting thin but significant layers and cannot detect surface ducts.
A relatively newer instrument used to measure the atmosphere’s refractive structure is called the refrac- tometer. This device is usually mounted on an aircraft and is capable of directly sampling the refractivity along the flight path. Refractometers detect much more detail than balloons do as the aircraft spirals through the lower atmosphere. However, the frequently large space-time variations of this detailed structure make much of it transient in nature. Thus, neither the balloon nor the airborne refractometer techniques are satisfactory in that the profiles they produce are not representative of a single point in space and time but rather over an extended region of both. Very thin but significant layers may be overlooked by one method and
over-detected by the other. Of utmost importance, soundings obtained from either method may not be representative of the actual atmospheric conditions traversed by a beam of radar energy.
The deficiencies of both systems are most severe in the very low altitudes of the evaporative duct where the lowest level measured by the airborne refractometer is limited by safety considerations to match the sampling void at these levels in the balloon system.
Clearly what is needed is a new sounding technique or method of determining refractive indicies for these very low altitudes to supplement theoretical calculations based on surface data. In an attempt to provide at least partial remedies to the problems of measuring low-level atmospheric structure, various modifications to existing techniques and other newer methods are being developed and tested throughout the military services, the National Oceanic and Atmospheric Administration, at universities and in private industry. Some of these methods include vertical pointing radars, acoustic sounders, tethered and delayed-release balloon soundings, instrumented portable and stationary towers and dropsondes (sondes dropped from aircraft or other airborne vehicles). Conventional radiosondes have been modified to increase the sensitivity and sampling rates of the temperature and humidity apparatus and certain errors have been largely eliminated. To date, however, there is no concrete proof or guideline as to which method of measurement is best under any particular weather or operational situation. The Pacific Missile Range is comparing some of the various conventional atmospheric sounding techniques to evaluate their relative merits in detecting the gross and finer scale atmospheric structure. The comparisons are being conducted under a variety of atmospheric and operational conditions using controlled-rise and tethered balloon techniques as well as aircraft operated methods employing special airborne sensors and Naval Avionics Facility, Indianapolis (NAF1), refractometers. The various measurement systems and approach are schematically shown in Figure 8.
Once the data are obtained from the various observation techniques operated in the same approximate volume of air space they are reduced by computer into comparative formats. The various refractive profiles derived are each applied to correct the radar data obtained while tracking the instrumented aircraft. Independent methods of determining true aircraft position are employed to provide a reference for a comparison of the various corrected tracks. When completed, it is expected that the study will help determine which sounding system is most representative of actual refractive conditions and which one leads to the most accurate correction of raw radar data for evaluations of
Don't Fall in the Radar Hole 65
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Satellites have opened up another large area for Potential measurement of low-level refractive condi- hons. Navy scientists at Point Mugu have theoretically 'knaonstrated the feasibility of using doppler shift data from available navigational satellites to reconstruct the gross features of significant refractive layers in the lower atrnosphere. Such a method would have the advantage °f requiring only a good receiver and a computer, however, doppler shift measurements would have to ^ accurate to one part in 107 in order to arrive at 1 solution that is sufficiently detailed to be useful.
The weather satellite can also be a useful tool in 'ktermining gross refractive conditions in the atmosphere. Meteorologists, familiar with the characteristic tractive profiles associated with various air masses and Houd-covered regions (particularly with stratus and stfatocumulus clouds) may, in the near future, be able t0 calculate cloud tops fairly accurately from satellite Hata and indicate the approximate refractive structure °ver large areas within which there are virtually no Afreet or routine meteorological observations. By studying the trends and movements of cloud patterns ,nd features from satellite picture to satellite picture, 'he meteorologist should be able to estimate refractive c°nditions of a generalized nature.
Even with adequate data and descriptions of the tractive environment, the problem of how to forecast tractive conditions for the future will be a difficult ®nc. But with conscientious use of present weather ^formation, theory, satellite pictures, climatology, and knowledge of refractive effects, the meteorologist can ^ake rough estimates of the location of major refrac- tlve layers up to 24 hours in advance.
level of training for the operator, performance level of the equipment, and temperature profiles. From this, the ship’s commanding officer or the force commander can make some tactical decisions concerning his ASW mission. A question the naval officer might ask then is, "Why can’t similar programs be established for radar refraction problems?”
They can and may soon be. If sufficient data and computer facilities are available, objective techniques can be developed and standardized to adequately forecast the regions of good or poor radar coverage. But before becoming operational, such tools would still have to be validated by comparing forecast conditions to the actual observed radar coverage conditions. Continuing efforts would require high priorities for the refraction effort which in turn requires the recognition of refraction as an important tactical weapon (education) and the coordination of many diverse efforts into one organized plan of action (direction).
Education must proceed at all levels from the Fleet commander down to the radar operator. Instruction booklets could be made available which present in layman’s terms, the refractive effects of the atmosphere and what can be done to minimize or maximize them. One such booklet is already available by the Fleet Combat Direction System Training Center Pacific, while others of a more comprehensive nature were
It was mentioned earlier that the problems encoun- tcred because of anomalous radar propagations are very Slfnilar to everyday problems encountered by sonarmen, 'b'er the years, however, concern over sound refraction 'n water has been generally much greater than electromagnetic wave refraction in the atmosphere. This em- Phasis is because optical visibility in the oceans is virtu- % zero, making sonar the only reliable means of Underwater detection. In air, where mistakes are probacy more apparent than in water, there are more alteratives for detection. In addition, the ocean is generally c°nsidered to be much more stable than the atmos- PHere, making it more amenable to simplified solutions, ^t any rate, much effort has gone into obtaining tem- Putature traces for the ocean areas and making predicts as to what range sonar contacts might be obtained t at what depths variable depth sonar transducers °u'd be used. Publications have been in existence for time that have as input the type sonar in use,
published by the former Navy Weather Research Facility. This kind of information must get down to the working level.
The second part of the problem, one of direction, has been the hardest to accomplish in the past but is now becoming a reality. A working group has been formed under the Joint Chiefs of Staff to review the state of refractive forecasts and related interests within the Navy, Air Force, and Army and to come up with a plan of action for solving the problem. The aim of this committee is to pool the results and knowledge of past studies concentrated within the Air Force and other services with a host of newer efforts in refraction and anomalous propagation largely concentrated within the Navy, with considerable help from private industry and government laboratories. Within the Navy at least, nearly all of these fragmented efforts are unfortunately still being conducted relatively independent of each other. Rather than advancing to a common goal or solution for the Navy as a whole, each agency is working on some aspect of the problem for their own specialized application within existing resources and stated missions. Nevertheless, the list of current efforts is impressive apd includes some promising techniques under development. These include programs by the Fleet Numerical Weather Central to forecast occurrences of evaporative and elevated ducts and radar coverage patterns (in the same manner as the SHARPS programs do for sonar). There are extensive studies of the geographical distribution of refractive parameters and their possible measurement from ships by the Environmental Prediction Research Facility. Objective techniques of applying refractive information for the Fleet are being studied by the Naval Research Laboratory, the Long Beach Naval Shipyard, Fleet activities on both U. S. coasts, and the Naval Electronics Laboratory Center which will also conduct studies into verification of refractive forecasts. The Pacific Missile Range is evaluating the techniques of sounding the atmosphere and obtaining refractive index parameters so we can know how to provide the best data for present operational needs and future forecasting tools that may be developed. Ultimately, line-of-sight radiometers and other remote sensing techniques may be applied to the measurement of refractive index along the true line of sight of a radar beam.
In the meantime, we must use our present methods.
A knowledgeable CIC officer can apply even meager information or qualitative impressions about the atmosphere to reach important and meaningful conclu
sions about the likelihood of encountering anomalous propagation of radar energy. For instance, while at sea, tactical guidelines in potentially critical situations can be developed by applying such simple thumb rules,s expecting strong super-refractive layers to:
► coincide with temperature inversions,
► be present just above the tops of extensive decks of stratus or stratocumulus clouds, and
► occur during warm, dry conditions over an open sea-
In places such as the Mediterranean, the presence of cloudless skies, light surface winds and reports of wat® dry air and high pressure extending over the operating area can be taken as forewarning of possible radar problems due to refraction. Over the eastern North Pacific Ocean, the presence of persistent stratus of stratocumulus cover should be sufficient evidence of strong super-refractive conditions just above cloud top* in that region. Dry winds like the Santa Ana of South' ern California and the khamsin of the Eastern Med1' terranean blowing over an oceanic surface are the atmosphere’s warning of extreme super-refractive conditions at very low levels. And wherever strong low-level refractive layers and topographic obstacles to the wind flow appear together, mysterious ghosts migh1 be anticipated. However imperfect it may be, this kind of simple environmental information might be assim1- lated and then used by the planning/cic officer to make the difference between success or failure.
The solution seems to be one of training, impk" menting interim measures in tactical doctrine and concentrating the fragmented efforts to solve the radar refraction problem under one manager or coordinator
Mr. Rosenthal graduated from New York University in 1959 with a Bacho lors degree in meteorology and received his Masters degree in meteorolog! from U.C.L.A. in 1963. He has been employed in the Geophysics Division at the Navy’s Pacific Missile Range since 1963. His duties as a meteorology at Point Mugu have included development of techniques for predicting a111* describing atmospheric conditions affecting PMR and naval operations. 1° 1971, he participated in the NATO Advanced Study Institute on Statistic*1 Methods and Instrumentation in Radio-Meteorology in Norway.
Captain Sherar has served in various meteorological billets both ashore an afloat including the USS Yorktoun (CVS-10), Headquarters Naval 3;f System Command, and the Pacific Missile Range. He was commissioned 1,1 1951 upon completion of flight training and subsequently served two tout* in all-weather fighter squadrons and as a fighter instructor. He received > B.S. (Meteorology) from Monterey in 1963 and an M.P.A. (Technolog) of Management) from The American University in 1970. He is presend) serving on the staff of the Commander, Naval Weather Service Command as Assistant Commander (Plans and Policy.)