Limitations Imposed on Radar by External Factors
A. Propagation of radio-frequency energy: Antenna height; bending of radio waves; reflection of radio waves from the sea; antenna pattern; attenuation of transmitted pulse; type of target.
B. Nonstandard propagation of radio waves--trapping: General features; effect on radar performance; meteorological factors that cause trapping; prediction of nonstandard propagation.
C. Weather: Roll and pitch of ship; sea return; formation of ice on antenna; storms; atmospheric noise.
A. PROPAGATION OF RADIO-FREQUENCY ENERGY
If it is assumed that radio waves travel along perfectly straight paths, the curvature of the earth prevents these straight rays from striking objects that are beyond the horizon. If the height of the antenna is increased, the point at which the lowest radiated ray is tangent to the curve of the earth is made more remote. That is, the horizon is extended. An object beyond the horizon can be seen if it is high enough that the ray tangent to the earth can strike it.
Figure 85 shows a nomogram that correlates radar antenna height and target altitude with the maximum range of the target as determined by the optical horizon. This chart is reasonably accurate for estimating radar ranges, but since changing propagation conditions may have considerable effect on the range of a radar, the nomogram cannot be relied on implicitly. For example, if the radar antenna is 100 feet above the water, and it is desired to find the maximum range at which a group of planes flying at 10,000 feet can be detected, a line is drawn as in figure 85 between the 100-feet point on the left vertical line and the 10,000-feet point on the right vertical line. The intersection on the middle vertical line indicates that the planes could be detected at a maximum range of 134 miles. However, this is no definite indication that the planes will be detected at this range; it simply shows that the geometry of the line of sight from the antenna is such that the planes may be detected at that range.
Radio waves, like light waves, must pass through the atmosphere to get from one place to another. The characteristics of the medium through which the waves pass affect the manner of their transmission. Thus, although it is sometimes assumed that both light and radio waves follow perfectly straight paths, the properties of the atmosphere are such that the waves are made to follow curved paths. One very common effect of the bending that light waves experience in passing through the atmosphere is apparent at sunrise and sunset. At this time the sun is very low, so that none of its light would be visible if the rays followed straight paths, but because the red components of the light are bent down to earth the sky is brightly colored. The propagation of radio waves is affected, then, by: the composition of the atmosphere, which controls the amount of bending, or refraction, that the waves experience in their passage; the diffraction that bends the waves over the horizon to some extent; and by the reflection of the radio waves from the surface of the sea. Other factors that affect propagation of radar pulses are the nature of the antenna pattern, the attenuation of the radar waves in their passage through the atmosphere, and the size and character of the target that the pulses strike.
The Heaviside layers, which are in the ionosphere 30 to 200 miles above the earth, reflect long-wave radio energy back to the earth, and so exert considerable effect on the propagation of frequencies below about 40 megacycles. However, the Heaviside layers are penetrated by the high frequencies used in radar and most VHF communications, so that these waves are not returned to the earth. Therefore the Heaviside layers have no appreciable effect on the propagation of radar pulses.
2. Bending of Radio Waves.
The quantity that indicates the degree of bending from a straight-line path is called the index of refraction. Refraction occurs whenever there is a change in the medium through which the waves are passing, as at the boundary between two substances. The measure of the bending that occurs at such a boundary is indicated by the change of index of refraction from one substance to the other. In the interior of a homogeneous material, in which the index of refraction is constant, light waves or radio waves travel in a straight line.
Figure 85--Nomogram correlating maximum detection range with radar antenna height and target altitude.
Basic to the understanding of the propagation of radio waves through the atmosphere is the fact that the atmosphere is not a homogeneous medium. The changeable characteristics of the atmosphere are temperature, pressure, and moisture content, and these factors vary with altitude. However, the change in characteristics of the atmosphere from one level to the next is small, so that a radio wave is refracted only a small amount in passing up through the atmosphere. Nevertheless, refraction is of great importance in radar operation and communications at frequencies above 30 megacycles.
Since the change in the physical properties of the atmosphere is normally gradual and continuous, the index of refraction also changes gradually with increasing height. Consequently, there is no sudden change in the direction of the radar waves; rather the change in direction is gradual and continuous. Radio waves passing through the lower atmosphere are usually bent downwards.
In order to establish a reference for the purpose of investigating the effect of the atmosphere on propagation, a so-called standard atmosphere has been postulated. The standard atmosphere is likely to be found in nature when the air is well mixed so that no unusual temperature or humidity gradients can exist. In the standard condition the temperature and moisture content of the air decrease uniformly with height. However, this standard atmosphere is not necessarily the normal atmosphere at any particular location. Above 10,000 or 12,000 feet the atmosphere is almost always of standard composition, but non-standard propagation conditions may often exist in the lower atmosphere. The conditions that lead to such nonstandard propagation and the effects on radar operation are discussed in section B of this chapter; the discussion in section A is based on standard propagation.
Since the water vapor in the atmosphere bends radio waves almost twenty times more than light waves, the distance to the apparent horizon for a radar set is approximately 15% greater than the distance to the optical horizon. Thus, in figure 86A the optical line of sight from point (1) is tangent to the earth at point (2), while the radar ray, which is more curved, is tangent to the earth at point (3). In many cases it is more convenient to draw the radar rays as straight lines as a means of simplifying the picture. It has been found that if the radius of the earth is considered to be 4/3 of the actual radius, then the radar rays may be drawn as straight lines when propagation conditions are standard, as in figure 86B. Often in plotting fade charts the antenna pattern is plotted on rectangular coordinates, in which the earth is assumed to be flat. The result of this assumption is that both the optical and radar rays appear to curve upward, as in figure 86C. The shape of the lobe of the radar antenna under these three conditions is shown in each part of figure 86. Note that the shape shown in A is true, except for the
Figure 86--Comparison of radar horizon with optical horizon.
Figure 87--Effect of diffraction on shadow.
exaggeration of the vertical dimensions, while in B and C the lobes show the distortion occasioned by changing the coordinate system for convenience in plotting. All three diagrams, however, are exactly equivalent, and they show the same thing.
Under standard conditions, radar pulses cannot penetrate the region which lies below the horizon ray since the radiated energy cannot enter this area either by bending or by reflection from the earth. However, measurements indicate that some of the pulse energy does enter this region because of diffraction of the energy around the earth's curvature.
Diffraction of radio waves can perhaps be explained most simply by discussing its effect on light. When white light is passed through very narrow, closely-spaced slits, diffraction causes the various components of the white light to be bent away from a straight path by an amount dependent on the wavelength of the light. Long wavelengths are bent more than short wavelengths and a spectrum is produced. This same bending effect can be observed in shadows cast by sunlight. The light rays from the sun are essentially parallel, yet a shadow of a ball does not have sharp clear edges, as in figure 87A, because of diffraction. When the rays pass close to the surface of the ball, they are bent inward slightly and so they penetrate the shadow, partially illuminating its edge, as in figure 87B. Since the wavelengths of the radio waves used in radar are much longer than those of visible light, the amount of bending caused by diffraction is also greater.
Although the radar energy diffracted around the curve of the earth usually is too weak to produce usable echoes, it can still be detected by a suitable receiver. Since low-frequency radio waves are bent more than high-frequency waves, a low-frequency radar can be intercepted by the enemy at a greater distance than a microwave radar, provided the two sets transmit comparable power. The principal effect of diffraction, then, is to extend the range of
Figure 88--Addition of sine waves.
Figure 89--Paths taken by radiation in reaching target.
possible interception of radar transmitted pulses by enemy surface vessels beyond the radar range.
3. Reflection of Radio Waves from the Sea.
The pulse of energy transmitted by a radar is carried through space by an alternating electromagnetic field that moves at the speed of light. At any point in space, the magnitude of the field varies from some maximum positive value through zero to the same maximum value in the negative direction, and then through zero in the positive direction again as the pulse of energy travels past the point. This variation takes place at a rate of several hundred million times per second during the radar pulse. It is convenient to represent such an alternating field by a sine curve, since the time-variation of the field carrying the pulse energy is of this character.
If energy arrives at a point in space by following two different paths, the resultant field at that point will be determined by the contributions from each of the two paths. Since both fields alternate at the same rate, the resultant field is determined not by simple addition, but by the combination of two sine waves.
If two sine waves vary exactly together as  and  in figure 88A, they are said to be in phase, or to have a phase relation of 0°. Since waves that bear this relationship always rise to maximum and pass through zero at exactly the same time, they add at every instant, so that one wave reinforces the other, making their sum the wave shown in figure 88A at  If the two sine waves are in opposition, as  and  in figure 88B, they are said to be 180° out of phase. Since wave  always increases in one direction at the same rate and at the same time that wave  increases in the other, one wave cancels the other, and their sum at all times is zero, as indicated by the horizontal line in figure 88B . If the waves are not a full 180° out of phase, as in figure 88C, partial cancellation takes place, depending on the phase relation, and the resultant wave is small.
Unless the radar antenna has a narrow beam that can be elevated, some of the radiated energy must hit the water and be reflected. In most cases, then, energy can arrive at a target T by both the direct path RT and the path RST involving reflection from the sea (figure 89). The nature of the reflection from the sea is such that the reflected wave is 180° out of phase with the incident wave. This change coupled with the fact that the two paths are not of the same length causes a phase difference between the two paths. Since this phase difference varies with the position of the target relative to the radar antenna, the strength of the energy at the target also varies with the target's position. For example, the phase relation of the two waves that arrive at point T in figure 89 is such that the fields reinforce, while at T' the fields cancel. If all points in space are considered, it will be found that many points exist at which complete cancellation occurs, and many other points where the reflected and direct waves reinforce. The area adjacent to a line joining the points of cancellation is called a null because echoes cannot be returned from targets within this area. Thus, the result of reflection of the radar pulse from the sea is to break the vertical antenna pattern into many rather narrow lobes which are separated by the nulls.
For a given frequency, the angular separation of the lobes and the height of the lobe closest to the surface of the water are dependent on the height of the antenna above the water. The higher the antenna is mounted, the closer the lowest lobe will be to the surface of the water, and the lobes will be more closely spaced. If the radar frequency is low and the antenna is mounted close to the surface of the water, the nulls will be rather wide, making several blind areas in the radar coverage. It would appear possible for a plane with a suitable intercept receiver to fly down a null by keeping in the region of minimum signal strength, thus remaining undetected until it reached a very close range. Tests in which evasive tactics of this sort have been tried were not highly successful, since it is very difficult to stay within the null area. The Japanese evidently have attempted this maneuver with partial success, although it is believed that they do not carry intercept receivers
specifically for this purpose. Since radar antennas are usually mounted reasonably high, the extent of the nulls is small, making it still more difficult for the plane to remain undetected for long.
The lobe pattern is affected by the radio frequency transmitted as well as by the height of the antenna. At very high frequencies, interference produces many narrow lobes that are separated by small angles. For a given antenna height, the lowest lobe will be closer to the water for a high frequency than for a low frequency. Compare figure 90A with figure 90B.
Below the lowest lobe of the antenna pattern is a region of practically complete cancellation. This condition prevents the air search radars, which are operated at a relatively low frequency, from detecting either surface vessels or low-flying airplanes at long ranges under standard weather conditions. Therefore, radar equipment which is to be used for surface search employs very high frequencies, usually in either the S or X band because the lowest lobe comes much closer to the line-of-sight boundary.
The protection against torpedo plane attack afforded by the air search radar usually is very poor. By the time the air search radar detects aircraft flying fifty or a hundred feet off the water, the plane has closed to such a range that there is little opportunity to intercept or to bring guns to bear. This serious limitation of low-frequency radar may be remedied partially by installing the antenna in the highest possible position on the ship. However, chief reliance for the detection of very low-flying airplanes at present must be placed on the low-angle search radar, such as the SM and SP, or on surface search radar. The SX general purpose radar should be very helpful for this application when the set becomes available.
In addition to producing many lobes and nulls in the radiation pattern, reflection of radar pulses allows the maximum range of detection to be nearly doubled. This increase results purely from the reflection of energy from the water; no change in characteristics of the radar is required. If no water reflection takes place, only one possible path, the direct one, exists between the radar and the target. When the target is near the maximum of a lobe, the pulse can reach the target by two paths and the echo can return to the radar antenna along the same two paths. Since more of the pulse energy can get to the target by two paths than by one, the echo is stronger, and a larger fraction of this stronger echo is returned to the radar by the two paths than if there were no reflection taking place from the water. As a result of this phenomenon, the coverage of the SM radar is approximately like that shown in figure 91. When the antenna beam is horizontal or tilted up by a small angle, some of the radiated energy strikes the water, and targets can be detected out to approximately 80 miles. However, when the beam is elevated so high that none of the energy is reflected from the water, targets can be detected only out to approximately 43 miles.
The phenomenon of producing lobes as a result of water reflection is not restricted only to radar.
Figure 90--Lobe diagrams under standard atmospheric conditions.
Figure 91--Effect of water reflection on radar coverage.
It must be realized that VHF communications energy also is confined to lobes such that at many places in space there will be nulls, or dead spots, in which communications will be very difficult if not impossible.
4. Antenna Pattern.
The width of an antenna beam in either the horizontal or vertical dimension is determined by the width and height of the radar antenna in wavelengths. Thus at long wavelengths, the directivity of the antenna is restricted by practical limitations on the maximum physical size that can be installed in a ship. Very narrow beams can be produced practically, therefore, only at very high frequencies.
The sharper the beam, the more concentrated the energy and therefore the greater the range obtainable with a given amount of transmitted power. The narrower the beam, the greater the bearing accuracy and bearing resolution possible. Thus low-frequency air-search radars inherently have relatively poor bearing accuracy and bearing resolution because the antenna beam cannot be made very sharp.
Although a radar antenna is highly directional, not all of the radiated energy is confined to the principal beam. Some energy, usually one per cent or less of the main lobe, is radiated in all directions, especially by the air search radar antennas (fig. 92). In spite of the fact that the radiation far off the axis of the main beam is so small, it is still sufficient to produce strong echoes from large nearby targets. If several large ships are within approximately a mile of the radar, they may produce saturation echoes no matter where the antenna is trained. These strong echoes will then prevent the small echoes from aircraft or small surface craft from being seen at very short ranges. If the receiver gain is decreased, the echoes from the small targets may be lost, while those from the large ships or nearby land are simply reduced to a value somewhat below saturation. Sensitivity-Time Control (STC), a circuit that automatically reduces receiver gain during the first few thousand yards of the sweep and then returns it to normal, should be very useful in minimizing the echoes returned in side lobes. However, in a partially land-locked harbor, the echoes returned from
Figure 92--Radiation pattern of air search radar antenna.
the land may be so strong that nearly the whole screen may be covered with land echoes. These echoes may be so strong that STC will be of little assistance, and reliance must be put in other circuits to enable the operator to read through the clutter.
In some cases it is desired to produce an especially broad beam in the vertical direction so that a large area can be scanned thoroughly. An antenna has been developed for this purpose which produces what is called a cosecant squared beam in the vertical direction. The horizontal beam width may be made as narrow as practical. The outstanding feature of a beam of this type is that the field intensity is essentially uniform along any horizontal line, such as A-B or C-D in figure 93. As a result, an airplane detected at D can fly to C without the echo amplitude increasing appreciably. Such a beam is useful where it is desired to know quickly whenever an airplane, irrespective of height, enters the area under search. A beam of this sort is desirable also because it minimizes ground and sea clutter on shipboard installations. However, in airborne radars that use this type of antenna beam the sea return may extend many miles beyond the limit observed on other sets.
Figure 93--Cosecant squared beam.
5. Attenuation of Transmitted Pulse.
The energy that is available for producing an echo decreases with range because the transmitted pulse spreads as it travels away from the radar. If the pulse is traveling toward a target at high altitude, the energy in a unit area decreases as the square of the range. The small fraction of the incident energy that is reradiated toward the radar also decreases as the square of the range. The net effect of this two-way attenuation is to cause the amount of energy returned to the radar from a target at high altitudes to vary inversely as the fourth power of the range.
For targets on the surface, however, the attenuation is even greater because of the partial cancellation inherent in low-angle radiation when water reflection takes place. For example, in figure 94, the direct and reflected paths are almost the same length because very small angles are involved. The path RST is always slightly longer than path RT, but with the 180° phase shift that takes place on reflection, partial cancellation reduces the energy to a small value. Only at very close ranges is the difference in path lengths great enough to permit the two fields to add over an appreciable area of the target. As a result, the effect of water reflection is to reduce the size of the echo from a distant surface target by an even greater amount than normal attenuation would indicate. It has been found that the received energy from distant
Figure 94--Low-angle radiation.
surface targets varies inversely as the eighth power of the range, approximately. Considerably greater attenuation is experienced beyond the horizon because only a very small part of the pulse is diffracted around the curve of the earth, and that is strongly affected by interference.
The atmosphere itself does not attenuate radiated electromagnetic energy appreciably if the wavelength is longer than approximately 10 centimeters. Therefore, the P and L bands suffer only the normal inverse fourth power attenuation with range. However, in the microwave region, the wavelength is so short that the gases and water vapor that make up the atmosphere begin to absorb energy from the passing electromagnetic waves. The attenuation so caused is in addition to the normal inverse fourth power attenuation caused by the spreading of the energy over a larger and larger area as the range increases. The attenuation caused by absorption is very small for the S band, but it becomes increasingly more important as the wavelength decreases. In addition to having relatively high absorption loss in clear weather, K band radar suffers a range decrease to approximately 50% of maximum in rainy weather, and there is a noticeable drop in the performance of X band sets because of rain. Radars that operate at frequencies in the S, L, and P bands do not encounter any increased absorption in rainy weather-- probably because the wavelength is large compared with the size of the raindrops.
6. Effect of Type of Target.
Radio frequency energy is reflected to some extent at any surface that presents a discontinuity. Reflection takes place best from a plane surface at right angles to the radar beam. Since a metal surface presents a very great discontinuity, it sends back a strong echo. Although the best echoes are obtained from conducting objects, non-conducting targets will also return an echo. Thus, echoes are sometimes received from wooden boats, birds, fish, and clouds, especially with microwave radar. Probably a low-lying wooden boat can be detected farther optically than by radar.
The amount of energy reflected back to the radar is nearly proportional to the effective area of the target as long as the target is large compared to the wavelength of the transmitted energy. Very small targets, such as submarine periscopes and small buoys, can therefore be detected at much greater range with microwave radar than with long wave. Large targets, such as high mountains, can be detected at very long ranges with all types of radar, so that transmitted power and antenna height rather than the frequency of the set are of paramount importance in making landfalls by radar.
A radar target is never a single point. If it is a large object, it is composed of a number of variously oriented surfaces and corners, each of which is responsible for a little of the echo returned from the target. Since the target rarely presents a flat vertical surface, the distances between the radar and the various parts of the target often will be slightly different. As a result of these small differences in range, the signal components returned by the many parts of the target may be out of phase by the time the echo returns to the radar. The strength of the whole echo signal will depend on whether these components partially cancel or reinforce each other. For example, with a radar operating on the X band the path lengths to and from various parts of a target need differ by less than a half inch to cause variation in the composite echo. As either the radar or the target changes orientation, the echo power may vary by a factor of as much as 100.
Since airplanes change aspect more rapidly than other types of targets, airplane echoes fluctuate more widely and more rapidly than those from other types of targets. It has been found that the reradiation pattern of an airplane is composed of many irregular narrow lobes, so that a very slight change of target angle may cause a very large increase or decrease in the echo height. In general, the echo from a plane is greater when the target angle is approximately 045°, 135°, 225°, or 315°, than at other angles. The rapid, irregular fluctuation in the size of the echoes returned to microwave radar by airplanes has been termed "glinting." This effect is not noticeable at low radar frequencies or on slow-moving targets because the pattern of cancellation and reinforcement of the echoes cannot vary rapidly.
The echoes from surface vessels fluctuate
considerably less than those from aircraft because, in large ships at least, a large part of the target area is the relatively flat side of the hull. When ships are hull down, or when they are small and rolling in the seas, the fluctuations are rapid and of relatively great amplitude. The target angle of surface targets has little effect on the fading of echoes at ranges beyond approximately 5,000 yards.
Land echoes are much steadier than those from either surface vessels or aircraft. However, even these echoes are often not perfectly steady because of movement of foliage in the wind. In general, a single isolated target on land, such as a standpipe or lighthouse should produce a very solid echo.
The amount of fluctuation of echoes from all types of targets is affected by the state of the surface of the sea. In a dead calm when there are no waves or swells, the fading is least. As the surface becomes more ruffled, the echoes fluctuate more because of the variation in the amount of energy that readies the target by way of the path involving reflection off the water.
Another factor that contributes to the fluctuation of echoes of a microwave radar is the large number of closely spaced lobes and nulls through which the target must move (see fig. 90B). This factor is of minor importance for long-wave radars because of the great extent and relatively small numbers of the lobes in the antenna pattern.
In many cases the atmosphere through which the radar pulse travels is not perfectly uniform. The radar waves are, therefore, not bent away from a straight path by the same amount at every point along their travel, nor is the amount of bending constant with time. Fluctuations caused by irregularities in the atmosphere are not apparent in calm air, but they become quite noticeable when the air is turbulent. The variation in the refraction of radio waves that is caused by turbulence in the air results in varying distribution of energy over the front of the moving pulse, and consequently in fluctuations of echo height.
The effect of fading of the echoes is minimized in some fire control radars by the use of automatic gain control (AGC). This type of circuit allows echoes from any selected single target to control the gain of the receiver. If the echo fades, the gain of the receiver is automatically increased so that the selected pip height is unchanged, but all the other echoes vary.
A special case of reflection takes place when radiation is directed at an object composed of two plane reflecting surfaces which are at right angles. Such a reflector is called a corner reflector. The property of a corner reflector which makes it of value is that any ray which can enter the corner is reflected out of the
Figure 95--Corner reflector.
corner along a path parallel to the incoming ray. The geometry of the corner reflector is shown in figure 95 to illustrate this effect. A ray AB which enters the corner at any angle 6 with the vertical side will be reflected from this surface at the same angle 6. Therefore, the angle at which the reflected ray strikes the horizontal side is 90-0, and the ray leaves the corner along CD. If this ray is projected back to B, it is clear that AB and DCB intersect the vertical plane at the same angle, so that these rays must always be parallel.
A reflector of this sort will produce a very strong echo on a radar because the energy that strikes the corner is reflected back directly to the radar without the excessive scattering that occurs with most targets. Metal corner reflectors are used often as artificial targets. However, such reflectors can frequently occur in nature, as in rock formations, sharp vertical cliffs that rise out of the water, or in the many perpendicular metal surfaces aboard ship. Artificial corner reflectors are usually made in the form of three sides of a cube rather than simply as two perpendicular planes. The larger the corner reflector is made, the greater will be the energy intercepted, which will produce a stronger echo.
Since corner reflectors return strong echoes for their relatively small size, they have found use in many applications. For example, they are used on aero-logical balloons that are tracked by radar, on special radar buoys, as artificial targets for use in boresighting radars, in target sleeves, on target rafts (fig. 96), and on life rafts. Targets which are rounded, as opposed to a corner reflector, so that no surfaces are perpendicular to each other probably will be hard to detect by radar.
Figure 96--Corner reflector for target raft.
B. NON-STANDARD PROPAGATION OF RADIO WAVES--TRAPPING
1. General Features.
When the atmosphere is well mixed so that no unusual temperature or humidity gradients are likely to exist, standard propagation conditions should prevail. Considerable mixing of the atmosphere occurs with high, gusty winds, or in a region of low barometric pressure which is usually accompanied by turbulence. Standard propagation is likely also when the water is colder than the air, or over land when cooling of the earth by radiation is limited by overcast skies. If rain is falling, the lower atmosphere in general will be saturated with water vapor, so that the existence of abnormal humidity gradients is prevented and propagation should be standard.
In the standard atmosphere both the air temperature and the moisture content decrease uniformly with height above the surface of the earth, so that the index of refraction of the atmosphere also decreases uniformly. However, the atmosphere is subject to many changes. The temperature may, for example, first increase with height and then begin to decrease. Such a situation is called a temperature inversion. More important, the moisture content may decrease markedly with height just above the sea. This latter effect, which is called a moisture lapse, either alone or in combination with a temperature inversion may produce a great change in the index of refraction of the lowest few hundred feet of the atmosphere. The altered characteristics of the atmosphere may result in an excessive bending of the radar waves that are passing through the lower atmosphere. In certain regions, notably in warm climates, excessive bending is observed as high as 5000 feet. The amount of bending in regions above this height is almost always that of normal atmosphere.
The atmosphere must be relatively calm in order to permit the existence of the conditions that produce this excessive bending. After a period of calm or of light breezes, the lower air may be stratified so that non-standard propagation is likely throughout a wide area. There will be formed within this area a sort of duct or waveguide in which the radar waves can be trapped. The most remarkable effect of such trapping is the extreme ranges that have been obtained as a result of it. Ships using 200-megacycle search equipment have detected land at ranges between 300 and 400 miles, and rather small ships have been detected at ranges up to 50 miles.
The duct in the atmosphere may be formed along the surface of the sea or elevated above the earth. In either case, the coverage of the radar will be changed by the great extension of detection range for targets within the duct. In figure 97, one possible type of deformation of a radar coverage pattern is illustrated. The duct illustrated acts very much like the waveguide that is used as the transmission line in surface search radar. The humidity gradient is such that the rays from the radar antenna are bent an excessive amount,
Figure 97--A possible effect of trapping.
and they are trapped in a zone near the surface of the water. Since the effect of the duct indicated in figure 97 is to guide the radiated energy around the curvature of the earth and back again to the antenna, the radar is able to detect the ship at a range much greater than normal.
If the moisture content of the air increases with height, the radar waves may be bent up instead of down. Non-standard propagation of this sort reduces the radar coverage instead of increasing it.
2. Effect on Radar Performance.
The position of the radar antenna relative to the duct in the atmosphere is one of the most important factors in controlling the effect of trapping. The radar pulse can be trapped and carried out to very distant targets and back to the radar if the antenna lies within the duct. If the antenna lies below the duct, trapping may take place, but only if the radiated rays from the antenna enter at a very small angle. For this reason, elevated ducts that are very high will not have any direct effect on radar performance. On the other hand, if the antenna is a small distance above the top of the duct, blind zones may be present in the low-level portion of the coverage pattern that do not exist during standard propagation conditions. However, if the antenna is high above the top of the duct, as an airplane flying over a ground-based duct, no effect on the airborne radar coverage will be apparent.
Since the ducts that are most important to the operation of shipboard radar are ground based, radars that have relatively low antenna heights are the most likely sets to experience trapping. Experiments and observations of shore radar installations in both the United States and England have indicated that low sites are rather generally affected by trapping while high sites experience the phenomenon relatively infrequently.
For some time after the discovery of trapping it was thought that the long ranges observed resulted from the diversion of a large amount of the radiated energy into the duct. Consequently, it was feared that the existence of trapping would cause a great decrease in coverage in the area above the duct. However, the conditions that exist in the most common type of trapping do not support this presumption. In spite of the great increase in ranges in the duct, the amount of energy trapped is small compared to the total energy radiated in the pulse. The increased range apparently is caused by the increased transmission efficiency that exists within the duct.
In many cases, coverage will be reduced for targets that are just above the duct. This reduction is caused by the waveguide action of the trapping layer. The radiated pulse is contained within the duct because the refractive power of the atmosphere within this region is such that the radiated rays are bent downward. Directly above the duct there may then be little or no energy, and targets flying there may not be detected. However, energy may reach targets that are well above the duct by travelling along paths that enter the trapping region at angles too great to be trapped.
The wavelength that can be trapped in the duct formed by non-standard propagation conditions varies with the height of the duct. If the duct is approximately 400 feet high, all radar frequencies can be trapped within it. However, if the duct is only 100 feet high, only microwave energy will be affected. A duct almost always exists within perhaps the first 20 feet above the water, but this is not yet of any practical use since only K band energy could be trapped within such a narrow region. There is fairly conclusive evidence of the rather general existence of a surface duct 50 to 65 feet high throughout the trade wind area of the Pacific ocean. This duct is capable of trapping S band and shorter wavelengths provided the antenna lies within the duct. Because of the characteristics of the ducts that are usually formed, radar frequencies
Figure 98--Regions in which refraction affects antenna patterns.
Figure 99--Coverage diagram showing deformation of lowest lobe.
below approximately 250 megacycles are the least likely to be trapped.
The amount of refraction, that is the amount of angular deflection of the rays, is very small and very rarely will exceed one degree. Such bending does not affect radar operation except in regions where the angle between the radiated ray and the horizontal is very small. When radar is used for AA fire control, the targets are usually at medium or short ranges, and the angle between the line of sight and the horizontal is usually larger than one or two degrees. Trapping has practically no effect on such applications of radar.
With air search radar the target may be an airplane fifty or a hundred miles away flying at an elevation of only a few thousand feet. In this case, the position angle of the target is only a fraction of a degree and for surface targets the position angle is even smaller. Only low-angle search is affected by meteorological conditions; performance of the radar is rarely affected by weather for targets above approximately 1° position angle (figure 98).
A coverage diagram for standard conditions is shown in figure 90A with height greatly exaggerated. Only the lowest three lobes are shown and the higher lobes appear compressed as compared with the lowest lobe. In figure 99 the lower part of the same diagram is drawn as it appears under a few conditions of trapping. The bottom part of the normal lowest lobe is shown by a broken line. The lines which separate the "blind zones" from the "detection zones" represent the range at which a medium bomber would just become visible to this particular radar set.
The diagrams clearly indicate the great extension of ranges in the duct, and also the moderate change in ranges--sometimes an extension, sometimes a reduction--above the duct. Another feature of some of these diagrams is the appearance of "skip ranges." A plane flying at an altitude of 500 feet, for example, would be detected early under the conditions shown in C and D. As the plane approaches, the echo will disappear from the indicator and reappear only at a range less than 20 miles. Similar conditions may exist for sea return. In B there may be echoes from waves close in and also from beyond 33 miles, but not in the space in between. For conditions shown in D, there will be echoes from very remote surface targets or land, but not from targets at intermediate ranges.
The lobe structure of an air search radar is not affected by trapping even though a duct high enough to trap P-band energy may be found in the atmosphere, since only the lowest lobe is deformed. Thus, trapping has no effect on the accuracy of altitude estimation by the use of fade charts. However, if altitude is estimated from the range that a plane is first detected, the estimate may be seriously in error. If the plane is flying in the duct, the estimated altitude is likely to be too high because the echo appears sooner than it would under standard conditions. On the other hand, if the target is flying just above the duct, the echo will be detected at a shorter range than normal, and the estimated altitude will be too low. The accuracy of measurement of altitude by radars that use lobe switching or a fanning beam should not be affected by trapping conditions, because when targets are at a position angle at which accurate
Figure 100--Variation of echo amplitude with constant radar performance.
measurement is possible, the radar waves make too great an angle with the horizontal for them to be trapped.
A very serious operational consequence of trapping is the misleading of radar operators as to the overall performance of the equipment. Long-range echoes caused by trapping have frequently been assumed to indicate good condition of the equipment when precisely the opposite may be the case. For example, figure 100 shows a part of a record of echo amplitude taken during an investigation of radar propagation. The performance of the radar was maintained at a constant level by the use of an echo box. The target was 30 miles away from the radar, with the path of the pulses passing over sea. Aside from the rise and fall of the tide, the variations in echo strength are due to changing characteristics of the targets or of the propagation pathway. The tremendous variation shown in figure 100 would certainly preclude the use of this target as a standard to check the performance level of a radar. At one point in the curve, the echo changed 200,000 fold (53db) within 2 1/2 hours. However, the phenomenon of trapping does not invalidate the measurement of echo height on nearby targets as a criterion of over-all set performance, since the echoes from objects well within the optical horizon are not affected appreciably by propagation variations.
Under conditions where the security of radar transmissions is involved, the possibility of trapping should be constantly kept in mind. When a duct is present, the enemy can intercept both radar pulses and VHF communications at far greater ranges than normal. When trapping is known to be present, and a choice of frequencies is available, it may be better to choose the lowest band since the high frequencies are the more likely to be trapped. Thus, decisions as to when to employ radar silence must be modified by consideration of propagation conditions.
Trapping may be the cause of apparently inexplicable failure in communications. Elevated ducts do not have much effect on ship-to-ship or ground-to-ship communications unless the duct is at low level. However, a plane flying within an elevated duct may have difficulty communicating with a plane outside the duct or with a distant ship because the duct may act as a barrier. When the temperature-inversion and moisture lapse are very steep over a relatively thin layer of the atmosphere, radio waves may be reflected directly from the bottom of the duct. Under this condition, which probably will be encountered only rarely, the lower frequency radar and communications waves may be reflected repeatedly between the duct and the sea, and so carried long distances, even without trapping taking place. There will be skip distances associated with this type of propagation because the nature of the reflection is such that the waves are returned to earth only at intervals. If a plane is flying above a thin, sharply bounded duct of this type, it may be impossible for communications to be established between the plane and the ship. In rare cases a phenomenon of this sort may also be responsible for failure of IFF. When communications from a plane fail, or the plane cannot make contact on a navigational aid, the pilot may be able to reestablish contact by changing altitude to get either above or below the duct.
When trapping causes echoes to be returned from targets at long ranges, the pips are likely to fluctuate far more than those from targets at short range. This fading is caused by changing propagation conditions within the duct. Such fading is of large amplitude, perhaps involving a change in echo power of as much as 1000 fold, and the echoes vary over a period of approximately 15 minutes. Violent fading of echoes' appears to be a characteristic feature of trapping conditions that may aid the operator in realizing when such conditions exist.
If the water vapor content of the air increases with elevation, the radar waves are bent up instead of down. This condition, which is referred to as substandard, can exist in some kinds of fog or when fog is about to form. Although not enough is yet known about radar propagation in fogs to state its full effect, it appears likely that nearly all fogs produce substandard conditions. The obvious danger of this condition is that the fog prevents ordinary optical search, and at the same time reduces the radar detection range on surface vessels and low-flying aircraft. Unfortunately, there is very little that the operator can do to combat the effect of fog except to run his set at the greatest possible sensitivity.
Airplanes often fly very low to avoid detection by radar. However, if a duct is formed near the surface of the water, such evasive action will probably be unsuccessful. If the presence and height of the duct were known, better protection against radar detection would be furnished if the planes were to fly just above the top of the duct.
The changing propagation conditions that radar waves can encounter, then, can have considerable effect on radar performance. To summarize, the effect of trapping on a radar may be to:
- Increase the range of detection for surface vessels and airplanes flying within a duct.
- Reduce the range of detection for airplanes flying just above a duct.
- Reduce the range of detection for surface vessels and low-flying planes in some types of foggy weather.
- Modify the height at which airplanes should fly to avoid detection by enemy radar.
- Cause errors in height finding for targets below position angles of 1°.
- Increase the extent of clutter from sea return, and thus reduce the operational efficiency.
- Increase the range to which a navigational aid can be effective.
- Increase the range at which radar signals can be heard.
- Modify the degree of jamming either suffered or inflicted.
Weather effects cannot be blamed indiscriminately for variable performance of radar. In general, inadequate adjustment or incipient electrical failure will cause a far greater decrease in range than any variation in the propagation conditions. For example, failure to detect a high-flying airplane at short range cannot be attributed to non-standard propagation; trapping affects the detection of targets only at low angles above the horizon.
3. Meteorological Factors That Cause Trapping.
The atmospheric conditions which allow standard propagation to take place are common. The peculiar structure of the atmosphere that produces trapping, especially for frequencies of 3,000 megacycles and higher, also occurs fairly often in many parts of the world. Several types of meteorological conditions can produce the temperature and humidity gradients necessary for trapping to take place.
Warm continental air blowing over a cooler sea leads to the formation of a duct by causing a temperature inversion as well as by evaporation of water from the cooler sea into the lower levels of the warm dry air. The base of such a duct is usually the sea surface with the trapping region extending several hundred feet upward. At higher latitudes--above 25° or 30°--this type of trapping is most prevalent along the eastern shores of continents. If it is present in the lower latitudes, the duct will be formed on the western coasts. This distribution with latitude results from the normal direction of the wind, which must be off-shore to produce the effect. Ducts of this sort will form only when there is a distinct temperature difference between the sea and the air blowing from the land. Hence, conditions are most favorable for trapping in the summer months. The duct is usually from 500 to 600 feet high, and it tends to remain level for a distance of 100 to 200 miles out to sea. Near the coast trapping will be found to be strongest just after noon, and weakest just before dawn. Fog usually is not associated with the meteorological conditions that produce this form of trapping, but at times a surface fog will be observed 5 to 10 miles off shore.
Over the open ocean, a surface duct may be formed by cool air blowing over a warmer sea. There is no temperature inversion associated with this phenomenon, and the entire effect is caused, apparently, by the evaporation of water into the lower levels of the atmosphere. Ducts of this sort are often created by easterly winds, such as the trade winds, that have blown for a long distance over the open sea. The height of the duct increases with the wind speed, and at the higher wind speeds typical of the Pacific trade belt--10 to 20 knots--ducts 50 to 60 feet high occur quite generally. These ducts seldom attain the height and intensity necessary to entrap the lower frequencies, but 5 and X band radars that have antennas within the duct show better range performance for the detection of surface vessels and low-flying aircraft than can be predicted from consideration of the characteristics of the radar alone. These ducts are important because they extend for long distances.
An elevated duct may form in an area of high barometric pressure because of the sinking and lateral spreading of the air, which is termed subsidence. When the air is warm and dry and the subsidence takes place over the sea, water is evaporated into the air, forming a moisture gradient that leads to the formation of a duct. Such ducts are always formed above the sea, with the base of the trapping layer ranging in elevation from a few thousand to 20,000 feet. Subsidence trapping can nearly always be found in the tropics; the ducts so produced usually are low and strong off the western coasts of continents in the trade winds latitudes, and high and weak near the equator. In areas where a monsoonal climate is found, subsidence trapping is weak or nonexistent in the moist, onshore, summer monsoon, and the duct is low and trapping strong during the dry, offshore, winter monsoon. When a duct is formed as the result of subsidence, the strength of the trapping varies throughout the day, being weak in the mid-forenoon, and strong after sunset.
Other meteorological conditions that may produce trapping are cooling of land at night by radiation and the mixing of two masses of air, as at a warm or cold front. The ducts formed by these effects are likely to be of such limited extent that they are unable to modify radar propagation by any appreciable amount.
4. Prediction of Nonstandard Propagation.
In spite of the fact that the relation of weather to the production of trapping conditions is not fully
understood, enough is known to permit a meteorologist to make a fairly reliable prediction. Considerable equipment is required to measure the several variables that must be known to determine the structure of the atmosphere, and rather specialized meteorological skills are needed to interpret the data. Since neither the skills nor the equipment is generally available to the fleet, reliable predictions are difficult to make on board ship. However, even in the absence of specialized equipment and personnel, it may sometimes be possible to predict the formation of ducts from observation of weather conditions, coupled with simple measurements that can be made on board any ship. Such an estimate cannot hope to be highly reliable at present, but continuing research on the problem may ultimately reveal a fairly simple relationship between weather and trapping that will allow propagation conditions to be evaluated routinely.
For a number of reasons the meteorological conditions in a region of high barometric pressure are favorable for the formation of ducts. Among the favorable factors are: subsidence, which creates temperature inversions, and which occurs in areas where the air is very dry so that evaporation can take place from the surface of the sea; calm conditions that prevent mixing of the lower layers of the atmosphere by turbulence, allowing thermal stratification to persist; and clear skies which permit nocturnal cooling over land.
The conditions in a barometric low, on the other hand, generally favor standard propagation. A lifting of the air, the opposite of subsidence, usually occurs in such regions and it is accompanied by strong winds. The combined effect of these factors is to destroy any local stratification of the atmosphere by a thorough mixing of the air. Moreover, the sky is usually overcast in a low pressure area, and nocturnal cooling is therefore negligible. Very often rain falls in a low pressure area, and the falling drops of water have the effect of destroying any non-standard humidity or temperature gradients that may have been established.
In all of the weather conditions that produce trapping, the atmosphere must be sufficiently stable to allow the necessary stratification of the atmosphere to be established and to persist. Thus, continued calm weather or moderate breezes are necessary. It must be emphasized, however, that even if weather conditions may favor the formation of ducts, they do not always produce them.
In terms of the readily observable phenomena, the weather conditions that may favor trapping are:
Although trapping conditions can occur at any place in the world, the climate and weather in some areas make their occurrence more likely. In some parts of the world, particularly those possessing a monsoonal type of climate, variation in the degree of trapping is mainly seasonal, and enormous fluctuations from day to day may not occur. In other parts of the world, especially those in which low barometric pressure areas recur often, the extent of nonstandard, propagation conditions varies considerably from day to day, even during the season of greatest prevalence.
- A moderate breeze that is warmer than the water, blowing from a continental land mass.
- Clear skies, little wind, and high barometric pressure.
- A cool breeze blowing over the open ocean far from large land masses, especially in the tropical trade wind belt.
- Smoke, haze, or dust fails to rise, but spreads out horizontally, which indicates quiet air in which a temperature inversion may exist.
- The air temperature at bridge level on the ship definitely exceeds that of the sea, or when the moisture content of the air at bridge level is considerably less than that just above the water, and the air is relatively calm.
Even though the geographical and seasonal aspects of trapping are not yet so firmly established that a map of the world can be drawn with trapping areas reliably delineated, it is possible to make a general summary.
- Atlantic Coast of the United States.--Along the northern part of this coast, trapping is common in summer, while in the Florida region the seasonal trend is the reverse, with a maximum in the winter season.
- Western Europe.--On the eastern side of the Atlantic, around the British Isles and in the North Sea, there is a pronounced maximum of trapping conditions in the summer months.
- Mediterranean Region.--Available reports indicate that the seasonal variation is very marked, with trapping more or less the rule in summer, while conditions are approximately standard in winter. Trapping in the central Mediterranean area is caused by flow of warm dry air from the south (Sirocco) which moves across the sea and thus provides an excellent opportunity for the formation of ducts. In the winter time, however, the climate in the central Mediterranean is more or less a reflection of Atlantic conditions and hence it is not favorable for duct formation.
- The Arabian Sea.--The dominating meteorological factor in this region is the Southwest Monsoon that blows from early June to mid-September and covers the whole Arabian Sea with
moist equatorial air up to considerable heights. When this meteorological situation is fully developed, no occurrence of trapping is to be expected. During the dry season, on the other hand, conditions are very different. Trapping is then the rule rather than the exception, and on some occasions extremely long ranges, up to 1,500 miles, have been observed on P-band radar on fixed echoes.
When the Southwest Monsoon sets in early in June, trapping disappears on the Indian side of the Arabian Sea. However, along the western coasts conditions favoring trapping may still linger. The Strait of Hormuz is particularly interesting as the monsoon there has to contend against the Shamal from the north. The Strait itself lies at the boundary between the two wind systems, and a front is formed with the warm, dry Shamal on top and the colder, humid monsoon underneath. As a consequence, conditions are favorable to the formation of an extensive duct which is of great importance to radar operation in the Strait of Hormuz.
- The Bay of Bengal.--The seasonal trend of trapping conditions is the same as in the Arabian Sea, with standard conditions occurring during the summer Southwest Monsoon, while trapping is found during the dry season.
- The Pacific Ocean.--This region appears to be one where, up to the present, least precise knowledge is available. However, there seems to be definite evidence of the frequent occurrence of trapping around Guadalcanal, the east coast of Australia, and around New Guinea. Observations along the Pacific coast of the United States indicate frequent occurrence of trapping, but no clear indication of its seasonal trend is available. The meteorological conditions in the Yellow Sea and the Sea of Japan, including the island of Honshu, are approximately like those off the northeastern coast of the United States. Therefore, trapping in this area should be common in the summer. Conditions in the South China Sea approximate those off the southeastern coast of the United States only during the winter months, when trapping can be expected. During the rest of the year, the Asiatic monsoon modifies the climate in this area and no data are available as to the prevalence of trapping during this time. The trade winds in the Pacific lead to the formation of rather low ducts quite generally over the open ocean.
1. Roll and Pitch of Ship.
The state of the weather had relatively little effect on the early, low-frequency radars, but as development continues and the fleet is supplied with microwave equipment that operates on very short wavelengths, the weather may have a profound effect on radar operation. However, one factor that affects all radars is the roll and pitch of the ship in which the radar is installed. The energy transmitted from a radar antenna is confined to a beam by the directivity, of the antenna. If the vertical width of this beam is narrow, then rolling of the ship may cause the radar to miss some targets at some times, as indicated in figure 101. In small ships the amount of roll is often great enough to prevent reliable use of radar. As a result, some of the more recent surface search equipments for small vessels, such as model SU radar, have antennas that are stabilized. Some of the new surface search radars for large vessels also have stabilized antennas in order to take advantage of the concentration of energy into a narrow beam that is possible when the beam is forced to scan the horizon irrespective of the movement of the ship. Stabilization of the antenna is necessary in radars that are used to determine altitude of airborne targets by measuring position angle. In the case of air search radars, however, the vertical beam width is usually no narrower than 20° so that the roll of the ship usually is insufficient to pull the beam off target.
Since some of the energy radiated by a radar strikes the surface of the sea, echoes are returned from the water when the surface is disturbed by waves.
Radar echoes from waves are called sea return. Those echoes show on a PPI screen as an irregular bright area around the center of the screen. The range to which sea return extends on the indicator
Figure 101--Effect of roll of ship on antenna coverage.
depends on the roughness of the ocean, the height of the antenna, and the peak power of the radar. In very calm water, sea return is of negligible importance. However, in very rough weather, it will be impossible to see targets within several miles of the radar because all echoes are lost in the clutter from sea return. Increasing the height of the antenna or the power radiated also causes an extension of the sea return.
Sea return is usually stronger upwind than downwind because the fronts of the waves are more nearly vertical, and therefore better reflectors, than the sloping backs. In addition, somewhat stronger echoes are obtained in a direction perpendicular to the waves than along the troughs. Consequently, it is frequently possible to estimate the direction of the surface wind from the oval shape of the sea return on a PPI screen.
The effect of sea return on the indicator can be considerably reduced by lowering the gain of the radar receiver. However, such an adjustment prevents the radar from detecting objects at a distance.
Station keeping by radar is made difficult by sea return, since the clutter on the screen often masks the echoes from nearby ships. A recent improvement in the design of radar receivers should almost completely prevent this effect. The addition of Sensitivity-Time Control (STC), Instantaneous Automatic Volume Control (IAVC), and Fast Time Constant (FTC) enable the radar to discriminate against echo signals, such as sea return, which extend over a long range in favor of the strong isolated signals returned from nearby ships.
Sensitivity-Time Control reduces the gain of the radar receiver for the first few miles of the sweep, and then restores full sensitivity for the remainder of the sweep. Thus the gain is low in the region from which the echoes are returned from waves, reducing the extent of sea return, yet full sensitivity is available to enable an effective long range search to be conducted.
Sea return echoes are reflected from a general area, so that in general these echoes cover a longer range than normal pips. The Instantaneous Automatic Volume Control circuit reduces the receiver gain for these longer echoes, but it maintains receiver sensitivity for the relatively short normal echoes. As a result, IAVC prevents sea return from forming a solid pattern around the center of a PPI scope, but it does not eliminate the clutter completely. (See fig. 136).
The Fast Time Constant video coupling circuit in the receiver also acts to prevent the passage of long echo pulses. It is designed to pass a normal echo with slight change, but it removes part of the long echoes so that they cannot build up a solid pattern on the PPI.
Of these three improvements, Sensitivity-Time Control is designed especially to reduce the effect of sea return, but the other circuits are helpful in the same purpose. However, both Instantaneous Automatic Volume Control and Fast Time Constant coupling are also of use against certain types of jamming and they tend to improve the range resolution of the radar. These circuits are currently being installed in SG radars as part of the modernization program (Field Change No. 50), and they are provided in most radars of recent design.
3. Formation of Ice on Antenna.
The effect of ice on radar antennas has been investigated only a little. Other than the mechanical load that it puts on the antenna training mechanism, there is apparently no difficulty presented by formation of ice on open antennas, such as the SC or SK arrays. Microwave radars in which energy is sent into the reflector through a plastic window, as in the SG and SJ, may be practically inoperative if ice forms across the whole area of the window, particularly if the ice is wet. It is unlikely that any ice can form directly on antennas that are covered by radomes--as the SF, SL, SO and SU--but a coating of ice on the radome may reduce radar performance appreciably. The reduction in performance caused by ice is. more serious the higher the radar frequency. Perfectly dry, hard-frozen ice seems to have little effect on system performance as long as the thickness of the coating is small compared to a wavelength, but a coating of water on the ice seems to cause a large reduction in range. This reduction appears to be caused equally well by either fresh or salt water.
As microwave radars have come into general use in recent years the cloud or storm echoes that are frequently seen on the indicators have attracted attention. The possibility of using microwave radar as an aid to meteorological forecasting was recognized early, and it is now being put to operational use.
Knowledge of the mechanism of reflection of radar pulses from storm clouds is still incomplete, but it is generally believed that the reflection is due to scattering by water droplets. The amount of scattering increases very rapidly with frequency, so that storm echoes are most frequently observed on S and X band radars. Cloud echoes are comparatively rare at frequencies below about 1,000 megacycles. Experience seems to indicate that clouds do not produce visible echoes unless the water drops within the cloud
Figure 102--Thunderstorm echoes on PPI.
are large enough to yield precipitation. This does not necessarily mean that there is rain at the ground at the place where storm echoes are observed because the drops may evaporate before they reach the ground, or rising currents of air may keep the drops afloat against gravity, a phenomenon very common in cumulus clouds and thunderheads.
Consequently, not all clouds can be seen on a radar scope. Stratus clouds give very little echo or perhaps none at all, and a general overcast is often invisible to the radar operator. Thunderstorms and frontal squalls usually produce intense illumination on the scope. Thundershowers are of two general types. The isolated air mass or convection type of storm produces a bright, dense echo surrounded by haze, as the echoes shown in the southern sector of figure 102A. The other type of thundershower occurs along a cold front, and may form a line squall. The echo from such a storm may show as a bright line if the storm is fully developed, or it may show as a group of bright spots caused by echoes from individual showers along the front, as in figure 102B.
Unless operators are trained to recognize these types of echoes, they may be interpreted as valid targets. However, if as a result of proper training such echoes are readily evaluated, it will be possible to vector aircraft around the storm areas and to maneuver carriers into clear areas where planes may be easily launched or landed.
Usually storm echoes are not so strong that they blank out the much sharper contours of a plane echo. If the echo from the storm should be so intense as to make the plane echo invisible, the air in this area would be so rough that it is improbable that a plane would intentionally fly in this part of the atmosphere. Therefore, it is unlikely that enemy planes will seek cover in such storms. Since radar can detect objects on each side of the storm area, little protection from radar detection is afforded airplanes by violent localized storms. Cloud echoes usually can be recognized by the operator by their fuzzy, hairy appearance on a type A indicator and by their nebulous, cloudy appearance on a PPI. If the pip is too sharply defined to allow ready recognition of the echo as a cloud contact, usually coordinating the target course and speed with wind data, along with position angle, will enable the echo to be evaluated properly.
Since clouds often extend over several miles and may rise to heights of 40,000 feet, they produce echoes at extremely long ranges, and the echoes may cover large areas on the indicator. In some unusual cases, nearly the entire PPI scope may be so covered with cloud echoes that it is impossible to see targets. Because cloud echoes, like sea return, are longer than the echoes from more solid targets, both IAVC and FTC are of considerable use in minimizing interference from clouds. For example, figure 103A shows a large area of the SG-3 PPI scope obscured with cloud echoes. The clouds that cover the center of the screen are quite dense, but those at 3 o'clock and at 9 o'clock show the nebulous appearance characteristic of cloud echoes. In B the same conditions are shown except that the radar is transmitting a one-quarter microsecond pulse instead of a one-microsecond pulse. The increased resolution produced by this change reduces the density of the cloud echoes, but they are still troublesome. In figure 103C, the transmitted pulse duration is again one microsecond,
Figure 103--Cloud echoes on SG-3 PPI.
but the IAVC circuit is switched on and the cloud echoes are considerably cut down. With both IAVC and FTC connected in the receiver, the cloud echoes are reduced to almost negligible proportions (fig. 103D).
5. Atmospheric Noise.
The frequencies used for radar are so high that atmospheric noise, or static, has little effect on its operation. The noise which shows on the indicator is produced in the early stages of the receiver. However, strong pulses, similar to noise pulses, have been observed on meter-wave radar indicators as a result of nearby lightning strokes. On occasion 200 megacycle radar has encountered serious interference from St. Elmo's Fire. The Aurora Borealis, which interferes so greatly with most communications, seems to have no effect on radar.
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