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8,0 CONDITIONS AFFECTING PATTERNS AND INTENSITIES Section 4 was concerned with the effects that snow or ice accumulated on the PAVE PAWS antenna could have on the integrity of the beam pattern. The conclusion was that the limits of Table III would not be exceeded. Fog or precipitation in the atmosphere can intercept radiation and absorb or scatter it. The phenomena are well known, have been carefully measured in other connections, and are readily subject to theoretical analysis. At the frequency of the PAVE PAWS radar, the effect of scatter- ing by precipitation is very small; its presence would not change the estimates in Table III. Weather conditions also create some refraction in the atmosphere especially near ground level. This causes a ducting phenomenon that can lead to detection by the radar of ground objects at longer range than usual. However, at the short ranges involved in this report, the effect is merely one of causing some downward bending of the rays with a curva- ture in the daytime of the order of three-quarters the curvature of the earth and somewhat more at night. The downward curvature can amount to several times the curvature of the earth on so-called "radiation nights" when there is no high cloud and ground fog may form. This effect is equivalent to raising the terrain at a distance of one kilometer from the radar by less than one meter, that is, by less than the height of a person. The effect is therefore unimportant to any estimates of radia- tion intensities at ground level nearby. The panel's conclusions refer to the design of the PAVE PAWS radar as given in the contractual specifications and as described to the panel by way of the information discussed and distributed at its meeting on September 7, l978. Some features of that design, e.g., the 3° horizon limit, that are emphasized in this report reside largely in software. The question arises: Can such features be altered? The answer is, of course, Yes. A more appropriate question is: Is there any incentive for altering the design of the radar in such a way as to invalidate the estimates in Table III? The answer is at least a qualified Yes. For example, the PAVE PAWS building and antenna are now designed for the addition of more active elements to the antenna, with the consequent increase of radiated power and antenna aperture. Reference l analyzes this higher powered version of the radar. The general effect of increased power on such 37
38 estimates as appear in Table III and in Section 4 is to increase them, but not to the extent that the radar power increases, because an increase in power is offset by the fact that with a larger aperture antenna, side- lobes can be narrower and be lower in intensity relative to the main beam. The incentives for increasing power and aperture are direct- The performance of the radar in its several missions would be improved. The change is a major one and would be costly. No defined program now exists for its accomplishment. Should such a change be made, the matter of radia- tion exposure in public areas would require attention comparable to that given the same matter in the present design. The panel notes that there is a possible incentive for changing the 3° horizon limits that prevail in the present PAVE PAWS design. Without trying to evaluate the exact degree of difficulty involved, the panel concludes that this change is easier and less costly to accomplish than would be an increase in power. An incentive for lowering the search horizon is that this could bring about the earlier detection of a hostile target. There is also an operational disincentive in that, at lower angles to the horizon, atmospheric effects tend to create more false targets, thus masking real ones. For other reasons also, lowering the search angle does not necessarily guarantee an earlier detection of a valid target. The balance between incentives and penalties is not sharp or clearcut. Most long range search systems tend to limit the lowest scan angle to one that keeps the first sidelobes above the horizontal. For the PAVE PAWS antenna, this would put the main beam at or above 2° from the horizontal. There is little incentive for dropping the main beam by l° (or even 2°), from its present 3°. The greatest improvement in detection time that can result occurs when the target sought is at long range so that its apparent angular velocity in elevation is the least. Typical tra- jectories viewed at ranges of 2,000 to 3,000 miles have angular velocities in elevation of the order of one milliradian (.06°) per second. De- pressing the beam by l° then advances initial detection by l6 seconds. Compared to the travel time of a ballistic target from detection to impact--more than l,000 seconds--advancing detection by 16 seconds can hardly be considered a powerful incentive. A target, once detected, must be kept under track for sufficient time for it to be classified as threatening and an impact prediction made. Detecting a target at 2° above the horizon rather than at 3° will not advance significantly and perhaps not advance at all the time at which tracking is completed to adequately determine a trajectory. The panel therefore finds no strong incentive to lower the horizon limits below those of the present design. One other point relative to Table III needs comment for complete- ness. The maximum power used for Table III was based on a nominal out- put of 322 watts per module, and average power on a duty cycle of 0.25. The long term duty cycle of 0.25 results from constraints in the tactical software that prevent scheduling of too many pulses. Quite apart from these software constraints, temperature monitors, voltage monitors, and overload protectors throughout the system will cut off
39 power supplies within a few seconds if the power demand on one face is as great as that represented by a duty cycle of 0.30. For these reasons, there is no possibility of a condition in which either power during a pulse or average power could materially exceed the 580kW and l46kW, respectively, which are the basis of Table III. While the panel was concerned essentially with the peak field in- tensity of the PAVE PAWS radar measured at ground sites where humans may be exposed, it turned during its final deliberations to the subject of passing aircraft. The panel observes that aircraft may fly, perhaps in- advertently, through the main beam of PAVE PAWS. Restrictions have been issued and widely distributed to caution aircraft from flying within one- half mile of the PAVE PAWS antenna. Beyond this boundary, the radiation flux in the main beam does not exceed the current U.S. occupational standard of lOmW/cm (average).
