National Academies Press: OpenBook

Radiation Intensity of the PAVE PAWS Radar System (1979)

Chapter: Pulse Patterns and Peak Power

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Suggested Citation:"Pulse Patterns and Peak Power." National Research Council. 1979. Radiation Intensity of the PAVE PAWS Radar System. Washington, DC: The National Academies Press. doi: 10.17226/19884.
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Suggested Citation:"Pulse Patterns and Peak Power." National Research Council. 1979. Radiation Intensity of the PAVE PAWS Radar System. Washington, DC: The National Academies Press. doi: 10.17226/19884.
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Suggested Citation:"Pulse Patterns and Peak Power." National Research Council. 1979. Radiation Intensity of the PAVE PAWS Radar System. Washington, DC: The National Academies Press. doi: 10.17226/19884.
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Suggested Citation:"Pulse Patterns and Peak Power." National Research Council. 1979. Radiation Intensity of the PAVE PAWS Radar System. Washington, DC: The National Academies Press. doi: 10.17226/19884.
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Suggested Citation:"Pulse Patterns and Peak Power." National Research Council. 1979. Radiation Intensity of the PAVE PAWS Radar System. Washington, DC: The National Academies Press. doi: 10.17226/19884.
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Suggested Citation:"Pulse Patterns and Peak Power." National Research Council. 1979. Radiation Intensity of the PAVE PAWS Radar System. Washington, DC: The National Academies Press. doi: 10.17226/19884.
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Suggested Citation:"Pulse Patterns and Peak Power." National Research Council. 1979. Radiation Intensity of the PAVE PAWS Radar System. Washington, DC: The National Academies Press. doi: 10.17226/19884.
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Suggested Citation:"Pulse Patterns and Peak Power." National Research Council. 1979. Radiation Intensity of the PAVE PAWS Radar System. Washington, DC: The National Academies Press. doi: 10.17226/19884.
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Suggested Citation:"Pulse Patterns and Peak Power." National Research Council. 1979. Radiation Intensity of the PAVE PAWS Radar System. Washington, DC: The National Academies Press. doi: 10.17226/19884.
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5, PULSE PATTERNS AND PEAK POWER In operation the PAVE PAWS radar tracks targets, searches for new targets, and probes the ionosphere to explore for conditions, such as aurorae, that are likely to affect propagation or create excessive clutter. The type of pulse or pulse burst that is emitted and the spacing of the bursts in time depend upon the function (search, track, or probe) and upon the range to the targets being tracked or to the regions being searched or probed. The pattern of pulses is therefore complex and dynamic, varying with the tactical situation. In addition to the variability in time of the pulse pattern itself there is further variability in the energy received at any fixed point on the ground, caused by the fact that each pulse is transmitted with the antenna pointing in a direction chosen for that pulse and at a frequency that is, typically, shifted from pulse to pulse. Therefore, a point on the ground sees radiation from a sidelobe pattern that changes with every transmission. It is in the nature of high gain antennas that the sidelobe pattern is "spiky" in the sense that it is characterized by narrow lobes separated by deep nulls. Designed as the PAVE PAWS antenna is, with particular attention to minimizing the large lobes, a pattern may have a few tens of lobes with peaks within 5dB of the design maximum (i.e., for PAVE PAWS, between 30 and 35dB below the main lobe). There- fore, the energy received at a point on the ground from the PAVE PAWS radar will be characterised by a complex pattern of pulses further modulated from pulse to pulse by a gain factor that is sharply variable. It is appropriate to consider (a) the maximum energy flux density observed during one pulse as viewed via one sidelobe, (b) the relation of this maximum flux density to average power density (since this bears on the problems of measurement), (c) the power spectrum (in frequency) of the envelope of the radiation seen by an observer, and (d) the likely recurrence of times of maximum fields. During the transmission of a single pulse, the PAVE PAWS antenna radiates about 580kW (l,792 x 322 watts). The gain of the antenna is 38.6dB. This means that any point beyond about 400 meters from the antenna the energy flux density on the axis of the main beam is 38.6dB greater than would be the case if that 580kW were radiated isotropically. That is, on-axis flux density is 38.6dB above 4.6 x l04 (= 5.8 x l05 x (4TT)"1) 20

21 watts per steradian, or 3.3 x l08 watts/steradian. At slightly over one mile from the antenna (5,500 ft.) this creates a (peak) flux density, in the main beam of l0 milliwatts per square cm. The maximum gain of a secondary sidelobe of the PAVE PAWS antenna is at least 30dB below the gain of the main lobe and hence is not greater than 8.6dB (= 38.6 - 30) above an isotropic radiator. The radiation flux density during a pulse along the axis of a secondary sidelobe is therefore 8.6dB above 4.6 x l04 watts/steradian. This, equates to a flux density of 3.3 x l0^ watts/steradian. At one kilo- meter from the antenna, 3.3 x l0^ watts/steradian creates a flux density of 33 microwatts/cm^ (33yW/cm^). This is a useful reference number; two of the points at which measurements have been made are located with unobstructed views of the radar at approximately this distance (Station 2, 3,900 feet, Station l, 3,l00 feet (see Section 6)) and only a few points of public access are less distant than this. The figure 33yW/cnr is a reference based on the nominal level of -30dB for a maximum secondary sidelobe* Given that the antenna elements remain within design tolerances, the probability is less than 5% that the maximum flux density be as large as 46yW/cm2 at one km (2dB above 33yw/cm2). We will call this the "worst-case" reference flux density. A reference flux represents the power during a pulse. It determines the gradient of the electric potential at the point at which the flux is measured by the simple relationship Potential gradient in volts/meter = 0.l (377 <f>)l/2k, where <j> is the reference flux in yW/cm'', 377 is the impedance of free space in ohms, and the factor 0.l results from the conversion of yW/cm^ to watts per meter^. The factor k is /~2~ if by "field" one means the maximum amplitude of a sinusoidally varying field. Engineers usually use k = l, nevertheless calling the result "peak field gradient." For consistency with other reports (Reference 3) we will use k = l, calling the result "power-equivalent peak field." Each pulse or short group of pulses emitted by the radar is followed by a listening period. (More details will follow in later discussion.) On the average the transmitter is operating at most one- quarter of the time (duty cycle - 0,25). At a maximum secondary sidelobe, then, the average energy flux density is one-quarter the maximum flux density. The maximum duration of any pulse emitted is .0l6 seconds (l6ms). The energy flux in joules/cm2 from such a pulse is then equal to the flux density in watts/era^ times .0l6. Table III exhibits for comparison purposes the relevant quantities expressed for a point that is at a distance R in km from the antenna. The reference levels in Table III represent conditions on the axis of a secondary sidelobe- At any point that receives energy only from secondary sidelobes, they describe upper bounds to the measurements that would be made of the indicated quantities. When the main beam of the PAVE PAWS radar is directed to eleva- tions higher than about 4.5°, no energy can reach the ground from first sidelobes, and the upper bounds in Table III apply to exposures or

22 Table III ESTIMATES OF RADIATION AND FIELD INTENSITY ON THE AXIS OF A SECONDARY SIDEL'OBE R > .5km " Nominal Worst-Case Maximum flux density, watts/ 3.3 x l05 4.6 x l05 steradian Maximum flux density at distance R km, in yW/cm2 33 R-2 46 R-2 Power-equivalent peak potential gradient at distance R km, in volts/m ll R-l l3 R-l Maximum energy density in one pulse, at R km, in joules/cm2 5.3 x l0-7R~2 7.4 x l0~7R-2 Average power density, at R km, in pW/cm2 8.2 R~2 ll R-2

23 measurements at points on the ground. With the main beam directed to its lowest elevation of 3o, fringes of the first sidelobes illuminate the grounds. In this case, the entries in Table III are not necessarily upper bounds. To state absolute upper bounds for the most unfavorably situated points on the ground near the radar, to which R > .5km, mul- tiply the power entries in Table III by 4 and the peak potential gradient by 2. The situation for R < .5km must be modeled in a different way and is generally more favorable than Table III suggests. The EPA has calculated the fields in the vicinity of PAVE PAWS for the case in which the main beam is at 3o elevation (Reference l). The calculations use a refined model of the radiation pattern of the antenna and take into account the topography of the local area. The panel considers the calculations to be conservative,* in that they assume that a point not illuminated by the main beam or by a first sidelobe necessarily falls on the axis of a secondary sidelobe--i.e., is a point to which Table III applies. They are also conservative, in that they assume always a clear line of sight between the radar and the point at which exposure is calculated. Exposures calculated in Reference l for points on the ground that are more than about 2km distant from the PAVE PAWS antenna are larger than those that would derive from Table III by substituting the proper value of R because of the effect of the first sidelobes. For the nearest points of public access, along the proposed extension of Route 25 and Route 6, the calculations of Reference l are in agreement with Table III. The calculations by the EPA are conservative in all respects. Table III is offered for purposes of illustration and not as a substi- tute for Reference l. It happens that Table III is conservative in the same way as Reference lT at nearby points of public access. The panel now turns to features of the radiation from PAVE PAWS other than simply power level. In the discussion, further light is shed on the conservatism in Reference l and in Table III in regard to average power levels. The operation of the PAVE PAWS radar is timed to a basic cycle that is 54 milliseconds long. Thus, l7 consecutive cycles of 54 milliseconds each are devoted to search and track functions; the eighteenth is then devoted to radar tests. A basic 54 millisecond cycle is called a "resource." A resource may consist of a period of about l6 milliseconds during which the transmitter is operating continuously or almost continuously, followed by 38 milliseconds of silence (transmitter off). Alternatively, a resource may be divided into shorter cycles, each of the same general form, consisting of a short transmission followed by a period of silence that is several times as long as the transmission. Each period of transmission has internal structure: pulses go out in rapid succession, typically in each of several directions and on different carrier frequencies. *By conservative the panel means from the point of view of concern about radiation exposure--that is, simplifications are used in making the calcu- lations so that the estimate of exposure is likely to be greater than the actual exposure.

24 The pulses themselves are frequency modulated ("chirped") for spectrum spreading--i.e., to increase bandwidth and increase processing gain upon reception^ Bandwidth in the search mode is l00kHz, in the track mode, lMHz. On either face of the radar, the maximum duty cycle during any resource is 16/54 = .30. The maximum possible duty cycle during any l8 consecutive resource intervals is then l7/l8 of this, or 0.28. Actually the duty cycle over a period of one second or more is con- trolled by considerations other than pulse pattern (see Section 8) and is not greater than 0.25. At a fixed point on the ground an observer samples each pulse through a filter defined by the sidelobe pattern. The gain of this filter depends upon the direction in which the main lobe is pointed and upon the carrier frequency used for that transmission of that pulse. Typically, these both change from pulse to pulse. The effect of this variable gain is twofold. First, it reduces the average power at the point of observation to something below the reference figures given in Table III, because these latter are based on the maximum gain of a secondary sidelobe, a gain of 8.6dB over isotropic. Second, it imposes an amplitude modulation of high peak-to-average ratio from pulse to pulse on the already complex pulse pattern. The two effects are dis- cussed somewhat separately below.. It is possible in principle to calculate from design data the average power density received at some given point as the radar runs through a specified cycle of operations. One simply calculates the average gain that characterizes the field of sidelobes at specified sampling points and carrier frequencies. To identify such a calcula- tion with an actual situation requires that a representative cycle of operations, or one that defines a practical maximum average gain, be specified. The model of a "worst-case" operation used in the EPA analysis and that implicit in Table III is one in which samples always fall at maximum secondary sidelobes. Because actual measurements of radiation intensity are being made at representative points on the ground, all one needs from an analysis is reassurance that such measurements represent either the actual in- tensity or an upper bound to the intensity of radiation intercepted at the point of measurement,. The discussion below shows that the meas- urements can exhibit high peak-to-average ratios in time, and under some operating conditions a "spiky" fine structure in angular distribution. However, Table III still provides conservative estimates for nearby points of public access. The most nearly regular and systematic operating mode of the radar is called enhanced search. In this mode, the main beam visits succes- sively l20 different positions at 3° .above the horizon, seeking targets at maximum range. This scan is not interrupted for other functions and repeats approximately every 2.5 seconds. This is then a mode in which the greatest exposure is likely to occur at nearby points on the ground and is the mode exhibiting the most nearly repetitive pattern of pulses. Most of the measurements to date have been made with the radar operating in a mode differing from enhanced search in two respects: (l) the normal pulse-to-pulse switching of carrier frequency is disabled

25 (so that one narrow-band measuring instrument suffices), and (2) scans at elevations from 3° to l0° above the horizon are measured to further explore the region near the main beam where sidelobes tend to concen- trate. To get a qualitative understanding of the exposure at one position on the ground during enhanced search, or as a measuring instrument might observe the modified enhanced search pattern, imagine that meas- uring points are set up at l20 locations around the radar uniformly spaced along the l20° of azimuth that is scanned during search. Each point is to be at 4° below the minimum scan elevation--i.e., at l° below the horizontal. Such points are representative of the nearby terrain at points of public access. With the main beam fixed at one azimuth and 3° elevation, the l20 measuring devices will then sample a line of sidelobes in azimuth-elevation space. One such array of samples calculated by the contractor under the nominal design conditions of the antenna is shown in Figure 5. By this curve perhaps l0 percent--!.e., l2--of the measuring points will be sampling sidelobe gains that are between 30dB and 35dB below the main lobe; another 5 percent on the "skirt" of a first side- lobe sample gains between -30dB and -25dB. Assume for the moment that these statistics of the curve, not the details, remain the same for other positions of the main beam. Then with the main beam at a different azimuth, some other 18 sampling points will experience simi- lar sidelobe gains. Under these conditions, as the main beam steps through all l20 of its scan positions, a given measuring point can be expected to fall lO percent of the time within 5dB of the peak of some secondary sidelobe and another 5 percent of the time on a fringe of a first sidelobe. If one computes the total power received by a line of instruments spaced uniformly in angle from 0° to 60o along the horizontal axis of Figure 5, it is 36.6dB below the main lobe corresponding to 6.6dB below the nominal 30dB (below main lobe) used in computing the reference levels of Table III. Hence, if the statistics of the curve of Figure 5 are representative of the statistics of every sidelobe pattern in the search repertoire, the effect of sidelobe filtering can be expected to reduce the average power observed at one point during enhanced search by 6_6dB below that exhibited in the last line of Table III. This would reduce the nominal reference average power density at one km, to l.8yW/cm2. This is about 6dB greater power density at lkm than what is shown by the measurements to date, (See Section 6). From this comparison one is encouraged to believe that Table III, as it refers to average power, is highly conservative and that the model of a "spiky" sidelobe pattern provided by the single curve of Figure 5 is probably not grossly raisrepresentative of the statistics of sidelobe gain seen at a fixed point on the ground as the main beam executes enhanced search. Other charts of sidelobe patterns support this latter conclusion. The "spiky" nature of the sidelobe pattern as it scans past a fixed point has the effect that average power as measured over a one second interval varies by several dB, just as a high-gain pulse

26 -20 •- LU CD U. O X. < -30 LU a. O IU CO m \ -40 Main Beam at 3° Elev., 0° Az. RMS= -36.6 dB Al .1 .2 .4 .5 .6 SIDELOBE PATTERNS .7 .8 .9 1.0 Figure 5 Calculated Radiation Intensity During Enhanced Search Courtesy of the United States Air Force

27 is intercepted during the measuring interval. Accordingly, it has been the practice to quote measurements of average power based on averaging intervals of l0 or more seconds of duration. When measurements are made under standard operating conditions--i.e., with the antenna searching and tracking and carrier frequencies being shifted—much longer inter- vals will be needed to get representative data on either maximum power or average power. Quite apart from the relation between peak values and average values, it is of interest to examine the modulation that appears at a fixed point on the ground on the envelope of the radio frequency carrier (a nominal 435MHz). In this discussion, the panel considers the carrier as modulated, first by the envelope of the pulse pattern, and then by the scanning of the sidelobes. In the enhanced search mode, each resource that is occupied by a transmission consists of a period of about l6ms, during which either two pulses of 8ms each or three pulses of 5ms each are emitted. No other transmission takes place during the remaining 38ms of that resource. So, l7 consecutive transmissions are followed by one more during which the transmitter operates only at low power for antenna tests. There are l20 distinct beam positions across the l20° sector that is being scanned. During a period of about 2.5 seconds, each position is visited once. The pattern of visits tends to repeat during subsequent similar intervals, but repetition is not exact until about 25 seconds have elapsed. In the pulse train of this enhanced search mode, there is clearly a repetitive element with a period of 54ms. The power spectrum of the envelope has a component at zero frequency, governed by the duty cycle. To a first approximation, it also has power concentrated at l8.5Hz (.05A~lHz) and at the multiples thereof. The effect of periodic inter- ruptions every .972 seconds (.972 = l8 x .054) introduces sidebands about these spectral lines, spaced every l.03Hz (.972~-'-Hz) . Even after the spreading of energy caused by these sidebands, the single line at zero frequency contains nearly 30 percent of the total power. Less than 6 percent of the power falls at l8.5Hz and its nearby sidebands. The simple and regular mode of enhanced search is not a likely one during normal operation. It is expected that, typically, only one out of two or one out of three consecutive resource intervals will be devoted to long range search. The intervening intervals of 54ms will be subdivided into shorter intervals, 27, l3.5, or 6.25ms long. This has the effect of reducing the spectral peak near l8.5Hz and of increasing the power around higher harmonics (37, 74, l48Hz) without reducing the concentration near zero Hz. Moreover, more sidebands are introduced about all of the spectral peaks because of the partially periodic recurrence of those resource intervals that are subdivided. These sidebands will also appear about the zero-fre- quency spectral peak. Imposing on this envelope a further modulation induced by pulse- to-pulse sampling of the sidelobe pattern will produce a final spectrum

28 of modulation that is the convolution of the envelope spectrum, as just sketched, with the spectrum of the time series that represents the succession of sidelobe gains. This latter time series will be highly random in character if for no other reason than that the carrier frequency changes at random from pulse to pulse. The effect of this sampling is then to introduce a flat loss (estimated crudely above to be 6.6dB), and a further spreading of the lines of the enve- lope spectrum into sidebands near each line. The .final power spectrum is dominated by power in the band from zero frequency to two or three Hz. The whole region from lOHz to 25Hz has less power in it than in the band from 0 to 3Hz. Energy in the range l5Hz to 20Hz is less than l percent of the total energy of the signal--i.e., corresponding to a signal of.average power at least 20dB lower than that shown in Table III. The panel was shown a sample strip-chart record of power measure- ments made by the contractor (actually, from Station 2 as discussed in Section 6.). Over a 40 second interval the record showed a fluc- tuating average power with a highly periodic and bi-modal structure. A basic fluctuation fairly regular in peak amplitude at about 0.4yW/cm^ showed a periodicity of two per second; superposed on this was a re- gular sequence of spikes regular in amplitude at about l.4yW/cm2, having a period of four per l0 seconds. These records were taken with the radar scanning in a search pattern resembling the enhanced search discussed above but containing only 60 beam positions rather than 120. It was stated that this was a typical record. It is fully consistent with what .the discussion above predicts for power measurements averaged over an interval of several times .05 seconds.

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