National Academies Press: OpenBook

Radiation Intensity of the PAVE PAWS Radar System (1979)

Chapter: Overview and Summary

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Suggested Citation:"Overview and Summary." 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:"Overview and Summary." 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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Page 5
Suggested Citation:"Overview and Summary." 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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Page 6

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2, OVERVIEW AND SUMMARY Radar systems like PAVE PAWS, operating from a fixed antenna rather than one that is physically movable or scans automatically, typically radiate beams of energy at microwave frequencies in short pulses or bursts of peak power. The PAVE PAWS antenna consists of a circular array of 5,354 elements, of which only half, or 2,677, are to be active when the facility begins operation in April l979, and of the active elements only l,792 are powered. At some future date, which is not yet determined, the entire antenna may be placed in operation. The beam of radiation is focused and pointed in a specific direction by controlling the way the individual elements radiate. If the beam is to be directed to the left of center (or "boresight"), the signals radiated from the elements on the left side of the array are delayed relative to those emitted from the elements on the right, the period of the delay increasing progressively across the array from right to left. PAVE PAWS has two antenna faces l20° apart covering from 347° (West of North) to 227° (47° West of South). When searching, the faces trans- mit in parallel the beams normally scanning by steps in a somewhat regu- lar sequence across each l20° sector at 3° above the horizon. When it is tracking an object in space, the beam can search anywhere in the horizontal sector, from 3° to 85° above the horizon. The actual pattern of beam positions followed during any one scanning and tracking cycle depends on the particular search mode selected and on the position of the target under surveillance. In normal operation, when the PAVE PAWS beam is at 3° above the horizon, the beam is always l00 feet or more above ground level at the nearest point of public access, because the radar is on high ground at 325 feet above sea level. The radar operates in the UHF (ultra high frequency) band at 420-450 MHz. No significant transmitted energy falls outside this band, which corresponds to wave lengths of from 7l-67 centimeters. Each of the l,792 transmitting elements of the array is connected to its own transmitter, and each element radiates about 320 watts of peak power dur- ing the one pulse. PAVE PAWS transmits a brief pulse or a short chain of pulses and then pauses while it awaits the returning echoes. The peak power of radar transmitted is about 580 kilowatts—derived by multiplying 320 watts by l,792 elements. The actual time sequence of pulses depends, during an interval, on the functions performed and the number of targets

tracked. The duty cycle never exceeds 0.25. Therefore, the average transmitted power never exceeds l45 kilowatts (0.25 of 580 kilowatts), which is, in round numbers, about three times the average power transmitted by a typical large TV station and somewhat more than the average power of a typical high power FM station. The most powerful TV and FM stations, by comparison, radiate more power than the PAVE PAWS radar. In December l977, EPA investigators stated their calculations of power density levels for PAVE PAWS in their Environmental Impact Analy- sis. The EPA defines a high power source as one where the power density of the main beam is lOOyW/cm2 at a distance of l00 meters from the antenna. TV and FM broadcast transmitters fall into this range, as do radar systems and satellite communications earth stations. The measurements of power density levels by the Air Force in l978, taken at various points up to 5 miles beyond the "exclusion" fence some l,000 feet from the PAVE PAWS radar, have been reviewed by the National Bureau of Standards as to the validity of the techniques used as well as the ambient levels of electromagnetic radiation. The measurements show that at the fence the microwave power densities averaged in the range of 5yW/cra2, with the levels decreasing at distances farther away from the radar. At the location where the public is most likely to be exposed--on Highway Route 6, some 3,450 feet from the radar--the measured intensity was 0.06yW/cm2. The Air Force measurements of PAVE PAWS indicate that the power density levels do not exceed O.lyW/cm2 at points beyond about l mile from the radar. After its examination of the PAVE PAWS design and technology and its review of the test measurements, the panel concludes, specifically, that the EPA has calculated valid upper limits for the radiation fields at ground level near the PAVE PAWS radar. The panel also finds that: o The PAVE PAWS radar, though of advanced design, uses technology that is well known and has been tested in other radar systems already in operation. There is nothing in the design parameters that exceeds the capabilities of today's technology. In particular, the techniques used in the design and analysis of the antenna are well tried. Experi- ence and measurement in other systems show that these techniques provide accurate estimates of antenna performance. o In all features of the radar and system design that were examined, thorough attention has been given to safeguard against malfunctions. The safeguards are built into the system. o It is in the basic nature of a phased-array antenna, such as the one used in the PAVE PAWS radar, that component or equipment failures are unlikely to cause radiation to be directed into public areas or in any undesignated direction in excess of the amounts estimated for normal operation. Independent monitoring devices and other protective features are designed into the system to detect malfunctions and, in the event, to shut the radar down. o The PAVE PAWS radar is controlled by digital computers. The computer programs as now designed provide for multiple independent tests of antenna steering orders. Orders issued by the computers to direct radiation into improper directions, such as too close to the horizon or

into the ground, are rejected. Indeed, -If that were to happen, the events would be reported to operating personnel by way of status dis- plays. o The design and testing of the programs for the control computers of the PAVE PAWS radar conform to modern practice. The techniques are similar to, and to some degree based upon, those used in the design of a similar radar system—the Perimeter Acquisition Radar (PAR) in North Dakota (see Section 3). PAR has operated successfully and reliably over a four year period. o The measurements being made by the contractor during installa- tion and test of the PAVE PAWS radar, and independent measurements as planned by the Air Force, and now in process, can give a reliable veri- fication of the estimates of performance to compare with design data, o Measurements made td date are consistent with predictions from design data. They demonstrate that radiation intensities at ground level are below the bounds estimated in Reference 2. In particular, they are far below the level of 10 milliwatts per square cm that is the currently accepted U.S. occupational safety level for human exposure.

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