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Despite some disadvantages, several features of radar and communications systems operating at these frequencies make them attractive in applications where small size, covertness, and high data rates are considerations.
Approved For Release 2004/05/13: CIA-RDP88-00893R0002001 00006-6 Approved For Release 2004/05/13: Cl A-RDP88-Q0893R0002001 00006-6 Millimeter waves, the frequencies between 30 and 300 GHz, are of interest because of the large bandwidths available and the possibility of designing transmitters and receivers in small packages. Large bandwidth means that a radar can have good range resolution or that a communication system can handle high data rates. In addition, there is less likelihood of interference among systems operating in the millimeter wave band because of sparse utilization of the band. Small package size allows the use of a radar, communication equipment, or radiometer in systems where space or weight is at a premium.
Balancing these advantages are a number of technological problems which restrict operation of receivers and transmitters in this frequency band. Transmitter power generally decreases with increasing frequency because the smaller dimensions and closer spacing cause electrical breakdown problems. Receiver sensitivity also worsens and components are difficult and costly to fabricate because of their small size.
Another limitation of systems operating at frequencies above 30 GHz is the severe attenuation introduced by propagation through rain and atmospheric gases. The atmospheric attenuation is not uniform with frequency. “Windows” exist where the attenuation is lower than at adjacent frequencies. One such window centered at 34 GHz is approximately 7 GHz wide; another window centered at 94 GHz is about 23 GHz wide and represents the next logical millimeter wave frequency band for systems which must operate in the atmosphere. If one operates outside the atmosphere, attenuation is not a problem. In fact, radars or communications systems for use in space probably would operate at about 60 GHz. At this frequency the oxygen absorption line in the atmosphere causes severe attenuation and makes the interception of emissions by hostile ground-based Sigint units unlikely.
In addition to radar and communications systems, radiometers (detectors of thermally generated electromagnetic radiation) have certain advantages when operated at frequencies above 30 GHz. In general a radiometer produces an output voltage which is a function (usually linear) of the difference between the target temperature sensed by the antenna and that of a reference. A radiometer is thus capable of passively detecting and perhaps tracking targets such as aircraft, ships, and satellites which have apparent temperatures that differ from those of their backgrounds. This antenna temperature contrast not only is a function of the targetto-background temperature ratio but also depends on the fraction of the antenna beam subtended by the target. Radiometer operation at millimeter wavelengths permits higher resolution ground maps for accurate navigation fixes. This advantage, however, must be weighed against the attenuation due to atmospheric absorption and scattering, which has the effect of blurring the contrast available at the target site.
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