A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum (2015) / Chapter Skim
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1 Introduction
1 Introduction
Pages 10-23
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Figure 1.2 depicts active sensing measurement scenarios relevant to Earth remote sensing. They include upward-looking backscatter measurements made by weather radars; downward-looking backscatter measurements made by satelliteborne radars; upward-looking backscatter measurements of the ionosphere, and bistatic observations of the ocean surface realized by measuring the bistatically scattered illumination due to an orbiting transmitter such as a Global Position 10
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This is in contrast to passive sensing, wherein the medium under observation is itself the transmission or emission source; a passive sensor consists of only a receiver con figured to measure the naturally generated thermal radiation from the observed medium or reflected radiation from other sources. THE RADIO SPECTRUM The electromagnetic spectrum (Figure 1.4)
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Ulaby and David G Long, Microwave Radar and Radiometric Remote Sensing, University of Michigan Press, Ann Arbor, Mich., 2014.
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Atmospheric transmissivity plays a key role in frequency selection for both active and passive sensing within or through the atmosphere. For example, because of water-vapor absorption near 22 and 183 GHz and oxygen absorption near 58 and 119 GHz, these frequencies are used almost exclusively for passive sensing observations of the atmosphere.
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Ulaby and David G Long, Microwave Radar and Radiometric Remote Sensing, University of Michigan Press, Ann Arbor, Mich., 2014.
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Ulaby and David G Long, Microwave Radar and Radiomet ric Remote Sensing, University of Michigan Press, Ann Arbor, Mich., 2014.
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FREQUENCY ALLOCATIONS The radio spectrum is used by many types of services, from radio and TV broadcasting to wireless phone communication; weather, military, and remote sensing radars; and radio and radar astronomy, among many others. The services concerned with scientific uses of the spectrum are listed in Table 1.2.
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, flash flood warnings, severe storm warnings, forensic meteorology for the insurance industry, risk mitigation, aviation/trans portation safety -- wind shear detection, television weather forecasts, mesoscale meteorology, numerical weather prediction via data assimilation, urban meteorology. Ionospheric Sounding Ionospheric plasma density structure, plasma dynamics, upper atmospheric heating, storms and substorms, radio propagation, ground-satellite communication impacts, radiation belt impacts on spacecraft, GPS disruptions, ground-induced currents in pipelines and power systems.
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. The Interdepartment Radio Advisory Committee (IRAC)
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Contained in the overall list are the bands commonly used for each of the five types of active sensors on satellite platforms -- namely, scatterometers, altimeters, imagers, precipitation radars, and cloud profiling radars. More detail about indi vidual bands is available in Appendix C.
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Current Planetary Radar Systems Arecibo, Puerto Rico 2.380 20 1.0 MW average Arecibo, Puerto Rico GBT, Green Bank, West Virginia VLA, Socorro, New Mexico LRO, Lunar orbit 0.430 0.6 2.5 MW peak, Arecibo, Puerto Rico 150 kW average GBT, Green Bank, West Virginia Goldstone, 7.190 80 80 kW average Goldstone, California California, GBT, Green Bank, West Virginia DSS-13 Arecibo, Puerto Rico Goldstone, 8.560 50 500 kW average Goldstone, California California, GBT, Green Bank, West Virginia DSS-14 Arecibo, Puerto Rico VLA, Socorro, New Mexico 10 VLBA sites B Potential New Planetary Radar Systems GBT, Green Bank, 8.6 100 500 kW average GBT, Green Bank, West Virginia West Virginia VLA, Socorro, New Mexico Goldstone, California Arecibo, Puerto Rico 4.6 50 500 kW average Arecibo, Puerto Rico GBT, Green Bank, West Virginia Goldstone, 8.60 120 1.0 MW average Goldstone, California California, DSS-14 GBT, Green Bank, West Virginia Arecibo, Puerto Rico NOTE: Close asteroid observations require bistatic operation.
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Ocean scattering depends on the spa tial spectrum of waves on the ocean surface relative to λ. Radar scattering by the ionosphere is strongly dependent on the plasma density, the strength of the ambient magnetic field, and the frequency and polarization direction of the incident radar wave.
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Advances in klystron technology leading to successful operation at higher frequencies and/or higher power has led to proposed upgrades of current radar astronomy facilities as noted in Table 1.4. Higher average power allows for wider bandwidths, which provide for better spatial resolution.
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Moreover, the preliminary evidence suggests that active sensing instruments do not normally cause RFI to other users but that other users have negatively impacted the performance of active sensing instruments. Another topic alluded to in Chapter 8 is RFI mitigation and the degree to which mitigation techniques have been applied successfully.
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