A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum (2015) / Chapter Skim
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3 Active Earth Remote Sensing for Ocean Applications
3 Active Earth Remote Sensing for Ocean Applications
Pages 54-76
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From page 54... ...
Applications include global weather prediction, storm and hurricane warning, wave forecasting, coastal storm surge, ship routing, commercial fishing, coastal current and wave monitoring, and climate change. For these ocean applications, active and passive microwaves provide a different and unique response to oceanic geophysical parameters that cannot be obtained using infrared or visible sensors.
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From page 55... ...
The discussion begins with a brief overview of the history of active microwave remote sensing for oceanic applications. HISTORY OF ACTIVE REMOTE SENSING FOR OCEAN APPLICATIONS Active microwave remote sensing began before World War II, in the 1930s and 1940s, when radar was exploited for military applications.
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From page 56... ...
Donald E Barrick was the first to offer a supporting theoretical description for the measurements of surface currents by radar.2 Follow-up studies provided the derivation and interpretation of the nonlinear theory for detecting surface gravity waves (i.e., ocean waves)
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From page 57... ...
The first use by NASA of microwave sensors for ocean observations occurred, primarily as a technology demonstration, on the Skylab mission in 1973-1974.9 This paved the way for NASA's first oceanographic satellite Seasat‑A, which was launched into a polar orbit in 1978.10 Seasat-A was the first Earth-orbiting satel lite to carry four complementary microwave experiments: the radar altimeter (ALT) 11 to measure ocean surface topography; the Seasat-A satellite scatterometer (SASS)
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From page 58... ...
In recent years, other scientific applications are now using microwave sensor data to study sea and glacial ice, ocean, and land ecology, and other nonoceanic applications. TECHNICAL BASIS FOR AIRBORNE/SPACEBORNE ACTIVE EARTH REMOTE SENSING OF OCEANS Because of the global coverage and frequent revisit time provided by polar satellites, they are the platform of choice for the vast majority of oceanic active remote sensing observations.
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From page 59... ...
For more than four decades, satellite altimeters have been widely used for the measurement of ocean topography to support oceanographic research and for near-real-time operational applications for wave forecasting, sea-level storm surge, and ocean circulation. A brief history of civilian remote sensing altimeters on spacecraft is presented in Table 3.2.
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From page 60... ...
essential tools for the study of ocean circulation.18 Satellite altimeters provide very precise measurements of the distance from the satellite to the surface and, coupled with precise orbit determination, very accurate surface topography can be derived. Precision altimetry missions began with TOPEX/Poseidon in 1992 and follow-on missions on Jason-1 and the Ocean Surface Topography Mission (OSTM)
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From page 61... ...
partnership has produced more than three decades worth of ocean topography data for studying ocean circulation and global climate change associated with sea level rise, which is a sensitive indicator of climate change. Scientific and operational oceanographic applications of altimetry data include determining the geoid; measuring ocean currents; monitoring ocean tides; mapping undersea topography, ocean surface wind speed, and significant ocean wave height; and monitoring large-scale ocean phenomena.
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From page 62... ...
, is now well established after four decades of research and development. An example of scatterometer derived global ocean winds is shown in Figure 3.3, where colors represent the wind speed and the arrows show the wind flow patterns.
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From page 63... ...
Ulaby and David G Long, Microwave Radar and Radiometric Remote Sensing, University of Michigan Press, Ann Arbor, Mich., 2014.
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From page 64... ...
To meet the requirement of global ocean coverage in 1-2 days, satellite scatterometers have wide swaths, which result in off-nadir antenna pointing; and the requirement for multiple-azimuth looks is needed in order to recover the surface wind direction from the anisotropic radar backscatter. Note that the antenna configurations are of two types: (1)
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From page 65... ...
Long, Microwave Radar and Radiometric Remote Sensing, University of Michigan Press, Ann Arbor, Mich., 2014. With permission of the authors.
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From page 66... ...
Figure 3.4 illustrates how SAR combines data collected from multiple antenna positions to synthesize a longer effective antenna, thereby achieving high along-track resolution. Three examples of satellite SAR images for oceanographic applications are given in Figures 3.5-3.7.
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From page 67... ...
ian radar to fly in space was on-board the oceanographic satellite Seasat in 1978.23 Several space shuttle SAR missions were flown through the 1980s, 1990s, and early 2000s, including the Shuttle Radar Topography Mission (SRTM) ,24 which used radar interferometry to measure global surface topography.
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From page 68... ...
See ESA News, "Wilkins Ice Shelf Under Threat," November 28, 2008, http://www.esa.int/Our_Activities/Space_News. Bistatic Scatterometry Sensors can usefully operate in a bistatic mode by measuring the signal trans mitted by manmade sources after the signal reflects or scatters off a medium or surface of interest.25 Earth remote sensing platforms will soon begin to take advan tage of this mode of operation in earnest.
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From page 69... ...
Shuttle radar NASA/JPL, USA missions ERS-1/2 1991-2000/ C (vv) European Remote ESA, Europe 1995-2001 Sensing Satellite (first European SAR satellite)
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From page 70... ...
Ulaby and David G Long, Microwave Radar and Radiometric Remote Sensing, Uni versity of Michigan Press, Ann Arbor, Mich., 2014; based on Moreira et al., 2013.
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From page 71... ...
GROUND-BASED HF RADARS The ground-based HF radar is a low-power, chirped, pulse-Doppler radar that measures Bragg scattering from ocean waves in range-gated surface pixels. Since surface gravity waves with wavelengths of 3-30 m are always present on the ocean's surface, Bragg scattering corresponds to frequencies from 3 to 50 MHz.
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From page 72... ...
Rose, On the application of HF ocean radar to the observation of temporal and spatial changes in wind direction, IEEE Journal of Oceanic Engineering 11(2)
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From page 73... ...
Frequently, HF derived surface current observations are combined with other satellite remote sensing data to examine the coastal ocean in entirely new ways. For example, a col laborative effort among radar operators along the California coast has investigated the utility of combining HF radar with satellite altimetry28 and NOAA's Advanced Very High Resolution Radiometer (AVHRR)
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From page 74... ...
Fortunately, over the oceans few users share the same frequency bands; how ever, near the coast significant RFI degradation has been experienced, especially for C-band scatterometers. C-band scatterometers use range gating and therefore transmit short pulses over nonarticulating fan beam antennas.
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From page 75... ...
operational and experimental uses of the technology is needed. At present there is a large installed base of HF/VHF coastal ocean radars in the United States, which are acquired, operated, and maintained almost exclusively by academic researchers for primarily scientific purposes, and the operations of most are subsidized financially by NOAA's Integrated Ocean Observing System (IOOS)
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From page 76... ...
Finding 3.3: Active microwave sensors provide unique ocean measurements for scientific and operational applications that are vital to the interests of the United States and complementary to passive microwave and visible and infrared sensors. For nearly four decades, ocean-sensing radar systems have coexisted with the global communications and radar infrastructure.
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