Abstract
Microwave radiometry has provided valuable spaceborne observations of Earth’s geophysical properties for decades. The recent SMOS, Aquarius, and SMAP satellites have demonstrated the value of measurements at 1400 MHz for observing surface soil moisture, sea surface salinity, sea ice thickness, soil freeze/thaw state, and other geophysical variables. However, the information obtained is limited by penetration through the subsurface at 1400 MHz and by a reduced sensitivity to surface salinity in cold or wind-roughened waters. Recent airborne experiments have shown the potential of brightness temperature measurements from 500–1400 MHz to address these limitations by enabling sensing of soil moisture and sea ice thickness to greater depths, sensing of temperature deep within ice sheets, improved sensing of sea salinity in cold waters, and enhanced sensitivity to soil moisture under vegetation canopies. However, the absence of significant spectrum reserved for passive microwave measurements in the 500–1400 MHz band requires both an opportunistic sensing strategy and systems for reducing the impact of radio-frequency interference. Here, we summarize the potential advantages and applications of 500–1400 MHz microwave radiometry for Earth observation and review recent experiments and demonstrations of these concepts. We also describe the remaining questions and challenges to be addressed in advancing to future spaceborne operation of this technology along with recommendations for future research activities.
Highlights
M ICROWAVE radiometry provides valuable observations of Earth’s geophysical properties, including those of the atmosphere, ocean, cryosphere, and land [1], [2]
Because microwave radiometry involves measurement of the naturally emitted thermal noise power, it is often performed in portions of the electromagnetic spectrum where anthropogenic radio transmissions are restricted [2]
The degradation is relatively slow: for example, flagging 75% of the time and frequency samples within an integration period degrades noise equivalent delta temperature (NEDT) by a factor of only two. These considerations suggest that microwave radiometers operating at 500−1400 MHz should be designed to achieve NEDT values that are a factor of two or more better than the NEDT corresponding to the desired science goals if no radio-frequency interference (RFI) is present, so that degradations in this parameter caused by RFI can still be tolerated
Summary
M ICROWAVE radiometry provides valuable observations of Earth’s geophysical properties, including those of the atmosphere, ocean, cryosphere, and land [1], [2]. Because microwave radiometry involves measurement of the naturally emitted thermal noise power (which occurs at very small power levels), it is often performed in portions (or bands) of the electromagnetic spectrum where anthropogenic radio transmissions are restricted [2] Despite these restrictions, no band is completely free of emissions from active services, due to the presence of both in-band (shared) and out-of-band signals. To address the challenge of RFI, additional subsystems have been developed that aim to separate man-made signals from thermal emission contributions (e.g., [2]–[13]) These signal detection and RFI filtering approaches can in some cases allow brightness temperature observations to continue in nonprotected portions of the spectrum.
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