Microwave Radiometry at Frequencies From 500 to 1400 MHz: An Emerging Technology for Earth Observations

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.

Measurement of SST and SSS Using Frequencies in the Range 0.3–2.0 GHz

“…The best approach may be to retrieve SST at frequencies below the singularity using a dedicated frequency without averaging. However, the use of several frequencies is essential to applications in the cryosphere such as profiling ice sheet temperature (for example, Johnson et al., 2021), which are likely to drive any future sensor and averaging is important for improving the retrieval of SSS in cold water (Le Vine & Dinnat, 2022). Assuming that a selection of frequencies will be available, the retrieval of SST can be tuned to select the best frequency available (e.g., in the presence of RFI) or the retrieval could use an average of several frequencies to reduce noise, a choice that can be made depending on experience with the retrieval under the specifics of the sensor and scene.…”

“…Operating in protected spectrum is necessary to detect natural thermal emission from the surface and avoid being overwhelmed by interference from manmade sources of radiation. These radiometers have demonstrated remote sensing of sea surface salinity (SSS) (Dinnat et al., 2019; Kao et al., 2018) and soil moisture (Bindlish et al., 2019; Chan et al., 2018] and the potential for monitoring ice thickness (Johnson et al., 2021). However, contemporary research suggests that in polar regions, where radio frequency interference (RFI) is less problematic it may be possible to operate over a wider bandwidth (Andrews et al., 2021).…”

Analysis of ice-sheet temperature profiles from low-frequency airborne remote sensing

“…The Ohio State University developed the Ultra-WideBand software-defined microwave RADiometer (UWBRAD) to investigate the proposal by Jezek and others (2015) that low-frequency, wide-band radiometers could sense physical temperatures deep within polar ice. UWBRAD was designed to observe circularly polarized 0.5–2 GHz thermal emissions in a nadir viewing geometry (Johnson and others, 2021). Given the heavy use of this portion of the radio spectrum, radio frequency interference (RFI) originating on board the aircraft or from external sources is removed using multiple processing algorithms (Andrews and others, 2018).…”

“…By observing brightness temperatures (Tb) over a wide band of microwave frequencies, the variations in penetration depth over the band enable inversion of the measured Tb spectra into the physical temperature depth profile. As discussed by several authors (Macelloni and others, 2018;Johnson and others, 2021), new technologies for low-frequency wide-band radiometric measurements are enabling continued and future airborne and spaceborne remote sensing of the physical temperature field of polar ice sheets. Yardim and others (2021) report an inversion of brightness temperature spectra from a 2017 airborne campaign over northwestern Greenland into a set of geophysical parameters necessary for calculating ice physical temperatures at depth.…”

Agenda Items of the World Radiocommunication Conference 2023 With a Potential Impact on Microwave Remote Sensing [Technical Committees]

“…◗ Secondar y harmonics from portions of the 694-960-MHz band place the L-band at 1,400-1,427 MHz at risk of interference. Also note that some passive instruments (e.g., wideband radiometers for cryosphere and salinity studies [5], [6]) are planned to operate at 0.5-2.0 GHz without allocation.…”