CryoRad from sea ice to ocean salinity, a new microwave approach could boost polar observations
Ken Jezek is the lead author, and Joel Johnson is a co-author.
Corresponding authors: Giovanni Macelloni, et al.
Wideband Radiometry From P to S Band for Monitoring Polar Regions
The work of Jezek and Johnson was supported in part by NASA’s Instrument Incubator Program.
Satellites have transformed how scientists track polar change—but some of the most important processes remain difficult to observe from space, especially those that depend on conditions below the surface of ice sheets and sea ice or within cold polar oceans. A new review in Proceedings of the IEEE argues that the next step in Earth observation may come from moving to lower frequencies and wider bandwidths: using wideband microwave radiometry across approximately 0.4 to 2 gigahertz (GHz) to strengthen monitoring in polar regions.
The reason is largely physical. Compared with the 1.4-GHz “protected band” used by missions such as SMOS, Aquarius, and SMAP, lower frequencies can probe deeper into natural materials. The paper notes that at 0.4 GHz, the emission depth is about 50% greater in wet soil than at 1.4 GHz; in compacted sea ice (up to 1 m thick), the increase is about 100%; and in ice sheets, it can exceed 300%. Deeper emissions increase sensitivity to subsurface properties—key for variables such as sea-ice thickness and internal ice-sheet temperature.
That matters because some current L-band capabilities reach a depth limit in thick ice. The review reports that ice-sheet temperature estimation at L band can become inaccurate below ~1000–1500 meters, and that temperature uncertainty can exceed 8 K beyond 2500 meters in some locations (though it is below 2 K in about 75% of cases). Wideband, lower-frequency measurements could help extend sensitivity to deeper layers.
The paper also highlights potential benefits for sea ice, where thickness is hard to measure globally and where salinity observations at scale have historically been limited. In modeling work summarized in the review, 0.5–2 GHz wideband radiometry is shown to enable simultaneous retrieval of sea-ice thickness and salinity, with retrieval errors below 20% for thickness and 15% for salinity across the season.
In polar oceans, the advantage is especially clear in cold water. The paper states that, in cold conditions, the sensitivity of brightness temperature to sea-surface salinity is about three times greater at 0.4 GHz than at 1.4 GHz, suggesting that salinity retrievals could improve if measurements include the low end of the band. In practice, the review notes that weekly sea-surface-salinity errors from current L-band missions rise to more than 1 practical salinity unit (pss) in cold polar regions—substantially larger than typical errors in warmer oceans.
In CryoRad-related preparatory simulations described in the paper, the authors report that under “common” polar-ocean conditions (SSS = 33 pss, SST = 0°C), salinity uncertainty could reach ~0.3 pss (given the study’s stated assumptions), and that the uncertainty reduction can be even stronger at lower salinities.
A wide band radiometer also offers practical advantages. While frequencies below 1.4 GHz can be more vulnerable to radio-frequency interference (RFI), the review notes that continuous sampling across narrow sub-bands can help isolate uncontaminated measurements. The authors also point to “spectral” retrieval approaches, analogous to hyperspectral methods, where rich frequency information can support retrieving multiple variables and reduce the risk of unstable (ill-conditioned) inversions.
The review connects these scientific motivations to emerging mission concepts. One example is CryoRad, selected in 2024 as one of four candidates for ESA’s Earth Explorer 12 program, built around a 0.4–2 GHz wideband radiometer. The paper describes goals, including complete coverage at high latitudes with maximum revisit times of ~10 days above 50°, and target product resolutions better than 15 km at 1.4 GHz and 50 km at 0.4 GHz.
Two Ohio State University authors, Kenneth C. Jezek, Professor Emeritus, School of Earth Sciences, and Joel T. Johnson, Professor, Electrical & Computer Engineering, contributed to the review. Jezek is a current PI of the Byrd Polar and Climate Research Center and his recent research interests include ultra-wideband radiometry for ice-sheet and sea-ice studies. Johnson is the Burn and Sue Lin Professor in Ohio State’s Department of Electrical and Computer Engineering and the ElectroScience Laboratory, with research interests in microwave remote sensing and electromagnetic theory.
The paper suggests that expanding from narrowband L-band sensing to wideband 0.4–2 GHz radiometry could improve key polar observations—especially where deeper penetration and better cold-water salinity sensitivity are needed—while also opening the door to new retrieval strategies and mission designs.
That matters because some current L-band capabilities reach a depth limit in thick ice. The review reports that ice-sheet temperature estimation at L band can become inaccurate below ~1000–1500 meters, and that temperature uncertainty can exceed 8 K beyond 2500 meters in some locations (though it is below 2 K in about 75% of cases). Wideband, lower-frequency measurements could help extend sensitivity to deeper layers.
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