Arctic sea ice is changing rapidly. Its retreat significantly impacts Arctic heat fluxes, ocean currents, and ecology, warranting the continuous monitoring and tracking of changes to sea ice extent and thickness. L-band (1.4 GHz) microwave radiometry can measure sea ice thickness for thin ice ≤1 m, depending on salinity and temperature. The sensitivity to thin ice makes L-band measurements complementary to radar altimetry which can measure the thickness of thick ice with reasonable accuracy. During the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, we deployed the mobile ARIEL L-band radiometer on the sea ice floe next to research vessel Polarstern to study the sensitivity of the L-band to different sea ice parameters (e.g., snow and ice thickness, ice salinity, ice and snow temperature), with the aim to help improve/validate current microwave emission models. Our results show that ARIEL is sensitive to different types of surfaces (ice, leads, and melt ponds) and to ice thickness up to 70 cm when the salinity of the sea ice is low. The measurements can be reproduced with the Burke emission model when in situ snow and ice measurements for the autumn transects were used as model input. The correlation coefficient for modeled Burke brightness temperature (BT) versus ARIEL measurements was approximately 0.8. The discrepancy between the measurements and the model is about 5%, depending on the transects analyzed. No explicit dependence on snow depth was detected. We present a qualitative analysis for thin ice observations on leads. We have demonstrated that the ARIEL radiometer is an excellent field instrument for quantifying the sensitivity of L-band radiometry to ice and snow parameters, leading to insights that can enhance sea ice thickness retrievals from L-band radiometer satellites (such as Soil Moisture Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP)) and improve estimates of Arctic sea-ice thickness changes on a larger scale.

This paper provides a high level overview of a multi-frequency Ground-Based Synthetic Aperture Radar operating at X-, C-, L-and P-bands. The system core is implemented using a flexible high performance Software Defined Radio, aided by a custom radio-frequency front-end. The capabilities of the system are demonstrated by measuring dense time-series of a vegetated area. The benefits of lower frequencies appear as a significant increase in phase stability and coherences at P-and L-bands, which is explained by the increased vegetation penetration depth of these bands compared of C-and X-bands. It is concluded that multi-frequency measurements closely spaced in time are valuable and suggest new applications in vegetated areas.

This work describes a system-level overview of a multi-frequency GBSAR built around a high performance software defined radio (SDR). The main goal of the instrument is to be employed as a demonstrator and experimental platform for multi-frequency GBSAR campaigns. The system is capable of operating in P, L, C and X-bands, and signal generation and digital signal processing are customizable and reconfigurable through software. An overview of the software and hardware and implementations of the system are presented. The operation of the system is demonstrated with two measuring campaigns showing focused amplitude images at different frequencies. It is shown how the usage of SDR for GBSAR systems is a viable design option.

<p>Passive and active electromagnetic methods such as L-Band radiometry and radar reflectometry have great potential to provide, from ground or as remote applications, soil moisture maps on local or regional scales. To calculate values of absolute water content from the measured dielectric constant, calibration data is necessary. This is usually acquired in the lab by weighing soil samples. The required drying lasts several days to weeks depending on the soil type. A large number of samples distributed over the entire investigation area is desirable in order to increase the accuracy of the derived moisture maps. This, however, requires increased effort and thus higher costs.</p><p>We suggest the use of nuclear magnetic resonance (NMR) to gather this kind of calibration data. NMR measures the water content in porous media directly by stimulating the proton spins of the water molecules. The amplitude of the received response signal is linearly correlated with the number of protons in the sensitive volume of the device, i.e., with the amount of water in it: zero is measured when water is absent, while 100% water corresponds to maximum signal amplitude. In contrast to conventional laboratory NMR, the single-sided NMR technology enables mobile tools that are easy to handle in the field. Absolute soil moisture data is collected just by placing the sensor at the size of a shoebox or suitcase on the ground and the result for a single spot is available after a few minutes instead of days when taking and drying samples.</p><p>We successfully tested the single-sided NMR technology at one of our L-Band radiometry test sites predominated by clayey loam. In addition to the quad-bike based passive areal L-Band data acquisition, pointwise single-sided NMR measurements were performed on a profile with the length of 600 m at 10 m spacing. The sensitive NMR volume was adjusted to a depth of 1 cm. A total of 10 samples were taken for verification and analysed in the lab. The absolute water contents provided by NMR excellently agree with those of the samples. Moreover, the NMR profile results are also in good agreement with the L-Band measurements on the same profile. Future investigations will focus on the feasibility of the single-sided NMR method for other soil types and on the interpretation of the NMR relaxation behavior, which allows estimating the water-filled pore size distribution. In addition to the water content, this additional information is useful to estimate water mobility and storage capacity in the topsoil. </p>