P-BAND ICE DIELECTRIC MEASUREMENT SYSTEM FOR THE CRYORAD MISSION

Y. Zhou¹, M. Schwank^1,2, F. Montomoli³, M. Brogioni³, G. Macellon³, M. Montagnat⁴, T. Casal⁵, M. Drusch⁵

¹Swiss Federal Research Institute WSL, ²Gamma Remote Sensing, ³CNR Institute of Applied Physics “N. Carrara”, 50019 Florence, Italy , ⁴Institut des Géosciences de l’Environnement, Université Grenoble Alpes, 38400 Saint-Martin-d’Hères, France, ⁵European Space Agency, Noordwijk, 2201 AZ, The Netherlands

CryoRad is a candidate mission under ESA’s Earth Explorer 12 program, aimed at advancing our understanding of the cryosphere and its role in the Earth system. The satellite will carry a novel P- to L-band wideband microwave radiometer (0.4–2 GHz) with continuous frequency scanning, representing the first deployment of a P-band radiometer in space. The mission has three mission primary objectives, which are 1) the ice sheet temperature profile from the top to the bottom; 2) the sea ice thickness volume and salinity in the range 0-1 m and 3) the sea surface salinity in cold water with improved accuracy. For ice sheets, CryoRad’s broad low microwave frequency coverage, particularly its inclusion of P-band (0.4-1GHz), enables temperature sensing at different depths within ice sheets down to several kilometers. To achieve this goal, an accurate knowledge of ice dielectric constant is of vital importance, as even a small error will accumulatively cause a large error in the modelled brightness temperature for an ice sheet with thousands-of-meters thickness. However, the information for the ice dielectric constant remains sparse, motivating the development of an accurate dielectric measurement system to support the fundamental concept, forward modeling and retrieval algorithms of CryoRad.

To meet this goal, we have developed a novel experimental set-up whose key component is a cylindrical cavity sensor. The cavity, which has a radius of 16.5 cm and a frequency resonance at around 700 MHz, is gold/silver-plated, making a very high Q of about 16500. The cavity’s top endplate is centrally hollowed with a hole of approximately 4.5 cm radius, allowing the ice sample to be placed inside the cavity. A cylindrical Teflon sample holder with a radius of about 3 cm is placed at the center of the cavity to secure the position of ice sample. The insertion hole is then sealed by a cap to preserve the resonance condition inside the cavity. Then the real and imaginary parts of the dielectric constant of the ice sample will be determined by the changes in the resonant frequency and the quality factor, Q, of the cavity, respectively. Since the ice dielectric constant has a very small imaginary part (in the order of 1e-3 – 1e-5), it is important to have a large sample volume to ensure enough change in Q that can be detected. Furthermore, this design allows the potential of the measurements of ice cores with radii of 1-3 cm made in lab or collected from the field in the future.

The design of the whole system is inspired by a previous system for seawater dielectric constant measurements [1] but fundamentally differs in their algorithms to extract the dielectric constant from resonant frequency and Q. While the previous seawater dielectric constant measurements rely on perturbation theory due to the extremely small volume of introduced seawater, the present system uses the exact solution of the field inside this three-layer cavity (air-sample holder-sample) to simulate the resonant frequency and Q. A customized software is developed according to this approach for analyzing the measurements. Initial results suggest an accuracy better than 2% can be reached for a Teflon sample (used for calibration) with an imaginary part of around 5e-4. The design, setup and initial results will be presented at the conference.

1. R. Lang, Y. Zhou, C. Utku and D. Le Vine, “Accurate measurements of the dielectric constant of seawater at L band”, Radio Science, 51 (1), 2-24, 2016