Simulated Ka- and Ku-band radar altimeter height and freeboard estimation on snow-covered Arctic sea ice

Owing to differing and complex snow geophysical properties, radar waves of different wavelengths undergo variable penetration through snow-covered sea ice. However, the mechanisms influencing radar altimeter backscatter from snow-covered sea ice, especially at Ka- and Ku-band frequencies, and the impact on the Ka- and Ku-band radar scattering horizon or the “track point” (i.e. the scattering layer depth detected by the radar re-tracker) are not well understood. In this study, we evaluate the Ka- and Ku-band radar scattering horizon with respect to radar penetration and ice floe buoyancy using a first-order scattering model and the Archimedes principle. The scattering model is forced with snow depth data from the European Space Agency (ESA) climate change initiative (CCI) round-robin data package, in which NASA's Operation IceBridge (OIB) data and climatology are included, and detailed snow geophysical property profiles from the Canadian Arctic. Our simulations demonstrate that the Ka- and Ku-band track point difference is a function of snow depth; however, the simulated track point difference is much smaller than what is reported in the literature from the Ku-band CryoSat-2 and Ka-band SARAL/AltiKa satellite radar altimeter observations. We argue that this discrepancy in the Ka- and Ku-band track point differences is sensitive to ice type and snow depth and its associated geophysical properties. Snow salinity is first increasing the Ka- and Ku-band track point difference when the snow is thin and then decreasing the difference when the snow is thick (>0.1 m). A relationship between the Ku-band radar scattering horizon and snow depth is found. This relationship has implications for (1) the use of snow climatology in the conversion of radar freeboard into sea ice thickness and (2) the impact of variability in measured snow depth on the derived ice thickness. For both (1) and (2), the impact of using a snow climatology versus the actual snow depth is relatively small on the radar freeboard, only raising the radar freeboard by 0.03 times the climatological snow depth plus 0.03 times the real snow depth. The radar freeboard is a function of both radar scattering and floe buoyancy. This study serves to enhance our understanding of microwave interactions towards improved accuracy of snow depth and sea ice thickness retrievals via the combination of the currently operational and ESA's forthcoming Ka- and Ku-band dual-frequency CRISTAL radar altimeter missions.

Polar Mesospheric Summer Echoes (PMSE) during artificial heating

<p>Polar Mesospheric Summer Echoes (PMSE) are regions of enhanced radar backscatter at 80 to 90 km that are assumed to form in the presence of neutral air turbulence and charged ice particles as a result of spatial variations in the electron density. Changes in the electron temperature, as can be generated by the EISCAT heater, influence the electron diffusivity as well as the charging of the ice particles and both are parameters that influence the radar scattering. In many cases, an overshoot effect [1] can be observed when the backscattered power is reduced during heater-on and rises above the initial signal during heater-off. We present observations made on the 11-12 and 15-16 of August 2018 with the EISCAT VHF radar during PMSE conditions. The EISCAT heating facility, operated at 5.423 MHz, was run in identical cycles where the heater was on for 48 seconds and off for 168 seconds. The observations clearly show the overshoot effect, caused by the cyclic heating of PMSE.  The surface charge of the ice particles increases during the heater-on intervals because of the higher electron temperature. As the heater is turned off the electrons are quickly cooled. The dust particles, however, still carry a higher charge, i.e. more electrons, so that the electrons cannot immediately obtain the initial density distribution. The typical result is that the electron density gradients are increased, which in turn lead to increased radar scattering, an overshoot. During the heater off phase, dust and plasma conditions are expected to relax back to undisturbed conditions. A theory was developed by Havnes [1] to explain the overshoot and we use a dusty plasma code [2] based on this theory to calculate the overshoot curves. They agree well with the average of the observational data. There is clear indication that during high precipitation the PMSE cloud is not affected by the heater and accordingly does not show an overshoot effect. </p><p> </p><p>1.     Havnes, O. (2004). Polar Mesospheric Summer Echoes (PMSE) overshoot effect due to cycling of artificial electron heating. Journal of Geophysical Research: Space Physics, 109(A2).</p><p>2.     Biebricher, A., Havnes, O., Hartquist, T. W., & LaHoz, C. (2006). On the influence of plasma absorption by dust on the PMSE overshoot effect. Advances in Space Research, 38(11), 2541-2550.</p>

FEKO™ Simulation of Radar Scattering from Objects in Low Earth Orbit for ISAR Imaging

Objects in low earth orbit such as CubeSats and the International Space Station (ISS) move with constant velocity along a linear trajectory when viewed from a ground-based radar. The small change in attitude of the object as it flies overhead permits the generation of an inverse synthetic aperture radar (ISAR) image. In this paper, Altair’s FEKO™ software is used to model the monostatic radar scattering from the ISS as a function of frequency and aspect angle. The computed data is used for generating a simulated ISAR image from a ground-based radar. The system design requirements for the radar are calculated from the radar equation.