Probing the cryosphere with an Earth Observation orbiting radar sounder

Abstract

The dynamic changes in the cryosphere, such as the ice calving fed by seaward flowing glaciers, have a significant impact on the society and environment, by contributing to the increase of the sea-level rise. This requires a continuous monitoring of the processes occurring within the ice-sheets to predict the dynamics and stability of the polar ice-caps. Radar sounders can acquire profiles of the subsurface (radargrams) that provide rich information for estimating the Essential Climate Variables (ECV) and better understanding the processes and structures in the ice. Subsurface targets detected from the radargrams can be used to determine: (i) the surface and the basal interface topography, ii) the ice sheet thickness and the internal layering, (iii) the characteristics of ice shelves and the position of the grounding line, (iv) the basal boundary conditions and ice flow regime, and (v) the ice mass balance. Currently, there are a number of airborne radar sounders (e.g., POLARIS [1] and MCoRDS [2]) that have been acquiring data over the Earth’s icy areas over the last decades. Significant information on the glacier structure and processes in Greenland and Antarctica have been extracted from the analysis of these radargrams [5-7]. However, these data are acquired seasonally and over specific regions, and thus they do not provide uniform coverage in space and time nor have homogeneous data quality. These limitations can be addressed by a satellite-mounted radar sounder, which would provide continuous, homogeneous, and uniform coverage of the cryosphere. The Earth-orbiting radar sounder should be designed to operate at a central frequency in the HF-VHF range with a bandwidth of 10 MHz, to have a penetration depth up to 5 km and a vertical resolution about 10 meters in ice. However, data acquired by orbiting radar sounders are subjected to the cosmic microwave background, path loss, relatively low transmit power, ionospheric effects, and poor resolution due to the altitude of the platform and the constraints on the bandwidth. In this paper, we analyze the detectability of subsurface targets in icy radargrams acquired by an orbiting radar sounder taking into account these limitations. To this purpose, we defined a simulation procedure for estimating radargrams acquired by such a radar sounder. Further, we analysed the simulated radargrams with existing techniques developed for airborne acquisitions, in order to understand the detectability of the subsurface targets. The simulation approach is based on reprocessing the available airborne radargrams to match the range of possible characteristics of the orbiting radar sounder. The simulation technique accounts for the radar and target parameters (e.g., the central frequency, bandwidth, sampling frequency, and the medium dielectric properties) that are related to the radargram characteristics (e.g., the resolution, received power, and sample spacing). Moreover, the cosmic microwave background noise is stochastically added to the simulated radargram to represent the real scenario. Simulated radargrams of a set of important ice subsurface targets are analyzed using state-of-the-art analysis techniques developed for airborne or planetary radar sounder data. The algorithms are applied to detect the important radargram targets, such as (i) the bedrock and the surface [3] - useful for estimating the ice sheet thickness; (ii) the number and strength of the layers [4, 6] - useful for understanding the paleoclimatic records and glacier aging; (iii) the ice-shelves for better understanding processes in the grounding zone; (iv) the basal features such as subglacial lakes [8] and refreezing ice [7] - useful for understanding the basal conditions and the mass balance. Our analysis confirms that an EO orbiting radar sounder can image important subsurface targets