A Controlled Experiment to Retrieve River Ice Characteristics from an Fmcw Radar System

A controlled experiment was conducted at the Cold Region Research and Engineering Laboratory (CRREL) facilities for retrieving freshwater ice characteristics using a Frequency Modulated Continuous Wave (FM-CW) radar system. The objectives of the experiment are 1) to try to unravel the complex interactions between a backscattered radar signal and ice cover characteristics such as thickness and air bubble content; and 2) to provide a comprehensive data set for testing a theoretical radar backscattering model currently under development at INRS-ETE in collaboration with ÉTS. The experiment was held from October 16 to 27 2006 in an indoor test basin (36.5m X 9.4m X 2.5m), where an ice sheet was grown from 0 to 24 cm thickness and monitored with a series of radar and geophysical measurements. At every 1.5 cm of ice growth, backscattering of the ice cover using an in-house FM-CW C-band radar was measured at incidence angles varying from 0 to 50 degrees for four polarisation combinations, HH, VV, HV, and VH. For each series of measurements, ground penetrating radar (GPR) profiles were obtained at 400, 900 and 1500 MHz, as well as Time Domain Reflectometry (TDR) measurements using horizontal probes installed at 7 cm and 18 cm below the ice surface and a 0-30 cm vertical probe. Ice thickness was measured by extracting small ice samples and snow resulting from air moisture condensation was characterized for depth, snow water equivalent and crystal size. Ice cores were also extracted to characterize their air bubbles content. Pre-processing of some of the radar data (filtering, fast Fourier transforms, etc.) has been made to ‘clean’ the data for further processing to extract ice cover characteristics. Preliminary analysis of the data indicates that the radar backscattered signal is notably affected by ice thickness. For example, the HH/HV ratio of power signals correlates with ice thickness between 8 and 24 cm with an R value of 0,95. Using Maxwell’s equations, it was shown that the lack of correlation at lower thicknesses (0-8 cm) may be due to multiple reflections causing interference patterns. As expected, the power of the backscattered signal decreases with increasing incidence angle for all polarization combinations, with HH signal displaying the sensitivity with incidence angle changes. The presence of snow at the ice surface marginally altered the backscattered signal. Preliminary investigations confirm previous findings that ice characteristics can be extracted from the spectral content of backscattered signal.