We investigated the feasibility of using multi-polarization Synthetic aperture radar (SAR) data to estimate the thickness of undeformed first-year ice. Analysis of the radar Signatures for the C- and L-bands Showed that the correlation between the ice thickness and VV-to-HH backscattering ratio is larger than the correlation between the ice thicknesses and the backscattering coefficients. This is in part because the ice Surface Salinity and hence the Surface reflection coefficient decreases as the ice thickens. The backscattering ratio had low Sensitivity to the Small-scale ice Surface roughness for the C-band and is almost independent of roughness at L-band. Given that the ratio is most Sensitive to ice Surface dielectric constants, which depend on Salinity, we developed an algorithm for retrieving the ice thickness that is based on the backscattering ratio and on the integral-equation-method (IEM) Surface Scattering model. Comparison of the observed and estimated ice thicknesses Showed that the correlation was much better when the thicknesses were estimated using the backscattering ratio than when the backscattering coefficient was used directly. The algorithm also performed better than previous retrievals using an empirical technique.

To better understand how internal radar echoes depend on ice-flow conditions and radar polarization, we surveyed two basins in East Antarctica using 179 MHz airborne radar. We compared radar echoes from three ice-flow conditions: parallel sheet flow in the main stream of a basin, convergent flow towards an ice stream, and longitudinal compression by nunataks. We detected a distinct zone of high radar scattering several hundred meters thick at middle depths in the latter two regions. This high-scattering zone was detected only when the radar polarization plane was parallel to the compression axis in ice. Such a high-scattering zone was not found in the parallel-flow region, regardless of the polarization. Using a recently developed theory of radar scattering in ice, we interpret the high-scattering zone as being caused by crystal-orientation-fabric alternations among adjacent ice layers due to difference in horizontal strain components. We argue that the spatial variation of the high-scattering zone is crucial for understanding past and present flow features.

Dual-frequency, multi-polarization airborne synthetic aperture radar (Pi-SAR; developed by the Communications Research Laboratory and National Space Development Agency of Japan) observations of the seasonal sea-ice region off the Okhotsk coast of Hokkaido, Japan, were carried out in February 1999 using X- and L-band radar frequencies with a resolution of 1.5 and 3.0 m. In conjunction with the SAR observations, the sea-ice thickness (draft) and velocity were measured by a moored Ice Profiling Sonar (IPS) and an Acoustic Doppler Current Profiler (ADCP). Tracks of the sea ice passing over the IPS were estimated from the time series of the ADCP ice-velocity and -direction data. Along these tracks, the SAR backscattering coefficient profiles were compared with the IPS ice-draft profiles. The results showed that the L-band SAR backs cattering profiles correlated well with the IPS ice-draft data, particularly in the thicker part (a few meters thick) of the rim of first-year ice, which had a large backscattering coefficient. Although the X-band SAR backscattering profiles did not correlate well with the IPS data, thin ice (<10 cm thick) showed a large backscattering coefficient. The L-band SAR and IPS data did not distinguish thin ice from open water.

To better understand how ice sheets respond to climate, we designed a new multi-frequency ice-penetrating radar system to investigate subsurface structures of ice sheets. The system is mounted on a single platform and handled by a single operator. Three radio frequencies, 30,60 and 179 MHz, were used. An underlying principle of these multi-frequency observations is that the lower frequencies are more sensitive to electrical conductivity changes, whereas the higher frequencies are more sensitive to dielectric permittivity fluctuations in the ice. The system is composed of three single-frequency pulse radars, a trigger-controller unit and a data-acquisition unit. The trigger controller is the key component of this system. It switches transmitters on at different timings to prevent mixing of signals among the three radars. The timing difference was set as 50 μs, which is equivalent to the two-way travel time for radio waves reflecting from 4250m below the surface. A field test was done along a 2000 km long traverse line in east Dronning Maud Land, Antarctica. The multi-frequency system successfully acquired data that are equivalent in quality to our earlier single-frequency measurements along the same traverse line. The details of the system and preliminary data are described.

Multi-frequency, multi-polarization airborne synthetic aperture radar (SAR) observations of sea ice in the southern Sea of Okhotsk were carried out in February 1999 in conjunction with RADARSAT SAR observations. The final goal of this study is to clarify the backscattering characteristics and to understand the scattering mechanisms of sea ice in the Sea of Okhotsk by using microwave multiparametric SAR. The airborne SAR (Pi-SAR) has two frequencies (X- and L-band) and multi-polarization (HH, VV, HV, VH) with 1.5 m (X-band) and 3.0 m (L-band) resolution. It was developed by the Communications Research Laboratory (X-band) and the National Space Development Agency of Japan (L-band). We show the frequency dependence and polarization properties of radar backscattering from sea ice. We find that it is possible to distinguish ice types by comparing backscattering from sea ice in the X- and L-bands. Investigation of the polarization characteristics at X-band was very useful for detecting the thin-ice area (e.g. nilas and gray ice).

Evidence of changing basal and internal ice properties near the grounding line was derived from airborne radio-echo-sounder observations of the ice sheet around the Sør Rondane Mountains, Antarctica. From the trailing figure of the bottom-echo signal, the roughness of the ice bottom near the grounding line was inferred. Results show that the specular components of scattering begin to appear on the ice-shelf side of the grounding line. Furthermore, double-trip echoes were observed with a strong scattering in the shelf area, and their boundary of occurrence was very close to the grounding line. This is evidence of interaction between ice and sea water at the bottom of the ice shelf. We also examined the occurrence of internal layered echoes. In most of the area around the mountains, internal echoes were observed continously, but they were not found at or close to the ice shelf. The boundary between the appearance and disappearance of internal-layer echoes is distinct, and occurs 20–30 km inland from the grounding line. These results suggest that some major change may occur in the internal ice on the inland side of the grounding line.