This paper presents some results obtained during the 2022 CIRFA arctic cruise concerning the measurements of the radar response of different types of sea-ice using a high-resolution Ground-Based radar system operating at C band. This MIMO device was able to directly measure tomograms, or slices of reflectivity in the elevation-ground plane, allowing to quantitatively appreciate the penetration of radar waves into sea-ice types having complex geometrical features, and to assess the dominant contributions measured by radar devices at C band.

In support of ESA's Mission Advisory Group for ROSE-L (Radar Observing System for Europe at L-band), a project team consisting of experts from operational ice services, universities and research institutes has been investigating the benefits of using data from L-band SAR in addition to C-band SAR imagery for monitoring of sea ice and detection of icebergs. The project work is still ongoing. Here, we introduce the technical background and present first results.

This paper discusses the alignment of Synthetic Aperture Radar (SAR) image pairs with one image acquired at L- and the other at C-band. For our study we used data from ALOS-2 PALSAR-2 and Sentinel-1 taken over sea ice in the Fram Strait. The use of multifrequency SAR data is beneficial for sea ice characterization and classification because it combines the advantages of the frequency dependence of signal penetration depths and backscattering characteristics due to small-scale ice properties. For drifting sea ice, however, an immediate combination of image pairs acquired with a time gap is usually not possible. Such cases require to consider the effects of ice drift. In this study we investigate the possibility to compensate for sea ice drift for separation in acquisition time ranging from a few hours to several days. The first results are promising and suggest that alignment of SAR image pairs obtained at different frequencies is possible even for larger time gaps between acquisitions if drift and deformation can be estimated reliably.

As part of the development of the Radar Observing System for Europe at L-Band (ROSE-L), potential improvements of sea ice monitoring through the synergistic use of L- and C-band are investigated in a project funded by ESA. The present note concentrates on one aspect of this project, namely a summary of related studies on the joint use of L- and C-band SAR imagery for the production of sea ice charts and the retrieval of parameters characterizing ice conditions and properties. One objective is to describe the gain that is achieved when using L-band images in addition to data acquired at C-band, as far as known today. In addition, gaps in the recent knowledge are listed.

This brief note introduces a study that aims at quantifying the gain for sea ice observations which is achieved when combining L- and C-band SAR imagery for the retrieval of ice conditions and iceberg occurrences. An L-band SAR mission is currently under discussion at the European Space Agency (ESA) as one of the six Copernicus High Priority Candidate Missions (HPCMs). Since the emphasis of the study is on operational monitoring and ice charting, it involves contributions from different ice services that are members of the International Ice Charting Group (IICWG). Preliminary results confirm findings of earlier investigations on the use of L-band for sea ice monitoring and extend them with first systematic investigations of iceberg detection at L-band.

In this paper, the retrieval of sea-ice surface heights from the interferometric TanDEM-X data is investigated. The data were acquired over fast and drifting ice in Fram Strait located between Greenland and Svalbard. Additional measurements of the sea-ice surface topography were carried out using a stereo camera and a laser altimeter. The comparison of the surface elevation retrieved from TanDEM-X imagery with the results of the stereo camera measurements revealed that sea-ice ridges greater than 0.5 m can be estimated with a root-mean-square error of 0.3 m or less with the error decreasing as a function of ridge height. Although the helicopter-borne laser data are only available as 1-D profiles with a much higher across-track spatial resolution than the TanDEM-X data, they proved to be useful for the validation. The need for multilook averaging to reduce the phase noise is identified as the main challenge in achieving the spatial resolution necessary for retrieving sea-ice surface topography using synthetic aperture radar interferometry.

In this paper we investigate interferometric pairs of SAR images acquired by Tandem-X with the monostatic pursuit and the alternative bistatic modes for sea ice segmentation. The individual SAR images are modelled as non-Gaussian, and from the modelled data different features are extracted, stacked together and clustered. The interferometric coherence is regarded as an additional feature and utilized for clustering. In addition to complementing the information extracted from the individual images, the interferometric coherence is found to be capable of discriminating between open water and sea ice, as well as between different ice types.

Icebergs represent hazards to maritime traffic and offshore operations. Satellite synthetic aperture radar (SAR) is very valuable for the observation of polar regions, and extensive work was already carried out on detection and tracking of large icebergs. However, the identification of small icebergs is still challenging especially when these are embedded in sea ice. In this paper, a new detector is proposed based on incoherent dual-polarization SAR images. The algorithm considers the limited extension of small icebergs, which are supposed to have a stronger cross-polarization and higher cross- over copolarization ratio compared to the surrounding sea or sea ice background. The new detector is tested with two satellite systems. First, RADARSAT-2 quad-polarimetric images are analyzed to evaluate the effects of high-resolution data. Subsequently, a more exhaustive analysis is carried out using dual-polarization ground-detected Sentinel-1a extra wide swath images acquired over the time span of two months. The test areas are in the east coast of Greenland, where several icebergs have been observed. A quantitative analysis and a comparison with a detector using only the cross-polarization channel are carried out, exploiting grounded icebergs as test targets. The proposed methodology improves the contrast between icebergs and sea ice clutter by up to 75 times. This returns an improved probability of detection.

Antarctica is surrounded by ice shelves and glaciers of different sizes. Satellite imagery shows different feature patterns (e.g. crevasses, rifts) at their surfaces, which control the shape and the size of icebergs that calve from their seaward edges. An edge detection method was used to map and classify the surface features, considering their orientation relative to the calving front. Calved icebergs can automatically be detected and then tracked on their way through the ocean using single and multi-polarized Synthetic Aperture Radar (SAR) images. Temporal gaps between subsequent SAR imagery can be closed by applying a simple wind-driven iceberg drift model.

This paper is focused on investigations of polarimetric C-band radar signatures of icebergs in sea-ice-covered ocean regions. The main objective is to assess the potential improvement of iceberg detection when using radar polarimetry. The dominant backscattering mechanisms of icebergs are deduced by evaluating different polarimetric parameters. Magnitudes of the cross-polarization ratios, the correlation coefficients between HH- and VV-polarized signals, and the entropy/alpha parameters indicate a strong contribution of volume scattering in many cases. Over most icebergs, the phase differences between HH- and VV-polarization are larger than zero. Spatial patterns of the polarimetric parameters differ from iceberg to iceberg and between different parameters. On some bergs, they only exhibit slight variations, whereas on others, they show noiselike textures, but also, more systematic changes are observed. Occasionally, radar intensities of icebergs are of similar magnitude as those of sea ice. Only for a number of these cases, the combined use of the investigated polarimetric parameters together with intensity improves the discrimination performance between icebergs and sea ice.

A short overview is given on the achievements made in sea ice monitoring during the last decade utilizing the technical possibilities of Envisat ASAR. In particular we discuss the improvements by employing the dual-polarization image mode, the benefits of the wide-swath capability, and the advantage to select between seven swaths at different incidence angle ranges when using the image mode.

Melt and thaw of snow, ice, and soil are important processes for the hydrological cycle as well as for energy fluxes. Remote sensing from space allows the monitoring of vast uninhabitated regions like the Siberian Arctic. This study presents results from monitoring of snowmelt events during spring, the thaw of the active layer on different geomorphological units on tundra land surfaces, as well as lake-ice decay from high resolution TerraSAR-X and quad-pol RADARSAT-2 timeseries acquired over the central Lena Delta, Siberia. The main findings are that X-band data do not show a backscatter difference between frozen and thawed soils, whereas the C-band backscatter rises in the order of 3 dB. Polarimetric decompositions show the spatial extend of snowmelt events and help to distinguish between different geomorphological units. The high spatial resolution of the TerraSAR-X images is extremely helpful to distinguish between land surfaces and the frequent ponds in the investigation area.

SIGNAL is an innovative earth exploration mission proposal with the main objective to estimate accurately and repeatedly topography and topographic changes associated with mass change or other dynamic effects on glaciers, ice caps and polar ice sheets. Elevation measurements are complemented with glacier velocity measurements, providing valuable additional information for a better understanding of the hydrology of glacierized basins and of the Arctic and Antarctic water cycle. SIGNAL is capable of monitoring all critical regions with a high spatial resolution and an adequate revisit time. This paper gives an overview about the actual mission design status and provides a brief description of the topography (DEM - digital elevation map) self-calibration strategy and the estimated global interferometric performance.

Airborne C- and L-band synthetic aperture radar (SAR) images were acquired for three test sites over different sea ice regimes around Svalbard in March 2007, complemented by optical imagery, environmental data, and ice observations. One objective was to use the high-resolution low-noise radar data for investigations on the technical performance of European Space Agency's Sentinel-1 mission for sea ice mapping, the other to assess the potential gain of additional use of L-band SAR data currently available from Advanced Land Observing Satellite (ALOS). The airborne SAR images were employed to simulate data products resembling the interferometric wide-swath mode (IWSM) of Sentinel-1 and ALOS PALSAR fine resolution mode (FRM) in order to quantify the information loss due to the higher noise level and coarser spatial resolution. At the IWSM noise level, zones of deformation, level ice, and new ice can be well separated at like-polarization. At cross-polarization, the noise level is too high for a robust automated classification including thin ice, but deformed and level ice can be discriminated. PALSAR FRM is on average better suited to distinguish deformed and level ice over all ice regimes. It was worse for separating new from thicker level ice at two test sites. For the third test site with very few patches of new ice, the PALSAR FRM showed larger intensity contrasts between new and level ice than Sentinel-1 IWSM. The spatial resolution provided by the IWSM and FRM is sufficient to identify most ice features and types without ambiguities. Typical characteristics of the imaged ice regimes are described.

C- and L-band airborne synthetic aperture radar (SAR) imagery acquired at like- and cross-polarizations over sea ice under winter conditions is examined with the objective to study the discrimination between level ice and ice deformation features. High-resolution low-noise data were analyzed in the first paper. In this second paper, the main topics are the effects of spatial resolution and signal-to-noise ratio. Airborne high-resolution SAR scenes are used to generate a sequence of images with increasingly coarser spatial resolution from 5 to 25 m, keeping the number of looks constant. The signal-to-noise ratio is varied between typical noise levels for airborne imagery and satellite data. Areal fraction of deformed ice and average deformation distance are determined for each image product. At L-band, the retrieved values of the areal fraction get larger as the image resolution is degraded. The areal fraction at C-band remains constant. The retrieved average distance between deformation features increases both at C- and L-bands as the image resolution gets coarser. The influence of noise becomes noticeable if its level is equal or larger than the average intensity backscattered from the level ice. The retrieval of deformation parameters using simulated images that resemble ERS-2 SAR, Envisat ASAR, and ALOS PALSAR data products is discussed. Basic differences between real and simulated ERS-2 SAR images are analyzed.

In this paper, we present a quantitative comparison of L- and C-band airborne synthetic aperture radar imagery acquired at like- and cross-polarizations over deformed sea ice under winter conditions. The parameters characterizing the deformation state of the ice are determined at both radar bands and at different polarizations. The separation of deformed and level ice is based on a target detection technique. The threshold is set such that image pixels with intensities equal to or larger than the highest 2% of the level-ice intensity distribution are classified as deformed ice, independent of the radar configuration and ice conditions. Optical imagery of sufficient quality for comparison is available only in a very few cases. To characterize the deformation state, the areal fraction of deformation features and the average distance between these features are evaluated. The values obtained for both parameters are very sensitive to the radar frequency. Aeral fractions are larger, and average distances are smaller at L-band than at C-band because of the much higher intensity contrast between the deformed and level ice at L-band. The differences between polarizations at one radar band are smaller but not always negligible. In comparison to optical images, it was observed that deformed-ice areas can be distinguished from level ice over their whole length and extension at L-band, whereas at C-band, often, only prominent parts are visible.

Spaceborne single-polarization C-band synthetic aperture radar (SAR) imagery is widely used to gather information about the state of the sea ice cover in the polar regions. C-band is regarded as a reasonable choice for all-season monitoring capabilities. For specific mapping tasks, however, other frequency bands can be more suitable. In the first part of this paper, the summary of a literature study dealing with the utilization of L-band SAR imagery for sea ice monitoring is presented. Investigations reveal that if deformation features such as ice ridges, rubble fields, and brash ice are to be mapped, L-band radar is superior in a number of cases. The second part of this paper addresses the comparison of JERS-1 and ERS-1 SAR images that were acquired over sea ice east of Svalbard and along the east coast of Greenland. The effects of the different frequencies, polarizations, and incidence angles of the two SAR systems are discussed. It is demonstrated that the images of both sensors complement one another in the analysis of ice conditions, resulting in a more detailed view of the sea ice cover state.

This study combines two satellite radar techniques, low-resolution C-/Ku-band scatterometer and high-resolution C-band synthetic aperture radar (SAR) for glaciological studies, in particular mass-balance estimations. Three parameters expressing the mean backscattering and its dependency on azimuth and incidence angle are used to describe and classify the Antarctic ice sheets backscattering behavior. Simple linear regression analyses are carried out between ground truth accumulation data and the SAR backscattering coefficient along continuous profile lines. From this we parameterize the accumulation rate separately for certain snow pack regimes. We find that SAR data can be used to map mass-balance changes, however only within limited areas. Applying this method therefore generally requires accurate ground truth for regional calibration together with additional information regarding mean air temperature or elevation. This investigation focuses on the area of Dronning Maud Land, Antarctica. We present the first high-resolution accumulation map based on SAR data for the surrounding area of the EPICA deep ice core drilling site Kohnen, which is compared to reliable ground truth records as well as to a surface-mass-balance map interpolated from these at low resolution.

Polarimetric SAR images acquired at C- and L-band over sea ice in the Greenland Sea, Baltic Sea, and Beaufort Sea have been analysed with respect to their potential for ice type classification. The polarimetric data were gathered by the Danish EMISAR and the US AIRSAR which both are airborne systems. A hierarchical classification scheme was chosen for sea ice because our knowledge about magnitudes, variations, and dependences of sea ice signatures can be directly considered. The optimal sequence of classification rules and the rules themselves depend on the ice conditions/regimes. The use of the polarimetric phase information improves the classification only in the case of thin ice types but is not necessary for thicker ice (above about 30 cm thickness)

Routine sea ice observations of ice type, concentration and thickness were carried out during a microwave remote sensing validation campaign in the Barents Sea aboard the US Coast Guard icebreaker Healy during October-November 2001. Ice near the edge was generally a near equal mixture of small to medium floes of thin first-year ice and approximately 2 meters thick multiyear ice, with large floes occurring deeper in the ice pack. These data are compared to estimates of ice concentration from several radarsat and SSM/I images to validate routine operational ice analysis performed at the National/Naval Ice Center in Washington, D.C. Observations of several different ice regimes observed in the field are compared to observed SAR backscatter characteristics. Results show that while some distinction between ice types can be made within the pack, near the ice edge unambiguous determination is difficult from SAR data alone. For the most accurate analysis of ice conditions from SAR in the marginal ice zone, the day to day evolution of the ice drift and growth should be monitored.

The microstructural characteristics of sea ice during autumn freeze-up in the Barents Sea were measured during a microwave remote sensing validation campaign aboard the US Coast Guard icebreaker Healy during October-November 2001. Ice samples were obtained from 16 ice floes and digital images of the thin and thick sections were taken to examine the characteristics of the brine and void structure. Due to the dynamic nature of the marginal ice zone, brine structure in first-year or thin ice was highly variable. Although brine pockets were typically elongated and tube-like, they could either be randomly oriented or aligned vertically, and may form either well-connected network or relatively isolated pockets. Some samples contained a significant fraction of air bubbles. Multi-year ice was typically very porous. Void diameters ranged from less than 0.1 mm to several centimeters and were often nonspherical. Of particular importance for microwave scattering, the larger voids may have contained either brine or air, depending on the fluid permeability of individual cores. A wide range of void fractions were observed in the upper, salt-free layers of the ice, which could account in part for the range of backscatter values observed for multi-year ice.

The paper addresses the detection of changes in multitemporal polarimetric radar images, focusing on small objects and narrow linear features. The images were acquired at C- and L-band by the airborne EMISAR system. It is found that the radar intensities are better suited for change detection than the correlation coefficient and the phase difference between the co-polarized channels. In the case of linear features, there is no obvious difference between the C- and L-bands , and slight variations of the flight tracks are acceptable at look angles larger than 35 degrees. Theoretical detection thresholds are evaluated from the statistical distribution of the intensity ratio due to speckle. For the linear features and for urban environments, the observed thresholds are larger than the theoretical predictions. This is interpreted as an effect of radar intensity variations on length scales smaller than the spatial image resolution. The signature of urban areas is very sensitive to deviations between the flight tracks, and the sensitivity is larger at C-band than at L-band. On the other hand, the intensity contrast between buildings and the urban background is smaller at L-band and larger at C-band. For change detection, thresholds may have to be chosen separately for each object class because the intensity ratios of different object classes vary differently as a function of time.

In radar signature analysis, the root mean square (RMS) surface slope is utilized to assess the relative contribution of multiple scattering effects. For an exponentially correlated surface, an effective RMS slope can be determined by truncating the high frequency tail of the roughness spectrum. The choice of the cutoff frequency and the effect on surface scattering simulations are discussed.

Stochastic surface models are useful for analyzing in situ roughness profiles and synthetic aperture radar (SAR) images of geological terrain. In this paper, two different surface models are discussed: surfaces with a stationary random roughness (conventional model) and surfaces with a power-law roughness spectrum (fractal model). In the former case, it must be considered that for short profiles (L<200l/sub 0/), the measured values of rms-height s and correlation length l may be significantly smaller than the intrinsic values s/sub 0/ and l/sub 0/. In the latter case, rms-height and correlation length depend on the profile length L, and the surface is better characterized by slope and offset of the roughness spectrum (which are independent of L), The sensitivity of the SAR signature to variations in surface roughness parameters is evaluated by means of theoretical scattering models. For smoother geological surfaces such as most arid terrain types, single scattering is dominant, which means that the roughness parameters can be determined from SAR data using comparatively simple algorithms. Multiple scattering processes on rough surfaces such as a'a lava and variations of the local incidence angle due to large-scale terrain undulations make the retrieval of roughness parameters by means of inverse modeling much more complex. Field data of surface roughness indicate that rougher geological surfaces may be in the diffractal regime at higher radar frequencies, in which the scattering characteristics deviate significantly from the patterns observed for stationary surfaces. On the basis of surface and scattering models, recently published observations of roughness data and radar signatures from volcanic, alluvial, and arid surfaces are examined.

This paper focuses on the spatially varying backscattering signature of an area of refrozen brash ice observed by a ship based scatterometer. The measurements were carried out as part of the Baltic Experiment for ERS-1 in 1994. The scatterometer was operated at 5.4 GHz at different incidence angles and polarizations. By analysing the scatterometer data over azimuth scans, it was found that the backscattering variabilities are not only due to fading but also contain a textural component. Surface height profiles were measured using a laser. The observed ice surface roughness was nonstationary over the measurement area. The ice surface can be approximated by adjacent patches of stationary roughness with patch dimensions of about 4.5 m. From the roughness spectra of different stationary patches, two roughness classes can be distinguished. The implications of estimating the roughness parameters from relatively short profile lengths is discussed and the effect on theoretical predictions of the backscattering coefficient is investigated. The texture variance is evaluated theoretically on the basis of the simulated backscattering coefficients of the two observed roughness classes and is found to compare with the scatterometer data.