Multi-temporal monitoring of wetland water levels in the Florida Everglades using interferometric synthetic aperture radar (InSAR)

Interferometric synthetic aperture radar (InSAR) techniques can successfully detect phase variations related to the water level changes in wetlands and produce spatially detailed high-resolution maps of water level changes. Despite the vast details, the usefulness of the wetland InSAR observations is rather limited, because hydrologists and water resources managers need information on absolute water level values and not on relative water level changes. We present an InSAR technique called Small Temporal Baseline Subset (STBAS) for monitoring absolute water level time series using radar interferograms acquired successively over wetlands. The method uses stage (water level) observation for calibrating the relative InSAR observations and tying them to the stage's vertical datum. We tested the STBAS technique with two-year long Radarsat-1 data acquired during 2006-2008 over the Water Conservation Area 1 (WCA1) in the Everglades wetlands, south Florida (USA). The InSAR-derived water level data were calibrated using 13 stage stations located in the study area to generate 28 successive high spatial resolution maps (50 m pixel resolution) of absolute water levels. We evaluate the quality of the STBAS technique using a root mean square error (RMSE) criterion of the difference between InSAR observations and stage measurements. The average RMSE is 6.6 cm, which provides an uncertainty estimation of the STBAS technique to monitor absolute water levels. About half of the uncertainties are attributed to the accuracy of the InSAR technique to detect relative water levels. The other half reflects uncertainties derived from tying the relative levels to the stage stations' datum.     

High resolution observations of Weddell Sea surface currents using ERS-l SAR sea-ice motion vectors

Two areas of the Weddell Sea, one in the south and one in the west, were chosen for a preliminary investigation of sea-ice motion tracking from ERSt Synthetic Aperture Radar (SAR) images during the Austral summer. Only a small number of images were processed, so a manual tracking method was used. In the 3–day period between SAR images the atmosphere warmed near the surface, which led to significant changes in radar backscatter from, and thus in contrast between, ice floes and the areas between them. It was therefore not always possible to track features from one image to the next. The tracked features were clearly identified in images which were sub-sampled at onesixteenth of the full resolution available. In the southern Weddell Sea images, many large floes were present which allowed a quite detailed pattern of the surface water circulation to be mapped as the ice motion was predominantly forced by the ocean currents during a period of low surface wind speeds. The observed circulation pattern agreed well with previous observations from hydrographic surveys in this area north of the Filchner Ice Shelf. In the western Weddell Sea images good tracers were hard to find, but it was still possible to detect the edge of the western boundary current of the Weddell Gyre. Continuous monitoring of sea-ice motion in these two areas using SAR imagery could be a useful means of detecting changes in surface water flow which may be linked to the rate of formation of Antarctic bottom water.     

Monitoring of large-scale deformation in mining areas using sub-band InSAR and the probability integral fusion method

The large-scale and rapid land subsidence that occurs in mining areas often leads to problems, such as densely spaced interference fringes and the temporal decorrelation of interferometric synthetic aperture radar (InSAR) interferograms. To solve these problems, sub-band InSAR is applied to monitor the large-scale deformation that occurs in mining areas. First of all, four different bandwidth images with three sub-band bandwidth parameters are used to extract simulated mining-induced subsidence with seven different deformation magnitudes. The results of the simulation experiment suggest the following conclusions. In monitoring subsidence with different deformation magnitudes using images with different bandwidths, an optimal monitoring value exists; wider image bandwidths lead to smaller optimal monitoring values and higher monitoring accuracies. Therefore, an appropriate sub-band bandwidth should be selected that depends upon the image bandwidth and the subsidence level to achieve optimal monitoring. The optimal sub-band bandwidth for monitoring subsidence of different magnitudes in mining areas is determined through simulation experiments, and these conclusions can provide a technical basis for selecting the appropriate sub-band bandwidth for the monitoring of subsidence in mining areas. Although sub-band InSAR can reduce the number of interference fringes and the difficulty of unwrapping, the simultaneous introduction of large amounts of noise leads to reduced monitoring precision, and the application of the probability integral method in the prediction of mine subsidence is more mature. Therefore, the combined use of sub-band InSAR and the probability integral fusion method to monitor mining-induced deformation is proposed in this paper. The probability integral method is used to perform noise peeling on the interferometric phases of the sub-bands to improve the monitoring accuracy of sub-band interferometry. Then, according to the results of the simulation experiment, the fusion method with the appropriate sub-band bandwidth parameters is applied to monitor the surface deformation associated with working face 52,304 from 2 December 2012 to 13 December 2012. Finally, the monitoring results are compared with the results of monitoring using conventional differential interferometric synthetic aperture radar (D-InSAR) and global positioning system (GPS) field survey data. The results show that the reliability and accuracy of the fusion method are much better than those of conventional D-InSAR in monitoring the large-scale deformation that occurs at the edges of subsidence basins.     

Seasonal variation of coherence in SAR interferograms in Kiruna,Northern Sweden

This article presents the results of a study conducted to quantify the seasonal variation of coherence in synthetic aperture radar (SAR) interferograms in Kiruna, Northern Sweden. In SAR interferometry (InSAR), coherence is an important concept that provides a good indication of the phase stability of the scatterers. Therefore, in this study, the degree of coherence is used as a parameter to identify the seasonal variation interferograms. For this study, 34 Radarsat-2 ultra-fine beam mode (U6D) images of the Kiruna area (67°51?N, 20°14?E) and the improved digital elevation model (DEM) created by merging the Radarsat-2 DEM and ASTER DEM were used to produce 561 differential interferograms. The interferograms were arranged in three different ways for the analysis, with the first including common master interferograms (with the summer master image), the second including the sequential interferograms that have the shortest temporal baseline, and the third accounting for all possible combinations of the interferograms (full network of interferograms). Following this step, the variation of coherence for forest areas, urban areas, and flat waste rock areas was studied. This study shows that interferograms generated for the Kiruna region exhibit seasonal variations in coherence due to the ground snow layer in winter. Furthermore, when there is water on the ground due to the melting of the snow layer (in the spring) or due to rains in autumn, the coherence is reduced considerably. Another significant feature is that there is a significant change in summer-to-summer coherence for some regions even over the course of a few years. Based on this study, it is clear that the winter Radarsat-2 U6D beam mode images are not suitable for differential interferometric SAR (DInSAR) deformation measurements in flat waste rock regions in Kiruna. It is expected that even with winter images, PSInSAR or CTM techniques will be able to provide better deformation measurements, but, in this study, those techniques were not assessed. The next step will be to study the seasonal variations in coherence in natural or man-made targets/persistent scatterers using persistent scatter InSAR (PSInSAR) or coherence target monitoring (CTM) techniques.     

A comparison of TerraSAR-X, RADARSAT-2 and ALOS-PALSAR interferometry for monitoring permafrost environments, case study from Herschel Island, Canada

Interferometric synthetic aperture radar (InSAR) data sets from TerraSAR-X, RADARSAT-2 and ALOS-PALSAR are compared for their ability to detect ground movement over the continuous permafrost site of Herschel Island, Yukon Territory, Canada. All three sensors maintain good coherence within a summer season and can be used to create summer displacement products. Stacking is advantageous for the TerraSAR-X and RADARSAT-2 data sets, although mottling, possibly an interaction of the SAR with vegetation, or residual tropospheric noise, is visible, reducing the reliability of the results. RADARSAT-2 and ALOS-PALSAR provide the most promising results with the ability to form one year interval interferograms. PALSAR can also form two and three year interval interferograms. Long interval data sets spanning 2007 to 2010 identify a band of movement of 20 to 30 cm/year along the north-east coast, and a region of movement of up to 5 cm/year near the northern tip of the island. The ability to form long interval displacement products holds the most promise for permafrost monitoring, since long-term trends are of greater interest for permafrost stability than short-term seasonal changes. TerraSAR-X data have the disadvantage that year to year interferograms cannot be formed. InSAR is not the ideal monitoring technique for the large thaw slumps of Herschel Island. Although general areas of instability can be identified, specific slump detection is limited by radar look direction, and the large and abrupt slump movement, often accompanied by disintegration and collapse of slump sections, causes loss of coherence in the InSAR data. Thaw slumps may require a different interferometric approach, such as slump extent mapping from coherence loss, or the installation of corner reflectors and point target techniques. The frequent revisit and high spatial resolution of TerraSAR-X provide the best chance of maintaining coherence over thaw slumps. In general, InSAR is more successful at identifying broad areas of subtle subsidence in gentle relief, areas of terrain instability, possibly due to permafrost thaw or ground ice melt and the removal of water volume, and prior to significant slumping.     

SAR interferometry over Baltic Sea ice

SAR interferometry (InSAR) offers new interesting possibilities for research in sea ice radar scattering and sea ice mechanics. A case study of this is presented from the Baltic Sea in late March 1992. Interferometric coherence is mainly dependent of the temporal characteristics of the scattering sources in sea ice. Different areas with different scattering properties were examined and the present data indicates that more field data is necessary to fully understand the InSAR coherence over sea ice. However, some interesting features were noted. Over low-salinity ice, backscattering and coherence seems to be related, high backscatter areas are more unstable than low backscatter areas. Over areas with surface roughness scattering, the scattering is relatively stable and also that a snow cover seems to retain the coherence over such areas. Interferometric phase measurements are dependent on small deformations of the ice pack. Fast ice which is (nearly) stationary experiences small discontinuous slips and deformations. Interferometric phase measurements are very sensitive to these slips, displacements and deformations and will provide new insight into the rheology for fast ice and how the fast ice starts to move. How the fast ice starts to move is one of the major problems in sea ice mechanics research and there is not much earlier data on the subject. In the present case, the ice was nearly stationary as the stresses were below the yield limit under the low forcing conditions. Two ice floe compressions have been observed and the strains are believed to be viscous with a viscosity value at approximately 1013-1014kg (ms). Both the interferometric phase and the coherence measurements over ice are believed to be of great value in future backscattering models and sea ice mechanics models.     

Efficient mitigation of atmospheric phase effects in repeat-pass InSAR measurements

The problem of atmospheric phase effects is currently one of the most important limiting factors for widespread application of repeat-pass interferometric synthetic aperture radar (InSAR) measurements. Due to the extraordinary complexity of the atmospheric inhomogeneity and turbulence, it is generally difficult to obtain satisfactory mitigation of the atmospheric phase effects in repeat-pass InSAR measurements. In recent years, several methods have been developed for mitigating the atmospheric phase effects. An effective approach is interferogram stacking, which is based on stacking independent interferograms. However, as many as 2n images are required to generate n interferograms and the atmospheric delay errors of the stacked interferogram decrease only with the square root of the number of interferograms in the conventional interferogram stacking method, which is not very efficient. In order to efficiently mitigate the atmospheric phase effects on the stacked interferogram in repeat-pass InSAR measurements, we propose a relay-interferogram stacking method. Compared with the conventional method, this method not only can efficiently mitigate atmospheric phase effects on the stacked interferogram, but also greatly decreases the number of required synthetic aperture radar (SAR) images. The key element is that the first and the last SAR images are selected from the periods of similar meteorological conditions. In addition, we present an application of the approach to the study of ground subsidence in the area around Beijing, China.     

InDeandCoE: A framework based on multi-scale feature fusion and residual learning for interferometric SAR remote sensing image denoising and coherence estimation

Synthetic aperture radar (SAR) interferometry is a high-resolution microwave remote sensing imaging method. Over the past two decades, many researchers working on remote sensing have applied this technology in various disciplines, including environmental monitoring, disaster monitoring, and elevation mapping. However, due to the existence of many influencing factors in the acquisition stage, such as atmospheric humidity and temperature, the reflected wave signals from the ground will be disturbed when received by remote sensing satellites. The presence of noise in interferograms is inevitable. Therefore, the accuracy of interferometric SAR phase denoising and coherence estimation has a decisive impact on the validity of subsequent processing results. In this paper, we pioneer the use of a nested U-net as a feature extractor for interferometric SAR phase and coherence. In addition, we build a phase filter and a coherence estimator by using the residual learning module. With the aim of determining the unique non-local similarity of InSAR images, we use non-local convolution and channel attention mechanisms to extract features in different dimensions of the interferogram. Through quantitative and qualitative experiments, the proposed method performs better in phase denoising and coherence estimation than state-of-the-art methods.     

Passive microwave imagery of sea ice at 33 GHz

Passive microwave (33 GHz) imagery has been examined to ascertain if changes in radiometric temperature can be used to detect and classify sea ice of varying age and thickness, and in various stages of formation, deformation, and weathering. The high-resolution imagery was compared with simultaneous aerial photography to allow for detailed correlation of surface features. The lowest radiometric temperatures were displayed by open water, new ice (presumably moist surface), and multiyear ice floes which suggested a history of high internal stress. The pattern of low radiometric temperatures in areas of multiyear ice suggest that internal stress may cause changes in crystal structure which result in lowered radiometric temperatures. The highest radiometric temperatures were displayed by thin ice (presumably without a wet surface), new ridges, and frozen melt ponds. The older thicker areas of first-year ice appeared to have lower radiometric temperatures than the thinner areas of first-year ice. Individual multiyear ice floes often displayed a wide range of radiometric temperatures. This variation has been attributed to changes caused by melting and erosional processes, and from stresses caused by ice floe collisions. Observations of microwave images of old ice floes indicate that they are somewhat transparent to microwave radiation, and that the primary source of the radiation is not from the surface but from within the ice column.     

Land deformation monitoring using coherent target-neighbourhood networking method combined with polarimetric information: a case study of Shanghai,China

In this article, an advanced approach for land deformation monitoring using synthetic aperture radar (SAR) interferometry combined with polarimetric information is presented. The linear and nonlinear components of the deformation, the error of the digital elevation model (DEM) and the atmospheric artefacts can be achieved by a coherent target (CT)-neighbourhood networking approach. In order to detect recent land deformation in Shanghai, China, 12 ENVISAT advanced synthetic aperture radar (ASAR) alternating polarization images acquired from January 2006 to August 2008 are employed for deformation analysis. Over a 2.5-year period, two deformation velocity fields from HH and VV modes over Shanghai are derived using the CT-neighbourhood networking SAR interferometry (InSAR), then integrated into a final deformation map by a fusion scheme. It is found that the annual subsidence rates in the study area range from??20 to 10 mm year^?1 and the average subsidence rate in the downtown area reaches??7.5 mm year^?1, which is consistent with the local government statistics published in 2007.     

多模式雷达在矿区沉降监测中的应用研究

合成孔径雷达差分干涉测量(DInSAR)技术在地表形变监测方面已得到广泛应用。介绍了将差分InSAR技术运用于矿区地表沉降监测,获得了河北峰峰煤矿地表Envisat/ASAR和ALOS/PALSAR的雷达形变干涉相位图,并对Envisat C波段和ALOS L波段的形变干涉相位图进行了相干特性和相位特性的分析。通过综合考虑C波段和L波段的优势与不足,将两者联合使用,实验表明利用多模式雷达数据对矿区地表沉降进行检测的可行性。同时,通过对雷达干涉相位图的分析,能够及时提供正在进行地下开采活动的矿区地理位置。     

Insight into ground deformations at Lascar volcano (Chile) from SAR interferometry, photogrammetry and GPS data: Implications on volcano dynamics and future space monitoring

We present a detailed study of Lascar volcano (Chile) based on the combination of satellite, aerial and ground-based data, in order (i) to better characterize the deformation style of Andean explosive volcanoes, and (ii) to provide new insights on the potential of space techniques to monitor active volcanic deformations on such edifices. Lascar is one of the most active volcanoes in Central Andes characterized by a recent cyclic activity. Additionally, it is located in favourable conditions for radar imaging. Lascar thus offers very good conditions for studying large to small scale ground deformations associated with volcano dynamics. The analysis of InSAR (Synthetic Aperture Radar interferometry) time series data from the European and Japanese satellites (ERS, JERS) acquired between 1993 and 2000, encompassing three eruptive events, confirmed the absence of broad far-field deformation signal. Thus during the recent activity of Lascar we discard significant magmatic input at depth. The following approaches were used to improve the InSAR signal / noise ratio in order to detect possible local deformation. We carried out a quantitative evaluation of the potential tropospheric contribution in INSAR interferograms for the Salar de Atacama-Lascar area using radar (ASAR-ENVISAT) and spectrometer (MODIS) data. We also used an accurate aerial photogrammetric and GPS constrained DEM in our InSAR data reprocessing. We find a co-eruptive ground-deformation confined into the summit crater for the 1995 eruption. This deformation has spatial dimension of 500 by 400 m and relates to a subsidence of crater floor up to 17 mm. We interpret it as pressure or volume decrease at subsurface levels below the active crater. Our study made it possible to image a new near-field volcanic deformation confined within the summit crater of the Lascar volcano. It also demonstrates that the combination of precise photogrammetry DEM and INSAR data can significantly improve our ability to remotely sense subtle surface deformation on these explosive volcanoes. This methodology might contribute to better understand volcano dynamics and to complement their monitoring in remote areas.     

Monitoring land deformation in Changzhou city (China) with multi-band InSAR data sets from 2006 to 2012

Over exploitation of groundwater in Changzhou city, China can cause land deformation, which in turn proves detrimental to the urban infrastructure. In this study, multi-band synthetic aperture radar (SAR) data sets (C-band Envisat ASAR, L-band ALOS PALSAR, and X-band COSMO-SkyMed) acquired from 2006 to 2012 were analysed using the synthetic aperture radar (SAR) interferometry (InSAR) time-series method to investigate the relationship between spatial–temporal distribution of land deformation and groundwater exploitation. Annual deformation rate inferred from multi-band interferograms ranges from ?58 to 24 mm year^?1. Levelling-survey data were used to validate the multi-band InSAR measurements. The results showed that these two types of measurements were generally in agreement. Correlating groundwater-table and multi-band InSAR measurements at six groundwater-well stations showed that with the rise of the water table, the land rebounded. But in some areas with larger subsidence, continual subsidence was observed even though the water table rose after the prohibition of groundwater exploitation. This may have been caused by the hysteresis effect due to the consolidation of strata (especially for the creep deformation). Our study provides scientific evidence on the management of groundwater extraction and the assessment of land-subsidence hazards.     

Monitoring of slow ground deformation by ERS radar interferometry on the Vauvert salt mine (France): Comparison with ground-based measurement

The differential SAR Interferometry (DInSAR) technique has been applied to a test site near Vauvert (France) to detect and monitor ground deformation. This site corresponds to the location of an industrial exploitation of underground salt using the solution mining technique. An area of subsidence has been observed using in situ measurements. Despite conditions unfavorable for InSAR because of the vegetal cover, we show that radar remote sensing observations provide valuable information which substantially improves our knowledge of the phenomenon. An adaptive phase filtering process has been used to improve the coherence level. In particular, our study shows that the geometry of the subsidence bowl is different to that previously assumed using ground-based techniques only. The size of the subsidence bowl (8 km) is larger than expected. This information will be useful for further modeling of the deformation and to improve the coverage of the in situ measurement networks. It also shows that radar interferometry can be used for the long-term monitoring of such sites and to predict potential environmental issues.     

Sea ice altimeter processing scheme at the EODC

Sea ice presents a serious impediment to both shipping and off-shore operations in the polar regions. Since sea ice conditions can change within a matter of hours, near real time monitoring is required. Airborne data are available in some areas, but collection is expensive and coverage limited. Satellite images can provide wider coverage, but cloud cover, darkness and the need for rapid processing and dissemination can limit their use. Information on sea ice cover over longer periods is needed for global climate monitoring. Microwave sensors provide the most practical means of monitoring global sea ice cover since they can operate both at night and day and observe through clouds. Previous studies have concentrated on the use of passive microwave data.

Here we discuss the routine monitoring of sea ice using the ERS-1 radar altimeter. The low data rate and somewhat simple nature of the data, lend themselves to the mapping of global sea ice cover and to operational applications.

We review the processing adopted at the U.K. EODC.     

Persistent Scatterers Interferometry Hotspot and Cluster Analysis (PSI-HCA) for detection of extremely slow-moving landslides

The synthetic aperture radar (SAR) interferometry (InSAR) technique has already shown its importance in landslide mapping and monitoring applications. However, the usefulness of traditional differential InSAR applications is limited by disturbing factors such as temporal decorrelation and atmospheric disturbances. The Persistent Scatterers Interferometry (PSI) technique is a recently developed InSAR approach. It generates stable radar benchmarks (namely persistent scatterers, PSI point targets) using a multi-interferogram analysis of SAR images. The PSI technique has the advantage of reducing temporal decorrelation and atmospheric artefacts. The PSI technique is suitable for the investigation of extremely slow-moving landslides due to its capability to detect ground displacements with millimetre precision. However, the interpretation of PSI outputs is sometimes difficult for the large number of possible persistent scatterers (PSs). A new approach of PSI Hotspot and Cluster Analysis (PSI-HCA) is introduced here in order to develop a procedure for mapping landslides efficiently and automatically. This analysis has been performed on PSs in hilly and mountainous areas within the Arno river basin (Italy). The aim is to use PSs processed from 4 years (2003–2006) of Radarsat images for identifying areas preferentially affected by extremely slow-moving landslides. The Getis–Ord G_i *?statistic is applied in the study for the PSI-HCA approach. The velocity of PSs is used as weighting factor and the G_i *?index is calculated for each single point target. The results indicate that both high positive and low negative G_i *?values imply the clustering of potential mass movements. High positive values suggest the moving direction towards the sensor along the satellite line-of-sight (LOS), whereas low negative values imply the movement away from the sensor. Furthermore, the kernel function is used to estimate PS density based on these derived G_i *?values. The output is a hotspot map which highlights active mass movements. This spatial statistic approach of PSI-HCA is considered an effective way to extract useful information from PSs at a regional scale, thus providing an innovative approach for rapid mapping of extremely slow-moving landslides over large areas.     

Integrated analysis of PALSAR/Radarsat-1 InSAR and ENVISAT altimeter data for mapping of absolute water level changes in Louisiana wetlands

Interferometric Synthetic Aperture Radar (InSAR) has been used to detect relative water level changes in wetlands. We developed an innovative method to integrate InSAR and satellite radar altimetry for measuring absolute or geocentric water level changes and applied the methodology to remote areas of swamp forest in coastal Louisiana. Coherence analysis of InSAR pairs suggested that the HH polarization is preferred for this type of observation, and polarimetric analysis can help to identify double-bounce backscattering areas in the wetland. ENVISAT radar altimeter-measured 18-Hz (along-track sampling of 417 m) water level data processed with regional stackfile method have been used to provide vertical references for water bodies separated by levees. The high-resolution (~ 40 m) relative water changes measured from ALOS PALSAR L-band and Radarsat-1 C-band InSAR are then integrated with ENVISAT radar altimetry to obtain absolute water level. The resulting water level time series were validated with in situ gauge observations within the swamp forest. We anticipate that this new technique will allow retrospective reconstruction and concurrent monitoring of water conditions and flow dynamics in wetlands, especially those lacking gauge networks.     

Spatial and temporal patterns in Arctic river ice breakup observed with MODIS and AVHRR time series

The timing of spring river-ice breakup, a major annual event for physical, biological, and human systems on Arctic rivers, has been used to infer regional climate variations over the past century or more. Most observations of ice breakup are recorded as point data taken from selected ground-based stations. It is unknown whether these point observations are fully representative of breakup patterns elsewhere along the course of a river. Here, daily time series of moderate resolution imaging spectroradiometer (MODIS) and advanced very high resolution radiometer (AVHRR) satellite images are used to remotely sense spatial and temporal patterns in ice breakup along 1600-3300 km lengths of the Lena, Ob', Yenisey, and Mackenzie Rivers. The first day of predominantly ice-free water is visually identified and mapped for ten years (1992-1993, 1995-1998, and 2000-2003), with a mean precision of ±1.75 days. The derived breakup dates show high correlation with ground-based observations, although a slight trend towards earlier satellite-derived dates can be traced to differences in the way ice breakup date is defined. Large ice jams are often observed, particularly at confluences, although smaller ice jams may not be visible due to the limited spatial resolution of the imagery used. At the watershed scale, spatial patterns in breakup seem to be primarily governed by latitude, timing of the spring flood wave, and location of confluences with major tributaries. Interestingly, channel-scale factors such as slope, width, and radius of curvature, which are known to influence ice breakup at the reach scale, do not appear to be major factors at the scale observed here. The degree of similarity between interannual trends in breakup date at distant points along a river is generally high, which supports the use of point-scale data to infer regional climate variations. This similarity does not hold true for the Mackenzie River, where substantial spatial differences in breakup trends are observed. A new variable, spatially integrated breakup date (d_i), uses weighted spatial averaging to provide a more encompassing measure of breakup timing. The Ob' and Yenisey Rivers show similar trends in spatially integrated breakup date from year to year. In contrast, the Mackenzie and Lena show a remarkably consistent negative correlation, here attributed to sea surface temperature anomalies associated with the Pacific Decadal Oscillation Index.     

Radar backscatter signatures of thin sea ice in the central Arctic

Monitoring the early stages of sea ice growth is vital because the changing sea ice cover controls the heat exchange between the ocean and air. New ice growth is also responsible for adding brine into the upper portion of the water column. Thin sea ice also significantly affects the albedo of the surface as it changes from a sea surface to an ice surface.

To investigate the ability of radars to map thin ice we performed radar backscatter measurements early in the fall freeze-up as part of the International Arctic Ocean Expedition '91 (IAOE'91(. We collected data over the thin ice types of light nilas, dark nilas, and pancake/slush ice using a ship-based, C-band FM radar with all four linear polarizations.

Our results indicate that radars must be able to measure σ⁰ as low as - 30 dB for VV polarization and - 34 dB for HH polarization. The noise-equivalent σ⁰ fortheERS-1 Synthetic-Aperture Radar is - 24 dB and for RADARS AT is -23 dB. This implies that these sensors are not capable of monitoring thin sea ice types such as dark nilas and grease ice but may be able to detect slightly thicker ice, such as light nilas and pancake ice, due to their higher backscatter.     

A comparison of Arctic sea ice freeboard products from Sentinel-3A and CryoSat-2 data

ABSTRACT

A number of satellite altimeters have been used to measure Arctic sea ice freeboard and to study its changes over the past decades (1992-present). In order to produce long-term time series of sea ice freeboard data set, it is essential to investigate the difference and consistency between different satellite-based sea ice freeboard data sets. Hence in this study, the comparison between ice freeboard products from altimeters on board Sentinel-3A and CryoSat-2 is constructed from February 2017 to January 2018 excluding summer months. The comparisons of echo waveform shapes and along-track radar freeboard estimates suggest that the freeboard difference between these two sensors is caused by the signal range bin number and the chosen retrackers for different surface types (leads and sea ice floes). Monthly gridded freeboard results show that mean values of two different satellite altimeters agree each other reasonably over the whole study period. In general, Sentinel-3A data set shows lower freeboard estimates than CryoSat-2 data set, this phenomenon is found in both First-Year ice (FYI) and Multi-Year ice (MYI) regions. No ice-type-related difference indicates the good consistency between Sentinel-3A and CryoSat-2 data sets. Over the whole period, mean freeboard estimates for the entire Arctic differs generally by not more than 0.07 m between Sentinel-3A and CryoSat-2. Compared to airborne Operation IceBridge (OIB) data, Sentinel-3A has closer sea ice freeboard estimates than CryoSat-2.