Radar Altimeter Research Articles

Radar altimetry observations have pinpointed 85 active subglacial lakes, shedding light on how water moves beneath the Antarctic Ice Sheet.

A review of the evolution of satellite radar altimetry for hydrological Monitoring: Water level and discharge estimation

Assessing the accuracy of satellite radar altimetry for inland water level monitoring

Microwave remote sensing (RS) allows one to obtain information at any time of day, regardless of solar illumination and in a wide range of meteorological conditions, which gives it an advantage over Earth RS methods using the optical and thermal ranges of electromagnetic radiation. At the same time, the potentialities of microwave methods are not fully utilized in commercial RS spacecraft. This paper analyzes trends in the development of microwave RS aimed at identifying the prospects for the use of these methods in commercial RS satellite constellations. Microwave radiometry, altimetry, scatterometry, and radar imaging are considered. It is shown that, due to government contracts, microwave radiometers will be massively used onboard U.S. small commercial weather satellites in the next few years. On-orbit testing of hyperspectral microwave radiometers, which are also of interest for national security and defense applications, may be expected in the coming years. The use of radar altimeters and scatterometers onboard small satellites is currently technically possible, but not in demand. These instruments are designed primarily for monitoring the ocean surface. The onboard use of altimeters and scatterometers may be caused by the needs of military meteorology and the need to improve the quality of weather forecasts in maritime theaters of war. The most actively developing line of microwave RS is space imagery using synthetic aperture radars. In the last decade, single large-size reconnaissance radar satellites have been replaced by multi-satellite constellations of small spacecraft. Of special interest is the development of operational digital models of the Earth's surface as one of the new areas of radar data application. A transition to higher-frequency bands for imaging, which has been outlined in a number of Chinese companies, will make it possible to achieve a higher spatial resolution at a lower cost.

Abstract. For the first time, a comparison of altimetry-derived snow depth estimates between dual-frequency spaceborne and near-coincident multi-frequency airborne estimates is conducted using data from the recent under-flight of a CryoSat-2 and ICESat-2 (CRYO2ICE) orbit by a simultaneous airborne campaign over the Weddell Sea in December 2022 carrying Ka-, Ku-, C/S-band radars and a scanning near-infrared lidar. From this unique combination of airborne sensors, the accuracy of snow depth captured by the near-coincident CRYO2ICE orbits can be evaluated. The CRYO2ICE snow depth achieved along the orbit was, on average, 0.34 m, which is within 0.01 m from passive-microwave-derived observations and 0.12 m from a model-based estimate. The retrieval methodology appears to play a significant role, which we suspect is highly dependent on the classification and filtration schemes applied to remove potentially ambiguous altimetry observations. Comparison with airborne snow depths at 25 km segments showed correlations of 0.51–0.53, a bias of 0.03 m, and root-mean-square deviation of 0.08 m when using the airborne lidar scanner as air–snow interface and C/S-band at maximum amplitude at the snow–ice interface. To understand how comparisons across ground, air, and space shall be conducted, especially in preparation for the upcoming dual-frequency radar altimeter mission Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL), it is critical that we investigate the impact of different scattering mechanisms at varying frequencies for diverging viewing geometries considering dissimilar spatial and range resolutions.

AltiCube+: A low-cost long fixed-baseline radar altimeter solution based on cubesats on-orbit assembly

Subglacial lake activity influences ice sheet flow, grounding line discharge and ice shelf basal melting. Although 146 active subglacial lakes have been detected in Antarctica via ice surface elevation change associated with their activity, only 36 fill-drain cycles have been observed worldwide, and knowledge of these mechanisms is limited. Here, we use a decade of CryoSat-2 radar altimetry to detect 85 active subglacial lakes in Antarctica, documenting 37 and 34 complete draining and filling events respectively. We delineate time-varying boundaries of subglacial lake activity and investigate their variability over time. Our observations increase the number of known active subglacial lakes in Antarctica by 58%, with six of these located within 8 km of the grounding zone. We observe five subglacial lake networks, with concurrent upstream drainage and downstream filling, and 25 clusters of lakes, improving our knowledge of interconnected subglacial hydrological pathways.

Electromagnetic Field Analysis and EMI Evaluation of Radio Altimeter and 5G Sub-6 GHz Band in Aircraft Environment

Field test campaign of Chandrayaan-3 radar altimeter

Abstract. Over the past three decades, there has been a 4.5-fold increase in the loss of ice from the Greenland and Antarctic Ice Sheets, resulting in an enhanced contribution to global sea level rise. Accurately tracking these changes in ice mass requires comprehensive, long-term measurements, which are only feasible from space. Satellite radar altimetry provides the longest near-continuous record of ice sheet surface elevation and volume change, dating back to the launch of ERS-1 in 1991, and maintained through the successive ERS-2, Envisat, CryoSat-2, and Sentinel-3 missions. To reliably constrain multi-decadal trends in ice sheet imbalance, and to place current observations within a longer-term context, requires continued efforts to optimise the processing of data acquired by the older historical missions and to evaluate the accuracy of these measurements. Here, we present new ERS-1, ERS-2, and Envisat altimeter datasets, comprising measurements of ice sheet elevation spanning two decades. This new observational record has been derived using consistent and improved retrieval methods, including enhancements to key Level-2 processing steps such as waveform retracking and echo relocation. Through comparison with independent airborne datasets, we undertake a comprehensive assessment of the accuracy of these measurements and demonstrate the improvements delivered relative to previously available products. With this updated processing, we find that all missions achieve sub-metre (<0.85 m) median elevation biases and dispersion of elevation differences relative to coincident airborne data. These new along-track datasets will be of benefit to a broad range of applications, including the quantification of ice sheet mass imbalance, investigations of the processes driving contemporary ice loss, and the constraint of numerical ice sheet models.

ABSTRACT River discharge is one of the most important hydrological quantities that provides freshwater for human use and ecosystems. Ground-based measurements of discharge are often expensive, and for many rivers globally, discharge data may exist but are not publicly available for various reasons. Therefore, spaceborne measurements have been pursued as alternatives. Satellite radar altimetry is an extremely valuable source of information for estimating river discharge. While numerous case studies exist, the question of which locations are preferable to build altimetry-based stage-discharge rating curves remains less studied. The goal of this study was to investigate this issue by constructing a large set of stage-discharge rating curves across the globe. We derived the water levels of global rivers wider than 300 m based on the Sentinel-3 altimeters. Based on historical discharge records and altimetry-derived water levels, 737 rating curves were constructed using a Bayesian hierarchical approach. The results indicate a median Kling-Gupta Efficiency (KGE) of 0.76 and a normalized root-mean-squared error (NRMSE) of 33.8%, demonstrating the reliability of this method. Notably, water level variability and width-to-depth ratio emerged as key factors influencing rating curve accuracy, suggesting that prioritizing river segments with significant water level fluctuations can enhance the precision of discharge estimation. These findings provide valuable guidance for selecting satellite tracks to augment global river discharge monitoring networks. The reliable rating curves could also facilitate near real-time forecasting of river discharge.

Abstract. The development and evolution of satellite altimetry and operational oceanography are very closely linked. By providing all weather, global, and real-time observations of sea level, a key variable to constrain ocean analysis and forecasting systems, satellite altimetry has had a profound influence on the advancement of operational oceanography. Over the past 20 years, satellite altimetry has been providing a continuous observation of the ocean in near-real time. From the launch of Jason-1 in 2001 to the launch of SWOT in 2022, satellite altimetry capabilities have been regularly improved through refinements of geophysical corrections and processing algorithms, including real-time processing, as well as through the evolution of altimeter radar technology (synthetic aperture radar (SAR) mode, swath altimetry). Resolution has also improved through the use of multiple altimeters and, now, swath altimetry. In parallel, major improvements in ocean prediction systems have occurred, from the Global Data Assimilation Experiment (GODAE) demonstration in the 2000s to the fully operational systems now serving a large range of applications in the 2020s. This paper provides an overview of the development and evolution of satellite altimetry and operational oceanography over the past 20 years in the context of the Mercator Ocean prediction centre, DUACS (data unification and altimeter combination system), and the EU Copernicus Marine Service. The impact of altimetry on the performances of ocean prediction systems (based on observing-system evaluations (OSEs) and observing-system simulation experiments (OSSEs)) is reviewed. The future contribution of swath altimetry to ocean prediction is also discussed. Prospects for the next decade are addressed as part of our conclusions.

Abstract. Satellite radar altimeters provide crucial insights into polar oceans and their sea ice cover, enabling the estimation of sea level, sea ice freeboard, and thickness. These retrieval algorithms depend on accurate discrimination between radar altimeter waveforms from sea ice and ocean surfaces in heterogeneous and dynamic surface conditions. A further and less mature step is classifying different sea ice types in addition to the ice/ocean discrimination. We aim to develop new methods for a novel multi-category sea ice and ocean surface classification directly from satellite radar altimeter data to improve sea ice climate data records. Traditional waveform representations are limited to a small set of parameters, leading to information loss. Moreover, machine learning models for sea ice classification often depend on supervised training, which is vulnerable to uncertainties in labeled data, especially in polar regions. To address these limitations, we explore self-supervised learning methods to optimize waveform representations, which can capture more detailed information for a classification with finer granularity. Furthermore, they do not require labeled data, which is not available at the spatial coverage and resolution of radar altimeter waveforms. We apply these techniques to SRAL data from the Sentinel-3 mission. We show that the information preserved in the latent space of an auto-encoder enhances the feature space of traditional waveform parameters, improving the subsequent classification process, when comparing our results to available sea ice charts and other remote sensing products. Our results demonstrate better generalization compared to supervised approaches.

Abstract Antarctic sea ice has seen recent rapid declines in extent, but it remains unclear whether this is accompanied by thinning. Due to the relative abundance and complexity of overlying snow on sea ice, radar altimetry methods routinely deployed for sea ice thickness estimation in the Arctic are difficult to apply in Antarctica. We present nadir‐looking radar waveforms from the first deployment of the KuKa surface‐based radar on Antarctic sea ice, specifically multi‐year sea ice in the Weddell Sea marginal ice zone with a thick snow cover. Coincident snow pits revealed thick layers of snow which were exposed to the summer melt season and superimposed ice. Our instrument detects only very small amount of co‐polarized radar backscatter from the sea ice surface, suggesting that conventional satellite altimeters may not always range to this interface. However, polarimetric snow depth determination performs well, with of 0.76 between measured and KuKa‐estimated snow depths.

A novel Multiple Persistent Peaks (MPP) retracker to improve global inland water level monitoring from satellite radar altimetry

Abstract. The surface elevation of the Greenland Ice Sheet is constantly changing due to the interplay between surface mass balance processes and ice dynamics, each exhibiting distinct spatiotemporal patterns. Here, we employ satellite and airborne altimetry data with fine spatial (1 km) and temporal (monthly) resolutions to document this spatiotemporal evolution from January 2003 to August 2023. To estimate elevation changes of the Greenland Ice Sheet (GIS), we utilize radar altimetry data from CryoSat-2 and EnviSat, laser altimetry data from the ICESat and ICESat-2, and laser altimetry data from NASA's Operation IceBridge Airborne Topographic Mapper. We produce continuous monthly ice surface elevation changes from January 2003 to August 2023 on a 1 km grid covering the entire GIS. We estimate cumulative ice loss of 4352 Gt ± 315 Gt (12.1±0.9 mm sea level equivalent) during this period, excluding peripheral glaciers. Between 2003 and 2023, the ice sheet land-terminating margin underwent a significant cumulative thinning of several meters. Ocean-terminating glaciers exhibited thinning between 20–40 m, with Jakobshavn Isbræ experiencing an exceptional thinning of nearly 70 m. This dataset of fine-resolution altimetry data in both space and time will support studies of ice mass loss and will be useful for GIS modeling. To validate our monthly mass changes of the Greenland ice sheet, we use mass change from satellite gravimetry and mass change from the input–output method. On multiannual timescales, there is a strong correlation between the time series, with R values ranging from 0.88 to 0.92 (https://doi.org/10.5061/dryad.s4mw6m9dh, Khan et al., 2025)

Abstract The advancement of multi-mission satellite altimeters has greatly enhanced the conventional approach of using a tide gauge station to establish a localised hydrographic datum. However, there are still several limitations in continually depending on a small number of sparsely distributed tide gauge stations and wide satellite altimeter track missions, even with its continuous monitoring of ocean data at local and worldwide coverage. Such limitation has left a region farther away from the satellite altimeter track and tide gauge stations without any hydrographic data. Hence, this study aims to develop Malaysia’s Continuous Hydrographic Datum (MyCHD) by combining the satellite altimeter, tide gauge station, and global hydrodynamic model (GHM). In addition, the reliability of MyCHD was also assessed to determine the rate of improvement by incorporating GHM as additional hydrographic data. The research methodology involves collecting data from the Department of Survey and Mapping Malaysia (DSMM) tide gauge stations along Malaysia’s coastline, as well as satellite altimetry data from TOPEX, Jason-1, Jason-2, and GEOSAT Follow-On (GFO) via the Radar Altimeter Database System (RADS). Additionally, Indian Ocean GHM data from Oregon State University (OSU) was also utilised. The tide gauge, satellite altimetry, and GHM datasets encompass 26 years of tidal observations, spanning from 1993 to 2018. All hydrographic data were processed using harmonic analysis in Unified Tidal Analysis and Prediction (UTide) within MATLAB to establish the hydrographic datum. The derived Lowest and Highest Astronomical Tide (LAT and HAT) from tide gauge, satellite altimetry, and GHM data were referred to the Mean Sea Level (MSL) for compatibility in data integration; these were designated as LATMSL and HATMSL, respectively. Then, using ArcGIS software, the LATMSL and HATMSL were interpolated using Inverse Distance Weighting (IDW). In contrast to the integration of tide gauge and satellite altimeter, the statistical assessment showed that the integration of tide gauge, satellite altimeter, and GHM has a better agreement with the DSMM tide gauges, with a Root Mean Square Error (RMSE) of ± 0.671 m for LATMSL and ± 0.370 m for HATMSL. In percentage terms, incorporating GHM data with tide gauge and satellite altimeter in establishing MyCHD has significantly improved its reliability by 17 % for LATMSL and 30 % for HATMSL respectively. In conclusion, the hydrographic datum is now available at any coordinate along Malaysia’s coast with the establishment of MyCHD (LATMSLand HATMSL).

An essential component of managing, administering, and conserving aquatic environments is the assessment and tracking of water quality. To illustrate the difficulties, constraints, and requirements for RS technology’s future integration into GIS mapping, this study examines the most recent developments in remotely sensedbased investigations on water quality monitoring. Several studies underline the utilities of different remote sensing technologies (e.g., Landsat TM/ETM+, hyperspectral imagery, radar altimetry) in monitoring water quality constituents’ turbidity, chlorophyll concentration, and sediment load. In this review, the revolution that remote sensing and GIS technologies have been highlighted, that have played crucial role in enhancing our ability to monitor water quality for improved catchment management practices which includes sustainable utilization of these critical resources. This scholarly review paper investigates the amalgamation of remote sensing and Geographic Information Systems (GIS) technologies for the proficient surveillance of water quality. It delineates a spectrum of remote sensing methodologies employed to evaluate critical water quality indicators, such as chlorophyll-, turbidity, and thermal variations in water. The work accentuates the significance of both spatial and temporal fluctuations in water quality evaluations, underscoring the necessity for the integration of remote sensing data with in situ measurements. Furthermore, it confronts the challenges and limitations inherent in contemporary methodologies, suggesting prospective research trajectories for the augmentation of water quality management practices. This scholarly review paper examines the innovations in the surveillance of water quality through the utilization of remote sensing and GIS. It elucidates diverse methodologies adopted for the assessment of water parameters, encompassing temperature, turbidity, and chemical composition while leveraging satellite imagery and aerial data. The synthesis of remote sensing with GIS enhances spatial analysis and visualization, thereby facilitating the effective governance of water resources. The manuscript reviews case studies that illustrate the efficacy of these technologies in real-time monitoring and informed decision-making for sustainable water quality management.

Egypt is confronted with a number of hazardous environmental incidents, mainly sea level rise (SLR) and land subsidence. The Nile Delta is a low-relief surface that is particularly vulnerable to flooding and SLR, making it important to study inundation scenarios for the region. Potential social and economic consequences of this anticipated sea encroachment were projected utilizing (1) crustal deformation calculations derived from the time series analysis using the Persistent Scatterer Interferometry (PSI) technique based on Least Squares Estimation. (a stack of 191 Sentinel-1 ascending scenes), and eight permanent stations of Global Navigation Satellite System (GNSS); both spanning the period 2014–2019, (2) SLR values using Satellite Altimetry, and (3) a high-resolution digital elevation model (TerraSAR-X/TanDEM-X). The The key findings of this study are summarized as follows; (1) large cities and urban regions adjacent to the two main active branches of the Nile Delta (Rosetta and Damietta) experienced the majority of subsidence rates, (2) the cities of Damietta, Mansoura and Port said (eastern side of the Nile Delta) experienced the maximum rates of subsidence (− 11 ± 0.6, − 8.9 ± 0.7, and − 6.3 ± 0.7 mm/year, respectively), (3) the cities of Shebin El Kom, Damanhour, Tanta, New-Damietta, Kafr El-Sheikh had moderate subsidence rates (− 3.2 ± 0.6, − 2.4 ± 0.7, − 4.2 ± 0.6, − 3.8 ± 0.7, − 3.2 ± 0.7 mm/year, respectively), (4) the Nile Delta subsidence seems to be dominated by anthropogenic reasons such as urbanization, ground water and hydrocarbon extraction, (5) the linear trend of sea level anomaly (SLA) from satellite altimetry data over the period from 1993 to 2019 along the Delta shoreline, the SLR is ~ 3.42 ± 0.5 mm/year, and (6) based on GIS tools and IDW interpolation, wide swaths of the northern Nile Delta would be flooded in the worst-case scenario, which would result in approximately 482 km2 being flooded in fifty years, 2433 km2 in one hundred years, and 3320 km2 in one hundred and fifty years due to the ongoing land subsidence and SLR of 3.4 mm/year.

Abstract Previously, the existence of seven active subglacial lakes was reported beneath Thwaites Glacier, one of West Antarctica's largest glaciers, predicted to significantly impact future sea level rise. In this study, we re‐evaluate the CryoSat‐2 satellite radar altimeter data from 2010 to 2022 and identify 27 subglacial lakes that were active during 2013–2014 and 2016–2018, including those previously observed. Our new observations reveal three lakes located downstream of the lakes identified previously that appear to drain 3 months prior to the drainage of upstream lakes in 2013. Despite being over 100 km apart, the observed lakes (from the farthest upstream to downstream) exhibited synchronized behaviors. The timing of lake activities shows a complex activity propagation pattern, featuring both upward and downward transmission, contrasting with the simpler, one‐directional flows observed in previous studies. While lake activities correlate with abrupt changes in ice speed near the grounding line, their impacts are limited, regardless of the total amount of drained water.