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Advances in Satellite Altimetry II

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A special issue of Remote Sensing (ISSN 2072-4292).

Deadline for manuscript submissions: closed (30 April 2024) | Viewed by 16228

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Prof. Dr. Stelios Mertikas

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Geodesy and Geomatics Engineering Lab, University Campus, Technical University of Crete, GR-73100 Chania, Crete, Greece
Interests: geodesy; satellite positioning, navigation; remote sensing; altimetry; calibration/validation; data analysis; sea level change; metrology; statistical process control
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Dr. Xiaoli Deng

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School of Engineering, Faculty of Engineering and Built Environment, The University of Newcastle (UON), University Drive, Callaghan, NSW 2308, Australia
Interests: satellite geodesy; astronomy and satellite positioning; satellite altimetry; coastal altimetry and its applications; satellite altimetry retracking algorithms; sea level rise; remote sensing; satellite geodesy in natural hazard mitigation
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Dr. Jérôme Benveniste

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European Space Agency (ESA-ESRIN), Directorate of Earth Observation Programmes, Largo Galileo Galilei, 1, I-00044 Frascati, Roma, Italy
Interests: earth observation; geodesy; geoid; oceanography; sea level; ocean dynamics; hydrology; river discharge; cryosphere; climate change; water cycle; GOCE; CryoSat; Sentinel-3; Sentinel-6; Sentinel-3NG-Topo; CRISTAL; MAGIC/NGGM
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Special Issue Information

Dear Colleagues,

For more than 40 years, satellite altimeters have observed the heights of the sea surface, rivers, lakes, ice, sea ice, etc., in respect to the center of the mass of the Earth, as well as the sea-ice freeboard, significant wave height (SWH) and the wind speed over the ocean. After the launch of the SEASAT mission in 1978, new altimetry technology and applications have emerged and expanded tremendously. Altimetry satellites, such as Sentinel-6 Michael Freilich, Sentinel-3, CryoSat-2, Jason-3, HY-2, Envisat, SARAL/AltiKa, IceSat-2, etc., observe and practically realize ranges by measuring time differences between the transmission and reception of an electromagnetic wave or photon. Earth surface heights are, thus, measured from space-borne instruments using radar or laser signals at an altitude of 800–1300 km with an accuracy of less than ± 1cm. New-generation sensors operate at different signal frequencies (Ku-band, Ka-Band and laser) and implement various measurement principles of pulse-limited, delay–Doppler (unfocused or fully focused), also called synthetic aperture radar (SAR), and SAR interferometric (with two antennas) altimetry.

Altimetry missions monitor the sea level, ocean dynamics, coastal regions, ice sheets, sea ice, inland waters (rivers, lakes, reservoirs, wetlands and discharge/runoff), terrain elevation, soil moisture and the marine geoid globally with a short or long revisit period. Many of these observed parameters of the Earth’s surface constitute essential variables for monitoring climate change. Nonetheless, to understand and predict climate variability and change, Earth observation satellites and observing systems have to generate data records of a sufficient length, consistency, continuity and stability. While satellite altimetry is moving into its operational phase with Sentinel-3 and Sentinel-6 of the European Copernicus Programme, a new principle of fiducial reference measurement systems has arisen to monitor the quality of data produced with metrology standards, to properly and seamlessly archive and distribute the retained data, and, most importantly, to closely follow the performance of observing systems along with their smooth integration of different satellite products.

In this Volume II of the Special Issue of “Advances in Satellite Altimetry”, we invite researchers and engineers from all disciplines to submit manuscripts presenting recent advances in the field of radar and laser altimetry, including recent and future altimetry missions (e.g., Sentinel-6 MF, ICESat, SWOT, Sentinel-3 Next-Generation, CRISTAL, Guanlan, HY-2, SmallSat constellations, etc.), their processing algorithms, calibration/validation, their contribution to the advancement of science and their applications in synergy with other sensors’ products. We also encourage the submission of review manuscripts exploiting historic altimetry records and their applications in the spatio-temporal monitoring of Earth’s systems on all scales.

This Special Issue is the second edition of the Special Issue “Advances in Satellite Altimetry”.

Prof. Dr. Stelios Mertikas
Dr. Xiaoli Deng
Dr. Jérôme Benveniste
Guest Editors

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Keywords

radar and laser altimetry
satellite altimetry
delay–Doppler (SAR) altimetry
interferometry
transponder, sea surface and lake surface calibration and validation
fiducial reference systems
remote sensing of ocean, inland water, cryosphere and mountain glaciers
integration of altimetry with other satellite sensors
sea level rise
surface currents and ocean dynamics with altimetry
coastal altimetry
polar altimetry
inland altimetry
marine geoid
ocean waves
wind speed over ocean

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Advances in Satellite Altimetry in Remote Sensing (19 articles)

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Research

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20 pages, 51888 KiB

Open AccessArticle

Introducing the Azimuth Cutoff as an Independent Measure for Characterizing Sea-State Dynamics in SAR Altimetry

by Ourania Altiparmaki, Samira Amraoui, Marcel Kleinherenbrink, Thomas Moreau, Claire Maraldi, Pieter N. A. M. Visser and Marc Naeije

Remote Sens. 2024, 16(7), 1292; https://doi.org/10.3390/rs16071292 - 6 Apr 2024

Cited by 2 | Viewed by 1599

Abstract

This study presents the first azimuth cutoff analysis in Synthetic Aperture Radar (SAR) altimetry, aiming to assess its applicability in characterizing sea-state dynamics. In SAR imaging, the azimuth cutoff serves as a proxy for the shortest waves, in terms of wavelength, that can be detected by the satellite under certain wind and wave conditions. The magnitude of this parameter is closely related to the wave orbital velocity variance, a key parameter for characterizing wind-wave systems. We exploit wave modulations exhibited in the tail of fully-focused SAR waveforms and extract the azimuth cutoff from the radar signal through the analysis of its along-track autocorrelation function. We showcase the capability of Sentinel-6A in deriving these two parameters based on analyses in the spatial and wavenumber domains, accompanied by a discussion of the limitations. We use Level-1A high-resolution Sentinel-6A data from one repeat cycle (10 days) globally to verify our findings against wave modeled data. In the spatial domain analysis, the estimation of azimuth cutoff involves fitting a Gaussian function to the along-track autocorrelation function. Results reveal pronounced dependencies on wind speed and significant wave height, factors primarily determining the magnitude of the velocity variance. In extreme sea states, the parameters are underestimated by the altimeter, while in relatively calm sea states and in the presence of swells, a substantial overestimation trend is observed. We introduce an alternative approach to extract the azimuth cutoff by identifying the fall-off wavenumber in the wavenumber domain. Results indicate effective mitigation of swell-induced errors, with some additional sensitivity to extreme sea states compared to the spatial domain approach. Full article

(This article belongs to the Special Issue Advances in Satellite Altimetry II)

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26 pages, 35515 KiB

Open AccessArticle

Optimal Configuration of Omega-Kappa FF-SAR Processing for Specular and Non-Specular Targets in Altimetric Data: The Sentinel-6 Michael Freilich Study Case

by Samira Amraoui, Pietro Guccione, Thomas Moreau, Marta Alves, Ourania Altiparmaki, Charles Peureux, Lisa Recchia, Claire Maraldi, François Boy and Craig Donlon

Remote Sens. 2024, 16(6), 1112; https://doi.org/10.3390/rs16061112 - 21 Mar 2024

Cited by 1 | Viewed by 1878

Abstract

In this study, the full-focusing (FF) algorithm is reviewed with the objective of optimizing it for processing data from different types of surfaces probed in altimetry. In particular, this work aims to provide a set of optimal FF processing parameters for the Sentinel-6 Michael Freilich (S6-MF) mission. The S6-MF satellite carries an advanced radar altimeter offering a wide range of potential FF-based applications which are just beginning to be explored and require prior optimization of this processing. In S6-MF, the Synthetic Aperture Radar (SAR) altimeter acquisitions are known to be aliased in the along-track direction. Depending on the target, aliasing can be tolerated or may be a severe impairment to provide the level of performance expected from FF processing. Another key aspect to consider in this optimization study is the unprecedented resolution of the FF processing, which results in a higher posting rate than the standard SAR processing. This work investigates the relationship between posting rate and noise levels and provides recommendations for optimal algorithm configurations in various scenarios, including transponder, open ocean, and specular targets like sea-ice and inland water scenes. The Omega–Kappa (WK) algorithm, which has demonstrated superior CPU efficiency compared to the back-projection (BP) algorithm, is considered for this study. But, unlike BP, it operates in the Doppler frequency domain, necessitating further precise spectral and time domain settings. Based on the results of this work, real case studies using S6-MF acquisitions are presented. We first compare S6-MF FF radargrams with Sentinel-1 (S1) images to showcase the potential of optimally configured FF processing. For highly specular surfaces such as sea-ice, distinct techniques are employed for lead signature identification. S1 relies on image-based lineic reconstruction, while S6-MF utilizes phase coherency of focalized pulses for lead detection. The study also delves into two-dimensional wave spectra derived from the amplitude modulation of image/radargrams, with a focus on a coastal example. This case is especially intriguing, as it vividly illustrates different sea states characterized by varying spectral peak positions over time. Full article

(This article belongs to the Special Issue Advances in Satellite Altimetry II)

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18 pages, 11662 KiB

Open AccessArticle

Estimating and Assessing Monthly Water Level Changes of Reservoirs and Lakes in Jiangsu Province Using Sentinel-3 Radar Altimetry Data

by Jia Xu, Min Xia, Vagner G. Ferreira, Dongmei Wang and Chongbin Liu

Remote Sens. 2024, 16(5), 808; https://doi.org/10.3390/rs16050808 - 26 Feb 2024

Cited by 7 | Viewed by 2737

Abstract

Generating accurate monthly estimations of water level fluctuations in reservoirs and lakes is crucial for supporting effective water resource management and protection. The dual-satellite configuration of Sentinel-3 makes it possible to monitor water level changes with great coverage and short time intervals. However, the potential of Sentinel-3’s Synthetic Aperture Radar Altimetry (SRAL) data to enable operational monitoring of water levels across Jiangsu Province on a monthly basis has not yet been fully explored. This study demonstrated and validated the use of Sentinel-3’s SRAL to generate accurate monthly water level estimations needed to inform water management strategies. The monthly water levels of lakes and reservoirs from 2017 to 2021 were produced using Sentinel-3 level-2 land products. Results showed that, compared with in situ data across eight studied lakes, all lakes presented R (Pearson correlation coefficient) values greater than 0.5 and Root Mean Square Error (RMSE) values less than 1 m. Notably, water level estimates for Tai Lake, Gaoyou Lake, and Luoma Lake were particularly accurate, with R above 0.9 and RMSE below 0.5 m. Furthermore, the monthly water level estimates derived from the Sentinel-3 data showed consistent seasonal trends over the multi-year study period. The annual water level of all lakes did not change significantly, except for Shijiu Lake, of which the difference between the highest and lowest water level was up to about 5 m. Our findings confirmed the water level observation ability of Sentinel-3. The accuracy of water level monitoring could be influenced by internal water level differences, terrain features, as well as the area and shape of the lake. Larger lakes with more altimetry sampling points tended to yield higher accuracy estimates of water level fluctuations. These results demonstrate that the frequent, wide-area coverage offered by this satellite platform provides valuable hydrological information, especially across remote regions lacking in situ data. Sentinel-3 has immense potential to support improved water security in data-scarce regions. Full article

(This article belongs to the Special Issue Advances in Satellite Altimetry II)

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42 pages, 18118 KiB

Open AccessArticle

The ESA Permanent Facility for Altimetry Calibration in Crete: Advanced Services and the Latest Cal/Val Results

by Stelios P. Mertikas, Craig Donlon, Costas Kokolakis, Dimitrios Piretzidis, Robert Cullen, Pierre Féménias, Marco Fornari, Xenophon Frantzis, Achilles Tripolitsiotis, Jérôme Bouffard, Alessandro Di Bella, François Boy and Jerome Saunier

Remote Sens. 2024, 16(2), 223; https://doi.org/10.3390/rs16020223 - 5 Jan 2024

Cited by 2 | Viewed by 2458

Abstract

Two microwave transponders have been operating in west Crete and Gavdos to calibrate international satellite radar altimeters at the Ku-band. One has been continuously operating for about 8 years at the CDN1 Cal/Val site in the mountains of Crete, and the other at the GVD1 Cal/Val site on Gavdos since 11 October 2021. This ground infrastructure is also supported at present by four sea-surface Cal/Val sites operating, some of them for over 20 years, while two additional such Cal/Val sites are under construction. This ground infrastructure is part of the European Space Agency Permanent Facility for Altimetry Calibration (PFAC), and as of 2015, it has been producing continuously a time series of range biases for Sentinel-3A, Sentinel-3B, Sentinel-6 MF, Jason-2, Jason-3, and CryoSat-2. This work presents a thorough examination of the transponder Cal/Val responses to understand and determine absolute biases for all satellite altimeters overflying this ground infrastructure. The latest calibration results for the Jason-3, Copernicus Sentinel-3A and -3B, Sentinel-6 MF, and CryoSat-2 radar altimeters are described based on four sea-surface and two transponder Cal/Val sites of the PFAC in west Crete, Greece. Absolute biases for Jason-3, Sentinel-6 MF, Sentinel-3A, Sentinel-3B, and CryoSat-2 are close to a few mm, determined using various techniques, infrastructure, and settings. Full article

(This article belongs to the Special Issue Advances in Satellite Altimetry II)

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26 pages, 30643 KiB

Open AccessArticle

Preliminary Results of Marine Gravity Recovery by Tiangong-2 Interferometric Imaging Radar Altimeter

by Meng Sun, Yunhua Zhang, Xiao Dong and Xiaojin Shi

Remote Sens. 2023, 15(19), 4759; https://doi.org/10.3390/rs15194759 - 28 Sep 2023

Cited by 4 | Viewed by 1649

Abstract

This paper presents for the first time the results of marine gravity recovery using the ocean observation data acquired by Tiangong-2 interferometric imaging radar altimeter (TG2 InIRA) which demonstrate not only the balanced accuracies of the north and east components of deflection of the vertical (DOV) as envisaged, but also the improved spatial resolutions of DOV compared with that by conventional altimeters (CAs). Moreover, much higher measurement efficiency owing to the wide-swath capability and the great potential in accuracy improvement of marine gravity field are also demonstrated. TG2 InIRA adopts the interferometry with short baseline and takes small incidence angles, by which wide-swath sea surface height (SSH) can be measured with high accuracy. Gravity recovery experiments in the Western Pacific area are conducted to demonstrate the performance, advantages and capability of TG2 InIRA. SSH data processing algorithms and DOV calculation have been designed by taking the wide-swath feature into account, based on which, the gravity anomalies are then calculated using the inverse Vening Meinesz formula. The derived gravity anomalies are compared with both the published gravity models and the shipborne gravity measurements. The results show that the accuracy of TG2 InIRA is equivalent to, or even a little better than, that of CAs. The fused gravity result using equal TG2 InIRA data and CAs data performs better than those using TG2 InIRA data alone or CAs data alone. Due to the signal bandwidth of TG2 InIRA is only 40 MHz which is much smaller than that of CAs, much higher accuracy can be hopefully achieved for future missions if larger signal bandwidth is used. Full article

(This article belongs to the Special Issue Advances in Satellite Altimetry II)

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Review

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37 pages, 4020 KiB

Open AccessReview

Inland Water Level Monitoring from Satellite Observations: A Scoping Review of Current Advances and Future Opportunities

by Stylianos Kossieris, Valantis Tsiakos, Georgios Tsimiklis and Angelos Amditis

Remote Sens. 2024, 16(7), 1181; https://doi.org/10.3390/rs16071181 - 28 Mar 2024

Cited by 12 | Viewed by 4660

Abstract

Inland water level and its dynamics are key components in the global water cycle and land surface hydrology, significantly influencing climate variability and water resource management. Satellite observations, in particular altimetry missions, provide inland water level time series for nearly three decades. Space-based remote sensing is regarded as a cost-effective technique that provides measurements of global coverage and homogeneous accuracy in contrast to in-situ sensors. The advent of Open-Loop Tracking Command (OLTC), and Synthetic Aperture Radar (SAR) mode strengthened the use of altimetry missions for inland water level monitoring. However, it is still very challenging to obtain accurate measurements of water level over narrow rivers and small lakes. This scoping systematic literature review summarizes and disseminates the research findings, highlights major results, and presents the limitations regarding inland water level monitoring from satellite observations between 2018 and 2022. Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline and through a double screening process, 48 scientific publications were selected meeting the eligibility criteria. To summarize the achievements of the previous 5 years, we present fundamental statistical results of the publications, such as the annual number of publications, scientific journals, keywords, and study regions per continent and type of inland water body. Also, publications associated with specific satellite missions were analyzed. The findings show that Sentinel-3 is the dominant satellite mission, while the ICESat-2 laser altimetry mission has exhibited a high growth trend. Furthermore, publications including radar altimetry missions were charted based on the retracking algorithms, presenting the novel and improved methods of the last five years. Moreover, this review confirms that there is a lack of research on the collaboration of altimetry data with machine learning techniques. Full article

(This article belongs to the Special Issue Advances in Satellite Altimetry II)

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