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15 pages, 10707 KiB

Open AccessArticle

Polar Ocean Tides—Revisited Using Cryosat-2

by Ole Baltazar Andersen, Stine Kildegaard Rose and Michael G. Hart-Davis

Remote Sens. 2023, 15(18), 4479; https://doi.org/10.3390/rs15184479 - 12 Sep 2023

Cited by 2 | Viewed by 1473

Abstract

With the availability of more than 9 years of Cryosat-2, it is possible to revisit polar ocean tides, which have traditionally been difficult to determine from satellite altimetry. The SAMOSA+ physical retracker is a stable retracker developed particularly for Cryosat-2. Being a physical [...] Read more.

With the availability of more than 9 years of Cryosat-2, it is possible to revisit polar ocean tides, which have traditionally been difficult to determine from satellite altimetry. The SAMOSA+ physical retracker is a stable retracker developed particularly for Cryosat-2. Being a physical retracker, it enables the determination of the sea state bias. Correcting for the sea state bias enables more reliable sea level estimates compared with traditional empirical retrackers used before. Cryosat-2 data have been analyzed for residual ocean tides to the FES2014 ocean tide model in the Arctic Ocean and Antarctic Ocean using the response formalism. We utilize data from the sub-cycle of Cryosat-2, which follows a repeating pattern of approximately 28.33 days. This sub-repeat period makes it an advantageous alias period for the majority of significant constituents. This allowed for the estimation and mapping of the major tidal constituents in the open ocean and also in floating ice shelves from data extracted from leads in the sea ice. A novel empirical ocean tide model designed specifically for the polar region, DTU22, is introduced. Our findings reveal substantial enhancements in semi-diurnal tides within the Arctic Ocean and improvement in diurnal constituents within the Southern Ocean. In the Southern Ocean, the diurnal constituents are particularly improved using the empirical model by more than a factor of two to around 3 cm for both constituents compared with FES2014b. These outcomes underscore the significance of incorporating the reprocessed and retracted Cryosat-2 data into tidal modeling, highlighting its pivotal role in advancing the field. Full article

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

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22 pages, 5753 KiB

Open AccessArticle

Mean Seasonal Sea Surface Height Variations in and around the Makassar Strait

by Kaoru Ichikawa

Remote Sens. 2023, 15(17), 4324; https://doi.org/10.3390/rs15174324 - 1 Sep 2023

Cited by 1 | Viewed by 1383

Abstract

Seasonal variations are significant in currents in the Makassar Strait, 80% of the Indonesian Throughflow (ITF) from the Pacific to the Indian Ocean, and they are in phase with both the monsoon and the sea surface height anomaly (SSHA) difference between two oceans. [...] Read more.

Seasonal variations are significant in currents in the Makassar Strait, 80% of the Indonesian Throughflow (ITF) from the Pacific to the Indian Ocean, and they are in phase with both the monsoon and the sea surface height anomaly (SSHA) difference between two oceans. However, dynamics are not well discussed since gridded SSHA products within the strait are less reliable because of both over-smoothing and contamination in coastal areas. In this study, therefore, 17 years of along-track Jason altimetry data with the ALES retracker are used without grid interpolation to investigate seasonal SSHA variations in and around the Makassar Strait. All SSHA variations are in phase from the southern Celebes Sea to the northern Java Sea through the Makassar Strait, but their amplitude decreases by the distance from the southern shallow area. These amplitude modulations produce the pressure gradient force, which is maximum to the north of 4°S reaching 1.5

\times

10⁻⁶ ms⁻², and that would be balanced with the bottom friction of the upper-layer ITF velocity, rather than the wind stress whose magnitude 5

\times

10⁻⁸ ms⁻² is too small. The SSHA difference between the two oceans is in phase but is isolated from the Makassar Strait by adjacent uncorrelated SSHA variations. Full article

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

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31 pages, 17869 KiB

Open AccessArticle

Towards the Mitigation of Discrepancies in Sea Surface Parameters Estimated from Low- and High-Resolution Satellite Altimetry

by Christopher K. Buchhaupt, Alejandro Egido, Douglas Vandemark, Walter H. F. Smith, Luciana Fenoglio and Eric Leuliette

Remote Sens. 2023, 15(17), 4206; https://doi.org/10.3390/rs15174206 - 27 Aug 2023

Cited by 2 | Viewed by 1361

Abstract

In this study, we present an extension to existing numerical retrackers of synthetic-aperture radar (SAR) altimetry signals. To our knowledge at the time of writing this manuscript, it offers the most consistent retrieval of geophysical parameters compared to low-resolution mode (LRM) retracking results. [...] Read more.

In this study, we present an extension to existing numerical retrackers of synthetic-aperture radar (SAR) altimetry signals. To our knowledge at the time of writing this manuscript, it offers the most consistent retrieval of geophysical parameters compared to low-resolution mode (LRM) retracking results. We achieve this by additionally estimating the standard deviation of vertical wave-particle velocities

σ_{v}

and a new parameter

u_{x}

, linked to a residual Doppler in the returned radar echoes, which can be related to wind speed and direction. Including this new parameter into the SAR stack retracker mitigates sea surface height estimation errors by up to two centimeters for Sentinel-6MF SAR mode results. Additionally, we found a closed-form equation to describe

u_{x}

as a function of eastward and northward wind variables, which allows mitigating the effects of this parameter on a SAR stack within level 1B processing and generating a lookup table to correct sea surface height estimates in SAR mode. This additionally opens up the door to estimating the wind speed and direction from SAR altimetry stacks. Additionally, we discuss how this new retracker performs with respect to different planned future baseline processor changes of Sentinel-6MF, namely F09 and F10, by attempting to imitate their level 2 processing. This is achieved by processing cycles 017 to 051 (nearly a full year) of Sentinel-6MF level 1A data on a global scale. We observe that the new retracking method is, on average, more accurate with respect to LRM. However, there is a slight increase in measurement noise due to the introduction of an additional parameter. To ensure that the results of the new retracker are not biased, we retrack using both the new method and the SINCS-OV ZSK retracker on Sentinel-6MF stack data produced in a Monte Carlo simulation. We analyze the simulation results with respect to accuracy, precision, and correlations between estimated parameters. We show that the accuracy of the new retracker is better than SINCS-OV ZSK but less precise, which could be related to higher correlation coefficients—especially with respect to the new parameter

u_{x}

—between estimated parameters. Full article

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

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14 pages, 3714 KiB

Open AccessCommunication

ICESat-2 for Coastal MSS Determination—Evaluation in the Norwegian Coastal Zone

by Matea Tomić and Ole Baltazar Andersen

Remote Sens. 2023, 15(16), 3974; https://doi.org/10.3390/rs15163974 - 10 Aug 2023

Cited by 2 | Viewed by 2071

Abstract

Radar satellite altimeters enable the determination of the mean sea surface to centimeter accuracy, which can be degraded in coastal areas because of the lack of valid altimetry observations due to land contamination and the altimeter footprint size. In 2018, the National Aeronautics [...] Read more.

Radar satellite altimeters enable the determination of the mean sea surface to centimeter accuracy, which can be degraded in coastal areas because of the lack of valid altimetry observations due to land contamination and the altimeter footprint size. In 2018, the National Aeronautics and Space Administration launched ICESat-2, a laser altimetry mission equipped with the Advanced Topographic Laser Altimeter System, providing measurements every 0.7 m in the along-track direction. Taking into account the complexity of the Norwegian coastline, this study aims to evaluate coastal observations from ICESat-2 in order to use it to update the existing mean sea surface for Norway, NMBU18. We, therefore, determined the mean sea surface using only ICESat-2 observations and compared it with mean sea level observations from 23 permanent tide gauges along the entire coast and 21 temporary tide gauges in Norway’s largest and deepest fjord, Sognefjorden. We also included two global mean sea surface models and NMBU18 for comparison. The results have shown that ICESat-2 is indeed able to provide more valid observations in the coastal zone, which can be used to improve the mean sea surface model, especially along the coast. Full article

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

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17 pages, 10271 KiB

Open AccessArticle

SARAL’s Full Mission Reprocessing: Improvement with the GDR-F Standard

by Ghita Jettou, Manon Rousseau, Fanny Piras, Mathilde Simeon and Ngan Tran

Remote Sens. 2023, 15(10), 2604; https://doi.org/10.3390/rs15102604 - 17 May 2023

Cited by 1 | Viewed by 1220

Abstract

Seven years (2013–2019) of the French/Indian mission SARAL altimetry data have been successfully reprocessed within the SALP contract supported by CNES to produce a new data set of GDR (Geophysical Data Record) using an updated, modern set of algorithms and models. The main [...] Read more.

Seven years (2013–2019) of the French/Indian mission SARAL altimetry data have been successfully reprocessed within the SALP contract supported by CNES to produce a new data set of GDR (Geophysical Data Record) using an updated, modern set of algorithms and models. The main objective of this article is to assess the quality of the reprocessed dataset and estimate the system’s performance using GDR-F products. To achieve this goal, the new dataset has been validated against the previous one (identified as GDR-T) using mono-mission metrics and comparisons to reference altimetry missions such as Jason-2 and Jason-3. The new data set shows a clear improvement in data quality. The product validation shows a reduction of the standard deviation of crossovers’ Sea Surface Height differences from 5.5 cm (GDR-T) to 5.2 cm (GDR-F). This paper presents the main processing changes and shows some of the results from the validation and quality-assurance processes. The major improvement of the GDR-F data set with respect to the previous one is due to the use of state-of-the-art orbit standards (POE-F) and geophysical corrections, including new tidal models, a new wet troposphere retrieval algorithm, and a new algorithm for sea state estimation. The intent of this paper is to highlight the overall benefit of this new dataset. Full article

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

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23 pages, 13085 KiB

Open AccessArticle

A New Method to Combine Coastal Sea Surface Height Estimates from Multiple Retrackers by Using the Dijkstra Algorithm

by Fukai Peng, Xiaoli Deng, Maofei Jiang, Salvatore Dinardo and Yunzhong Shen

Remote Sens. 2023, 15(9), 2329; https://doi.org/10.3390/rs15092329 - 28 Apr 2023

Cited by 4 | Viewed by 1792

Abstract

To increase data availability and accuracy in the coastal zone, especially in the last 5 km to the coast, we present a SCMR (Seamless Combination of Multiple Retrackers) processing strategy to combine sea surface height (SSH) estimates from waveform retrackers of SGDR MLE4, [...] Read more.

To increase data availability and accuracy in the coastal zone, especially in the last 5 km to the coast, we present a SCMR (Seamless Combination of Multiple Retrackers) processing strategy to combine sea surface height (SSH) estimates from waveform retrackers of SGDR MLE4, ALES, WLS3 and MB4 for Jason-3 and Saral missions, and of SAMOSA and SAMOSA+ for Sentinel-3A mission in the Australian coastal zone. The SCMR does not require the waveform classification result. It includes two steps: (1) estimating and removing the SSH bias due mainly to the significant wave height (SWH) difference-dependent height differences, and (2) determining the optimal along-track SSH profile by using the Dijkstra algorithm. In the study region, the results show that the SCMR increases the data availability by up to 15% in the last 5 km to the coast and reduces the noise level by 28–34% at the spatial scales < 2.5 km. The validation results against tide gauges show that SCMR-derived SSH estimates achieve a better accuracy than that from any single retracker, with the improvement percentage of 6.26% and 4.94% over 0–10 km and 20–100 km distance bands, respectively. Full article

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

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20 pages, 30009 KiB

Open AccessArticle

Accurate Discharge Estimation Based on River Widths of SWOT and Constrained At-Many-Stations Hydraulic Geometry

by Bin Du, Taoyong Jin, Dong Liu, Youkun Wang and Xuequn Wu

Remote Sens. 2023, 15(6), 1672; https://doi.org/10.3390/rs15061672 - 20 Mar 2023

Cited by 2 | Viewed by 2894

Abstract

River discharge monitoring is an important component of the hydrology objectives of Surface Water and Ocean Topography mission (SWOT). River discharge can be estimated Solely using river widths and At Many-stations Hydraulic Geometry (AMHG), but the accuracy is low due to the parameters [...] Read more.

River discharge monitoring is an important component of the hydrology objectives of Surface Water and Ocean Topography mission (SWOT). River discharge can be estimated Solely using river widths and At Many-stations Hydraulic Geometry (AMHG), but the accuracy is low due to the parameters of At a-station Hydraulic Geometry (AHG) given by AMHG deviate from the truth. In view of this, a Constrained At-Many-Stations Hydraulic Geometry (CAMHG) is proposed to optimize AHG parameters. The performance of CAMHG is verified in three reaches of the Yangtze River using river widths derived from SWOT. After using CAMHG, the relative root mean square error (RRMSE) of estimated discharge reduce 100.1% to 24.4%, 1137.1% to 49.9% and 48.6% to 45.5% for Hankou, Shashi and Luoshan respectively. In addition, CAMHG can also weaken the accuracy difference of estimated discharge in dry and wet seasons benefited from its more reliable AHG parameters. Thus, the proposed CAMHG can dramatically improves the accuracy of discharge estimations and it is meaningful for the discharge calculation after SWOT data release. Full article

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

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31 pages, 36347 KiB

Open AccessArticle

Absolute Calibration of the Chinese HY-2B Altimetric Mission with Fiducial Reference Measurement Standards

by Stelios P. Mertikas, Mingsen Lin, Dimitrios Piretzidis, Costas Kokolakis, Craig Donlon, Chaofei Ma, Yufei Zhang, Yongjun Jia, Bo Mu, Xenophon Frantzis, Achilles Tripolitsiotis and Lei Yang

Remote Sens. 2023, 15(5), 1393; https://doi.org/10.3390/rs15051393 - 1 Mar 2023

Cited by 5 | Viewed by 2476

Abstract

This research and collaboration work aims at the calibration and validation (Cal/Val) of the Chinese HY-2B satellite altimeter based upon two permanent Cal/Val facilities: (1) the China Altimetry Calibration Cooperation Plan in Qingdao, Bohai Sea and the Wanshan islands, China and (2) the [...] Read more.

This research and collaboration work aims at the calibration and validation (Cal/Val) of the Chinese HY-2B satellite altimeter based upon two permanent Cal/Val facilities: (1) the China Altimetry Calibration Cooperation Plan in Qingdao, Bohai Sea and the Wanshan islands, China and (2) the permanent facility for altimetry calibration established by the European Space Agency in Crete, Greece. The HY-2B satellite altimeter and its radiometer have been calibrated and monitored using uniform, standardized procedures, as well as protocols and best practices, and they also built upon trusted and indisputable reference standards at both Cal/Val infrastructures in Europe and China. The HY-2B altimeter is thus monitored in a coordinated, absolute, homogeneous, long-term and worldwide manner. Calibration of altimeters is accomplished by examining satellite observations in open seas against reference measurements. Comparisons are established through precise satellite positioning, water level observations, GPS buoys and reference models (geoid, mean dynamic topography, earth tides, troposphere and ionosphere), all defined at the Cal/Val sites. In this work, the final uncertainty for the altimeter bias will be attributed to several individual sources of uncertainty, coming from observations in water level, atmosphere, absolute positioning, reference surface models, transfer of heights from Cal/Val sites to satellite observations, etc. Through this project, the procedures, protocols and best practices, originally developed in the course of the ESA FRM4ALT project, are updated, upgraded and followed at both Cal/Val facilities in Europe and China. All in all, the HY-2B satellite altimeter observes the sea level quite well and within its specifications. Full article

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

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26 pages, 13399 KiB

Open AccessArticle

Estimation of Arctic Sea Ice Thickness from Chinese HY-2B Radar Altimetry Data

by Maofei Jiang, Wenqing Zhong, Ke Xu and Yongjun Jia

Remote Sens. 2023, 15(5), 1180; https://doi.org/10.3390/rs15051180 - 21 Feb 2023

Cited by 4 | Viewed by 2391

Abstract

Sea ice thickness (SIT) is an important parameter in the study of climate change. During the past 20 years, satellite altimetry has been widely used to observe sea ice thickness. The Chinese Haiyang-2B (HY-2B) radar altimeter, launched in October 2018, can provide data [...] Read more.

Sea ice thickness (SIT) is an important parameter in the study of climate change. During the past 20 years, satellite altimetry has been widely used to observe sea ice thickness. The Chinese Haiyang-2B (HY-2B) radar altimeter, launched in October 2018, can provide data up to 80.6° latitude and can be used as a supplementary means to observe polar sea ice. Reliable HY-2B SIT products will contribute to the sea ice community. In this study, we aimed to assess the Arctic sea ice thickness retrieval ability of the HY-2B radar altimetry data. We processed the HY-2B radar altimetry data from January 2019 to April 2022 and used the processed data to retrieve the Arctic SIT. The Alfred Wegener Institute (AWI) CryoSat-2 (CS-2) SIT products were used to calibrate the HY-2B SIT estimates with a linear regression method. The Goddard Space Flight Center (GSFC) CS-2, Jet Propulsion Laboratory (JPL), and GSFC ICESat-2 (IS-2) SIT products were used to validate the HY-2B calibrated SIT estimates. The HY-2B calibrated SIT estimates have good, consistent spatial distributions with the CS-2 and IS-2 SIT products. The comparison with the IS-2 and IS-2 SIT products shows the root-mean-square error (RMSE) and bias for the HY-2B SIT estimates are significantly reduced after calibration. The HY-2B SIT estimates were also validated using the ice thickness data from Operation IceBridge (OIB) and the ice draft data from the Beaufort Gyre Exploration Project (BGEP). Finally, the monthly variations of the HY-2B SIT estimates were analyzed. Results show that the HY-2B calibrated SIT estimates are reliable, especially when the SIT values are lower than 3 m. The HY-2B altimetry data is a possible source for sea ice thickness data at lower latitudes and will help us better understand the sea ice response to climate change. Full article

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

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22 pages, 3564 KiB

Open AccessArticle

The Impact of Altimetry Corrections of Sentinel-3A Sea Surface Height in the Coastal Zone of the Northwest Atlantic

by Anrijs Abele, Sam Royston and Jonathan Bamber

Remote Sens. 2023, 15(4), 1132; https://doi.org/10.3390/rs15041132 - 18 Feb 2023

Cited by 3 | Viewed by 2129

Abstract

Corrections to altimeter-measured sea surface height anomalies (SSHA) have a larger proportional effect for synthetic aperture radar (SAR) altimetry than conventional, pulse-limited altimetry because of lower range noise. Here, we quantified the impact of the current generation of altimeter corrections in the coastal [...] Read more.

Corrections to altimeter-measured sea surface height anomalies (SSHA) have a larger proportional effect for synthetic aperture radar (SAR) altimetry than conventional, pulse-limited altimetry because of lower range noise. Here, we quantified the impact of the current generation of altimeter corrections in the coastal zone of the Northwest Atlantic, a region with significant dynamic activity. In this study, we used the sea level variance analysis to determine the change in variance for the altimeter corrections—range, geophysical, and mean surface—compared to the baseline. We also evaluated the performance of two coastal retrackers, ALES (empirical) and SAMOSA++ (fully analytical), against the SSHA from the Radar Altimeter Database System (RADS), which uses the standard SAR retracker. Tide corrections caused the largest change in sea level variance, followed by wet tropospheric corrections and sea state bias. Most non-standard altimeter corrections failed to reduce the sea level variance and performed markedly worse closer to the coast. Coastal retrackers showed a higher deviation from the standard SSHA closer to the coast, especially when the backscatter coefficient was high and the significant wave height was low. We conclude that further development of coastal corrections is needed. Contrary to our prior expectation, we found that standard altimetry corrections appear to perform as well as alternative more advanced/tailored corrections. Full article

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

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22 pages, 2851 KiB

Open AccessArticle

Ongoing Development of the Bass Strait GNSS/INS Buoy System for Altimetry Validation in Preparation for SWOT

by Boye Zhou, Christopher Watson, Benoit Legresy, Matt A. King and Jack Beardsley

Remote Sens. 2023, 15(1), 287; https://doi.org/10.3390/rs15010287 - 3 Jan 2023

Cited by 2 | Viewed by 2637

Abstract

GNSS equipped buoys remain an important tool in altimetry validation. Progressive advances in altimetry missions require associated development in such validation tools. In this paper, we enhanced an existing buoy approach and gained further understanding of the buoy dynamics based on in situ [...] Read more.

GNSS equipped buoys remain an important tool in altimetry validation. Progressive advances in altimetry missions require associated development in such validation tools. In this paper, we enhanced an existing buoy approach and gained further understanding of the buoy dynamics based on in situ observations. First, we implemented the capability to separate the ambiguity fixing strategy for different constellations in the processing software TRACK. A comparison between GPS and GNSS solutions suggested up to 3 cm reduction in the root mean square of the buoy minus co-located mooring SSH residuals over the selected sidereal periods. Then, comparison between double differencing and precise point positioning solutions suggested a possible common mode error external to GNSS processing. To assess buoy performance in different ocean conditions and sea states, GNSS and INS observations were used during periods where external forcings (waves, current and wind) were not interacting substantially. For the deployments investigated, no significant relationship was found, noting the maximum significant wave height and current velocity was ~2.3 m and ~0.3 m/s, respectively. In the lead up to the validation required for the SWOT mission, these results place important bounds on the performance of the buoy design under real operating conditions. Full article

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

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26 pages, 5111 KiB

Open AccessEditor’s ChoiceArticle

Evaluation of Sentinel-6 Altimetry Data over Ocean

by Maofei Jiang, Ke Xu and Jiaming Wang

Remote Sens. 2023, 15(1), 12; https://doi.org/10.3390/rs15010012 - 21 Dec 2022

Cited by 9 | Viewed by 2633

Abstract

The Sentinel-6 Michael Freilich (S6-MF) satellite was launched on 21st November 2020. Poseidon-4, the main payload onboard S6-MF, is the first synthetic aperture radar (SAR) altimeter operating in an interleaved open burst mode. In this study, the sea surface height (SSH), [...] Read more.

The Sentinel-6 Michael Freilich (S6-MF) satellite was launched on 21st November 2020. Poseidon-4, the main payload onboard S6-MF, is the first synthetic aperture radar (SAR) altimeter operating in an interleaved open burst mode. In this study, the sea surface height (SSH), significant wave height (SWH) and wind speed observations from the Poseidon-4 Level 2 altimetry products from November 2021 to October 2022 are assessed. The assessment contains synthetic aperture radar mode (SARM) as well as low-resolution mode (LRM) data. The SSH assessment is conducted using range noise, sea level anomaly (SLA) spectral analysis and crossover analysis, whereas the SWH and wind speed assessments are performed against NDBC buoy data and other satellite altimetry missions. The performance of the Sentinel-6 altimetry data is compared to those of Sentinel-3A/B and Jason-3 altimetry data. The 20 Hz range noise is 3.07 cm for SARM and 6.40 cm for LRM when SWH is 2 m. The standard deviation (STD) of SSH differences at crossovers is 3.76 cm for SARM and 4.27 cm for LRM. Compared against the NDBC measurements, the Sentinel-6 SWH measurements have a root-mean-square error (RMSE) of 0.361 m for SARM and an RMSE of 0.225 m for LRM. The Sentinel-6 wind speed measurements show an RMSE of 1.216 m/s for SARM and an RMSE of 1.323 m/s for LRM. We also present the impacts of ocean waves on parameter retrievals from Sentinel-6 SARM data. The Sentinel-6 SARM data are sensitive to wave period and direction as well as vertical velocity. It should be paid attention to in the future. Full article

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

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22 pages, 4356 KiB

Open AccessArticle

Impact of Satellite Attitude on Altimetry Calibration with Microwave Transponders

by Costas Kokolakis, Dimitrios Piretzidis and Stelios P. Mertikas

Remote Sens. 2022, 14(24), 6369; https://doi.org/10.3390/rs14246369 - 16 Dec 2022

Cited by 4 | Viewed by 2216

Abstract

Satellite altimetry plays a key role in monitoring changes in sea level and climate change. The quality of satellite altimetry products is commonly ensured through dedicated calibration. One such calibration is with microwave transponders acting as ground reference point targets. It is common [...] Read more.

Satellite altimetry plays a key role in monitoring changes in sea level and climate change. The quality of satellite altimetry products is commonly ensured through dedicated calibration. One such calibration is with microwave transponders acting as ground reference point targets. It is common practice that satellite ranges between the transponder phase center and the satellite center of gravity (CoG) are compared against the true geometric ranges to determine bias. Transponder ranges are, however, realized by the two phase centers of the altimeter and the ground transponder. So, to make this comparison feasible, the space origin of the measured range is transferred from the altimeter phase center (APC) to the satellite CoG by applying a constant offset, usually referred to as “CoG correction”. Instead of a fixed “CoG correction”, this work introduces the actual vector between APC and CoG in space, by examining the satellite attitude. Thus, the observed and geometric distances to the transponder are both referred to the APC. The case of Jason-3 and Sentinel-6A Michael Freilich (Sentinel-6A MF) with two transponders on Crete (CDN1) and Gavdos (GVD1) islands is examined. At first, the attitude of Jason-3 is determined by its quaternions. Then, analysis reveals that the transponder bias is correlated with the Jason-3 satellite attitude. The revised calibration brings about bias changes which fluctuate from about −2 mm to 1 mm in range and from

- 110 μ

s to +110

μ

s in datation for Jason-3. Spectral analysis on the bias differences between the revised and conventional transponder calibrations reveals constituents with periods of 117, 39 and 23 days. Finally, the revised methodology on crossover calibrations over the GVD1 transponder results in an improvement between the mean bias of the ascending and descending orbits by 12% for Jason-3 and by 14% (preliminary) for Sentinel-6A MF. Full article

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

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13 pages, 7323 KiB

Open AccessArticle

Detection of Bering Sea Slope Mesoscale Eddies Derived from Satellite Altimetry Data by an Attention Network

by Yuyuan Zhang, Na Liu, Zhiyuan Zhang, Min Liu, Long Fan, Yunbo Li, Lei Yang, Lina Lin and Hongxia Chen

Remote Sens. 2022, 14(19), 4974; https://doi.org/10.3390/rs14194974 - 6 Oct 2022

Cited by 1 | Viewed by 1692

Abstract

In the Bering Sea slope, ocean eddies are essential physical processes that carry nutrients to the shelf. The development of the satellite altimeter has facilitated the observation of oceanic eddies. Attention networks are used as the core algorithm for eddy detection to suppress [...] Read more.

In the Bering Sea slope, ocean eddies are essential physical processes that carry nutrients to the shelf. The development of the satellite altimeter has facilitated the observation of oceanic eddies. Attention networks are used as the core algorithm for eddy detection to suppress feature responses in irrelevant non-eddy areas, which can address the issue of sample imbalance in high-latitude ocean eddies. Furthermore, data from both the sea surface height (SSH) and geostrophic velocity were employed as model inputs to integrate more eddy-related properties. The results of ocean eddy detection using this method and the dataset allowed more eddies to be detected than with traditional vector geometry-based methods and only SSH-based models. This study also incorporated the results of multiple deep learning models to increase both the overall and single-day eddy detection efficiency. As a result, the algorithms in this paper show that attention networks and geostrophic velocity data are both appropriate for high-latitude ocean eddy identification. This makes a contribution to the further application of deep learning methods to satellite altimetry data. Full article

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

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31 pages, 24754 KiB

Open AccessArticle

Latest Altimetry-Based Sea Ice Freeboard and Volume Inter-Annual Variability in the Antarctic over 2003–2020

by Florent Garnier, Marion Bocquet, Sara Fleury, Jérôme Bouffard, Michel Tsamados, Frédérique Remy, Gilles Garric and Aliette Chenal

Remote Sens. 2022, 14(19), 4741; https://doi.org/10.3390/rs14194741 - 22 Sep 2022

Cited by 4 | Viewed by 2790

Abstract

The relatively stable conditions of the sea ice cover in the Antarctic, observed for almost 40 years, seem to be changing recently. Therefore, it is essential to provide sea ice thickness (SIT) and volume (SIV) estimates in order to anticipate potential multi-scale changes [...] Read more.

The relatively stable conditions of the sea ice cover in the Antarctic, observed for almost 40 years, seem to be changing recently. Therefore, it is essential to provide sea ice thickness (SIT) and volume (SIV) estimates in order to anticipate potential multi-scale changes in the Antarctic sea ice. For that purpose, the main objectives of this work are: (1) to assess a new sea ice freeboard, thickness and volume altimetry dataset over 2003–2020 and (2) to identify first order impacts of the sea ice recent conditions. To produce these series, we use a neuronal network to calibrate Envisat radar freeboards onto CryoSat-2 (CS2). This method addresses the impacts of surface roughness on Low Resolution Mode (LRM) measurements. During the 2011 common flight period, we found a mean deviation between Envisat and CryoSat-2 radar freeboards by about 0.5 cm. Using the Advanced Microwave Scanning Radiometer (AMSR) and the dual-frequency Altimetric Snow Depth (ASD) data, our solutions are compared with the Upward looking sonar (ULS) draft data, some in-situ measurement of the SIMBA campaign, the total freeboards of 6 Operation Ice Bridge (OIB) missions and ICESat-2 total freeboards. Over 2003–2020, the global mean radar freeboard decreased by about −14% per decade and the SIT and SIV by about −10% per decade (considering a snow depth climatology). This is marked by a slight increase through 2015, which is directly followed by a strong decrease in 2016. Thereafter, freeboards generally remained low and even continued to decrease in some regions such as the Weddell sea. Considering the 2013–2020 period, for which the ASD data are available, radar freeboards and SIT decreased by about −40% per decade. The SIV decreased by about −60% per decade. After 2016, the low SIT values contrast with the sea ice extent that has rather increased again, reaching near-average values in winter 2020. The regional analysis underlines that such thinning (from 2016) occurs in all regions except the Amundsen-Bellingshausen sea sector. Meanwhile, we observed a reversal of the main regional trends from 2016, which may be the signature of significant ongoing changes in the Antarctic sea ice. Full article

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

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18 pages, 3835 KiB

Open AccessArticle

Validation of an Empirical Subwaveform Retracking Strategy for SAR Altimetry

by Marcello Passaro, Laura Rautiainen, Denise Dettmering, Marco Restano, Michael G. Hart-Davis, Florian Schlembach, Jani Särkkä, Felix L. Müller, Christian Schwatke and Jérôme Benveniste

Remote Sens. 2022, 14(16), 4122; https://doi.org/10.3390/rs14164122 - 22 Aug 2022

Cited by 9 | Viewed by 3035

Abstract

The sea level retrievals from the latest generation of radar altimeters (the SAR altimeters) are still challenging in the coastal zone and areas covered by sea ice and require a dedicated fitting (retracking) strategy for the waveforms. In the framework of the European [...] Read more.

The sea level retrievals from the latest generation of radar altimeters (the SAR altimeters) are still challenging in the coastal zone and areas covered by sea ice and require a dedicated fitting (retracking) strategy for the waveforms. In the framework of the European Space Agency’s Baltic + Sea Level (ESA Baltic SEAL) project, an empirical retracking strategy (ALES + SAR), including a dedicated sea state bias correction, has been designed to improve the sea level observations in the Baltic Sea, characterised by a jagged coastline and seasonal sea ice coverage, without compromising the quality of open ocean data. In this work, the performances of ALES + SAR are validated against in-situ data in the Baltic Sea. Moreover, variance, crossover differences and power spectral density of the open ocean data are evaluated on a global scale. The results show that ALES + SAR performances are of comparable quality to the ones obtained using physical-based retrackers, with relevant advantages in coastal and sea ice areas in terms of quality and quantity of the sea level data. Full article

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

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Review

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21 pages, 3501 KiB

Open AccessReview

A Review of Marine Gravity Field Recovery from Satellite Altimetry

by Zhen Li, Jinyun Guo, Bing Ji, Xiaoyun Wan and Shengjun Zhang

Remote Sens. 2022, 14(19), 4790; https://doi.org/10.3390/rs14194790 - 25 Sep 2022

Cited by 22 | Viewed by 3401

Abstract

Marine gravity field recovery relies heavily on satellite altimetry. Thanks to the evolution of altimetry missions and the improvements in altimeter data processing methods, the marine gravity field model has been prominently enhanced in accuracy and resolution. However, high-accuracy and high-resolution gravity field [...] Read more.

Marine gravity field recovery relies heavily on satellite altimetry. Thanks to the evolution of altimetry missions and the improvements in altimeter data processing methods, the marine gravity field model has been prominently enhanced in accuracy and resolution. However, high-accuracy and high-resolution gravity field recovery from satellite altimeter data remains particularly challenging. We provide an overview of advances in satellite altimetry for marine gravity field recovery, focusing on the impact factors and available models of altimetric gravity field construction. Firstly, the evolution of altimetry missions and the contribution to gravity field recovery are reviewed, from the existing altimetry missions to the future altimetry missions. Secondly, because the methods of altimeter data processing are of great significance when obtaining high-quality sea surface height observations, these improved methods are summarized and analyzed, especially for coastal altimetry. In addition, the problems to be resolved in altimeter data processing are highlighted. Thirdly, the characteristics of gravity recovery methods are analyzed, including the inverse Stokes formula, the inverse Vening Meinesz formula, Laplace’s equation, and least squares collocation. Furthermore, the latest global marine gravity field models are introduced, including the use of altimeter data and processing methods. The performance of the available global gravity field model is also evaluated by shipboard gravity measurements. The root mean square of difference between the available global marine gravity model and shipboard gravity from the National Centers for Environmental Information is approximately 5.10 mGal in the low-middle latitude regions, which is better than the result in high-latitude regions. In coastal areas, the accuracy of models still needs to be further improved, particularly within 40 km from the coastline. Meanwhile, the SDUST2021GRA model derived from the Shandong University of Science and Technology team also exhibited an exciting performance. Finally, the future challenges for marine gravity field recovery from satellite altimetry are discussed. Full article

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

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22 pages, 3043 KiB

Open AccessReview

Satellite Altimetry: Achievements and Future Trends by a Scientometrics Analysis

by Lei Yang, Lina Lin, Long Fan, Na Liu, Lingyong Huang, Yongsheng Xu, Stelios P. Mertikas, Yongjun Jia and Mingsen Lin

Remote Sens. 2022, 14(14), 3332; https://doi.org/10.3390/rs14143332 - 11 Jul 2022

Cited by 14 | Viewed by 5666

Abstract

Scientometric reviews, facilitated by computational and visual analytical approaches, allow researchers to gain a thorough understanding of research trends and areas of concentration from a large number of publications. With the fast development of satellite altimetry, which has been effectively applied to a [...] Read more.

Scientometric reviews, facilitated by computational and visual analytical approaches, allow researchers to gain a thorough understanding of research trends and areas of concentration from a large number of publications. With the fast development of satellite altimetry, which has been effectively applied to a wide range of research topics, it is timely to summarize the scientific achievements of the previous 50 years and identify future trends in this field. A comprehensive overview of satellite altimetry was presented using a total of 8541 publications from the Web of Science Core Collection covering the years from 1970 to 2021. We begin by presenting the fundamental statistical results of the publications, such as the annual number of papers, study categories, countries/regions, afflictions, journals, authors, and keywords, in order to provide a comprehensive picture of satellite altimetry research. We discuss the co-occurrence of the authors in order to reveal the global collaboration network of satellite altimetry research. Finally, we utilised co-citation networks to detect the development trend and associated crucial publications for various specific topics. The findings show that satellite altimetry research has been changed immensely during the last half-century. The United States, France, China, England, and Germany made the most significant contributions in the field of satellite altimetry. The analysis reveals a clear link between technology advancements and the trend in satellite altimetry research. As a result, wide swath altimetry, GNSS-reflectometry, laser altimetry, terrestrial hydrology, and deep learning are among the most frontier study subjects. The findings of this work could guide a thorough understanding of satellite altimetry’s overall development and research front. Full article

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

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13 pages, 1841 KiB

Open AccessTechnical Note

Quantifying Multifrequency Ocean Altimeter Wind Speed Error Due to Sea Surface Temperature and Resulting Impacts on Satellite Sea Level Measurements

by Ngan Tran, Douglas Vandemark, François Bignalet-Cazalet and Gérald Dibarboure

Remote Sens. 2023, 15(13), 3235; https://doi.org/10.3390/rs15133235 - 22 Jun 2023

Cited by 1 | Viewed by 1338

Abstract

Surface wind speed measurements from a satellite radar altimeter are used to adjust altimeter sea level measurements via sea state bias range correction. We focus here on previously neglected ocean radar backscatter and subsequent wind speed variations due to sea surface temperature (SST) [...] Read more.

Surface wind speed measurements from a satellite radar altimeter are used to adjust altimeter sea level measurements via sea state bias range correction. We focus here on previously neglected ocean radar backscatter and subsequent wind speed variations due to sea surface temperature (SST) change that may impact these sea level estimates. The expected error depends on the radar operating frequency and may be significant at the Ka band (36 GHz) frequency chosen for the new Surface Water and Ocean Topography (SWOT) satellite launched in December 2022. SWOT is expected to revolutionize oceanography by providing wide-swath Ka band observations and enhanced spatial resolution compared to conventional Ku band (14 GHz) altimetry. The change to the Ka band suggests a reconsideration of SST impact on wind and sea level estimates, and we investigate this in advance of SWOT using existing long-term Ku and Ka band satellite altimeter datasets. This study finds errors up to 1.5 m/s in wind speed estimation and 1.0 cm in sea level for AltiKa altimeter data. Future SWOT data analyses may require consideration of this dependence prior to using its radar backscatter data in its sea level estimation. Full article

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

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