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Advances in Satellite Altimetry and Its Application

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

Deadline for manuscript submissions: closed (31 August 2019) | Viewed by 54767

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Dr. Denise Dettmering

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Technical University of Munich, Deutsches Geodätisches Forschungsinstitut (DGFI-TUM), Arcisstraße 21, 80333 Munich, Germany
Interests: satellite altimetry; space geodesy; oceanography; hydrology and surface water storage; ionosphere modelling

Dr. Marcello Passaro

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Technical University of Munich, Deutsches Geodätisches Forschungsinstitut (DGFI-TUM), Arcisstraße 21, 80333 Munich, Germany
Interests: satellite oceanography; radar altimetry; sea level research

Prof. Dr. Alexander Braun

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Department of Geological Sciences and Geological Engineering, Queen’s University, 36 Union St, Kingston, ON K7L 3N6, Canada
Interests: geophysics; space geodesy; earth systems
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Special Issue Information

Dear Colleagues,

Since 1978, numerous satellite altimetry missions have observed the Earth with different orbital configuration, sampling characteristics, sensors and measurement principles. Currently, seven contemporaneous satellites provide elevation and elevation change observations of various Earth systems including the oceans, hydrosphere, solid Earth and cryosphere. Today, the operational, precise, absolute, and near-global determination of the sea surface is a critical component in ocean modelling and ocean dynamics on different scales from global mean sea level change to internal waves and tide modelling. Since 2010, a new-generation of altimeters (Cryosat-2 and Sentinel-3) was developed to provide higher spatial resolution SAR Altimetry (also known as Delay-Doppler mode). These missions enabled coastal altimetry with observations as close as a few hundred meters off the coast. In addition, satellite altimetry allows for monitoring of water level variations of inland water bodies as well as height variations of sea ice, ice sheets and mountain glaciers. Geodetic applications such as gravity field modeling, unification of height systems and monitoring of vertical land motion are other examples of the wide range of altimetry applications.

In this Special Issue, we invite researchers from all disciplines to submit manuscripts presenting recent advances in the field of radar and laser altimetry, including new and future altimetry missions (e.g., ICESat-2 and SWOT) and their applications. We further encourage review manuscripts exploiting the historic altimetry records and their applications in spatio-temporal monitoring of Earth systems on all scales.

Dr. Denise Dettmering
Dr. Marcello Passaro
Prof. Alexander Braun
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Remote Sensing is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

Radar altimetry
Satellite altimetry
Delay-doppler altimetry
Remote sensing of the oceans
Sea level rise
Surface currents
Coastal applications
Polar areas
Inland altimetry
Marine geoid

Published Papers (11 papers)

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Editorial

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3 pages, 170 KiB

Open AccessEditorial

Editorial for Special Issue “Advances in Satellite Altimetry and Its Application”

by Denise Dettmering, Marcello Passaro and Alexander Braun

Remote Sens. 2019, 11(24), 2913; https://doi.org/10.3390/rs11242913 - 05 Dec 2019

Cited by 4 | Viewed by 2198

Abstract

This special issue compiles studies from different disciplines presenting recent advances in the field of radar and laser altimetry including new and future altimetry missions and their applications. It comprises eight research papers as well as one review paper, and covers method development [...] Read more.

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

Research

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23 pages, 8445 KiB

Open AccessArticle

Observation-Based Attitude Realization for Accurate Jason Satellite Orbits and Its Impact on Geodetic and Altimetry Results

by Mathis Bloßfeld, Julian Zeitlhöfler, Sergei Rudenko and Denise Dettmering

Remote Sens. 2020, 12(4), 682; https://doi.org/10.3390/rs12040682 - 19 Feb 2020

Cited by 14 | Viewed by 3541

Abstract

For low Earth orbiting satellites, non-gravitational forces cause one of the largest perturbing accelerations. During a precise orbit determination (POD), the accurate modeling of the satellite-body attitude and solar panel orientation is important since the satellite’s effective cross-sectional area is directly related to the perturbing acceleration. Moreover, the position of tracking instruments that are mounted on the satellite body are affected by the satellite attitude. For satellites like Jason-1/-2/-3, attitude information is available in two forms—as a so-called nominal yaw steering model and as observation-based (measured by star tracking cameras) quaternions of the spacecraft body orientation and rotation angles of the solar arrays. In this study, we have developed a preprocessing procedure for publicly available satellite attitude information. We computed orbits based on Satellite Laser Ranging (SLR) observations to the Jason satellites at an overall time interval of approximately 25 years, using each of the two satellite attitude representations. Based on the analysis of the orbits, we investigate the influence of using preprocessed observation-based attitude in contrast to using a nominal yaw steering model for the POD. About 75% of all orbital arcs calculated with the observation-based satellite attitude data result in a smaller root mean square (RMS) of residuals. More precisely, the resulting orbits show an improvement in the overall mission RMS of SLR observation residuals of 5.93% (Jason-1), 8.27% (Jason-2) and 4.51% (Jason-3) compared to the nominal attitude realization. Besides the satellite orbits, also the estimated station coordinates benefit from the refined attitude handling, that is, the station repeatability is clearly improved at the draconitic period. Moreover, altimetry analysis indicates a clear improvement of the single-satellite crossover differences (6%, 15%, and 16% reduction of the mean of absolute differences and 1.2%, 2.7%, and 1.3% of their standard deviations for Jason-1/-2/-3, respectively). On request, the preprocessed attitude data are available. Full article

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

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29 pages, 7724 KiB

Open AccessEditor’s ChoiceArticle

Arctic Ocean Sea Level Record from the Complete Radar Altimetry Era: 1991–2018

by Stine Kildegaard Rose, Ole Baltazar Andersen, Marcello Passaro, Carsten Ankjær Ludwigsen and Christian Schwatke

Remote Sens. 2019, 11(14), 1672; https://doi.org/10.3390/rs11141672 - 14 Jul 2019

Cited by 43 | Viewed by 10294

Abstract

In recent years, there has been a large focus on the Arctic due to the rapid changes of the region. Arctic sea level determination is challenging due to the seasonal to permanent sea-ice cover, lack of regional coverage of satellites, satellite instruments ability to measure ice, insufficient geophysical models, residual orbit errors, challenging retracking of satellite altimeter data. We present the European Space Agency (ESA) Climate Change Initiative (CCI) Technical University of Denmark (DTU)/Technischen Universität München (TUM) sea level anomaly (SLA) record based on radar satellite altimetry data in the Arctic Ocean from the European Remote Sensing satellite number 1 (ERS-1) (1991) to CryoSat-2 (2018). We use updated geophysical corrections and a combination of altimeter data: Reprocessing of Altimeter Product for ERS (REAPER) (ERS-1), ALES+ retracker (ERS-2, Envisat), combination of Radar Altimetry Database System (RADS) and DTUs in-house retracker LARS (CryoSat-2). Furthermore, this study focuses on the transition between conventional and Synthetic Aperture Radar (SAR) altimeter data to make a smooth time series regarding the measurement method. We find a sea level rise of 1.54 mm/year from September 1991 to September 2018 with a 95% confidence interval from 1.16 to 1.81 mm/year. ERS-1 data is troublesome and when ignoring this satellite the SLA trend becomes 2.22 mm/year with a 95% confidence interval within 1.67–2.54 mm/year. Evaluating the SLA trends in 5 year intervals show a clear steepening of the SLA trend around 2004. The sea level anomaly record is validated against tide gauges and show good results. Additionally, the time series is split and evaluated in space and time. Full article

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

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19 pages, 18263 KiB

Open AccessArticle

SAR-Mode Altimetry Observations of Internal Solitary Waves in the Tropical Ocean Part 2: A Method of Detection

by Adriana M. Santos-Ferreira, José C. B. da Silva and Meric Srokosz

Remote Sens. 2019, 11(11), 1339; https://doi.org/10.3390/rs11111339 - 04 Jun 2019

Cited by 9 | Viewed by 3214

Abstract

It is demonstrated that the synthetic aperture radar altimeter (SRAL) on board of the Sentinel-3A can detect short-period internal solitary waves (ISWs) with scales of the order of a kilometer. A variety of signatures owing to the surface manifestations of the ISWs are apparent in the SRAL Level-2 products over the ocean. These signatures are identified in several geophysical parameters, such as radar backscatter (

σ_{0}

) and sea level anomaly (SLA). Radar backscatter is the primary parameter in which ISWs can be identified owing to the measurable sea surface roughness perturbations in the along-track direction resulting from the sharpened SRAL footprint. The SRAL footprint is sufficiently small (300 m in the along-track direction) to capture radar power fluctuations over successive wave crests and troughs, which produce rough and slick surface patterns arrayed in parallel bands with scales of a few kilometers along-track. Furthermore, it was possible to calculate the mean square slope (

\bar{s^{2}}

) for the dual-band (Ku and C bands) altimeter of Sentinel-3, which made the ISW signatures unambiguously identified because of the large

\bar{s^{2}}

variations in exact synergy with ocean and land color instrument (OLCI) images. Hence, the detection method is validated in cloud-free sun glint OLCI images. It is shown that both

σ_{0}

and SLA yield realistic estimates for routine observation of ISWs with the SRAL. The detection method that is used relies on the parameter

\bar{s^{2}}

which is calculated from

σ_{0}

. This is a significant improvement from previous observations recently reported for conventional pulse-limited altimeters (Jason-2). An algorithm is developed to be used in any ocean region. Wavelets were applied for a first analysis of the

\bar{s^{2}}

variations because ISWs can be readily identified in high-frequency signals. Other geophysical parameters such as SLA were used to exclude phenomena that are unlikely to be ISWs. Full article

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

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17 pages, 5560 KiB

Open AccessArticle

Coastal Waveform Retracking in the Slick-Rich Sulawesi Sea of Indonesia, Based on Variable Footprint Size with Homogeneous Sea Surface Roughness

by Xifeng Wang, Kaoru Ichikawa and Dongni Wei

Remote Sens. 2019, 11(11), 1274; https://doi.org/10.3390/rs11111274 - 29 May 2019

Cited by 10 | Viewed by 2600

Abstract

Waveforms of radar altimeters are often corrupted due to heterogeneous sea surface roughness within footprints, such as slicks. In past studies, subwaveform retrackers such as the adaptive leading edge subwaveform retracker (ALES) which use only a section of the waveform have been proposed. However, it is difficult to choose a reasonable estimation window from an individual waveform. In the present study, a post-processed subwaveform retracker is proposed which identifies the waveforms of surrounding along-track points. The size of the estimation window is variable and is determined to keep the sea surface roughness within the corresponding footprint homogeneous. The method was applied to seven years of 20 Hz Jason-2 altimeter data over the slick-rich Sulawesi Sea of Indonesia and compared with ALES and sensor geophysical data record (SGDR) products. The standard deviation of the sea surface dynamic heights was around 0.13 m, even without spatial smoothing or some geophysical corrections. This is only 75% and 25% of the ALES and SGDR results, respectively. Moreover, all retrievals of the range, SWH, and sigma0 include less outliers than the other products. These results indicate that the variable estimation windows determined in the present study can adapt well to the variation of sea surface roughness. Full article

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

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19 pages, 4263 KiB

Open AccessArticle

Reconstruction of the Surface Inshore Labrador Current from SWOT Sea Surface Height Measurements

by Zhimin Ma and Guoqi Han

Remote Sens. 2019, 11(11), 1264; https://doi.org/10.3390/rs11111264 - 28 May 2019

Cited by 5 | Viewed by 3301

Abstract

Utilizing a high-resolution (2-km) coastal ocean model output off Eastern Newfoundland, this paper explores the potential for reconstructing the sea surface height (SSH) and the surface inshore Labrador Current from high-resolution SSH data of the upcoming Surface Water and Ocean Topography (SWOT) satellite mission. The model results are evaluated against in-situ data from tide gauges and nadir altimetry for the period from June to October, 2010. The hourly model SSH output is used as true SSH and sampled along-swath with expected measurement errors by using a SWOT simulator, which produces SWOT-like data. We reconstruct half-day SSH fields from the SWOT-like data using optimal interpolation and average them into weekly fields. The average normalized root-mean-square difference between the weekly reconstructed SSH field and the model SSH filed is 0.07 for the inshore Labrador Current. Between the geostrophic surface current derived from the reconstructed SSH field and the model surface current, the average normalized root-mean-square difference is 0.26 for the inshore Labrador Current. For the surface unit-depth transport of the inshore Labrador Current, the normalized root-mean-square differences are 0.32–0.38 between the reconstructed current and the model current. Full article

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

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12 pages, 12574 KiB

Open AccessArticle

An Attempt to Observe Vertical Land Motion along the Norwegian Coast by CryoSat-2 and Tide Gauges

by Martina Idžanović, Christian Gerlach, Kristian Breili and Ole Baltazar Andersen

Remote Sens. 2019, 11(7), 744; https://doi.org/10.3390/rs11070744 - 27 Mar 2019

Cited by 9 | Viewed by 3325

Abstract

Present-day climate-change-related ice-melting induces elastic glacial isostatic adjustment (GIA) effects, while paleo-GIA effects describe the ongoing viscous response to the melting of late-Pleistocene ice sheets. The unloading initiated an uplift of the crust close to the centers of former ice sheets. Today, vertical land motion (VLM) rates in Fennoscandia reach values up to around 10 mm/year and are dominated by GIA. Uplift signals from GIA can be computed by solving the sea-level equation (SLE),

\dot{S}

\dot{N}

−

\dot{U}

. All three quantities can also be determined from geodetic observations: relative sea-level variations (

\dot{S}

) are observed by means of tide gauges, while rates of absolute sea-level change (

\dot{N}

) can be observed by satellite altimetry; rates of VLM (

\dot{U}

) can be determined by GPS (Global Positioning System). Based on the SLE,

\dot{U}

can be derived by combining sea-surface measurements from satellite altimetry and relative sea-level records from tide gauges. In the present study, we have combined 7.5 years of CryoSat-2 satellite altimetry and tide-gauge data to estimate linear VLM rates at 20 tide gauges along the Norwegian coast. Thereby, we made use of monthly averaged tide-gauge data from PSMSL (Permanent Service for Mean Sea Level) and a high-frequency tide-gauge data set with 10-min sampling rate from NMA (Norwegian Mapping Authority). To validate our VLM estimates, we have compared them with the independent semi-empirical land-uplift model NKG2016LU_abs for the Nordic-Baltic region, which is based on GPS, levelling, and geodynamical modeling. Estimated VLM rates from 1 Hz CryoSat-2 and high-frequency tide-gauge data reflect well the amplitude of coastal VLM as provided by NKG2016LU_abs. We find a coastal average of 2.4 mm/year (average over all tide gauges), while NKG2016LU_abs suggests 2.8 mm/year; the spatial correlation is 0.58. Full article

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

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25 pages, 5294 KiB

Open AccessArticle

Analysis of Retrackers’ Performances and Water Level Retrieval over the Ebro River Basin Using Sentinel-3

by Qi Gao, Eduard Makhoul, Maria Jose Escorihuela, Mehrez Zribi, Pere Quintana Seguí, Pablo García and Mònica Roca

Remote Sens. 2019, 11(6), 718; https://doi.org/10.3390/rs11060718 - 25 Mar 2019

Cited by 35 | Viewed by 5690

Abstract

Satellite altimeters have been used to monitor river and reservoir water levels, from which water storage estimates can be derived. Inland water altimetry can, therefore, play an important role in continental water resource management. Traditionally, satellite altimeters were designed to monitor homogeneous surfaces such as oceans or ice sheets, resulting in poor performance over small inland water bodies due to the contribution from land contamination in the returned waveforms. The advent of synthetic aperture radar (SAR) altimetry (with its improved along-track spatial resolution) has enabled the measurement of inland water levels with a better accuracy and an increased spatial resolution. This study aimed to retrieve water levels from Level-1B Sentinel-3 data with focus on the minimization of the land contamination over small- to middle-sized water bodies (130 m to 4.5 km), where continuous clean waveforms rarely exist. Three specialized algorithms or retrackers, together with a new waveform portion selection method, were evaluated to minimize land contamination in the waveforms and to select the nadir return associated with the water body being overflown. The waveform portion selection method, with consideration of the Digital Elevation Model (DEM), was used to fit the multipeak waveforms that arise when overflying the continental water bodies, exploiting a subwaveform-based approach to pick up the one corresponding to the nadir. The performances of the proposed waveform portion selection method with three retrackers, namely, the threshold retracker, Offset Center of Gravity (OCOG) retracker and two-step SAR physical-based retracker, were compared. No significant difference was found in the results of the three retrackers. However, waveform portion selection using DEM information great improved the results. Time series of water levels were retrieved for water bodies in the Ebro River basin (Spain). The results show good agreement with in situ measurements from the Ebro Reservoir (approximately 1.8 km wide) and Ribarroja Reservoir (approximately 400 m wide), with unbiased root-mean-square errors (RMSEs) down to 0.28 m and 0.16 m, respectively, depending on the retracker. Full article

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

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18 pages, 5308 KiB

Open AccessArticle

2D Frequency Domain Fully Focused SAR Processing for High PRF Radar Altimeters

by Pietro Guccione, Michele Scagliola and Davide Giudici

Remote Sens. 2018, 10(12), 1943; https://doi.org/10.3390/rs10121943 - 03 Dec 2018

Cited by 16 | Viewed by 5248

Abstract

Fully-focusing of radar altimeters is a recent concept that has been introduced to allow further improvement of along-track resolution in high pulse repetition frequency (PRF) radar altimeters. The straight potentiality of this new perspective reflects into a more accurate estimation of geophysical parameters in some applications such as sea-ice observation. However, as documented in a recent paper, such capability leaves unsolved the problem of the high computational effort required. In this paper, we face the problem of adapting for altimeters the Omega-Kappa SAR focusing algorithm that is performed in the two-dimensional wavenumber domain, accounting for the difference existing between SAR and altimeter from geometry (looking and swath width) and instrument (echoes are deramped onboard on receiving) point of view. Simulations and an application using in-orbit data show the effectiveness of the proposed approach and the highly reduced computational effort. Full article

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17 pages, 5234 KiB

Open AccessArticle

An Improved Digital Elevation Model of the Lunar Mons Rümker Region Based on Multisource Altimeter Data

by Fei Li, Chang Zhu, Weifeng Hao, Jianguo Yan, Mao Ye, Jean-Pierre Barriot, Qing Cheng and Tao Sun

Remote Sens. 2018, 10(9), 1442; https://doi.org/10.3390/rs10091442 - 10 Sep 2018

Cited by 9 | Viewed by 4339

Abstract

Mons Rümker is the primary candidate region for the lunar landing mission of Chang’E-5. We propose a data processing method that combines multisource altimeter data and we developed an improved digital elevation model (DEM) of the Mons Rümker region with a horizontal resolution of 256 pixels per degree. The lunar orbiter laser altimeter (LOLA) onboard the lunar reconnaissance orbiter (LRO) acquired 884 valid orbital benchmark data with a high precision. A special crossover adjustment of 156 orbital profiles from the Chang’E-1 laser altimeter (LAM) and 149 orbital profiles from the SELenological and ENgineering Explorer (SELENE) laser altimeter (LALT) was applied. The radial residual root mean square (RMS) of the LAM was reduced from 154.83 ± 43.60 m to 14.29 ± 27.84 m and that of the LALT was decreased from 3.50 ± 5.0 m to 2.75 ± 4.4 m. We used the adjusted LAM and LALT data to fill the LOLA gaps and created the merged LOLA + LAM and LOLA + LALT DEMs. The merged LOLA + LAM DEM showed distortions because of the horizontal geolocation errors in the LAM data. The merged LOLA + LALT DEM was closer to the ground truth than the LOLA-only DEM when validated with the images of the LRO camera (LROC). Full article

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

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Review

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47 pages, 11531 KiB

Open AccessEditor’s ChoiceReview

Retrieving Sea Level and Freeboard in the Arctic: A Review of Current Radar Altimetry Methodologies and Future Perspectives

by Graham D. Quartly, Eero Rinne, Marcello Passaro, Ole B. Andersen, Salvatore Dinardo, Sara Fleury, Amandine Guillot, Stefan Hendricks, Andrey A. Kurekin, Felix L. Müller, Robert Ricker, Henriette Skourup and Michel Tsamados

Remote Sens. 2019, 11(7), 881; https://doi.org/10.3390/rs11070881 - 11 Apr 2019

Cited by 39 | Viewed by 9618

Abstract

Spaceborne radar altimeters record echo waveforms over all Earth surfaces, but their interpretation and quantitative exploitation over the Arctic Ocean is particularly challenging. Radar returns may be from all ocean, all sea ice, or a mixture of the two, so the first task is the determination of which surface and then an interpretation of the signal to give range. Subsequently, corrections have to be applied for various surface and atmospheric effects before making a comparison with a reference level. This paper discusses the drivers for improved altimetry in the Arctic and then reviews the various approaches that have been used to achieve the initial classification and subsequent retracking over these diverse surfaces, showing examples from both LRM (low resolution mode) and SAR (synthetic aperture radar) altimeters. The review then discusses the issues concerning corrections, including the choices between using other remote-sensing measurements and using those from models or climatology. The paper finishes with some perspectives on future developments, incorporating secondary frequency, interferometric SAR and opportunities for fusion with measurements from laser altimetry or from the SMOS salinity sensor, and provides a full list of relevant abbreviations. Full article

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

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