Centre National d'Etudes Spatiales Research Articles

IGS real-time service for global ionospheric total electron content modeling

Benefiting from global multi-frequency and multi-constellation GNSS measurements provided by the experimental International GNSS real-time service (IGS RTS), a predicting-plus-modeling approach employed by Chinese Academy of Sciences (CAS) for the routine generation of real-time global ionospheric maps (RT-GIM) is first reported. Along with RT-GIMs generated by Universitat Politecnica de Catalunya (UPC), the quality of CAS and UPC RT-GIMs in IONEX format is assessed during a low soar activity period from September 2017 to December 2019. The differential slant total electron contents (dSTEC) derived from 50 GPS stations of the IGS and Jason-3 vertical TECs (VTEC) over the ocean are used as references. In comparison with different reference TECs, CAS and UPC RT-GIMs are approximately 1.7–4.9% and 8.6–12.5% worse than the respective post-processed GIMs CASG and UQRG, respectively. Using RTCM ionospheric data streams from CAS, Centre National d’Etudes Spatiales (CNES) and UPC, the first experimental IGS combined RT-GIM is generated and validated in actual real-time conditions. Compared to Jason-3 VTEC measurements available during the period of common availability, from October 2018 to April 2019, RT-GIM discrepancies present similar relative RMS errors, which are 33, 36, 36 and 38% for CNES, combined one, UPC and CAS, respectively. Aside from a better understanding of the influence of working in the original IONEX versus RTCM ionospheric formats, the update to a new experimental adaptation of RT strategy is highlighted by UPC, and the computation of multi-layer RT-GIM is emphasized by CAS in view of the inadequacy of single-layer ionospheric assumption in the presence of large latitudinal gradients.

Mitigation of receiver biases in ionospheric observables from PPP with ambiguity resolution

PPP (Precise Point Positioning) is a GNSS (Global Navigation Satellite Systems) positioning method that requires SSR (State Space Representation) corrections in order to provide solutions with an accuracy of centimetric level. The so-called RT-PPP (Real-time PPP) is possible thanks to real-time precise SSR products, for orbits and clocks, provided by IGS (International GNSS Service) and its associate analysis centers such as CNES (Centre National d'Etudes Spatiales). CNES SSR products also enable RT-PPP with integer ambiguity resolution. In GNSS related literature, PPP with ambiguity resolution (PPP-AR) in real-time is often referred as PPP-RTK (PPP – Real Time Kinematic). PPP-WIZARD (PPP - With Integer and Zero-difference Ambiguity Resolution Demonstrator) is a software that is made available by CNES. This software is capable of performing PPP-RTK. It estimates slant ionospheric delays and other GNSS positioning parameters. Since ionospheric effects are spatially correlated by GNSS data from active networks, it is possible to model and provide ionospheric delays for any position in the network coverage area. The prior knowledge ionospheric delays can reduce positioning convergence for PPP-RTK users. Real-time ionospheric models could benefit from highly precise ionospheric delays estimated in PPP-AR. In this study, we demonstrate that ionospheric delays obtained throughout PPP-AR estimation are actu ally ionospheric observables. Ionospheric observables are biased by an order of few meters caused by the receiver hardware biases. These biases prohibit the use of PPP-WIZARD ionospheric delays to produce ionospheric models. Receiver biases correction is essential to provide ionospheric delays while using PPP-AR based ionospheric observables. In this contribution, a method was implemented to estimate and mitigate receiver hardware biases influence on slant ionospheric observables from PPP-AR. In order to assess the proposed approach, PPP-AR data from 12 GNSS stations were processed over a two-month period (March and April 2018). A comparison between IGS ionospheric products and PPP-AR based ionospheric observables corrected for receiver biases, resulted in a mean of differences of −39 cm and 51 cm standard deviation. The results are consistent with the accuracy of the IGS ionospheric products, 2–8 TECU, considering that 1 TECU is ~16 cm in L1. In another analysis, a comparison of ionospheric delays from 5 pairs of short baselines GNSS stations found an agreement of 0.001 m in mean differences with 22 cm standard deviation after receiver biases were corrected. Therefore, the proposed solution is promising and could produce high quality (1–2 TECU) slant ionospheric delays. This product can be used in a large variety of modeling approaches, since ionospheric delays after correction are unbiased. These results indicate that the proposed strategy is promising, and could benefit applications that require accuracy of 1–2 TECU (~16–32 cm in L1).

Real-time clock comparison and monitoring with multi-GNSS precise point positioning: GPS, GLONASS and Galileo

The precise point positioning (PPP) technique is widely used in time and frequency applications. Because of the real-time service (RTS) project of the International GNSS Service, we can use the PPP technique for real-time clock comparison and monitoring. As a participant in the RTS, the Centre National d’Etudes Spatiales (CNES) implements the PPPWIZARD (Precise Point Positioning with Integer and Zero-difference Ambiguity Resolution Demonstrator) project to validate carrier phase ambiguity resolution. Unlike the Integer-PPP (IPPP) of the CNES, fixing ambiguities in the post-processing mode, the PPPWIZARD operates in the real-time mode, which is also called real-time IPPP (RT-IPPP). This paper focuses on applying the RT-IPPP for real-time clock comparison and monitoring. We review the principle of real-time clock comparison and monitoring, and introduce the methodology of the RT-IPPP technique. The observations of GPS, GLONASS and Galileo were processed for the experiments. Five processing modes were provided in the experiment to analyze the benefits of ambiguity resolution and multi-GNSS. In the clock comparison experiment, the average reduction ratios of standard deviations with respect to the G PPP mode range from 9.7% to 35.0%. In the clock monitoring experiment, G PPP mode can detect clock jumps whose magnitudes are larger than 0.9 ns. The RT-IPPP technique with GRE PPP AR (G) mode allows for the detection of any clock jumps larger than 0.6 ns. For frequency monitoring, G PPP mode allows detection of frequency changes larger than 1.1 × 10−14. When the RT-IPPP technique is applied, monitoring with GRE PPP AR (G) mode can detect frequency changes larger than 6.1 × 10−15.

Field and spaceborne imagery data for evaluation of the paleo-stress regime during formation of the Jurassic dike swarms in the Kalateh Alaeddin Mountain area, Shahrood, north Iran

Dike swarms are commonly linked with extensional structures in diverse geodynamic environments. Mafic dyke swarms are typically used to reconstruct the paleo-stress fields of a given region. These dikes are considered paleo-stress indicators and excellent time marker (if related geochronological data are available) of the local and regional stress fields. In the Middle-Late Jurassic, swarms of mafic dikes emplaced into the Neoproterozoic schists and amphibolites in the Kalateh Alaeddin Mountain area in south Shahrood, north Iran. These dikes with different thicknesses show a general east–west strike direction, with mostly a steep dip angle. In this paper, we present structural data of these dike swarms for the sake of assessing the paleo-stress state and the magma pressure ratio at the time of their emplacement. Field and structural data are integrated with ASTER Global Digital Elevation Model (GDEM) and Centre National d’Etudes Spatiales (CNES)/SPOT imagery data, to extract important parameters of the investigated dikes and controlling fault/joint sets. Orientation of the principal paleo-stress axes, quantification of the stress ratio, and the associated magma pressure ratio (driving stress ratio) were calculated using the stereographic projection and Mohr’s circle reconstruction techniques. The results reveal that the maximum paleo-stress component (σ1) was sub-vertical and the intermediate (σ2) and minimum (σ3) paleo-stresses components were sub-horizontal in N264° E and N173° E trends, respectively. Due to the low value of the driving stress ratio (R = 0.05), these dikes developed perpendicular to the minimum principal stress (in E–W direction). The stress ratio value (o = 0.66) indicates a moderately oblate stress ellipsoid. The orientation of the principal paleo-stress axes and the oblate ellipsoid are indicative of the dike emplacement during a N–S-directed tectonic extension, in agreement with the Jurassic subsidence phase and N–S stretching described for the Kalateh Alaeddin Mountain area.

Performance Assessment of BDS Real-Time Precise Point Positioning Based on SSR Corrections

In recent years, real-time precise point positioning (RTPPP) using the BeiDou Navigation Satellite System (BDS) is facing new developments thanks to the dissemination of BDS real-time state space representation (SSR) corrections provided by some institutions. However, the quality and accuracy levels of these corrections remain unclear because they have not been provided for long. In this paper, methods of data accessing for RTPPP are introduced, and the quality of BDS real-time products provided by the Centre National d'Etudes Spatiales (CNES) and the performance of BDS-RTPPP are assessed and validated. It is indicated that the products currently suffer from the issues of unavailability and discontinuity, along with limited product accuracy. BDS-RTPPP is then implemented based on these corrections to validate positioning performance. For 6-h observations, the average final result of static positioning reaches 1.31, 2.15, and 6.18 cm in the north, east, and up components, respectively, and the average convergence time is 73.4 min. As for kinematic positioning, an average accuracy of 4.76, 6.70, and 7.07 cm can be achieved after an average convergence time of 86.1 min.

Performance Evaluation of Three Atmospheric Density Models on HY-2A Precise Orbit Determination Using DORIS Range-Rate Data

Abstract Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) has become a matured and accurate space geodetic technique after more than two decades of development. DORIS’s main objective is precise orbit determination (POD) for the low earth orbit (LEO) satellite. For the POD of Haiyang-2A (HY-2A) satellite, a DORIS receiver was equipped on this mission. There are several nonconservative forces acting on a Haiyang-2A (HY-2A) satellite with an altitude of about 970 km. Out of the factors, the atmospheric drag is the most dominant and uncertain one during the process of POD with the dynamic method. The comparison between the computed orbits of HY-2A using DORIS range-rate data and the precise orbits computed by the Centre National d’Etudes Spatiales (CNES) has been performed, and the DORIS range-rate residuals have been analyzed in detail in normal period and frequent solar activity period. Tests demonstrated that the relative optimal atmospheric density model was the empirical MSIS-86 model for the HY-2A satellite. The root mean squares (RMSs) of orbit difference between the derived orbits using the MSIS-86 model and the CNES orbits were 0.0173 m in the radial direction and 0.0619 m in three dimensions. The RMS of the range-rate residual improved 0.024 mm/s during the frequent solar activity period with the atmospheric drag coefficient solved every three hours and then every six hours using the MSIS-86 model. This study will provide valuable references for POD and orbit prediction for the LEO satellites, especially for the other HY-2 series satellites of China.

THE BIRTH OF SPACE OCEANOGRAPHY: TECHNOLOGICAL QUESTIONS AND CLIMATOLOGICAL OPPORTUNITY (UNITED STATES, FRANCE, 1950–1980)

ABSTRACT This article analyzes the formation of space oceanography as a scientific specialty, in France and in the United States. Throughout much of its history, oceanography has relied upon a broad range of instrumentation (bathyscaphes, tide gauges, and so forth). The importance of instrumentation meant that many of the exchanges during major scientific meetings in the 1960s focused on engineering problems. As a result, institutional investments by NASA and the French space agency, the Centre National d'Études Spatiales (CNES) supported advances in instrumentation. The emergence of the climate change issue made it possible to merge several factors (technical, geopolitical, and institutional) into the specialty of space oceanography. The birth of a scientific specialty requires the conjunction of several determining factors: a powerful disciplinary basis, a technological innovation resulting in major advances, and scientific politicians capable of tackling new problems. The development of space oceanography provides an excellent example of the origin of a new scientific specialty. The purpose of this article is to trace the history of the combination of the technical and scientific factors that resulted in the origin of space oceanography. This article will focus on specific events that led to the origin of space oceanography, in particular on a series of meetings organized by researchers interested in very specific technical questions. Many of the classical questions of oceanography could be addressed and dealt with by developing and using new instruments (e.g. radar altimeters). To document how the specialty of space oceanography developed, I propose to follow American and French examples of the transformations induced by space oceanography. Such a comparison makes it possible to measure the differential scientific ‘maturity’ of a nascent speciality. The emergence of climate change research, starting in the 1970s, reorganized many of the oceanographic research questions and legitimized the extension of the discipline into geospatial research. One goal of this article is to understand how this development of science policy influenced interactions between different disciplines.

Laser geodetic satellites: a high-accuracy scientific tool

Satellite Laser Ranging (SLR) began in the mid-1960s on satellites of opportunity with retro-reflectors intended as a part of intercomparison tests of satellite tracking techniques. Shortly thereafter, data from these satellites began to work their way into geodetic solutions and dedicated geodesy experiments. By early 1970s when future requirements for centimeter accuracy were envisioned, planning began for dedicated, spherical retro-reflector geodetic satellites. Built with high mass-to-area ratios, these satellites would have important applications in gravity field modeling, station geolocation and fiducial reference systems, Earth rotation, and fundamental physics. Early geodetic satellites were Starlette, launched in 1975 by Centre National d’Etudes Spatiales (CNES), and LAGEOS in 1976 by the National Aeronautics and Space Administration (NASA). Recent geodetic satellites include LARES, launched in 2012, and LARES-2 under development, both by the Italian Space Agency (ASI). Today, a complex of these ‘geodetic satellites’ from low to high altitude Earth orbits supports many space geodesy requirements. This paper will discuss the evolution of the geodetic satellites from the early days, through current programs and out to future needs as we approach our goal for millimeter accuracy.

A Simplified BDS Broadcast Ephemeris and State Space Representative (SSR) Matching Method for BDS-Only Real-Time Precise Point Positioning (PPP)

As opposed to the GPS real-time precise point positioning (PPP) with sophisticated orbit/clock recovery method, BDS real-time PPP applications were limited by the fact that no internationally recognized BDS issue of data (IOD) calculation and matching method was defined. This paper proposes a simplified BDS broadcast ephemeris and state space representative (SSR) matching method for the BDS precise orbit and clock recovery in real time. Unlike the existing method, the proposed method does not need to perform IOD calculation which involves several bitwise operations. Instead, it only needs to monitor the BDS SSR IOD updating status. Comparison result shows that the proposed method can reduce 6.97% computational time for the 24-hour real-time orbit/clock product recovery. Using the proposed matching method, Centre National d’Etudes Spatiales (CNES) BDS real-time orbit and clock product from day of year (DOY) 119 to 128 2018 were evaluated. By comparing the 10-day dataset with German Research Centre for Geosciences Potsdam (GFZ) final products, we could see that BDS real-time orbit accuracies were greatly improved, i.e. 1.644-6.143 m real-time precise orbit for Geosynchronous Earth Orbit (GEO) satellites, 0.059-0.286 m for Inclined Geosynchronous Satellite Orbit (IGSO)/Medium Earth Orbit (MEO) satellites. In the meantime, 0.12-2.32 ns real-time clock precision was obtained. Both static and kinematic tests were adopted to evaluate BDS-only real-time PPP using CNES real-time corrections. The results indicated that cm-level horizontal and < 0.1 m vertical precisions were obtained in static experiment. As for the kinematic test, < 0.1 m horizontal and dm-level vertical precisions were obtained in an unmanned aerial vehicle experiment.

The Smithsonian Astrophysical Observatory (SAO) and the Centre National d’Études Spatiales (CNES): contributions to the international laser ranging network

The Smithsonian Astrophysical Observatory (SAO) and the Centre National d’Etudes Spatiales (CNES) worked closely together in the early years of the space program to deploy satellite laser ranging (SLR) systems at overseas sites to enhance global coverage to support specific missions. The data were routinely made available for use by the science community for programs in geodesy, gravity field, atmospheric physics, and ultimately for oceanography and geodynamics. SAO and CNES organized campaigns for international participation and loaned each other equipment to enhance the network. SAO and CNES provided technical expertise and advice to a number of other groups as they planned and deployed SLR systems. In this paper, we will discuss the history and the role of the two institutions in the building of the international SLR network.

Performance assessment of uncombined precise point positioning using Multi-GNSS real-time streams: Computational efficiency and RTS interruption

The International GNSS Service (IGS) has established the Real-time Services (RTS) that support the real-time precise point positioning (RT-PPP) and related applications. Due to the increasing number of unknown parameters and network instability issues, both the computational efficiency and the RTS stream outage are the key points to realize real-time uncombined GNSS PPP. To improve the computational efficiency of uncombined PPP, the recursive Kalman based on single observation is implemented rather than the standard Kalman filter. The differences between the RTS orbits and the corresponding ultra-rapid predicted orbits reveal that the ultra-rapid predicted orbits can be used as alternatives of the GNSS real-time orbits. During the RTS clock outage period, the linear and sinusoidal terms are determined by the ultra-rapid observed part in advance, while the constant term of clock offsets is determined by the latest available RTS clock corrections. The PPP results have revealed that the proposed prediction method is much more effective than that of directly using the ultra-rapid predicted products. Even when the prediction duration reaches up to 1 h, the 3D positioning accuracy of GNSS RT-PPP is improved by 51.91% compared with the ultra-rapid scheme. Compare to the GPS-only kinematic PPP, the 3D positioning accuracy of GNSS kinematic PPP using CNES (Centre National d’Études Spatiales) RTS products is improved from 8.29 cm to 6.60 cm, and the mean convergence time of achieving 3D position accuracy better than 10 cm is shortened by about 44.99%.

Assessment of Multiple GNSS Real-Time SSR Products from Different Analysis Centers

The real-time State Space Representation (SSR) product of the GNSS (Global Navigation Satellite System) orbit and clock is one of the most essential corrections for real-time precise point positioning (PPP). In this work, the performance of current SSR products from eight analysis centers were assessed by comparing it with the final product and the accuracy of real-time PPP. Numerical results showed that (1) the accuracies of the GPS SSR product were better than 8 cm for the satellite orbit and 0.3 ns for the satellite clock; (2) the accuracies of the GLONASS (GLObalnaya NAvigatsionnaya Sputnikovaya Sistema) SSR product were better than 10 cm for orbit RMS (Root Mean Square) and 0.6 ns for clock STD (Standard Deviation); and (3) the accuracies of the BDS (BeiDou Navigation Satellite System) and Galileo SSR products from CLK93 were about 14.54 and 4.42 cm for the orbit RMS and 0.32 and 0.18 ns for the clock STD, respectively. The simulated kinematic PPP results obtained using the SSR products from CLK93 and CLK51 performed better than those using other SSR products; and the accuracy of PPP based on all products was better than 6 and 10 cm in the horizontal and vertical directions, respectively. The real-time kinematic PPP experiment carried out in Beijing, Tianjin, and Shijiazhuang, China indicated that the SSR product CLK93 from Centre National d’Etudes Spatiales (CNES) had a better performance than CAS01. Moreover, the PPP with GPS + BDS dual systems had a higher accuracy than those with only a GPS single system.

CloudSat and CALIPSO within the A-Train: Ten Years of Actively Observing the Earth System

Abstract One of the most successful demonstrations of an integrated approach to observe Earth from multiple perspectives is the A-Train satellite constellation. The science enabled by this constellation flourished with the introduction of the two active sensors carried by the National Aeronautics and Space Administration (NASA) CloudSat and the NASA–Centre National d’Études Spatiales (CNES) Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellites that were launched together on 28 April 2006. These two missions have provided a 10-yr demonstration of coordinated formation flying that made it possible to develop integrated products and that offered new insights into key atmospheric processes. The progress achieved over this decade of observations, summarized in this paper, clearly demonstrate the fundamental importance of the vertical structure of clouds and aerosol for understanding the influences of the larger-scale atmospheric circulation on aerosol, the hydrological cycle, the cloud-scale physics, and the formation of the major storm systems of Earth. The research also underscored inherent ambiguities in radiance data in describing cloud properties and how these active systems have greatly enhanced passive observation. It is now clear that monitoring the vertical structure of clouds and aerosol is essential, and a climate data record is now being constructed. These pioneering efforts are to be continued with the Earth Clouds, Aerosol and Radiation Explorer (EarthCARE) mission planned for launch in 2019.

A Study on Re-entry Predictions of Uncontrolled Space Objects for Space Situational Awareness

The key risk analysis technologies for the re-entry of space objects into Earth’s atmosphere are divided into four categories: cataloguing and databases of the re-entry of space objects, lifetime and re-entry trajectory predictions, break-up models after re-entry and multiple debris distribution predictions, and ground impact probability models. In this study, we focused on reentry prediction, including orbital lifetime assessments, for space situational awareness systems. Re-entry predictions are very difficult and are affected by various sources of uncertainty. In particular, during uncontrolled re-entry, large spacecraft may break into several pieces of debris, and the surviving fragments can be a significant hazard for persons and properties on the ground. In recent years, specific methods and procedures have been developed to provide clear information for predicting and analyzing the re-entry of space objects and for ground-risk assessments. Representative tools include object reentry survival analysis tool (ORSAT) and debris assessment software (DAS) developed by National Aeronautics and Space Administration (NASA), spacecraft atmospheric re-entry and aerothermal break-up (SCARAB) and debris risk assessment and mitigation analysis (DRAMA) developed by European Space Agency (ESA), and semi-analytic tool for end of life analysis (STELA) developed by Centre National d’Etudes Spatiales (CNES). In this study, various surveys of existing re-entry space objects are reviewed, and an efficient re-entry prediction technique is suggested based on STELA, the life-cycle analysis tool for satellites, and DRAMA, a re-entry analysis tool. To verify the proposed method, the re-entry of the Tiangong-1 Space Lab, which is expected to re-enter Earth’s atmosphere shortly, was simulated. Eventually, these results will provide a basis for space situational awareness risk analyses of the re-entry of space objects.

Climate impact on malaria in northern Burkina Faso.

The Paluclim project managed by the French Centre National d'Etudes Spatiales (CNES) found that total rainfall for a 3-month period is a confounding factor for the density of malaria vectors in the region of Nouna in the Sahel administrative territory of northern Burkina Faso. Following the models introduced in 1999 by Craig et al. and in 2003 by Tanser et al., a climate impact model for malaria risk (using different climate indices) was created. Several predictions of this risk at different temporal scales (i.e. seasonal, inter-annual and low-frequency) were assessed using this climate model. The main result of this investigation was the discovery of a significant link between malaria risk and low-frequency rainfall variability related to the Atlantic Multi-decadal Oscillation (AMO). This result is critical for the health information systems in this region. Knowledge of the AMO phases would help local authorities to organise preparedness and prevention of malaria, which is of particular importance in the climate change context.

An investigation into the performance of real-time GPS+GLONASS Precise Point Positioning (PPP) in New Zealand

Abstract This paper presents an investigation into the performance of real-time Global Navigation Satellite Systems (GNSS) Precise Point Positioning (PPP) in New Zealand. The motivation of the research is to evaluate the feasibility of using PPP technique and a satellite based augmentation system such as the Japanese Quasi-Zenith Satellite System (QZSS) to deliver a real-time precise positioning solution in support of a nation-wide high accuracy GNSS positioning coverage in New Zealand. Two IGS real-time correction streams are evaluated alongside with the PPP correction messages transmitted by the QZSS satellite known as MDC1. MDC1 corrections stream is generated by Japan Aerospace Exploration Agency (JAXA) using the Multi-GNSS Advanced Demonstration tool for Orbit and Clock Analysis (MADOCA) software and are currently transmitted in test mode by the QZSS satellite. The IGS real-time streams are the CLK9B real-time corrections stream generated by the French Centre National D’études Spatiales (CNES) using the PPP-Wizard software, and the CLK81 real-time corrections stream produced by GMV using their MagicGNSS software. GNSS data is collected from six New Zealand CORS stations operated by Land Information New Zealand (LINZ) over a one-week period in 2015. GPS and GLONASS measurements are processed in a real-time PPP mode using the satellite orbit and clock corrections from the real-time streams. The results show that positioning accuracies of 6 cm in horizontal component and 15 cm in vertical component can be achieved in real-time PPP. The real-time GPS+GLONASS PPP solution required 30 minutes to converge to within 10 cm horizontal positioning accuracy.

Evaluating soil moisture retrievals from ESA's SMOS and NASA's SMAP brightness temperature datasets

Two satellites are currently monitoring surface soil moisture (SM) using L-band observations: SMOS (Soil Moisture and Ocean Salinity), a joint ESA (European Space Agency), CNES (Centre national d'études spatiales), and CDTI (the Spanish government agency with responsibility for space) satellite launched on November 2, 2009 and SMAP (Soil Moisture Active Passive), a National Aeronautics and Space Administration (NASA) satellite successfully launched in January 2015. In this study, we used a multilinear regression approach to retrieve SM from SMAP data to create a global dataset of SM, which is consistent with SM data retrieved from SMOS. This was achieved by calibrating coefficients of the regression model using the CATDS (Centre Aval de Traitement des Données) SMOS Level 3 SM and the horizontally and vertically polarized brightness temperatures (TB) at 40° incidence angle, over the 2013–2014 period. Next, this model was applied to SMAP L3 TB data from Apr 2015 to Jul 2016. The retrieved SM from SMAP (referred to here as SMAP_Reg) was compared to: (i) the operational SMAP L3 SM (SMAP_SCA), retrieved using the baseline Single Channel retrieval Algorithm (SCA); and (ii) the operational SMOSL3 SM, derived from the multiangular inversion of the L-MEB model (L-MEB algorithm) (SMOSL3). This inter-comparison was made against in situ soil moisture measurements from >400 sites spread over the globe, which are used here as a reference soil moisture dataset. The in situ observations were obtained from the International Soil Moisture Network (ISMN; https://ismn.geo.tuwien.ac.at/) in North of America (PBO_H2O, SCAN, SNOTEL, iRON, and USCRN), in Australia (Oznet), Africa (DAHRA), and in Europe (REMEDHUS, SMOSMANIA, FMI, and RSMN). The agreement was analyzed in terms of four classical statistical criteria: Root Mean Squared Error (RMSE), Bias, Unbiased RMSE (UnbRMSE), and correlation coefficient (R). Results of the comparison of these various products with in situ observations show that the performance of both SMAP products i.e. SMAP_SCA and SMAP_Reg is similar and marginally better to that of the SMOSL3 product particularly over the PBO_H2O, SCAN, and USCRN sites. However, SMOSL3 SM was closer to the in situ observations over the DAHRA and Oznet sites. We found that the correlation between all three datasets and in situ measurements is best (R>0.80) over the Oznet sites and worst (R=0.58) over the SNOTEL sites for SMAP_SCA and over the DAHRA and SMOSMANIA sites (R=0.51 and R=0.45 for SMAP_Reg and SMOSL3, respectively). The Bias values showed that all products are generally dry, except over RSMN, DAHRA, and Oznet (and FMI for SMAP_SCA). Finally, our analysis provided interesting insights that can be useful to improve the consistency between SMAP and SMOS datasets.

Development of Innovative Mechanical Flexible Solar Array Architecture

Flexible solar array architecture is proposed instead of rigid one, for high power solar array used from space tug mission to GEO satellite one, but also for specific scientific mission. THALES ALENIA SPACE has developed with Centre National d’Etudes Spatiales (CNES) support, innovative mechanical flexible solar array architecture, using 22 m long carbon tape-springs.The first part of the study was to design, predict, verify carbon tape-springs mechanical behaviour during deployment and coiling phases at room temperature. THALES ALENIA SPACE built mechanical flexible solar array mock-up full scale width / half scale length and performed deployment and rolling-up tests.The second part was to design, predict carbon tape-springs mechanical behaviour with large temperature range due to solar array missions. THALES ALENIA SPACE used again flexible solar array mock-up, performed deployment/coiling tests with innovative tape springs.This 11th ESPC paper describes results of these development activities and the THALES ALENIA SPACE forecast on this flexible solar array market.

An Inquiry on Contrast Enhancement Methods for Satellite Images

Enhancement algorithms are absolutely necessary for the visualization of both shadowed and bright image regions. Defining algorithms that permit to visualize them simultaneously without altering the image content is therefore extremely relevant for remote sensing applications. In this paper, we present the results of two successive benchmarks which tested the performance of the state-of-the-art contrast enhancement and tone-mapping algorithms applied to satellite images. Experts from the French Space Agency Centre National d'Etudes Spatiales (CNES), Service Regional de Traitement d'Image et de Teledetection (SERTIT), and two European universities assessed the quality and fidelity of the results of several state-of-the-art enhancement algorithms on the excerpts from seven images (five Pleiades and two simulated 30-cm images). The first benchmark permitted to tighten the procedure and the selection of the test images for the second one, and to make a first selection of concurrent algorithms. The second benchmark not only included the best algorithms selected by the first benchmark but also added even more competitors in the tone-mapping class. The results of both benchmarks were coherent. They point a particular retinex-based algorithm as the best compromise between the competitive requirements of a contrast enhancement in dark regions and a preservation of detail in bright parts.

Satellite radar altimetry water elevations performance over a 200 m wide river: Evaluation over the Garonne River

For at least 20 years, nadir altimetry satellite missions have been successfully used to first monitor the surface elevation of oceans and, shortly after, of large rivers and lakes. For the last 5–10 years, few studies have demonstrated the possibility to also observe smaller water bodies than previously thought feasible (river smaller than 500m wide and lake below 10km2). The present study aims at quantifying the nadir altimetry performance over a medium river (200m or lower wide) with a pluvio-nival regime in a temperate climate (the Garonne River, France). Three altimetry missions have been considered: ENVISAT (from 2002 to 2010), Jason-2 (from 2008 to 2014) and SARAL (from 2013 to 2014).Compared to nearby in situ gages, ENVISAT and Jason-2 observations over the lower Garonne River mainstream (110 km upstream of the estuary) have the smallest errors, with water elevation anomalies root mean square errors (RMSE) around 50cm and 20cm, respectively. The few ENVISAT upstream measurements have RMSE ranging from 80cm to 160cm. Over the estuary, ENVISAT and SARAL water elevation anomalies RMSE are around 30cm and 10cm, respectively.The most recent altimetry mission, SARAL, does not provide river elevation measurements for most satellite overflights of the river mainstream. The altimeter remains “locked” on the top of surrounding hilly areas and does not observe the steep-sided river valley, which could be 50–100m lower. This phenomenon is also observed, for fewer dates, on Jason-2 and ENVISAT measurements. In these cases, the measurement is not “erroneous”, it just does not correspond to water elevation of the river that is covered by the satellite. ENVISAT is less prone to get ‘locked’ on the top of the topography due to some differences in the instrument measurement parameters, trading lower accuracy for more useful measurements. Such problems are specific to continental surfaces (or near the coasts), but are not observed over the open oceans, which are flatter.To overcome this issue, an experimental instrument operating mode, called the DIODE/DEM tracking mode, has been developed by CNES (Centre National d’Etudes Spatiales) and has been tested during few Jason-2 cycles and during the first SARAL/AltiKA cycle. This tracking mode “forces” the instrument to observe a target of interest, i.e. water bodies. The example of the Garonne River shows, for one SARAL ground track, the benefit of the DIODE/DEM tracking mode for a steep-sided river reach, which is not detected using the nominal instrument operating mode. Yet, this mode relies on ancillary datasets (a priori global DEM and global land/water mask), which are critical to obtain river valley observation. The ultimately computed elevations along the satellite tracks, loaded on board, should have an absolute vertical accuracy around 10m (or better). This case also shows, when the instrument is correctly observing the river valley, that the altimeter can detect water bodies narrower than 100m (like an artificial canal).In agreement with recent studies, this work shows that altimeter missions can provide useful water elevation measurements over a 200m wide river with RMSE as low as 50cm and 20cm, for ENVISAT and Jason-2 respectively. The seasonal cycle can be observed with the temporal sampling of these missions (35 days and 10 days, respectively), but short term events, like flood events, are most of the time not observed. It also illustrates that altimeter capability to observe a river is highly dependent of the surrounding topography, the observation configuration, previous measurements and the instrument design. Therefore, it is not possible to generalize at global scale the minimum river width that could be seen by altimeters.This study analyzes, for the first time, the potential of the experimental DIODE/DEM tracking mode to observe steep-sided narrow river valleys, which are frequently missed with nominal tracking mode. For such case, using the DIODE/DEM mode could provide water elevation measurements, as long as the on board DEM is accurate enough. This mode should provide many more valid measurements over steep-sided rivers than currently observed.