Showing papers in "Advances in Space Research in 2017"

Abstract: Over the past five years, the International GNSS Service (IGS) has made continuous efforts to extend its service from GPS and GLONASS to the variety of newly established global and regional navigation satellite systems. This report summarizes the achievements and progress made in this period by the IGS Multi-GNSS Experiment (MGEX). The status and tracking capabilities of the IGS monitoring station network are presented and the multi-GNSS products derived from this resource are discussed. The achieved performance is assessed and related to the current level of space segment and user equipment characterization. While the performance of orbit and clock products for BeiDou, Galileo, and QZSS still lags behind the legacy GPS and GLONASS products, continued progress has been made since launch of the MGEX project and already enables use of the new constellations for precise point positioning, atmospheric research and other applications. Directions for further research are identified to fully integrate the new constellations into routine GNSS processing. Furthermore, the active support of GNSS providers is encouraged to assist the scientific community in the generation of fully competitive products for the new constellations.

Abstract: The nature of dark matter (DM) and dark energy (DE) which is supposed to constitute about 95% of the energy density of the universe is still a mystery. There is no shortage of ideas regarding the nature of both. While some candidates for DM are clearly ruled out, there is still a plethora of viable particles that fit the bill. In the context of DE, while current observations favour a cosmological constant picture, there are other competing models that are equally likely. This paper reviews the different possible candidates for DM including exotic candidates and their possible detection. This review also covers the different models for DE and the possibility of unified models for DM and DE. Keeping in mind the negative results in some of the ongoing DM detection experiments, here we also review the possible alternatives to both DM and DE (such as MOND and modifications of general relativity) and possible means of observationally distinguishing between the alternatives.

Abstract: The physics of energetic particle propagation in magnetized environments plays a crucial role in both the processes of acceleration and transport of cosmic rays. Recent theoretical developments in the field of cosmic ray research have been mainly in the direction of exploring non-linear aspects of the processes in which these particles are involved, namely the action of cosmic rays on the environment in which the transport and/or acceleration take place. When cosmic rays propagate outside of the acceleration region, such action is mainly in two forms: (1) they generate hydromagnetic waves, through streaming instabilities, leading to a dependence of the scattering properties of the medium on the spectrum and spatial distribution of the energetic particles, and (2) they exert a dynamical action on the plasma, which may cause the launching of cosmic ray driven Galactic winds. In this article we discuss these and other recent developments and how they compare with the bulk of new observations on the spectra of primary nuclei (mainly H and He) and secondary to primary ratios, such as the B/C ratio and the p ¯ / p ratio, and the positrons ratio e + / ( e - + e + ) . We also comment on some radically new models of the origin of CRs, in which the physical meaning of the secondary to primary ratios is not the same as in the standard model.

Abstract: Arctic sea ice is a major element of the Earth’s climate system. It acts to regulate regional heat and freshwater budgets and subsequent atmospheric and oceanic circulation across the Arctic and at lower latitudes. Satellites have observed a decline in Arctic sea ice extent for all months since 1979. However, to fully understand how changes in the Arctic sea ice cover impact on our global weather and climate, long-term and accurate observations of its thickness distribution are also required. Such observations were made possible with the launch of the European Space Agency’s (ESA’s) CryoSat-2 satellite in April 2010, which provides unparalleled coverage of the Arctic Ocean up to 88°N. Here we provide an end-to-end, comprehensive description of the data processing steps employed to estimate Northern Hemisphere sea ice thickness and subsequent volume using CryoSat-2 radar altimeter data and complementary observations. This is a sea ice processor that has been under constant development at the Centre for Polar Observation and Modelling (CPOM) since the early 1990s. We show that there is no significant bias in our satellite sea ice thickness retrievals when compared with independent measurements. We also provide a detailed analysis of the uncertainties associated with our sea ice thickness and volume estimates by considering the independent sources of error in the retrieval. Each month, the main contributors to the uncertainty are snow depth and snow density, which suggests that a crucial next step in Arctic sea ice research is to develop improved estimates of snow loading. In this paper we apply our theory and methods solely to CryoSat-2 data in the Northern Hemisphere. However, they may act as a guide to developing a sea ice processing system for satellite radar altimeter data over the Southern Hemisphere, and from other Polar orbiting missions.

Abstract: Scramjet is found to be the efficient method for the space shuttle. In this paper, numerical simulation is performed to investigate the fundamental flow physics of the interaction between an array of fuel jets and multi air jets in a supersonic transverse flow. Hydrogen as a fuel is released with a global equivalence ratio of 0.5 in presence of micro air jets on a flat plate into a Mach 4 crossflow. The fuel and air are injected through streamwise-aligned flush circular portholes. The hydrogen is injected through 4 holes with 7dj space when the air is injected in the interval of the hydrogen jets. The numerical simulation is performed by using the Reynolds-averaged Navier–Stokes equations with Menter’s Shear Stress Transport (SST) turbulence model. Both the number of air jets and jet-to-freestream total pressure ratio are varied in a parametric study. The interaction of the fuel and air jet in the supersonic flow present extremely complex feature of fuel and air jet. The results present various flow features depending upon the number and mass flow rate of micro air jets. These flow features were found to have significant effects on the penetration of hydrogen jets. A variation of the number of air jets, along with the jet-to-freestream total pressure ratio, induced a variety of flow structure in the downstream of the fuel jets.

Abstract: 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 500 m wide and lake below 10 km2). The present study aims at quantifying the nadir altimetry performance over a medium river (200 m 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 50 cm and 20 cm, respectively. The few ENVISAT upstream measurements have RMSE ranging from 80 cm to 160 cm. Over the estuary, ENVISAT and SARAL water elevation anomalies RMSE are around 30 cm and 10 cm, 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 50–100 m 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 10 m (or better). This case also shows, when the instrument is correctly observing the river valley, that the altimeter can detect water bodies narrower than 100 m (like an artificial canal). In agreement with recent studies, this work shows that altimeter missions can provide useful water elevation measurements over a 200 m wide river with RMSE as low as 50 cm and 20 cm, 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.

Abstract: Certainly, lineament mapping occupies an important place in several studies, including geology, hydrogeology and topography etc. With the help of remote sensing techniques, lineaments can be better identified due to strong advances in used data and methods. This allowed exceeding the usual classical procedures and achieving more precise results. The aim of this work is the comparison of ASTER, Landsat-8 and Sentinel 1 data sensors in automatic lineament extraction. In addition to image data, the followed approach includes the use of the pre-existing geological map, the Digital Elevation Model (DEM) as well as the ground truth. Through a fully automatic approach consisting of a combination of edge detection algorithm and line-linking algorithm, we have found the optimal parameters for automatic lineament extraction in the study area. Thereafter, the comparison and the validation of the obtained results showed that the Sentinel 1 data are more efficient in restitution of lineaments. This indicates the performance of the radar data compared to those optical in this kind of study.

Abstract: In the coastal ocean zones, satellite altimetry data processing and interpretation poses specific difficulties, due to the interaction of the radar signal with land topography, inaccuracies in some of the geophysical corrections and to the fast changes in the sea level. In order to optimize the completeness and the accuracy of the sea surface height information derived from satellite altimetry in coastal ocean areas, a dedicated post-processing software, called X-TRACK, has been developed by the Center of Topography of the Ocean and Hydrosphere in Toulouse. It is tailored for extending the use of altimetry data to coastal ocean applications and provides freely available along-track Sea Level Anomaly time series that cover today all the coastal oceans. Here, we present the improvements made in version 2016 of X-TRACK and show the gain in near-coastal data accuracy using in situ tide gauge observations. The correlations between altimeter and tide gauge sea level anomalies are higher (by 15% in average) compared with the previous version of X-TRACK. Three examples of applications are shown. The recent evolutions done in the X-TRACK processing result in an improved observation of the seasonal variations of the boundary circulation in the Bay of Biscay. Along Western Africa, sea-level variations derived from X-TRACK data are observed closer to land (5 km) compared to AVISO (10 km), and the sea-level statistics are more robust due to the larger and more stable data availability. Along-track empirical tidal constants derived from X-TRACK Sea Level Anomaly time series are also used to validate tidal models. By improving the altimetric data accuracy in coastal areas, we extend the field of marine applications.

Abstract: Autonomous close proximity operations are an arduous and attractive problem in space mission design. In particular, the estimation of pose, motion and inertia properties of an uncooperative object is a challenging task because of the lack of available a priori information. This paper develops a novel method to estimate the relative position, velocity, angular velocity, attitude and the ratios of the components of the inertia matrix of an uncooperative space object using only stereo-vision measurements. The classical Extended Kalman Filter (EKF) and an Iterated Extended Kalman Filter (IEKF) are used and compared for the estimation procedure. In addition, in order to compute the inertia properties, the ratios of the inertia components are added to the state and a pseudo-measurement equation is considered in the observation model. The relative simplicity of the proposed algorithm could be suitable for an online implementation for real applications. The developed algorithm is validated by numerical simulations in MATLAB using different initial conditions and uncertainty levels. The goal of the simulations is to verify the accuracy and robustness of the proposed estimation algorithm. The obtained results show satisfactory convergence of estimation errors for all the considered quantities. The obtained results, in several simulations, shows some improvements with respect to similar works, which deal with the same problem, present in literature. In addition, a video processing procedure is presented to reconstruct the geometrical properties of a body using cameras. This inertia reconstruction algorithm has been experimentally validated at the ADAMUS (ADvanced Autonomous MUltiple Spacecraft) Lab at the University of Florida. In the future, this different method could be integrated to the inertia ratios estimator to have a complete tool for mass properties recognition.

Abstract: The problem of spacecraft attitude stabilization control system with limited communication and external disturbances is investigated based on an event-triggered control scheme. In the proposed scheme, information of attitude and control torque only need to be transmitted at some discrete triggered times when a defined measurement error exceeds a state-dependent threshold. The proposed control scheme not only guarantees that spacecraft attitude control errors converge toward a small invariant set containing the origin, but also ensures that there is no accumulation of triggering instants. The performance of the proposed control scheme is demonstrated through numerical simulation.

Abstract: Unlike previous altimetric missions, the CryoSat-2 altimeter features a novel Synthetic Aperture Radar (SAR) mode that allows higher resolution and more accurate altimeter-derived parameters in the coastal zone, thanks to the reduced along-track footprint. The scope of this study is to quantify regionally the skills of CryoSat-2 SAR altimetry for distances to coast smaller than 10 km, during the mission lifetime and at different time scales. The validated geophysical altimeter parameters are the sea surface height above the ellipsoid, the significant sea wave height and wind speed, all computed at 20 Hz. These have been compared to in situ and regional model data along the coasts of German Bight and West Baltic Sea during a time interval of almost six years, from July 2010 to March 2016, to investigate both instantaneous and seasonal behaviour. From CryoSat-2 FBR (Full Bit Rate) data, a Delay-Doppler processing and waveform retracking tailored specifically to the coastal zone has been carried out, by applying a Hamming window and zero-padding, using an extended vertical swath window in order to mitigate tracker errors. Moreover, a dedicated SAMOSA-based coastal retracker (here referred to as SAMOSA+) has also been implemented. Since one of the highest remaining uncertainties in the altimeter parameters estimated in coastal shallow waters arises from residual errors in the applied range and geophysical corrections, innovative and high resolution solutions for ocean tide model, geoid, mean sea surface and wet tropospheric correction have been selected. As CryoSat-2 SAR and LRM (Low Rate Mode) modes are not collocated in time, in order to quantify the improvement in the coastal zone with respect to pulse-limited altimetry, 20 Hz PLRM (pseudo-LRM) data from CryoSat-2 FBR were built and retracked, adopting the ALES adaptive sub-waveform approach, with a numerical Brown-based retracker, here referred to as TALES. The cross-validation proves the good consistency between PLRM and SAR sea level anomaly in the coastal zone. The regional ocean model (BSH) shows the highest agreement with the SAR sea level anomaly, with a standard deviation of the differences (stdd) of 24 cm, whereas the corresponding value with respect to PLRM is 55 cm. Distance to coast plots show that land contamination begins to affect sea level and wave measurements at 2 km from the coast in SAR and at 3.5 km in PLRM TALES. The analysis of monthly mean time-series shows that SAR Altimetry is able to measure the sea level monthly mean in the coastal zone of the region of interest, during the entire mission, more precisely than PLRM. The cross-validation against in situ data also proves the higher accuracy of SAR SAMOSA+ compared to PLRM TALES in the coastal zone, with average SLA stdd of 4.4 cm and 8.4 cm respectively.

Abstract: This paper proposes a fractional order sliding mode control for the deployment of tethered space systems with the consideration of uncertainty of external disturbances and unmodeled system dynamics. The proposed fractional order sliding mode control consists of two sub-sliding manifolds that are defined separately for the actuated and unactuated states. This, in turn, generates a control scheme to make all states move toward to the desired states. The stability analysis of the proposed control law indicates not only all states converge to the desired states at equilibrium but also disturbances caused by the uncertainty can be suppressed satisfactorily. Parametric studies are conducted to investigate the influences of fractional order and sub-sliding manifold of unactuated states on the performance of the proposed control law. The performance is compared with the sliding mode, PD and fractional order PD control laws for a baseline scenario of tether deployment. The proposed control law performs better than others in the settling time and the maximum pitch angle control in the presence of unwanted disturbances. Effectiveness and robustness of the proposed control law are demonstrated by computer simulations.

Abstract: The level of solar modulation at different times (related to the solar activity) is a central question of solar and galactic cosmic-ray physics. In the first paper of this series, we have established a correspondence between the uncertainties on ground-based detectors count rates and the parameter ϕ (modulation level in the force-field approximation) reconstructed from these count rates. In this second paper, we detail a procedure to obtain a reference ϕ time series from neutron monitor data. We show that we can have an unbiased and accurate ϕ reconstruction ( Δ ϕ / ϕ ≃ 10 % ). We also discuss the potential of Bonner spheres spectrometers and muon detectors to provide ϕ time series. Two by-products of this calculation are updated ϕ values for the cosmic-ray database and a web interface to retrieve and plot ϕ from the 50’s to today ( http://lpsc.in2p3.fr/crdb ).

Abstract: The atmospheric effects that influence on the signal registered by remote sensors might be minimized in order to provide reliable spectral information. In aquatic systems, the application of atmospheric correction aims to minimize such effects and avoid the under or overestimation of remote sensing reflectance (Rrs). Accurately Rrs provides better information about the state of aquatic system, it means, establishing the concentration of aquatic compounds more precisely. The aim of this study is to evaluate the outputs from several atmospheric correction methods (Dark Object Subtraction – DOS; Quick Atmospheric Correction – QUAC; Fast Line-of-sight Atmospheric Analysis of Hypercubes – FLAASH; Atmospheric Correction for OLI ‘lite’ – ACOLITE, and Provisional Landsat-8 Surface Reflectance Algorithm – L8SR) in order to investigate the suitability of Rrs for estimating total suspended matter concentrations (TSM) in the Barra Bonita Hydroelectrical Reservoir. To establish TSM concentrations via atmospherically corrected Operational Land Imager (OLI) scene, the TSM retrieval model was calibrated and validated with in situ data. Thereby, the achieved results from TSM retrieval model application demonstrated that L8SR is able to provide the most suitable Rrs values for green and red spectral bands, and consequently, the lowest TSM retrieval errors (Mean Absolute Percentage Error about 10% and 12%, respectively). Retrieved Rrs from near infrared band is still a challenge for all the tested algorithms.

Abstract: Our knowledge of the internal structure of asteroids is, so far, indirect – relying entirely on inferences from remote sensing observations of the surface, and theoretical modeling of formation and evolution. What are the bulk properties of the regolith and deep interior? And what are the physical processes that shape asteroid internal structures? Is the composition and size distribution observed on the surface representative of the bulk? These questions are crucial to understand small bodies’ history from accretion in the early Solar System to the present, and direct measurements are needed to answer these questions for the benefit of science as well as for planetary defense or exploration. Radar is one of the main instruments capable of sounding asteroids to characterize internal structure from sub-meter to global scale. In this paper, we review the science case for direct observation of the deep internal structure and regolith of a rocky asteroid of kilometer size or smaller. We establish the requirements and model dielectric properties of asteroids to outline a possible instrument suite, and highlight the capabilities of radar instrumentation to achieve these observations. We then review the expected science return including secondary objectives contributing to the determination of the gravitational field, the shape model, and the dynamical state. This work is largely inherited from MarcoPolo-R and AIDA/AIM studies.

Abstract: In response to the changing world of GNSS, the International GNSS Service (IGS) has initiated the Multi-GNSS Experiment (MGEX). As part of the MGEX project, initial precise orbit and clock products have been released for public use, which are the key prerequisites for multi-GNSS precise point positioning (PPP). In particular, precise orbits and clocks at intervals of 5 min and 30 s are presently available for the new emerging systems. This paper investigates the benefits of multi-GNSS for PPP. Firstly, orbit and clock consistency tests (between different providers) were performed for GPS, GLONASS, Galileo and BeiDou. In general, the differences of GPS are, respectively, 1.0–1.5 cm for orbit and 0.1 ns for clock. The consistency of GLONASS is worse than GPS by a factor of 2–3, i.e. 2–4 cm for orbit and 0.2 ns for clock. However, the corresponding differences of Galileo and BeiDou are significantly larger than those of GPS and GLONASS, particularly for the BeiDou GEO satellites. Galileo as well as BeiDou IGSO/MEO products have a consistency of 0.1–0.2 m for orbit, and 0.2–0.3 ns for clock. As to BeiDou GEO satellites, the difference of their orbits reaches 3–4 m in along-track, 0.5–0.6 m in cross-track, and 0.2–0.3 m in the radial directions, together with an average RMS of 0.6 ns for clock. Furthermore, the short-term stability of multi-GNSS clocks was analyzed by Allan deviation. Results show that clock stability of the onboard GNSS is highly dependent on the satellites generations, operational lifetime, orbit types, and frequency standards. Finally, kinematic PPP tests were conducted to investigate the contribution of multi-GNSS and higher rate clock corrections. As expected, the positioning accuracy as well as convergence speed benefit from the fusion of multi-GNSS and higher rate of precise clock corrections. The multi-GNSS PPP improves the positioning accuracy by 10–20%, 40–60%, and 60–80% relative to the GPS-, GLONASS-, and BeiDou-only PPP. The usage of 30 s interval clock products decreases interpolation errors, and the positioning accuracy is improved by an average of 30–50% for the all the cases except for the BeiDou-only PPP.

Abstract: Sentinel-1A is the first satellite of the European Copernicus programme Equipped with a Synthetic Aperture Radar (SAR) instrument the satellite was launched on April 3, 2014 Operational since October 2014 the satellite delivers valuable data for more than two years The orbit accuracy requirements are given as 5 cm in 3D In order to fulfill this stringent requirement the precise orbit determination (POD) is based on the dual-frequency GPS observations delivered by an eight-channel GPS receiver The Copernicus POD (CPOD) Service is in charge of providing the orbital and auxiliary products required by the PDGS (Payload Data Ground Segment) External orbit validation is regularly performed by comparing the CPOD Service orbits to orbit solutions provided by POD expert members of the Copernicus POD Quality Working Group (QWG) The orbit comparisons revealed systematic orbit offsets mainly in radial direction (approx 3 cm) Although no independent observation technique (eg DORIS, SLR) is available to validate the GPS-derived orbit solutions, comparisons between the different antenna phase center variations and different reduced-dynamic orbit determination approaches used in the various software packages helped to detect the cause of the systematic offset An error in the given geometry information about the satellite has been found After correction of the geometry the orbit validation shows a significant reduction of the radial offset to below 5 mm The 5 cm orbit accuracy requirement in 3D is fulfilled according to the results of the orbit comparisons between the different orbit solutions from the QWG

Abstract: Distributed Space Missions such as formation flight and constellations, are being recognized as important Earth Observation solutions to increase measurement samples over space and time. Cubesats are increasing in size (27U, ∼40 kg in development) with increasing capabilities to host imager payloads. Given the precise attitude control systems emerging in the commercial market, Cubesats now have the ability to slew and capture images within short notice. We propose a modular framework that combines orbital mechanics, attitude control and scheduling optimization to plan the time-varying, full-body orientation of agile Cubesats in a constellation such that they maximize the number of observed images and observation time, within the constraints of Cubesat hardware specifications. The attitude control strategy combines bang-bang and PD control, with constraints such as power consumption, response time, and stability factored into the optimality computations and a possible extension to PID control to account for disturbances. Schedule optimization is performed using dynamic programming with two levels of heuristics, verified and improved upon using mixed integer linear programming. The automated scheduler is expected to run on ground station resources and the resultant schedules uplinked to the satellites for execution, however it can be adapted for onboard scheduling, contingent on Cubesat hardware and software upgrades. The framework is generalizable over small steerable spacecraft, sensor specifications, imaging objectives and regions of interest, and is demonstrated using multiple 20 kg satellites in Low Earth Orbit for two case studies – rapid imaging of Landsat’s land and coastal images and extended imaging of global, warm water coral reefs. The proposed algorithm captures up to 161% more Landsat images than nadir-pointing sensors with the same field of view, on a 2-satellite constellation over a 12-h simulation. Integer programming was able to verify that optimality of the dynamic programming solution for single satellites was within 10%, and find up to 5% more optimal solutions. The optimality gap for constellations was found to be 22% at worst, but the dynamic programming schedules were found at nearly four orders of magnitude better computational speed than integer programming. The algorithm can include cloud cover predictions, ground downlink windows or any other spatial, temporal or angular constraints into the orbital module and be integrated into planning tools for agile constellations.

Abstract: It is clear that we can benefit from multi-constellation GNSS in precise relative positioning. On the other hand, it is still an open problem how to combine multi-GNSS signals in a single functional model. This study presents methodology and quality assessment of selected methods allowing for multi-GNSS observations combining in relative kinematic positioning using baselines up to tens of kilometers. In specific, this paper characterizes loose and tight integration strategies applied to the ionosphere and troposphere weighted model. Performance assessment of the established strategies was based on the analyses of the integer ambiguity resolution and rover coordinates’ repeatability obtained in the medium range instantaneous RTK positioning with the use of full constellation dual frequency GPS and Galileo signals. Since full constellation of Galileo satellites is not yet available, the observational data were obtained from a hardware GNSS signal simulator using regular geodetic GNSS receivers. The results indicate on similar and high performance of the loose, and tight integration with calibrated receiver ISBs strategies. These approaches have undeniable advantage over single system positioning in terms of reliability of the integer ambiguity resolution as well as rover coordinate repeatability.

Abstract: The regulation of the carbon cycle is a critical ecosystem service provided by forests globally. It is, therefore, necessary to have robust techniques for speedy assessment of forest biophysical parameters at the landscape level. It is arduous and time taking to monitor the status of vast forest landscapes using traditional field methods. Remote sensing and GIS techniques are efficient tools that can monitor the health of forests regularly. Biomass estimation is a key parameter in the assessment of forest health. Polarimetric SAR (PolSAR) remote sensing has already shown its potential for forest biophysical parameter retrieval. The current research work focuses on the retrieval of forest biophysical parameters of tropical deciduous forest, using fully polarimetric spaceborne C-band data with Polarimetric SAR Interferometry (PolInSAR) techniques. PolSAR based Interferometric Water Cloud Model (IWCM) has been used to estimate aboveground biomass (AGB). Input parameters to the IWCM have been extracted from the decomposition modeling of SAR data as well as PolInSAR coherence estimation. The technique of forest tree height retrieval utilized PolInSAR coherence based modeling approach. Two techniques – Coherence Amplitude Inversion (CAI) and Three Stage Inversion (TSI) – for forest height estimation are discussed, compared and validated. These techniques allow estimation of forest stand height and true ground topography. The accuracy of the forest height estimated is assessed using ground-based measurements. PolInSAR based forest height models showed enervation in the identification of forest vegetation and as a result height values were obtained in river channels and plain areas. Overestimation in forest height was also noticed at several patches of the forest. To overcome this problem, coherence and backscatter based threshold technique is introduced for forest area identification and accurate height estimation in non-forested regions. IWCM based modeling for forest AGB retrieval showed R2 value of 0.5, RMSE of 62.73 (t ha−1) and a percent accuracy of 51%. TSI based PolInSAR inversion modeling showed the most accurate result for forest height estimation. The correlation between the field measured forest height and the estimated tree height using TSI technique is 62% with an average accuracy of 91.56% and RMSE of 2.28 m. The study suggested that PolInSAR coherence based modeling approach has significant potential for retrieval of forest biophysical parameters.

Abstract: The Mini-EUSO instrument is a UV telescope to be placed inside the International Space Station (ISS), looking down on the Earth from a nadir-facing window in the Russian Zvezda module. Mini-EUSO will map the earth in the UV range (300–400 nm) with a spatial resolution of 6.11 km and a temporal resolution of 2.5 μs, offering the opportunity to study a variety of atmospheric events such as transient luminous events (TLEs) and meteors, as well as searching for strange quark matter and bioluminescence. Furthermore, Mini-EUSO will be used to detect space debris to verify the possibility of using a EUSO-class telescope in combination with a high energy laser for space debris remediation. The high-resolution mapping of the UV emissions from Earth orbit allows Mini-EUSO to serve as a pathfinder for the study of Extreme Energy Cosmic Rays (EECRs) from space by the JEM-EUSO collaboration.

Abstract: For more than 25 years, satellite radar altimetry has provided continuous information on the state of the cryosphere and on its contribution to global sea-level rise. The technique typically delivers maps of ice-sheet elevation and elevation change with 3–10 km spatial resolution and seasonal to monthly temporal resolution. Here we show how the interferometric mode of CryoSat-2 can be used to map broad (5 km-wide) swaths of surface elevation with fine (500 m) spatial resolution from each satellite pass, providing a step-change in the capability of satellite altimetry for glaciology. These swaths of elevation data contain up to two orders of magnitude more surface elevation measurements than standard altimeter products, which provide single elevation measurements based on the range to the Point-Of-Closest-Approach (POCA) in the vicinity of the sub-satellite ground track. The swath elevations allow a more dense, statistically robust time series of elevation change to be formed with temporal resolution of a factor 5 higher than for POCA. The mean differences between airborne altimeter and CryoSat-2 derived ice sheet elevations and elevation rates range from −0.93 ± 1.17 m and 0.29 ± 1.25 m a−1, respectively, at the POCA, to −1.50 ± 1.73 m and 0.04 ± 1.04 m a−1, respectively, across the entire swath. We demonstrate the potential of these data by creating and evaluating elevation models of: (i) the Austfonna Ice Cap (Svalbard), (ii) western Greenland, and (iii) Law Dome (East Antarctica); and maps of ice elevation change of: (iv) the Amundsen Sea sector (West Antarctica), (v) Icelandic ice caps, and (vi) above an active subglacial lake system at Thwaites Glacier (Antarctica), each at 500 m spatial posting – around 10 times finer than possible using traditional approaches based on standard altimetry products.

Abstract: Gossamer structures for innovative space applications, such as solar sails, require technology that allows their controlled and thereby safe deployment. Before employing such technology for a dedicated science mission, it is desirable, if not necessary, to demonstrate its reliability with a Technology Readiness Level (TRL) of six or higher. The aim of the work presented here is to provide reliable technology that enables the controlled deployment and verification of its functionality with various laboratory tests, thereby qualifying the hardware for a first demonstration in low Earth orbit (LEO). The development was made in the Gossamer-1 project of the German Aerospace Center (DLR). This paper provides an overview of the Gossamer-1 mission and hardware development. The system is designed based on the requirements of a technology demonstration mission. The design rests on a crossed boom configuration with triangular sail segments. Employing engineering models, all aspects of the deployment were tested under ambient environment. Several components were also subjected to environmental qualification testing. An innovative stowing and deployment strategy for a controlled deployment, as well as the designs of the bus system, mechanisms and electronics are described. The tests conducted provide insights into the deployment process and allow a mechanical characterization of that deployment process, in particular the measurement of the deployment forces. Deployment on system level could be successfully demonstrated to be robust and controllable. The deployment technology is on TRL four approaching level five, with a qualification model for environmental testing currently being built.

Abstract: A perspective is given of the solar modulation of galactic cosmic rays in the heliosphere during the extraordinary quiet solar minimum period from 2006 to 2009. This is done in the context of the total modulation of cosmic rays in a global heliosphere. Such an approach has become possible since the observation of galactic cosmic rays made beyond the heliopause by Voyager 1 so that together with very precise observations at higher energies at the Earth, more reliable local interstellar spectra can be established. Combined with the results from comprehensive modeling, a global view unfolds. The requirements for such an approach to the modeling of solar modulation are discussed. Computed, modulated spectra for protons and electrons are shown, for 2006–2009 together with computed radial and latitudinal gradients for protons, in comparison with observations where available. Predictions are made for the energy ranges not covered by the 2006–2009 observations. Respectively, the modulation factor for protons, electrons and positrons are given for energies as low as 5 MeV for the mentioned period. The computed electron to positron ratio is presented as applicable to solar minimum modulation conditions. The differences in the modulation of protons, electrons and positrons are illustrated for such conditions and the main contributions to this global process, including particle drifts, are discussed.

Abstract: With the recent revitalization of GLONASS constellation and two newly emerging constellations of BeiDou and Galileo, multi-constellation integration has become a trend in Global Navigation Satellite System (GNSS) development. In order to provide a good indication of the positioning and navigation performance under the current GNSS constellations for the global users, the availability of four-constellation integration with GPS, GLONASS, BeiDou and Galileo should be investigated. In this study, the availability with the use of 31 GPS satellites, 24 GLONASS satellites, 18 BeiDou satellites and 11 Galileo satellites on a global scale is evaluated in terms of the number of visible satellites, the Position Dilution of Precision (PDOP) and the percentage of time span over which the position solutions can be acquired over the total time span during approximately a repeat cycle of orbits for all types of satellites. The effects of compatibility between different satellite systems on availability are discussed. In addition, datasets collected at 59 globally distributed four-system stations on 30 consecutive days are employed to fully assess the performance of four-constellation integrated dual-frequency precise point positioning (PPP), single-frequency PPP and single point positioning (SPP). The results indicate that the multi-constellation integration can significantly improve the availability and positioning accuracy. The enhancement of compatibility can also improve the availability. The availability can reflect the positioning performance.

Abstract: To fulfill the mission of targets recognition over sea, a bi-satellite cluster composed of an autonomous low resolution satellite (LRS) leading the formation for targets detection and a trailing agile high resolution satellite (HRS) for targets recognition is considered. This paper focuses on the development of a method that is able to generate a schedule plan onboard the HRS taking into account the information received from the LRS, which amounts to solving an agile earth observation satellite (AEOS) scheduling problem. The main contributions of this paper are two folds: a mathematical model for formulating the AEOS scheduling problem, and an anytime branch and bound algorithm for problem solution. Experimental results on a set of representative scenarios show that the proposed algorithm is effective which promotes significantly the bi-satellite cluster to improve the efficiency of targets recognition over sea as opposed to traditional methods where a large number of satellites are required to work coordinately. In particular, in a scenario over a 500 km × 2000 km sea area involving 25 targets, the performance of the bi-satellite cluster amounts to the coordination of 30 high resolution satellites.

Abstract: Knowledge about the rotation properties of space debris objects is essential for the active debris removal missions, accurate re-entry predictions and to investigate the long-term effects of the space environment on the attitude motion change. Different orbital regions and object’s physical properties lead to different attitude states and their change over time. Since 2007 the Astronomical Institute of the University of Bern (AIUB) performs photometric measurements of space debris objects. To June 2016 almost 2000 light curves of more than 400 individual objects have been acquired and processed. These objects are situated in all orbital regions, from low Earth orbit (LEO), via global navigation systems orbits and high eccentricity orbit (HEO), to geosynchronous Earth orbit (GEO). All types of objects were observed including the non-functional spacecraft, rocket bodies, fragmentation debris and uncorrelated objects discovered during dedicated surveys. For data acquisition, we used the 1-meter Zimmerwald Laser and Astrometry Telescope (ZIMLAT) at the Swiss Optical Ground Station and Geodynamics Observatory Zimmerwald, Switzerland. We applied our own method of phase-diagram reconstruction to extract the apparent rotation period from the light curve. Presented is the AIUB’s light curve database and the obtained rotation properties of space debris as a function of object type and orbit.

Abstract: Government space agencies, including NASA and ESA, are conducting preliminary studies on building alternative space-habitat systems for deep-space exploration. Such studies include development of advanced technologies for planetary surface exploration, including an in-depth understanding of the use of local resources. Currently, NASA plans to land humans on Mars in the 2030s. Similarly, other space agencies from Europe (ESA), Canada (CSA), Russia (Roscosmos), India (ISRO), Japan (JAXA) and China (CNSA) have already initiated or announced their plans for launching a series of lunar missions over the next decade, ranging from orbiters, landers and rovers for extended stays on the lunar surface. As the Space Odyssey is one of humanity’s oldest dreams, there has been a series of research works for establishing temporary or permanent settlement on other planetary bodies, including the Moon and Mars. This paper reviews current projects developing extra-terrestrial construction, broadly categorised as: (i) ISRU-based construction materials; (ii) fabrication methods; and (iii) construction processes. It also discusses four categories of challenges to developing an appropriate construction process: (i) lunar simulants; (ii) material fabrication and curing; (iii) microwave-sintering based fabrication; and (iv) fully autonomous and scaled-up construction processes.

Abstract: Multipath is one of the primary error sources in high precision GNSS applications. Since it is highly correlated with observation environments, the multipath effect is difficult to be parameterized with an empirical model or eliminated by current differencing techniques. A sophisticated multipath extraction and mitigation technique is proposed. The technique uses the spectrum density of the time series of single-difference (SD) phase residuals to identify which portions of the observation environments contribute the various multipath constituents. Wavelet analysis is used to extract the time-varying frequency and magnitude contents of multipath. Multipath templates are built to assess the performance of ambiguity resolution before and after multipath mitigation. Using GPS data measured at the Forth Road Bridge in Scotland, we identify that there are two types of multipath with different affecting characteristics on the bridge. The initial analysis reveals that the correlations between adjacent days remain higher than 80% for both carrier phase and pseudorange multipath. Further comparisons indicate that the standard deviations of the residuals are reduced roughly by 30% for most of the satellites when multipath templates are applied, whereas the reductions of the mean standard deviations of the coordinate components, from 13 consecutive days, maintain stable at about 30% for a 1.5 km baseline and 45% for a 36 m baseline. It is also evident that ambiguity resolution has significant improvement with applying multipath mitigation, contributing to more accurate and reliable ambiguity results in high-precision deformation monitoring.

Abstract: Solar radiation pressure (SRP) is the dominant non-gravitational perturbation of global navigation satellite system (GNSS) satellites. In the absence of detailed surface models, empirical SRP models, such as the Empirical CODE Orbit Model (ECOM), are widely used in practice for GNSS orbit determination but may require an undue number of parameters to properly describe the actual motion. Building up on previous research for spacecraft in yaw-steering (YS) attitude, analytical expressions for the SRP acceleration in orbit-normal (ON) attitude are established based on a generic box-wing model, and related to the corresponding parameters of the ECOM. The results are used to obtain an a priori SRP model for the QZS-1 satellite of the Quasi Zenith Satellite System (QZSS), which achieves a modeling accuracy of about 1 nm/s 2 using as little as 6 parameters. To compensate remaining modeling deficiencies, we combine the analytical a priori model with a complementary set of five empirical parameters based on an ECOM-type formulation. QZS-1 orbits based on the resulting “semi-analytical” SRP model exhibit a better than 10 cm RMS consistency with satellite laser ranging measurements for both YS and ON attitude modes, which marks a 2–4 times improvement over legacy orbit products without a priori model.