Showing papers in "Advances in Space Research in 2016"

Abstract: The Asteroid Impact & Deflection Assessment (AIDA) mission is a joint cooperation between European and US space agencies that consists of two separate and independent spacecraft that will be launched to a binary asteroid system, the near-Earth asteroid Didymos, to test the kinetic impactor technique to deflect an asteroid. The European Asteroid Impact Mission (AIM) is set to rendezvous with the asteroid system to fully characterise the smaller of the two binary components a few months prior to the impact by the US Double Asteroid Redirection Test (DART) spacecraft. AIM is a unique mission as it will be the first time that a spacecraft will investigate the surface, subsurface, and internal properties of a small binary near–Earth asteroid. In addition it will perform various important technology demonstrations that can serve other space missions. The knowledge obtained by this mission will have great implications for our understanding of the history of the Solar System. Having direct information on the surface and internal properties of small asteroids will allow us to understand how the vaious processes they undergo work and transform these small bodies as well as, for this particular case, how a binary system forms. Making these measurements from up close and comparing them with ground-based data from telescopes will also allow us to calibrate remote observations and improve our data interpretation of other systems. With DART, thanks to the characterization of the target by AIM, the mission will be the first fully documented impact experiment at asteroid scale, which will include the characterization of the target’s properties and the outcome of the impact. AIDA will thus offer a great opportunity to test and refine our understanding and models at the actual scale of an asteroid, and to check whether the current extrapolations of material strength from laboratory-scale targets to the scale of AIDA’s target are valid. Moreover, it will offer a first check of the validity of the kinetic impactor concept to deflect a small body and lead to improved efficiency for future kinetic impactor designs. This paper focuses on the science return of AIM, the current knowledge of its target from ground-based observations, and the instrumentation planned to get the necessary data.

Abstract: Desertification is a serious ecological, environmental, and socio-economic threat to the world, and there is a pressing need to develop a reasonable and reproducible method to assess it at different scales. In this paper, the Hogno Khaan protected area in Mongolia was selected as the study area, and a quantitative method for assessing land cover change and desertification assessment was developed using Landsat TM/ETM+ data on a local scale. In this method, NDVI (Normalized Difference Vegetation Index), TGSI (Topsoil Grain Size Index), and land surface albedo were selected as indicators for representing land surface conditions from vegetation biomass, landscape pattern, and micrometeorology. A Decision Tree (DT) approach was used to assess the land cover change and desertification of the Hogno Khaan protected area in 1990, 2002, and 2011. Our analysis showed no correlation between NDVI and albedo or TGSI but high correlation between TGSI and albedo. Strong correlations (0.77–0.92) between TGSI and albedo were found in the non-desertification areas. The TGSI was less strongly correlated with albedo in the low and non desertification areas, at 0.70 and 0.92; respectively. The desertification of the study area is increasing each year; in the desertification map for 1990–2002, there is a decrease in areas of zero and low desertification, and an increase in areas of high and severe desertification. From 2002 to 2011, areas of non desertification increased significantly, with areas of severe desertification also exhibiting increase, while areas of medium and high desertification demonstrated little change.

Abstract: Previous generations of X-ray observatories revealed a group of massive binaries that were relatively bright X-ray emitters. This was attributed to emission of shock-heated plasma in the wind–wind interaction zone located between the stars. With the advent of the current generation of X-ray observatories, the phenomenon could be studied in much more detail. In this review, we highlight the progress that has been achieved in our understanding of the phenomenon over the last 15 years, both on theoretical and observational grounds. All these studies have paved the way for future investigations using the next generation of X-ray satellites that will provide crucial information on the X-ray emission formed in the innermost part of the wind–wind interaction.

Abstract: The Swarm mission consists of three satellites orbiting the Earth at low orbital altitudes. The onboard GPS receivers, star cameras, and laser retro-reflectors make the Swarm mission an interesting candidate to explore the contribution of Swarm GPS data to the recovery of both the static and time-variable gravity fields. We use 1.5 years of Swarm GPS and attitude data to generate kinematic positions of high quality to perform gravity field determination using the Celestial Mechanics Approach. The generated gravity fields reveal severe systematic errors along the geomagnetic equator. Their size is correlated with the ionospheric density and thus strongly varying over the analyzed time period. Similar to the findings of the GOCE mission, the systematic errors are related to the Swarm GPS carrier phase data and may be reduced by rejecting GPS data affected by large ionospheric changes. Such a measure yields a strong reduction of the systematic errors along the geomagnetic equator in the gravity field recovery. Long wavelength signatures of the gravity field may then be recovered with a similar quality as achieved with GRACE GPS data, which makes the Swarm mission well suited to bridge a potential gap between the current GRACE and the future GRACE Follow-On mission.

Abstract: For more than 14 years, the Gravity Recovery and Climate Experiment (GRACE) mission has provided information about Earth’s gravity field with unprecedented accuracy. The twin satellites GRACE-A and GRACE-B are both equipped with a three-axis electrostatic accelerometer, measuring the non-gravitational forces acting on the spacecraft. In order to make use of the uncalibrated Level-1B accelerometer (ACC1B) data during gravity field recovery, bias and scale parameters have to be estimated. The proposed calibration method is a two-step approach and makes use of modeled non-conservative accelerations. The simulated non-conservative accelerations serve as reference for the a priori accelerometer calibration, i.e. for the ACC1B data. During gravity field recovery the calibration parameters are re-estimated. Several calibration parameters for the GRACE accelerometers using different methods have already been published. The aim of our study was primarily to analyze the temperature-dependent behavior of the accelerometer scale factors and biases, and the impact of the parametrization of scale factors and biases on the recovered gravity field solutions; but not to obtain calibrated accelerometer data. Within the ITSG-Grace2016 release, the accelerometer biases are estimated daily using uniform cubic basis splines (UCBS), the scale factors are also estimated daily using a fully-populated scale factor matrix. Therefore, not only the scale factors in along-track, cross-track, and radial direction are estimated, but also the non-orthogonality of the accelerometer axes (cross-talk) and the misalignment between the Accelerometer Frame (AF) and Science Reference Frame (SRF) are taken into account. The time evolution of the estimated calibration parameters over the whole GRACE period (2002-04 to 2016-01) shows a clear temperature-dependency for both scale factors and biases. Using this new approach, the estimates of the C20 coefficient significantly improve, with results now comparable to Satellite Laser Ranging (SLR) solutions. Based on the achieved results, we suggest the presence of a clear temperature-dependent behavior and the presence of off-diagonal elements in the accelerometer scale factor matrix.

Abstract: This study investigates the use of the non-linear model predictive control (NMPC) strategy for a kinematically redundant space robot to approach an un-cooperative target in complex space environment. Collision avoidance, traditionally treated as a high level planning problem, can be effectively translated into control constraints as part of the NMPC. The objective of this paper is to evaluate the performance of the predictive controller in a constrained workspace and to investigate the feasibility of imposing additional constraints into the NMPC. In this paper, we reformulated the issue of the space robot motion control by using NMPC with predefined objectives under input, output and obstacle constraints over a receding horizon. An on-line quadratic programming (QP) procedure is employed to obtain the constrained optimal control decisions in real-time. This study has been implemented for a 7 degree-of-freedom (DOF) kinematically redundant manipulator mounted on a 6 DOF free-floating spacecraft via simulation studies. Real-time trajectory tracking and collision avoidance particularly demonstrate the effectiveness and potential of the proposed NMPC strategy for the space robot.

Abstract: Long considered as the “odd man out” among X-ray emitting Be stars, γ Cas (B0.5e IV) is now recognized as the prototype of a class of stars that emit hard thermal X-rays. Our classification differs from the historical use of the term “ γ Cas stars” defined from optical properties alone. The luminosity output of this class contributes significantly to the hard X-ray production of massive stars in the Galaxy. The γ Cas stars have light curves showing variability on a few broadly-defined timescales and spectra indicative of an optically thin plasma consisting of one or more hot thermal components. By now 9–13 Galactic ≈ B0-1.5e main sequence stars are judged to be members or candidate members of the γ Cas class. Conservative criteria for this designation are for a ≈ B0-1.5e III–V star to have an X-ray luminosity of 1032–1033 ergs s−1, a hot thermal spectrum containing the short wavelength Lyα Fe XXV and Fe XXVI lines and the fluorescence FeK feature all in emission. If thermality cannot be demonstrated, for example from either the presence of these Lyα lines or curvature of the hard continuum of the spectrum of an X-ray active Be star, we call them γ Cas candidates. We discuss the history of the discovery of the complicated characteristics of the variability in the optical, UV, and X-ray domains, leading to suggestions for the physical cause of the production of hard X-rays. These include scenarios in which matter from the Be star accretes onto a degenerate secondary star and interactions between magnetic fields on the Be star and its decretion disk. The greatest aid to the choice of the causal mechanism is the temporal correlations of X-ray light curves and spectra with diagnostics in the optical and UV wavebands. We show why the magnetic star-disk interaction scenario is the most tenable explanation for the creation of hard X-rays on these stars.

Abstract: Broadcast ionospheric model is currently an effective approach to mitigate the ionospheric time delay for real-time Global Navigation Satellite System (GNSS) single-frequency users. Klobuchar coefficients transmitted in Global Positioning System (GPS) navigation message have been widely used in various GNSS positioning and navigation applications; however, this model can only reduce the ionospheric error by approximately 50% in mid-latitudes. With the emerging BeiDou and Galileo, as well as the modernization of GPS and GLONASS, more precise ionospheric correction models or algorithms are required by GNSS single-frequency users. Numerical analysis of the initial phase and nighttime term in Klobuchar algorithm demonstrates that more parameters should be introduced to better describe the variation of nighttime ionospheric total electron content (TEC). In view of this, several schemes are proposed for the improvement of Klobuchar algorithm. Performance of these improved Klobuchar-like models are validated over the continental and oceanic regions during high (2002) and low (2006) levels of solar activities, respectively. Over the continental region, GPS TEC generated from 35 International GNSS Service (IGS) and the Crust Movement Observation Network of China (CMONOC) stations are used as references. Over the oceanic region, TEC data from TOPEX/Poseidon and JASON-1 altimeters are used for comparison. A ten-parameter Klobuchar-like model, which describes the nighttime term as a linear function of geomagnetic latitude, is finally proposed for GNSS single-frequency ionospheric corrections. Compared to GPS TEC, while GPS broadcast model can correct for 55.0% and 49.5% of the ionospheric delay for the year 2002 and 2006, respectively, the proposed ten-parameter Klobuchar-like model can reduce the ionospheric error by 68.4% and 64.7% for the same period. Compared to TOPEX/Poseidon and JASON-1 TEC, the improved ten-parameter Klobuchar-like model can mitigate the ionospheric delay by 61.1% and 64.3% in 2002 and 2006, respectively.

Abstract: This paper presents a method for rapid generation of three-dimensional low-thrust trajectories that utilizes Fourier series for shaping the position vector. The generated trajectories are feasible with respect to the given thrust acceleration constraints. An objective function is defined representing the overall mission cost, i.e. minimum Δ V . Four missions from Earth to Mars, the near Earth asteroid 1989ML, comet Tempel 1 and asteroid Dionysus are considered for assessing the performance of the algorithm. The selected missions present a range of various difficulties with different levels of thrust acceleration constraints. The Fourier series technique is flexible in generating various shapes rather than using one global shape. The proposed method is capable of rapid generation of sub-optimal feasible trajectories that are totally different from and comparable to the solutions of the state-of-the-art three-dimensional shape-based methods. This feature is quite favorable at the preliminary stages of low-thrust mission designs where various trajectory alternatives are required. The results also show that the obtained trajectories can be used as initial guesses for high fidelity optimal control tools.

Abstract: In preparation of the 2014 realization of the International Terrestrial Reference Frame (ITRF2014), the International DORIS Service delivered to the International Earth Rotation and Reference Systems Service a set of 1140 weekly solution files including station coordinates and Earth orientation parameters, covering the time period from 1993.0 to 2015.0. The data come from eleven DORIS satellites: TOPEX/Poseidon, SPOT2, SPOT3, SPOT4, SPOT5, Envisat, Jason-1, Jason-2, Cryosat-2, Saral and HY-2A. In their processing, the six analysis centers which contributed to the DORIS combined solution used the latest time variable gravity models and estimated DORIS ground beacon frequency variations. Furthermore, all the analysis centers but one excepted included in their processing phase center variations for ground antennas. The main objective of this study is to present the combination process and to analyze the impact of the new modeling on the performance of the new combined solution. Comparisons with the IDS contribution to ITRF2008 show that (i) the application of the DORIS ground phase center variations in the data processing shifts the combined scale upward by nearly 7–11 mm and (ii) thanks to estimation of DORIS ground beacon frequency variations, the new combined solution no longer shows any scale discontinuity in early 2002 and does not present unexplained vertical discontinuities in any station position time series. However, analysis of the new series with respect to ITRF2008 exhibits a scale increase late 2011 which is not yet explained. A new DORIS Terrestrial Reference Frame was computed to evaluate the intrinsic quality of the new combined solution. That evaluation shows that the addition of data from the new missions equipped with the latest generation of DORIS receiver (Jason-2, Cryosat-2, HY-2A, Saral), results in an internal position consistency of 10 mm or better after mid-2008.

Abstract: A numerical model for the solar modulation of cosmic rays, based on the solution of a set of stochastic differential equations (SDEs), is used to illustrate the effects of modifying the heliospheric magnetic field, particularly in the polar regions of the heliosphere. SDE-based models are well suited for such studies so that new insights are gained. To this end, the differences in the modulation brought about by each of three choices for the heliospheric magnetic field, i.e. the unmodified Parker field, the Smith–Bieber modified field, and the Jokipii–Kota modified field, are studied as typical well-known cases. It is illustrated that although both these modifications change the Parker field satisfactorily in the polar regions of the heliosphere, the Smith–Bieber modification is more effective in reducing cosmic ray drift effects in these regions. The features of these two modifications, as well as the effects on the solar modulation of cosmic rays, are illustrated qualitatively and quantitatively. In particular, it is shown how the Smith–Bieber modified field is applied in a cosmic ray modulation model to reproduce observational proton spectra from the PAMELA mission during the solar minimum of 2006–2009. These SDE-based results are compared with those obtained in previous studies of this unusual solar minimum activity period and found to be in good qualitative agreement.

Abstract: The main objective of this paper is to introduce the STABLE (Surface flux Transport And Babcock–LEighton) solar dynamo model. STABLE is a 3D Babcock–Leighton/Flux Transport dynamo model in which the source of poloidal field is the explicit emergence, distortion, and dispersal of bipolar magnetic regions (BMRs). Here we describe the STABLE model in more detail than we have previously and we verify it by reproducing a 2D mean-field benchmark. We also present some representative dynamo simulations, focusing on the special case of kinematic magnetic induction and axisymmetric flow fields. Not all solutions are supercritical; it can be a challenge for the BL mechanism to sustain the dynamo when the turbulent diffusion near the surface is ⩾ 10 12 cm2 s−1. However, if BMRs are sufficiently large, deep, and numerous, then sustained, cyclic, dynamo solutions can be found that exhibit solar-like features. Furthermore, we find that the shearing of radial magnetic flux by the surface differential rotation can account for most of the net toroidal flux generation in each hemisphere, as has been recently argued for the Sun by Cameron and Schussler (2015) .

Abstract: Several lines of evidence suggest that the velocity amplitude in global simulations of solar convection, U , may be systematically over-estimated. Motivated by these recent results, we explore the factors that determine U and we consider how these might scale to solar parameter regimes. To this end, we decrease the thermal diffusivity κ along two paths in parameter space. If the kinematic viscosity ν is decreased proportionally with κ (fixing the Prandtl number P r = ν / κ ), we find that U increases but asymptotes toward a constant value, as found by Featherstone and Hindman (2016). However, if ν is held fixed while decreasing κ (increasing P r ), we find that U systematically decreases. We attribute this to an enhancement of the thermal content of downflow plumes, which allows them to carry the solar luminosity with slower flow speeds. We contrast this with the case of Rayleigh–Benard convection which is not subject to this luminosity constraint. This dramatic difference in behavior for the two paths in parameter space (fixed P r or fixed ν ) persists whether the heat transport by unresolved, near-surface convection is modeled as a thermal conduction or as a fixed flux. The results suggest that if solar convection can operate in a high- P r regime, then this might effectively limit the velocity amplitude. Small-scale magnetism is a possible source of enhanced viscosity that may serve to achieve this high- P r regime.

Abstract: The effects of dynamical coupling between the rotational (attitude) and translational (orbital) motion of spacecraft near small Solar System bodies is investigated This coupling arises due to the weak gravity of these bodies, as well as solar radiation pressure The traditional approach assumes a point-mass spacecraft model to describe the translational motion of the spacecraft, while the attitude motion is considered to be completely decoupled from the translational motion The model used here to describe the rigid-body spacecraft dynamics includes the non-uniform rotating gravity field of the small body up to second degree and order along with the attitude dependent terms, solar tide, and solar radiation pressure This model shows that the second degree and order gravity terms due to the small body affect the dynamics of the spacecraft to the same extent as the orbit–attitude coupling due to the primary gravity (zeroth order) term Variational integrators are used to simulate the dynamics of both the rigid spacecraft and the point mass The small bodies considered here are modeled after Near-Earth Objects (NEO) 101955 Bennu, and 25143 Itokawa, and are assumed to be triaxial ellipsoids with uniform density Differences in the numerically obtained trajectories of a rigid spacecraft and a point mass are then compared, to illustrate the impact of the orbit–attitude coupling on spacecraft dynamics in proximity of small bodies Possible implications on the performance of model-based spacecraft control and on the station-keeping budget, if the orbit–attitude coupling is not accounted for in the model of the dynamics, are also discussed An almost globally asymptotically stable motion estimation scheme based solely on visual/optical feedback that estimates the relative motion of the asteroid with respect to the spacecraft is also obtained This estimation scheme does not require a model of the dynamics of the asteroid, which makes it perfectly suited for asteroids whose properties are not well known

Abstract: The integration of Global Positioning System (GPS) with Inertial Navigation System (INS) has been very intensively developed and widely applied in multiple areas. To further enhance the reliability and availability of GPS/INS integrated navigation in GPS challenging environment, range observation through ultra-wideband (UWB) is introduced in GPS/INS tightly coupled navigation. An improved robust Kalman filter is proposed and used to resist the influence of gross error from UWB observation in GPS/UWB/IMU tightly coupled navigation. The variance of the squared Mahalanobis distance in moving window is calculated, which brings as new judgement factor for gross errors in order to decrease the rate of false outlier identification. A simulation analysis shows that the improved robust Kalman filter is able to correctly identify gross errors and the rate of false judgment as zero. In order to validate the new robust filter, a real experiment is conducted. The results indicate that the improved robust Kalman filter used in GPS/UWB/INS tightly coupled navigation is able to remove the harmful effect of gross error in UWB observation. It clearly illustrates that the improved robust Kalman filter is very effective, and all the simulated small and large gross errors added to UWB distance observation are successfully identified.

Abstract: Debris removal in Earth orbits is an urgent issue to be faced for space exploitation durability. Among different techniques, tethered-nets present appealing benefits and some open points to fix. Former and latter are discussed in the paper, supported by the exploitation of a multibody dynamics tool. With respect to other proposed capture mechanisms, tethered-net solutions are characterised by a safer capturing distance, a passive angular momentum damping effect and the highest flexibility to unknown shape, material and attitude of the target to interface with. They also allow not considering the centre of gravity alignment with thrust axis as a constraint, as it is for any rigid link solution. Furthermore, the introduction of a closing thread around the net perimeter ensures safer and more reliable grasping and holding. In the paper, a six degrees of freedom multibody dynamics simulator is presented: it was developed at Politecnico di Milano – Department of Aerospace Science and Technologies – and it is able to describe the orbital and attitude dynamics of tethered-nets systems and end-bodies during different phases, with great flexibility in dealing with different topologies and configurations. Critical phases as impact and wrapping are analysed by simulation to address the tethered-stack controllability. It is shown how the role of contact modelling is fundamental to describe the coupled dynamics: it is demonstrated, as a major novel contribution, how friction between the net and a tumbling target allows reducing its angular motion, stabilizing the system and allowing safer towing operations. Moreover, the so-called tethered space tug is analysed: after capture, the two objects, one passive and one active, are connected by the tethered-net flexible link, the motion of the system being excited by the active spacecraft thrusters. The critical modes prevention during this phase, by means of a closed-loop control synthesis is shown. Finally, the connection between flexible dynamics and capture system design is highlighted, giving engineering answers to most challenging open points to lead to a ready to flight solution.

Abstract: A regional cross-calibration between the first Delay–Doppler altimetry dataset from CryoSat-2 and a retracked Envisat dataset is here presented, in order to test the benefits of the Delay–Doppler processing and to expand the Envisat time series in the coastal ocean. The Indonesian Seas are chosen for the calibration, since the availability of altimetry data in this region is particularly beneficial due to the lack of in situ measurements and its importance for global ocean circulation. The Envisat data in the region are retracked with the Adaptive Leading Edge Subwaveform (ALES) retracker, which has been previously validated and applied successfully to coastal sea level research. The study demonstrates that CryoSat-2 is able to decrease the 1-Hz noise of sea level estimations by 0.3 cm within 50 km of the coast, when compared to the ALES-reprocessed Envisat dataset. It also shows that Envisat can be confidently used for detailed oceanographic research after the orbit change of October 2010. Cross-calibration at the crossover points indicates that in the region of study a sea state bias correction equal to 5% of the significant wave height is an acceptable approximation for Delay–Doppler altimetry. The analysis of the joint sea level time series reveals the geographic extent of the semiannual signal caused by Kelvin waves during the monsoon transitions, the larger amplitudes of the annual signal due to the Java Coastal Current and the impact of the strong La Nina event of 2010 on rising sea level trends.

Abstract: This report is the response to a request by the Committee on Space Research of the International Council for Science to prepare a roadmap on observation and integrated Earth-system science for the coming ten years. Its focus is on the combined use of observations and modelling to address the functioning, predictability and projected evolution of interacting components of the Earth system on timescales out to a century or so. It discusses how observations support integrated Earth-system science and its applications, and identifies planned enhancements to the contributing observing systems and other requirements for observations and their processing. All types of observation are considered, but emphasis is placed on those made from space. The origins and development of the integrated view of the Earth system are outlined, noting the interactions between the main components that lead to requirements for integrated science and modelling, and for the observations that guide and support them. What constitutes an Earth-system model is discussed. Summaries are given of key cycles within the Earth system. The nature of Earth observation and the arrangements for international coordination essential for effective operation of global observing systems are introduced. Instances are given of present types of observation, what is already on the roadmap for 2016–2025 and some of the issues to be faced. Observations that are organised on a systematic basis and observations that are made for process understanding and model development, or other research or demonstration purposes, are covered. Specific accounts are given for many of the variables of the Earth system. The current status and prospects for Earth-system modelling are summarized. The evolution towards applying Earth-system models for environmental monitoring and prediction as well as for climate simulation and projection is outlined. General aspects of the improvement of models, whether through refining the representations of processes that are already incorporated or through adding new processes or components, are discussed. Some important elements of Earth-system models are considered more fully. Data assimilation is discussed not only because it uses observations and models to generate datasets for monitoring the Earth system and for initiating and evaluating predictions, in particular through reanalysis, but also because of the feedback it provides on the quality of both the observations and the models employed. Inverse methods for surface-flux or model-parameter estimation are also covered. Reviews are given of the way observations and the processed datasets based on them are used for evaluating models, and of the combined use of observations and models for monitoring and interpreting the behaviour of the Earth system and for predicting and projecting its future. A set of concluding discussions covers general developmental needs, requirements for continuity of space-based observing systems, further long-term requirements for observations and other data, technological advances and data challenges, and the importance of enhanced international co-operation.

Abstract: The Beidou Navigation Satellite System (BDS) manages to estimate simultaneously the orbits and clock offsets of navigation satellites, using code and carrier phase measurements of a regional network within China. The satellite clock offsets are also directly measured with Two-way Satellite Time Frequency Transfer (TWSTFT). Satellite laser ranging (SLR) residuals and comparisons with the precise ephemeris indicate that the radial error of GEO satellites is much larger than that of IGSO and MEO satellites and that the BDS orbit accuracy is worse than GPS. In order to improve the orbit determination accuracy for BDS, a new orbit determination strategy is proposed, in which the satellite clock measurements from TWSTFT are fixed as known values, and only the orbits of the satellites are solved. However, a constant systematic error at the nanosecond level can be found in the clock measurements, which is obtained and then corrected by differencing the clock measurements and the clock estimates from orbit determination. The effectiveness of the new strategy is verified by a GPS regional network orbit determination experiment. With the IGS final clock products fixed, the orbit determination and prediction accuracy for GPS satellites improve by more than 50% and the 12-h prediction User Range Error (URE) is better than 0.12 m. By processing a 25-day of measurement from the BDS regional network, an optimal strategy for the satellite-clock-fixed orbit determination is identified. User Equivalent Ranging Error is reduced by 27.6% for GEO satellites, but no apparent reduction is found for IGSO/MEO satellites. The SLR residuals exhibit reductions by 59% and 32% for IGSO satellites but no reductions for GEO and MEO satellites.

Abstract: The impressive development of global numerical simulations of turbulent stellar interiors unveiled a variety of possible differential rotation (solar or anti-solar), meridional circulation (single or multi-cellular), and dynamo states (stable large scale toroidal field or periodically reversing magnetic fields). Various numerical schemes, based on the so-called anelastic set of equations, were used to obtain these results. It appears today mandatory to assess their robustness with respect to the details of the numerics, and in particular to the treatment of turbulent sub-grid scales. We report on an ongoing comparison between two global models, the ASH and EULAG codes. In EULAG the sub-grid scales are treated implicitly by the numerical scheme, while in ASH their effect is generally modeled by using enhanced dissipation coefficients. We characterize the sub-grid scales effect in a turbulent convection simulation with EULAG. We assess their effect at each resolved scale with a detailed energy budget. We derive equivalent eddy-diffusion coefficients and use the derived diffusivities in twin ASH numerical simulations. We find a good agreement between the large-scale flows developing in the two codes in the hydrodynamic regime, which encourages further investigation in the magnetohydrodynamic regime for various dynamo solutions.

Abstract: The present work is the second of a three-part review of space weather in Latin America, specifically observing its evolution in three countries (Argentina, Brazil and Mexico). This work comprises a summary of scientific challenges in space weather research that are considered to be open scientific questions and how they are being addressed in terms of instrumentation by the international community, including the Latin American groups. We also provide an inventory of the networks and collaborations being constructed in Latin America, including details on the data processing, capabilities and a basic description of the resulting variables. These instrumental networks currently used for space science research are gradually being incorporated into the space weather monitoring data pipelines as their data provides key variables for monitoring and forecasting space weather, which allow these centers to monitor space weather and issue watches, warnings and alerts.

Abstract: We present a set of thermophysical property models for lunar regolith. Data from over 25 sources in the literature are integrated and fit with regression models for the following properties: composition, density, specific heat, latent heat of melting/fusion, thermal conductivity, electrical conductivity, optical absorption length, Gibbs Free Energy and Enthalpy of Formation. The models are based on data from Apollo samples and high-temperature molten regolith simulants, extending significantly beyond existing models in the literature. Furthermore, separate regression models are presented for Mare and Highlands regolith to demonstrate the effect of composition and to allow the models to be tailored to a wide range of applications. These models can enable more consistent, informed analysis and design of lunar regolith processing hardware and can also support lunar geological simulations. In addition to regression models for each material property, the raw data are presented to allow for further interpretation and fitting as necessary.

Abstract: The aim of this article is to study how the differential rotation of solar-like stars is influenced by rotation rate and mass in presence of magnetic fields generated by a convective dynamo. We use the ASH code to model the convective dynamo of solar-like stars at various rotation rates and masses, hence different effective Rossby numbers. We obtained models with either prograde (solar-like) or retrograde (anti-solar-like) differential rotation. The trends of differential rotation versus stellar rotation rate obtained for simulations including the effect of the magnetic field are weaker compared with hydro simulations ( Δ Ω ∝ ( Ω / Ω ⊙ ) 0.44 in the MHD case and Δ Ω ∝ ( Ω / Ω ⊙ ) 0.89 in the hydro case), hence showing a better agreement with the observations. Analysis of angular momentum transport revealed that the simulations with retrograde and prograde differential rotation have opposite distribution of the viscous, turbulent Reynolds stresses and meridional circulation contributions. The thermal wind balance is achieved in the prograde cases. However, in retrograde cases Reynolds stresses are dominant for high latitudes and near the top of the convective layer. Baroclinic effects are stronger for faster rotating models.

Abstract: Mechanisms to mitigate climate change in tropical countries such as India require information on forest structural components i.e., biomass and carbon for conservation steps to be implemented successfully. The present study focuses on investigating the potential use of a one time, QuadPOL ALOS PALSAR L-band 25 m data to estimate above-ground biomass (AGB) using a water cloud model (WCM) in a wildlife sanctuary in India. A significant correlation was obtained between the SAR-derived backscatter coefficient ( σ °) and the field measured AGB, with the maximum coefficient of determination for cross-polarized (HV) σ ° for Shorea robusta , and the weakest correlation was observed with co-polarized (HH) σ ° for Tectona grandis forests. The biomass of S. robusta and that of T. grandis were estimated on the basis of field-measured data at 444.7 ± 170.4 Mg/ha and 451 ± 179.4 Mg/ha respectively. The mean biomass values estimated using the WCM varied between 562 and 660 Mg/ha for S. robusta ; between 590 and 710 Mg/ha for T. grandis using various polarized data. Our results highlighted the efficacy of one time, fully polarized PALSAR data for biomass and carbon estimate in a dense forest.

Abstract: Recent experience with prominent formation flying missions in Low Earth Orbit (LEO), such as GRACE and TanDEM-X, has shown the feasibility of precise relative navigation at millimeter and sub-millimeter levels using GPS carrier phase measurements with fixed integer ambiguities. However, the robustness and availability of the solutions provided by current algorithms may be highly dependent on the mission profile. The main challenges faced in the LEO scenario are the resulting short continuous carrier phase tracking arcs along with the observed rapidly changing ionospheric conditions, which in the particular situation of long baselines increase the difficulty of correct integer ambiguity resolution. To reduce the impact of these factors, the present study proposes a strategy based on a reduced-dynamics filtering of dual-frequency GPS measurements for precise baseline determination along with a dedicated scheme for integer ambiguity resolution, consisting of a hybrid sequential/batch algorithm based on the maximum a posteriori and integer aperture estimators. The algorithms have been tested using flight data from the GRACE, TanDEM-X and Swarm missions in order to assess their robustness to different formation and baseline configurations. Results with the GRACE mission show an average 0.7 mm consistency with the K/Ka-band ranging measurements over a period of more than two years in a baseline configuration of 220 km. Results with TanDEM-X data show an average of 3.8 mm consistency of kinematic and reduced-dynamic solutions in the along-track component over a period of 40 days in baseline configurations of 500 m and 75 km. Data from Swarm A and Swarm C spacecraft are largely affected by atmospheric scintillation and contain half cycle ambiguities. The results obtained under such conditions show an overall consistency between kinematic and reduced-dynamic solutions of 1.7 cm in the along-track component over a period of 30 days in a variable baseline of approximately 60–175 km. An analysis of one orbital period excluding a region where errors due to atmospheric scintillation occur, shows a consistency between kinematic and reduced-dynamic solutions of 3 mm in the along-track direction.

Abstract: The mitigation of ionospheric delay is still of crucial interest in GNSS positioning, especially in precise solutions such as instantaneous RTK positioning. Thus, several effective algorithms and functional models were developed, and also numerous investigations of ionospheric correction properties in RTK positioning have been performed so far. One of the most highly effective approaches in precise relative positioning is the application of the ionosphere-weighted model with network-derived corrections. This contribution investigates the impact of the accuracy of the network ionospheric corrections on time-to-fix in RTK-OTF positioning. Also, an attempt has been made to estimate the desirable accuracy of the network ionospheric corrections, allowing for reliable instantaneous ambiguity resolution. The experiment is based on a multi-baseline GPS RTK positioning supported with network-derived ionospheric corrections for medium length baselines. The results show that in such scenario, the double-differenced ionospheric correction residuals should not exceed ∼1/3 of the L1 wavelength for successful single-epoch ambiguity resolution.

Abstract: Arbitrary amplitude nonlinear low frequency electrostatic soliton and supersoliton structures are studied in magnetized four-component auroral plasmas composed of a cold singly charged oxygen-ion fluid, Boltzmann distribution of hot protons and two distinct group of electron species. Using the Sagdeev pseudo-potential technique, the characteristics of obliquely propagating nonlinear structures are investigated analytically and numerically. The model supports the evolution of soliton and supersoliton structures in the auroral acceleration region. Depending on the parametric region, the positive and negative potential solitons coexists at lower Mach numbers, but at higher Mach numbers only negative potential solitons and supersolitons can exist. The presence of hot protons restricted the Mach number of the nonlinear structures to exist only at the subsonic region. The present investigation concurs with the Swedish Viking satellite observations in the auroral region.

Abstract: Recent studies of the space debris population in low Earth orbit (LEO) have concluded that certain regions have already reached a critical density of objects. This will eventually lead to a cascading process called the Kessler syndrome. The time may have come to seriously consider active debris removal (ADR) missions as the only viable way of preserving the space environment for future generations. Among all objects in the current environment, the SL-8 (Kosmos 3M second stages) rocket bodies (R/Bs) are some of the most suitable targets for future robotic ADR missions. However, to date, an autonomous relative navigation to and capture of an non-cooperative target has never been performed. Therefore, there is a need for more advanced, autonomous and modular systems that can cope with uncontrolled, tumbling objects. The guidance, navigation and control (GNC) system is one of the most critical ones. The main objective of this paper is to present a preliminary concept of a modular GNC architecture that should enable a safe and fuel-efficient capture of a known but uncooperative target, such as Kosmos 3M R/B. In particular, the concept was developed having in mind the most critical part of an ADR mission, i.e. close range proximity operations, and state of the art algorithms in the field of autonomous rendezvous and docking. In the end, a brief description of the hardware in the loop (HIL) testing facility is made, foreseen for the practical evaluation of the developed architecture.

Abstract: We describe a novel data-processing and analysis pipeline for optical observations of moving objects, either of natural (asteroids, meteors) or artificial origin (satellites, space debris). The monitoring of the space object populations requires reliable acquisition of observational data, to support the development and validation of population models and to build and maintain catalogues of orbital elements. The orbital catalogues are, in turn, needed for the assessment of close approaches (for asteroids, with the Earth; for satellites, with each other) and for the support of contingency situations or launches. For both types of populations, there is also increasing interest to detect fainter objects corresponding to the small end of the size distribution. The ESA-funded StreakDet (streak detection and astrometric reduction) activity has aimed at formulating and discussing suitable approaches for the detection and astrometric reduction of object trails, or streaks, in optical observations. Our two main focuses are objects in lower altitudes and space-based observations (i.e., high angular velocities), resulting in long (potentially curved) and faint streaks in the optical images. In particular, we concentrate on single-image (as compared to consecutive frames of the same field) and low-SNR detection of objects. Particular attention has been paid to the process of extraction of all necessary information from one image (segmentation), and subsequently, to efficient reduction of the extracted data (classification). We have developed an automated streak detection and processing pipeline and demonstrated its performance with an extensive database of semisynthetic images simulating streak observations both from ground-based and space-based observing platforms. The average processing time per image is about 13 s for a typical 2k-by-2k image. For long streaks (length >100 pixels), primary targets of the pipeline, the detection sensitivity (true positives) is about 90% for both scenarios for the bright streaks ( SNR > 1 ), while in the low-SNR regime, the sensitivity is still 50% at SNR = 0.5 .

Abstract: BVRI photometric observations of Geosynchronous Earth Orbit (GEO) objects were conducted with the 15 m Cassini Telescope located in Loiano, Italy The observatory is operated by the INAF (National Institute for Astrophysics) Astronomical Observatory of Bologna, Italy The Ritchey–Chretien optical system is equipped with the BFOSC (Bologna Faint Object Spectrograph and Camera), a multipurpose instrument for imaging and spectroscopy, with an EEV CCD (1340 × 1300 pixel) This paper deals with the results of the photometric observations of several targets from the SSN (Space Surveillance Network) catalog that were acquired in May and December 2013 In particular: • 1 piece of debris from Ekran: SSN 29014 • 1 piece of debris from LES 8: SSN 13753 • 5 SL-12 rocket bodies: SSN 38104, 17125, 20926, 17705 and 27444 • 2 IUS rocket bodies: SSN 19913, 21641 • 3 operational GEO satellite: SSN 34810, 27509, 28912 • 1 non-operational GEO satellites: SSN 02653 Observations of Landolt standard fields were performed for calibration purposes In addition, long exposures with sidereal tracking with no filter have been taken where the object image is trailed to study the brightness variability over timescales of a second This paper describes the results of the code developed in order to detect the primary frequencies of the object’s brightness variation