Showing papers in "Advances in Space Research in 2015"

Abstract: There is a growing appreciation that the environmental conditions that we call space weather impact the technological infrastructure that powers the coupled economies around the world. With that co ...

Abstract: This paper presents a contemporary review of vertical coupling in the atmosphere and ionosphere system induced by internal waves of lower atmospheric origin. Atmospheric waves are primarily generated by meteorological processes, possess a broad range of spatial and temporal scales, and can propagate to the upper atmosphere. A brief summary of internal wave theory is given, focusing on gravity waves, solar tides, planetary Rossby and Kelvin waves. Observations of wave signatures in the upper atmosphere, their relationship with the direct propagation of waves into the upper atmosphere, dynamical and thermal impacts as well as concepts, approaches, and numerical modeling techniques are outlined. Recent progress in studies of sudden stratospheric warming and upper atmospheric variability are discussed in the context of wave-induced vertical coupling between the lower and upper atmosphere.

Abstract: We present in detail the scientific objectives in fundamental physics of the Space-Time Explorer and 4. It carries out tests of different aspects of the Einstein Equivalence Principle using atomic clocks, matter wave interferometry and long distance time/frequency links, providing fascinating science at the interface between quantum mechanics and gravitation that cannot be achieved, at that level of precision, in ground experiments. We especially emphasize the specific strong interest of performing equivalence principle tests in the quantum regime, i.e. using quantum atomic wave interferometry. Although STE-QUEST was finally not selected in early 2014 because of budgetary and technological reasons, its science case was very highly rated. Our aim is to expose that science to a large audience in order to allow future projects and proposals to take advantage of the STE-QUEST experience.

Abstract: Multi-constellation GNSS precise point positioning (PPP) first became feasible back to 2007 but with only two constellations, namely GPS and GLONASS. With the availability of more satellites and precise orbit and clock products from BeiDou and Galileo, it is possible now to investigate PPP with four constellations, namely GPS, BeiDou, GLONASS and Galileo. This research aims at investigating the quad-constellation PPP for position determination and analyzing its positioning performance. A quad-constellation PPP model is developed to simultaneously process the observations from all the four GNSS systems. The developed model is also applicable to the PPP processing with observations from single, dual or triple constellations. The analysis on PPP accuracy and convergence time is conducted based on data processing results from both static and kinematic tests of single-constellation and multi-constellations. The three-hour static positioning results indicate that the BeiDou-only PPP accuracy is worse than the GPS-only PPP. The RMSs of position errors for BeiDou-only PPP are 5.2 cm, 2.7 cm and 8.3 cm in east, north and up directions while the ones for GPS-only PPP are 3.9 cm, 1.6 cm and 5.7 cm. The GPS/BeiDou PPP improves the positioning accuracy by 28%, 6% and 7% and reduces the convergence time by 26%, 13% and 14% over the GPS-only PPP in three coordinate components, respectively. The GPS/GLONASS PPP achieves slightly better performance than the GPS/BeiDou PPP. The triple-constellation PPP further increases the positioning accuracy and decreases the convergence time over the dual-constellation PPP. The improvement of positioning performance is not significant after adding Galileo due to currently limited number of satellites. Similar to the static positioning, the quad-constellation kinematic PPP also significantly improves the positioning performance in contrast with single-constellation and dual-constellations. The time varying characteristics of the time differences between the four systems are also investigated. The results indicate that the system time differences of GPS with BeiDou, GLONASS and Galileo are very stable over time with STD values of better than 1.1 ns.

Abstract: This article discusses the attitude modes employed by present Global (and Regional) Navigation Satellite Systems (GNSSs) and the models used to describe them along with definitions of the constellation-specific spacecraft body frames. A uniform convention for the labeling of the principal spacecraft axes is proposed by the International GNSS Service (IGS), which results in a common formulation of the nominal attitude of all GNSS satellites in yaw-steering mode irrespective of their specific orbit and constellation. The conventions defined within this document provide the basis for the specification of antenna phase center offsets and variations in a multi-GNSS version of the IGS absolute phase center model in the ANTEX (antenna exchange) format. To facilitate the joint analysis of GNSS observations and satellite laser ranging measurements, laser retroreflector array coordinates consistent with the IGS-specific spacecraft frame conventions are provided in addition to representative antenna offset values for all GNSS constellations.

Abstract: The mass density of Earth’s thermosphere (∼90–600 km altitude) is a critical parameter for low Earth orbit prediction because of the atmospheric drag on satellites in this region. In this review, we first survey techniques for measuring thermospheric density, empirical models that provide a synthesis of historical data, and physical models that simulate the environment by solving fluid equations. We then review the climate and weather features that are observed in thermospheric density (including its response to solar forcing) and summarize recent studies of these features. The review is focused on results published between 2000 and 2014, which coincides with a period of extensive accelerometer measurements of density and accompanying research; some historical context is also provided.

Abstract: The Multi-GNSS Experiment (MGEX) of the International GNSS Service (IGS) aims at the data collection and analysis of all available satellite navigation systems. In particular the new global and regional satellite navigation systems are of interest, i.e., the European Galileo, the Chinese BeiDou, the Japanese QZSS as well as satellite based augmentation systems. This article analyzes the orbit and clock quality of the Galileo products of four MGEX analysis centers for a common time period of 20 weeks. Orbit comparisons of the individual analysis centers have a consistency at the 5–30 cm level. Day boundary discontinuities range from 4 to 28 cm whereas 2-day orbit fit RMS values vary between 1 and 7 cm. The accuracy evaluated by satellite laser ranging residuals is on the one decimeter level with a systematic bias of about −5 cm for all analysis centers. In addition, systematic errors on the decimeter level related to solar radiation pressure mismodeling are present in all orbit products. Due to the correlation of radial orbit errors with the clock parameters, these errors are also visible as a bump in the Allan deviation of the Galileo satellite clocks at the orbital frequency.

Abstract: The European Space Agency (ESA) Swarm mission was launched on 22 November 2013 to study the dynamics of the Earth’s magnetic field and its interaction with the Earth system. The mission consists of three identical satellites, flying in carefully selected near polar orbits. Two satellites fly almost side-by-side at an initial altitude of about 480 km, and will descend due to drag to around 300 km during the mission lifetime. The third satellite was placed in a higher orbit of about 530 km altitude, and therefore descends much more slowly. To geolocate the Swarm observations, each satellite is equipped with an 8-channel, dual-frequency GPS receiver for Precise Orbit Determination (POD). Onboard laser retroreflectors provide the opportunity to validate the orbits computed from the GPS observations using Satellite Laser Ranging (SLR) data. Precise Science Orbits (PSOs) for the Swarm satellites are computed by the Faculty of Aerospace Engineering at Delft University of Technology in the framework of the Swarm Satellite Constellation Application and Research Facility (SCARF). The PSO product consists of both a reduced-dynamic and a kinematic orbit solution. After a short description of the Swarm GPS data characteristics, the adopted POD strategy for both orbit types is explained and first PSO results from more than one year of Swarm GPS data are presented. Independent SLR validation shows that the reduced-dynamic Swarm PSOs have an accuracy of better than 2 cm, while the kinematic orbits have a slightly reduced accuracy of about 4–5 cm. Orbit comparisons indicate that the consistency between the reduced-dynamic and kinematic Swarm PSO for most parts of the Earth is at the 4–5 cm level. Close to the geomagnetic poles and along the geomagnetic equator, however, the kinematic orbits show larger errors, which are probably due to ionospheric scintillations that affect the Swarm GPS receivers over these areas.

Abstract: An estimated orbit error budget for the Jason-1 and Jason-2 GDR-D solutions is constructed, using several measures of orbit error. The focus is on the long-term stability of the orbit time series for mean sea level applications on a regional scale. We discuss various issues related to the assessment of radial orbit error trends; in particular this study reviews orbit errors dependent on the tracking technique, with an aim to monitoring the long-term stability of all available tracking systems operating on Jason-1 and Jason-2 (GPS, DORIS, SLR). The reference frame accuracy and its effect on Jason orbit is assessed. We also examine the impact of analysis method on the inference of Geographically Correlated Errors as well as the significance of estimated radial orbit error trends versus the time span of the analysis. Thus a long-term error budget of the 10-year Jason-1 and Envisat GDR-D orbit time series is provided for two time scales: interannual and decadal. As the temporal variations of the geopotential remain one of the primary limitations in the Precision Orbit Determination modeling, the overall accuracy of the Jason-1 and Jason-2 GDR-D solutions is evaluated through comparison with external orbits based on different time-variable gravity models. This contribution is limited to an East–West “order-1” pattern at the 2 mm/y level (secular) and 4 mm level (seasonal), over the Jason-2 lifetime. The possibility of achieving sub-mm/y radial orbit stability over interannual and decadal periods at regional scales and the challenge of evaluating such an improvement using in situ independent data is discussed.

Abstract: The nonlinear wave excitations arising from the orbital motion of a charged space debris object in the plasma environment of the Low Earth Orbital (LEO) region are investigated In the weakly nonlinear and dispersive limit it is shown that the moving debris induced excitations can be described by a forced Korteweg de Vries (fKdV) type model equation This equation predicts a striking phenomenon of excitation of advancing solitary waves (precursor solitons) in the upstream region of the moving object apart from weak dispersive excitations (wake fields) in the down stream region The detection of such precursor solitons by in situ or remote means could provide a valuable diagnostic tool for detection of charged debris objects in the less than 10 cm size range which are normally difficult to observe by optical means The conditions for the excitation of such solitons through a numerical solution of the fKdV equation are delineated and the relevance and impact of these waves for the dynamical study of orbital space debris are discussed

Abstract: Active debris removal is one of the promising techniques that will decrease the population of large, non-functional spacecraft (space debris) on orbit. Properties of space debris should be taken into account during planning an active debris removal mission. In this paper the thrusting phase of tethered deorbit of large space debris with flexible appendages is considered. The goal of the work is to investigate the mutual influence of the tether vibrations and the vibrations of flexible appendages during thrusting phase. A mathematical model of the space tug and the towed space debris with flexible appendages is developed. Parameters of the system are determined with assumptions that the system is moving in straight line, avoiding high amplitude vibrations of flexible appendages. The expression of the discriminant indicates that the vibrations of the tether and flexible appendages influence each other. A critical tether stiffness exists for the given space tug mass that should be avoided.

Abstract: The retrieval of the three geophysical parameters – sea surface height above the reference ellipsoid (SSH), significant wave height (SWH) and wind speed at 10 m above the sea surface (U10) – is the main goal of satellite altimetry and of primary importance for climate research. The Synthetic Aperture Radar (SAR) altimetry is expected to provide improved precision and along-track resolution compared to the conventional low-resolution mode (LRM) radar altimetry. CryoSat-2 enables a quantitative comparison of SAR and Pseudo-LRM (PLRM) data derived respectively from a coherent and an incoherent processing of the same SAR echoes. In this paper we perform their cross-validation and validation against in situ and model data to derive precision and accuracy at 1 Hz in open ocean, at distances larger than 10 km from the coast. The analysis is performed in the German Bight during 2011 and 2012. Both the PLRM and the SAR scheme include waveform zero-padding and identical environmental, geophysical, and atmospheric attenuation corrections. A Look Up Table is additionally used in SAR to correct for approximations of the Point Target Response (PTR) applied in the retracking procedure. The regional cross-validation analysis proves the good consistency between PLRM and SAR data, with no bias and rms differences of 3 cm, 21 cm, and 0.26 m/s for SSH, SWH, and U10, respectively. The precision of SSH and SWH is higher in SAR than in PLRM (by a factor of 2), while the precision of U10 is 1.4 times better in PLRM than in SAR. At 2 m waveheight, the SAR precision is 0.9 cm for SSH, 6.6 cm for SWH. and 5.8 cm/s for U10. The in situ analysis shows that SSH and U10 have comparable accuracy in SAR and PLRM, while SWH has a significantly higher accuracy in SAR. With a maximum distance of 20 km between altimeter and in situ data, the minimum values obtained for their rms differences are 7 cm, 14 cm, and 1.3 m/s for SAR and 6 cm, 29 cm, and 1.4 m/s for PLRM.

Abstract: This work describes the innovation activities performed in the field of space education since the academic year 2009/10 at the Technical University of Madrid (UPM), in collaboration with the Spanish User Support and Operations Center (E-USOC), the center assigned by the European Space Agency (ESA) in Spain to support the operations of scientific experiments on board the International Space Station. These activities have been integrated within the last year of the UPM Aerospace Engineering degree. A laboratory has been created, where students have to validate and integrate the subsystems of a microsatellite using demonstrator satellites. In parallel, the students participate in a Project Based Learning (PBL) training process in which they work in groups to develop the conceptual design of a space mission. One student in each group takes the role of project manager, another one is responsible for the mission design and the rest are each responsible for the design of one of the satellite subsystems. A ground station has also been set up with the help of students developing their final thesis, which will allow future students to perform training sessions and learn how to communicate with satellites, how to receive telemetry and how to process the data. Several surveys have been conducted along two academic years to evaluate the impact of these techniques in engineering learning. The surveys evaluate the acquisition of specific and generic competences, as well as the students’ degree of satisfaction with respect to the use of these learning methodologies. The results of the surveys and the perception of the lecturers show that PBL encourages students’ motivation and improves their results. They not only acquire better technical training, but also improve their transversal skills. It is also pointed out that this methodology requires more dedication from lecturers than traditional methods.

Abstract: The present study investigates variation of the ionospheric total electron content (TEC) in the low latitude Indian sub-continental region from the GPS observations and its comparison with the global ionosphere maps (GIMs), standard international reference ionosphere (IRI 2012), and the standard plasmasphere–ionosphere model (SPIM) for the period from November 2011 to October 2012 that corresponds to the progressive phase towards the midst of the solar cycle-24. Observations during quiet period show diurnal maximum of TEC occurring around 14:00–16:00 IST, with relatively broader and longer duration of local maximum at Bangalore and behave reversely towards Delhi. The secondary maximum of TEC was markedly noticeable at Bangalore during the months of March and September, and only in the month of September at Hyderabad and Mumbai. However, the relatively higher TEC during December month than the June is ascribed to the winter anomaly which is more prevalent during the high solar activity periods. The prevailing instability in latitudes of anomaly crest during January 2012 is possibly due to the seasonal variation of lunar tidal effects, modulating the EEJ strength at the equator. The studies covered the period of a strong geomagnetic storm during 6–11 March 2012 (SYM-H: −149 nT) which resulted in positive deviation of GPS-TEC at Bangalore ( ↑ 20%), Hyderabad ( ↑ 22%), and Lucknow ( ↑ 94%) compared to the mean quiet days level. The relatively large deviation of TEC at Lucknow could be attributed to the poleward shifting of the anomaly crest, manifested by enhanced fountain effect at the equator. Studies confirm excellent agreement (80–85%) of GPS-TEC with IGS-GIM at Bangalore and Hyderabad with the exception of the night-time hours (Deviations >50%). However relatively larger deviation of GPS-TEC from GIM-TEC at Delhi could be due to the unavailability of IGS stations in the proximity of the position. Predictions of the SPIM model (extension of IRI up to GPS altitude) exhibit much higher deviation from the in situ GPS observations as well as GIM and IRI outputs during quiet periods. Correspondingly, either of the models (IRI and SPIM) did not respond well to the arrival of the sudden storm commencements (SSCs) during the storm period (6–11 March 2012). When SPIM is used instead of IRI, the overestimation from GPS-TECs are further exaggerated by 13–18% (December solstice), 27–37% (March equinox), 15–31% (June solstice), and 20–32% (September equinox) during peak hours of the period. We attribute the relatively more deviation of the SPIM than the IRI model possibly due to its plasmaspheric extension to the IRI model by adding the Russian SMI model of high latitude characteristics. Hence, we emphasize the further improvement in the model with due consideration of the driving forces at play in the region, for reliable predictions of the low latitude ionosphere.

Abstract: In this article, we present a global median model of the ionospheric F2-layer peak height (hmF2), which we named Satellite and Digisonde Model of the F2 layer (SDMF2). This model is based on the radio-occultation data of the satellite missions CHAMP (2001–2008), GRACE (2007–2011), COSMIC (2006–2012) as well as the ionospheric sounding data from the 62 Earth-based Digisonde sounders (1987–2012). As the input parameters, the model uses the year, month and time UT as well as the geographic coordinates and F10.7 index averaged over the 3 Sun rotations (F10.7A). The SDMF2 model is based on the spherical functions decomposition with the 12 harmonics for the longitude and the 8 ones for the modified dip latitude (MODIP). For the diurnal variations, we used the 3 Fourier harmonics. We assumed that the dependency of hmF 2o nF10.7A index is logarithmic. The model accurately reproduces both the spatial and temporal behavior of the monthly hmF2 median. The root-mean-square (RMS) and the mean relative deviations (MRD) from the original data are MRD � 3.7%, RMS � 14.3 km and MRD � 5.4%, RMS � 23.4 km for the periods of low and high solar activity, respectively. The large initial dataset allows achieving the higher accuracy than International Reference Ionosphere model (IRI), and this is confirmed by comparing the SDMF2 model with independent data. 2015 COSPAR. Published by Elsevier Ltd. All rights reserved.

Abstract: We propose a very simple physical mechanism responsible for the formation of the Low Ionization Line part of the Broad Line Region in Active Galactic Nuclei. It explains the scaling of the Broad Line Region size with the monochromatic luminosity, including the exact slope and the proportionality constant, seen in the reverberation studies of nearby sources. The scaling is independent from the mass and accretion rate of an active nucleus. The mechanism predicts the formation of a dust-driven wind in the disk region where the local effective temperature of a non-illuminated accretion disk drops below 1000 K and allows for dust formation. We explore now the predictive power of the model with the aim to differentiate between this model and the previously proposed mechanisms of the formation of the Broad Line Region. We discuss the expected departures from the universal scaling at long wavelength, and the role of the inclination angle of the accretion disk in the source. We compare the expected line profiles with Mg II line profiles in the quasars observed by us with the SALT telescope. We also discuss the tests based on the presence or absence of the broad emission lines in low luminosity active galaxies. Finally, we discuss the future tests of the model to be done with expected ground-based observations and satellite missions.

Abstract: Vertical total electron content (vTEC) values computed using IRI-2012 and IRI Plas models have been compared with diurnal GPS vTEC data derived from European mid-latitude GPS station Potsdam. Comparative data-model analysis does not reveal good performance in vTEC representation. It was found that new extension of IRI model – IRI Plas – cannot represent correctly the vTEC variations over European midlatitudes and mainly overestimates GPS vTEC especially for low and moderate solar activity. In order to estimate the source of the data-model discrepancies, the case-study with detailed analysis of the model simulated electron density profiles was done. It was obtained that all models do not represent correctly the topside profile part and tend to overestimate the electron density higher than F2 peak. So, the main problem of the IRI vTEC representation is not situated in the plasmaspheric part, its absence in IRI model or its presence in IRI Plas model, the main source of the resulted discrepancies is still in the IRI topside ionosphere representation.

Abstract: This paper describes the latest version of the International Reference Ionosphere (IRI) model. IRI-2012 includes new models for the electron density and ion densities in the region below the F-peak, a storm-time model for the auroral E-region, an improved electron temperature model that includes variations with solar activity, and for the first time a description of auroral boundaries. In addition, the thermosphere model required for baseline neutral densities and temperatures was upgraded from MSIS-86 to the newer NRLMSIS-00 model and Corrected Geomagnetic coordinates (CGM) were included in IRI as an additional coordinate system for a better representation of auroral and polar latitudes. Ongoing IRI activities towards the inclusion of an improved model for the F2 peak height hmF2 are discussed as are efforts to develop a “Real-Time IRI”. The paper is based on an IRI status report presented at the 2013 IRI Workshop in Olsztyn, Poland. The IRI homepage is at IRImodel.org .

Abstract: In this paper, we consider trajectory tracking control of a two-link flexible manipulator model in space. Two variables of joint angle and elastic deformation are partly decoupled by a nonlinear decoupling feedback control method. An extended state observer is introduced to estimate nonlinear terms of the two-link flexible manipulator system. Based on a back-stepping method, a nonlinear controller is designed for the flexible manipulator system. Finally, some simulation results are given to demonstrate the effectiveness of the developed techniques in this paper.

Abstract: SAR waveforms from Cryosat-2 are processed using primary peak empirical retrackers to determine the sea surface height in the Arctic. The empirical retrackers investigated are based on the combination of the traditional OCOG (Offset Center of Gravity) and threshold methods with primary peak extraction. The primary peak retrackers involve the application of retracking algorithms on just the primary peak of the waveform instead of the complete reflected waveform. These primary peak empirical retrackers are developed for Cryosat-2 SAR data. This is the first time SAR data in the Arctic are processed using such primary peak retrackers. The sea surface heights determined are compared with the sea surface heights generated by the ESA Retracker as available in the Cryosat-2 Level-2 dataset from 2012. Performance of the primary peak retrackers is also compared with the traditional OCOG, threshold and five parameter beta retrackers. In the case of SAR-lead data, it is concluded that the proposed primary peak retrackers work better as compared with the traditional retrackers (OCOG, threshold, five parameter beta) as well as the ESA Retracker.

Abstract: We studied the contribution of the global plasmaspheric and ionospheric electron content (PEC and IEC) into total electron content (TEC). The experimental PEC was estimated by comparison of GPS TEC observations and FORMOSAT-3/COSMIC radio occultation IEC measurements. Results are retrieved for the winter solstice (January and December 2009) conditions. Global maps of COSMIC-derived IEC, PEC and GPS TEC were compared with Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) results. In addition, we used GSM TIP model results in order to estimate the contribution of plasmaspheric electron content into TEC value at the different altitudinal regions. The advantages and problems of the outer ionospheric/plasmaspheric parameters (O+/H+ transition height, TEC and electron density at height above F2 layer peak) representation by the IRI (International Reference Ionosphere) model are discussed.

Abstract: Future manned missions to the moon will require the ability to build structures using the moon’s natural resources. The geopolymer binder described in this paper (Lunamer) is a construction material that consists of up to 98% lunar regolith, drastically reducing the amount of material that must be carried from Earth in the event of lunar construction. This material could be used to fabricate structural panels and interlocking blocks that have radiation shielding and thermal insulation characteristics. These panels and blocks could be used to construct living quarters and storage facilities on the lunar surface, or as shielding panels to be installed on crafts launched from the moon surface to deep-space destinations. Lunamer specimens were manufactured in the laboratory and compressive strength results of up to 16 MPa when cast with conventional methods and 37 MPa when cast using uniaxial pressing were obtained. Simulation results have shown that the mechanical and chemical properties of Lunamer allow for adequate radiation shielding for a crew inside the lunar living quarters without additional requirements.

Abstract: The usage of cumulative luminosity distributions, constructed thanks to the long-term observations available through wide field hard X-ray imagers, has been recently exploited to study the averaged high energy emission (>17 keV) from supergiant fast X-ray transients (SFXTs) and classical Supergiant High Mass X-ray Binaries (SgXBs). Here, we take advantage of the long term monitorings now available with Swift/XRT to construct for the first time the cumulative luminosity distributions of a number of SFXTs and the classical SgXB IGR J18027-2016 in the soft X-ray domain with a high sensitivity focusing X-ray telescope (0.3–10 keV). By complementing previous results obtained in the hard X-rays, we found that classical SgXBs are characterized by cumulative distributions with a single knee around ∼ 1036–1037 erg s−1, while SFXTs are found to be systematically sub-luminous and their distributions are shifted at significantly lower luminosities (a factor of ∼ 10–100). As the luminosity states in which these sources spend most of their time are typically below the sensitivity limit of large field of view hard X-ray imagers, we conclude that soft X-ray monitorings carried out with high sensitivity telescopes are particularly crucial to reconstruct the complete profile of the SFXT cumulative luminosity distributions. The difference between the cumulative luminosity distributions of classical SgXBs and SFXTs is interpreted in terms of accretion from a structured wind in the former sources and the presence of magnetic/centrifugal gates or a quasi-spherical settling accretion regime in the latter.

Abstract: Global Positioning System (GPS) signal-to-noise ratio (SNR) measurements can be employed to retrieve environmental variables in multipath reception conditions, whereby direct or line-of-sight transmission is received simultaneously with coherent reflections thereof. Previous GPS SNR multipath studies of soil moisture and snow depth have focused on the legacy GPS L1 and L2 bands. In the present paper, short-delay, near-grazing incidence multipath from the L5-band GPS SNR is assessed for its value in detecting soil moisture and snow depth. The L5 signal will become more important in the future because of compatibility and interoperability among the different global satellite navigation systems. The L5 results are compared with L2C estimates to determine whether the L2C–L5 differences (given their differing power budgets and their modulation properties) are significant. To address these questions, measurements and simulations were employed. A physically-based multipath simulator was enhanced to investigate the differences between parameter retrievals for the L2C and the L5 GPS signals. Parameter retrievals from synthetic observations for different scenarios were compared. Comparisons included varying reflector height, surface material, and surface roughness. Measurements from two GPS stations in Colorado, USA, were used to retrieve soil moisture and snow depth conditions. Over a 153-day period that encompassed the catastrophic 2013 Colorado flooding event, L2-derived volumetric soil moisture had an RMS difference of 0.042 cm3/cm3 while the L5 RMS difference was 0.034 cm3/cm3 with respect to in-situ data (values of volumetric soil moisture range between 0.04 and 0.34 cm3/cm3). In a separate 483-day campaign, L5-derived snow depth estimates were compared to L2C-derived values and found strongly correlated, deviating from a one-to-one relationship by only 0.00001 ± 0.0064 cm/cm. These results indicate the absence of any detectable biases in L5 as compared to L2C for retrieving soil moisture and snow depth from GPS SNR multipath observations.

Abstract: Additional observations from other GNSS s can augment GPS precise point positioning (PPP) for improved positioning accuracy, reliability and availability. Traditional multi-GNSS PPP model requires the estimation of inter-system bias (ISB) parameter. Based on the scaled sensitivity matrix (SSM) method, a quantitative approach for assessing parameter assimilation, we theoretically prove that the ISB parameter is not correlated with coordinate parameters and it can be assimilated into clock and ambiguity parameters. Thus, removing ISB from multi-GNSS PPP model does not affect coordinate estimation. Based on this analysis, we develop a simplified and unified model for multi-GNSS PPP, where ISB parameter does not need to be estimated and observations from different GNSS systems are treated in a unified way. To verify the new model, we implement the algorithm to the self-developed software to process 1 year GPS/GLONASS data of 53 IGS (International GNSS Service) worldwide stations and 1 month GPS/BDS data of 15 IGS MGEX (Multi-GNSS Experiment) stations. Two types of GPS/GLONASS and GPS/BDS combined PPP solution are performed, one is based on traditional model and the other implements the new model. RMSs of coordinate differences between the two type of solutions are few μm for daily static PPP and less than 0.02 mm for GPS/GLONASS kinematic PPP in the North, East and Up components, respectively. Considering the millimeter-level precision of current GNSS PPP solutions, these statistics demonstrate equivalent performance of the two solution types.

Abstract: This paper focuses on performance analysis and accuracy enhancement of long-term position time series of a regional network of GPS stations with two near sub-blocks, one block of 8 stations in Cascadia region and another block of 14 stations in Southern California We have analyzed the seasonal variations of the 22 IGS site positions between 2004 and 2011 The Green’s function is used to calculate the station-site displacements induced by the environmental loading due to atmospheric pressure, soil moisture, snow depth and nontidal ocean The analysis has revealed that these loading factors can result in position shift of centimeter level, the displacement time series exhibit a periodic pattern, which can explain about 1270–2178% of the seasonal amplitude on vertical GPS time series, and the loading effect is significantly different among the two nearby geographical regions After the loading effect is corrected, the principal component analysis (PCA)-based block spatial filtering is proposed to filter out the remaining common mode error (CME) of the GPS time series The results show that the PCA-based block spatial filtering can extract the CME more accurately and effectively than the conventional overall filtering method, reducing more of the uncertainty With the loading correction and block spatial filtering, about 6834–7320% of the vertical GPS seasonal power can be separated and removed, improving the reliability of the GPS time series and hence enabling better deformation analysis and higher precision geodetic applications

Abstract: The future space debris environment will be governed by the production of fragments coming from massive breakups. In order to identify the most relevant parameters influencing the long term evolution of the environment and to assess the criticality of selected space objects in different regions of the circumterrestrial space, a large parametric study was performed. In this framework some indicators were produced to quantify and rank the relevance of selected fragmentations on the long term evolution of the space debris population. Based on the results of the fragmentation studies, a novel analytic index, the Criticality of Spacecraft Index (CSI), aimed at ranking the environmental criticality of abandoned objects in Low Earth Orbit (LEO), was formulated. It takes into account the physical characteristics of a given object, its orbit and the environment where this is located. The results corresponding to a sample of LEO objects in the initial population at the epoch of January 1, 2020 and mass larger than 100 kg are shown.

Abstract: Particles count rates at given Earth location and altitude result from the convolution of (i) the interstellar (IS) cosmic-ray fluxes outside the solar cavity, (ii) the time-dependent modulation of IS into Top-of-Atmosphere (TOA) fluxes, (iii) the rigidity cut-off (or geomagnetic transmission function) and grammage at the counter location, (iv) the atmosphere response to incoming TOA cosmic rays (shower development), and (v) the counter response to the various particles/energies in the shower. Count rates from neutron monitors or muon counters are therefore a proxy to solar activity. In this paper, we review all ingredients, discuss how their uncertainties impact count rate calculations, and how they translate into variation/uncertainties on the level of solar modulation ϕ (in the simple Force-Field approximation). The main uncertainty for neutron monitors is related to the yield function. However, many other effects have a significant impact, at the 5–10% level on ϕ values. We find no clear ranking of the dominant effects, as some depend on the station position and/or the weather and/or the season. An abacus to translate any variation of count rates (for neutron and μ detectors) to a variation of the solar modulation ϕ is provided.

Abstract: In this paper, first, we develop new analytical solutions to hypersonic gliding problem. In the derivation of these solutions, we propose an innovative method based on spectral decomposition for solving a special type of linear system with variable coefficients, where the system matrix can be expressed as the product of a scale function and a constant matrix. Next, we design an entry guidance based on these analytical solutions. In the guidance, the downrange analytical expression is used to plan the longitudinal reference profile satisfying the downrange requirement in real time. Two bank reversals are needed to eliminate the crossrange error. The first is planned by the crossrange analytical expression such that the second is at a specified point near the end of the flight. After the first bank reversal is performed, the second is slightly corrected using the trajectory simulation. Because the longitudinal reference profile and bank reversals are planned onboard, the entry guidance can handle various urgent tasks and deal well with large dispersions in the initial conditions, aerodynamic model and atmospheric model.

Abstract: A detailed study is made of celestial observations recorded in East Asian and European history around A.D. 774–5 and 993–4. These include reports of supernovae, comets, sunspots and aurorae. The aim is to attempt to provide astronomical evidence in support of the measured 14C increases in tree rings at these dates. However, the results prove to be far from convincing, leaving the physical cause of the increases still in doubt.