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GPS satellites undergo surface contamination on the solar array coverglasses from repeated arcing events. Using NASA Air Force Spacecraft Charging Analyzer Program (Nascap-2 K) spacecraft charging ...

Global Navigation Satellite Systems (GNSSs) have greatly developed over the last decade, so that, in addition to GPS and GLONASS, we have seen the emergence of the European Galileo and the Chinese BeiDou systems. There are presently a sufficient number of satellites in each GNSS constellation for single-system positioning purposes. Satellite visibility and single point positioning (SPP) are two basic evaluation tools in the field of GNSS data analysis. We have assessed the performance of the currently available GNSS constellations in terms of satellite visibility and SPP with the single-system approach. Visibility analysis results show that with 5- and 10-degree elevation mask angles, GPS-only, GLONASS-only, Galileo-only, and BeiDou-3-only medium earth orbit satellites in view have more limited availability in middle latitudes than in polar areas. Hybrid constellations including BeiDou-2 and combined BeiDou-2/3 exhibit a dominant longitudinal and latitudinal visibility pattern over the Asia-Pacific region. Ionosphere Free (IF) linear combination of code observations and precise orbits and clocks were used in SPP analysis. SPP results show that the horizontal and vertical RMSE values for Galileo-only case are more stable and are lower compared to the GPS-only, GLONASS-only, and BeiDou-only (BDS-2 and BDS-3) cases. Also, BeiDou performance has been significantly improved after middle of the 2020.

After the successful establishment of the NewSpace sector in the Low Earth Orbit (LEO), it is only a matter of time before commercial systems seek to access the lower Medium Earth Orbit (MEO). A cost-efficient way is to perform orbit-raising strategies using a propulsion system onboard the spacecraft. However, lower cost and time-to-market are often prioritised to the detriment of system reliability, which leads to heightened risk of producing space debris. In this paper, orbit raising strategies are analysed with the goal of fail-safe compliance with the European Code of Conduct for Space Debris Mitigation (ECOC), which demands a de-orbit within 25 years after the space system’s operational phase. The used data is based on orbit propagation and re-entry extrapolation tools of NASA and ESA. ROMEO, a satellite mission of the University of Stuttgart for technology demonstration and a research platform in MEO, proposed to launch in 2024, is used as an exemplary mission for the simulations. As part of the project, a strategy was developed which maximises the lifetime of the satellite, and eliminates the potential of it ending up as long-term space debris. Thus, the risks behind using technology demonstrations and commercial systems are reduced, and a first step to a more affordable access to MEO is established.

Medium Earth orbit is subject to destabilizing resonances due to third body effects from the Moon and Sun. This behavior can be leveraged for debris mitigation in this regime. This paper will explore how feasible this technique is and whether satellites could be targeted into such impact trajectories at their end-of-life. In the paper, six targets are examined in detail to determine what level of uncertainty in targeting these satellites will achieve a reentry trajectory some decades in the future. This study will also characterize the sensitivity to initial conditions for this orbital regime and the feasibility of leveraging these naturally occurring dynamics.

With the great development of Global Navigation Satellite System (GNSS), multi-GNSS constellations (GPS, BDS, GLONASS, and Galileo) are able to provide users with more accurate positioning result. For civil aviation, to guarantee user’s safety, multi-constellation GNSS needs to meet the integrity requirement. Using conservative error models, Multiple Hypothesis Solution Separation (MHSS) Advanced Receiver Autonomous Integrity Monitoring (ARAIM) is proposed to evaluate GNSS integrity. Current multipath error model in ARAIM algorithm is based on data of GPS Medium Earth Orbit (MEO) satellites. But BDS is a hybrid constellation. For BDS II, there has 5 Geosynchronous Earth Orbit (GEO) satellites. Previous studies have shown that the multipath effect of GEO satellites has statistical characteristics different from MEO satellites. Meanwhile, the multipath magnitude of GEO satellites is larger than that of MEO satellites. This paper mainly focuses on validating whether the multipath error model in ARAIM algorithm is conservative enough for GEO satellites. In this paper, Code Minus Carrier (CMC) residuals are calculated for BDS GEO dual-frequency signals. Then the Standard Deviation (STD) of CMC residuals can be conservatively estimated by bounded Cumulative Distribution Function (CDF). After eliminating interference from receiver noise, STD of GEO multipath can be obtained. Comparing the STD of GEO multipath effect with ARAIM multipath error model, a conclusion could be drawn that current multipath error model in ARAIM algorithm is no longer able to conservatively estimate the statistical characteristics of GEO satellites.

Energetic electron measurements and spacecraft charging are of great significance for theoretical research in space physics and space weather applications. In this paper, the energetic electron detection package (EEDP) deployed on three Chinese navigation satellites in medium Earth orbit (MEO) is reviewed. The instrument was developed by the space science payload team led by Peking University. The EEDP includes a pinhole medium-energy electron spectrometer (MES), a high-energy electron detector (HED) based on ΔE-E telescope technology, and a deep dielectric charging monitor (DDCM). The MES measures the energy spectra of 50−600 keV electrons from nine directions with a 180°×30° field of view (FOV). The HED measures the energy spectrum of 0.5−3.0 MeV electrons from one direction with a 30° cone-angle FOV. The ground test and calibration results indicate that these three sensors exhibit excellent performance. Preliminary observations show that the electron spectra measured by the MES and HED are in good agreement with the results from the magnetic electron-ion spectrometer (MagEIS) of the Van Allen Probes spacecraft, with an average relative deviation of 27.3% for the energy spectra. The charging currents and voltages measured by the DDCM during storms are consistent with the high-energy electron observations of the HED, demonstrating the effectiveness of the DDCM. The observations of the EEDP on board the three MEO satellites can provide important support for theoretical research on the radiation belts and the applications related to space weather.

The operational Terrestrial Reference Frames (TRFs) realized through the evaluation of broadcast ephemerides for GPS, GLONASS, Galileo, BeiDou-2 and BeiDou-3 have been compared to IGS14, the TRF realized by the International GNSS Service (IGS). The TRFs realized by the GPS, GLONASS, Galileo, and BeiDou-2 and BeiDou-3 broadcast ephemerides are the orbital realizations of WGS 84 (G1762′), PZ90.11, GTRF19v01, and BDCS respectively. These TRFs are compared using up to 56 days of data (21 July-14 Sept 2019) at a 5 or 15-min rate. The operational TRFs are compared to IGS14 in a 7-parameter similarity (Helmert) transformation. Numerical results show that the operational GNSS TRFs differ from IGS14 at a level no greater than 4 cm for Galileo, 6 cm for GPS and BeiDou-3, 13 cm for GLONASS, and 48 cm for a limited set of BeiDou-2 Medium Earth Orbit (MEO) vehicles.

The analysis of the short-circuit current and open-circuit voltage performed for solar arrays of satellites operating in medium earth orbit reflects the essential exceeding the short-circuit current degradation rate over the open-circuit voltage degradation rate. The study demonstrated also that there is another factor of non-radiation degradation resulted in additional degradation of the solar array short-circuit current.

Large-scale non-geostationary orbit (NGSO) satellite communication systems (SCSs) are emerging to play an important role for future global wireless communication. However, downlink transmissions from these mega-constellation SCSs can cause disruptive radio frequency interference (RFI) to the radio astronomy systems (RASs) on the ground or in space. Therefore, the satellite-based RASs including low earth orbit (LEO)-based or medium earth orbit (MEO)-based RASs are further considered to lower the impact from SCSs on the higher orbit onto the ground RASs and the lower orbit RASs. In this paper, we first analyze and compare the impact of RFI on LEO satellite-based RAS and MEO satellite-based RAS. The RFI results show that for both systems, RFI from higher orbit SCSs can totally undermine the RAS continuum observation adjacent to the SCS downlink bands when using the emission mask defined by National Telecommunications and Information Administration (NTIA). To solve the observation issues for both systems, we find out the required SCS emission mask for each RAS so that both systems can avoid RFI. Secondly, this paper also investigates three typical radio astronomy metrics such as maximum baseline distance (MBD), the number of simultaneously observing telescopes, and the signal to interference plus noise power ratio (SINR) performance. Our evaluation results show that the satellite-based RASs have greater MBD, thus, better spatial resolution, than the ground-based RAS. Due to the greater antenna gain of the ground telescopes, the SINR performance of satellite-based RASs may have limitations comparatively. However, because of more RFI sources at ground RASs in reality, satellite-based RASs have better chances to avoid RFI impact and potential SINR advantages. Additionally, we also explore the advantages of NGSO satellite-based RAS from a communication side. Our analysis shows that the large-scale NGSO satellite-based RAS can offer more spectrum access to both SCS and RAS.

This chapter focuses on navigation from low earth orbit (LEO). It opens with a brief history of early US and Russian satellite navigation systems in LEO. The chapter discusses satellites in LEO today, their role in navigation, and LEO constellations on the horizon. It provides the needed mathematical background and discusses the role of LEO in satellite navigation today, showcasing the performance of the Satelles Iridium‐based GPS augmentation system and demonstrating the benefit of stronger signals. The chapter looks to the future of LEO in navigation and demonstrates that the scale of the proposed Broadband LEO constellations, coupled with the more benign LEO radiation environment, can deliver the positioning performance of GPS with commercial‐off‐the‐shelf components. It provides an analysis of the LEO radiation environment compared to medium Earth orbit.

The construction of the third generation of the Chinese BeiDou navigation satellite system (BDS), which provides global positioning, navigation, and timing (PNT) services, will be completed in 2020. As BDS satellites contain the geostationary orbit (GEO), inclined geosynchronous (IGSO), and medium earth orbit (MEO), orbital manoeuvres occur more frequently. Thus, orbit manoeuvre detection is important for continuous and reliable PNT services. A BDS orbit manoeuvre detection method based on the combination of global positioning system (GPS) and BDS observations is proposed in this study. The standard deviations of the observation residuals of the epoch-differenced velocity estimation of the single BDS system and combined GPS + BDS system were compared to determine the beginning and end of the orbit manoeuvre. The results show that the orbital manoeuvre leads to a position, velocity, and receiver clock error bias of approximately tens to hundreds of metres, several centimetres per second, and tens to hundreds of metres, respectively. Based on the proposed method, the start and end times can be detected in real time and the usable observation time can be extended by >157 min.

As space technologies mature, the cost of inserting materiel into orbit continues to decrease. Advances in the miniaturization of hardware and decreasing launch costs have resulted in space becomin...

This paper describes a Markov decision process approach to a robust spacecraft mission control policy that maximizes the expected value of science reward assuming a circular orbit. The control policy that governs mission steps can be computed off-board or onboard depending upon the availability of communication bandwidth and on-board computational resources. This paper considers a sample science mission, where the spacecraft collects data from celestial objects viewable only within a certain orbit true anomaly window. Science data collection requires the spacecraft to slew its instrument(s) toward each target, and continue pointing in the direction of the target while the spacecraft traverses its orbit. Robustness and stochastic optimization of scientific reward, is achieved at the cost of computational complexity. Approximate dynamic programming (ADP) is exploited to reduce the computational time and effort to manageable levels and to treat larger problem sizes. The proposed ADP algorithm partitions the state-space based on true anomaly regions, enabling grouping of adjacent science targets. Results of a simulation case study demonstrate that our proposed ADP approach performs quite well for reasonable ranges of key problem parameters.

This chapter presents a concise system‐level overview of constellations, operational configurations, and signaling characteristics of a global navigation satellite system (GNSS). GNSSs use medium Earth orbit for good visibility/availably of the satellite and provide moderate Doppler for position, velocity, and timing determination. GNSS augmentation or regional satellite‐based navigation systems are often placed in geostationary or inclined geosynchronous over the region to be services. Global position system (GPS) time is derived from a composite or “paper” clock that consists of all operational monitor station and satellite atomic clocks. The GPS control segment coordinates the update of coordinated universal time (UTC) parameters at a future upload in order to maintain a proper continuity of the t UTC timescale. The chapter demonstrates how to go about calculating the user position, given ranges (pseudoranges) to satellites, the known positions of the satellites, and ignoring the effects of clock errors, receiver errors, and propagation errors.

Summary In this paper, a three‐layered medium Earth orbit (MEO), geostationary Earth orbit (GEO), and inclined geosynchronous orbit (IGSO) satellite network (IGMSN) is presented. Based on the idea ...

Positioning, Navigation, and Timing (PNT) supports critical infrastructure with its most known service: Global Positioning System (GPS). The presence of signals broadcast from satellites in Medium Earth Orbit (MEO), results in low signal strength from the receiver making it vulnerable to intentional and unintentional disruptions which could lead to public sector economic and safety losses including transportation services, property damage, environmental damage, injury, and loss of confidence for the system. With guidance from current legislation, the main goal within these field demonstrations is to find a suitable vendor(s) to offer complementary service in the event of GPS disruptions with an emphasis for a scenario-based demonstration rather than a technological based demonstration. The purpose of the demonstration was to gather information on PNT technologies at a high Technology Readiness Level (TRL) that can work in the absence of GPS. Six key constructs were implemented when considering potential stakeholders including timing performance, positioning performance, Timing-terrestrial broadcast, PNT-Terrestrial Broadcast, and PNT broadcast. Across eleven vendors, five timing scenarios were assessed including: 72 Hour Bench Static Timing, Static Outdoor Timing, Static Indoor Timing, Static Basement Timing, and eLORAN Reference Station Offset. Four positioning scenarios were also developed to assess vendor systems based on these five attributes: Dynamic Outdoor Positioning with Holds, 3D Positioning, Static Outdoor Positioning, and Static Indoor Positioning. The purpose of the positioning scenarios was to exemplify: coverage, two-dimensional (2D) dynamic positioning/three-dimensional (3D) dynamic positioning, static positioning including service availability/accuracy, and static positioning under long term service availability/accuracy under normal conditions as well as challenged GPS signal conditions in relevance to multimodal transportation and other critical infrastructure applications From the vendors’ technological demonstration, four key findings were issued and the decision to achieve resilient PNT service across all categories is to pursue multiple technologies while promoting diversity across PNT functions in areas including transportation and further infrastructure sectors.

Soil moisture is an important geophysical parameter for studying terrestrial water and energy cycles. It has been proven that Global Navigation Satellite System Interferometry Reflectometry (GNSS-IR) can be applied to monitor soil moisture. Unlike the Global Positioning System (GPS) that has only medium earth orbit (MEO) satellites, the Beidou Navigation Satellite System (BDS) also has geosynchronous earth orbit (GEO) satellites and inclined geosynchronous satellite orbit (IGSO) satellites. Benefiting from the distribution of three different orbits, the BDS has better coverage in Asia than other satellite systems. Previous retrieval methods that have been confirmed on GPS cannot be directly applied to BDS MEO satellites due to different satellite orbits. The contribution of this study is a proposed multi-satellite soil moisture retrieval method for BDS MEO and IGSO satellites based on signal-to-noise ratio (SNR) observations. The method weakened the influence of environmental differences in different directions by considering satellite repeat period. A 30-day observation experiment was conducted in Fengqiu County, China and was used for verification. The satellite data collected were divided according to the satellite repeat period, and ensured the response data moved in the same direction. The experimental results showed that the BDS IGSO and MEO soil moisture estimation results had good correlations with the in situ soil moisture fluctuations. The BDS MEO B1I estimation results had the best performance; the estimation accuracy in terms of correlation coefficient was 0.9824, root mean square error (RMSE) was 0.0056 cm3cm−3, and mean absolute error (MAE) was 0.0040 cm3cm−3. The estimations of the BDS MEO B1I, MEO B2I, and IGSO B2I performed better than the GPS L1 and L2 estimations. For the BDS IGSO satellites, the B1I signal was more suitable for soil moisture retrieval than the B2I signal; the correlation coefficient was increased by 19.84%, RMSE was decreased by 42.64%, and MAE was decreased by 43.93%. In addition, the BDS MEO satellites could effectively capture sudden rainfall events.

Global navigation satellite system (GNSS) signals are used throughout the world for position, navigation, and timing. As these signals travel to Earth's surface from Medium Earth Orbit they can encounter ionospheric scintillation caused by ionospheric plasma irregularities, especially at high latitudes. This ionospheric scintillation is detrimental to GNSS signal strength, accuracy, and confidence. Incoherent scatter radars like Resolute Bay Incoherent Scatter Radar (RISR) are excellent tools for measuring the ionospheric conditions that surround the path of incoming GNSS signals. This paper outlines a process that utilizes Systems Toolkit (STK) and Matrix Laboratory (MATLAB) to streamline the process of identifying valid future conjunctions between GPS signals and RISR radar beams to advance study of the impact of ionospheric scintillations on GNSS signals at high latitudes.

Currently, low Earth orbit (LEO) satellites are attracting great attention in the navigation enhancement field because of their stronger navigation signal and faster elevation variation than medium Earth orbit (MEO) satellites. To meet the need for real-time and precise positioning, navigation and timing (PNT) services, the first and most difficult task is correcting errors in the process of precise LEO orbit and clock offset determination as much as possible. Launched in 29 September 2018, the CentiSpace-1 (CS01) satellite is the first experimental satellite of LEO-based navigation enhancement system constellations developed by Beijing Future Navigation Technology Co. Ltd. To analyze the impact of the attitude model, carrier phase wind-up (PWU) and phase center variation (PCV) on precise LEO orbit and clock offset in an LEO-based navigation system that needs extremely high precision, we not only select the CS01 satellite as a testing spacecraft, but also the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO). First, the dual-frequency global positioning system (GPS) data are collected and the data quality is assessed by analyzing the performance of tracking GPS satellites, multipath errors and signal to noise ratio (SNR) variation. The analysis results show that the data quality of GRACE-FO is slightly better than CS01. With residual analysis and overlapping comparison, a further orbit quality improvement is possible when we further correct the errors of the attitude model, PWU and PCV in this paper. The final three-dimensional (3D) root mean square (RMS) of the overlapping orbit for GRACE-FO and CS01 is 2.08 cm and 1.72 cm, respectively. Meanwhile, errors of the attitude model, PWU and PCV can be absorbed partly in the clock offset and these errors can generate one nonnegligible effect, which can reach 0.02~0.05 ns. The experiment results indicate that processing the errors of the attitude model, PWU and PCV carefully can improve the consistency of precise LEO orbit and clock offset and raise the performance of an LEO-based navigation enhancement system.

Carreno-Luengo, H., et al., The Institute of Electrical and Electronics Engineers (IEEE) Geoscience and Remote Sensing Society (GRSS) created the GRSS 'Standards for Earth Observation Technical Committee' to advance the usability of remote sensing products by experts from academia, industry, and government through the creation and promotion of standards and best practices. In February 2019, a Project Authorization Request was approved by the IEEE Standards Association (IEEE-SA) with the title 'Standard for Spaceborne Global Navigation Satellite Systems Reflectometry (GNSS-R) Data and Metadata Content.' At present, 4 GNSS constellations cover the Earth with their navigation signals: The United States of America (USA) Global Positioning System GPS with 31 Medium Earth Orbit (MEO) operational satellites, the Russian GLObal'naya NAvigatsionnaya Sputnikovaya Sistema GLONASS with 24 MEO operational satellites, the European Galileo with 24 MEO operational satellites, and the Chinese BeiDou-3 with 3 Inclined GeoSynchronous Orbit (IGSO), 24 MEO, and 2 Geosynchronous Equatorial Orbit (GEO) operational satellites. Additionally, several regional navigation constellations increase the number of available signals for remote sensing purposes: the Japanese Quasi-Zenith Satellite System QZSS with 1 GSO and 3 Tundra-type orbit operational satellites, and the Indian Regional Navigation Satellite System IRNSS with 3 GEO and 4 IGSO operational satellites. On the other hand, there are different GNSS-R processing techniques, instruments and spaceborne missions, and a wide variety of retrieval algorithms have been used. The heterogeneous nature of these signals of opportunity as well as the numerous working methodologies justify the need of a standard to further advance in the development of GNSS-R towards an operational Earth Observation technique. In particular, the scope of this working group is to develop a standard for data and metadata content arising from past, present, and future spaceborne missions such as the United Kingdom (UK) TechDemoSat-1 TDS-1, and the National Aeronautics and Space Administration (NASA) CYclone Global Navigation Satellite System CYGNSS constellation coordinated by the University of Michigan (UM). In this article we describe the scene study, including fundamental aspects, scientific applications, and historical milestones. The spaceborne standard is under development and it will be published in IEEE-SA., 10.13039/100000104-National Aeronautics and Space Administration (NASA) Science Mission Directorate with the University of Michigan (Grant Number: NNL13AQ00C)

In this paper, the recently developed machine learning (ML) approach to improve orbit prediction accuracy is systematically investigated using three ML algorithms, including support vector machine (SVM), artificial neural network (ANN), and Gaussian processes (GPs). In a simulation environment consisting of orbit propagation, measurement, estimation, and prediction processes, totally 12 resident space objects (RSOs) in solar-synchronous orbit (SSO), low Earth orbit (LEO), and medium Earth orbit (MEO) are simulated to compare the performance of three ML algorithms. The results in this paper show that ANN usually has the best approximation capability but is easiest to overfit data; SVM is the least likely to overfit but the performance usually cannot surpass ANN and GPs. Additionally, the ML approach with all the three algorithms is observed to be robust with respect to the measurement noise.

Global positioning system (GPS) satellites are strongly suspected to arc into the space environment. Arcing on spacecraft due to differential charging can be blamed for a host of operational anomalies resulting in the loss of billions of dollars and productivity. In addition to anomalous behavior on spacecraft, arcing can be accused of an otherwise unexplained degradation in solar power production of GPS satellites while on orbit. In the study presented here, it will be shown that arcs on a GPS-representative solar array coupon under representative medium earth orbit/geosynchronous earth orbit (MEO/GEO)-like conditions result in contamination of solar panel coverglasses. This finding supports the argument that arc-induced contamination is a prime suspect in explaining the mysterious excess power loss of GPS satellites on orbit. Further, optical mapping suggests that this arcing occurs primarily at the edges of the sample panel where silicone adhesive with glass microspheres was exposed. X-ray photoelectron spectroscopy (XPS) reveals that the concentration of Si near the arc sites is increased, suggesting that the adhesive is ionized during the electrostatic discharge and coats the surface of the solar panel, resulting in reduced optical transmission through the coverglass and therefore reducing power production. X-ray depth profiling suggests that the silicon detected is due to surface contamination, and electrostatic simulations indicate that glass microspheres intended to reduce adhesive mass and produce a uniform bond-line may be exacerbating the arcing problem.

This paper presents a novel approach to test surface material charging under electron flux representative of space conditions below 400 keV. The experimental protocol used to test space material electrical properties consists in adapting the electron flux distribution to mimic worst case spectra met in space. Input environments are taken from Kp > 5 specifications at geosynchronous orbit (GEO) and from the analysis of both RBSP and LANL spacecraft data at middle Earth orbit (MEO) and GEO. Experimental results using GEO specifications show that the surface potential strongly depends on the energetic particle flux. Above a given > 100-keV electron flux, the radiation-induced conductivity tends to govern the charging dynamics. A numerical analysis based on the environments selected in this paper shows that charging levels could be larger at MEO than at GEO.

The objective of this paper is to validate and evaluate the orbit accuracy of the BeiDou Navigation System (BDS) using satellite laser ranging (SLR) data over a 5-year time series. The microwave-based precise orbits of BDS derived from four Analysis Centers (ACs) are validated from 2013 to 2018, including three Multi-GNSS EXperiment (MGEX) ACs from WHU, CODE and GFZ, as well as a fourth AC, known as ISC in China, with the corresponding products designated as WUM, COM, GBM and ISC. The validated orbits of BDS include the Geostationary Earth Orbit (GEO) satellite of C01, the Inclined Geo-Synchronous Orbit (ISGO) satellites of C08, C10, and C13, as well as the Medium Earth Orbit (MEO) satellite of C11. In addition, the performances of the BDS orbits during noneclipse, eclipse and Yaw Maneuver (YM) period are evaluated. Finally, the dependencies of SLR residuals on the satellite nadir angle and on the Sun elevation angle β are analyzed in detail. The results demonstrated that (1) the optimal orbit accuracies of C01, C08, C10, C11 and C13 are 522.8 mm from WUM, 53.3 mm from ISC, 54.3 mm from ISC, 38.2 mm from ISC and 51.5 mm from COM, respectively. And the orbits derived from ISC have more stable and excellent accuracy than the other ACs due to the advantages of its orbit combination algorithm. (2) Due to the deficiencies of the Solar Radiation Pressure and the YM modeling for BDS satellites from four ACs, the microwave-based orbit accuracy of BDS IGSO and MEO satellites is significantly deteriorated during the eclipse period, especially in the YM period, while the accuracy of GEO C01 during the eclipse period is better than that during the noneclipse period. (3) The dependency of SLR residuals on the satellite nadir angle has been analyzed, but limited by the number and type of all the validated BDS satellites, the nadir-dependency offset of the SLR residuals cannot be well assigned from the characteristics of the SLR detector system, or the Laser Retro-reflector Arrays. (4) By analyzing the dependency of SLR residuals on the Sun elevation angle β , some commonalities of the patterns derived from four ACs have been found for BDS IGSO and MEO satellites, and the patterns are similar to that of the GLONASS, rather than those of GPS and Galileo.

Extend lifetime of near-earth orbit satellites by on-orbit refueling has significant economic benefits, while the orbit deployment strategy of the on-orbit servicing (OOS) system which implements the refueling task directly affects the efficiency of on-orbit refueling mission. Three orbit deployment modes were proposed, including ground deployment, low earth orbit (LEO) deployment and the same orbit deployment. For the client satellites to be refueled on near-earth orbit including LEO, medium earth orbit (MEO) and geosynchronous orbit (GEO), the three deployment modes were compared quantitatively based on two evaluation indicators: the refueling response time and the economic benefits. The comparing results showed that for MEO and GEO client satellites, it is appropriate to adopt the same orbit deployment mode, while for LEO client satellites, ground deployment mode is more suitable. As the maximum economic benefits can be earned to refuel GEO client satellites, the orbit deployment scheme of OOS system for refueling GEO client satellites was further studied. A new architecture of OOS system called “1+N” consist of 1 fuel storage station and N refueling vehicles was proposed. The fuel storage station carries a great deal of fuel, running on orbit steadily. The refueling vehicles maneuver on GEO and implement the refueling operations for GEO client satellites. If the vehicles run out of fuel, they arrive at the station to be refueled. The weight and orbital altitude of fuel storage station, the number, weight, orbital altitude and orbital phase of refueling vehicles are the key parameters of the orbit deployment scheme. The mathematical model for optimizing the scheme was constructed, which takes shorten the refueling response time and lower the cost of OOS system as multi-objective. The calculation process of the optimization was explained. By analyzing the optimization results, the optimal orbit deployment scheme of OOS system containing 1 fuel storage station and 4 to 6 refueling vehicles was proposed.

A realistic stochastic model is the prerequisite in global navigation satellite system (GNSS) positioning applications. Considering that Beidou satellite navigation system (BDS) consists of geostationary earth orbit (GEO), inclined geosynchronous satellite orbit (IGSO) and medium earth orbit (MEO) satellites operating in different orbits with different heights, only one model being used in stochastic modeling is probably inadequate, which is the usual case for Global Positioning System (GPS). In this contribution, a new strategy of stochastic modeling for hybrid constellation BDS precise relative positioning is proposed, namely multiple elevation-dependent models (MEDM). In MEDM, parameters of three elevation-dependent models for GEO, IGSO, MEO satellites are estimated independently. After that, variances of phase and code observations for different orbits satellites can be estimated, then the stochastic model specific for BDS can be constructed in real time. The experimental results reveal that, when MEDM is compared with the sole elevation-dependent model and the carrier-to-noise power density ratios (C/N0)-dependent model, the maximum improvements in ambiguity resolution success rates (SR) in 5, 6, 7, 8 available satellites cases are about 0.2%, 13%, 3%, 1% for B1 frequency, and 1%, 6%, 2.5%, 0.2% for B2 frequency, respectively. The overall improvements in baseline solutions by using MEDM are in millimeter or submillimeter level. Hence, MEDM is more suitable for hybrid constellation of BDS in precise relative positioning applications.

Summary This paper has deeply investigated the performance signature of modulation techniques based low earth orbit (LEO)/medium earth orbit (MEO) intersatellite optical wireless communication syst...

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

The space environment consists of various complex phenomena, which could have a strong influence on the spacecraft operation in different aspects. Since the very beginning of space exploration, numerous studies have been done on the space environment. However, most of the existing literature focuses on the investigation of the details of environmental phenomena, while the space environment has rarely been discussed from the perspective of orbits types. Therefore, a comprehensive review on analyzing and comparing the environmental characteristics among diverse orbits in space is of great significance. In this paper, the main components of the space environment are introduced, including the neutral atmosphere, the plasma environment, the radiation environment, the macroscopic particle environment, the geomagnetic field, the temperature field, and the solar activities. The relations of the various space environmental components are also discussed. The dominant environmental components and their effects on spacecraft in different orbits, i.e., the geosynchronous orbit (GEO), the low earth orbit (LEO), the medium earth orbit (MEO), and the high earth orbit (HEO), are investigated, respectively. The space environment that should be taken into particular consideration is summed up to facilitate the design of the spacecraft in a specific orbit.

—Complementary solutions to the Medium Earth Orbit (MEO) Global Navigation Satellite Systems (GNSS) are more and more in demand to be able to achieve seamless positioning worldwide, in outdoor as well as in indoor scenarios, and to cope with increased interference threats in GNSS bands. Two of such complementary systems can rely on the emerging Low Earth Orbit (LEO) constellations and on the terrestrial long-range Internet of Things (IoT) systems, both under rapid developments nowadays. Standalone positioning solutions based on such systems complementary to GNSS can be beneficial in situations where GNSS signal is highly affected by interferences, such as jammers and spoofers, while hybrid GNSS and nonGNSS solutions making use of LEO and terrestrial IoT signals as signals of opportunity can improve the achievable positioning accuracy in a wide variety of scenarios. Comparative research of performance bounds achievable through MEO, LEO, and terrestrial IoT signals are still hard to find in the current literature. It is the goal of this paper to introduce a unified framework to compare these three system types, based on geometry matrices and error modeling, and to present a performance analysis in terms of Geometric Dilution of Precision (GDOP) and positioning accuracy bounds. Index Terms—Medium Earth orbit (MEO), global navigation satellite systems (GNSS), low Earth orbit (LEO) satellites, Internet of Things (IoT) terrestrial network, positioning, geometric dilution of precision (GDOP). Citation R. M. Ferre and E. S. Lohan, "Comparison of MEO, LEO, and terrestrial IoT configurations in terms of GDOP and achievable positioning accuracies," inIEEE Journal of Radio Frequency Identification, doi: 10.1109/JRFID.2021.3079475. Year 2021 Version Final version Link to publication https://ieeexplore.ieee.org/document/9430943 Published in IEEE JOURNAL OF RADIO FREQUENCY IDENTIFICATION DOI 10.1109/JRFID.2021.3079475 License This work is licensed under a Creative Commons Attribution 4.0 License. .

Compared to the BeiDou regional navigation satellite system (BDS-2), the BeiDou global navigation satellite system (BDS-3) includes the newly designed B1C and B2a signals, which are compatible with the L1 and L5 frequencies of the global positioning system (GPS). Considering that the precise orbit determination (POD) of the BDS-3 constellation is currently restricted to the legacy B1I and B3I signals, we reported the POD performance of BDS-3 satellites using B1C and B2a dual-frequency measurements. Nine globally distributed Multi-GNSS Experiment (MGEX) stations were selected to determine the orbits of BDS-3 satellites during the period of July 2019. The results show that B1C/B2a-based POD enables an average three-dimensional root-mean-square error (3D RMS) of 24.2 cm, and the precision is better than 6 cm in the radial component in a comparison of two-day overlapping arcs. Satellite laser ranging (SLR) validation achieves an overall precision of 6.8 cm in RMS differences. Compared to the B1I/B3I-based POD results, the quality of B1C/B2a-based orbits is improved by approximately 9% across the whole BDS-3 constellation, indicating that the new B1C/B2a signals can be employed for the superior POD performance of BDS-3 satellites. Moreover, we investigated the behavior of the solar radiation pressure (SRP), which is generally considered one of the primary error sources in BDS-3 medium earth orbit (MEO) dynamic orbit determination. The ECOM7 SRP model has a better POD performance in continuous yaw steering (CYS) mode than the ECOM5 and ECOM9 SRP models. The results also show that there is no degradation in the orbit precision of BDS-3 MEO satellites when the elevation angle of the sun above the orbital plane (β angle) varies within the range of − 4° to + 4°.

The next generation of aviation will operate in a data-driven environment. Aviation’s destiny is now coupled with the Internet of Things (IoT)- wherein devices and autonomous systems have an inherent need for continuous network connectivity. For next generation technology to reach its full potential, a network connection should be available anywhere, anytime. Unmanned Aerial Vehicles (UAVs or drones) are increasing in number and large companies are designing drone delivery systems. A next generation Unmanned Aerial System (UAS) will require a more flexible network that can deliver data in non-traditional environments. However, looking beyond UAS flight operations, the aviation industry at large needs to consider integrating new services into the Air Navigation Service Provider’s (ANSP’s) enterprise communications architectures-both to get low-cost communication to remote areas for UAS operations and as a cost-effective last mile alternative for high-availability, low latency services.These developments will require a communication network that is scalable to the needs of the future. Low and Medium Earth Orbit (LEO/MEO) satellite constellations can address the challenges of a rapidly evolving future by being able to provide communications beyond the existing commercial and public infrastructure, to include a global broadband, low-latency footprint. Remote regions typically require costly site-specific infrastructure solutions to provide last-mile connectivity. By utilizing lower orbits, the constellation can provide high data rates and low latency on the links by bringing the edge of broadcasting closer to the Earth versus a Geosynchronous Orbit (GEO). The telecommunications industry is at a crossroads for determining the network design for tomorrow’s aviation requirements. The introduction of UAVs into the National Airspace System (NAS) will present the challenge of locating and surveilling these small and elusive aircraft. The time is now to build a new infrastructure; a Satellite Communication (SatCom) network, that achieves rapid global broadband coverage, versus localized terrestrial networks.

The modeling of the earth-satellite propagation channel requires experimental data that is usually obtained by monitoring continuous wave (CW) signal sources installed in satellites (beacons). Although there is considerable experience in the development of beacon receivers for geostationary satellites, this is not the case for Medium Earth Orbit (MEO) or Low Earth Orbit (LEO) constellations. The main considerations for the development of propagation receivers for monitoring MEO satellites are addressed in this article. The target constellation is presented with a description of the satellite orbits, characteristics of the beacon payload and the project requirements. The receiver's requirements are then analyzed, starting with the antenna, satellite tracking, beacon detection and propagation data handling.

Beyond line-of-sight communication is important from a tactical point of view, as it yields network-centric capability and data link between multi-generation machines. Satellite communication (SATCOM) is the only technology that provides this capability. The SATCOM modem is an important communication component that provides the capability to convert baseband data to radio frequency (RF) band and vice versa. The commercial off-the-shelf (COTS) modems for SATCOM have well-known packet and communication standards resulting in non-confidentiality and limited user control for customized usage. Considering recent trends in modern digital communication systems, namely 4G/5G networks, satellite constellations and tactical communication, it is evident that reliable and low latency communication between multi-generation machines is inevitable. Keeping in view the limitations of COTS modems, in this paper, we present a prototype design of a software-defined radio based SATCOM modem for geostationary satellites with a bent-pipe satellite transponder. The proposed design accommodates multiple provisions such as selection of variable-length message signal, formation of packet structure, inclusion of cyclic redundancy check, implementation of Reed-Solomon codes for short block-lengths at various rates, direct sequence spread spectrum with various modulation schemes including binary phase shift-keying (BPSK), quadrature phase shift-keying (QPSK) and 8PSK at various transmission rates and reliability requirements. In addition, the proposed design incorporates anti-jamming capability and user anonymity along with self-reliance. The prototype provides a testing platform for communication in both static and dynamic environments in different orbits such as geostationary Earth orbit (GEO), medium Earth orbit (MEO), and low Earth orbit (LEO).

The BeiDou navigation satellite system (BDS) tracks medium earth orbit (MEO) satellites using only regional tracking stations in China. As a result, the broadcast clock accuracy of the MEO satellites decreases rapidly during the invisible arcs because of the lack of available observations. The inter-satellite link (ISL) technology of the third generation of BDS (BDS-3) can be used to extend the visible arcs of MEO satellites and to measure the relative inter-satellite clock in nearly real time. We propose a broadcast clock approach for BDS-3 by integrating observations from regional tracking stations and ISLs. The clock error between satellites is obtained through centralized estimation based on ISLs. The Ka-band hardware delay is calibrated by taking the double difference between ISL-centralized clock and the Multi-satellite Precise Orbit Determination clock. The deviation between the ISL-centralized clock and the BeiDou time is obtained using only one Two-way Satellite Time Comparison station or anchor station. To validate the algorithms, we analyze clock estimation and prediction accuracy, hardware delay stability, and time synchronization accuracy. The results show that the frequency stability of the BDS-3 onboard passive hydrogen maser (PHM) and rubidium atomic frequency standard (RAFS) is competitive to those of the GPS IIF RAFS and Galileo FOC PHM and better than those of GPS IIR RAFS. The root-mean-square error of the 2-h clock prediction is better than 0.25 ns, and the validation result relative to the post-processed precise clock product is better than 0.4 ns. The time synchronization accuracy of better than 1 ns can be obtained based on only one TSTC station or an anchor station, and the standard deviation of Ka-band hardware delay is about 0.12 ns. It is believed that the ISL and the proposed algorithms will bring a significant upgrade in the estimation of BDS-3 broadcast clock; the broadcast clock accuracy will be greatly improved, and reliance on the ground segment will also be reduced significantly.

An in-depth and comprehensive assessment of new observations from BDS-3 satellites is presented, with the main focus on the Carrier-to-Noise density ratio (C/N0), the quality of code and carrier phase observations for B1C and B2a signal. The signal characteristics of geosynchronous earth orbit (GEO), inclined geosynchronous satellite orbit (IGSO) and medium earth orbit (MEO) satellites of BDS-3 were grouped and compared, respectively. The evaluation results of the new B1C and B2a signals of BDS-3 were compared with the previously B1I/B2I/B3I signals and the interoperable signals of GPS, Galileo and quasi-zenith satellite system (QZSS) were compared simultaneously. As expected, the results clearly show that B1C and B2a have better signal strength and higher accuracy, including code and carrier phase observations. The C/N0 of the B2a signal is about 3 dB higher than other signals. One exception is the code observation accuracy of B3I, which value is less than 0.15 m. The carrier precision of B1C and B2a is better than that of B1I/B2I/B3I. Despite difference-in-difference (DD) observation quantity or zero-base line evaluation is adopted, while B1C is about 0.3 mm higher carrier precision than B2a. The BDS-3 MEO satellite and GPS, Galileo, and QZSS satellites have the same level of signal strength, code and phase observation accuracy at the interoperable frequency, namely 1575.42 MHz and 1176.45 MHz which are very suitable for the co-position application.

As pre-launch antenna calibrations are not available for GPS and GLONASS satellites, the high correlation between the terrestrial scale and phase center offset (PCO) prevents a reliable estimation of the terrestrial scale with GNSS (Global Navigation Satellite System) technology. Fortunately, the ground calibrated PCO values for Galileo, BeiDou navigation satellite system (BDS), and QZSS have been released, making a reliable estimation of the terrestrial scale with GNSS possible. In the third reprocess (repro3) of International GNSS Service (IGS), the terrestrial scale derived with Galileo, has been used. To evaluate the consistency of the terrestrial scale derived from the BDS-released PCOs as well as Galileo-released ones, and to incorporate BDS into IGS repro3 as well as operational legacy analysis, the phase center variations (PCV) and PCO for BDS medium earth orbit (MEO) and inclined geostationary orbit (IGSO) satellites are estimated to be consistent with GPS/GLONASS antenna offsets in two frames, i.e., IGb14 and IGS R3, considering robot calibrations of the ground receiver antenna models for BDS released by Geo++. We observe that the average offset of Z-PCOs achieves +98.8 mm between BDS official released and the estimated PCOs in IGb14 frame for BDS-3 MEO satellites, whereas the average offset for Z-PCO is about +174.1 mm (about −1.27 ppb at the height of BDS MEO satellites) between the solutions in IGSR3 and IGb14 frame. The phase center solutions are evaluated with orbit boundary disclosures (OBD) as well as the global station coordinates. The orbit consistency benefits from the PCO/PCV estimates, particularly for BDS-2 MEO satellites, of which the 3D RMS (root mean square) OBD is reduced by 50%, whereas 3D OBD achieves about 90.0 mm for BDS-3 MEO satellites. Moreover, the scale bias between BDS-derived station coordinates and IGS legacy solutions in IGb14 frame is reduced from +0.446 ± 0.153 ppb to +0.012 ± 0.112 ppb using PCO/PCV estimates in IGb14, instead of the BDS official released values. Additionally, the residuals of the BDS-derived station heights (after the Helmert transformation) are slightly reduced from 9.65 to 8.62 mm. On the other hand, about +0.226 ± 0.175 ppb is observed between BDS-only coordinate solutions derived from PCO/PCV estimates in IGSR3 frame and the IGS repro3 initial combination. These results demonstrate that the scale inconsistency derived from BDS and Galileo released PCOs is about +1.854 ± 0.191 ppb, and a good consistency of PCO/PCC estimates for BDS in IGb14 and IGSR3 frame with other systems of GPS/ GLONASS antenna offsets is achieved.

Aiming to analyze the influence of earth-atmosphere radiation on imaging characteristics of space object. A scene of space object motion and detection was designed by Satellite Tool Kit (STK) where visible light imagers mounted on geosynchronous earth orbit (GEO)/medium earth orbit (MEO) satellites were treated as observation platform, highly elliptical orbit (HEO) satellite was treated as object. Equivalent magnitude models of space object and earth-atmosphere radiation, and formulation for signal-to-noise ratio (SNR) of space object were derived by adopting infinitesimal method, according to spatial relationship between space object, earth, sun, and observation platform. The variation of equivalent magnitude between object and earth-atmosphere radiation, as well as the SNR were analyzed when tracking detector and gazing detector were arranged on observation platform. Simulation results indicate that the SNR of object on low orbit observation platform is higher than that on high orbit observation platform, the SNR of the former is 1.1 orders of magnitude higher than the latter on average, while the average imaging SNR of the latter is 1.9. Tracking detector’s object SNR is higher than gazing detector, the difference is largest when object enters or leaves detecting field of view, yet it is the smallest when object is close to detecting field of view. Moreover, the value of SNR obtained by simulation provides a guidance for the detection and recognition of space object, as well as a way of reduction of earth-atmosphere radiation.

The recent decades have seen an increasing interest in Medium Earth Orbit and Low Earth Orbit satellite constellations. However, there is little information on the delay variation characteristics of these systems and the resulting impact on high layer protocols. To fill this gap, this paper simulates a constellation that exhibits the same delay characteristics as the already deployed Iridium but considers closer bandwidths to constellation projects’. We identify five major sources of delay variation in polar satellite constellations with different occurrence rates: elevation, intra-orbital handover, inter-orbital handover, orbital seam handover and Inter-Satellite Link changes. We simulate file transfers of different sizes to assess the impact of each of these delay variations on the file transfer. We conclude that the orbital seam is the less frequent source of delay and induces a larger impact on a small file transfers: the orbital seam, which occurs at most three times during 24 h, induces a 66% increase of the time needed to transmit a small file. Inter-orbital and intra-orbital handovers occur less often and reduce the throughput by approximately ∼ 8% for both low and high throughput configurations. The other sources of delay variations have a negligible impact on small file transfers, and long file transfers are not impacted much by the delay variations.

For monitoring MEO (Medium Earth Orbit) space environment on orbit, for acquiring the time and space distribution of plasma parameters and the correlation between space plasma environment and typical surface potential to provide environment detection data for early warning of the surface of charged risk caused by the space plasma environment, for extracting key environment indicators causing surface charging, for building surface charged risk early warning environment standards, two programs were designed that the space plasma detectors working on MEO and GEO orbit, forming high and low collocation. This program starts from MEO and develops on GEO space plasma detection. Therefore, in order to adapt to the changing external environment from MEO to GEO, it is necessary to carry out adaptive improvement and comparative analysis for design (especially for structural design). In this paper, two structural design schemes are compared and their rationality is verified by dynamic simulation from data of modal analysis and frequency response for sine sweeping vibration and mechanical test. The final results and analysis point to a conclusion that the improvement of structure design scheme adapted to the large change from MEO to GEO satellite platform, which gave great support to the space plasma detector.

GPS is a critical tool often used in terrestrial location and navigation. However, GPS relies on a system of satellites in medium earth orbit and is prone to dropouts at higher latitudes, and may be susceptible to spoofing or other attacks. Thus, it is prudent to examine possible alternatives. Recently, star tracking has been of interest in global positioning. There are many implementations of a star tracker using an imaging-based optical system to capture star locations in order to estimate position using celestial navigation techniques. These imaging systems are performance limited by blurring imposed by atmospheric turbulence, platform jitter, and measurement noise. An interferometric system for locating stars can be used as an alternative to accurately locate stars in the sky. The use of one or more 3 or 4-aperture interferometers may allow for greater noise immunity in the phase error induced by atmospheric turbulence, and allows a larger baseline to be used as compared to the diameter of a single lens imaging system in some configurations. Such a system could sample at a rate higher than the Greenwood frequency, which helps reduce error in boresight angle retrieval. In the system described herein, the performance limitations would be dominated by atmospheric tilt and boresight angle retrieval. When the boresight angle can be retrieved, the system could be reasonably expected to produce a position estimate RMS error of less than 30 meters.

An autonomous spacecraft navigation scheme based on the stellar aberration and the starlight gravitational deflection is presented in the paper. The stellar aberration provides spacecraft velocity information in the plane perpendicular to the star line-of-sight unit vector, while the starlight gravitational deflection is sensitive to the spacecraft position relative to celestial bodies. For autonomous spacecraft navigation, a practical approach to collect measurements is to observe the change of inter-star angles caused by the stellar aberration and the starlight gravitational deflection through an optical interferometer. A measurement equation is established to describe the relation between the state vector and the observations. Then it is used together with the dynamical model for the design of an extended Kalman filter that provides spacecraft kinematic state estimation. To improve the navigation performance, an optical path delay bias estimation and compensation are implemented in the extended Kalman filter. It is shown via simulation that, for a medium Earth orbit (MEO) satellite, the position and velocity errors of the presented scheme are on the order of 150 m and 0.02 m/s with the inter-star angle measurement accuracy of about 1 mas.

Two-Line Elements (TLEs) issued by the space-track catalog are still the most extensive public data source for space debris tracking to date. However, TLEs accuracy at epoch is typically larger than 1 km in Low Earth Orbit (LEO) and 3 km in Medium Earth Orbit (MEO). Therefore, TLEs are too coarse to enable collision avoidance maneuvers. The present work aims at correcting TLEs orbits at epoch to enable operational conjunction assessment and help satellite operators better protect their assets at moderate cost. Using only Moon-Earth and Sun-Earth distance time series, as well as 2018 TLE data for 14 GPS satellites and SGP4 propagation over a short period, we were able to correct the TLEs of the 29 operational GPS satellites by 65% on average for year 2019, reducing the 3D RMS error at epoch from 2.2 km to 680 m. In other words, we improved TLEs accuracy from 5 km to 1.5 km with a 95% confidence level.

The BDS-3 preliminary system was declared functional on December 27, 2018. It enables positioning service on a global scale. This study conducts a comprehensive investigation of the BDS-3 global service from the perspective of positioning, including satellite ephemeris, benefits of the new BDS-3 satellites, and the new signal effect on three positioning modes: single-point positioning (SPP), precise-point positioning (PPP) and real-time kinematic (RTK). First, the broadcast and precise ephemeris availabilities of 18 BDS-3 Medium Earth orbit (MEO) satellites from January to June 2019 are investigated. Due to the shorter operational timespans of BDS-3 satellites, their broadcast and precise ephemeris files retrieved from Multi-GNSS Experiment (MGEX) have 1.2–19.4% and 10.9–13.77% discontinuities, respectively, while the discontinuities of BDS-2 satellite broadcast and precise ephemeris are only 0.1 and 2.1%. Second, the BDS-3 satellite orbit accuracies and clock precisions are significantly improved, i.e., below 0.5 m and 1.82 ns compared to 2 m and 2.91 ns for BDS-2 because of the inter-satellite links of BDS-3 satellites. Third, an average of 4.4–5.7 BDS satellites can be observed at stations located in North/South America, and 8.9 for the European station FFMJ located in Germany after the BDS-3 global system announcement. As a result, continuous positioning is feasible in these regions. As to four Asia–Pacific stations, their SPP accuracies are improved by 12.1–60.2% compared to those of the BDS-2-only solution. Meanwhile, the PPP and RTK convergence times of Asia–Pacific stations are also shortened. Last but not least, the new B1C signal does not bring convincing improvement to PPP and RTK positioning accuracies in this study, due to the limited number of available B1C new signals at this time.

In this study, an uncombined precise point positioning (PPP) model was established and was used for estimating fractional cycle bias (FCB) products and for achieving ambiguity resolution (AR), using GPS, BDS-2, and Galileo raw observations. The uncombined PPP model is flexible and efficient for positioning services and generating FCB. The FCBs for GPS, BDS-2, and Galileo were estimated using the uncombined PPP model with observations from the Multi-GNSS Experiment (MGEX) stations. The root mean squares (RMSs) of the float ambiguity a posteriori residuals associated with all of the three GNSS constellations, i.e., GPS, BDS-2, and Galileo, are less than 0.1 cycles for both narrow-lane (NL) and wide-lane (WL) combinations. The standard deviation (STD) of the WL combination FCB series is 0.015, 0.013, and 0.006 cycles for GPS, BDS-2, and Galileo, respectively, and the counterpart for the NL combination FCB series is 0.030 and 0.0184 cycles for GPS and Galileo, respectively. For the BDS-2 NL combination FCB series, the STD of the inclined geosynchronous orbit (IGSO) satellites is 0.0156 cycles, while the value for the medium Earth orbit (MEO) satellites is 0.073 cycles. The AR solutions produced by the uncombined multi-GNSS PPP model were evaluated from the positioning biases and the success fixing rate of ambiguity. The experimental results demonstrate that the growth of the amount of available satellites significantly improves the PPP performance. The three-dimensional (3D) positioning accuracies associated with the PPP ambiguity-fixed solutions for the respective only-GPS, GPS/BDS-2, GPS/Galileo, and GPS/BDS-2/Galileo models are 1.34, 1.19, 1.21, and 1.14 cm, respectively, and more than a 30% improvement is achieved when compared to the results related to the ambiguity-float solutions. Additionally, the convergence time based on the GPS/BDS-2/Galileo observations is only 7.5 min for the ambiguity-fixed solutions, and the results exhibit a 53% improvement in comparison to the ambiguity-float solutions. The values of convergence time based on the only-GPS observations are estimated as 22 and 10.5 min for the ambiguity-float and ambiguity-fixed solutions, respectively. Lastly, the success fixing rate of ambiguity is also dramatically raised for the multi-GNSS PPP AR. For example, the percentage is approximately 99% for the GPS/BDS-2/Galileo solution over a 10 min processing period. In addition, the inter-system bias (ISB) between GPS, BDS-2, and Galileo, which is carefully considered in the uncombined multi-GNSS PPP method, is modeled as a white noise process. The differences of the ISB series between BDS-2 and Galileo indicate that the clock datum bias of the satellite clock offset estimation accounts for the variation of the ISB series.

The Advanced Processing and Communications team at Los Alamos National Laboratory has designed and manufactured a new system controller that complies with the 6U SpaceVPX (ANSI/VITA 78) specification and can function as a command- and data-handling single-board computer. The design meets the radiation hardness requirements for application in geosynchronous (GEO) and medium earth orbit (MEO), employs QMLV and Class-S components, has a conduction cooling frame, and is mechanically hardened against typical shock and vibration profiles encountered during launch. The system controller is based on the space grade GR740 quadcore LEON4 processor ASIC with a MicroChip RTG4 field programmable gate array (FPGA) to support hardware coprocessing and supply the gigabit /s serializer-deserializers (SerDes) needed for the VPX control and data planes. This module was designed to allow interoperability between OpenVPX (ANSI/VITA 65) and SpaceVPX so that lower cost hardware from the commercial world can be used during the prototyping process instead of more expensive flight like hardware. This design has 1 GByte of SDRAM with an additional ½ GByte of error detection and correction memory (EDAC). Since SDRAM is susceptible to single event functional interrupts (SEFIs), the design team used byte-wide aspect ratio memories with individual power control for recovery. This paper will discuss the performance, power consumption, and status of this design.

This paper presents power-optimal guidance for a planar space solar power satellite (SSPS). Power-optimal guidance is the attitude trajectory that maximizes the solar power transmitted by the SSPS. Planarity is important because it couples the orientations of the SSPS’s photovoltaic and antenna surfaces. Hence, the transmitted power depends on the relative geometry between the SSPS, the sun, and the receiving station. The orientation that maximizes power transfer changes as this relative geometry changes. Both single- and dual-sided SSPS architectures are considered. A single-sided SSPS has one photovoltaic surface and one antenna surface. A dual-sided SSPS is a single-sided SSPS with at least one additional photovoltaic or antenna surface. Geometric arguments show that a dual-sided SSPS has superior performance to a single-sided SSPS. Power-optimal guidance is then presented for the special cases of SSPSs in geostationary Earth orbit, medium Earth orbit, and low Earth orbit transmitting to an equatorial receiving station at the time of Earth’s vernal equinox. These examples emphasize important solution properties, including the need for large slew maneuvers, and they show that, even though system efficiency decreases as orbit altitude decreases, reduced path losses actually increase the amount of received energy per unit aperture area. This has significant system implications for future space solar power missions.

To meet the needs of step-by-step construction of inclined geosynchronous orbit (IGSO)/medium earth orbit (MEO) constellation in the future and to improve the robustness of the constellation in case of losing connections of some satellites, this paper studies the incomplete IGSO/MEO constellation network topology design technology, analyses the inter-satellite link accessibility, and proposes a multilayer satellite network link-building strategy based on dynamic programming. This paper also designs the methods of link-building for intra-layer links among IGSO satellites, intra-layer links among MEO satellites and inter-layer links between IGSO satellites and MEO satellites. The simulation results show that this method can well construct dynamic inter-satellite network topology with less link switches, higher network transmission bandwidth and wider access user coverage.

In the realm of increasing demand for stable and excellent connectivity around the world, researches are done heavily to boost up the connectivity and transmission distance. Therefore, in this research work, we have made the realization of an inter-satellite optical wireless channel, wavelength division multiplexed system implemented with polarization interleaving technique in order to realize an 8-channel system capable of transmitting data rate of 20 Gbps per channel for the transmission distance of LEO, MEO, and GEO satellite orbits. The designed system is capable of data transmission with mitigated channel non linearities and acceptable SNR, BER, and Q-factor for the transmission distance of 40,000 km, 10,000 km, and 5000 km covered under the geostationary equatorial orbit, medium earth orbit, and low earth orbit, respectively. The system realized here is also considered to work efficiently under 1 µ radian to 3 µ radian transmitter and receiver pointing error losses. Optimal results can be observed on the basis of transmitted and received optical power spectrum, SNR values, total power received, eye-opening diagrams, BER, and Q-factor.

Summary In this paper, a methodology for the generation of cloud coverage time series correlated on both temporal and spatial domains and the estimation of cloud‐free line‐of‐sight (CFLOS) probabil...