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With the deployment of the global BeiDou navigation satellite system (BDS-3) constellation completed, the positioning service performance is significantly improved relative to the regional system (BDS-2). As one of the key prerequisite products for precise point positioning (PPP), the combined BDS-2/BDS-3 real-time satellite clock estimation method is investigated in this study. Since the code hardware delays of BDS-2/BDS-3 satellites might be inconsistent for some receiver types, the inter-system bias (ISB) is considered in the combination although the overlapping B1I/B3I signal is employed. The combination processing is comprehensively evaluated in terms of satellite tracking coverage, clock precision, and convergence time. The experiment results indicate that the standard deviation (STD) precision of the estimated clocks for BDS-2 medium Earth orbit (MEO) satellites is improved by 52.0% from 0.150 ns to 0.072 ns due to the improvement of satellite tracking coverage. Introducing the ISB parameter as piece-wise constants improves the STD of all clocks from 0.076 ns to 0.067 ns and BDS-3 clocks achieve a significant improvement from 0.065 ns to 0.053 ns. The real-time clock estimation is demonstrated to be robust for the clock datum transformation, and it converges after one hour to a stable precision but needs a long time to achieve the best performance. The BDS real-time clocks are estimated with the proposed strategy to support real-time PPP based on the predicted satellite orbits. Results show that the clock STD is about 0.1 ns for most BDS satellites and the positioning accuracy of about 10 cm is achieved after the convergence time of about half an hour. The clock precision for BDS-2 geostationary Earth orbit (GEO) satellites is degraded to 0.303 ns due to the impact of poor orbit prediction errors. The exclusion of GEO satellites improves the positioning accuracy by about 15% and reduces the convergence time by 22–34%.

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.

The service areas of the BeiDou Navigation Satellite System (BDS) are China, the Asia–Pacific, and the world via three stages of development. The BDS constellation is designed to maintain a geostationary Earth orbit (GEO), inclined geosynchronous orbit (IGSO), and medium Earth orbit (MEO). These orbital maneuvers yield certain difficulties for data processing, especially for the BeiDou satellites, such as a decrease in the real-time service performance, which causes a missing precise orbit product from the maneuvered satellites. The frequencies of the orbital maneuvers for the GEO and IGSO satellites are higher than those for the MEO satellites. The maneuvering time and strategy cannot be obtained by common users due to secrecy, which can yield a decline in the service performance of the BDS. Thus, in this study, we analyzed the variation in the orbital semimajor axis for the satellites. The long-term variation in the orbital semimajor axis changes linearly. The short-term variation in the orbital semimajor axis also has periodicity, which can be described by the sum of sine functions. According to the long-term variation in the orbital semimajor axis for the satellites, we propose a detection method for in-plane orbital maneuvering of the GEO, IGSO, and MEO satellites. We then propose a detection method for out-of-plane maneuvering of the GEO satellites. BDS and Global Positioning System (GPS) data from the broadcast ephemeris were analyzed to verify the proposed methods. The experimental results from 2013 to 2019 show that the frequency of orbital maneuvering is approximately once a month for the GEO satellites, once every six months for the IGSO satellites, and once every 1.4 years for the MEO satellites.

BeiDou Navigation Satellite System (BDS) is composed of satellites in geostationary Earth orbit (GEO), medium Earth orbit (MEO) and inclined geosynchronous orbit (IGSO). However, the orbit determination of geostationary Earth orbits and of geosynchronous orbits (GSO) with small inclination angle and small eccentricity is a challenging task that is addressed in this paper using Extended Kalman Filter (EKF). The satellite positions were predicted in Earth-centred inertial (ECI) reference frame when propagated through Keplerian model and perturbation force model for different values of right ascension of ascending node (RAAN). Root mean square (RMS) errors of 9.61 cm, 6.73 cm and 11.46 cm were observed in ECI X, Y and Z satellite position coordinates of GSO respectively, whereas, the RMS errors for GEO satellite were 8.89 cm, 7.92 cm, and 0.93 cm respectively in ECI X, Y and Z coordinates; for perturbation force model with maximum value of RAAN when compared with dynamic orbit determination model. Kolmogorov-Smirnov test for EKF reported a p-value > 0.05, indicating a good fit of perturbation force model for orbit propagation. Orbit determination using EKF with perturbation force model were compared with that using EKF with Kepler's model. Wilcoxon Rank Sum test was used to compare the residuals from EKF algorithm through Kepler's model and perturbation force model. EKF with Perturbation force model showed improvement in predicting the satellite positions as compared to Kepler's model. EKF with Perturbation force model was further applied to International GNSS Service (IGS) station data and kilometre level accuracy was achieved. RMS errors of 0.75 km, 2.53 km and 1.91 km were observed in ECI X, Y and Z satellite position coordinates of GSO, respectively, whereas, the RMS errors for GEO satellite were 3.89 km, 4.20 km and 6.66 km respectively in ECI X, Y and Z coordinates for perturbation force model.

Short Message Communication (SMC) is a featured service of BeiDou Navigation Satellite System (BDS). After its successful deployment in 2003, Regional Short Message Communication (RSMC) service has been continuously serving China and its neighboring countries and regions, especially in life safety scenarios. In this paper, the architecture of the Global Short Message Communication (GSMC) system is proposed based on the medium earth orbit (MEO) constellation and the crosslinks of the global BeiDou navigation system (BDS-3). Three subtypes of GSMC service, i.e. positioning report, emergency search and rescue (SAR) and regular SMC are designed in accordance with the technical characteristic of integration of navigation and communication in BDS-3, which supports future wide applications of GSMC. The performance of the designed GSMC system is analyzed by numerical calculations. As BDS-3 was officially announced completion on July 31, 2020, GSMC has been providing initial service. First test results of the in orbit GSMC payloads are also presented in the paper to verify the designed capabilities. Preliminary results also show that the requirements of Global Maritime Distress and Safety System (GMDSS) can also be fulfilled.

To realize the smooth transition from regional BeiDou Navigation Satellite System (BDS-2) to the global one (BDS-3), the integration of BDS-2 and BDS-3 is important for providing continuous, stable and reliable positioning, navigation and timing (PNT) services for global users. This work used 154 globally distributed multi-GNSS (Global Navigation Satellite System) experiment stations spanning 30 days to analyze the satellite availability and positioning performance of uncombined precise point positioning (UC-PPP) under current BDS-2 and BDS-3 constellations. We focused on three issues: the influence of BDS-3 receiver tracking abilities, the positioning performance among different areas, and the benefit of multi-frequency observations. The results show that the elliptical zone caused by poor BDS-2 satellite visibility is disappeared when the evenly distributed BDS-3 medium earth orbit satellites are introduced. When BDS-3 are integrated with BDS-2, the area with the Position Dilution of Precision (PDOP) less than 2 can be expanded to 75° S-75° N and 30° E-150° W. The positioning performance of BDS-3 and BDS-2/BDS-3 UC-PPP are seriously affected by the receiver tracking abilities of BDS-3 signals. When the maximum pseudo-random noise sequences (PRNs) of BDS-3 satellites tracked by stations are within 30 or 37, the positioning accuracy of static UC-PPP can be improved by 22.94% or 8.27% due to the integration of BDS-2 and BDS-3. Besides, the most improvement of BDS-2 and BDS-3 integration is achieved in Asia-Pacific regions, especially for the kinematic UC-PPP or the poor receiver tracking abilities of BDS-3. Similar to the multi-frequency BDS-2 UC-PPP, the benefit of multi-frequency signals for BDS-3 or BDS-2/BDS-3 UC-PPP is also non-vital. The three-dimensional positioning accuracy of BDS-2/BDS-3 multi-frequency UC-PPP in static mode and kinematic mode are 2.24 cm and 5.39 cm, while the corresponding convergence time are 49.62 min and 73.80 min, respectively. Compared with BDS-2, both the positioning accuracy and the convergence time of BDS-2/BDS-3 joint UC-PPP are improved by approximately over 50%, which indicates that BDS-3 has a great potential to provide high-quality PNT services as other global navigation satellite systems.

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.

Submillimeter interferometry has the potential to image supermassive black holes on event horizon scales, providing tests of the theory of general relativity and increasing our understanding of black hole accretion processes. The Event Horizon Telescope (EHT) performs these observations from the ground, and its main imaging targets are Sagittarius A* in the Galactic Center and the black hole at the center of the M87 galaxy. However, the EHT is fundamentally limited in its performance by atmospheric effects and sparse terrestrial $(u,v)$-coverage (Fourier sampling of the image). The scientific interest in quantitative studies of the horizon size and shape of these black holes has motivated studies into using space interferometry which is free of these limitations. Angular resolution considerations and interstellar scattering effects push the desired observing frequency to bands above 500 GHz. This paper presents the requirements for meeting these science goals, describes the concept of interferometry from Polar or Equatorial Medium Earth Orbits (PECMEO) which we dub the Event Horizon Imager (EHI), and utilizes suitable space technology heritage. In this concept, two or three satellites orbit at slightly different orbital radii, resulting in a dense and uniform spiral-shaped $(u,v)$-coverage over time. The local oscillator signals are shared via an inter-satellite link, and the data streams are correlated on-board before final processing on the ground. Inter-satellite metrology and satellite positioning are extensively employed to facilitate the knowledge of the instrument position vector, and its time derivative. The European space heritage usable for both the front ends and the antenna technology of such an instrument is investigated. Current and future sensors for the required inter-satellite metrology are listed. Intended performance estimates and simulation results are given., Comment: 23 pages, 10 figures

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.

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.

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.

On December 27, 2018, the Beidou-3 System (BDS-3) has completed the deployment of 18 Medium Earth Orbit (MEO) satellites combined as a space constellation. In addition to the augmentation information for the new system signals B1C and B2a, the BDS-3 is compatible with the three augmentation information broadcast by the BDS-2 system for B1I and B3I signals: equivalent clock error correction, User Differential Ranging Error (UDRE) and Grid Ionosphere Vertical Error (GIVE). In this paper, the observation data of Beidou monitoring network are used to analyze the pseudo-range observation quality of the smooth transition signals B1I and B3I of BDS-2 and BDS-3. At the same time, the relationship between the equivalent clock error correction and the prediction error of the satellite clock is analyzed by using the Two-Way Satellite Time and Frequency Transfer (TWSTFT) data. The results show that the correlation between the equivalent clock error correction and the monitored clock error by using the TWSTFT data is greater than 60%. We calculate the UDRE by using the equivalent clock error correction. The results show that the satellite equivalent clock error correction can improve the accuracy of User Equivalent Range Error (UERE) by about 50%. This paper also compares the positioning accuracy of the BDS-2 satellites with the BDS-2 satellites combined BDS-3 satellites. The results show that the three-dimensional positioning accuracy is improved by about 30% after the BDS-3 satellites are added.

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.

A new model is presented to resolve cycle slips detection for triple-frequency observations of BeiDou navigation satellite system (BDS) in this article when pseudorange observations are missing or insufficiently accurate under harsh or special situations. Based on the first-order time-difference geometry-free (GF) pseudorange-phase combination model, the new cycle slips detection and correction method based on the triple-frequency carrier phase and Doppler observations is proposed. With analyses on the two common sampling intervals (30 s and 1 s), it can be concluded that the optimal combination coefficients of the proposed model relate to sampling intervals. Combinations [4, -2, -3], [-1, -5,6], and [-3,6, -2] are selected to detect and correct cycle slips for 30 s sampling interval, while combinations [0, -1,1], [1,0, -1], and [-3,2,2] are selected for 1 s sampling interval. The validity of the phase-Doppler combination model under the static condition and the steady ionosphere with 30 s sampling interval and 1 s sampling interval is verified by two static experiments. Results show that the phase-Doppler combination model can achieve the same performance as the pseudorange-phase combination model. All the small, insensitive, and large cycle slips added to the three types of BDS satellites which separately belong to Geostationary Earth Orbit (GEO), Inclined Geosynchronous Orbit (IGSO), and Medium Earth Orbit (MEO) are detected and corrected successfully by the proposed model.

From November 2017 to December 2018, Beidou navigation satellite system (BDS) completed the basic system construction of Beidou-3 system (BDS-3) and opened the initial service, BDS-3 satellites are equipped with Ka-band inter-satellite link (ISL) payloads, satellite autonomous orbit determination (OD) and time synchronization (TS) can be realized through ISL technology. At present, 18 BDS-3 medium earth orbit (MEO) satellites have been formally networked, and a series of ground-satellite link (GSL) test and connection test of Ka-band ISL signals are carried out, through the test, the working state and functional performance of the ISL payloads can be evaluated. This article focuses on the GSL measurements and performance evaluation of the ISL signals of BDS-3, the basic architecture of the GSL test platform is given, the principle of Ka-band GSL measurement and basic data processing methods and performance evaluation methods are introduced. Obtained the GSL observation data of 18 BDS-3 MEO satellites using the Ka-band ground station of the National Time Service Center (NTSC), and evaluate the performance of the GSL ranging and time synchronization. The result shows that GSL ranging accuracy of 18 MEO satellites is better than 0.02m and GSL time synchronization accuracy is better than 0.2ns. Thus, this meets the requirements of ISL system. The assessment methods and analysis results presented in this article can provide the reference of the function and performance verification for inter-satellite link system, and provide technical and data accumulation for the time synchronization extension application based on ISL signals.

Chinese BeiDou Navigation Satellite System (BDS) as a mixed constellation consists of not only medium earth orbit (MEO) satellites but also inclined geosynchronous orbit and geostationary earth orbit (GEO) satellites, of which the GEO satellites remain visible and stationary in the Asia–Pacific region almost all the time so as to better ensure the reflected signals stability and continuity, compared to using only MEO satellites in previous Global Navigation Satellite Systems Reflectometry remote sensing research. This article demonstrates ocean altimetry capability using reflected signals from BDS GEO satellite with coastal experiments conducted in Weihai, Shandong, China, on July 6 and July 9, 2019. By using only one GEO satellite, the precision of delay Doppler map and phase altimetry is up to decimeter (close to millimeter) level. And the results show that BDS GEO-R technique can be used to monitor small changes in ocean altimetry with high temporal resolution.

Global Navigation Satellite Systems (GNSS) satellites are constantly orbiting the Earth in a Medium Earth Orbit (MEO) at an altitude of approximately 20,200 km. GNSS satellites in MEO continuously direct signals towards Earth for terrestrial users, but some signals are bent or refracted back into space. Low Earth Orbiting (LEO) satellites equipped with the proper equipment can receive these radio occultations (RO) and transmit them to ground stations for processing and dissemination. RO data produces profiles or atmospheric soundings of water vapor, temperature, and pressure. This research uses RO profiles to expose the uncertainty of moisture in the tropical troposphere. The tropical troposphere’s moisture uncertainty is critical for determining if convection occurs and assessing updraft strength and buoyancy in the planetary boundary layer to determine intensity. Moisture uncertainty above the planetary boundary layer impacts how detrimental entrainment can be for the generation of thunderstorms. Cloud Model 1 (CM1) numerical simulations demonstrate how moisture uncertainty will affect thunderstorm forecasts. Quantifying uncertainty in the tropical troposphere and demonstrating its effects is critical for improving forecasts in the tropics, where the Navy is commonly operating. Lieutenant Commander, United States Navy Approved for public release. Distribution is unlimited.

Onboard satellite clocks are the basis of Global Navigation Satellite Systems (GNSS) operation, and their revolution periods are at the level of 2 per day (about 12 h) in the case of the Medium Earth Orbit (MEO) satellites. In this work, the authors analysed the entire BeiDou Navigation Satellite System (BDS) space segment (BDS-2 and BDS-3) in terms of the occurrence of periodic, repetitive signals in the clock products, and checked if they coincide with the orbital periods or their multiples. The Lomb-Scargle (L-S) power spectrum was used as a tool to determine the periods present in the BDS clock products, allowing for analyses based on incomplete input data; in this case, the incomplete data were the phase data with jumps and outliers removed. In addition, continuous wavelet transform (CWT) was used to produce a time−frequency representation showing the more complex behaviour of the satellite clock products. As shown in the case of geostationary and geosynchronous inclined orbit satellites, the main period was 23.935 h, while for the Medium Earth Orbit it was 12.887 h, with the BDS satellite orbital period being 12 h 53 m (12.883 h). Some effects connected with reference clock swapping are also visible in the power spectrum. The conducted analyses showed that the BDS-2 satellite clocks have much higher noise than the BDS-3 satellite clocks, meaning that the number of designated periods is greater, but their reliability is significantly lower. BDS-3 satellites have only been in operation for a very short time, thus this is the first analysis to include this type of data. Moreover, such a wide and complex analysis has not been carried out to date.

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.

International audience; Global Navigation Satellite Systems (GNSSs) are the most widespread technology for Position Navigation and Timing (PNT). They have been traditionally deployed exploiting Medium Earth Orbit (MEO) or Geosynchronous Orbit (GSO) satellites. To meet future demands and overcome MEO and GSO limitations, GNSSs based on Low Earth Orbit (LEO) constellations have been investigated as a radical system change. Although characterized by a higher Doppler effect, a PNT service supplied by LEO satellites can provide received signals that are about 30 dB stronger. Moreover, broadband LEO constellations and the forthcoming mega-constellations can be exploited to deliver a piggybacked PNT service. With this cost-effective solution, a PNT service might be subject to substantial bandwidth limitations. A narrowband implementation of the so-called metasignal approach in GNSS receivers, namely Virtual Wideband (VWB), can tolerate harsh Doppler conditions while reducing the processed bandwidth. It is thus suited to a secondary PNT service delivered by LEO satellites. The aim of this work is to show the applicability of the VWB architecture to signals provided by a piggybacked PNT service, hosted on a mega-constellation. Through this case study we demonstrate the capability of this implementation to bear high Doppler conditions while empowering the potential of LEO PNT.

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 order to analyse the contribution of inter-satellite ranging observation to the BeiDou regional navigation satellite system (BDS-2) satellite orbit determination under the condition of regional ground tracking stations, first, the methods of using the observation model, the dynamic parameters dilution of precision (DPDOP), and the satellite position dilution of precision (SPDOP) to describe the contribution of inter-satellite ranging observation to orbit determination accuracy are proposed. Then, based on the measured observation data of the regional ground tracking stations located in China, as well as simulated inter-satellite ranging observation data, the contribution of inter-satellite ranging observation to orbit determination accuracy for the three types of satellites used by BDS-2 is analysed. The results show that the inter-satellite ranging observation improves the solution strength of the dynamic parameters by adding redundant observations without adding new parameters to be estimated, which effectively improves the orbit determination accuracy of the along-track component of Geostationary Earth Orbit (GEO) satellite and the cross-track and radial components of Inclined Geosynchronous Orbit (IGSO) and Medium Earth Orbit (MEO) satellites. In addition, under the condition of the regional tracking stations, the inter-satellite ranging observation compensates for the disadvantage that the satellite cannot be tracked by the whole arc segment. The accuracy and continuous stability of the orbit determination solution and the geometry structure of satellites that can be observed by stations and other satellites are significant improved, for which the average DPDOP values of GEO, IGSO, and MEO satellites are improved 50.2%, 73.0%, and 72.1%, respectively.

A growing number of satellites are operating on inclined geosynchronous orbits (IGSOs) with inclination typically much higher than that of traditional geosynchronous orbits (GEOs). Several recent studies have considered the long-term evolution of IGSOs. Unlike traditional GEO disposal orbits, IGSO disposal orbits can undergo large excursions in eccentricity due to the effect of luni-solar gravity perturbations. For specific ranges of initial orbital elements, perigee can reach the Earth's atmosphere, resulting in vehicle reentry. A previously published study by the authors on Tundra orbits, a specific class of IGSO with critical inclination and moderate eccentricity, demonstrated that orbital lifetime can be reduced below 200 years (in some cases below 25 years) and that the corresponding collision probability with background objects can be significantly reduced below that for traditional GEO disposal orbits. This paper presents a study of a broad range of IGSO disposal orbits. Three cases of IGSO disposal orbit were considered: (1) near circular orbits (motivated by the BeiDou and IRNSS constellations); (2) orbits with intermediate eccentricity (motivated by the QZS constellation); and (3) orbits with larger eccentricity (motivated by the Sirius Tundra constellation). For each case, a large number of long-term propagations using the high-precision code TRACE were performed. Disposal orbit initial inclination and right ascension of ascending node (RAAN) were parametrically varied. The Aerospace Debris Environment Projection Tool (ADEPT) suite was used to determine collision probability with inactive background objects and with operational satellites in GEO, medium Earth orbit (MEO), and low Earth orbit (LEO). Study results show that orbital lifetime of IGSOs can be reduced to less than 200 years for all three IGSO cases considered if initial inclination is high enough. The minimum required inclination depends on initial RAAN. An orbital lifetime less than 200 years offers effective reduction of probability of collision with inactive background objects (by 0.7–1.8 orders of magnitude) and can be achieved in a wider disposal orbit design space than would be needed to reduce orbital lifetime to less than 25 years. For near circular IGSOs, collision probability with operational satellites will be higher at high inclinations than at low inclinations, but there are options for mitigation. When inclination is not high enough to achieve reentry, a high eccentricity storage disposal orbit may be a consideration if collision probability is lower than for other options.

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.

Continuous observation of polar regions from space remains an important unsolved technical challenge of great interest for the international meteorological community. This capacity would allow achieving global continuous coverage once combined with the geostationary (GEO) satellite network. From a practical point of view, continuous coverage of polar regions with a small number of spacecraft can be obtained from a constellation of satellites either in highly elliptical orbits (HEO) or in medium Earth orbits (MEO). The study compares HEO and MEO satellite constellations for their capacity to provide continuous imaging of polar regions as function of the viewing zenith angle (VZA) and evaluates the corresponding latitude limits that ensure sufficient overlap with GEO imagery. Earlier studies assumed the latitude boundary of 60° and the VZA range 70°–85° depending on the space mission focus: meteorological purposes or communications. From the detailed analysis of meteorological retrieval requirements, this study suggests that the overlap of the GEO and polar observing systems (HEO or MEO) should occur down to the latitude band 45°–50° with a maximum VZA ranging between 60° and 64°. This coverage requirement can be met with two sets of three-satellite HEO constellations (one for each polar area) or a six-satellite MEO constellation. The 12-h Molniya and 14-, 15-, and 16-h HEO systems have been analyzed and determined to meet these revised requirements. The study demonstrates that the six-satellite 24-h MEO system can provide a suitable solution, which is also beneficial from the point of view of ionizing radiation and image acquisition geometry. Among the HEO systems, the 16-h HEO has some advantages relative to other HEO systems from the point of view of spatial coverage and space radiation.

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.

The Medium Earth Orbit (MEO) region hosts satellites for navigation, communication, and geodetic/space environmental science, among which are the Global Navigation Satellites Systems (GNSS). Safe and efficient removal of debris from MEO is problematic due to the high cost for maneuvers needed to directly reach the Earth (reentry orbits) and the relatively crowded GNSS neighborhood (graveyard orbits). Recent studies have highlighted the complicated secular dynamics in the MEO region, but also the possibility of exploiting these dynamics, for designing removal strategies. In this paper, we present our numerical exploration of the long-term dynamics in MEO, performed with the purpose of unveiling the set of reentry and graveyard solutions that could be reached with maneuvers of reasonable DV cost. We simulated the dynamics over 120-200 years for an extended grid of millions of fictitious MEO satellites that covered all inclinations from 0 to 90deg, using non-averaged equations of motion and a suitable dynamical model that accounted for the principal geopotential terms, 3rd-body perturbations and solar radiation pressure (SRP). We found a sizeable set of usable solutions with reentry times that exceed ~40years, mainly around three specific inclination values: 46deg, 56deg, and 68deg; a result compatible with our understanding of MEO secular dynamics. For DV, 39 pages, 23 figures

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.

Five BDS-3 satellites were launched successfully in 2015 and 2016, including two new-generation BDS-3 IGSO satellites I1-S and I2-S, and three new-generation BDS-3 Medium Earth Orbit (MEO) satellites M1-S, M2-S and M3-S. The BDS-3 satellites are able to transmit several new navigation frequencies including B1C at 1575.42 MHz, B2a at 1176.45 MHz, and B2b at 1207.14 MHz, in addition to B1 (1561.098 MHz) and B3 (1268.520 MHz) frequencies transmitted by BDS-2 satellites. Since the satellite-induced and elevation-dependent code biases were observed for the 14 older BDS-2 satellites (C01-C14), a comprehensive analysis and characterization of the code observations for the BDS-3 satellites were conducted based on observation data collected at 14 stations including 13 of the International GNSS Monitoring and Assessment System (IGMAS) and one station named SGG0 operated at the School of Geodesy and Geomatics (SGG) of Wuhan University. The Multipath (MP) combination, Melbourne-Wubbena (MW) combination and wide-lane uncalibrated phase delays (UPDs) are calculated to verify the existence and magnitude of the satellite-induced code bias for the BDS-3 satellites. The elevation-dependent polynomial models of MP combination for BDS-3 satellites were also established in order to analyze the characteristic of satellite-induced bias observed at different receiver types or different frequencies. For comparison, the MP combination of a newly launched BDS-2 satellite I6 was also calculated. The results indicate that the satellite-induced code bias is negligible for the BDS-3 satellites while newly launched BDS-2 satellite I6 has similar elevation-dependent code bias as the 14 older BDS-2 satellites.

There are code biases on the pseudo-range observations of the Beidou Navigation Satellite System (BDS) that range in size from several decimeters to larger than one meter. These biases can be divided into two categories, which are the code biases in the pseudo-range observations of Inclined Geo-Synchronous Orbit (IGSO) satellites and Medium Earth Orbit (MEO) satellites and the code biases in the pseudo-range observations of Geosynchronous Earth Orbit (GEO) satellites. In view of the code bias of the IGSO/MEO satellites, the code bias correction model is established using the weighted least square curve fitting method. After the correction, the code biases of the IGSO and MEO satellites are clearly mitigated. A methodology of correcting GEO code bias is proposed based on the empirical mode decomposition (EMD)-wavelet transform (WT) coupled model. The accuracies of the GEO multipath combination of the B1, B2 and B3 frequencies are improved by 39.9%, 17.9%, and 29.4%, respectively. Based on the corrections above, the ten days observations of three Multi-GNSS Experiment (MGEX) stations are processed. The results indicate that the convergence time of the precise point positioning (PPP) can be improved remarkably by applying a code bias. The mean convergence time can be improved by 14.67% after the IGSO/MEO code bias correction. By applying the GEO code bias, the mean convergence time can be further improved by 17.42%.

Eight new-generation BeiDou satellites (BeiDou-3) have been launched into Medium Earth Orbit (MEO), allowing for global coverage since March 2018, and they are equipped with new hydrogen atomic clocks and updated rubidium clocks. Firstly, we analyzed the signals for the carrier-to-noise-density ratio (C/N0) and pseudorange multipath (MP) by using international GNSS (Global Navigation Satellite System) Monitoring and Assessment System (iGMAS) station data, and found that B1C has a lower C/N0, and B2a has the same level of C/N0 as the B1I and B3I signals. For pseudorange multipath, compared with the BeiDou-2 satellites, the obvious systematic variation of MP scatters related to the elevation angle is greatly improved for the BeiDou-3 and BeiDou-3e satellites signals. For the signals of the BeiDou-3 satellites, the order of the Root Mean Square (RMS) values of multipath and noise is B3I

Communication satellites have a much longer propagation delay than terrestrial communication networks such as cellular or WiFi. In addition, as the carrier frequency moves up, mobile satellite communications show worse performances than the conventional fixed satellite communications. The mobile satellite service (MSS) has not been actively pursued with long latency at high-frequency bands for future applications. In this paper, the adverse impact of long propagation delay in the conventional satellite system is investigated with various user mobility and Doppler-shifted carrier frequency. The satellite network is modeled as a basic delayed feedback channel system and the communication performance is analyzed under delayed channel state information (CSI) for assessing the system feasibility in mobile conditions. The results of performance analysis are provided at high-frequency bands with high-speed user movement, specifically on the outage probability and the channel capacity exploiting three types of channel models: conventional land mobile satellite (LMS) channel models of E. Lutz and C. Loo, and Nakagami fading model. In the circumstance with various user speeds, system performances are evaluated with different propagation delays in the LMS channel models and for line-of-sight (LOS) components in the Nakagami fading. In addition, the conventional models are compared depending on different altitudes for geostationary orbit (GEO), medium earth orbit (MEO), and low earth orbit (LEO) satellites, as well as high-altitude platforms (HAP).

Design and analysis of quadrifilar helical antenna is presented in this paper. The proposed antenna is designed for cube-sats in the low earth and medium earth orbit applications. It is a combination of 4 helical antennas, each separated by 90o, with left-handedness and it is designed for operation at 4.5GHz. The proposed antenna has 11.11% impedance bandwidth. The antenna is designed in 2 steps to analyze its proper radiation pattern and input characteristics. The first step of designing is selection of a ground plane with perfect dimensions, which can operate at desired frequency. While the second step is to analyses the antenna performance for different helix angle using proper ground plane dimensions. The gain versus frequency curve has been obtained for desired frequency and it is showing more than 5 dB gain at resonant frequency 4.5 GHz. Thus, based on desired application proposed antenna has been designed.

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. .

The paper presents results of precise orbit and Earth parameter determination based on full-scale simulations of a potential future GNSS, with the name of Kepler, recently proposed by the German Aerospace Center. The space segment of the system consists of 24 Galileo-like Medium Earth Orbit (MEO) satellites and 6 Low Earth Orbit (LEO) satellites equipped with ultra-stable optical clocks. All spacecraft carry terminals for two-way optical inter-satellite links (ISLs) enabling high-rate data communication, constellation-wide clock synchronization and precise absolute ranging. The clock synchronization is assumed to create an extremely stable reference time scale based on ensemble of the optical clocks. A series of simulation scenarios are defined, starting from the Galileo constellation as benchmark, and gradually incorporating additional features including LEOs, ISLs and synchronized clocks to assess the impact on orbits, singnal-in-space range errors (SiSREs) and a geodetic quality Earth parameters like polar coordinates, length of day (LOD) and geocenter coordinates. Precise orbit determination (POD) is done at the observation level using the same standards and setups as in real data processing with ground network containing up to 18 stations. The MEO SiSRE, the most relevant to navigation applications, obtained for the Kepler constellation with perfect models and 18 ground stations was found to be 0.009 cm, 160 times better than 1.4 cm found for Galileo. It was demonstrated, that for operational POD for the Kepler system the ground network can be significantly reduced to only two stations if Earth rotation parameters are to be estimated, or to just one otherwise. A SiSRE value of 0.037 cm was obtained with two stations, and 0.018 cm with one station. For more realistic results a number of modeling errors are being considered. These concern observations – multipath, GNSS code hardware delays and ISL range biases, the dynamics – solar radiation pressure (SRP), gravity field, air drag, MEO antenna thrust, Earth tide potential, and the geometry – ocean loading and phase center offsets (PCOs) of space GNSS antennas. Any other satellite and station equipment contribution to mismodeling has been disregarded. It is shown, that for the Kepler the modeling errors are clearly visible in ISL observation residuals, in contrast to Galileo case, where they are to a large extent absorbed by the large number of estimated clock parameters. This was exploited to efficiently mitigate the dynamic modeling errors by estimation of empirical accelerations. With this setup the MEO SiSRE of 5 cm was obtained for Galileo and 0.3 cm, 0.4 cm and 0.7 cm for Kepler with 18, two and one ground station, respectively. The key to obtaining such low values are ISLs and synchronized satellite clocks. In the Kepler constellation with a single ground station the ISL range biases (RBs) up to 5 mm and 20 mm increase the SiSRE to 0.74 cm and 1.67 cm. Estimation of the ISL RBs is not recommended in the presence of modeling errors since they are significantly biased, which leads to increase of SiSRE to 2.2 cm. The Earth parameter determination, based on global network of 124 observing IGS stations, also benefits significantly from the Kepler constellation. The initial results show that the impact of modeling errors on Earth parameters can, as with POD, be effectively reduced in the Kepler system. The 30 cm error in integrated LOD, which has an impact on UT1-UTC, obtained for Galileo due to combined modeling errors could be reduced to 0.5 cm for the Kepler system. The same is true for the Z-coordinate of the geocenter, whose 14 mm error in the MEO-only solution could be reduced to 0.2 mm in the Kepler solution. The Earth parameters completely free of bias are obtained when the number of the modeling errors is reduced to only two: MEO SRP and PCOs. The ISL range biases up to 5 mm were found to have negligible impact on the estimated EPs. The largest differences (up to 0.2 mm) are found for UT1-UTC, for all other parameters they do not exceed 0.02 mm. The estimation of PCOs, being an integral part of Earth parameter determination, also benefits enormously from the Kepler design. It was demonstrated, that using only ten days of data they could be estimated with millimeter accuracy for MEOs and sub-millimeter accuracy for LEOs in all three spacial directions even in the presence of ISL range biases of up to 5 mm. Such accuracy of PCOs is not achievable with the Galileo constellation.

The positioning, navigation, and timing (PNT) service of the Global Navigation Satellite System (GNSS) is developing in the direction of real time and high precision. However, there are some problems that restrict the development of real-time and high-precision PNT technology. Satellite orbit maneuvering is one of the factors that reduce the reliability of real-time navigation products, especially the high-frequency orbit maneuvering of geostationary earth orbit (GEO) and inclined geosynchronous orbit (IGSO) satellites. The BeiDou Navigation Satellite System (BDS) constellation is designed to contain GEO, IGSO, and medium earth orbit (MEO). These orbit maneuvers bring certain difficulties for data processing, especially for BeiDou satellites, such as decreased real-time service performance, which results in real-time navigation products including unusable maneuvered satellites. Additionally, the performance of real-time navigation products will decrease because the orbit maneuvers could not be known in advance, which diminishes the real-time PNT service performance of BDS for users. Common users cannot obtain maneuvering times and strategies owing to confidentiality, which can lead to a decline in the BDS real-time service performance. Thus, we propose a method to predict orbit maneuvers. BDS data from the broadcast ephemeris were analyzed to verify the availability of the proposed method. In addition, the results of real-time positioning were analyzed by using ultra-rapid orbit products, demonstrating that their reliability is improved by removing maneuvered satellites in advance. This is vital to improve the reliability of real-time navigation products and BDS service performance.

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.

With the rapid development of satellite technology and the need to satisfy the increasing demand for location-based services, in challenging environments such as indoors, forests, and canyons, there is an urgent need to improve the position accuracy in these environments. However, traditional algorithms obtain the position solution through time redundancy in exchange for spatial redundancy, and they require continuous observations that cannot satisfy the real-time location services. In addition, they must also consider the clock bias between the satellite and receiver. Therefore, in this paper, we provide a single-satellite integrated navigation algorithm based on the elimination of clock bias for broadband low earth orbit (LEO) satellite communication links. First, we derive the principle of LEO satellite communication link clock bias elimination; then, we give the principle and process of the algorithm. Next, we model and analyze the error of the system. Subsequently, based on the unscented Kalman filter (UKF), we model the state vector and observation vector of our algorithm and give the state and observation equations. Finally, for different scenarios, we conduct qualitative and quantitative analysis through simulations, and the results show that, whether in an altimeter scenario or non-altimeter scenario, the performance indicators of our algorithm are significantly better than the inertial navigation system (INS), which can effectively overcome the divergence problem of INS; compared with the medium earth orbit (MEO) constellation, the navigation trajectory under the LEO constellation is closer to the real trajectory of the aircraft; and compared with the traditional algorithm, the accuracy of each item is improved by more than 95%. These results show that our algorithm not only significantly improves the position error, but also effectively suppresses the divergence of INS. The algorithm is more robust and can satisfy the requirements of cm-level real-time location services in challenging environments.