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Earth Observation (EO)satellite constellation design deserves further investigation for optimizing configurations that enhance space mission performances. In recent years, there has been considerable interest in reducing the System Response Time (SRT)of EO satellites - the interval between request submission and availability of the image product - in order to achieve rapid response in case of natural or man-made disasters or matters involving defense and natural security. This key performance indicates to the user when, after the request submission, the image produced will be available to him. The best way to improve this performance metric is using heterogeneous constellations, where two different functional constellations are cross-linked; one is mainly for imaging and the other is a communication constellation that is dedicated to relaying commands delivery from Earth station to imaging satellites and data collection back to Earth. This scheme has been proposed before in the previous work to explore its potential enhancement of system performance, or to evaluate the network performances by comparing candidate relay constellations for servicing remote sensing satellites. However, methods for satellite constellation design of this scheme have not been introduced. Since the best heterogeneous configuration may require studying several constellation combinations, this paper presents a framework capable of generating thousands of heterogeneous constellation configurations based on predefined Design Variable (DV)ranges and sizing those configurations in terms of the predefined Measure of Performances (MOPs). Using Systems Tool Kit (STK)and its various add-on modules, we introduce multiple solutions to configure both the imaging and relay constellations of the heterogeneous constellation systems that can achieve their objectives and improve the overall system performance by reducing the SRT. One of these solutions is an imaging constellation of 8 satellites equally distributed in 2 different planes, a Sun-Synchronous Orbit (SSO)and a Mid-Inclination Orbit (MIO). We select this constellation based on the global daily coverage percentage and the satellite optical sensor parameters. In order to reduce the maximum SRT, we select a relay constellation in a Medium Earth Orbit (MEO)on an Equatorial plane and a location of a Ground Station (GS)as a receiving and transmitting Earth site.

Statistical Orbit Determination techniques are used to find satellite positions for various navigation applications. The present paper reports the simulation study of the statistical orbit determination algorithm developed for geostationary and geosynchronous orbits for BeiDou constellation. BeiDou Navigation Satellite System (BDS) space constellation is composed of geostationary earth orbit (GEO), medium earth orbit (MEO) and inclined geosynchronous satellite orbit (IGSO). However, predicting the orbital parameters for geostationary earth orbits with zero inclination and for geosynchronous satellite orbits (GSO) with small inclination angle and small eccentricity is a challenging task [1]. Thus this paper is focused on mainly satellite orbit determination techniques for GEO and GSO orbits in the BDS.

An enabling technology platform, the Payload Interface Unit (PIU) is a simple, straight forward secure payload processor design that can service a variety of payloads and operate across virtually any orbit. The PIU design uses low-power electronics, low-mass structure and packaging, economical components and simple architecture to provide a highly capable interface unit. The PIU incorporates industry-standard digital interface(s) for both the payload and the Host Spacecraft and will be designed for standard spacecraft bus voltage. The PIU supports on-orbit operation in any Earth orbit, including Low Earth Orbit, Medium Earth Orbit, Highly Elliptical Orbit, and Geostationary Earth Orbit. The PIU architecture is capable of both embedding the payload data into the Host Spacecraft command and telemetry stream, or a dedicated communication link. This paper describes the implementation of the PIU, opportunities for further efficiency gains, and potential future capability growth.

Since November 2017, eight BeiDou global navigation system (BDS-3) satellites equipped with Ka-band inter-satellite link (ISL) payloads have been launched into medium earth orbit. We present the precise orbit determination (POD) for BDS-3 satellites using both L-band satellite-ground and Ka-band ISL observations. The satellite-ground tracking data are collected from the international GNSS Monitoring and Assessment System stations. The data period is DOY (day of year) 127---156, 2018. The BDS-3 ISL measurements are described by a dual one-way observation model. After transforming the dual one-way observations to the same epoch, clock-free and geometry-free observables can be obtained by the addition and subtraction of dual one-way observations. Using the geometry-free observables, the ISL measurement noise is analyzed and confirmed to be less than 10?cm. Using the clock-free observables and ground tracking data, the precise orbits are determined together with combined ISL hardware delays. For the estimates of hardware delays, the mean STD is 0.08?ns. For the satellite orbits, the ground-only POD solutions are also computed for comparison. When using 16 globally distributed ground stations, the addition of the ISLs improves the POD performance. For example, the 3D RMS of orbit overlap differences is reduced from 15.9?cm to 9.2?cm, yielding an improvement of 42% compared to ground-only POD. When only 6 stations in China are used for POD, the addition of ISLs enables the 3D RMS to be reduced from 85.4?cm to 14.8?cm with a greater improvement of 83%.

The Single-frequency Single-epoch double-differenced baseline resolution technique of Global Positioning System (GPS) provides a good opportunity for monitoring the displacement or deflection behavior of bridges under different loading conditions in real-time. However, for single GPS, a high success rate baseline solution is difficult to achieve due to the lack of sufficient visible satellites and the low accuracy of float solutions. Galileo Satellite Navigation System (Galileo) has 14 medium earth orbit satellites (as of May 2018) that can be used to supplement GPS. The frequency bands of Galileo overlap with that of GPS on E1/L1 and E5a/L5, which is conducive to the combination of observations in integration positioning. Accordingly, Galileo augmenting GPS is an effective and necessary approach to improve the positioning availability and reliability. Moreover, using the baseline length constraint can improve the accuracy of float solutions, narrow the search space, and finally increase the success rate of ambiguity resolution and positioning. The single-frequency single-epoch double-differenced GPS/Galileo mathematical model with baseline length constraint is deduced in this paper. Two sets of GNSS real bridge data were used for further analysis on the improvement of GPS/Galileo with baseline length constraint when compared to single GPS. Finally, a Fast Fourier Transformation (FFT) algorithm was adopted for precisely detecting the local dominant frequencies of XB, YB, and ZB direction of the two stations.

Currently, the Global Navigation Satellite System (GNSS) mainly uses the satellites in Medium Earth Orbit (MEO) to provide position, navigation, and timing (PNT) service. The weak navigation signals limit its usage in deep attenuation environments, and make it easy to interference and counterfeit by jammers or spoofers. Moreover, being far away to the Earth results in relatively slow motion of the satellites in the sky and geometric change, making long time needed for achieved centimeter positioning accuracy. By using the satellites in Lower Earth Orbit (LEO) as the navigation satellites, these disadvantages can be addressed. In this contribution, the advantages of navigation from LEO constellation has been investigated and analyzed theoretically. The space segment of global Chinese BeiDou Navigation Satellite System consisting of three GEO, three IGSO, and 24 MEO satellites has been simulated with a LEO constellation with 120 satellites in 10 orbit planes with inclination of 55 degrees in a nearly circular orbit (eccentricity about 0.000001) at an approximate altitude of 975 km. With simulated data, the performance of LEO constellation to augment the global Chinese BeiDou Navigation Satellite System (BeiDou-3) has been assessed, as one of the example to show the promising of using LEO as navigation system. The results demonstrate that the satellite visibility and position dilution of precision have been significantly improved, particularly in mid-latitude region of Asia-Pacific region, once the LEO data were combined with BeiDou-3 for navigation. Most importantly, the convergence time for Precise Point Positioning (PPP) can be shorted from about 30 min to 1 min, which is essential and promising for real-time PPP application. Considering there are a plenty of commercial LEO communication constellation with hundreds or thousands of satellites, navigation from LEO will be an economic and promising way to change the heavily relay on GNSS systems.

By taking advantage of the additional combined signals introduced by triple-frequency GNSS, we propose a cycle slip detection and correction method based on the traditional extra-wide-lane Hatch–Melbourne–Wubbena (HMW) combination and also modified HMW combinations. Instead of using the combined code signals directly in the traditional HMW combination, the modified HMW combination adopts the original code signals and one combined phase signal with corrected cycle slips to eliminate the ionospheric bias and reduce the effect of the noise induced by the code measurement. To determine the optimally combined signals and the corresponding coefficients in the modified HMW combination, four constrained conditions are proposed based on the maximum acceptable ionospheric bias and measurement noise of the combination in the process of cycle slip detection. Two optimally combined signals are selected; however, the second best signal cannot maintain a 100% success rate when epoch intervals are increased, due to the effect of the remaining ionospheric bias. To solve this problem, a scale factor is introduced to balance the corrected percentage of the ionospheric bias and the amplification of the measurement noise. These selected signals are further tested with real triple-frequency GPS and BDS observations. Results show that the proposed method can provide a 100% success rate in detecting cycle slips in the observations with large epoch intervals (up to 30 s) from medium earth orbit satellites with elevation angles above 5°, as well as inclined geosynchronous orbit and geostationary orbit satellites with elevation angles above 20°.

Satellite quantum communications have rapidly evolved in the past few years, culminating in the proposal, development, and deployment of satellite missions dedicated to quantum key distribution and the realization of fundamental tests of quantum mechanics in space. However, in comparison with the more mature technology based on fiber optics, several challenges are still open, such as the capability of detecting, with high temporal accuracy, single photons coming from orbiting terminals. Satellite laser ranging, commonly used to estimate satellite distance, could also be exploited to overcome this challenge. For example, high repetition rates and a low background noise can be obtained by determining the time-of-flight of faint laser pulses that are retro-reflected by geodynamics satellites and then detected on Earth at the single-photon level. Here we report an experiment with regard to achieving a temporal accuracy of approximately 230 ps in the detection of an optical signal of few photons per pulse reflected by satellites in medium Earth orbit, at a distance exceeding 7500 km, by using commercially available detectors. Lastly, the performance of the Matera Laser Ranging Observatory is evaluated in terms of the detection rate and the signal-to-noise ratio for satellite quantum communications., Comment: 7 pages, 4 figures, part of the "QUANTUM KEY DISTRIBUTION AND BEYOND" featured issue of the Journal of the Optical Society of America B

The message service is the most distinctive part of the BeiDou System (BDS), whose coverage area will be expanded from China & surroundings to the global surface space at BDS-3 stage. This is an innovative integration of a global mobile communications service in a Global Navigation Satellite System (GNSS), with great practical value and significant strategy. However, BeiDou Global Message service relies on the global Medium Earth Orbit (MEO) satellite constellation to achieve the global beam-forming, the space-borne antenna has smaller size and lower receiving gain than regional beam-forming, so a key issue in designing Global Message is the uplink signal synchronization segment to ensure the space-borne message receiver achieves fast and reliable capture of short burst signals at very low carrier-to-noise ratio (C/N0). In this paper comparing with the uplink channel in Radio Determination Satellite Service (RDSS) based on the BDS Geostationary Earth Orbit (GEO) satellites, the signal condition parameters of the optional BeiDou Global Message are calculated. Then the design scheme of the shortest length synchronization segment based on the constant false alarm and specified detection probability is given. According to this, a fast acquisition algorithm for the uplink message receiver based on MEO satellite is proposed. Finally, through software simulation analysis and ground equipment testing the proposed signal design scheme is proved, as well as the fast acquisition algorithm with high-reliability reception of short burst signals in the condition of signal-to-noise ratio lower than 34 dBHz. The research results of this paper provide important technical support for the overall design of Global Message service in BDS-3, and will be further tested in the subsequent actual applications. It is also hoped to provide theoretical basis for the continuous optimization of Global Messages in the future BDS.

With the advent of multiple Global Navigation Satellite Systems (GNSS), the feasibility of navigation using GNSS observations beyond Low Earth Orbits has been explored in recent years. An estimation algorithm is one of the critical components of a well-designed navigation system, and it must be judiciously chosen for a particular application. The non-linearity of spacecraft dynamics in a highly elliptical orbit is high and if the apogee of the orbit is higher than Medium Earth Orbit, the state vector of the spacecraft motion may be only partially observable due to the limited availability and low Signal power of GNSS signals. In this scenario, variants of the Unscented Kalman Filter (UKF) can provide better estimation performance. In this paper, GNSS-based navigation of spacecraft in elliptical and High Earth Orbits using Single Propagation type UKFs are demonstrated. These estimation algorithms provide UKF like estimation performance and require around 90% less computation time than the conventional UKF.

The multipath of BDS-2 medium earth orbit (MEO) satellites and BDS-3 satellites is analyzed. The multipath bias of BDS-2 satellites is corrected by second order polynomial fitting. By calculating Pearson correlation coefficient between multipath and elevation angle, it is found that the correlation coefficient between multipath and elevation angle of BDS-3 satellites is less than 0.1, and the correlation is not strong. Further, the fourth polynomial fitting model is used to verify that there is no code multipath bias for BDS-3 satellites. By comparing the Root Mean Square (RMS) of BDS-3 satellites code multipath, it is found that the multipath of B1C signal is the largest, approaching 0.5 m, while that of other signals is better than 0.3 m.

Compared with the traditional geosynchronous orbit earth and medium earth orbit satellite communication systems, the low earth orbit (LEO) satellite communication system has the real-time communication capability of all time, all weather and all terrain, which is the only way to achieve seamless coverage of global mobile communication. A seasonal autoregressive integrated moving average (S-ARIMA) model is used for satellite load forecast. Traffic load measured by the first N satellites is processed to obtain stationary sequence via difference and seasonal difference. Then on the basis of traffic load forecast, the corresponding resource management strategy is selected in light load scenario or heavy load scenario. Light load scenario is limited-coverage, satellite parameter adjustment can enlarge the coverage of beams to reduce resource waste as far as possible. Heavy load scenario is limited-capacity, satellite parameter adjustment can narrow beam coverage and reduce the amount of traffic load. Simulation results show that S-ARIMA model can accurately forecast the load of satellites, and the proposed method can improve resource utilization and reduce energy consumption of satellites.

On June 23, 2020, the final satellite of the third generation Beidou satellite navigation system (BDS-3) was launched, which represents the BDS-3 is fully operational and can provide global services. Due to the fact that BDS-3 constellation mainly consists of Medium Earth Orbit (MEO) satellites, we only discussed the in-orbit atomic clock performance of MEO, and compared to that of the latest generation of GPS as well as Galileo satellites. Based on the precise satellite clock product provided by German Research Center for Geosciences (GFZ), a level of 0.2 ns fitting precision is obtained, and it is better than BDS-2. The periodic terms of satellite clock are analyzed by means of spectrum analysis. Except some mutual periodic terms, several exclusive periodic terms, approximately 8 h, 6 h and 4.8 h harmonics, are found in passive hydrogen masers (PHMs) of BDS-3, and possible reasons are given. Clock systematic characteristics, such as frequency accuracy and drift rate are examined and found that PHMs have better result than rubidium atomic clocks. Finally, frequency stability is evaluated: all BDS-3 satellite clocks can reach the order of 10-15 at the average interval of 86400 s, which is much better than BDS-2 and comparable to the latest type of GPS III as well as Galileo satellites.

© 2021 Elsevier Ltd As of the end of 2019, 46.4% of the world's population does not have regular access to the Internet. Bringing the more than 3.5 billion individuals still unconnected online is the primary goal for multiple international organizations, including the ITU and the UN Broadband Commission. Two important barriers that restrict connectivity are the lack of infrastructure and affordability. To address these barriers, several novel concepts that involve spaceborne and airborne platforms have been proposed to provide connectivity at a lower cost (improve affordability) to a wider reach of people (extend infrastructure). We develop a techno-economic methodology to assess the potential impact of space and aerial concepts in expanding connectivity to uncovered and under-served regions. In particular, constellations of geostationary orbit (GEO) satellites, large constellations of medium Earth orbit (MEO) and low Earth orbit (LEO) satellites, and high- and low-altitude aerial platforms are studied. Results show that under the current scenario, the impact of space and aerial systems in terms of expanding connectivity would be rather modest; the current cost of satellite technology (~$200 per Mbps/month) are affordable for less than 1% of the uncovered and under-served population in the countries of interest. In a future scenario in 8–10 years, space systems have the highest potential to bring uncovered and under-served populations online, being a viable technology for 24% of the population in these countries.

With the development of the global navigation satellite system(GNSS), the hourly ultra-rapid products of GNSS are attracting more attention due to their low latency and high accuracy. A new strategy and method was applied by the Shanghai Astronomical Observatory (SHAO) Analysis Center (AC) of the international GNSS Monitoring and Assessment Service (iGMAS) for generating 6-hourly and 1-hourly GNSS products, which mainly include the American Global Positioning System (GPS), the Russian Global’naya Navigatsionnaya Sputnikova Sistema (GLONASS), the European Union’s Galileo, and the Chinese BeiDou navigation satellite system (BDS). The 6-hourly and 1-hourly GNSS orbit and clock ultra-rapid products included a 24-h observation session which is determined by 24-h observation data from global tracking stations, and a 24-h prediction session which is predicted from the observation session. The accuracy of the 1-hourly orbit product improved about 1%, 31%, 13%, 11%, 23%, and 9% for the observation session and 18%, 43%, 45%, 34%, 53%, and 15% for the prediction session of GPS, GLONASS, Galileo, BDS Medium Earth Orbit (MEO), Inclined Geosynchronous Orbit (IGSO), and GEO orbit, when compared with reference products with high accuracy from the International GNSS service (IGS).The precision of the 1-hourly clock products can also be seen better than the 6-hourly clock products. The accuracy and precision of the 6-hourly and 1-hourly orbit and clock verify the availability and reliability of the hourly ultra-rapid products, which can be used for real-time or near-real-time applications, and show encouraging prospects.

Real-time precise orbit/clock product is fundamental to BeiDou Navigation Satellite System (BDS) real-time precise point positioning (PPP). In order to explore achievable performance of BDS real-time orbit/clock products and thus offer usage recommendations for BDS real-time PPP users, this paper studies BDS real-time orbit/clock corrections provided by four International GNSS Service (IGS) Analysis Centers (ACs), i.e. Wuhan University (WHU), Deutsches Zentrum fur Luft- und Raumfahrt (DLR), GeoForschungsZentrum Potsdam (GFZ) and Centre National d’ Etudes Spatiales (CNE) for seven consecutive days staring from day of year (DOY) 324, 2019. Firstly, the availability of BDS-2 real-time orbit/clock corrections is studied. Larger than 85% availabilities are identified for all ACs. During the test period, DLR does not provide all Geosynchronous Earth Orbit (GEO) satellites’ corrections, whereas CNE does not provide C14/C16′s corrections. Secondly, the BDS real-time orbit accuracy is analyzed. Results indicate that the real-time orbit accuracy of GEO satellites is at meter level, and those of Inclined Geosynchronous Satellite Orbit (IGSO)/Medium Earth Orbit (MEO) satellites are at decimeter level. Among all ACs, the accuracy of CNE’s three-dimensional (3D) orbit correction is the best, which is 2.391/0.278/0.271 m for GEO/IGSO/MEO satellites, respectively. DLR has the largest orbit errors of 1.116/0.504 m for IGSO/MEO satellites. Thirdly, the BDS real-time clock offset precision is investigated. On the one hand, CNE’s real-time corrections still perform best with 1.19/0.28/0.32 ns clock offset precision for GEO/IGSO/MEO satellites, respectively. On the other hand, DLR’s real-time clock is the worst with 2.61/1.91 ns IGSO/MEO clock offset precision. Finally, all corrections are applied to estimate 47 Multi-GNSS Experiment (MGEX) station’s BDS-2 real-time PPP on DOY 330, 2019. Results show an average of 0.53–1.46 h convergence time and 8.8–10.9 cm positioning accuracy in the static mode. In the meantime, the average convergence time in the kinematic mode is 2.11–9.84 h, much longer than the static counterpart. The corresponding converged positioning accuracy is 30.7–68.0 cm. To sum up, WHU/GFZ/CNE’s real-time corrections are proven with satisfied orbit/clock performance, thus are recommended for BDS-2 real-time PPP.

Differential code bias (DCB) is one of the main error sources of the positioning, navigation and timing services and slant total electron content extraction. The DCB can usually be estimated together with ionospheric model coefficients from the data of International GNSS Service stations. However, the precision of the BeiDou Navigation Satellite System (BDS) satellite DCBs determined with GNSS data is still lower compared with that of GPS. Apart from the sparsely distributed tracking stations of BDS, the quality of BDS observables also matters. Moreover, the determined DCB will have different characteristics for geostationary orbit (GEO), inclined geosynchronous orbit (IGSO) and medium earth orbit (MEO) satellites because of different observation times, geometric structures and orbit repeat periods of the different orbit types. In this study, we mainly analyze the factors affecting the accuracy of BDS satellite DCBs. First, a piece-wise local ionospheric model based on a two-step method is proposed to obtain the BDS DCBs. Second, the impact of BDS satellite-induced code bias variations on the ionospheric observable and DCB determination is analyzed in detail. Finally, we focus on the analysis of the cause of different day-to-day scattering results for GEO, IGSO and MEO satellite DCBs. The performance of the proposed algorithm and the satellite-induced factors affecting the accuracy of BDS DCBs are assessed by about 3 months of GNSS data in 2015 from 42 MGEX stations. Results show that the proposed algorithm is able to estimate BDS satellite DCBs precisely. The DCB day-to-day stability of BDS satellites is improved by 40.6% compared with DCB products of Institute of Geodesy and Geophysics (IGG), China. After the satellite-induced bias is eliminated by a correction model, the precision of BDS DCBs improves. In particular, the value of B1–B3 DCB day-to-day scattering for MEO satellites decreased by 28.4%. In addition, we determined that the lower DCB day-to-day stability of MEO satellites compared with that of IGSO satellites may be mainly attributed to the different satellite orbit repeat periods.

This letter focuses on exploiting parameters, including peak power spectral density (PSD), integrated power, mean frequency, and spectrum width of reflected BeiDou signals in the frequency domain, to retrieve wind speed. The PSD of reflected signals is estimated after choosing proper parameters of the Welch method. Then, the features of PSD are illustrated, and a method is proposed based on fitting estimated PSD with a Gaussian function to evaluate the aforementioned PSD parameters. The estimated parameters of collected BeiDou medium earth orbit (MEO)/inclined geosynchronous satellite orbit (IGSO) and geostationary orbit (GEO) data are fitted with in situ wind speed using polynomial. Fitting results show that the peak PSD, mean frequency, and spectrum width of reflected signals from GEO have more evident dependence on wind speed, compared with MEO/IGSO. To obtain the most accurate results, the impact of delay used in lagging direct replica to align reflected signals is analyzed. Results show that regardless of delay locating within the interval of $[\tau_{0}-\tau_{c},\tau_{0}+\tau_{c}]$ , the better root-mean-square error (rmse) of less than 1.7 m/s and the larger coefficient of determination over 0.8 can be obtained by retrieving from the spectrum width, compared with peak PSD and mean frequency, whose optimal results, with rmse values of 2.03 and 1.70 m/s and coefficient of determination values of 0.73 and 0.81, are obtained as delay is $\tau_{0}-0.85\tau_{c}$ and $\tau_{0}+0.9\tau_{c}$ , respectively, where $\tau_{0}$ is the delay of specular reflection, and $\tau_{c}$ is the length of the B1 code.

The basic performance of space-surface bistatic synthetic aperture radar (SS-BSAR) using BeiDou satellites as transmitters of opportunity is presented. The transmitted signal, the satellite trajectory, and the observation time are analyzed to demonstrate the potential of BeiDou satellites to be used as transmitters of SS-BSAR. When a BeiDou medium earth orbit (MEO) satellite is used, the signal-to-noise ratio and the resolution are examined for different SS-BSAR cases, where the receiver is fixed on the ground or mounted on a moving platform. Since the parameters of the BeiDou geostationary earth orbit (GEO) satellite and the inclined geosynchronous satellite orbit (IGSO) satellite are different from those of the MEO satellite, the peculiarities of SS-BSAR with the GEO or IGSO satellite as transmitter are analyzed and then compared with the case of MEO satellite. In order to show the performance difference of SS-BSAR with BeiDou satellites as transmitters, comparisons with other global navigation satellite system (GNSS) transmitters are also made. The theoretical results show that SS-BSAR using BeiDou satellites as transmitters can achieve different sounding performance and can provide some new potential applications compared to other GNSS transmitters.

Relativistic electrons can penetrate spacecraft shielding and can damage satellite components. Spacecraft in medium Earth orbit pass through the heart of the outer radiation belt and may be exposed to large fluxes of relativistic electrons, particularly during extreme space weather events. In this study we perform an extreme value analysis of the daily average internal charging currents at three different shielding depths in medium Earth orbit as a function of L∗ and along the orbit path. We use data from the SURF instrument on board the European Space Agency's Giove-A spacecraft from December 2005 to January 2016. The top, middle, and bottom plates of this instrument respond to electrons with energies >500 keV, >700 keV, and >1.1 MeV, respectively. The 1 in 10 year daily average top plate current decreases with increasing L∗ ranging from 1.0 pA cm−2 at L∗=4.75 to 0.03 pA cm−2 at L∗=7.0. The 1 in 100 year daily average top plate current is a factor of 1.2 to 1.8 larger than the corresponding 1 in 10 year current. The 1 in 10 year daily average middle and bottom plate currents also decrease with increasing L∗ ranging from 0.4 pA cm−2 at L∗=4.75 to 0.01 pA cm−2 at L∗=7.0. The 1 in 100 year daily average middle and bottom plate currents are a factor of 1.2 to 2.7 larger than the corresponding 1 in 10 year currents. Averaged along the orbit path the 1 in 10 year daily average top, middle, and bottom plate currents are 0.22, 0.094, and 0.094 pA cm−2, respectively.

The R2CGGTTS software tool developed at the Royal Observatory of Belgium (ROB) to provide clock solutions in the standard Common GNSS Generic Time Transfer Standard (CGGTTS) has been extended to BeiDou Navigation Satellite System (BDS). The BDS includes satellites in three different orbits: 1) Medium Earth Orbit (MEO); 2) Inclined Geosynchronous Satellite Orbit (IGSO); and 3) Geostationary Earth Orbit (GEO). This paper presents first results obtained with this upgraded software, and a comparison between common view (CV) time transfer solutions obtained with either BDS, or GPS or Galileo. These preliminary results indicate that the BeiDou MEO satellites give time transfer results with a higher noise than the GPS results. This additional noise is shown to be due to some elevation-dependent delay in the BDS code measurements. Some biases were furthermore pointed out between the CV results obtained with the different BeiDou MEO satellites when the receivers used in the two stations are of different make. These biases may reach some nanoseconds, and find most probably their origin in the receiver hardware or firmware. It is shown additionally that using the BeiDou IGSO satellites and the GEO satellites, although increasing the number of observations, especially in the Asia-Pacific region, introduces a significant time transfer noise in the CV results.

In order to simplify the attitude control for inclined geosynchronous orbit (IGSO) and medium earth orbit (MEO) satellites of the BeiDou Navigation Satellite System (BDS) in eclipse seasons, two attitude modes, namely yaw-steering (YS) and orbit-normal (ON) mode are used. Significant accuracy degradation is observed for the orbits determined with the purely empirical CODE solar radiation pressure (SRP) model when these satellites switch to the ON mode. In addition, even though BDS IGSO satellites are in the YS mode, the orbits determined with the CODE SRP model show undesirable systematic errors that depend on the elevation angle β of the sun above the satellite orbit plane and on the argument angle μ of satellite with respect to the midnight point in the orbit plane as identified from satellite laser ranging residuals. We present the yaw attitude model used for the bus of BDS IGSO and MEO satellites, and constrain the mode-switch conditions by the β and μ angles. In order to overcome the deficiency of the purely empirical CODE SRP model for precise orbit determination (POD) of BDS IGSO and MEO satellites in the ON mode, an additional constant acceleration bias with tight constraint of 1.0 × 10ź10 m/s2 in the along-track direction has been introduced to the CODE SRP model, and it is denoted as the C5a model. Although the orbit accuracy of IGSO and MEO satellites is significantly improved in the ON mode, the β- and μ-dependent systematic orbit errors of BDS IGSO are not reduced. Hence, with the presented yaw attitude model of the satellite bus and two assumed orientations of solar panels, the adjustable box-wing (ABW) model has been modified. Two modified ABW models are compared with the purely empirical CODE and C5a model. Based on the analysis of real data of 2014, the C5a model shows the best performance in the ON mode among the four SRP models. Although two modified ABW models show a rather worse performance for POD in the ON mode, particularly in the cross-track and radial direction, the β- and μ-dependent systematic orbit errors of BDS IGSO satellites are reduced. This provides a new insight and a possible way to improve the orbits of BDS IGSO and MEO satellites.

Space solar-power satellites may one day transmit large amounts of high-powered microwave beams to Earth from geostationary Earth orbit. New spacecraft can be designed to prosper from this new available source of energy by equipping future spacecraft with a rectenna array or hybrid photovoltaic/rectenna array system. This idea offers prospective benefits, such as extending the mission lifetime of the spacecraft through increasing the life of the power generation function and reducing the power generation units mass. This paper explores the integration of rectenna array and hybrid photovoltaic/rectenna array systems into future spacecraft and compares this method to the conventional photovoltaic array method, considering mission and component life, mass, area, and energy harvesting capabilities. Cost is compared between conventional photovoltaic array and the rectenna array. This paper shows that the integrated hybrid photovoltaic/rectenna array power generation unit seems a viable option in obtaining the ...

It is well known that many GPS satellites have experienced power degradation in excess of what would be expected from radiation damage on their solar arrays. Originally, contamination was suspected, based on the time behavior and rate of degradation seen. Calorimeters were included on some satellites, and a temperature rise commensurate with the power loss was detected, but subsequent efforts to eliminate line-of-sight paths from the contamination sources were unfruitful in reducing the power degradation. The hypothesis put forward here is that the contamination is the result of solar array arcing over a GPS lifetime. In this paper, we examine this hypothesis, using known characteristics of space solar array arcs, to place limits on the arc rate that would be sufficient to produce the contamination-induced power losses seen on GPS satellites to date. Comparisons with United States Nuclear Detonation Detection System signals on GPS satellites are consistent with at least some of the United States Nuclear D...

Because of the unknown systematic errors and special satellite constellations in the Beidou system (BDS), it is difficult to quickly and reliably determine the ambiguity over long-range baselines in continuously operating reference station (CORS) network. This study seeks to improve the effectiveness and reliability of BDS ambiguity resolution (AR) by combining the geometry-free and ionosphere-free (GFIF) combination and multipath mitigation algorithm. The GFIF combination composed with three-frequency signals is free of distance-dependent errors and can be used to determine the narrow lane ambiguity. The presence of multipath errors means that not all ambiguities can be correctly achieved by rounding the averaged GFIF ambiguity series. A multipath model of the single-differenced (SD) GFIF combination from the previous period is established for each individual satellite. This model is subtracted from the SD GFIF combination for the current day to remove the effects of multipath errors. Using three triangle networks with lengths of approximately 120, 80 and 50 km, we demonstrate that the proposed method improves the AR performance. The ambiguity averaged first fixing time is typically less than 1801 s for inclined geosynchronous orbit (IGSO) and medium earth orbit (MEO) satellites and less than 2007 s for the $$\sim $$ 42 $$^{\circ }$$ elevation geostationary earth orbit (GEO) C02 satellite. However, it is more time consuming for the low-elevation GEO satellites C04 ( $$\sim $$ 18 $$^{\circ }$$ ) and C05 ( $$\sim $$ 28 $$^{\circ }$$ ). Kalman filtering is used to estimate the troposphere delays and two unfixed ambiguities by employing the ionosphere-free observations of all ambiguity-fixed/unfixed satellites. The experimental results show that only tens of seconds are required for AR in around 90 km baselines.

The use of Global Positioning System (GPS) observables provides the primary and advantageous technique for satellite orbit determination in low earth orbit (LEO), in particular when onboard autonomy is required. A wide range of LEO spacecraft equipped with GPS receivers have been launched into space for different applications. Aiming at determining the position and velocity of the satellite in real-time, a consider Kalman filter (CKF)-based reduced-dynamic orbit determination (RDOD) is introduced in this paper. In the CKF, orbit dynamic model is simplified to meet the space-borne computational limitations. The atmospheric drag and solar radiation pressure coefficients are considered rather than estimated in the conventional RDOD strategy. Only a lower order and degree of an earth gravity model is used in the orbit model. However, the propagation of the covariance of the consider parameters is able to absorb the unmodeled and dismodeled perturbations. Therefore, the filter could become convergent with desirable orbit determination performance. The CKF-RDOD method is implemented with a set of the Gravity Recovery and Climate Experiment flight data. The solutions indicate that this proposed method could achieve satisfactory precision orbit determination with approximately 1.5 m level of three-dimensional root-mean-square error using GPS broadcast messages in real-time scenarios.

Stochastic models are important in global navigation satellite systems (GNSS) estimation problems. One can achieve reliable ambiguity resolution and precise positioning only by use of a suitable stochastic model. The BeiDou system has received increased research focus, but based only on empirical stochastic models from the knowledge of GPS. In this paper, we will systematically study the estimation, assessment and impacts of a triple-frequency BeiDou stochastic model. In our estimation problem, a single-difference, geometry-free functional model is used to extract pure random noise. A very sophisticated structure of unknown variance matrix is designed to allow the estimation of satellite-specific variances, cross correlations between two arbitrary frequencies, as well as the time correlations for phase and code observations per frequency. In assessing the stochastic models, six data sets with four brands of BeiDou receivers on short and zero-length baselines are processed, and the results are compared. In impact analysis of stochastic model, the performance of integer ambiguity resolution and positioning are numerically demonstrated using a realistic stochastic model. The results from ultrashort (shorter than 10 m) and zero-length baselines indicate that BeiDou stochastic models are affected by both observation and receiver brands. The observation variances have been modeled by an elevation-dependent function, but the modeling errors for geostationary earth orbit (GEO) satellites are larger than for inclined geosynchronous satellite orbit (IGSO) and medium earth orbit (MEO) satellites. The stochastic model is governed by both the internal errors of the receiver and external errors at the site. Different receivers have different capabilities for resisting external errors. A realistic stochastic model is very important for achieving ambiguity resolution with a high success rate and small false alarm and for determining realistic variances for position estimates. To the best of our knowledge, this paper is the first comprehensive study on such stochastic models used specifically with BeiDou data.

The Hatch---Melbourne---Wubbena (HMW) combination is a geometry-free and ionospheric-free (first-order) function that has been widely used for cycle slip detection and ambiguity resolution in GNSS dual-frequency data processing. We investigate the characteristics of HMW combinations for BeiDou navigation satellite system (BDS) observations for different types of satellites. We first study the characteristics of BDS undifferenced (UD) HMW combinations. Obvious systematic errors are found on BDS UD HMW combinations. The characteristics of single-differenced (SD) and double-differenced (DD) HMW combinations are then analyzed. The results indicate that the biases do not cancel in single-differencing between two satellites. On the contrary, they are amplified because of the superposition of the systematic biases of two satellite signals. In single-differencing between two receivers, different characteristics are found for different satellites types depending on baseline length. For inclined geostationary orbit (IGSO) and Medium Earth Orbit (MEO), the systematic biases cancel for single-differencing between receivers or double-differencing over short and medium baselines; however, they do not cancel in single-differencing between receivers or double-differencing over long baselines. Regarding geostationary earth orbit (GEO), the systematic biases do not cancel for single-differencing between receivers or double-differencing over both short and long baselines. In addition, we analyze the sources of the systematic biases of the BDS HMW combination. The systematic biases of IGSO and MEO HMW combinations mainly originate from multipath errors at the satellite; the systematic biases of GEO HMW combinations may also originate from multipath errors. Finally, we study the influence of the systematic errors on wide-lane ambiguity resolution by analyzing the consistency of the fractional part of the averaged SD HMW combinations and the fixing rate of DD wide-lane ambiguity resolution. The results are compared with those of GPS.

This paper introduces architectures for next-generation high throughput satellite HTS systems comprising various satellite payload options, ground terminal advances, and scalable system-level software control and management techniques. It describes a model to estimate aggregate system capacity as a function of radio band, available spectrum, spot beams, waveforms, and payload capability, including antenna size, power, and digital/analog connectivity across various links and availability objectives. This system model has been used to evaluate aggregate capacity of representative Ka-Band low earth orbit and geosynchronous orbit systems. A system implementation approach is described for next-generation HTS systems based on widely used Industry standards. Modulation and coding techniques are based on Digital Video Broadcasting - S2 extensions DVB-S2X, which comprises spectrally efficient modulation schemes combined with low-rate codes. Several implementation technologies are analyzed related to configurable onboard payload and ground-based, software-defined resource control and management, key enablers of next-generation HTS systems. Basic architectural building blocks are introduced for design of end-to-end systems across low earth orbit, medium earth orbit, and geosynchronous orbit satellite constellations, with and without onboard processing and inter-satellite links, and including several efficient scenarios to achieve lossless handovers. Copyright © 2016 John Wiley & Sons, Ltd.

Two classic empirical solar radiation pressure (SRP) models, the Extended Center for Orbit Determination in Europe (CODE) Orbit Model ECOM 5 and ECOM 9 have been widely used for Global Positioning System (GPS) Medium Earth Orbit (MEO) satellites precise orbit determination (POD). However, these two models are not suitable for BeiDou Geostationary Earth Orbit (GEO) satellites due to their special attitude control mode. With the experimental design method this paper proposes a new empirical SRP model for BeiDou GEO satellites, which is featured by three constant terms in DYX directions, two sine terms in DX directions and one cosine term in the Y direction. It is the first time to reveal that the periodic terms in the D direction are more important than those in YX directions for BeiDou GEO satellites. Compared with ECOM 5 and ECOM 9, the BeiDou GEO satellite orbits are significantly stabilized with the new SRP force model. The average orbit overlapping root mean square (RMS) achieved by the proposed model is 7.5 cm in the radial component, which is evidently improved over those of 37.4 and 13.2 cm for ECOM 5 and ECOM 9, respectively. In addition, the correlation coefficients between GEO orbit overlaps precision and the elevation angle of the Sun have been decreased to −0.12, 0.21, and −0.03 in radial, along-track and cross-track components by using the proposed model, while they are −0.94, −0.79 and −0.29 for ECOM 5 and −0.70, 0.21 and 0.10 for ECOM 9. Moreover, the standard deviation (STD) of Satellite Laser Ranging (SLR) data residuals for the GEO satellite C01 is reduced by 37.4% and 16.1% compared with those of ECOM 5 and ECOM 9 SRP models.

The Van Allen radiation belts surrounding the Earth are filled with MeV-energy electrons. This region poses ionizing radiation risks for spacecraft that operate within it, including those in geostationary orbit (GEO) and medium Earth orbit. To provide alerts of electron flux enhancements, 16 prediction models of the electron log-flux variation throughout the equatorial outer radiation belt as a function of the McIlwain L parameter were developed using the multivariate autoregressive model and Kalman filter. Measurements of omnidirectional 2.3 MeV electron flux from the Van Allen Probes mission as well as >2 MeV electrons from the GOES 15 spacecraft were used as the predictors. Model explanatory parameters were selected from solar wind parameters, the electron log-flux at GEO, and geomagnetic indices. For the innermost region of the outer radiation belt, the electron flux is best predicted by using the Dst index as the sole input parameter. For the central to outermost regions, at L ≧ 4.8 and L ≧ 5.6, the electron flux is predicted most accurately by including also the solar wind velocity and then the dynamic pressure, respectively. The Dst index is the best overall single parameter for predicting at 3 ≦ L ≦ 6, while for the GEO flux prediction, the KP index is better than Dst. A test calculation demonstrates that the model successfully predicts the timing and location of the flux maximum as much as 2 days in advance and that the electron flux decreases faster with time at higher L values, both model features consistent with the actually observed behavior.

In contrast to the US Global Positioning System (GPS), the Russian Global Navigation Satellite System (GLONASS) and the European Galileo, the developing Chinese BeiDou satellite navigation system (BDS) consists of not only Medium Earth Orbit (MEO), but also Geostationary Orbit (GEO) as well as Inclined Geosynchronous Orbit (IGSO) satellites. In this study, the Precise Point Positioning (PPP) and PPP with Integer Ambiguity Resolution (IAR) are obtained. The contributions of these three different types of BDS satellites to PPP in Asia–Pacific region are assessed using data from selected 20 sites over more than four weeks. By using various PPP cases with different satellite combinations, in general, the largest contribution of BDS IGSO among the three kinds of BDS satellites to the reduction of convergence time and the improvement of positioning accuracy, particularly in the east direction, is identified. These PPP cases include static BDS only solutions and static/kinematic ambiguity-float and -fixed PPP with the combination of GPS and BDS. The statistical results demonstrate that the inclusion of BDS GEO and MEO satellites can improve the observation condition and result in better PPP performance as well. When combined with GPS, the contribution of BDS to the reduction of convergence time is, however, not as significant as that of GLONASS. As far as the positioning accuracy is concerned, GLONASS improves the accuracy in vertical component more than BDS does, whereas similar improvement in horizontal component can be achieved by inclusion of BDS IGSO and MEO as GLONASS.

The outer electron belt poses a significant radiation hazard to spacecraft due to internal charging and dose effects. We present a new user-friendly model to characterise the worst-case environment built on data from the Giove-A spacecraft. We use instrument data based on a novel technique of direct charging current measurements, uncontaminated by protons and unaffected by dead-time, to create the model. The model provides integral or differential electron flux spectra with the following input parameters: percentile 90%, 99% or 100%; L-shell range 3-8; Magnetic Latitude ( B/B 0 ) range 1-100; Energy range 0.5-3 MeV. Comparisons with other models and independent data sets show that our “Model of Outer Belt Electrons for Dielectric Internal Charging” (MOBE-DIC) provides a sound worst-case specification for mission planners and design engineers. We find that, in medium Earth orbit particularly, MOBE-DIC indicates a harder electron spectrum than either AE9 or FLUMIC.

This paper deals with the analysis of local Love and Shida numbers (parameters h2 and l2) values of the Australian Yarragadee and Mount Stromlo satellite laser ranging (SLR) stations. The research was conducted based on data from the Medium Earth Orbit (MEO) satellites, LAGEOS-1 and LAGEOS-2, and Low Earth Orbit (LEO) satellites, STELLA and STARLETTE. Data from a 60-month time interval, from 01.01.2014 to 01.01.2019, was used. In the first research stage, the Love and Shida numbers values were determined separately from observations of each satellite, the obtained values of h2, l2 exhibit a high degree of compliance, and the differences do not exceed formal error values. At this stage, we found that it was not possible to determine l2 from the data of STELLA and STARLETTE. In the second research stage, we combined the satellite observations of MEO (LAGEOS-1+LAGEOS-2) and LEO (STELLA+STARLETTE) and redefined the h2, l2 parameters. The final values were adopted, and further analyses were made based on the values obtained from the combined observations. For the Yarragadee station, local h2 = 0.5756 &plusmn, 0.0005 and l2 = 0.0751 &plusmn, 0.0002 values were obtained from LAGEOS-1 + LAGEOS-2 and h2 = 0.5742 &plusmn, 0.0015 were obtained from STELLA+STARLETTE data. For the Mount Stromlo station, we obtained the local h2=0.5601 &plusmn, 0.0006 and l2 = 0.0637 &plusmn, 0.0003 values from LAGEOS-1+LAGEOS-2 and h2 = 0.5618 &plusmn, 0.0017 from STELLA + STARLETTE. We found discrepancies between the local parameters determined for the Yarragadee and Mount Stromlo stations and the commonly used values of the h2, l2 parameters averaged for the whole Earth (so-called global nominal parameters). The sequential equalization method was used for the analysis, which allowed to determine the minimum time interval necessary to obtain stable h2, l2 values. It turned out to be about 50 months. Additionally, we investigated the impact of the use of local values of the Love/Shida numbers on the determination of the Yarragadee and Mount Stromlo station coordinates. We proposed to determine the stations (X, Y, Z) coordinates in International Terrestrial Reference Frame 2014 (ITRF2014) in two computational versions: using global nominal h2, l2 values and local h2, l2 values calculated during this research. We found that the use of the local values of the h2, l2 parameters in the process of determining the stations coordinates influences the result.

The output frequency of an on-board atomic clock changes with a change in the external magnetic field as it is a magnetic sensitive device. In this paper, the phase comparison method is employed for the main and standby atomic clocks to evaluate the frequency stability of 10 Medium Earth Orbit (MEO for short) navigation satellite atomic clocks equipped with hydrogen atomic clocks. The average short-term stability is higher than 1×10−12 τ −1/2, and the daily stability is higher than 7×10−15. Based on this stability level, this paper presents a preliminary evaluation of the magnetic frequency shift in the atomic clock in the navigation satellite. The magnetorquer can cause a shift in the magnetic frequency of the atomic clock to an order of 1×10−13. In addition, the continuous operation of the magnetic torque converter for 1 h will cause the prediction error of the clock difference to be 0.4 ns. In this paper, the working state of the on-orbit magnetorquer is equivalent to the magnetic frequency shift noise, and its influence on the stability of the on-board atomic clock is simulated and analyzed. The simulation results show that the magnetic frequency shift noise worsens the long-term stability of the on-board atomic clock, especially for those with higher stability. Furthermore, the possible ways of minimizing the effect of the magnetic frequency shift on the atomic clock are discussed.

Two methods are widely used for evaluating the precision of satellite clock products, namely the single-satellite method (SSM) and the multi-satellite method (MSM). In the satellite clock product evaluation, an important issue is how to eliminate the timescale difference. The SSM selects a reference satellite to eliminate the timescale difference by between-satellite differencing, but its evaluation results are susceptible to the gross errors in the referenced satellite clock offsets. In the MSM, the timescale difference is first estimated and then removed. Unlike the GPS, the BeiDou Navigation Satellite System (BDS) consists of three types of satellites, namely geosynchronous earth orbit (GEO), inclined geosynchronous orbit (IGSO), and medium earth orbit (MEO) satellites. The three types of satellites have uneven orbital accuracy. In the generation of satellite clock products, the orbital errors are partly assimilated into the clock offsets. If neglecting the orbital accuracy difference of the three types of BeiDou satellites, the MSM will obtain biased estimates of the timescale difference and finally affect the clock product evaluation. In this study, an improved multi-satellite method (IMSM) is proposed for evaluating the real-time BDS clock products by removing the assimilated orbital errors of the three types of BDS satellites when estimating the timescale difference. Three real-time BDS clock products disseminated by three different International GNSS Service (IGS) analysis centers, namely CLK16, CLK20, and CLK93, over a period of two months are used to validate this method. The results indicate that the assimilated orbital errors have a significant impact on the estimation of the timescale difference. Subsequently, the IMSM is compared with the SSM in which the referenced satellite is rigorously chosen, and their RMS difference is only 0.08 ns, which suggests that the evaluation results obtained by the IMSM are accurate. Compared with the traditional MSM, the IMSM improves the RMS by 0.16, 0.11, and 0.07 ns for CLK16, CLK20, and CLK93, respectively. Finally, three real-time BDS clock products are evaluated using the proposed method, and results reveal a significant precision difference among them.

We proposed an extended geometry and probability model (EGAPM) to analyze the performance of various kinds of (Global Navigation Satellite System) GNSS+ constellation design scenarios in terms of satellite visibility and dilution of precision (DOP) et al. on global and regional scales. Different from conventional methods, requiring real or simulated satellite ephemerides, this new model only uses some basic parameters of one satellite constellation. Verified by the reference values derived from precise satellite ephemerides, the accuracy of visible satellite visibility estimation using EGAPM gets an accuracy better than 0.11 on average. Applying the EGAPM to evaluate the geometry distribution quality of the hybrid GNSS+ constellation, where highly eccentric orbits (HEO), quasi-zenith orbit (QZO), inclined geosynchronous orbit (IGSO), geostationary earth orbit (GEO), medium earth orbit (MEO), and also low earth orbit (LEO) satellites included, we analyze the overall performance quantities of different constellation configurations. Results show that QZO satellites perform slightly better in the Northern Hemisphere than IGSO satellites. HEO satellites can significantly improve constellation geometry distribution quality in the high latitude regions. With 5 HEO satellites included in the third-generation BeiDou navigation satellite system (BDS-3), the average VDOP (vertical DOP) of the 30&deg, N&ndash, 90&deg, N region can be decreased by 16.65%, meanwhile satellite visibility can be increased by 38.76%. What is more, the inclusion of the polar LEO constellation can significantly improve GNSS service performance. When including with 288 LEO satellites, the overall DOPs (GDOP (geometric DOP), HDOP (horizontal DOP), PDOP (position DOP), TDOP (time DOP), and VDOP) are decreased by about 40%, and the satellite visibility can be increased by 183.99% relative to the Global Positioning System (GPS) constellation.

In recent years, the development of new constellations including Galileo, BeiDou Navigation Satellite System (BDS) and Quasi-Zenith Satellite System (QZSS) have undergone dramatic changes. Since January 2018, about 30 satellites of the new constellations have been launched and most of the new satellites have been included in the precise orbit and clock products provided by the Multi Global Navigation Satellite System (Multi-GNSS) Experiment (MGEX). Meanwhile, critical issues including antenna parameters, yaw-attitude models and solar radiation pressure models have been continuously refined for these new constellations and updated into precise MGEX orbit determination and precise clock estimation solutions. In this context, MGEX products since 2018 are herein assessed by orbit and clock comparisons among individual analysis centers (ACs), satellite laser ranging (SLR) validation and precise point positioning (PPP) solutions. Orbit comparisons showed 3D agreements of 3–5 cm for Galileo, 8–9 cm for BDS-2 inclined geosynchronous orbit (IGSO), 12–18 cm for BDS-2 medium earth orbit (MEO) satellites, 24 cm for BDS-3 MEO and 11–16 cm for QZSS IGSO satellites. SLR validations demonstrated an orbit accuracy of about 3–4 cm for Galileo and BDS-2 MEO, 5–6 cm for BDS-2 IGSO, 4–6 cm for BDS-3 MEO and 5–10 cm for QZSS IGSO satellites. Clock products from different ACs generally had a consistency of 0.1–0.3 ns for Galileo, 0.2–0.5 ns for BDS IGSO/MEO and 0.2–0.4 ns for QZSS satellites. The positioning errors of kinematic PPP in Galileo-only mode were about 17–19 mm in the north, 13–16 mm in the east and 74–81 mm in the up direction, respectively. As for BDS-only PPP, positioning accuracies of about 14, 14 and 49 mm could be achieved in kinematic mode with products from Wuhan University applied.

The satellite-induced code bias variation of geostationary satellite orbit satellites and medium earth orbit satellites of the second-generation BeiDou Navigation Satellite System (BDS-2) exceeds 1 m, which severely affects the accuracy and stability of the ambiguity resolution and high-precision positioning. With the development of the third-generation BDS (BDS-3) with a new system design and new technology, analysis of the satellite-induced code variation characteristics of BDS-3 has become increasingly important. At present, many scholars have explored the satellite-induced code bias of BDS-3, but most of them focus on BDS-3 experimental satellites via normal geodetic antenna. Compared to normal geodetic antenna, the 40-m dish antenna from the National Time Service Center can accurately detect satellite-induced code variations with low noise and high gain. Thus, observational data from fifteen BDS-3 medium earth orbit satellites are collected with the B1I/B2b/B3I/B1C/B2a frequency bands on the day of year (DOY) 199&ndash, 206 in 2019, the PRN numbers of which are C19/C20/C21/C22/C23/C24/C25/C26/C27/C28/C30/C32/C33 /C35/C37, via the 40 m dish antenna to analyze the code bias variation characteristics. The results show that the obvious satellite-induced elevation‑dependent code bias variations exist in the B1I/B2b/B3I/B1C/B2a frequency bands of C28, compared with other satellites. Similarly, the multipath (MP) combination of B3I has an obvious elevation‑dependent variation within a range of 0.1 m for C21/C24/C27/C28/C37 and elevation‑dependent variation of the B2a and B2b frequency bands also exists in most satellites with a range of 0.1 m. However, the MP combination values of some satellites are asymmetric with respect to elevation, which is different from BDS-2 satellites and especially obvious for BDS-3 satellites B1I and BIC frequency bands with elevation‑dependent variations of 0.2 m, indicating that the code bias variation is not uniquely related to elevation, especially for the B1I/BIC frequency bands. What&rsquo, s more, the satellite-induced code bias variation of the BDS-3 satellites is greatly reduced compared with that of the BDS-2 satellites. In addition, the similar code bias variation appears at the Xia1 station with a normal geodetic antenna of B1I/B1C/B3I/B2a/B2b of C21, B3I/B2a/B2b of C24 and B2b of C28 among B1I/B1C/B3I/B2a/B2b of C21/C24/C27/C28/C37. The influence of the BDS-3 satellite-induced elevation‑dependent code bias on precision positioning and ambiguity fixing is worth further study using different antennas or receivers.

According to altitude, the orbits of satellites constellations can be divided into geostationary Earth orbit (GEO), medium Earth orbit (MEO), and low Earth orbit (LEO) constellations. We propose to use a Walker star constellation with polar orbits, at any altitude, to emulate the geostationary orbit with zenith paths at any latitude. Any transmitter/receiver will be linked to a satellite as if the site were at the equator and the satellite at the local zenith. This constellation design can have most of the advantages of the current GEO, MEO, and LEO constellations, without having most of their drawbacks. Doppler phenomena are largely minimized because the connected satellite is always seen almost at the local zenith. The extra free-space loss, due to the fixed pointing of all antennas, is at most 6 dBs when the satellite enters or leaves the service area. The connections among satellites are easy because the positions in the orbital plane and in adjacent planes are constant, although with variable distances. No steering antennas are required. The tropospheric propagation fading and scintillations are minimized. Our aim is to put forth the theoretical ideas about this design, to which we refer to as the geostationary surface (GeoSurf) constellation.

Signal-In-Space User Range Errors (SIS UREs) are assumed to be overbounded by a normal distribution with a standard deviation represented by the User Range Accuracy (URA). The BeiDou Navigation Satellite System (BDS) broadcast URA is not compatible with the historical SIS URE performance that affects the Advanced Receiver Autonomous Integrity Monitoring (ARAIM) False Alert Probability (Pfa) and availability evaluation. This study compares the BDS broadcast and precise ephemeris from 1 March 2013 to 1 March 2017 to obtain SIS UREs. Through analyzing the statistical characteristics of the SIS UREs, we obtain the standard deviation &sigma, URE for the accuracy and continuity and &sigma, URA used for the integrity of the SIS UREs. The results show that the broadcast &sigma, URA of 2 m cannot completely overbound SIS UREs for all BDS satellites, but the &sigma, URA of 2.4 m can. Then, we use the &sigma, URA of 2.4 m to evaluate the ARAIM Pfa and availability. The results show that the Pfa may increase to 2 &times, 10&minus, 5 and exceed its limit by an order of magnitude. We also consider the differences between the SIS UREs of Geostationary Earth Orbit (GEO), Inclined Geo-Synchronous Orbit (IGSO), and Medium Earth Orbit (MEO). The results indicate that all Pfa values calculated by the computed &sigma, URE are less than the Pfa in the Integrity Support Message (ISM) for the worst-performing GEO satellite. The approximately 55% Pfa calculated by the computed &sigma, URE is less than the Pfa in ISM for the worst-performing IGSO satellite. Most Pfa values calculated by the computed &sigma, URE is less than the Pfa in the ISM for the worst-performing MEO satellite. For BDS satellites, the Pfa is mainly affected by &sigma, URE. When the &sigma, URA of 2.4 m is used to evaluate the availability, the computed availability is lower than the availability calculated by the broadcast &sigma, URA/&sigma, URE and the greatest degradation can reach 25%.

1. Introduction The DLR and GFZ joint project ADVANTAGE (Advanced Technology for Navigation and Geodesy 1,2 ) aims at studying the potential of optical ranging and time transfer technology for future satellite navigation systems. The proposed constellation consists of 24 Medium Earth Orbit (MEO) navigation satellites and 4 Low Earth Orbit (LEO) satellites devoted to generate the time scale and synchronize the clocks of the system using two-way optical links. The MEO constellation is essentially similar to the current GALILEO system which assures long term orbit stability and limits the number of constellation keeping maneuvers. The LEO part consists of satellites on circular polar orbits on two perpendicular orbital planes (two LEOs per plane) with a mutual phasing that assures the LEO-MEO links to be optimal for the synchronization of the system time. Using GFZ’s Earth Parameters and Orbit System – Orbit Computation (EPOS-OC) software package we simulate two data types which will be used in precise orbit determination (POD): (1) GNSS type L-band ranging data (pseudo-range and carrier phase) between both, MEOs and LEOs (satellite-to-satellite tracking data) and MEOs and a network of 124 ground stations, (2) highly precise ranges between neighboring MEOs as well as between MEO and LEO satellites. 2. Objectives The main goal of this research is to find the impact of inter-satellite links and ultra-stable time on POD of the proposed constellation. In particular we want to asses the benefit of the inclusion of the LEOs for POD of the navigation satellites and to investigate possibilities of reducing the number of ground stations required to achieve satisfactory orbit accuracies. A realistic simulation must also include systematic modeling errors like solar radiation pressure, antenna thrust and phase center offset etc. We simulate them to find their influence on the performance of the recovered orbits and probable impacts on their use in geodetic applications like gravity field determination, remote sensing applications, Earth observation (water vapor, reflectometry) and precise point positioning. 3. Simulation In the data simulation step we use standards and models from daily processing of real GNSS data. That includes GPS-like attitude, ROCK4 solar radiation pressure model, EIGEN-6C gravity field model, Earth and ocean tide models, ocean loading etc. The antenna thrust is simulated as constant radial acceleration with small periodic variations. The GNSS range data are simulated on 2 GALILEO frequencies E1 and E5. For the purpose of orbit determination ionosphere-free combinations are established thereof and endowed with Gaussian noise of 50 cm for pseudo-ranges as well as 3 and 5 mm for carrier phases of LEOs and MEOs, respectively. To obtain a realistic number of carrier phase ambiguities, cycle slips are simulated as well. The system time is assumed to be ultra-stable for all the MEO and LEO satellites. Therefore the clock offsets are simulated as constant values with zero mean and small Gaussian noise of 0.3 mm. 4. Orbit recovery In the POD step we use the integrated approach. Here the simulated data from all satellites, MEOs and LEOs, and from all ground stations are processed simultaneously. The parameters of interest, like the orbits, are estimated in a differential orbit improvement process following the least-squares (LS) principle. To achieve the goals of this research, we define various orbit recovery scenarios involving different combinations of MEOs, LEOs and the ground network with different data types and a selection of modeling errors. In particular, in view of the proposed constellation we consider POD using: a) MEOs + LEOs only, b) MEOs + LEOS + a limited number of ground stations. For each constellation set-up we investigate the impact of modeling errors, ambiguity fixing and inclusion of precise inter-satellite range links. One of the most important properties of the proposed navigation system is the very stable timekeeping, which results in predictable clock offsets of the space segment. To asses the benefit of stable clocks, we use three methods to obtain the clocks. In the first one all clock offsets are estimated epoch by epoch, which is the standard case in current day GNSS data processing. A known drawback is the necessity to estimate a large number of clock parameters. In the second approach we make use of ultra-stable clocks and model them as being piece-wise constant. The ensuring significant reduction of the number of adjustable parameters makes the LS solution more accurate and more reliable on the one hand. On the other hand the modeling errors become more visible in observation residuals, orbits and other parameters, as they are not absorbed by the large number of clock parameters as in the first method. In the third approach we consider a theoretical case when all space clocks are known and can be fixed. Finally we summarize the results with focus on the orbit accuracy. Footnotes: 1. This project is supported by the Helmholtz-Gemeinschaft Deutscher Forschungszentren e.V. under grant numbers ZT-0007 (ADVANTAGE, Advanced Technologies for Navigation and Geodesy). 2. The project ADVANTAGE is a joint project of DLR and the GFZ German Research Centre for Geosciences. It aims at defining a future system for navigation, geodesy and metrology.

The introduction of an intersatellite link, also called a crosslink, is considered a promising technique for improving the reliability and integrity of a global navigation satellite system. As one of the most rapidly developing satellite navigation systems, the BeiDou system launched from March 2015 to February 2016 an in-orbit validation constellation that includes two inclined geosynchronous orbit satellites and three medium earth orbit satellites equipped with an intersatellite link in the Ka band. We modeled the intersatellite measurements of BeiDou and evaluated the ranging performance of the intersatellite link based on the analysis of in-orbit measurement data. We used both residual analysis and external data comparison to assess the data. The results show that the ranging precision of the BeiDou intersatellite link is within 10 cm and is determined mainly by the thermal noise of the receiver. Moreover, the drift rate of the group delay of the transceiver channel is within 1 cm per day.

The basic system of the BeiDou global navigation satellite system (BDS-3) with 18 satellites has been deployed since December 2018. As the primary frequency standard, BDS-3 satellites include two clock types with the passive hydrogen maser (PHM) and the rubidium atomic frequency standard (RAFS). Based on the final precise orbit and clock product from Xi’an Research Institute of Surveying and Mapping (XRS), the atomic clock performance of BDS-3 satellites is evaluated, including the frequency accuracy, frequency drift rate, and frequency stability, and compared with GPS block IIF satellites with RAFS, Galileo satellites with PHM, and BDS-2 satellites. A data auto-editing procedure to preprocess clock data and assess the clock performance is developed, where the assessed results are derived at each continuous data arc and the outliers are excluded properly. The stability of XRS product noise is given by using some stations equipped with high-precision active hydrogen masers (AHM). The best stability is 8.93 × 10−15 and 1.85 × 10−15 for the averaging time of 10,000 s and 1 day, which is basically comparable to one-third of the in-orbit PHM frequency stability. The assessed results show the average frequency accuracy and drift rate of BDS-3 with RAFS are slightly worse while the stability is better than BDS-2 medium earth orbit (MEO) satellites. The 10,000 s stability is better but the 1-day stability is worse than GPS, which may be related to the performance of the BDS-3 RAFS clock. As for BDS-3 with PHM, the frequency accuracy is slightly worse than Galileo PHM satellites; the drift rate, when excluding C34 and C35, is basically comparable to Galileo and significantly better than GPS satellites; the stability is comparable to Galileo, where the 10,000 s stability is slightly worse than Galileo and better than GPS. The 1-day stability among BDS-3 PHM, GPS IIF RAFS, and Galileo PHM satellites is basically comparable.

The satellite-induced systematic biases were identified to exist in the code observations from BeiDou navigation satellite system (BDS) satellites using multipath (MP) combinations. The current correction model for satellite-induced code bias (SICB) does not take into account the consistency of MP combinations, which limits the accuracy of the developed model. Both the cycle slips and different tracking of a satellite at different stations can affect the absolute values of MP combinations, although the variations remain unchanged. An improved SICB piecewise linear correction model as a function of elevations is proposed. We estimate the model parameters for each frequency and for each satellite. The single-difference of MP combinations in the domain of elevation angles is carried out to remove the unknown ambiguities and stable hardware delays so that the SICB modeling is free of the effects of MP combination inconsistency. In addition, a denser elevation node separation of 1&deg, rather than the 10&deg, usually employed by the traditional model, is used to describe the more precise SICB variations. The SICB corrections show significant differences among orbit types and frequency bands. The SICB variations have much less effect on Inclined Geosynchronous Orbit (IGSO) satellites than on Medium Earth Orbit (MEO) satellites for the regional BDS (BDS-2). The B1 signal has the largest SICB corrections, which can be up to 0.9 m close to zenith for BDS-2 MEO satellites, and the B2 signal follows. After adding the SICB corrections to the code observations, the elevation-dependent code biases vanish, and we can obtain improved code observations. After applying the improved SICB correction model, the root mean square (RMS) values of MP combination time series are reduced by 7%, 6% and 2%, and 18%, 14% and 5% on the B1, B2 and B3 frequencies for the BDS-2 IGSO and MEO satellites, respectively. For comparison, we also establish the traditional SICB correction model. With the traditional SICB correction model, the corresponding RMS MP combinations are smaller than those of uncorrected MP series, but slightly larger than those of corrected MP series using the improved SICB correction model. To validate the effectiveness and correctness of our proposed model, single-frequency precise point positioning (PPP) processing with BDS-2 MEO and IGSO satellites is conducted. An accuracy improvement of 24%, 19% and 89%, and 7%, 7% and 6% for the single-frequency PPP applying the improved SICB corrections over the case without SICB corrections and the case using the traditional SICB corrections in east, north and vertical directions is achieved, respectively. Although only centimeter-level SICB variations could be observed for the two legacy signals B1 and B3 and the three new navigation signals B1C, B2a and B2b transmitted by the satellites of global BDS demonstration system (BDS-3S), we still establish an effective SICB correction model on the B1 and B3 frequencies for BDS-3S IGSO satellites, and the RMS MP combinations are reduced by 1&ndash, 4% after applying the improved SICB corrections.

In this chapter, we provide a thorough review of medium Earth orbit (MEO) satellites, highlighting their applications, peculiarities, and the role that they may play in the implementation of 5G for satellite networks. In particular, we will explain why MEO satellites are a new paradigm, how to tackle the challenges related to their usage, and how they fit into the 5G context. In this perspective, we will show how diversity combining and handover are key functionalities for their successful integration. Towards describing the 5G paradigm with MEO satellites, the chapter first provides a high-level description of the satellite characteristics, the services that have been deployed, and also possible future applications. Further, a high-level description of all the atmospheric effects affecting typical MEO communications is included. Finally, a critical review of handover techniques (state-of-the-art, trade-offs, and future challenges), as well as a review of combining techniques (theoretical performance in a MEO scenario, advantages, drawbacks, and trade-offs), will be presented.

The dual-system compatible Global Navigation Satellite System(GNSS) occultation sounder (GNOS) for Beidou Navigation Satellite System (BDS) and Global Positioning System(GPS) was launched into orbit with FY3C satellite on September 23, 2013, in which we can get an important ionospheric parameter: F2-layer peak electron density (NmF2), it can limit the maximum feasible frequency for ground signal propagation, as well as defining the minimum required frequency for transionospheric radio wave propagation, so it's a crucial parameter for charactering ionosphere and the space weather. In this presentation, we verify the precision of ionospheric BDS radio occultation (BDSRO) data obtained by FY3C/GNOS through NmF2 comparisons between BDSRO and ionosondes. The results show that the correlation coefficient of NmF2 data between BDSRO and ionosondes is 0.96, the bias is 10.21 % and the standard deviation (std) is 19.61 %. The NmF2 precision of BDSRO at daytime is higher than that of nighttime; When Beidou satellites are at Inclined Geosynchronous Satellite Orbit (IGSO), its precision is higher than that at Geosynchronous orbit (GEO) and Medium Earth Orbit (MEO).