Geosynchronous Orbit Satellites Research Articles

Earth shadow time/accelerometer/GNSS-based geosynchronous transfer orbit determination method for electric propulsion satellite

Autonomous orbital maneuvering of an electric propulsion satellite in geosynchronous transfer orbit (GTO) requires more accurate autonomous orbit determination (OD). Implementing the traditional autonomous OD method for the electric propulsion satellites may result in the accumulation of measurement errors and dynamic errors from orbit extrapolation, which significantly restrict the OD accuracy. Fortunately, an electric propulsion satellite in GTO can sense the transit time in or out of the Earth shadow according to the output power change of the solar array. Therefore, an Earth shadow time/accelerometer/GNSS-based geosynchronous transfer OD method for electric propulsion satellites is proposed in this paper. First, a system state model with dynamics is established for the entire orbit transfer process. Second, measurement models that use the measurement information from the accelerometer, GNSS, and the transit time into or out of the Earth shadow are established. Third, a sequential filtering estimation approach is designed to solve the problem of asynchronous data fusion for autonomous OD. Comparison simulations demonstrate the effectiveness and superiority of the proposed method.

Evaluation of fengyun geosynchronous orbit and GPM satellites precipitation products over the southeastern Tibetan plateau

ABSTRACT Based on the hourly precipitation data from the observation stations, the accuracy of hourly QPEs from Fengyun geostationary satellites FY-4A, FY-2 H and multi-satellite joint IMERG-Early precipitation products over the southeastern Tibetan Plateau were compared and assessed. And the relationship between the accuracy of various satellites and terrain were discussed. The results show that: (1) The QPEs of FY-4A, FY-2 H and IMERG-Early can reflect the temporal and spatial distribution characteristics of real precipitation; (2) The QPEs of FY-4A and FY-2 H overestimate the precipitation in the researched region, and the overestimation of FY-4A was more obvious, while IMERG-Early QPE underestimates the precipitation. IMERG-Early QPE showed the best overall accuracy performance. (3) All three satellite QPEs were less accurate in July and August. (4) The accuracy of the QPEs of FY-4A and FY −2 H were better than that of IMERG-Early QPE in heavy rainfall. (5) The accuracy of three satellite QPEs increase with the increase of altitude. The results can provide reference for selecting satellite QPEs for operational applications in the southeastern Tibetan Plateau.

Effects of Equatorial Plasma Bubbles on Multi-GNSS Signals: A Case Study over South China

Equatorial plasma bubbles (EPBs) occur frequently in low-latitude areas and have a non-negligible impact on navigation satellite signals. To systematically analyze the effects of a single EPB event on multi-frequency signals of GPS, Galileo, GLONASS, and BDS, all-sky airglow images over South China are jointly used to visually determine the EPB structure and depletion degree. The results reveal that scintillations, or GNSS signal fluctuations, are directly linked to EPBs and that the intensity of scintillation is positively correlated with the airglow depletion intensity. The center of the airglow depletion often corresponds to stronger GNSS scintillation, while the edge of the bubble, which is considered to have the largest density gradient, corresponds to relatively smaller scintillation instead. This work also systematically analyzes the responses of multi-constellation and multi-frequency signals to EPBs. The results show that the L2 and L5 frequencies are more susceptible than the L1 frequency is. For different constellations, Galileo’s signal has the best tracking stability during an EPB event compared with GPS, GLONASS, and BDS. The results provide a reference for dual-frequency signal selection in precise positioning or TEC calculation, that is, L1C and L2L for GPS, L1C and L5Q for Galileo, L1P and L2C for GLONASS, and L1P and L5P for BDS. Notably, BDS-2 is significantly weaker than BDS-3. And inclined geosynchronous orbit (IGSO) satellites have abnormal data error rates, which should be related to the special signal path trajectory of the IGSO satellite.

Performance Evaluation and Application Field Analysis of Precise Point Positioning Based on Different Real-Time Augmentation Information

The most commonly used real-time augmentation services in China are the International GNSS Service’s (IGS) real-time service (RTS), PPP-B2b service, and Double-Frequency Multi-Constellation (DFMC) service of the BeiDou Satellite-Based Augmentation System (BDSBAS) service. However, research on the performance evaluation, comparison, and application scope of these three products is still incomplete. This article introduces methods for obtaining real-time augmentation information and real-time orbit and clock offset recovery. Based on real-time orbit and clock offset accuracy, positioning accuracy, and positioning availability, this article systematically evaluates the performance and analyzes the application fields of Centre National d’Études Spatiales (CNES), PPP-B2b, and BDSBAS augmentation information. The results of the evaluation revealed that the radial accuracy of the CNES and PPP-B2b real-time orbit product is consistent, and the Root Mean Square (RMS) is better than 5 cm. The CNES real-time orbit product can achieve centimeter-level accuracy in both along-track and cross-track components, surpassing PPP-B2b’s decimeter-level accuracy. Both services demonstrate consistent accuracy in the real-time clock offset, with PPP-B2b showing similar standard deviations (STDs) of 0.16 ns for different satellites. However, for CNES, the STD of the real-time clock offset varies, with values of 0.10 ns, 0.19 ns, and 0.60 ns, respectively, for GPS, BDS-3 Medium Earth Orbit (MEO), and BDS-3 Inclined Geosynchronous Satellite Orbit (IGSO) satellites. Centimeter-level accuracy is achieved after convergence and positioning availability exceeds 99% for CNES and PPP-B2b services. Therefore, the difference between the two services in application areas depends on the acquisition of augmentation information. However, BDSBAS, which concentrates on code observations, demonstrates inferior performance in real-time orbit, clock offset, positioning accuracy, and positioning availability when compared to the other two services. Its primary application is in the aviation and maritime domains, where there is a greater need for service integrity, continuity, and reliability.

Gravity field recovery of inter-satellite links between Beidou navigation satellite system (BDS) and LEO based on geodesy and time reference in space (GETRIS)

The goal of this contribution is to construct the inter-satellite links (ISLs) between Global Navigation Satellite System (GNSS) and Low Earth Orbit (LEO) satellite to recover the Earth’s gravity field based on the concept of Geodesy and Time Reference in Space (GETRIS). Taking the constellation of the Beidou Navigation Satellite System (BDS) as an example, several types of high-low satellite-to-satellite tracking (H-L SST) modes combining BDS and GETRIS are used to research the tracking mode of high orbit satellites and the distribution of LEO satellite constellations to explore the optimal mode for gravity field recovery. The effects of multiple H-L SST modes on the accuracy of gravity field recovery are studied. In addition, the contribution of each error source is also analyzed. Finally, this study varies LEO satellite constellations to show reduced errors in the temporal gravity field solutions for the improved ISLs. The results showed that the Inclined GeoSynchronous Orbit (IGSO) satellite greatly improved the Geosynchronous orbit (GEO) satellite tracking LEO satellite mode in the high latitude regions. Compared with the Medium Earth Orbit (MEO) satellite tracking LEO satellite, the error of gravity field recovered by combining with GEO, IGSO and MEO can be reduced by 16 %. The largest error is the non-tidal signal, followed by the ocean tidal model difference, and the smallest is the instrument error. For a single LEO satellite, the combination model is more sensitive to the satellite orbital altitude, the cumulative error was decreased from 5.08 mm (550 km) to 0.33 mm (250 km). For LEO constellations, adding inclined orbit satellites can improve the accuracy of the solution to some extent. But the optimal solution can be obtained by the constellation composed of polar orbit satellites with different Right Ascension of Ascending Node (RAAN), the cumulative errors are 0.27 mm (two satellites), 0.19 mm (three satellites), and 0.19 mm (four satellites).

Investigating the Global Performance of the BDS-2 and BDS-3 Joint Real-Time Undifferenced and Uncombined Precise Point Positioning Using RTS Products from Different Analysis Centers

Compared to the traditional ionospheric-free (IF) precise point positioning (PPP) model, the undifferenced and uncombined (UU) PPP has the advantages of lower observation noise and the ability to obtain ionospheric information. Thanks to the IGS (International GNSS Service), real-time service (RTS) can provide RT vertical total electron content (VTEC) products, and an enhanced RT UU-PPP based on the RT-VTEC constraints can be achieved. The global performance of the BeiDou Navigation Satellite System-2 (BDS-2) and BDS-3 joint RT UU-PPP using different RTS products was investigated. There is not much difference in the RTS orbit accuracy of medium earth orbit (MEO) satellites among all analysis centers (ACs), and the optimal orbit accuracy is better than 5, 9, and 7 cm in the radial, along-track, and cross-track directions, respectively. The orbit accuracy of inclined geosynchronous orbit (IGSO) satellites is worse than that of MEO satellites. Except for CAS of 0.46 ns, the RTS clock accuracy of MEO satellites for other ACs achieves 0.2–0.27 ns, and the corresponding accuracy is about 0.4 ns for IGSO satellites. In static positioning, due to the limited accuracy of RT-VTEC, the convergence time of the enhanced RT UU-PPP is longer than that of RT IF-PPP for most ACs and can be better than 25 and 20 min in the horizontal and vertical components, respectively. After convergence, the 3D positioning accuracy of the static RT UU-PPP is improved by no more than 8.7%, and the optimal horizontal and vertical positioning accuracy reaches 3.5 and 7.0 cm, respectively. As for the kinematic mode with poor convergence performance, with the introduction of RT-VTEC constraints, the convergence time of RT UU-PPP can be slightly shorter and reaches about 55 and 60 min in the horizontal and vertical components, respectively. Both the horizontal and vertical positioning accuracies of the kinematic RT UU-PPP can be improved and achieve around 7.5 and 10 cm, respectively.

Analysis of BDS inter-satellite link ranging performance

The Ka-band inter-satellite link (ISL) is a key technology of BeiDou Global Navigation Satellite System (BDS-3) that can improve the system performance. Based on 30 days of ISL observations, we analyze the BDS-3 ISL ranging performance and the related influencing factors, such as satellite manufacturer, satellite orbit type and link type. The fitting residuals of the ISL geometry-free observations are used to characterize the measurement noise, and the post-fit ISL residuals from precise orbit determination (POD) and precise clock estimation (PCE) are used to further validate the ranging performance. The fitting residuals follow a normal distribution, and the average root mean square (RMS) value for all 421 links is approximately 2.5 cm. The satellite manufacturers, China Academy of Space Technology (CAST) and Shanghai Engineering Center for Microsatellites (SECM), have an obvious impact on the ISL measurement noise. The RMS of the ISL fitting residuals for the links established by the two CAST satellites is only 1.45 cm, while it can reach 3.34 cm for the links established by the two SECM satellites. This correlation between ISL ranging performance and the satellite manufacturer is also found in the POD and PCE post-fit ISL residuals. The RMS of PCE residuals is about 3.1 cm, while the POD residuals are larger with the RMS of 5.5 cm due to the effect of orbital errors, especially for the Inclined Geosynchronous Orbit (IGSO) and Geostationary Orbit (GEO) satellites. Overall, the ranging accuracy of the BDS-3 ISL is better than 5 cm according to the above analyses. Furthermore, the BDS-3 satellite orbit and clock results are evaluated by internal consistency check and external comparison to analyze the application of ISL observations.

Surface unequal charging effect of GEO satellite

When a satellite is in orbit, the plasma environment interacts with the surface material, and the surface charge accumulation of different structures is inconsistent,resulting in an unequal charging effect. In serious cases, this leads to electrostatic discharge and affects the normal operation of the satellite. In this study, a three-dimensional model of a geosynchronous orbit (GEO) satellite is constructed, and the particle cloud grid algorithm (Particle-In-Cell,PIC) was used to simulate the charging process and the unequal charged state between the satellite panels in the solar wind plasma environment and the hot plasma environment. The simulation results show that in the solar wind plasma environment, the charging balance potential of solar panels is low and the charging difference between panels is small, which does not affect the normal operation of the satellite. However, when the satellite is in an hot plasma environment, if some parts of the satellite suddenly encounter extremely strong interference, the charging balance of the satellite in the solar wind plasma will be destroyed, and there will be a large potential difference between the irradiated part and the adjacent structure, which can easily cause electrostatic discharge to harm the normal operation of the satellite.

Finding a Needle in a Haystack: Faint and Small Space Object Detection in 16-Bit Astronomical Images Using a Deep Learning-Based Approach

With the increasing interest in space science exploration, the number of spacecraft in Earth’s orbit has been steadily increasing. To ensure the safety and operational integrity of active satellites, advanced surveillance and early warning of unknown space objects such as space debris are crucial. The traditional threshold-based filter for space object detection heavily relies on manual settings, leading to limitations such as poor flexibility, high false alarm rates, and weak target detection capability in low signal-to-noise ratios. Therefore, detecting faint and small objects against a complex starry background remains a formidable challenge. To address this challenge, we propose a novel, intelligent, and accurate detection method called You Only Look Once for Space Object Detection (SOD-YOLO). Our method includes the following novel modules: Multi-Channel Histogram Truncation (MHT) enhances feature representation, CD-ELAN based on Central Differential Convolution (CDC) facilitates learning contrast information, the Space-to-Depth (SPD) module replaces pooling layer to prevent small object feature loss, a simple and parameter-free attention module (SimAM) expands receptive field for Global Contextual Information, and Alpha-EIoU optimizes the loss function for efficient training. Experiments on our SSOD dataset show SOD-YOLO has the ability to detect objects with a minimum signal-to-noise ratio of 2.08, improves AP by 11.2% compared to YOLOv7, and enhances detection speed by 42.7%. Evaluation on the Spot the Geosynchronous Orbit Satellites (SpotGEO) dataset demonstrates SOD-YOLO’s comparable performance to state-of-the-art methods, affirming its generalization and precision.

Enhanced precise orbit determination for GPS and BDS-2/3 with real LEO onboard and ground observations

Due to the current geographical situation in China that it is difficult to achieve globally well distributed ground monitoring stations for Chinese BeiDou Navigation System (BDS), the BDS Medium Earth Orbit (MEO) satellite orbital accuracy can be seriously affected by the insufficient length of continuous observation arc based on regional ground network. What is more, the highly static characteristics of BDS Geosynchronous Orbiting (GEO) satellites resulting in meter-level orbit determination accuracy is far from satisfactory for precise positioning services. In order to overcome these problems, the low Earth orbit (LEO) satellites serving as fast-moving space monitoring stations can be used as a good supplement for the current BDS ground monitoring stations layout. In this study, real measured spaceborne data from two LEO satellites, namely Fengyun-3D (FY-3D) and Tianjin University No. 1 (TJU-01), are collected and studied for the augmentation of BDS precise orbit determination (POD). The multipath errors of spaceborne code observations are comprehensively analyzed. It is found that not only BDS-2 relevant satellite-induced code bias but also near-field multipath errors, especially distinct in GPS, exist in a spaceborne environment. The piece-wise linear model is therefore established and applied to correct the code observations to reduce the multipath errors in the integrated POD processing. The GPS and BDS-2/3 POD solutions are compared with orbital overlapping differences and Satellite Laser Ranging (SLR) residuals validation. It is demonstrated that including the two LEO satellites can improve the orbit determination accuracy of GPS and BDS-2/3 significantly under a regional ground network, especially in the along component. The average overlapping orbital Root Mean Square (RMS) is decreased by 18.4 %, 34.3 %, 24.9 %, 21.4 %, and 7.8 % for the GPS, GEO, Inclined Geosynchronous Orbit (IGSO), BDS-2 MEO, and BDS-3 MEO satellites, respectively, in the along component. The RMS of SLR residuals is decreased from 37.1 cm to 17.8 cm for the GEO satellites, and the overall improvement is about 8 %-12 % for the BDS MEO satellites. There has also been a slight improvement under a global network, with an average improvement of 2–3 cm in the SLR residuals RMS. However, significant systematic errors are also observed in the SLR residuals in BDS GEO, IGSO, and MEO satellites, and are expected to be improved with refined observation models based on more LEO satellites, i.e., the phase center variation (PCV).

An optimization framework with improved auction-based initialization for highly constrained on-orbit servicing mission planning

The on-orbit servicing (OOS) mission planning problem for multiple geosynchronous earth orbit (GSO) satellites is a critical issue due to its huge economic value. However, given the complexity of the space scenario and the subjective nature of the formulation process, the problem is highly constrained in actual use, and the performance of conventional optimization algorithms decreases due to fuel or mission deadline constraints. With the goal of enhancing solution quality and speeding up the solution process, this paper proposes a novel optimization framework. The problem is decomposed into three sub-problems and then solved using the proposed three-level algorithm to lessen its complexity. In addition, a new initialization method is embedded to improve global search capabilities. Notably, considering the high computation efficiency of the auction algorithm (AA), an improved version is designed to generate primary solutions, which introduces the grouping and reallocation mechanism. Then, several variations of the primary solutions are used as the initial variables in the optimization. Numerical simulations demonstrate that the proposed methodology has higher computational efficiency and convergence performance than conventional strategies.

Design of VO2-based spacecraft smart radiator with low solar absorptance

Thermochromic film with the ability to self-adaptively switch emittance is important for spacecraft missions experiencing rapidly increasing demand. Current research on thermochromic films primarily focuses on optimizing emittance tunability, while the solar absorptance of these films remains high, limiting their practical application in spacecraft thermal control. In this work, a VO2-based spacecraft smart radiator with low solar absorptance and high emittance tunability is proposed. The smart radiator is a multilayer structured film composed of a switchable resonator (i.e., a superposed Fabry-Perot cavity) and a solar reflector (i.e., a Distributed Bragg Reflector). The performance of the smart radiator is optimized by combining the Rigorous Coupled Wave Analysis method and Genetic Algorithm, achieving a normal solar absorptance of 0.121 and emittance tunability of 0.825. The solar absorptance and emittance tunability of the smart radiator remain excellent even at large incident angles. The underlying mechanisms involved in the smart radiator are attributed to the combination of superposed Fabry-Perot resonance and multiple solar reflectance. In addition, we provide a numerical example using a geosynchronous orbit satellite to demonstrate that the regulation range of the net heat dissipation flux density can increase from 146.5–463.9 W/m2 by using Optical Solar Reflector to 0–494.6 W/m2 when using the proposed smart radiator within the permissible temperature range (263.15–318.15 K) of spacecraft. This VO2-based spacecraft smart radiator with outstanding properties which outperform the existing thermochromic films reveals the promising potential of thermochromic films for spacecraft intelligent thermal control applications.

An improved satellite-induced code bias correction model for BDS un-GEO satellites in consideration of elevation and azimuth angles

The satellite-induced code bias (SICB) resulting in code-phase divergences of nearly 1.0 m has been observed in the code measurements of BeiDou navigation satellite system (BDS). Since the SICB is proved to be elevation- and frequency-dependent, several SICB correction models have been proposed to remove the systematic bias, but most of them only consider elevation angles, overlooking the influence of azimuth angles, which would limit the accuracy of the correction models, especially for Medium Earth Orbit (MEO) satellites. To deal with this problem, we proposed an improved SICB piecewise correction model as a function of elevation and azimuth angles and estimated parameters for each BDS un-GEO (Geostationary Earth Orbit) satellite and each available frequency with node-separation of 1°. The epoch-difference method is employed to remove the effect of stable terms, such as phase-ambiguities, receiver- and satellite-dependent hardware delays. Moreover, to reduce the effect of code noise and multipath at low-elevation conditions, the threshold of elevation angle for the improved model is determined based on the statistics of Multipath (MP) combinations. Besides, weight-average algorithm is used to obtain the optimal MP deviations and improve the accuracy of proposed correction model in consideration of signal-to-noise (SNR). The experiment results with real GNSS data show that the SICB models are more suitable for BDS-2 MEO satellites than Inclined Geosynchronous Orbit (IGSO) satellites. The code measurements on B1 signal are more susceptible to the SICB than that on B3 signal. After applying the improved SICB correction models, the RMS errors of MP series are significantly reduced for BDS-2 un-GEO satellites. However, for BDS-3 satellites, only centimeter-level SICB deviations can be detected for all frequency-bands, and the RMS of MP variations have slight changes after applying the SICB corrections, indicating that the systematic bias in MP combinations is not obvious, especially for BDS-3 MEO satellites.

Satellite Clustering for Non-Terrestrial Networks: Concept, Architectures, and Applications

Recently, megaconstellations with a massive number of low Earth orbit (LEO) satellites are being considered as a possible solution for providing global coverage due to relatively low latency and high throughput compared to geosynchronous orbit (GEO) satellites. However, as the number of satellites and operators participating in the LEO constellation increases, inter-satellite interference will become more severe, which may yield marginal improvement or even decrement in network throughput. In this article, we introduce the concept of satellite clusters that can enhance network performance through satellites’ cooperative transmissions. The characteristics, formation types, and transmission schemes for the satellite clusters are highlighted. Simulation results evaluate the impact of clustering from coverage and capacity perspectives, showing that when the number of satellites is large, the performance of clustered networks outperforms the unclustered ones. The viable network architectures of the satellite cluster are proposed based on the 3rd Generation Partnership Project (3GPP) standard. Finally, the future applications of clustered satellite networks are discussed.

Estimating inter-satellite link Ka-band antenna phase center offsets to improve BDS-3 satellite orbit determination

Satellite antenna phase center offset (PCO) correction is essential not noly for L-band ground-tracking GNSS high-precision data processing, but also for Ka-band inter-satellite link (ISL) observations. The Ka-band ISL and L-band satellite-ground observations of 90 days from 25 November 2020 to 23 February 2021 are used for the Ka-band PCO estimation and precise orbit determination (POD) validation for BDS-3 satellites. The results show that the standard deviations of the estimated Ka-band PCOs for 29 BDS-3 satellites are 1.73, 2.63 and 3.58 cm in the X, Y and Z components, respectively. The accuracy of the Ka-band PCO estimates for medium Earth orbit satellites (MEO) is better than those of inclined geosynchronous orbit (IGSO) and geostationary Earth orbit (GEO) satellites due to a better observation geometry. The POD validation results indicate that using the estimated Ka-band PCOs can improve the ISL-based orbit determination performance. Using Ka-band ISL and L-band satellite-ground observations with 40 global stations for combined POD, the orbit overlap comparison shows that the average three-dimensional (3D) root mean square (RMS) error for 29 BDS-3 satellites is decreased from 12.6 to 6.6 cm with a reduction of 47.6%, and satellite laser ranging (SLR) validation for 4 BDS-3 satellites shows an accuracy improvement from 6 to 5 cm in RMS. When using ISL and only 7 regional ground stations are used, the average 3D RMS of orbit overlap errors is decreased from 15.9 to 10.9 cm with a reduction of 31.6%, and the RMS of SLR residuals for 4 BDS-3 satellites is decreased from 6.7 to 5.6 cm. For ISL-only POD, the average 3D RMS of orbit overlap errors is decreased from 18.0 to 12.4 cm with a reduction of 31.1%, and the RMS of SLR residuals for 4 BDS-3 satellites is decreased from 7.1 to 6.9 cm.

An on‐orbit calibration method for time delay of inter‐satellite link transponder in tracking and data relay satellite

AbstractThe tracking and data relay satellite (TDRS) in geosynchronous orbit (GEO) can provide ranging and communication services for user spacecraft in low Earth orbit (LEO) through the inter‐satellite link (ISL). The time delay of TDRS ISL transponder in orbit differs from its pre‐launch state and changes due to the complex space environment, adding significant errors to the user spacecraft's orbit determination based on four‐way ranging data. The authors have developed a calibration system and proposed an efficient on‐orbit calibration method for the ISL transponder's time delay in TDRS. Experimental results demonstrate the effectiveness of the authors’ method, and the RMS of calibration residuals is less than 0.5 m.

Wave optics approach to solar cell BRDF modeling with experimental results.

Light curve analysis is often used to discern information about satellites in geosynchronous orbits. Solar panels, comprising a large part of the satellite's body, contribute significantly to these light curves. Historically, theoretical bidirectional reflectance distribution functions (BRDFs) have failed to capture key features in the scattered light from solar panels. In recently published work, a new solar cell BRDF was developed by combining specular microfacet and "two-slit" diffraction terms to capture specular and periodic/array scattering, respectively. This BRDF was experimentally motivated and predicted many features of the solar cell scattered irradiance. However, the experiments that informed the BRDF were limited to a single laser wavelength, single beam size, and single solar cell sample. In addition, the BRDF was not physics based and therefore, physical insight into what causes certain features in the scattered irradiance was not evident. In this work, we examine solar cell scattering from first principles and derive a simple physics-based expression for the scattered irradiance. We analyze this expression and physically link terms to important scattering features, e.g., out-of-plane phenomena. In addition, we compare our model with experimental data and find good agreement in the locations and behaviors of these features. Our new model, being more predictive by nature, will allow for greater flexibility and accuracy when modeling reflection from solar cells in both real-world and experimental situations.

A Method for Measuring Gravitational Potential of Satellite’s Orbit Using Frequency Signal Transfer Technique between Satellites

We introduce an approach for the direct measurement of the gravitational potential (GP) along the trajectory of a satellite, with a specific focus on Low-Earth Orbit (LEO) satellites. A LEO satellite communicates with several Geosynchronous Equatorial Orbit (GEO) satellites via frequency signal links. The GP difference can be measured in real-time using the gravitational frequency shift approach by equipping both LEO and GEO satellites with precise atomic clocks. Since the GP at the high orbits of the GEO satellites can be precisely determined by the present gravitational field model EGM2008, the GP along the LEO satellite’s trajectory can be determined. In this study, simulation experiments were conducted, featuring a GRACE-type satellite as the LEO satellite in communication with three equidistant GEO satellites. The results indicated that the accuracy of the GP measurements along the LEO satellite’s trajectory primarily depends on the precision of the onboard atomic clocks. Supposing optical atomic clocks attain an instability level of 1×10−17τ−1/2 (τ in seconds), we determined the GP distribution covered by the LEO satellite’s trajectories with 30-day observations. Then, we determined a gravitational field at the centimeter level based on the GP distribution. The GP data derived from the trajectory of a LEO satellite can be utilized to establish temporal gravitational fields, which have broad applications in different disciplines.

The Extraction and Characterization of Pseudorange Multipath Based on BDS-3 Multi-Frequency Observations.

Global Navigation Satellite System (GNSS) observations are subject to various errors during their propagation process. A reasonable correction of these errors can improve the positioning, navigation, and timing (PNT) service capability. The impact of multipaths on pseudorange observations can reach a decimeters or even meters level. However, their mechanism is complex and there is currently no universally accepted high-precision correction model. The correlation between the pseudorange multipaths (MP) of BDS-2 satellites and satellite elevation has been confirmed, while there have been fewer analyses of the MP characteristics for different frequencies of BDS-3 satellites. The broadcasting of multi-frequency observations in BDS-3 should theoretically make the extracted MP more accurate compared to traditional methods. Based on this, in this contribution, a multi-frequency MP extraction algorithm based on the least squares principle is proposed, which can simultaneously eliminate the influence of higher-order ionospheric delay. The analytical expression for only eliminating first-order ionospheric delay is successfully derived. Subsequently, the characteristics of the MPs extracted from different frequency combinations and the impact of combination noise on the extraction accuracy are discussed. The influence of second-order ionospheric delay on the MPs for each frequency under different combination noises, as well as the periodic behavior exhibited in long-term observations of the BDS-3 medium earth orbit (MEO) and inclined geosynchronous orbit (IGSO) satellites, are also analyzed. Finally, the correlations between the MPs of each frequency of BDS satellite and elevation are quantitatively analyzed based on observations from 35 stations. Overall, this work has positive implications for the study of the MP characteristics of BDS-3 and subsequent modeling efforts.

High-bandwidth coherent optical communication over 10.3 km of turbulent air.

We demonstrate 111.8Gb/s coherent optical communication throughput over a 10.3km folded free-space laser range. Folded links are low complexity to establish and provide a high uptime for testing equipment. The communication signals were sourced from an un-modified commercial off-the-shelf transceiver intended for long-haul fiber networks. Wavelength dependence was explored by testing 52 optical C-band channels over the course of an evening. In the future, such high-bandwidth communications will be used in feeder links from satellites in geosynchronous orbit. Optical power measurements of the received signal are compared with atmospheric theory to determine the turbulence strength exhibited and therefore the applicability of the laser range to space-to-ground links. We show that the high-uptime, 10.3km laser range is suitable for testing high-bandwidth space-to-ground optical communication systems intended for links to geosynchronous orbit at 20°-50° elevation.