Low Earth Orbit Communication Research Articles

Analysis of multipath effects on Low Earth Orbit navigation satellites

Abstract With the large-scale emergence of Low-Earth Orbit (LEO) internet constellations, the direct or indirect utilization of LEO communication constellations for navigation and positioning has become a hot topic of current research. However, the multipath effect remains a critical factor influencing positioning accuracy. Compared to traditional Medium-Earth Orbit (MEO) satellites, LEO satellites move swiftly, resulting in faster multipath variations, which renders traditional multipath envelope analysis methods inapplicable. This study proposes a novel analytical model tailored to the multipath effect characteristics of LEO satellites. Through this model, we have observed that the multipath variations of high-frequency signals in LEO rapidly fluctuate near zero, offering the potential to reduce errors through smoothing methods. Furthermore, the study reveals that the impact of the multipath effect is positively correlated with orbital altitude and inversely correlated with carrier frequency. This study provides a new perspective for understanding the multipath effect characteristics of LEO satellites and offers necessary theoretical support and practical guidance for improving the accuracy of navigation and positioning systems based on LEO satellites. Future research can further explore multipath suppression methods and promote the widespread application of LEO satellites in the field of navigation and positioning.

Research on AIS algorithm based on factor graph inference in Doppler localization

Abstract In this paper, an adaptive incremental smoothing (AIS) algorithm based on factor graph inference is proposed to solve the problem of improving the Doppler positioning accuracy of low-Earth orbit communication satellites. Traditional methods such as iterative least squares (ILS) algorithm and Extended Kalman filter (EKF) have certain limitations in real-time Doppler localization. The AIS algorithm uses factor graph inference to identify interference factors, and adopts adaptive smoothing to reduce the impact of estimation errors and promote adaptive noise estimation. The ground station Starlink satellite constellation static positioning was taken as an example, and the simulation was carried out on the STK software simulation platform to verify the feasibility and effectiveness of the algorithm. The experimental results show that the positioning accuracy of the AIS algorithm based on factor graph inference is significantly improved compared with the ILS and EKF algorithms in real-time Doppler positioning solutions. The aim of this research is to improve the positioning accuracy of low-orbit satellites and provide a reference for practical applications.

Integrated precise orbit determination of GPS, BDS, and partial satellites selected from LEO constellation

Traditional precise orbit determination (POD) for Global Navigation Satellite System (GNSS) typically relies on a considerable number of global ground stations. The rapid development of large Low Earth Orbit (LEO) communication constellations in recent years presents an opportunity for integrated POD of LEO and GNSS satellites. The participation of abundant observations from onboard GNSS receivers in GNSS POD can significantly enhance the orbit precision, availability, and integrity of GNSS satellites. However, the improvement in orbit precision comes with a notable increase in computation time when a substantial number of LEO satellites are involved. In this study, we propose a method for selecting LEO satellites based on the LEO geometric dilution of precision (LGDOP) value. By identifying the LEO satellite combination corresponding to the minimum LGDOP, partial LEO satellites evenly distributed are selected from the entire LEO constellation. Using the semi-realistic simulation approach, only onboard observations of the LEO constellation are simulated to enhance GPS and BDS POD. The average accuracy of orbit determination for simulated LEO satellites is 2.27 cm in 3D directions, comparable to FengYun-3 but inferior to GRACE-FO. When onboard observations from 10 LEO satellites, selected using the minimum LGDOP method, participate in the integrated POD, the orbit precision can be improved by 10 %-14 % compared with the sequence selection method. Consequently, the LGDOP selection method optimizes the distribution of LEO satellites and demonstrates superior effectiveness in enhancing GPS and BDS POD with an equivalent number of LEO satellites. Involving 20–30 LEO satellites in the solution can enhance the orbit precision of GPS and BDS satellites by approximately 40 % and 80 % for global and regional ground stations, respectively. Increasing the number of participating LEO satellites further does not significantly improve the GPS and BDS orbit precision but does notably increase the computation time, which exceed 12 h for regional stations and 14 h for global stations. Thus, selecting approximately 20–30 LEO satellites using the proposed method effectively balances orbit determination precision and computation efficiency.

Multiple Nodes Co-Carrier Cooperative Transmission in LEO Communication Networks: Developing the Diversity Gain of Satellites.

Low Earth orbit (LEO) satellite communication (SATCOM) networks have gradually been recognized as an efficient solution to enhance ground-based wireless networks. As one of the main characteristics of LEO SATCOM, the beam-edge area could be covered by multiple satellite nodes. In this case, user terminals (UTs) located at the beam-edge have the chance to connect one or more LEO satellites. To develop the diversity gain of multiple nodes in the overlapping area, we propose two high spectral efficiency cooperative transmission strategies, i.e., directly combining (DC) and selection combining (SC). In the DC scheme, signals arrived at the UT simultaneously could be combined into one enhanced signal. For downlink time division multiplexing, the SC scheme enables the UT to select the strongest signal path. Further, as there exists a significant channel gain difference of the beam-center and beam-edge areas, UTs in these two areas can be allocated in one resource block. In this case, we derive co-carriers based on DC and SC, respectively. To deeply analyze the novel methods, we study the ergodic sum-rate and outage probability while the outage diversity gain is further provided. Simulation results show that the co-carrier-based DC method has the ability to provide a higher ergodic sum-rate while the SC method performs better in terms of the outage probability.

Analysis and Design of Starlink-like Satellite Constellation

Satellite constellations are widely used for communication, navigation, and Earth observation purposes. They provide good ground coverages and serve better for these needs. Of all the configurations, the Walker constellation is extensively applied in many navigation satellite systems and some low Earth orbit communication constellations, since it can be easily designed and has good coverage. Despite of these advantages, the satellites in Walker constellation generally have different ground tracks. When multiple Walker constellations are to be coordinated, in terms that the orbital planes precess synchronously with the same satellite mean motion Ω1, the semi-major axes a of these Walker constellations would be significantly different even when the orbital inclinations differ by a small amount. The Space Exploration Corp (SpaceX) claimed a new constellation design in a patent for their multi-shell Starlink satellite constellation. Constellation shells with different inclinations have small altitude differences, which facilitates regulatory approval and deployment. Satellites in the same shell can also be easily designed to share the same ground track. Although they claimed these features in the patent, SpaceX shared little technical details regarding how to design these constellations. Here in this paper, we analyze the features of the Starlink constellation, and try to find a practical approach to design a Starlink-like constellation, as well as how to determine the rules for inter-satellite links within the constellation.

Cross-Domain Fusion Constellation Design of Communication, Navigation and Remote Sensing

Low earth orbit (LEO) mega-constellations have once again triggered a wave of space-based system construction. On the one hand, LEO communication, LEO navigation, LEO remote sensing constellations and so on are proposed. On the other hand, with the continuous development of software-defined satellite and intelligent satellite technology, space-based systems are developing in the direction of multi-function, integration and cross-domain integration. The whole space-based system is no longer the traditional working mode of a single functional constellation, but a genral cross-domain fusion constellation (CDFC) system for complex tasks. Like the terrestrial global Internet, the space-based system will serve as a global infrastructure for integrating communication, navigation and remote sensing, that is, the intelligent space-based system, to provide services for the global demand. The traditional method of designing constellation for a certain type of function is no longer applicable to this type of constellation design. To solve this problem, this paper proposes a design and optimization method of cross-domain fusion constellation of communication, navigation and remote sensing based on reverse design. The paper optimizes the CDFC through resource coverage. Through experiments, we prove that the number of satellites in the CDFC can be reduced by 30.60% compared with the independent and combined constellations in each domain, and the coverage and service performance of the constellation can be improved. The cost can be reduced by 18.31% compared with the combined constellation. When the same number of satellites is used, the resource coverage of the cross-domain fusion constellation is increased by at least eight times.

An imperative role of 6G communication with perspective of industry 4.0: Challenges and research directions

The united nations have set a target of meeting sustainability by 2030 in terms of social, economic, and environmental. Digitalization has proven to be a significant approach to meeting the goal of sustainability. Along with digitalization and Industry 4.0, it was also stated that 6G communication is beneficial for lowering carbon emissions, effective management of natural resources, etc. Based on this motivation, this is the first study to attempt to present the significance of 6G communication from the perspective of Industry 4.0. The fundamental technologies of 6G wireless communications at the physical layer are emphasized, and also discussed enabling technologies of Industry 4.0 for 6G communication in detail. The recommendations and challenges are discussed for future directions. 6G for low earth orbit (LEO) communication and data center operations for 6G network nodes, and 6G network with aerial computing are the recommendation presented in the study. The novelty of study is, it integrates and discussed the sustainability aspect, the vision of the 6G network with fundamental technologies, and Industry 4.0 in detailed. The recommendations and challenges presented in the study motivate the researchers to carry out the study for future research in implementing 6G communication for achieving sustainability with industry 4.0.

Very Compact Waveguide Orthomode Transducer in the K-Band for Broadband Communication Satellite Array Antennas

A very compact waveguide orthomode transducer (OMT) is described in this paper. The design is characterized with a twofold rotationally symmetric cross-section in the probing area, adapted from a side-coupling OMT design, simultaneously enabling low port-to-port coupling and high cross-polarization discrimination (XPD) over a fractional bandwidth of about 15-20%. Compared to previously reported compact waveguide OMTs, the proposed design is simpler, thus facilitating its manufacture at millimeter-wave frequencies. The concept is demonstrated with a design in the K-band for a broadband communication satellite downlink over the frequency band of 17.3-20.2 GHz. For test purposes, transitions to standard waveguide WR42 are included, and the OMT is assembled with a conical horn antenna. The measured reflection and coupling coefficients are below -19.5 dB and -22.9 dB, respectively, over the nominal bandwidth, and they are in good agreement with the simulation's results. The on-axis XPD, measured in an anechoic chamber, is better than 30 dB over the nominal bandwidth, which is also in line with simulations. The proposed waveguide OMT may be designed to fit in a lattice below one wavelength at the highest operating frequency, which is desirable for dual-polarized closely spaced array antennas in low and medium Earth orbit communication satellite systems. The simple mechanical design of the proposed OMT makes it particularly appealing for additive manufacturing techniques, as demonstrated with a variant of the design having folded single-mode waveguides, which preserves the RF properties of the original design.

Multi-parameter influenced acquisition model with an in-orbit jitter for inter-satellite laser communication of the LCES system.

With the development of large low earth orbit (LEO) communication constellations, the efficiency of laser inter-satellite link (ISL) establishing become the bottleneck for subsequent large-scale launch and rapid networking applications of LEO communication constellations. Hence, we establish the pointing jitter error structure of LEO communication experiment satellites (LCES) system. The error structure can be used to trace the source of errors and evaluate the in-orbit jitter. And we derive an analytical expression of the acquisition probability density function (PDF) which comprehensively considering the influence of the scanning region, the pointing jitter error, the overlap factor and the in-orbit jitter error. The multi-parameter influenced acquisition model is validated by Monte Carlo (MC) simulations and semi-physical tests. The results reveals that the multi-parameter influenced acquisition model can be used to guide the in-orbit ISL establishing.

Nonhomogeneous Stochastic Geometry Analysis of Massive LEO Communication Constellations

Providing truly ubiquitous connectivity requires development of low Earth orbit (LEO) satellite Internet, whose theoretical study is lagging behind network-specific simulations. In this paper, we derive analytical expressions for the downlink coverage probability and average data rate of a massive inclined LEO constellation in terms of total interference power’s Laplace transform in the presence of fading and shadowing, ergo presenting a stochastic geometry-based analysis. We assume the desired link to experience Nakagami- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$m$ </tex-math></inline-formula> fading, which serves to represent different fading scenarios by varying integer <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$m$ </tex-math></inline-formula> , while the interfering channels can follow any fading model without an effect on analytical tractability. To take into account the inherent non-uniform distribution of satellites across different latitudes, we model the LEO network as a nonhomogeneous Poisson point process with its intensity being a function of satellites’ actual distribution in terms of constellation size, the altitude of the constellation, and the inclination of orbital planes. From the numerical results, we observe optimum points for both the constellation altitude and the number of orthogonal frequency channels; interestingly, the optimum user’s latitude is greater than the inclination angle due to the presence of fewer interfering satellites. Overall, the presented study facilitates general stochastic evaluation and planning of satellite Internet constellations without specific orbital simulations or tracking data on satellites’ exact positions in space.

In-flight performance analysis of the navigation augmentation payload on LEO communication satellite: a preliminary study on WT01 mission

ABSTRACT With the development of the Low Earth Orbit (LEO) communication constellations, it has become a hot area of research to provide additional navigation augmentation services. Limited by volume, weight, power consumption, and running time, the in-flight performance of navigation augmentation payload remains to be investigated. In this paper, we analyze the data quality of on-board GNSS observation and evaluate the precision of short-arc dynamic Precise Orbit Determination (POD) performance based on the WangTong-01 (WT01) mission. Furthermore, the downlink navigation measurement data of WT01 satellites are analyzed and compared with the GNSS observations. The results show that the average multipath errors of the WT01 on-board GPS L1, L2 and BeiDou Satellite Navigation System (BDS) B1, B2 code observation are 0.54, 0.74, 0.65, and 0.58 m, respectively. The short-arc dynamic POD three-dimensional (3D) overlapping accuracy is 7.1 cm. The average multipath errors of downlink navigation signal Z1 and Z2 are 0.81 and 0.80 m, respectively, which at the same order of magnitude as GNSS signals. The maximum Carrier-to-Noise Ratio (C/N0) value of WT01 downlink measurement data can reach 60 dB Hz, which is much stronger than GNSS and indicates the navigation signals of LEO satellites can meet the basic requirement of navigation augmentation.

Link Budget Analysis for LEO Satellites Based on the Statistics of the Elevation Angle

Link budgets are widely applied to evaluate communication links for low Earth Orbit (LEO) satellites. However, approaches to calculate the received power from LEO satellites have followed similar procedures to those for Geostationary (GEO) satellites and other fixed distance wireless systems, ignoring the satellite mobility that causes continuous changes in the path length and in the elevation angle. Link budgets found in the literature for LEO communication systems have commonly opted to characterize the best and worst-cases of the received signal; however, this common approach tells little about how often those cases occur, and little can be inferred about the expected received power and its measures of dispersion. This article introduces an innovative methodology to evaluate LEO link budgets using the long term statistics and probabilities of occurrence of the elevation angle, which is characterized in this work through a random variable. This characterization of the elevation angle through a random variable allows the calculation of the expected value of the received power, its standard deviation, quantiles, among other quantities. The received power is essential to calculate other link indicators such as the carrier-to-noise ratio (C/N), and with the proposed methodology it can be now calculated considering the path length and elevation angle variability that occur for LEO satellites.

Модели массового обслуживания с простейшими потоками для низкоорбитальной спутниковой системы

The relevance of the work is associated with the active deployment of low-orbit communication systems and the expansion of research in the field of corresponding satellite systems. A promising low-orbit communication system based on relay satellites with the function (RSRFs) of routing message packets is considered. The low earth orbit communications systems use the BGP protocol and the AAA functionality at the ground station. For assessing the characteristics of RSRF inter-satellite paths, a scenario was created for the message packets arrival from a group of inter-satellite paths to one subscriber path. The corresponding analytical models have been developed using the mathematical apparatus of queuing systems with the simplest flows of requests and exponential distribution of the service time. The RSRF characteristics of a promising low-orbit communication system are predicted. It is proposed to make the mathematical apparatus of analytical models more complicated to take into account the dynamics of displacements and failures of the RSRF in a low-orbit communication system.

Paradigm Shift in the Satellite Communications Sector and its Implications for Low-Earth Orbit Communications

Paradigm Shift in the Satellite Communications Sector and its Implications for Low-Earth Orbit Communications

Exclusion zone minimization and optimal operational mode selection for co‐existent geostationary and non‐geostationary satellites

SummaryThe number of satellites has been increasing in both geostationary (GEO) and non‐geostationary (NGEO) earth orbits. Due to the limited availability of spectrum resources, the interference risk among these satellite networks has been increasing consequently. In such a scenario, the protection of existent GEO transmissions is crucial. In this paper, the co‐existence downlink interference from a typical low earth orbit (LEO) constellation to earth stations of GEO satellites is examined for minimization of exclusion zone on the equatorial region. Two different operational scenario based on modulation and coding (MODCOD) with/without spread spectrum for the LEO system are considered. A multiobjective optimization problem (MOP) is formulated for nondominant solutions set based on exclusive angle minimization and bandwidth utilization of the LEO link. It is shown that the exclusive angle can be reduced up to 21.3% and 19.6%, compared with the initial anchor point at the transmission bit rates of 100 and 200 Mbps, respectively. The proposed optimal operational setting minimizes the interference risk to the GEO satellite network as well as maintains quality of service (QoS) for the LEO communication network. The results provide optimal operational mode selection for LEO satellite operators and/or decision makers.

Optical-to-NIR magnitude measurements of the Starlink LEO Darksat satellite and effectiveness of the darkening treatment

Aims. We aim to measure the Sloan r′, Sloan i′, J, and Ks magnitudes of Starlink’s STARLINK-1130 (Darksat) and STARLINK-1113 low Earth orbit (LEO) communication satellites and determine the effectiveness of the Darksat darkening treatment from the optical to the near-infrared (NIR). Methods. Four observations of Starlink’s LEO communication satellites, Darksat and STARLINK-1113, were conducted on two nights with two telescopes. The Chakana 0.6 m telescope at the Ckoirama observatory (Chile) observed both satellites on 5 Mar. 2020 (UTC) and 7 Mar. 2020 (UTC) using a Sloan r′ and Sloan i′ filter, respectively. The ESO VISTA 4.1 m telescope with the VIRCAM instrument observed both satellites on 5 Mar. 2020 (UTC) and 7 Mar. 2020 (UTC) in the NIR J-band and Ks-band, respectively. Results. The calibration, image processing, and analysis of the Darksat images give r ≈ 5.6 mag, i ≈ 5.0 mag, J ≈ 4.2 mag, and Ks ≈ 4.0 mag when scaled to a range of 550 km (airmass = 1) and corrected for the solar incidence and observer phase angles. In comparison, the STARLINK-1113 images give r ≈ 4.9 mag, i ≈ 4.4 mag, J ≈ 3.8 mag, and Ks ≈ 3.6 mag when corrected for range, solar incidence, and observer phase angles. The data and results presented in this work show that the special darkening coating used by Starlink for Darksat has darkened the Sloan r′ magnitude by 50%, Sloan i′ magnitude by 42%, NIR J magnitude by 32%, and NIR Ks magnitude by 28%. Conclusions. The results show that both satellites increase in reflective brightness with increasing wavelength and that the effectiveness of the darkening treatment is reduced at longer wavelengths. This shows that the mitigation strategies being developed by Starlink and other LEO satellite operators need to take into account other wavelengths, not just the optical. This work highlights the continued importance of obtaining multi-wavelength observations of many different LEO satellites in order to characterise their reflective properties and to aid the community in developing impact simulations and developing mitigation tools.

First observations and magnitude measurement of Starlink’s Darksat

Aims. We measured the Sloan g′ magnitudes of the Starlink’s STARLINK-1130 (Darksat) and 1113 low Earth orbit (LEO) communication satellites to determine the effectiveness of the Darksat darkening treatment at 475.4 nm. Methods. Two observations of the Starlink’s Darksat LEO communication satellite were conducted on 2020/02/08 and 2020/03/06 using Sloan r′ and g′ filters, respectively. A second satellite, STARLINK-1113, was observed on 2020/03/06 using a Sloan g′ filter. The initial observation on 2020/02/08 was a test observation conducted when Darksat was still in the process of manoeuvring to its nominal orbit and orientation. Based on the successful test observation, the first main observation took place on 2020/03/06, along with an observation of the second Starlink satellite. Results. The calibration, image processing, and analysis of the Darksat Sloan g′ image gives an estimated Sloan g′ magnitude of 7.46 ± 0.04 at a range of 976.50 km. For STARLINK-1113, an estimated Sloan g′ magnitude of 6.59 ± 0.05 at a range of 941.62 km was found. When scaled to a range of 550 km and corrected for the solar and observer phase angles, a reduction by a factor of two is seen in the reflected solar flux between Darksat and STARLINK-1113. Conclusions. The data and results presented in this work demonstrate that the special darkening coating used by Starlink for Darksat has darkened the Sloan g’ magnitude by 0.77 ± 0.05 mag when the range is equal to a nominal orbital height (550 km). This result will serve members of the astronomical community who are actively modelling the satellite mega-constellations to ascertain their actual impact on both amateur and professional astronomical observations. Both concurrent and subsequent observations are planned to cover the full optical and NIR spectrum using an ensemble of instruments, telescopes, and observatories.

LEO-Augmented GNSS Based on Communication Navigation Integrated Signal.

Low Earth Orbit (LEO) is of great benefit for the positioning performance of Global Navigation Satellite System (GNSS). To realize the system of LEO-augmented GNSS, three methods to integrate communication and navigation signal for LEO communication system with the least influence on the communication performance are analyzed. The analysis adopts the parameters of IRIDIUM signal as restrictions. This paper gives quantitative comparison of these methods considering CN0(carrier noise power spectral density rate) margin, pseudorange accuracy, Doppler accuracy, and communication loss. For method 1, a low-power navigation signal is added to the communication signal. For method 2, the navigation signal is launched in one or more frames. For method 3, the navigation signal is launched in the frequency band separated to the communication signal. The result shows that the pseudorange accuracy of method 2 is far below method 1 and method 3. However, the difference of Doppler accuracy among the three methods can be emitted. Detailed analysis shows that method 1 is practicable when the communication and navigation signal power rate is 15 dB. It achieves the balance of pseudorange accuracy and bit error rate (BER) performance under this condition. Comprehensive comparison of these methods is given in the last. The result shows that the CN0 margin of the navigation signal for method 3 can be 13.04 dB higher than method 1, based on the accuracy threshold considered in this paper. Methods 1 and 3 have the advantage of high accuracy and high CN0 margin respectively. However, method 3 causes high communication capacity loss. Considering that the main disadvantage of GNSS signals is low CN0, method 3 is a good choice for the LEO-augmented GNSS system. Methods 1 and 3 can be combined to realize both high accuracy and high CN0 margin if possible.

Beam Coverage Comparison of LEO Satellite Systems Based on User Diversification

To achieve global network coverage and the need for high-speed communication, the idea of providing Internet access from space has made a strong comeback in recent years. The low earth orbit (LEO) communication satellite constellation is once again on the stage of the world with its unique features and new technology. In order to provide faster and more affordable communication resources, low-orbit satellites need be customized to design satellites. The beam coverage design is essential to the user-customized design. This paper combines the user traffic demand model and the low-orbit satellite beam coverage model to analyze the impact of beam coverage characteristics on the performance of low-orbit satellite systems. The user traffic model bases on the user simulative distribution (uniform, normal) and the user geographic distribution (according to the AIS and ADS-B historical data acquired by STU-2B and STU-2C which are the LEO satellites launched in Sep, 2015, Jiuquan, China). The beam coverage model compares the OneWeb system to the SpaceX system. The beam coverage model takes the variability in performance induced by atmospheric conditions for the user links into account. Follow that this paper proposes a system method to simulate the two satellite system which described by the throughput, delay, access probability. Finally, the sensitivity of beam coverage to user diversification is summarized and discussed.

A Focal Curve Design Method for Rotman Lenses With Wider Angular Scanning Range

This letter describes a novel focal curve design method for two-dimensional Rotman lenses. The proposed design method offers some additional degrees of freedom in the design of such beamformers and enables to extend significantly their angular scanning range, providing a solution to overcome the resulting focal arc and inner focal lens contour obstruction. A specific design, based on a phase-only model, is described, and numerical results are discussed. Good scanning performance over a very wide angle (± 50°) is demonstrated with phase-aberrations one order of magnitude lower than those of an equivalent standard Rotman lens. This improved beamformer design is of interest for low Earth orbit communication satellite antennas with a field-of-view ranging typically from ±40° to ±60°.