A Review on Precise Orbit Determination of Various LEO Satellites

The need for precise orbit determination (POD) has grown significantly due to the increased amount of space-based activities appearing at an accelerating pace. POD has a positive contribution in achieving the requirements of Low-Earth Orbit (LEO) satellite mission which includes improved reliability and continuity. In this paper, we will review the POD approaches of various LEO satellites and discuss the accuracy levels obtained as well as the methods and algorithms used to achieve the POD of LEO satellites. With recent advancements in miniature space technology, a greater number of smaller low-cost satellites are launched into the LEO for various purposes. Furthermore, development in the Global Navigation Satellite Systems (GNSS) and chipsets played a vital role in revolutionizing the GNSS receiver technology. Lower-cost, smaller size but yet high performing GNSS receivers need to be implemented also in CubeSats in addition to the various terrestrial applications. POD using onboard GNSS receiver data will benefit the development of several upcoming space applications in the field of navigation systems, telecommunication, remote sensing, and earth observation. In the future, it is anticipated that LEO-based satellites enabled by POD can also offer positioning capabilities that will enhance GNSS and create vast opportunities for users with new features and possibilities to the navigation field.© 2021 Copyright for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0). CEUR Workshop Proceedings (CEUR-WS.org)fi=vertaisarvioitu|en=peerReviewed

Precise orbit determination of LEO satellites : a systematic review

The need for precise orbit determination (POD) has grown significantly due to the increased amount of space-based activities taking place at an accelerating pace. Accurate POD positively contributes to achieving the requirements of Low-Earth Orbit (LEO) satellite missions, including improved tracking, reliability and continuity. This research aims to systematically analyze the LEO–POD in four aspects: (i) data sources used; (ii) POD technique implemented; (iii) validation method applied; (iv) accuracy level obtained. We also present the most used GNSS systems, satellite missions, processing procedures and ephemeris. The review includes studies on LEO–POD algorithms/methods and software published in the last two decades (2000–2021). To this end, 137 primary studies relevant to achieving the objective of this research were identified. After the investigation of these primary studies, it was found that several types of POD techniques have been employed in the POD of LEO satellites, with a clear trend observed for techniques using reduced-dynamic model, least-squares solvers, dual-frequency signals with undifferenced phase and code observations in post-processing mode. This review provides an understanding of the various POD techniques, dataset utilized, validation techniques, and accuracy level of LEO satellites, which have interest to developers of small satellites, new researchers and practitioners.© The Author(s) 2023. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.fi=vertaisarvioitu|en=peerReviewed

Orbit determination of Sentinel-6A using the Galileo high accuracy service test signal

The High Accuracy Service (HAS) is an upcoming addition to the Galileo service portfolio that offers free correction data for precise point positioning in real-time. Beyond terrestrial and aeronautical applications, precise orbit determination (POD) of satellites in low Earth orbit (LEO) has been proposed as a potential use case for HAS corrections in view of their global availability. Based on HAS data collected during a test campaign in September 2021, the benefit of HAS corrections is assessed for real-time, onboard navigation as well as near real-time POD on the ground using GNSS observations of the Sentinel-6A LEO satellite. Compared to real-time POD using only broadcast ephemerides, performance improvements of about 40%, 10%, and 5% in terms of 3D position error can already be achieved for GPS-only, GPS + Galileo, and Galileo-only navigation. While Galileo processing benefits only moderately from the HAS correction data during the early tests in view of an already excellent Open Service performance, their use is highly advantageous for GPS processing and enables dual-constellation navigation with balanced contributions of both GNSSs for improved robustness. For near real-time offline POD, HAS corrections offer reduced latency or accuracy compared to established ultra-rapid GNSS orbit and clock products as well as independence from external sources

Precise Onboard Time Synchronization for LEO Satellites

Onboard time synchronization is an important requirement for a wide range of low Earth orbit (LEO) missions such as altimetry or communication services, and extends to future position, navigation, and timing (PNT) services in LEO. For GNSS-based time synchronization, continuous knowledge about the satellite's position is required and, eventually, the quality of the position solution defines the timing precision attainable through GNSS measurements. Previous research has shown that real-time GNSS orbit determination of LEO satellites can achieve decimeter-level accuracy. This paper characterizes the performance of GNSS-based real-time clock synchronization in LEO using the satellite Sentinel-6A as a real-world case study. The satellite's ultra-stable oscillator (USO) and triple-frequency GPS/Galileo receiver provide measurements for a navigation filter representative of real-time onboard processing. Continuous evaluation of actual flight data over 14 days shows that a 3D orbit root-mean-square (RMS) error of 11 cm and a 0.9-ns clock standard deviation can be achieved

Precise Orbit Determination of CubeSats

CubeSats are faced with some limitations, mainly due to the limited onboard power and the quality of the onboard sensors. These limitations significantly reduce CubeSats' applicability in space missions requiring high orbital accuracy. This thesis first investigates the limitations in the precise orbit determination of CubeSats and next develops algorithms and remedies to reach high orbital and clock accuracies. The outputs would help in increasing CubeSats' applicability in future space missions

Performance Assessment of Navigation Using Carrier Doppler Measurements from Multiple LEO Constellations

The goal of this work is to characterize a novel navigation method which uses carrier Doppler shift measurements from LEO satellites. An ever-growing reliance on the GNSS has coincided with an increase in ways it can be degraded or denied, whether naturally occurring or man-made. These potentially disastrous threats to traditional navigation and timing have necessitated new technologies to augment GNSS in the case of an outage. LEO constellations, whose size and higher signal power make them potentially useful for navigation, are one technology that has been explored. The navigation algorithms detailed in this research use Doppler measurements from 8 or more LEO satellites to simultaneously solve for position, clock offset, velocity, and clock offset rate. Through simulation, a user-satellite geometry analysis is conducted for a number of emerging LEO constellations, as well as navigation simulations with the same constellations. Results are presented which show promise from both a satellite geometry perspective and PVT solution convergence perspective

Investigation into the accuracy of single frequency precise point positioning (PPP)

This thesis investigates the major errors and processes affecting the performance of a viable, standalone point positioning technique known as single frequency Precise Point Positioning (PPP). The PPP processing utilises both single frequency code and carrier phase GPS observables. The mathematical model implemented is known as the code and quasi-phase combination. Effective measures to improve the quality of the positioning solutions are assessed and proposed. The a priori observations sigma (or standard deviation) ratio in the sequential least squares adjustment model plays a significant role in determining the accuracy and precision of the estimated solutions, as well as the solutions convergence time. An "optimal" observations sigma ratio is found using an empirical approach, whereby different sigma ratios are tested and evaluated. It is concluded that an a priori code and quasi-phase sigma ratio of 1:50 provides optimal performance irrespective of the ionospheric conditions and the location of the GPS receiver. This is an innovative attribute of the research. The feasibility of using Regional Ionosphere Maps (RIMs) to improve the accuracy of the single frequency PPP solutions is also examined. The performance of the RIMs is evaluated as a function of geographical locations and different ionospheric conditions. The quality of the estimated positioning solutions based on the RIMs is then compared to those using the Broadcast model and the Global Ionosphere Maps. It is concluded that the RIMs are advantageous for GPS stations located in the low latitude regions and also during periods of high ionospheric activity. The single frequency PPP solutions convergence is investigated with respect to i) satellite clock corrections at different sampling rates, ii) varying observation sampling intervals, and iii) the different tropospheric delay mitigation methods. It is found that the clock corrections and observations sampling intervals have minimal impacts on the solutions convergence time. However, in order to improve the time of convergence, the use of a modelled tropospheric delay (instead of estimating the tropospheric delay as part of the solutions) is recommended. The viability of using the various International GNSS Service (IGS) satellite orbit and clock corrections in single frequency PPP processing, particularly the near real-time and real-time products, is evaluated. The outcomes of this study demonstrate the potential benefits of the near real-time and real-time corrections for high accuracy point positioning. Numerical validations have been carried out using GPS data collected from different receiver types and qualities, i.e. geodetic grade, medium-cost, and low-cost receivers. The results suggest that single frequency PPP has the potential to provide 0.1m to 0.9m point positioning accuracy in post-processing mode. For real-time scenario, point positioning accuracy of about 1m to 2m can be expected. Despite the encouraging results, PPP is a challenging positioning technique and users should be aware of its limitations. The accuracy of the PPP solutions is dependent on the quality of the GPS measurements and corrections products used, as well as the capacity of the processing engine. It is anticipated this research will provide valuable guidelines for high accuracy point positioning using a single frequency GPS receiver

A Sensitivity Study of POD Using Dual-Frequency GPS for CubeSats Data Limitation and Resources

Making use of dual-frequency (DF) global navigation satellite system (GNSS) observations and good dynamic models, the precise orbit determination (POD) for the satellites on low earth orbits has been intensively investigated in the last decades and has achieved an accuracy of centimeters. With the rapidly increasing number of the CubeSat missions in recent years, the POD of CubeSats were also attempted with combined dynamic models and GNSS DF observations. While comprehensive dynamic models are allowed to be used in the postprocessing mode, strong constraints on the data completeness, continuity, and restricted resources due to the power and size limits of CubeSats still hamper the high-accuracy POD. An analysis of these constraints and their impact on the achievable orbital accuracy thus needs to be considered in the planning phase. In this study, with the focus put on the use of DF GNSS data in postprocessing CubeSat POD, a detailed sensitivity analysis of the orbital accuracy was performed w.r.t. the data continuity, completeness, observation sampling interval, latency requirements, availability of the attitude information, and arc length. It is found that the overlapping of several constraints often causes a relatively large degradation in the orbital accuracy, especially when one of the constraints is related to a low duty-cycle of, e.g., below 40% of time. Assuming that the GNSS data is properly tracked except for the assumed constraints, and using the International GNSS Service (IGS) final products or products from the IGS real-time service, the 3D orbital accuracy for arcs of 6 h to 24 h should generally be within or around 1 dm, provided that the limitation on data is not too severe, i.e., with a duty-cycle not lower than 40% and an observation sampling interval not larger than 60 s