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A special issue of Remote Sensing (ISSN 2072-4292). This special issue belongs to the section "Satellite Missions for Earth and Planetary Exploration".

Deadline for manuscript submissions: 15 February 2025 | Viewed by 19284

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Special Issue Editors

Prof. Dr. Kan Wang

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Guest Editor

1. National Time Service Center, Chinese Academy of Sciences, Xi’an 710600, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
3. Key Laboratory of Time Reference and Applications, Chinese Academy of Sciences, Xi’an 710600, China
Interests: high-precision GNSS positioning and navigation; LEO POD and clock determination; LEO-augmented PNT services; integrity monitoring; PPP-RTK
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Ahmed El-Mowafy

E-Mail Website
Guest Editor

School of Earth and Planetary Sciences, Curtin University, Bentley, WA 6102, Australia
Interests: positioning and navigation using GNSS; precise point positioning; integration of GNSS with other sensors; integrity monitoring and quality control; estimation theory

Dr. Xuhai Yang

E-Mail Website
Guest Editor

1. National Time Service Center, Chinese Academy of Sciences, Xi’an 710600, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
Interests: high-precision time and frequency transfer; orbit measurement; precise orbit determination

Special Issue Information

Dear Colleagues,

With tens of thousands of LEO satellites launched or to be launched in the coming decades, the LEO augmentation in the ground-based Positioning, Navigation and Timing (PNT) service has become a hot topic in recent years. Compared with MEO or GEO satellites of different GNSSs, the LEO-augmented PNT service can benefit from the increased satellite numbers, the strengthened signal strength, and the rapid geometry change, which improves the estimation precision, enlarges the service area, and shortens of convergence time of, e.g., precise point positioning (PPP). In this Special Issue, we aim to study diverse components that contribute to the LEO-augmented PNT service, including (but not limited to) signal processing and analysis, sensor calibration/validation, and determination and prediction of the LEO satellite orbits and clocks. In addition, this Special Issue aims to explore the possible benefits of the PNT service and its integrity monitoring brought by the LEO mega-constellations, including strategy design, algorithm development, and data analysis based on simulations and experiments.

In this Special Issue, original research articles and reviews are welcome. Research areas may include (but are not limited to) the following:

LEO POD and prediction;
Integrated LEO-GNSS POD;
LEO satellite clock analysis and prediction;
LEO-augmented GNSS positioning and navigation;
Frequency and time transfer under LEO augmentation;
LEO signal processing and analysis;
Integrity monitoring of LEO satellite orbits and clocks;
Integrity monitoring of LEO-augmented ground-based positioning;
Quality control of LEO signals;
Constellation design of LEO satellites;
Calibration/validation of spaceborne and terrestrial sensors related to LEO satellite signals.

We look forward to receiving your contributions.

Dr. Kan Wang
Prof. Dr. Ahmed El-Mowafy
Dr. Xuhai Yang
Guest Editors

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Keywords

Low Earth orbit (LEO)
Precise orbit determination (POD)
Time and frequency transfer
Positioning
Navigation
Integrity monitoring
Quality control
Signal processing
Sensor calibration

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Published Papers (7 papers)

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24 pages, 8658 KiB

Open AccessArticle

Effects of Topside Ionosphere Modeling Parameters on Differential Code Bias (DCB) Estimation Using LEO Satellite Observations

by Yifan Wang, Mingming Liu, Yunbin Yuan, Guofang Wang and Hao Geng

Remote Sens. 2023, 15(22), 5335; https://doi.org/10.3390/rs15225335 - 13 Nov 2023

Viewed by 957

Abstract

Given the potential of low-earth orbit (LEO) satellites in terms of navigation enhancement, accurately estimating the differential code bias (DCB) of GNSS satellites and LEO satellites is an important research topic. In this study, to obtain accurate DCB estimates, the effects of vertical total electron content (VTEC) modeling parameters of the topside ionosphere on DCB estimation were investigated using LEO observations for the first time. Different modeling parameters were set in the DCB estimations, encompassing modeling spacing in the dynamic temporal mode and degree and order (D&O) in spherical harmonic modeling. The DCB precisions were then evaluated, and the impacts were analyzed. Thus, a number of crucial and beneficial conclusions are drawn: (1) The maximum differences in the GPS DCB estimates after adopting different modeling spacings and different D&Os exhibit that the different modeling spacings or D&Os both affect the GPS DCB estimates and their root-mean square (RMS), and the effects of the two are at the same level. (2) The maximum differences in receiver DCBs using different modeling spacings indicate that the modeling spacing has a significant impact on the receiver DCBs, compared with GPS DCBs. Whereas, the maximum differences in receiver DCBs with different modeling D&Os are inferior to the differences in the GPS DCBs. That is, the modeling spacing has a greater impact on the LEO DCBs than those of the modeling D&O. (3) The experimental results indicate that the GPS DCB estimates using a modeling spacing of 12H have higher precisions than the others, whereas LEO receiver DCBs using a spacing of 4H or 6H obtain optimal STD. In terms of modeling D&O, adopting 8D&O in the LEO-based VTEC modeling can attain superior estimates and precisions for both GPS and LEO DCBs. The research conclusions can provide references for LEO-augmented DCB estimation. Full article

(This article belongs to the Special Issue LEO-Augmented PNT Service)

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22 pages, 7949 KiB

Open AccessArticle

LEO Satellite Clock Modeling and Its Benefits for LEO Kinematic POD

by Kan Wang, Ahmed El-Mowafy and Xuhai Yang

Remote Sens. 2023, 15(12), 3149; https://doi.org/10.3390/rs15123149 - 16 Jun 2023

Cited by 1 | Viewed by 1776

Abstract

High-accuracy Low Earth Orbit (LEO) satellite clock and orbital products are preconditions to realize LEO augmentation for high-accuracy GNSS-based positioning on the ground. There is a high correlation between the orbit and clock parameters in the kinematic Precise Orbit Determination (POD) process. While future LEO satellites are planned to be equipped with better clocks, the benefits of modeling high-stability LEO satellite clocks are not yet thoroughly investigated, particularly when mid- to long-term systematic effects induced by the complex LEO relativistic effects and the external environment remain in the clocks. Through clock modeling, this study attempts to reduce not only the short-term noise of radial kinematic orbits, but also mis-modeled effects caused by, e.g., real-time GNSS orbital and clock errors. To explore the benefits of clock modeling, the clocks need to be first detrended by the mid- to long-term systematic effects. While over-detrending limits the orbital improvements, weak detrending would also hamper strong clock modeling and easily lead to performance degradations. A balance between the strengths of the detrending and the model thus needs to be investigated for different clock types. In this study, the Piece-Wise Linear (PWL) model of different time lengths and a 2.5-state filter with different strengths (h values) are tested using real data from GRACE FO-1 with an Ultra-Stable Oscillator (USO) on board. Using the CNES real-time GPS products, it was found that when detrending the clocks with a smoothing window of 300 to 500 s, one could generally expect an improvement larger than 10% in the estimation of radial orbits when applying a PWL model with a length from 300 to 1200 s. Improvements of this size can also be expected when using the 2.5-state model with h₋₁ (for Flicker Frequency Noise) from 10⁻²⁸ to 10⁻³⁰. Full article

(This article belongs to the Special Issue LEO-Augmented PNT Service)

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21 pages, 4498 KiB

Open AccessArticle

Real-Time LEO Satellite Orbits Based on Batch Least-Squares Orbit Determination with Short-Term Orbit Prediction

by Kan Wang, Jiawei Liu, Hang Su, Ahmed El-Mowafy and Xuhai Yang

Remote Sens. 2023, 15(1), 133; https://doi.org/10.3390/rs15010133 - 26 Dec 2022

Cited by 12 | Viewed by 3101

Abstract

The augmentation of the Global Navigation Satellite System (GNSS) by Low Earth Orbit (LEO) satellites is proposed as an effective method to improve the precision and shorten the convergence time of Precise Point Positioning (PPP). Serving as navigation satellites in the future, LEO satellites need to be provided with their high-accuracy orbits in real-time. This would potentially enable the high-accuracy real-time LEO satellite clock determination, and eventually facilitate the high-accuracy ground-based positioning. Studies have been performed to achieve such real-time orbits using a Kalman filter in both the kinematic and reduced-dynamic modes. Batch Least-Squares (BLS) adjustment delivers more stable orbits in near-real-time, as it performs better phase screening. However, it suffers from longer delays compared to the Kalman filter. With the LEO satellite orbit prediction strategies improved over time, this latency can be bridged by short-term orbit prediction. In this study, using real-time GNSS satellite products, the real-time LEO satellite orbits are obtained based on the batch least-squares adjustment and short-term prediction. LEO ephemeris parameters are generated within specific prediction time windows. Using real data from the 500 km GRACE C satellite and 810 km Sentinel-3B satellite, the near-real-time BLS Precise Orbit Determination (POD) results exhibit good accuracy with an Orbital User Range Error (OURE) of 2–4 cm using different real-time GNSS products. A range of delays of the BLS POD processes are assumed, based on tests performed on different processing machines, leading to various prediction windows, from 3–8 min to 12–17 min that correspond to the real-time usage. The orbital prediction errors are shown to be highly correlated with the orbital height and the prediction time. The computational efficiency thus becomes essential to reduce the prediction errors for a certain LEO satellite. For advanced processing units leading to a prediction window shorter or equal to 6–11 min, one can expect a total real-time orbital error budget of 3–5 cm, provided that an appropriate prediction strategy is applied and high-quality GNSS products are used. For a given fitting interval, the ephemeris fitting errors are generally related to the number of ephemeris parameters and the orbital height. Compared with the prediction errors, the ephemeris fitting errors do not play a significant role in the total error budget when using 22 ephemeris parameters. Full article

(This article belongs to the Special Issue LEO-Augmented PNT Service)

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22 pages, 7500 KiB

Open AccessArticle

Integrity Monitoring of PPP-RTK Positioning; Part II: LEO Augmentation

by Kan Wang, Ahmed El-Mowafy, Wei Wang, Long Yang and Xuhai Yang

Remote Sens. 2022, 14(7), 1599; https://doi.org/10.3390/rs14071599 - 26 Mar 2022

Cited by 15 | Viewed by 3052

Abstract

Low Earth orbit (LEO) satellites benefit future ground-based positioning with their high number, strong signal strength and high speed. The rapid geometry change with the LEO augmentation offers acceleration of the convergence of the precision point positioning (PPP) solution. This contribution discusses the influences of the LEO augmentation on the precise point positioning—real-time kinematic (PPP-RTK) positioning and its integrity monitoring. Using 1 Hz simulated data around Beijing for global positioning system (GPS)/Galileo/Beidou navigation satellite system (BDS)-3 and the tested LEO constellation with 150 satellites on L1/L5, it was found that the convergence of the formal horizontal precision can be significantly shortened in the ambiguity-float case, especially for the single-constellation scenarios with low precision of the interpolated ionospheric delays. The LEO augmentation also improves the efficiency of the user ambiguity resolution and the formal horizontal precision with the ambiguities fixed. Using the integrity monitoring (IM) procedure introduced in the first part of this series of papers, the ambiguity-float horizontal protection levels (HPLs) are sharply reduced in various tested scenarios, with an improvement of more than 60% from 5 to 30 min after the processing start. The ambiguity-fixed HPLs can generally be improved by 10% to 60% with the LEO augmentation, depending on the global navigation satellite system (GNSS) constellations used and the precision of the ionospheric interpolation. Full article

(This article belongs to the Special Issue LEO-Augmented PNT Service)

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26 pages, 9037 KiB

Open AccessArticle

Stability of CubeSat Clocks and Their Impacts on GNSS Radio Occultation

by Amir Allahvirdi-Zadeh, Joseph Awange, Ahmed El-Mowafy, Tong Ding and Kan Wang

Remote Sens. 2022, 14(2), 362; https://doi.org/10.3390/rs14020362 - 13 Jan 2022

Cited by 6 | Viewed by 3416

Abstract

Global Navigation Satellite Systems’ radio occultation (GNSS-RO) provides the upper troposphere-lower stratosphere (UTLS) vertical atmospheric profiles that are complementing radiosonde and reanalysis data. Such data are employed in the numerical weather prediction (NWP) models used to forecast global weather as well as in climate change studies. Typically, GNSS-RO operates by remotely sensing the bending angles of an occulting GNSS signal measured by larger low Earth orbit (LEO) satellites. However, these satellites are faced with complexities in their design and costs. CubeSats, on the other hand, are emerging small and cheap satellites; the low prices of building them and the advancements in their components make them favorable for the GNSS-RO. In order to be compatible with GNSS-RO requirements, the clocks of the onboard receivers that are estimated through the precise orbit determination (POD) should have short-term stabilities. This is essential to correctly time tag the excess phase observations used in the derivation of the GNSS-RO UTLS atmospheric profiles. In this study, the stabilities of estimated clocks of a set of CubeSats launched for GNSS-RO in the Spire Global constellation are rigorously analysed and evaluated in comparison to the ultra-stable oscillators (USOs) onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC-2) satellites. Methods for improving their clock stabilities are proposed and tested. The results (i) show improvement of the estimated clocks at the level of several microseconds, which increases their short-term stabilities, (ii) indicate that the quality of the frequency oscillator plays a dominant role in CubeSats’ clock instabilities, and (iii) show that CubeSats’ derived UTLS (i.e., tropopause) atmospheric profiles are comparable to those of COSMIC-2 products and in situ radiosonde observations, which provided external validation products. Different comparisons confirm that CubeSats, even those with unstable onboard clocks, provide high-quality RO profiles, comparable to those of COSMIC-2. The proposed remedies in POD and the advancements of the COTS components, such as chip-scale atomic clocks and better onboard processing units, also present a brighter future for real-time applications that require precise orbits and stable clocks. Full article

(This article belongs to the Special Issue LEO-Augmented PNT Service)

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25 pages, 4667 KiB

Open AccessArticle

Integrity Monitoring of PPP-RTK Positioning; Part I: GNSS-Based IM Procedure

by Kan Wang, Ahmed El-Mowafy, Weijin Qin and Xuhai Yang

Remote Sens. 2022, 14(1), 44; https://doi.org/10.3390/rs14010044 - 23 Dec 2021

Cited by 10 | Viewed by 3737

Abstract

Nowadays, integrity monitoring (IM) is required for diverse safety-related applications using intelligent transport systems (ITS). To ensure high availability for road transport users for in-lane positioning, a sub-meter horizontal protection level (HPL) is expected, which normally requires a much higher horizontal positioning precision of, e.g., a few centimeters. Precise point positioning-real-time kinematic (PPP-RTK) is a positioning method that could achieve high accuracy without long convergence time and strong dependency on nearby infrastructure. As the first part of a series of papers, this contribution proposes an IM strategy for multi-constellation PPP-RTK positioning based on global navigation satellite system (GNSS) signals. It analytically studies the form of the variance-covariance (V-C) matrix of ionosphere interpolation errors for both accuracy and integrity purposes, which considers the processing noise, the ionosphere activities and the network scale. In addition, this contribution analyzes the impacts of diverse factors on the size and convergence of the HPLs, including the user multipath environment, the ionosphere activity, the network scale and the horizontal probability of misleading information (PMI). It is found that the user multipath environment generally has the largest influence on the size of the converged HPLs, while the ionosphere interpolation and the multipath environments have joint impacts on the convergence of the HPL. Making use of 1 Hz data of Global Positioning System (GPS)/Galileo/Beidou Navigation Satellite System (BDS) signals on L1 and L5 frequencies, for small- to mid-scaled networks, under nominal multipath environments and for a horizontal PMI down to

2 \times 10^{- 6}

, the ambiguity-float HPLs can converge to 1.5 m within or around 50 epochs under quiet to medium ionosphere activities. Under nominal multipath conditions for small- to mid-scaled networks, with the partial ambiguity resolution enabled, the HPLs can converge to 0.3 m within 10 epochs even under active ionosphere activities. Full article

(This article belongs to the Special Issue LEO-Augmented PNT Service)

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15 pages, 4385 KiB

Open AccessTechnical Note

Downlink Analysis of a Low-Earth Orbit Satellite Considering an Airborne Interference Source Moving on Various Trajectory

by Eunjung Kang, YoungJu Park, JungHoon Kim and Hosung Choo

Remote Sens. 2024, 16(2), 321; https://doi.org/10.3390/rs16020321 - 12 Jan 2024

Viewed by 1122

Abstract

This paper analyzes low-Earth-orbit (LEO) satellite downlinks when an airborne interference source moves parallel to the satellite trajectory by considering the relative angle differences between the satellites and the interference sources. To make the experimental interference situations more like actual environments, the LEO trajectories are obtained from two-line element set (TLE) data. Airborne interference sources with various altitudes move parallel to the LEO trajectories, and a jamming to signal (J/S) ratio is calculated based on the relative angle differences between the ground station, the LEO satellite, and the interference source. To accurately calculate the J/S ratio, we should apply the sidelobe gain from which the interference signal enters to the ground station antenna. In order to calculate the relative angle difference ψ, the coordinates of the satellite and the interference source are converted from the World Geodetic System 1984 (WGS84) to the ground station-centered east–north-up (ENU) system. The resulting J/S ratio demonstrates that the distance and the relative angle difference ψ between the ground stations, LEO satellite, and airborne interference source appear to be important factors causing changes in the J/S ratio. Among them, the relative angle difference ψ, which determines the sidelobe gain of the ground station antenna, is the most significant factor affecting the J/S ratio variation. Full article

(This article belongs to the Special Issue LEO-Augmented PNT Service)

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