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GNSS Constellation Specific Monthly Analysis Summary: February 2025

Apr 2025 | No Comment

The analysis performed in this report is solely his work and own opinion. State Program: U.S.A (G); EU (E); China (C) “Only MEO- SECM satellites”; Russia (R); Japan (J); India (I)

Narayan Dhital

Actively involved to support international collaboration in GNSSrelated activities. He has regularly supported and contributed to different workshops of the International Committee on GNSS (ICG), and the United Nations Office for Outer Space Affairs (UNOOSA). As a professional employee, the author is working as GNSS expert at the Galileo Control Center, DLR GfR mbH, Germany

Introduction

The article is a continuation of monthly performance analysis of the GNSS constellation. Please refer to previous issues for past analysis. Regarding applications, a short overview on the satellite control to maintain the attitude of GNSS satellites with respect to the mission requirement is provided. The implications on the precise use cases are also highlighted.

Analyzed Parameters for February, 2025

(Dhital et. al, 2024) provides a brief overview of the necessity and applicability of monitoring the satellite clock and orbit parameters.
a.. Satellite Broadcast Accuracy, measured in terms of Signal-In-Space Range Error (SISRE) (Montenbruck et. al, 2010).
b. SISRE-Orbit (only orbit impact on the range error), SISRE (both orbit and clock impact), and SISRE-PPP (as seen by the users of carrier phase signals, where the ambiguities absorb the unmodelled biases related to satellite clock and orbit estimations. Satellite specific clock bias is removed) (Hauschlid et.al, 2020)
c. Clock Discontinuity: The jump in the satellite clock offset between two consecutive batches of data uploads from the ground mission segment. It is indicative of the quality of the satellite atomic clock and associated clock model.
d. URA: User Range Accuracy as an indicator of the confidence on the accuracy of satellite ephemeris. It is mostly used in the integrity computation of RAIM.
e. GNSS-UTC offset: It shows stability of the timekeeping of each constellation w.r.t the UTC
f. Satellite Attitude Quaternions: The optimal control of the GNSS satellite attitude is mission critical and the awareness of the orientation of the satellites is important for the user level PVT solutions. Quaternions are used to represent the attitude of the GNSS satellites.

Note:- :-for India’s IRNSS there are no precise satellite clocks and orbits as they broadcast only 1 frequency which does not allow the dual frequency combination required in precise clock and orbit estimation; as such, only URA and Clock Discontinuity is analyzed.

Due to data glitches, the satellite clock jump statistics are not provided for this month. The processing will be recovered and provided together with the month of March.

On a positive side, a brief overview on the satellite atomic clock stability of the newly operational Galileo satellites GSAT0232 GSAT0226 (PRNs: E16 and E23) are provided. As it can be seen in the plot below (based on the 24 hours data on the 14th of February, 2025), the Passive Hydrogen Maser (PHM) clocks of both satellites behave as expected. The PHM of other Galileo satellites, including the E06 and E29 that were launched in 2024 as well, are provided as references. The relatively poor stability of RFS clock in satellites E11 and 19 are also shown.

(f) Satellite Attitude Quaternions

The inertial attitude of a GNSS satellite is determined by the mission requirement to point the navigation antenna towards the Earth while keeping the solar panels optimally towards the Sun. As a result, the satellite control mechanism continuously rotates the satellite around the Earthpointing axis, ensuring the solar panel axis remains perpendicular to the Sun’s direction. This method is known as the Yaw Steering (YS) attitude control. However, this mode requires rapid yaw-slews of up to 180 degrees when the Sun is near the orbital plane. In such cases, an orbit normal (ON) mode is often preferred, where the satellite body is fixed in the local orbital frame and the solar panel rotation axis is kept perpendicular to the orbital plane. The ON is also applied for the Geostationary satellites. Figure F(a) shows the sketch of YS and ON mode adopted from (Guo et.al, 2017). (Montenbruck et.al, 2015) provides detail on the attitude of the satellites relating the body-reference frame and orbit plane. When satellites enter noon maneuver or shadow crossing regimes, their actual attitudes can deviate from nominal values. Improper attitude models can lead to errors due to the wind-up effect and satellite antenna Phase Center Offset (PCO), deteriorating positioning accuracy. While the improvement in modeled attitude for multi-GNSS solutions might be modest, it can be significant for single positioning and in urban areas with limited satellite tracking.

The Euler angles that represent the rotation of the satellite suffers from the gimble lock and singularities, where two of the three axis aligns rendering loss of the degree of freedom. The use of quaternions overcome that problem and is also efficient for representing the attitudes. The satellite attitude quaternions q = (q₀, q₁, q₂, q₃) are provided by IGS ACs to ensure consistency in the attitude model used by both the network and user end. These quaternions describe the transformation between the Terrestrial Frame and the Satellite Body-Frame, ensuring accurate positioning.

This ensures accurate positioning and minimizes errors due to the wind-up effect and PCO (IGS et.al, 2019) 4Such implementation in the user algorithm that ingest the attitude quaternions provided by the ACs improves the PVT solution. It is not envisaged to demonstrate in this article and viewers are referred to (Yuang et.al, 2025) for relevant analysis. However, a short demonstration on the usability of the attitude quaternions to understand the behavior of various satellites in different orbits is provided in Figure F 2.

The computed angle as shown in the y-axis is with respect to the satellite body frame (nadir pointing vector and the along track vector). It is not based on the orbit reference frame which explains the offset of the yaw angle. However, the evolution of the yaw behavior for the 24 hours is explainable through the computed angle. The distinct evolution is seen for J07 and C60 which are the Geostationary satellites that follow the orbit normal attitude control of the satellite. Similarly, the IGSO based satellites (C10 and J04) show varying yaw angle with slow change as the orbit period is close to 24 hours while the MEO based satellites show 2 cycles of varying yaw angles per 24 hours matching the orbit period close to 12 hours. The behavior of the satellite attitude in the shadow crossing and low beta angle (angle of the orbit plan with respect to the Sun direction) period was provided in previous months analysis (February-March 2024).

Monthly Performance Remarks:

1. Satellite Clock and Orbit Accuracy:
▪ The performance of all constellations is relatively stable and unchanged from previous month.
▪ The data glitch in the computation prevented the analysis of the satellite clock jump. However, an overview on the newly operational Galileo satellite clocks (E16 and E23) was provided. The stability of the PHM of E16 and E23 looks as expected.
▪ The URA for I06 showed a little more scatter in comparison to previous months.

2. UTC Prediction (GNSS-UTC):
▪ All constellations show stable UTC prediction with minor variations. It is showing a good consistency in last months.

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Note: References in this list might also include references provided to previous issues.