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GNSS Constellation Specific Monthly Analysis Summary: March 2025

May 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. As an addition, there is a section that explores the modernization and evolution of the satellites in the past one year.

Analyzed Parameters for March 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. Evolution of GPS and Galileo Satellites: The GNSS system is consistently modernizing and evolving. The last 1 year represents a couple of key milestones in the modernization of the satellite systems and their performances.

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.

Important notes: the author sincerely apologies for an error in the statistics represented for Galileo constellation SISRE for the month of January and February 2025. The SISRE and SISRE-PPP are: 25 cm and 24 cm, instead of 15 cm and 14 cm for both months.

Due to data glitches, the satellite clock jump statistics for February 2025 was not provided. However, it was resolved, and the statistics represent the similar characteristics as shown for March 2025 in the table.

(f) Evolution of GPS and Galileo Satellites: Looking Back to Last 1 Year

It has been more than 1 year since the monthly GNSS performance analysis reporting started. In this time, a couple of interesting evolutions in the satellite systems and their obtained performances are detected. As a legacy system, GPS has consistently modernized its satellite infrastructure including the signals and broadcast messages. Through the monthly monitoring it was observed that there was a significant improvement in the SISRE (upto 30 cm performance enhancement with LNAV dual frequency) from March 2024 onwards. This is mostly attributed to the switch in the atomic clock from Cs standard to Rb standards for satellite G08 and G10. In addition, it has been mentioned (Montenbruck et. al, 2024) that upload rate from the mission control segment has been improved that rendered the better performances. The relatively weak stability of some of the GPS satellites (the performance of the Rb clocks varied among satellites) constrained the achievable SISRE to around 45 cm prior to this evolution. To overcome this issue, the additional mission control segment supported the generation of navigation message upload twice a day rather than once a day. The combined impact is clearly visible in the clock and SISRE performance.

In the following Allan deviation plots (Figure Fa and Fb), the comparison between the atomic clock stability for a single representative day in January 2024 and March 2025 is shown. The performance after the switch to the Rb standard is improved significantly for the integration time. It is observed that the stochastic clock variation over the fitting interval of 10,000 seconds is improved by 10 folds. modified Allan deviation is improved by 10 folds for integration time of 10,000 seconds. This corresponds to, using c.dt(τ)=c.ADEV (τ).τ, around 25 cm of uncertainty between the actual clock and the prediction through the polynomial model.

For Galileo systems, there was a massive upgrade in the ground segment to the Ground Segment System Build 2.0 (SB 2.0). It includes a modernized Ground Mission Segment (GMS) with improved resilience, enhanced Public Regulated Service (PRS), and robust security monitoring through the Security Operations Centre at Galileo Security Monitoring Centers. Additionally, cybersecurity protections have been upgraded, laying the groundwork for future developments like the transition to Galileo Second Generation.

From 11th March at 13:39 UTC, Galileo satellites started transmitting the first navigation message from this new system. The Galileo is a system relied upon by four billion users every day and hence, cannot afford any significant downtime. The strategic upgrade phases of the system were finely executed without any noticeable changes in the service performances (InsideGNSS et.al, 2024). The mycoordinates monthly reporting summary (please refer to each issue of mycoordinates in 2024) did not detect any changes in March. In fact, the Galileo performance has remained consistent and solid throughout the last 12 months supporting the facts claimed in the article.

The evolution of the Beidou and QZSS systems will be provided in the next month’s issue. Both systems consist of different satellite orbits and their associated atomic clock characteristics. Due to highly varying satellite orbits for inclined and geosynchronous satellites, the impact of force models including gravitational and solar radiation forces are more pronounced.

Monthly Performance Remarks:

1. Satellite Clock and Orbit Accuracy:
▪ The performance of all constellations is relatively stable and unchanged from previous month.
▪ The satellite clock jumps identified a couple of issues in GPS satellites and one issue in Galileo E12 satellite.
▪ The improvement in the GPS satellite broadcast ephemeris from the switch in the atomic clock standards and the increased upload rate from the mission control center is verified.
▪ The URA for I02 showed a little more scatter in comparison to previous months. It suggests a degraded confidence in its satellite orbit. 2. UTC Prediction (GNSS-UTC):
▪ GPS and Beidou showed some variations in comparison to previous months.

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