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GNSS Constellation Specific Monthly Analysis Summary: September 2025

Nov 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 GNSS related 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

This article continues the monthly performance analysis of the GNSS constellation. Readers are encouraged to refer to previous issues for foundational discussions and earlier results. In addition, there is a short overview on the recent anomaly and degradation in the UTC time dissemination parameter from GNSS broadcast messages.

Analyzed Parameters for June 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. UTC Offset Anomaly and Degradation: It shows the anomaly in the difference between GNSS Time and Coordinated Universal Time (UTC) and is critical for converting GNSS Time to UTC in timing applications

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.

(F) Time Transfer Degradation: UTC Offset Anomaly and Degradation

The objective of this section is to highlight the necessity to monitor the GNSS-UTC broadcast parameter, as we have been doing every month in this column. Two examples from recent time are used to demonstrate the uses cases of the monitoring. On January 26, 2016, a GPS UTC anomaly occurred when 15 satellites began broadcasting incorrect UTC offset parameters— specifically the A0 and A1 coefficients (A1 impacted due to reference UTC week number set to 0) used to convert GPS Time to UTC (see Equations 1-2 below). This led to a sudden ~13 microsecond discrepancy between GPS-derived UTC and actual UTC (see Table F1). The root cause was traced to a software update in the GPS ground control segment that mishandled the decommissioning of satellite SVN23. Although SVN23 was no longer active, its outdated UTC correction parameters were inadvertently propagated to active satellites during the update. This caused receivers that relied on the broadcast UTC offset to apply a faulty correction, resulting in timing errors (Yao et.al, 2016) Galileo and QZSS satellites were unaffected at the time, as they use independent timekeeping systems and did not broadcast erroneous UTC corrections during this period.

While this anomaly was mostly detected by the time laboratories, users of GPS timing receivers and GPS disciplined oscillators, it also pushed the GPS control segment for more robust approach in its UTC dissemination. For the positioning and navigation, however, the impact was not seen as the observables and residuals from the GPS signals can be used without the UTC information. Only the receivers that used the UTC offset to GPS from the navigation messages to convert from GPS to UTC time were impacted by the anomaly.

In the second example of the degradation of the GNSS-UTC dissemination, the events in August–December 2023, for Galileo constellation can be considered. The Galileo System Time (GST) drifted from UTC by up to 20 nanoseconds (see Figure F1), but still within the 30 ns requirement set by Galileo Open Service standard (GSC et.al, 2023). The degradation probably drifted the time synchronization process based only on GST. However, the prevalence of the multi-GNSS system meant it was not a big concern for the timing applications. To conclude, when these broadcast parameters (Equation 2) are incorrect/ degraded, the computed UTC will deviate from true UTC. Not all receivers are impacted: positioning receivers use GNSS system time directly and are unaffected in terms of location accuracy. Timing receivers, however, become vulnerable if they apply the broadcast UTC offset without validation. NIST’s UTC(NIST) was notably impacted in 2016 because several of its GPS-disciplined receivers trusted the faulty broadcast data. Other UTC(k) labs, especially those using multi GNSS receivers or internal sanity checks, either avoided the error or recovered quickly. The Galileo 2023 anomaly similarly exposed the importance of cross constellation validation and robust receiver design in safeguarding UTC(k) integrity.

The anomaly and degradation highlighted with these two examples reflect that even modern GNSS constellations can suffer timing inconsistencies. The physical realization of the time in the dedicated laboratories can also have issues that propagate to the values broadcast through the satellite. In our monthly monitoring of GNSS-UTC broadcast, such big anomaly or degradation have not been noticed so far, which is on a positive note, but some drifts in values for IRNSS and GLONASS have been noticed from time to time.

Monthly Performance Remarks:

1. Satellite Clock and Orbit Accuracy:
• The performance of GPS, Galileo and Beidou remained similar. The clock of Galileo improved by a small Figure F 1: The evolution of the UTC time dissemination offset as broadcast by different GNSS constellations. Galileo constellation had an unstable period during the second half of 2023. The offset computed by the UTC(k) of the Galileo system had a large deviation (up to 18 ns as indicated in the daily navigation message broadcast value). margin. GPS PRN 20 is resumed from 30th September and had a large clock jump on the day when SVN 51 started in transition to PRN20. NANU was released for the operational change. QZSS analysis is excluded for further consistency check in processing and to identify degradation in orbits of one of the QZSS satellites.
• IRNSS URA values for all satellites indicate improved accuracy.
• IRNSS clocks show I02 satellite has consistently good clock and I10 satellite has consistently the worst clock (note: among IRNSS satellite clocks)

2. The UTC Prediction (GNSS-UTC):
• All constellation, except IRNSS, provided relatively stable predictions of the GNSS-UTC. IRNSS started to drift heavily in the second half of the month

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