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GNSS Constellation Specific Monthly Analysis Summary: January 2026

Mar 2026 | 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 new analysis regarding Indian regional navigation systems (NAVIC and GAGAN) that will be featured regularly.

Analyzed Parameters for January 2026

(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. Status Update on the IRNSS/ NAVIC: A technical overview on the IRNSS/NAVIC system, its coverage and performance analysis is provided

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) IRNSS/NAVIC Status

The Indian Regional Navigation Satellite System (IRNSS), operationally known as NavIC, has progressed from an experimental concept into a functioning regional navigation and timing system. Designed to provide continuous coverage over India and surrounding regions, NavIC employs a hybrid constellation of geostationary (GEO) and inclined geosynchronous (IGSO) satellites, enabling consistently high elevation angles across the subcontinent. The original constellation consisted of seven satellites—three GEO spacecraft located near 32.5°E, 83°E and 131.5°E, and four IGSO satellites inclined at approximately 29° with ground tracks centered over India.

NavIC initially transmitted signals on L5 and S-band, with the more recent NVS-01 generation adding L1 capability to improve interoperability and user-segment adoption. Early spacecraft relied on imported SpectraTime rubidium atomic clocks, several of which experienced failures; the complete loss of all three clocks on IRNSS-1A rendered the satellite unusable for navigation. These shortcomings led ISRO to develop indigenous clock technologies, culminating in the launch of NVS-01 in 2023 carrying the first Indian-built rubidium atomic clock—an important milestone in strengthening long-term system resilience.

In its early operational years, NavIC demonstrated strong performance, with ISRO reporting meter-level dual frequency positioning accuracy and timing stability at the tens of nanoseconds level. High elevation geometry, robust dual-frequency ionospheric correction, and stable broadcast ephemerides contributed to an adequate regional performance as long as sufficient healthy satellites were available.

Constellation Geometry and Its Implications

An analysis using the LCK400IND IGS station (Lucknow) provides insight into NavIC’s real-world behavior. The GEO/IGSO architecture typically ensures visibility of four to five satellites at moderate to high elevation angles. Figure Fa illustrates this geometry: GEO satellites (PRNs 3, 6, and 10) appear nearly stationary from the user location, while IGSO satellites (PRNs 2 and 9) trace elongated figure8 ground tracks due to their inclination.

However, the limited constellation size introduces recurring daily geometric weaknesses. As shown in Figure Fb, PRN 2 and PRN 9 align along nearly the same line of sight twice per day, creating near-singularity events. During these intervals the geometry matrix becomes poorly conditioned, DOP values rise sharply, and the covariance inflates— amplifying the impact of even modest measurement noise. Although these events do not necessarily cause position solution outages, they reduce robustness and consistency.

This effect appears clearly in the user-domain results. Figure Fc shows the dual-frequency NavIConly single-point positioning (SPP) solution for DOY 131/2024, where two pronounced divergences near 40,000 s and 80,000 s correspond precisely to the near-alignment periods. In comparison, Figure Fd presents the GPS-only DF SPP solution for the same day, demonstrating stable behavior due to superior geometry and higher-quality broadcast ephemerides—highlighting the structural advantage of larger constellations.

Signal Strength Characteristics

Carrier-to-noise density ratios recorded at LCK400IND (Figure Fe) show:
• GEO satellites provide consistent, stable C/N- values due to their nearly fixed line of sight.
• IGSO satellites exhibit periodic C/N- variations driven by their elevation and azimuth cycles.
• L5 signals are consistently stronger than Sband, resulting in more robust tracking and lower code measurement noise.

In recent years, however, intermittent jamming and interference events have become increasingly evident in L1 band and in some cases other NavIC frequency bands. These events manifest as abrupt C/N- degradation, elevated measurement noise, and occasional loss of lock. While often short-lived, such interference exacerbates performance degradation during geometry-limited periods.

Overall System Health and Reliability

NavIC continues to meet its intended regional performance levels when:
• at least four satellites are available,
• geometry is favorable, and
• signal conditions are stable.

However, overall system reliability has declined due to:
• aging satellites and partial payload degradations,
• historical clock failures,
• slow constellation replenishment,
• increased radio-frequency interference, and
• inherent geometric limitations of a seven satellite regional system.

The introduction of indigenous atomic clocks represents a pivotal advancement, but NavIC currently remains in a transition phase. Without timely satellite replacements, expanded signal capability, and enhanced interference mitigation, the gap between the system’s architectural potential and realized operational performance will continue to widen.

A new analysis of NavIC and GAGAN characteristics and performance will be included each month to help readers better understand the discussions presented above.

Monthly Performance Remarks:

1. Satellite Clock and Orbit Accuracy:
• The performance looked like previous months.
• A new analysis of NavIC and GAGAN characteristics and performance will be included each month to help readers better understand the discussions presented above.

2. The UTC Prediction (GNSS-UTC):
•IRNS and QZSS have no BRDC UTC values in the BRDC messages.

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