GNSS constellation specific monthly analysis summary: December 2023

Feb 2024 | No Comment

Coordinates starts a regular column on the accuracy assessment of satellite orbits and atomic clocks on board of all GNSS constellations (GPS, Galileo, Glonass, Beidou, QZSS and IRNSS)

Narayan Dhital

is 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

The global push for economic, political and strategic power have led to the build up of multiple independent satellite navigation systems. Right now, there exists a mixture of legacy and gradually modernized navigation satellites with varying performances. The new initiative in this column will attempt to regularly characterize the system performance and evolution of each state program. As a starter, a couple of remarks can already be made on the recent progress of the Indian GNSS. The IRNSS recently entered the phase of second generation satellites. In May 2023, the first NAVIC satellite, NVS- 01 (PRN: I10) was launched and the satellite clock performance is already showing promising results in comparison to the 1st generation IRNSS satellites. Also, it is the first IRNSS satellite with a L1 signal allowing dual frequency navigation. This column will attempt to regularly highlight such progresses and more importantly highlight the performance evolution of all systems and whenever deemed essential, will provide a separate article to dissect into details the detected performance improvement or an anomaly.

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)

Introduction

The compatibility and interoperability of the GNSS is a key to the digital transformation of the global economy driven by GNSS based PNT solutions. In that regard, it is indispensable to have a continuous monitoring of relevant system parameters of all GNSS in a single view.

The pioneering services provided by GPS system have demonstrated a range accuracy improvement from 4.5 m rms to below 1 m rms. For GLONASS, the improvement has seen the range accuracy brought down to below 2 m rms from 20 m rms. The latest development of Galileo has seen its performance vastly improved in comparison to the legacy system. Currently, the Galileo system provides below 0.5 m range accuracy (Hauschlid et. al, 2020). In parallel to Galileo, the global constellation of the Beidou system has also provided highly accurate ranging services comparable to Galileo. A new addition to the GNSS, the Japanese QZSS has also started to provid a strong performance. From the Indian region, the first generation of IRNSS and the second generation (NAVIC) are also on their way to achieve strong range accuracy. With such continuous evolution and modernization of legacy GNSS, mostly in terms of atomic clock stability, upgrade in mission segment prediction of the satellite orbits and stable system timekeeping, it is foreseen to have a gradual improvement in the system performances.

The range accuracy, as mentioned herewith, is interpreted as the Signal in Space (SiS) Range Error (SISRE). The SISRE describes the statistical uncertainty of the modeled pseudorange due to errors in the broadcast satellite orbit and clock information. It is driven by the space segment characteristics (e.g., clock stability and predictability of orbital motion), as well as the mission control segment capabilities (orbit and clock determination algorithm, distribution of monitoring stations, and upload capacity). The positioning accuracy at the user level is directly dictated by the SISRE in conjunction with the dilution of precision (Montenbruck et. al, 2010). Stable time keeping is the main aspect of the GNSS that drives the services and for this, the atomic clocks onboard each satellite are vital. Even though they are highly stable, there are numerous factors like ageing, sudden breakdowns, radiation or temperature that impact the synchronization between the satellite broadcast signals derived using the atomic clock and the GNSS system reference time. It is important to have a continuous monitoring of the performance of satellite atomic clocks and mission segment clock prediction to leverage the full benefits of multi-GNSS systems. Such monitoring can also generate valuable information for the optimization of PNT algorithms, for example a priori assumption on the noise characteristics of satellite clocks and pseudorange errors. Besides, a continuous monitoring of the stability of steered GNSS time to predicted UTC, through GNSS-UTC information, can aid users of PNT services.

The system performance is also characterized by two additional parameters: i) URA, which is a prediction of the minimum standard deviation of the unbiased Gaussian distribution, which overbounds the SISRE prediction. In a case of larger URA values, the system can be considered either unusable or degraded with high range error and ii) the clock discontinuity which is the offset in the clock prediction between two batches of messages during the handover. A larger clock discontinuity can violate the assumption of short-term variation in satellite clock offset process noise and degrade the PNT solution.

Analyzed Parameters
a. Satellite Broadcast Accuracy, measured in terms of Signal-In-Space Range Error (SISRE)
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)
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. Modified Allan Deviation: The stability of each satellite atomic clock. It is indicative of short term and long-term stability of each type of atomic clock used on-board the satellite

(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)

Monthly performance remarks

A greater performance of the satellite atomic clock and the mission control segment can be attributed to the best SISRE achieved by the Galileo system. The Allan deviation plots also demonstrate the greater stability of atomic clocks of Galileo satellites. Their achieved performance is closely followed by Beidou and GPS. QZSS system is right up there, albeit a fewer number of satellites. GLONASS system is lagging a bit behind. The performance of the IRNSS system, detected to have larger clock discontinuities and some instances of degraded URA, suggest there is enough room for improvement. The addition of new frequency and the launch of 2nd generation satellites look promising.