GNSS constellation specific monthly analysis summary: February 2024

Mar 2024 | No Comment

The analysis performed in this report is solely the author’s work and his 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. (Dhital et. al, 2024a, and Dhital et. al, 2024b) provides previous months analysis. In this month’s issue, the monitoring of Galileo satellite attitude maneuver during low beta angle is the only addition.

Analyzed Parameters for February, 2024

(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). Due to the data latency of 2 weeks for precise satellite clocks and orbits, at the time of current analysis, only 01-18 February, 2024 time frame is used.
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. ERPs Accuracy (xp, yp and UT1): It shows the prediction accuracy of the linear model at 1 day interval (or the discontinuity between two batches of data) and the accuracy w.r.t precise products from IERS (IERS et. al, 2024). Due to the required latency of IERS C04 products, Dec 16, 2023 to January 16, 2024 time frame is selected.
g. Satellite Yaw Maneuver: It shows the behavior of satellites that are operating under the low sun angle w.r.t the orbital plane and their yaw rate to quickly align the solar panels towards the sun direction (Liu et. al, 2022)

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.

Remark: The introduction to the ERPs and the broadcast navigation format are available in (Steigenberger et. al, 2022 and IGS et.al, 2021). Regarding the performance obtained by analyzing the 13 days of data, it can be observed that the accuracy of GNSS broadcast ERPs does not have drastic differences among the constellation and remain consistent to the results reported in January (Dhital et. al, 2024b). A special mention is needed for GPS accuracy due to its reference epoch in broadcast data not matching the IERS C04 (00 h UTC) reference epoch. As the IERS reference epoch is propagated with a simpler linear model, without accounting for semidiurnal tidal effects, and whereas the GPS ICD specifies the consideration of such variations, the direct comparison with GPS data will have a bias. (Steigenberger et. al, 2022) shows such bias could be in the range of 1 mas for polar coordinates and upto 0.5 msec for dUT1. When the bias is subtracted from the accuracy for GPS, the values will be at least similar, if not better, to other constellations (QZSS and IRNSS). The only interesting difference in this analysis is a relatively larger values for GPS in comparison to what was observed in (Dhital et. al, 2024b). The relatively lower accuracy and higher discontinuities of BDS are attributed to the lower update rate of the ERP prediction model from the mission segment. It is observed that the update period ranges up to 7 days or even more in some cases. The overall performance looks similar to the results provided in the literature (Liu M et. al, 2023; Liu W et. al, 2023).

Note:- ERPs are introduced only in RINEX version 4. Galileo and GLONASS do not provide ERPs in the broadcast messages.

(g): GNSS satellite nominal and nonnominal yaw attitude (GPS and Galileo)

The basic introduction to the orientation of GNSS satellites is provided in (Dhital et. al, 2024b). It also covers the relation between the Sun elevation angle (beta angle) and the nominal yaw angle of the satellite.

In February, there were no GPS satellites under the low beta angle ( > -1° and < 1°). However, there were Galileo satellites from orbital plane C with lower beta angle and hence, undergoing noon and midnight turn maneuver.

E03, E04, E05, E07, E08, E09, and E19 from 01 Feb until 07 Feb had beta angle > -4° and < 4° and from 02 to 04 Feb > -1° and < 1°.

As presented in different literature (Cao et. al, 2018; Liu et.al, 2022, Sylvain et. al, 2021), the yaw rate of the Galileo satellites depends on the satellite type and the beta angle. The nominal yaw rate is close to 0.01°/sec and during the orbit noon and midnight maneuver, the rate increases to 0.07°/sec depending on the beta angle. The lower the beta angle higher the yaw rate. To monitor the yaw rate of the identified satellites undergoing the midnight and noon maneuver, two cases are monitored. In the first case, a day is selected with beta angle below 1°. In the second case, a day is selected with beta angle above 4° but below 6°. Galileo E05 satellite is examined for both cases and is shown in Figure (g). The yaw rate is close to 0.07°/sec on the 3rd of Feb where the beta angle is below 1°. Similarly, the yaw rate reduces below 0.03°/sec when the beta angle began to go above 4°.

Monthly Performance Remarks:
1. Satellite Clock and Orbit Accuracy
• For GPS, the satellite clock and orbit accuracy shows consistent performance in comparison to January 2024 (Dhital et. al, 2024b). There were multiple satellites (G02, G08, G10 and G27) in maneuvers and non-healthy status. As a result, large satellite clock discontinuities and orbit degradation were observed. G27 became usable again for the first time since the 30th December 2023.
• For Galileo, a large satellite clock discontinuity was observed for E30 on the 9th of February. Other parameters showed consistent performances in comparison to January.
• For GLONASS, no large clock offsets were detected unlike in January. The performance looked only marginally better in February.
• For BDS and QZSS, the performance looks very much the same as in December 2023. For QZSS, there are days with better orbit quality and some days with degraded performance. A 24 hours periodic degradation is seen for J03 from 08 to 10 Feb.
• For IRNSS, the overall constellation clock discontinuity and the URA statistics look similar to the performance in January. I10 consistently broadcast a few epochs with varying confidences in the ephemerides.

2. UTC Prediction (GNSS-UTC):

Among the GNSS, Galileo and GLONASS showed more variations in comparison to January 2024. That being said, the GAL UTC prediction is still kept within 3 ns.

References

Cao X, Zhang S, Kuang K, Liu T (2018) The impact of eclipsing GNSS satellites on the precise point positioning, Remote Sensing 10(1):94

Dhital N (2024a) GNSS constellation specific monthly analysis summary, Coordinates, Vol XX, Issue 1 pages 20-22

Dhital N (2024b) GNSS constellation specific monthly analysis summary, Coordinates, Vol XX, Issue 2 pages 9-12

Hauschlid A, Montenbruck O (2020) Precise realtime navigation of LEO satellites using GNSS broadcast ephemerides, ION

IERS C04 (2024) https://hpiers.obspm. fr/iers/eop/eopc04/eopc04.1962-now

IGS (2021) RINEX Version 4.00 https:// files.igs.org/pub/data/format/rinex_4.00.pdf

Li M, Wang Y, Li W (2023) performance evaluation of real-time orbit determination for LUTAN-01B satellite using broadcast earth orientation parameters and multi-GNSS combination, GPS Solutions, Vol 28, article number 52

Li W, Chen G (2023) Evaluation of GPS and BDS-3 broadcast earth rotation parameters: a contribution to the ephemeris rotation error Montenbruck O, Steigenberger P, Hauschlid A (2014) Broadcast versus precise ephemerides: a multi-GNSS perspective, GPS Solutions

Liu T, Chen H, Jiang Weiping (2022) Assessing the exchanging satellite attitude quaternions from CNES/ CLS and their application in the deep eclipse season, GPS Solutions 26(1)

Montenbruck O, Steigenberger P, Hauschlid A (2014) Broadcast versus precise ephemerides: a multi-GNSS perspective, GPS Solutions

Steigenberger P, Montenbruck O, Bradke M, Ramatschi M (2022) Evaluation of earth rotation parameters from modernized GNSS navigation messages, GPS Solutions 26(2)

Sylvain L, Banville S, Geng J, Strasser S (2021) Exchanging satellite attitude quaternions for improved GNSS data processing consistency, Vol 68, Issue 6, pages 2441-2452