GNSS Constellation Specific Monthly Analysis Summary: September 2024

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. The two new Galileo satellites (GSAT0225-PRN E29) and GSAT0227-PRN E06) which are declared operational and usable from 05 September, 2024 are included in the analysis. Furthermore, the time transfer method using GNSS pseudorange measurements is introduced in this month’s analysis. The impact of each constellation in time synchronization and dissimination applications will be further analyzed in next issues.

Analyzed Parameters for August, 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).
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. Time Transfer Performance: The analysis shows the performance of different GNSS system and time link methods for timing and synchronization applications including the realization of the UTC.

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

GNSS has been used for the realization of International Atomic Time (TAI) and UTC for some decades now. The technology is heavily used, due to high precision with low cost, in time synchronization and dissemination in almost all digital technologies in the modern world. The realization of the UTC standard requires the continuous monitoring of the accuracy and stability of the time transfer among the contributing precise time laboratories from around the world. Similarly, such monitoring is also required for the use of GNSS for time synchronization and dissemination (Wang et.al, 2014). The BIPM is the organization responsible for the realization of UTC standard using different methods, the GNSS time transfer with common view being one of them. The CGGTTS analysis tool (BIPM et.al, 2024 c) from BIPM is used in common view mode to understand how different GNSS constellations behave in terms of their application for time transfer. Using the dual frequency ionospheric free code measurements, the time link between NPLI (responsible for time dissemination in India) and NIMT (responsible for time dissemination in Thailand) is checked with GPS, Galileo and Beidou constellations. The data is provided as part of the capacity building program by BIPM and the source is referenced here (BIPM et.al, 2024 b). In the figure (f), it can be observed that all constellations provide accuracy in the level of several nanoseconds which is excellent and good enough for the vast majority of timing and synchronization applications. There indeed appears to be systematic biases among the constellations as they are using different signals and have varying behavior in terms of signal noise and delays (BIPM et.al, 2024 a). Overall, the biases in common view time transfer can be explained by the time transfer noise, multipath noise and the hardware system delays at the two sites. The ephemeris errors are common and canceled out while the ionospheric error is almost removed with the iono-free combination. Acknowledging these, it can be concluded that the time transfer accuracy and precision in terms of stability & noise look reasonable for each constellation. This monitoring has multiple applications, one being the direct calibration of the time link, such that potential systematic offset can be identified and removed and then the link can be reliably used for the time synchronization between remote clocks. At the same time any anomalies within the time laboratories or the reference timing source can be detected. For example, in the Figure (f), the GPS based time transfer (green color) is impacted by an outlier and this is due to the misbehaving satellite G12 (only the average of all satellites is shown in the figure) and when this satellite is excluded the noise level gets better. Other time transfer methods including all in view with precise clock & orbit products, PPP with carrier phase, ambiguity fixed solutions, multi-frequencies combination and multi-constellation multi frequencies have the potential to provide highly enhanced performance in terms of accuracy and stability and will be assessed in the upcoming issues of the article.

Monthly Performance Remarks:
1. Satellite Clock and Orbit Accuracy:
▪ For Galileo, the performance looked similar to the past months. There is, however, a slight degradation in GPS. Galileo E27 had an outage due to maintenance in the atomic clock.
▪ For GLONASS, the overall performance seems to have improved in terms of orbits and clocks in comparison to earlier months in 2024 and looked similar to last month
▪ For BDS and QZSS, the performance looks very much the same as in the past.
▪ For IRNSS, I06 has the poorest performance in terms of URA and satellite clock jumps.

2. UTC Prediction (GNSS-UTC):
▪ All constellations show better stability in comparison to previous months. Galileo started to converge to the nominal range after it diverged slightly last month.

References

Alonso M, Sanz J, Juan J, Garcia, A, Casado G (2020) Galileo Broadcast Ephemeris and Clock Errors Analysis: 1 January 2017 to 31 July 2020, MDPI

Alonso M (2022) Galileo Broadcast Ephemeris and Clock Errors, and Observed Fault Probabilities for ARAIM, Ph.D Thesis, UPC

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 (2024) GNSS constellation specific monthly analysis summary, Coordinates, Vol XX, Issue 1, 2, 3, 4

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

Guo F, Zhang X, Wang J (2015) Timing group delay and differential code bias corrections for BeiDou positioning, J Geod,

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 realtime 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 multiGNSS perspective, GPS Solutions

Montenbruck O, Hauschlid A (2014 a) Differential Code Bias Estimation using Multi-GNSS Observations and Global Ionosphere Maps, ION

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

Walter T, Blanch J, Gunning K (2019) Standards for ARAIM ISM Data Analysis, ION

Wang N, Li Z, Montenbruck O, Tang C (2019) Quality assessment of GPS, Galileo and BeiDou-2/3 satellite broadcast group delays, Advances in Space Research

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

Data sources and Tools:

https://cddis.nasa.gov (Daily BRDC); http://ftp.aiub.unibe.ch/ CODE_MGEX/CODE/ (Precise Products); BKG “SSRC00BKG” stream; IERS C04 ERP files

(The monitoring is based on following signals- GPS: LNAV, GAL: FNAV, BDS: CNAV-1, QZSS:LNAV IRNSS:LNAV GLO:LNAV (FDMA))

Time Transfer Through GNSS Pseudorange Measurements: https://elearning.bipm.org/login/index.php Allan Tools, https://pypi.org/project/ AllanTools/gLAB GNSS, https:// gage.upc.edu/en/learning-materials/ software-tools/glab-tool-suite.

NASA selects lunar relay contractor for lunar communication and navigation services NASA has awarded a contract to Intuitive Machines, LLC of Houston, to support the agency’s lunar relay systems as part of the Near Space Network, operated by the agency’s Goddard Space Flight Center in Greenbelt, Maryland. This Subcategory 2.2 GEO to Cislunar Relay Services is a new firm-fixedprice, multiple award, indefinitedelivery/indefinite-quantity task order contract. The contract has a base period of five years with an additional 5-year option period, with a maximum potential value of $4.82 billion. Lunar relays will play an essential role in NASA’s Artemis campaign to establish a long-term presence on the Moon. These relays will provide vital communication and navigation services for the exploration and scientific study of the Moon’s South Pole region. Without the extended coverage offered by lunar relays, landing opportunities at the Moon’s South Pole will be significantly limited due to the lack of direct communication between potential landing sites and ground stations on Earth. The lunar relay award also includes services to support position, navigation, and timing capabilities, which are crucial for ensuring the safety of navigation on and around the lunar surface. Under the contract, Intuitive Machines also will enable NASA to provide communication and navigation services to customer missions in the near space region. The initial task award will support the progressive validation of lunar relay capabilities/services for Artemis. NASA anticipates these lunar relay services will be used with human landing systems, the LTV (lunar terrain vehicle), and CLPS (Commercial Lunar Payload Services) flights. As lunar relay services become fully operational, they will be integrated into the Near Space Network’s expanding portfolio, enhancing communications and navigation support for future lunar missions. By implementing these new capabilities reliance on NASA’s Deep Space Network will be reduced. NASA’s goal is to provide users with communication and navigation services that are secure, reliable, and affordable, so that all NASA users receive the services required by their mission within their latency, accuracy, and availability requirements. This is another step in NASA partnering with U.S. industry to build commercial space partners to support NASA missions, including NASA’s long-term Moon to Mars objectives for interoperable communications and navigation capabilities. This award is part of the Space Communications and Navigation (SCaN) Program and will be executed by the Near Space Network team at NASA Goddard. www.nasa.gov