GNSS Constellation Specific Monthly Analysis Summary: June 2024

Aug 2024 | 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 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. In this month’s issue, there is an additional analysis of GPS and Galileo fault events based on the monitoring of URA in the context of RAIM and Advanced RAIM (ARAIM).

Analyzed Parameters for June, 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. Satellite Fault Analysis for ARAIM: The knowledge of satellite and constellation plays an important role in the realization of ARAIM. Together with the SISRE, it allows characterization of the system behavior and determination of service commitments of each GNSS constellation

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) Satellite Fault Analysis for ARAIM

probability with which the unfaulted errors can occur and still meet the requirements of the integrity.The faulted and un-faulted state are separated by this probability and for which the Gaussian inflation factor, K, is computed as 4.42. The conceptual computation of the value is provided in the literature (Walter, et.al, 2019). The K factor along with the URA value for each satellite provides a threshold to

(similar to SISRE computed in this monthly analysis report but as instantaneous value and worst user location) and URA are very critical parameters for understanding the probabilities of fault events for each GNSS constellation. While the last month’s issue focused on the statistical distribution of URA dissemination by each constellation, in this monthly analysis the focus is put on the fault event analysis. It can be observed in following figures (f (1)- f (4)) that each constellation (GPS and Galileo) has satellites in fault state, where the users were not timely informed about the degradation of the performances. As per the Galileo SDD 2022, the constant value of 6 m is used for URA (or SISA) in the threshold computation. This is to be changed in the future where the corresponding broadcast URA (or SISA) has to be used for fault monitoring. For GPS, the corresponding URA from the broadcast message is used in the following analysis

The above events are examples of real GPS and Galileo fault types, triggered by a GPS satellite clock jump of about 25 ns and a Galileo satellite clock ramp reaching above 200 ns, in the past but only a continuous evaluation and estimates of probabilities of fault occurrence can ensure reliable services. In the current status, the GPS and Galileo satellite fault probability is well within the specified value (10^-5/hour/sat and 3 ×10^-5/hour/sat, resp.) in the service definition. Looking forward, the target of the ARAIM concept is to enable aviation integrity services from RNP 0.3 (lateral navigation) upto LPV-200 (vertical navigation) by a sole means of on-board integrity monitoring using Dual Frequency Multi-Constellation (DFMC) GNSS. Rather than broadcast the conservative URA as is the case in RAIM, the ARAIM concept expects to relax the URA values, provide a bias term and the multiplier terms to separately bound the SIS accuracy and integrity. In doing so, the availability and continuity can be maintained to meet the required specifications of RNP 0.3 and LPV-200. The deployment of initial ARAIM is planned for 2025. For that, a continous monitoring of the satellite & constellation performances and also the analysis of potential fault events are pivotal tasks. Even though the Galileo constellation has a relatively short history in comparison to the GPS, the performance is getting better and looks promising to push forward the DFMC application in aviation integrity. It is also to be noted that the GPS constellation did not have any faults between 2021 and early 2022, but there were couple of faults in 2022 and 2023 (it was analyzed in this report) pointing towards the necessity of a continuous service monitoring.

Monthly Performance Remarks:
1. Satellite Clock and Orbit Accuracy:
• For GPS, the satellite clock and orbit accuracy shows similar performance as in May 2024. There were couple of satellite outages and NANU and removed from the analysis.
• For Galileo, all parameters showed consistent performances.
• For GLONASS, the performance looked similar to the past months. There were couple of satellites unusable which were removed in the analysis.
• For BDS and QZSS, the performance looks very much the same as in the past.
• For IRNSS, the notable difference in this month’s performance is the URA for I03. There is no prediction available for more than one-third of the total URA broadcast.

2. UTC Prediction (GNSS-UTC):
• Not much difference in comparison to the last month analysis but Glonass showed drifting values.

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 real-time orbit determination for LUTAN-01B satellite using broadcast earth orientation parameters and multiGNSS 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 multiGNSS 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.