GNSS


GNSS constellation specific monthly analysis summary: January 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 broadcast messages of all GNSS have evolved to provide further useful parameters in the last decade. In this article, besides baseline performance summary, an overview is provided to one of such relatively new parameters the Earth Rotation Parameters (ERPs) that are being sent out by satellites of GPS, BDS, QZSS, and IRNSS. It is now possible to apply the ERPs in real-time applications, thanks to the broadcast messages, for reference frame transformation between earth-fixed and inertial frames. All 4 constellations broadcast decent-quality ERPs with different update frequencies. In this month’s performance analysis summary, the broadcast ERPs constituting pole coordinates (xp and yp) and the difference between Universal Time 1 (UT1) and Universal Time Coordinated (UTC) are reported in addition to the satellite orbit and clock-related parameters.

The article attempts to also monitor the behavior of satellite attitude in terms of yaw angle. Even though such information is not critical for users applying only broadcast messages in the PVT applications, it is essential for precise applications where errors in satellite antenna phase center offset, carrier-phase wind-up, and solar radiation pressure modelling can not be ignored. In this month’s performance analysis, only GPS constellation is considered. In the future analysis, other constellations will be included.

Analyzed Parameters for 01-31 January, 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. 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 1 month of data, it can be observed that the accuracy of GNSS broadcast ERPs does not have drastic differences among the constellation. 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 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 non-nominal yaw attitude (GPS Constellation)

The GNSS satellites orient their satellite body-fixed reference

For January 2024, the yaw angles attained by all satellites for all combination of Sun elevation angles and the orbital angles are provided in Fig (g) (i) (here yaw angle and beta angle are shown as positive values only). The relation between the beta angle and the nominal yaw angle is straightforward: the lower the Sun-elevation angle, the higher the required yaw angle to orient the solar panels towards the Sun direction. This means GNSS satellites cannot follow a nominal yaw-steering whenever the Sun elevation angle relative to the orbital plane gets too low and the yaw rate required to keep the satellite solar panels pointing towards the Sun exceeds the maximum satellite yaw rate. The strategies on how GNSS satellites perform rate-limiited yaw-steering are different for each type of spacecraft and only partly documented for public users. The nominal yaw attitude has two singularities when the satellite is nearest to the sun (noon turn) and farthest away in the shadow of the Earth. For such low beta angels, the satellites start yawing with a higher yaw rate than the maximum satellite hardware yaw rate. The approach on how each GNSS satellite executes rate-limited yaw maneuver is different for each satellite.

(g) (i): GPS satellite nominal attitude yaw angle for different Sun elevation angle and orbital angle In January, the following satellites were maneuvering under the low beta angle ( > -1° and < 1°):

G08 (IIF): Day 04-07; G15 (IIR-M): Day 27-29; G17 (IIR-M): Day 10-13; G19 (IIR): Day 13- 15; G29 (IIRM): Day 11-14

As presented in different literature (Cao et. al, 2018; Liu et.al, 2022, Sylvain et. al, 2021), the yaw rate of the GPS 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 0.08°/sec for GPS IIF and IIIA satellites and 0.18°/sec for GPS IIR/RM satellites. To monitor the yaw rate of the identified satellites undergoing the midnight maneuver, satellites from IIF and IIR-M blocks are selected for January 2024 (note: no satellites from block III-A, are in the low beta angle during this month). For the IIF, the only available satellite undergoing mid-night maneuver is G08 and for this satellite, 08 January is selected where the beta angle is above 1° (to have a different beta angle in the analysis). For the IIR block, satellite G19 is selected for 14 Jan where the beta angle is below 1°. The yawing rates for G19 and G08 are computed to be 0.18°/sec and 0.05°/ sec. The beta angle for GPS 08 on 08 Jan was from -2.11° to -1.21°, which is the reason for the slower yaw rate. With an even smaller beta angle below 1 °, it is expected to yaw by more than 0.08°/sec.

(g) (ii): GPS satellite yaw maneuver for midnight turn during low beta angles

Note:- The yaw angle available from the BKG “SSRC00BKG” stream is used for this analysis. For higher precision applications where an accurate model of GNSS yaw attitude is needed, quaternions provided by IGS Analysis Centers can also be used to derive the yaw angle and to execute the reference frame transformations.

Monthly Performance Remarks:
1. Satellite Clock and Orbit Accuracy
• For GPS, the satellite clock and orbit accuracy shows improved performance in comparison to December 2023 (Dhital et. al, 2024). There were multiple satellites in maneuvers and non-healthy status. GPS satellite PRN 27 was removed from the analysis for a whole month due to bad data. It was declared unused in late December 2023 but came back to nominal status again. However, the orbit and clock data are erroneous. Overall, the SISRE value looked better due to improved satellite clock performance.
• For Galileo, there was a slight improvement in already good satellite clock performances. However, the orbit looked to go down by 7 cm in comparison to December 2023. In contrast to GPS, the SISRE for Galileo is impacted by the fluctuation in the orbit quality. It is noteworthy to point towards January 22, where multiple satellites including E30, E15, E3, E11, and E12 had relatively large discontinuity (> 4 ns).
• For GLONASS, there were numerous days (02, 06, 14-17 January) where the specific portion of the time had large clock offsets and in a few cases large orbit outliers. The reasons for such errors have not been identified in the analysis and will be updated after investigation in future issues. As a first step, a robust strategy to identify the errors in broadcast ephemeris data logging in RINEX is required. For January, the orbit performance after removing above mentioned days looks similar to December 2023 but the SISRE looks slightly degraded (mostly likely due to degraded clocks on some of the days).
• 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. This might be due to the nature of IGSO and will be looked into in detail in future analysis.
• For IRNSS, the overall constellation clock discontinuity is slightly degraded in comparison to December 2023. The URA statistics also suggest more broadcast data with degraded accuracies.:

2. UTC Prediction (GNSS-UTC):

Among the GNSS, only GLONASS showed different results in comparison to December 2023. The UTC prediction accuracy for GLONASS appears to be more stable. In this month’s analysis, QZSS is also added, which is not regularly present in the IGS RINEX V3 broadcast. For the available days, the offset remains very much the same..

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

Hauschlid A, Montenbruck O (2020) Precise real-time 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 multiGNSS 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

Data sources:

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))

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