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  • Sparse ground network  (1)
  • UT1‐UTC  (1)
  • 1
    Publication Date: 2024-01-12
    Description: Very Long Baseline Interferometry (VLBI) intensive (INT) sessions are critical for the rapid determination and densification of Universal Time 1‐Coordinate Universal Time (UT1‐UTC), which plays an important role in satellite geodesy and space exploration missions and is not predictable over longer time scales. Due to the limited observation geometry of INT sessions with two to three stations observing about 1 hr, tropospheric gradients cannot be estimated, which degrades the UT1‐UTC precision. We investigate the impact of tropospheric ties at Global Navigation Satellite Systems (GNSSs) and VLBI co‐located stations in INT sessions from 2001 to 2021. VLBI and GNSS observations are combined on the observation level. The results are evaluated by using both UT1‐UTC and Length of Day (LOD) from consecutive sessions. We demonstrate a better agreement of 10%–30% when comparing the derived LOD to GNSS LOD for INT1, INT2, and VGOS‐2 sessions; whereas, the agreement is not improved when directly comparing UT1‐UTC to the IERS Earth Orientation Parameters (EOPs) product, potentially because INT sessions also contribute to IERS EOP products. The major impact comes from tropospheric gradient ties, whereas applying zenith delay ties does not improve or even deteriorate UT1‐UTC agreement. Gradient ties also introduce systematic biases in UT1‐UTC by around −3 to −5 μs, except for the Russian INT sessions. Regression analysis shows that the east gradient introduces systematic effects in UT1‐UTC for sessions involving Germany and USA (Hawaii), whereas for Germany–Japan and Russian sessions, the north gradient also contributes systematically.
    Description: Plain Language Summary: Universal Time 1‐Coordinate Universal Time (UT1‐UTC) gives the time difference of UT1, defined by Earth's rotation, and UTC, defined by atomic clocks. UT1‐UTC is essential for real‐time navigation and space exploration. The variation of the first‐order negative time derivative of UT1‐UTC, Length of Day (LOD), is induced by mass redistribution, including tides of the solid Earth and oceans, the liquid core of the Earth and atmospheric variation, and climate events such as El Niño. Very Long Baseline Interferometry (VLBI) observing active galactic nuclei is the only space geodetic technique that can determine UT1‐UTC unambiguously. The 1‐hr intensive (INT) sessions, designed for the rapid determination and densification of UT1‐UTC, are performed daily with two VLBI radio telescopes. Due to the limited observation geometry, tropospheric gradients cannot be modeled in INT sessions, deteriorating UT1‐UTC estimates. We demonstrate an improvement of 10%–30% in LOD by applying tropospheric ties at VLBI and Global Navigation Satellite Systems co‐locations, especially the tropospheric gradients ties. Tropospheric gradient ties also introduce a systematic effect of −3 to −5 μs on UT1‐UTC, especially the east gradient. Our study shows that tropospheric ties should be adopted in future VLBI analysis for optimal UT1‐UTC products.
    Description: Key Points: Tropospheric ties are applied in a Global Navigation Satellite System–Very Long Baseline Interferometry (GNSS–VLBI) integrated solution analyzing VLBI intensive (INT) sessions from 2001 to 2021. Length of Day (LOD) of IVS INT sessions shows a better agreement by 10%–30% when compared to GNSS LOD product, mainly due to gradient ties. Gradient ties, especially the east one, introduce systematic biases of −3 to −5 μs in Universal Time 1‐Coordinate Universal Time of IVS INT sessions.
    Description: Helmholtz OCPC Program
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: https://www.iers.org/IERS/EN/DataProducts/EarthOrientationData/eop.html
    Description: http://doi.org/10.17616/R3RD2H
    Keywords: ddc:526 ; intensive sessions ; UT1‐UTC ; tropospheric ties ; GNSS ; VLBI ; integrated processing
    Language: English
    Type: doc-type:article
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  • 2
    Publication Date: 2024-02-14
    Description: The precise orbit determination (POD) of Global Navigation Satellite System (GNSS) satellites and low Earth orbiters (LEOs) are usually performed independently. It is a potential way to improve the GNSS orbits by integrating LEOs onboard observations into the processing, especially for the developing GNSS, e.g., Galileo with a sparse sensor station network and Beidou with a regional distributed operating network. In recent years, few studies combined the processing of ground- and space-based GNSS observations. The integrated POD of GPS satellites and seven LEOs, including GRACE-A/B, OSTM/Jason-2, Jason-3 and, Swarm-A/B/C, is discussed in this study. GPS code and phase observations obtained by onboard GPS receivers of LEOs and ground-based receivers of the International GNSS Service (IGS) tracking network are used together in one least-squares adjustment. The POD solutions of the integrated processing with different subsets of LEOs and ground stations are analyzed in detail. The derived GPS satellite orbits are validated by comparing with the official IGS products and internal comparison based on the differences of overlapping orbits and satellite positions at the day-boundary epoch. The differences between the GPS satellite orbits derived based on a 26-station network and the official IGS products decrease from 37.5 to 23.9 mm (34% improvement) in 1D-mean RMS when adding seven LEOs. Both the number of the space-based observations and the LEO orbit geometry affect the GPS satellite orbits derived in the integrated processing. In this study, the latter one is proved to be more critical. By including three LEOs in three different orbital planes, the GPS satellite orbits improve more than from adding seven well-selected additional stations to the network. Experiments with a ten-station and regional network show an improvement of the GPS satellite orbits from about 25 cm to less than five centimeters in 1D-mean RMS after integrating the seven LEOs.
    Description: Chinese Government Scholarship http://dx.doi.org/10.13039/501100010890
    Keywords: ddc:526 ; POD ; Integrated processing ; Sparse ground network ; GPS ; LEOs ; GRACE ; Jason ; Swarm
    Language: English
    Type: doc-type:article
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