Abstract
The Sentinel-6 (or Jason-CS) altimetry mission provides a long-term extension of the Topex and Jason-1/2/3 missions for ocean surface topography monitoring. Analysis of altimeter data relies on highly-accurate knowledge of the orbital position and requires radial RMS orbit errors of less than 1.5 cm. For precise orbit determination (POD), the Sentinel-6A spacecraft is equipped with a dual-constellation GNSS receiver. We present the results of Sentinel-6A POD solutions for the first 6 months since launch and demonstrate a 1-cm consistency of ambiguity-fixed GPS-only and Galileo-only solutions with the dual-constellation product. A similar performance (1.3 cm 3D RMS) is achieved in the comparison of kinematic and reduced-dynamic orbits. While Galileo measurements exhibit 30–50% smaller RMS errors than those of GPS, the POD benefits most from the availability of an increased number of satellites in the combined dual-frequency solution. Considering obvious uncertainties in the pre-mission calibration of the GNSS receiver antenna, an independent inflight calibration of the phase centers for GPS and Galileo signal frequencies is required. As such, Galileo observations cannot provide independent scale information and the estimated orbital height is ultimately driven by the employed forces models and knowledge of the center-of-mass location within the spacecraft. Using satellite laser ranging (SLR) from selected high-performance stations, a better than 1 cm RMS consistency of SLR normal points with the GNSS-based orbits is obtained, which further improves to 6 mm RMS when adjusting site-specific corrections to station positions and ranging biases. For the radial orbit component, a bias of less than 1 mm is found from the SLR analysis relative to the mean height of 13 high-performance SLR stations. Overall, the reduced-dynamic orbit determination based on GPS and Galileo tracking is considered to readily meet the altimetry-related Sentinel-6 mission needs for RMS height errors of less than 1.5 cm.
Highlights
Sentinel-6 is a multi-national satellite mission aiming to provide continuity of service for sea surface altimetry in the third decade of the current century (Scharroo et al 2016; Donlon et al 2021)
This study aims to characterize the precise orbit determination (POD) accuracy achievable for Sentinel-6A with dual-constellation GNSS tracking from the PODRIX receiver
Page 9 of 17 109 science telemetry on November 27, 2020. Throughout this period, GNSS data were collected with the “nominal” PODRIX receiver, which was connected to the Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) ultrastable oscillator (USO) and configured for “mixed,” block-specific GPS signal types
Summary
Sentinel-6 is a multi-national satellite mission aiming to provide continuity of service for sea surface altimetry in the third decade of the current century (Scharroo et al 2016; Donlon et al 2021). The prime instrument is a Poseidon-4 Ku/C-band altimeter (Cullen and Francis 2014; Donlon et al 2021), which offers both low- and high-resolution modes and represents a major evolution over the Poseidon-3 instruments of Jason-2 and -3. Ionospheric path delays in the altimeter measurements can be compensated and the total electron content below the spacecraft can be measured. For precise orbit determination (POD), Sentinel-6 carries a Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS; Auriol and Tourain 2010) receiver, a redundant pair of PODRIX GPS/Galileo receivers, and a laser retro-reflector array (LRA)
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