Abstract
Ocean tide aliasing is one of the largest error sources in satellite gravimetry. Despite its importance, the aliasing mechanism of ocean tides in satellite gravimetry is only partially understood. This paper explains tidal aliasing as a two-step mechanism. The primary aliasing is caused by orbit undersampling of original tidal signals. The secondary aliasing is due to undersampling of the primary aliasing signals through gravity recovery in discrete time intervals. The two-step aliasing mechanism is demonstrated through a closed-loop numerical simulation. The aliasing of the tidal constituents \(M_2\), \(N_2\), \(S_2\), \(K_2\), \(O_1\), \(P_1\), \(Q_1\) and \(K_1\) in CHAMP, GOCE, GRACE and GRACE Follow-On missions is analysed. The primary alias periods of individual constituents change from a few to hundreds of days depending on the orbital geometry. The long-term \(S_2\) aliasing may cause bias in gravity fields derived by GOCE data if the \(S_2\) tide is not well-represented by the ocean tide model applied in the data processing. Comparing the primary aliasing properties of CHAMP with that of GRACE indicates that tuning orbit inclination can improve aliasing properties, i.e. shorter aliasing periods and less couplings with annual/semi-annual signals. The strict definition of secondary alias periods is compromised in case of GRACE and GRACE Follow-On due to non-constant recovery periods, which results in irregular sampling and blurs the sharp spectral lines that are described by the two-step mechanism. The two-step aliasing mechanism can also be used in understanding aliasing of other periodic signals observed by satellite gravimetry.
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Data Availability Statement
The data we used in simulation is ocean tide model EOT08a. The updated version, EOT11a, is available at website page: https://doi.org/10.1594/PANGAEA.834232. We emphasize our closed-loop simulations do not rely on any specific ocean tide model. Inputs to the simulation are the orbital parameters (shown in text) and the known tidal spectral lines. Two-line element data of CHAMP, GOCE, GRACE and GRACE Follow-On missions are available at Space-Track.org.
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Acknowledgements
The first author thanks the China Scholarship Council (CSC) for the financial support of her PhD research. Part of this research topic was also supported by the ESA project “Additional Constellation & Scientific Analysis of the Next Generation Gravity Mission Concept” with the contract No. 4000118480/16/NL/FF/gp. We also thank all the reviewers and editors for their clear and constructive comments, as their efforts help us improve the quality of the paper.
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WL designed the research, performed the research and wrote the paper; NS supervised the research and revised the paper.
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Liu, W., Sneeuw, N. Aliasing of ocean tides in satellite gravimetry: a two-step mechanism. J Geod 95, 134 (2021). https://doi.org/10.1007/s00190-021-01586-6
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DOI: https://doi.org/10.1007/s00190-021-01586-6