Mitigation of temporal aliasing effects in the context of a next generation gravity mission
19/09/2016 | 15:00 | Session 1: New missions and concept performance
Author(s): Ilias Daras, Markus Hauk, Roland Pail and Anna Purkauser
Ilias Daras, Markus Hauk, Roland Pail and Anna Purkauser
Future gravity field missions are expected to use new generation instruments of unprecedented accuracy that will positively affect the error budget of space-borne gravity field recovery as we know it today. In the error budget of future mission concepts, temporal aliasing effects of tidal and non-tidal nature are expected to constitute the major contributors. Therefore, the mitigation of temporal aliasing effects is crucial in order to be able to exploit the new sensor accuracies to their full extent.
At first, an assessment of temporal aliasing effects in the context of next generation gravity missions is performed. Temporal aliasing effects mainly result from the following direct or indirect sources:
- Under-sampling of signals of interest (e.g. hydrology)
- Uncertainties in the de-aliasing models (e.g. high frequency atmospheric and oceanic signals)
- Uncertainties in the ocean tide models
This study addresses all three sources in a joint analysis, quantifies the individual contributions to their error budget as well as the correlations among each other, and defines a new strategy for gravity field processing that mitigates their effect. Additionally, we discuss the different aspects of classical de-aliasing versus observation of high-frequent signals based on full-scale closed-loop simulations. We focus on the analysis and validation of high-frequent gravity field estimates from single and double pairs and the capability of these observations to replace the classical de-aliasing approach at least for the non-tidal part. The investigations are performed for satellite formations such as the Bender-type constellation consisting of one (near-)polar pair and one inclined pair, as well as the GETRIS constellation, which assumes a high-precision inter-satellite link between high-flying GEO and/or GNSS satellites and an ensemble of low Earth orbiters (LEOs).
A method called the Wiese approach (Wiese , 2013) that results in reduction of temporal aliasing effects is investigated for a Bender-type constellation. This method employs a gravity field processing technique that co-estimates low resolution gravity fields at short time intervals together with the higher resolution gravity field which is sampled at a longer time interval. The approach opens a big search space for parameterization choices, which is thoroughly investigated. The effect of each choice is assessed, and the long-term as well as the short-term fields are validated. The method is also extended in order to retrieve even more signal variability at higher frequencies, by parameterizing in multiple time periods instead of single (Daras, 2016). A fine-tuning is performed in terms of temporal sampling and maximum resolution of the short-term gravity fields that leads to the biggest reduction of aliasing errors. The contribution of individual aliasing components to the error-budget is assessed, and the effectivity of the Wiese parameterization to reduce their effects is demonstrated. Concerning the mitigation of the aliasing effects due to the uncertainties in the ocean tide models, several methods are investigated both from a methodological and a numerical point of view in a full-scale closed-loop environment. As a further aspect of this work, possible correlations between a dedicated ocean tide co-parameterization with other parameters (Wiese, empirical accelerations, etc.) and their impact on the gravity solution shall be analyzed in detail.
Finally, it is demonstrated that future gravity satellite missions of a Bender-type constellation processed with the proposed parameterization, have the possibility to estimate the full spectrum of non-tidal geophysical processes that comprise system Earth, including variations of atmosphere and ocean. The averaged long-term fields, together with the short-term low-resolution fields (new by-product) of this content, comprise an innovative product that may open doors to new fields of application.