Abstract
Geopotential models derived from Gravity Recovery and Climate Experiment (GRACE) mission measurements are significantly obscured by the presence of a systematic artifact, known as longitudinal stripes. Based on our previous work (Peidou and Pagiatakis, 2020) we provide an in-depth analysis of the latitudinal sampling characteristics of GRACE and we reveal the intriguing sampling mechanism that creates sub-Nyquist artifacts (stripes). Because the sub-Nyquist artifacts are poorly understood, we provide a simple simulation example to elucidate the mechanism of the sub-Nyquist artifact generation. Subsequently, we randomly select June 2009 daily GPS precise science orbits for GRACE-A to produce ground tracks to sample the low frequency disturbing potential (geoid) along the parallel of ϕ=10° N. The sampled geoid is then deinterlaced in space to produce a monthly data sequence whose detailed analysis shows that the sub-Nyquist artifacts (stripes) are produced from a critical sampling rate of the low degree gravitational field that is related to the ratio m/n of two mutually prime integers, where m is the number of days it takes to have a nearly repeat orbit and n is the number of complete orbits in one day. We perform extensive analyses of GRACE Level-2 data over a period of eight years to show the variability in the orbital characteristics that are directly linked to the orbit resonances (via integers m and n). It turns out that during short repeat cycle resonances the stripes are amplified. Finally, to minimize the presence of stripes in Level-2 data products, it is recommended that orbits of future missions should be designed to avoid the critical m/n ratios while appropriately monitoring and adjusting them during the mission. For completed missions, or missions that are already active, force modelling the latitudinal low frequency disturbing potential may be a viable and most preferred approach to filtering.
Highlights
The sampled geoid is deinterlaced in space to produce a monthly data sequence whose detailed analysis shows that the sub-Nyquist artifacts are produced from a critical sampling rate of the low degree gravitational field that is related to the ratio m/n of two mutually prime integers, where m is the number of days it takes to have a nearly repeat orbit and n is the number of complete orbits in one day
We provide a typical sub-Nyquist artefact example that may show up in any experimental time or data series, and we link this example to Gravity Recovery and Climate Experiment (GRACE) sampling process with reference to the Lorentz reciprocity theorem
The example we presented in this study (Section 5) had a clear resonant structure at 107/7
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. These bands are usually referred to as longitudinal (north-south) stripes and they have been present in the geopotential models since the release of the first stream of temporal and static GRACE-based models These stripe artefacts are created while the spacecraft measures (samples) the gravitational field the longitudinal structure of the artefacts depends on the orbit of the mission (GRACE followed a near polar orbit). The need for an in-depth understanding of the source of stripes is further re-enforced by the upcoming Generation Gravity Missions (NGGM) [5,12,13], and by the requirement to develop mitigation strategies to prevent their presence This is precisely one of the main drives of this contribution that leads to recommendations on how to eliminate them either from the existing measurements or via a careful design of the orbits of future missions. We provide recommendations for mitigating the presence of stripes in future missions
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