ALBERT

Description: 〈span〉〈div〉SUMMARY〈/div〉The goal of next generation gravity missions (NGGM) is to improve the monitoring of mass transport in the Earth system by an increased space–time sampling capability as well as higher accuracies of a new generation of instrumentation. They should be able to fulfil the scientific and societal needs of providing high-resolution short-time gravity field solutions for geophysical applications like for for example service applications such as flood and drought monitoring and forecast or applications in water management. To facilitate this need a near-real time (NRT) processing scheme based on a coparametrization of low-resolution daily and longer-term gravity field solution, combined with a sliding window averaging, was set up. In contrast to other strategies that are usually based on Kalman filtering, the proposed NRT concept is independent of any prior information about the temporal gravity field, and does not require any regularization. The enhanced spatial-temporal resolution opens the possibility to self-dealias high-frequency atmospheric and oceanic signals, and additionally provides gravity field solutions on short timescales. In order to quantify the capabilities of the proposed NRT approach, a numerical closed-loop simulation of a low-low satellite-to-satellite tracking (ll-sst) mission for a two-pair Bender-type constellation with realistic noise assumptions was performed. While for the daily parametrization a spherical harmonics degree and order of 15 turns out to be a favourable choice, by applying the sliding window NRT approach stable daily gravity field estimates up to degree/order 50 with latencies of down to 1 d could be achieved.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The goal of next generation gravity missions (NGGM) is to improve the monitoring of mass transport in the Earth system by an increased space-time sampling capability as well as higher accuracies of a new generation of instrumentation. They should be able to fulfil the scientific and societal needs of providing high-resolution short-time gravity field solutions for geophysical applications like for e.g. service applications such as flood and drought monitoring and forecast or applications in water management. To facilitate this need a near-real time (NRT) processing scheme based on a co-parametrization of low-resolution daily and longer-term gravity field solution, combined with a sliding window averaging, was set up. In contrast to other strategies that are usually based on Kalman filtering, the proposed NRT concept is independent of any prior information about the temporal gravity field, and does not require any regularization. The enhanced spatial-temporal resolution opens the possibility to self-dealias high-frequency atmospheric and oceanic signals, and additionally provides gravity field solutions on short timescales. In order to quantify the capabilities of the proposed NRT approach, a numerical closed-loop simulation of a low-low satellite-to-satellite tracking (ll-sst) mission for a two-pair Bender-type constellation with realistic noise assumptions was performed. While for the daily parameterization a spherical harmonics degree and order of 15 turns out to be a favourable choice, by applying the sliding window NRT approach stable daily gravity field estimates up to degree/order 50 with latencies of down to 1 day could be achieved.〈/span〉

Description: Temporal gravity retrieval simulation results of a future Bender-type double pair mission concept, performed by five processing centers of a Sino-European study team, have been inter-compared and assessed. They were computed in a synthetic closed-loop simulation world by five independent software systems applying different gravity retrieval methods, but were based on jointly defined mission scenarios. The inter-comparison showed that the results achieved a quite similar performance. Exemplarily, the root mean square (RMS) deviations of global equivalent water height fields from their true reference, resolved up to degree and order 30 of a 9-day solution, vary in the order of 10% of the target signal. Also, co-estimated independent daily gravity fields up to degree and order 15, which have been co-estimated by all processing centers, do not show large differences among each other. This positive result is an important pre-requisite and basis for future joint activities towards the realization of next-generation gravity missions.

Description: Temporal gravity retrieval simulation results of a future Bender-type double pair mission concept, performed by five processing centers of a Sino-European study team, have been inter-compared and assessed. They were computed in a synthetic closed-loop simulation world by five independent software systems applying different gravity retrieval methods, but were based on jointly defined mission scenarios. The inter-comparison showed that the results achieved a quite similar performance. Exemplarily, the root mean square (RMS) deviations of global equivalent water height fields from their true reference, resolved up to degree and order 30 of a 9-day solution, vary in the order of 10% of the target signal. Also, co-estimated independent daily gravity fields up to degree and order 15, which have been co-estimated by all processing centers, do not show large differences among each other. This positive result is an important pre-requisite and basis for future joint activities towards the realization of next-generation gravity missions.

Description: The GRACE mission has demonstrated a tremendous potential for observing mass changes in the Earth system from space for climate research and the observation of climate change. Future mission should on the one hand extend the already existing time series and also provide higher spatial and temporal resolution that is required to fulfil all needs placed on a future mission. To analyse the applicability of such a Next Generation Gravity Mission (NGGM) concept regarding hydrological applications, two GRACE-FO-type pairs in Bender formation are analysed. The numerical closed loop simulations with a realistic noise assumption are based on the short arc approach and make use of the Wiese approach, enabling a self-de-aliasing of high-frequency atmospheric and oceanic signals, and a NRT approach for a short latency. Numerical simulations for future gravity mission concepts are based on geophysical models, representing the time-variable gravity field. First tests regarding the usability of the hydrology component contained in the Earth System Model (ESM) by the European Space Agency (ESA) for the analysis regarding a possible flood monitoring and detection showed a clear signal in a third of the analysed flood cases. Our analysis of selected cases found that detection of floods was clearly possible with the reconstructed AOHIS/HIS signal in 20% of the tested examples, while in 40% of the cases a peak was visible but not clearly recognisable.

Description: The GRACE and GRACE-FO missions have been observing time variations of the Earth's gravity field for more than 15 yr. For a possible successor mission, the need to continue mass change observations have to be balanced with the ambition for monitoring capabilities with an enhanced spatial and temporal resolution that will enable improved scientific results and will serve operational services and applications. Various study groups performed individual simulations to analyse different aspects of possible NGGMs from a scientific and technical point of view. As these studies are not directly comparable due to different assumptions regarding mission design and instrumentation, the goal of this paper is to systematically analyse and quantify the key mission parameters (number of satellite pairs, orbit altitude, sensors) and the impact of various error sources (AO, OT models, post-processing) in a consistent simulation environment. Our study demonstrates that a single-pair mission with laser interferometry in a low orbit with a drag compensation system would be the only possibility within the single-pair options to increase the performance compared to the GRACE/GRACE-FO. Tailored post-processing is not able to achieve the same performance as a double-pair mission without post-processing. Also, such a mission concept does not solve the problems of temporal aliasing due to observation geometry. In contrast, double-pair concepts have the potential to retrieve the full AOHIS signal and in some cases even double the performance to the comparable single-pair scenario. When combining a double-pair with laser interferometry and an improved accelerometer, the sensor noise is, apart from the ocean tide modelling errors, one of the limiting factors. Therefore, the next big step for observing the gravity field globally with a satellite mission can only be taken by launching a double pair mission. With this quantification of key architecture features of a future satellite gravity mission, the study aims to improve the available information to allow for an informed decision making and give an indication of priority for the different mission concepts.

Description: The goal of next-generation gravity missions (NGGM) is to improve the monitoring of mass transport in the Earth system by an increased space-time sampling capability as well as higher accuracies of a new generation of instrumentation, but also to continue the monitoring time series obtained by past and current missions such as GRACE and GRACE Follow-On. As the likelihood of three satellite pairs being simultaneously in orbit in the mid-term future increased, we have performed a closed-loop simulation to investigate the impact of a third pair in either polar or inclined orbit as an addition to a Bender-type constellation with NGGM instrumentation. For the additional pair, GRACE-like as well as NGGM instrumentation was tested. The analysis showed that the third pair mainly increases the redundancy of the monitoring system but does not significantly improve de-aliasing capabilities. The best-performing triple-pair scenario comprises a third inclined pair with NGGM sensors. Starting with a Bender-type constellation of a polar and an inclined satellite pair, simulation results indicate an average improvement of 11% in case of adding the third pair in a near-polar orbit, and of 21% for the third pair placed in an inclined orbit. The most important advantage of a multi-pair constellation, however, is the possibility to recover daily gravity fields with higher spatial resolution. In the case of the investigated triple-pair scenarios, a meaningful daily resolution with a maximum spherical harmonic degree of 26 can be achieved, while a higher daily parametrization up to degree 40 results in spatial aliasing and thus would need additional constraints or prior information.