Live display program

The signal acquisition by the two different GRACE-like satellite pairs in a Bender configuration - polar and inclined, is dissimilar to each other. This difference is attributable to differing relative sampling geometry and global coverage. While the polar pair covers the entire globe, the inclined pair does not cover the higher latitudes leaving a local discontinuity around the poles in acquired signal (better known as the Polar Gap problem). Similarly, due to its north-south orientation, the polar pair can capture well the features that are predominantly oriented in the east-west direction. We simulated a Bender configuration using ESA's Earth System Model to see how the two satellite pairs contributed to the spherical harmonic coefficients. The general pattern was that the polar orbit contributed strongly to the zonal coefficients and the tesserals around it (near-zonal coefficients) while the inclined orbit contributed strongly to the other tesseral and the sectorial coefficients, which is well known. We also found out that the weak zonal and near-zonal inclined pair contributions lay inside a wedge in the spectral space, very similar to the polar gap error wedge. We want to discern how the satellites' relative geometry, particularly the polar gap issue in the inclined pair of a bender configuration, affects the solution's spectral resolution. In this study, we model the contribution coefficients of the polar and inclined pairs as a function of orbit geometries, employing the semi-analytical framework based on inclination functions. We hope that this will help in understanding the spectral resolution of the next generation gravity missions.

How to cite: Yadav, A., Devaraju, B., Weigelt, M., and Sneeuw, N.: Revisiting the sampling problem of satellite gravimetry – a perspective from the Bender configuration, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-38, https://doi.org/10.5194/gstm2020-38, 2020.

In the context of an increased public interest in climate-relevant processes, a number of studies on Next Generation Gravity Missions (NGGMs) have been commissioned to better map mass transport processes on Earth. On the basis of the successfully completed gravity field missions CHAMP, GOCE and GRACE as well as the current satellite mission GRACE-FO, different concepts were examined for their feasibility and economic efficiency. The focus is on increasing the spatiotemporal resolution while simultaneously reducing the known error effects such as the aliasing of temporal gravity fields due to under-sampling of signals and uncertainties in ocean tide models. An additional inclined pair to a GRACE-like satellite pair (Bender constellation) is the most promising solution. Since the costs for a realization of the Bender constellation are very high, this contribution focuses on alternative concepts in the form of different constellations and formations of small satellites. The latter includes both satellite pairs and chains consisting of trailing satellites. The aim is to provide a cost-effective alternative to the previous gravity field satellites while simultaneously increasing the spatiotemporal resolution and minimizing the above mentioned error effects. In numerical closed-loop simulations, various scenarios will be conducted which differ in orbit parameters like shape and number of orbits and the number of satellites per orbit and instrument performances. Additionally, the impacts from the co-parametrization of non-tidal short periodic temporal gravity field signal on the gravity field solutions (so-called Wiese approach), obtained by the different concepts, will be investigated. In particular the possibilities and limits with multiple satellites pairs for achieving the highest possible spatial and temporal resolution in (sub-)daily temporal gravity fields shall be analyzed in detail which is crucial for macrosocial tasks in water balance estimation and water management.

How to cite: Pfaffenzeller, N. and Pail, R.: Constellations and formations of small satellites in NGGM concepts, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-44, https://doi.org/10.5194/gstm2020-44, 2020.

Following the recommendations of the 2019 CNES scientific prospective seminar, a pre-Phase-A study was launched in January 2020 at CNES on the concept of the MARVEL mission. MARVEL proposed to carry out in a single mission the survey of earth mass transfers with increased precision and the determination of the terrestrial reference system, thanks to two constellations of satellites at 500 and 7000 km altitude equipped with "radial" measurement links between the constellations. The results of the first six months of this pre-Phase-A will be presented.

Our work has mainly focused on two axes:
- a closed-loop numerical simulation to assess the scientific performance of a large number of orbital configurations, including radial links but also "Bender" and "Pendulum" configurations;
- a technological study to bring the precision of a laser inter-satellites chronometry link to the micrometric level.

This work has, so far, highlighted:
1 / the poorer performance for the determination of the gravity field of the "radial" type measurements compared to a "pendular" configuration in which the satellites are at the same low altitude with the ascending nodes of their orbits slightly offset so that the measurements between the spacecraft are alternatively oriented to the right and to the left of the track, thus improving the geometric configuration compared to an in-line pair of satellites;
2 / the technological possibility of achieving by chronometric laser link, over a few hundred or even a few thousand km, the precision of 1 micrometer in distance and 0.1 micrometer/s in relative velocity (which is necessary for the achievement of the scientific objectives).

The pre-Phase-A MARVEL, taking into account the "Mass Change" studies on the NASA side and "NGGM" on the ESA side, is therefore reoriented towards a "pendulum" type mission, abandoning the objective of determining jointly the terrestrial reference system.

How to cite: Lemoine, J.-M., Mandea, M., Meyssignac, B., Blazquez, A., Lopes, L., Michaud, J., Balmino, G., Bourgogne, S., Samain, E., Costes, V., and Bruinsma, S.: MARVEL mission proposal: The latest update, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-47, https://doi.org/10.5194/gstm2020-47, 2020.

The actual NASA Earth Science Decadal Survey Report highlights mass transport monitoring as one of five top priorities in Earth Observation for the next decade. To realize such as Mass Change Mission (MCM) NASA is seeking for international partnership. A future continuation of the very successful technological and scientific GRACE/GRACE-FO partnership between the U.S. and Germany is in the involved partners’ highest interest and would be based on a strong heritage in the fields of satellite manufacturing, laser ranging interferometry (LRI) or science data utilization.

The goal of a study, jointly performed in summer 2020 between DLR, industry and HGF and MPG scientists, was to bundle up an attractive scientific and technological German package for further discussions with NASA which 1) compares the cost and benefit of technical modifications with respect to GRACE-FO, 2) is not only attractive for a future MCM but also for the Laser Interferometer Space Antenna (LISA) and 3) strengthens at the same time Germany´s role towards ESA´s Next Generation Gravity Mission (NGGM) implementation.

An ICARUS (International Cooperation for Animal Research Using Space) payload system on a future polar-orbiting GRACE-like “GRACE-ICARUS” (or short “GRACE-I”) mission could provide a much-desired scientific extension of biodiversity monitoring, which is another designated observable in NASA´s Decadal Survey.

Three mission options have been investigated:

The nominal spacecraft separation for all options was 220 km at an altitude of 490 km with free-decay (Options 1a, 2a and 3a), or alternatively 420 km maintained using electric propulsion (Options 1b, 2b and 3b).

The presentation summarizes the most important findings of our study and discusses possible steps forward towards implementation with NASA.

How to cite: Flechtner, F. and the GRACE-I Team: Realization of a satellite mission “GRACE-I” for parallel observation of changing global water resources and biodiversity, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-6, https://doi.org/10.5194/gstm2020-6, 2020.

We have been investigating the science performance for different gravity mission constellation architectures using smallsats. Small satellite systems are increasingly being used in scientific missions, due to their increase in affordability and improvement in performance over the past years. A small satellite constellation of GRACE-like pairs or other inter satellite ranging configurations would allow for improved spatial and temporal resolution as well as allowing for a high inherent system redundancy and a lower overall cost. Additionally, such a mission architecture would be more robust to failure since a constellation is insensitive to single point failures and individual satellites can be replaced at a lower cost.
The design of such a mission architecture is not straightforward due to the vast search space that needs to be considered. In this work we make use of a multi-objective evolutionary algorithm that is population-based and metaheuristic based on Darwinian theory in order to identify future GRACE-like constellations that are optimized to retrieve sub-monthly time-varying gravity field events.

How to cite: Deccia, C. M. A., Wiese, D. N., Loomis, B. D., and Nerem, R. S.: Design of GRACE-like Small Satellite Constellations for Improved Temporal Gravity Measurements., GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-54, https://doi.org/10.5194/gstm2020-54, 2020.

A point mass on the surface of the Earth gives the highest frequency content for orbiting gravimetry, with  the maximum frequency for gradiometers or satellite-to-satellite tracking determined by orbital altitude.  Frequency-domain expressions are found for  measurements of a point-like source on the surface of the Earth.  The response of orbiting gradiometers such as GOCE and satellite-to-satellite tracking missions such as GRACE-FO are compared. The optimal signal-to-noise ratio as a function  of noise in the measurement apparatus is computed, and from that the minimum detectable mass is inferred. The point mass magnitude that gives signal-to-noise ratio = 3 is for GOCE  M_3=200 Gton and  for the laser ranging interferometer measurement on GRACE-FO  M_3= 0.5 Gton. For the laser ranging interferometer measurement, the optimal filter for detecting point-like masses has a passband of 1 to 20 mHz,  differing from the 0.3 to 20 mHz admittance filter of Ghobadi-Far et al. (2018), which is not specialized for detecting point-like masses. M_3 for  future GRACE-like missions with different orbital parameters and improved instrument sensitivity is explored, and the optimum spacecraft separation is found.

How to cite: Spero, R.: Mass Sensitivity or Gravimetric Satellites, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-19, https://doi.org/10.5194/gstm2020-19, 2020.

The 2017-2027 US National Academy of Sciences Decadal Survey for Earth Science and Applications from Space classified mass change as one of five designated observables having the highest priority in terms of Earth observations required to better understand the Earth system over the next decade.  In response to this designation, NASA initiated multi-center studies with an overarching goal of defining observing system architectures for each designated observable.  Here, we discuss the progress made and future plans for the Mass Change Designated Observable study. Progress includes the development of a Science and Applications Traceability Matrix (SATM), the definition of three different architectural classes that have potential to be responsive to the designated science objectives, and a framework to quantitatively link the performance of specific architectures to the SATM.  We will describe a Value Framework that has been developed to assess the value of each architecture in terms of science return, cost, schedule, and technical maturity.  Results assessing the value of 50+ architecture variants will be shown and discussed. Finally, the current status of the study process, and future plans will be discussed.

How to cite: Wiese, D. and the NASA Mass Change Designated Observable Study Team: The NASA Mass Change Designated Observable Study Team, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-36, https://doi.org/10.5194/gstm2020-36, 2020.