The Mass-change And Geoscience International Constellation (MAGIC) is a future satellite gravity mission jointly investigated by ESA and NASA. It arose from NASA’s Mass Change Designated Observable (MCDO) and ESA’s Next Generation Gravity Mission (NGGM) studies. The joint specifications for the MAGIC mission are documented in the Mission Requirements Document (MRD). On ESA side, the MAGIC concept is currently investigated in two parallel industry Phase A studies as well as a science support study.

In the framework of the science support study, various potential mission constellations are investigated regarding their scientific outcome in terms of gravity retrieval errors and the provided space-time sampling of the Earth’s gravity field. This is accomplished using numerical closed-loop simulations. Numerous single- and double-pair low-low satellite-to-satellite tracking configurations are analyzed and compared among each other as well as to the International Union of Geodesy and Geophysics (IUGG) threshold and target user requirements. In particular, the improvement of the individual scenarios with respect to a GRACE-like single polar pair mission is evaluated.

With respect to the mission design, the effect of pendulum versus in-line formations, the accelerometer instrument specifications, the inter-satellite distance and the orbit altitude are investigated. For all simulated mission scenarios, the impact of temporal aliasing on the resulting gravity field solutions is evaluated by comparing the simulation results with and without time-variable gravity signals taken into account. As orbits with stable subcycles and constant longitude drift rates are assumed, the MAGIC mission will also allow to compute short-term gravity field solutions of a few days. This requirement for the orbits allows the usage of the observed gravity field data for a wider range of applications such as short-term applications.

Based on our analysis, recommendations with respect to the investigated mission design parameters are deduced. Especially, the considerable benefit of Bender-type double-pair formations is emphasized.

The primary goal of GRACE/GRACE-FO and potential future multi-pair satellite missions such as MAGIC (Mass-change And Geoscience International Constellation, jointly investigated by ESA and NASA), is the precise retrieval of variations within the Earth’s hydro- and cryosphere. These particular signal components, however, are widely superimposed by other geophysical signals, e.g. related to tidal and non-tidal loading of the atmosphere and oceans, which feature higher amplitudes and temporal frequencies and thus induce temporal aliasing. For Bender-type mission scenarios (GRACE-type polar satellite pair in combination with an additional satellite pair flying at a lower inclination) parametrization schemes aiming to further reduce the aliasing effects have already been proposed e.g. by Wiese et al. (2011). For single-pair missions, the state of the art is the background-model-based de-aliasing which does not fully prevent temporal aliasing due to inevitable imperfection of the applied models.

In this contribution we review the so-called Wiese parametrization scheme and discuss its potential drawbacks. Based on these findings we present a novel, entirely data-driven self-de-aliasing approach which can be applied to single- as well as multi-pair-based data processing. This method consists of (a) the estimation of short-term low-resolution gravity solutions allowing to capture short-periodic signal components and to prevent it from aliasing into the long-term (e.g. monthly) solution by (b) reducing the observations by these interval fields. The functionality of this method is verified through numerical closed-loop simulations. It is shown that this approach is of primary benefit for single-pair mission scenarios, where the Wiese scheme does not yield any benefit at best. In case of Bender-type multi-pair scenarios this approach yields an at least similar gain in retrieval performance compared to Wiese. However, this gain can be further maximized by introducing additional estimation steps for various interval lengths which simultaneously results in reliable stand-alone short-period gravity solutions.

Groundwater (GW) is the world’s largest distributed freshwater storage for mankind, ecosystems, and is a key resource for industrial and agricultural demands. Due to its fundamental role in the Earth's water and energy cycles, groundwater has been declared as an Essential Climate Variable (ECV) by GCOS, the Global Climate Observing System. However, within Copernicus - the European Earth Observation Programme - there is no service available yet to deliver data on this fundamental resource, nor is there any other data source worldwide that operationally provides information on changing groundwater resources in a consistent way, observation-based, and with global coverage. Therefore, the Global Gravity-based Groundwater Product (G3P) project aims at developing an operational global groundwater service as a cross-cutting extension of the existing Copernicus - the European Earth Observation Programme - portfolio. G3P capitalizes from the unique capability of GRACE and GRACE-FO satellite gravimetry as the only remote sensing technology to monitor subsurface mass variations, and from other satellite-based water storage products that are already part of the Copernicus portfolio, to provide a data set of groundwater storage change for large areas with global coverage. G3P is obtained by using a mass balance approach, i.e., by subtracting satellite-based water storage compartments (WSCs) such as snow water equivalent, root-zone soil moisture, glacier mass, and surface water storage from GRACE/GRACE-FO monthly terrestrial water storage anomalies (TWSA). Compatibility of the observation-based WSCs with TWSA is achieved by a filtering process, where optimal filter types were derived by analyses of spatial correlation patterns. G3P groundwater variations are provided for almost two decades (from 2002 to the present), with the monthly resolution, and at a 0.5-degree spatial resolution globally. In this contribution, we also illustrate preliminary results of the G3P data set and of its uncertainties, as well as its evaluation by independent groundwater data. This study has been run in the context of the European Union’s Horizon 2020 research project G3P (Global Gravity-based Groundwater Product, grant agreement nº 870353).

The German-American satellite missions GRACE (Gravity Recovery and Climate Experiment) and its successor GRACE-Follow-On provide the unique opportunity to observe the total water storage (TWS) variations over the continents since 2002. With this nearly 20 years of data record, trends in water storage can be investigated beyond the strong declining trends of the ice sheets and glaciers. While there is an overall negative trend in continental water storage at the global scale, Africa is the only continent with an overall positive trend in TWS. In this contribution, we have a detailed look into the water storage changes and trends in Africa. We make an attempt of explaining these trends by comparison to other hydrological observations such as precipitation.
The long-term TWS increase in Africa is most pronounced in the East-African rift and the Niger River Basin. Some other regions such as Madagaskar exhibit a (statistically significant) negative TWS trend. Furthermore, the trends are not monotonous over time. For example, the increasing trend in East Africa only started around the year 2006 and accelerated after 2012. On the other hand, South Africa wetted until 2012 and is again drying since then.
In this study, we divide the African continent into climatically similar regions and investigate the regional mean TWS signals. As they are more complex than a linear trend and (sinusoidal annual and semiannual) seasonality, we employ the STL method (Seasonal Trend decomposition based on Loess). In this way, turning points are identified in the so-called trend component to mark major trend changes.
The observed TWS changes in Africa are mostly caused by changing precipitation patterns, observed for example with the GPCP (Global Precipitation Climatology Project) data set. In some regions such as South Africa, the correlation between precipitation and TWS change is evident whereas other regions show a more complex relationship between these two variables.

An overview of Spire’s unique GNSS-R altimetry product is provided in this presentation.

Spire operates a constellation of smallsats equipped with an advanced GNSS receiver designed to collect radio occultation observations. In 2019, the receivers were configured to additionally measure GNSS reflections at grazing angles, i.e., between 5 and 30 degrees. Nguyen et al. [2020] demonstrated the feasibility of phase delay altimetry using coherent reflections, following a number of prior studies (e.g., Martín-Neira [1993], Cardellach et al. [2004]). An in-house processing pipeline was subsequently established to generate Level 2 grazing angle altimetry products on an operational basis. As of November 2021, over 20 satellites continuously measure GNSS-R grazing angle reflection events in areas of high-coherence, i.e., calm waters and glaciated surfaces. Three satellites were added to the constellation in early November 2021.

Everyday, over 550 altimetry profiles are produced, spanning dozens to several thousand kms, without any inherent geographic limitation (i.e., no polar gap). Relative heights are provided with respect to a reference model. Over the oceans and sea ice, the large majority of profiles (>80%) can recover a reference surface model determined by a Mean Sea Surface and tides to within 50 centimeters. Bulk assessment of properties of altimetry profiles show that reflections occurring over sea ice are particularly long and coherent. Another opportune application of GNSS-R grazing angle altimetry pertains to land retrievals, for example in areas where reflections transition into glaciated land, such as the coastal regions in Antarctica. There, height retrievals over land surfaces have been shown to follow dynamic topographic changes and agree well with a reference DEM. Coherent retrievals are additionally available over higher altitude regions of the ice sheets, as well as glaciers and inland water bodies. We will also present the results of systematic assessments of more demanding altimetry applications such as sea ice freeboard height and sea level anomalies.

Given the wide array of potential scientific applications, the global and sustainable nature of Spire data collection, the availability of ancillary altimetry datasets and colocated measurements, and the unique measurement system (grazing angle GNSS-R from spaceborne platforms), there is a large body of work still to be accomplished. Studies by the geodesy community are encouraged and the data are freely available in agency portals such as through NASA CSDAP and ESA Earthnet.

Moreover, Spire data are not limited to Level 2 altimetry products. First, Level 1 products contain GNSS phase information alongside SNR and geometry information (position of transmitter, receiver, etc.). Complementary studies in the wider scientific community have already utilized such Spire reflection data to extract river heights and long-wavelength behavior of sea ice freeboard. Second, raw IF are also available upon request for dedicated studies over areas such as the Amazon, Great Lakes, etc. These types of lower level data are particularly useful to investigate components of the retrieval algorithm [Wang et al., 2020]. One example is phase unwrapping in high-noise environments, such as high elevation reflections (above 20 degrees) and/or rough sea state conditions. Furthermore, for low elevation reflections, an important source of error stems from the estimation of tropospheric delay, which has so far been mitigated via different approaches (different mapping functions, different atmospheric models, empirical corrections), with varying degrees of success.

Lastly, Spire satellites are continually undergoing modifications to improve and expand grazing angle observations. For example, multi-GNSS collection capability was recently implemented and tested on one satellite. One month of data collections showed that Galileo and GLONASS-based height retrievals were of the same quality and nearly as frequent and geographically diverse as those from GPS collections.

In conclusion, the Spire altimetry and reflections dataset presents a unique opportunity to 1) study the Earth’s surface and its evolution in a manner that is complementary to existing altimetry missions, 2) utilize the novel grazing angle dataset from space-based assets to research GNSS-R altimetry retrieval algorithms, and 3) harness a large volume of data (hundreds of thousands of altimetry profiles over oceans, sea ice, and land) to enable data mining in a diverse set of conditions.



References

Cardellach, E., Ao, C. O., De la Torre Juárez, M., & Hajj, G. A. (2004). Carrier phase delay altimetry with GPS-reflection/occultation interferometry from low Earth orbiters. Geophysical Research Letters, 31(10), L10402.
Martín-Neira, M., (1993), A passive reflectometry and interferometry system (PARIS): Application to ocean altimetry, ESA J., vol. 17, no. 4, pp. 331–355.
Nguyen, V. A., et al. (2020), Initial GNSS Phase Altimetry Measurements From the Spire Satellite Constellation, Geophys. Res. Letters, vol. 47, no. 15.
Wang, Y., B. Breitsch and Y. T. J. Morton, "A State-Based Method to Simultaneously Reduce Cycle Slips and Noise in Coherent GNSS-R Phase Measurements From Open-Loop Tracking," in IEEE Transactions on Geoscience and Remote Sensing, vol. 59, no. 10, pp. 8873-8884, Oct. 2021, doi: 10.1109/TGRS.2020.3036031.

The global mean sea level (GMSL) changes result from the sum of the steric and ocean mass global mean changes. Assessing the GMSL budget allows to cross-validate the consistency and stability of the observing systems involved. Many independent studies have shown that the GMSL budget is closed within uncertainties over the 1993-2016 time period. However, the sea level budget appears no longer closed after 2016 when using Jason-3 altimetry, Argo and GRACE/GRACE Follow-On gravimetric data. This non-closure may result from errors in one or several components of the sea level budget (altimetry-based GMSL, Argo-based steric sea level or GRACE-based ocean mass). We have investigated possible sources of errors affecting Jason-3 and Argo data. Concerning altimetry data, comparisons of Jason-3 GMSL time series with other altimetry missions show agreement within 0.4 mm/yr of standard uncertainty over 2016-2020. However, the comparisons of Jason-3 wet tropospheric correction with other altimetry missions and with climate data records show that Jason-3 radiometer is likely to drift. Such drift could be responsible for about 30 % of the non-closure of the budget. Concerning Argo in-situ data, a good agreement is found between all available thermosteric products. However, a decrease in the global mean halosteric sea level is observed since 2016 with strong discrepancies between the different data providers. A halosteric decrease corresponds to a salinity increase which is in contradiction with the global freshening of the oceans. This non-physical behaviour is attributed to uncorrected salinity measurement drifts and is responsible for about 40 % of the budget non-closure. Given that the halosteric component is expected to be negligible in global average, this spurious contribution should be neglected in the budget. The budget closure is significantly improved by taking into account a Jason-3 radiometer drift and assessing the sea level budget using only the thermosteric and ocean mass components. The remaining budget residual trends could be due to potential errors in the other components (i.e. thermosteric component based on Argo in-situ data, global mean ocean mass component based on GRACE and GRACE Follow-On satellite gravimetric data, missing physical contribution) that should be further investigated.