Why Simulations Matter for the Mass Change and Geosciences International Constellation (MAGIC)

For more than two decades, satellites have been tracking how Earth's water moves—whether it's ice melting, groundwater levels changing, or sea levels rising. The original Gravity Recovery and Climate Experiment (GRACE) satellite mission (2002–2017) and its successor, GRACE Follow-on (GRACE-FO, since 2018), have provided a continuous record of these mass changes. To keep this critical data flowing, the U.S. and German space agencies (NASA and DLR) are developing a new mission, GRACE-C (Continuation), due for launch at the end of 2028. In addition, the European Space Agency (ESA) is preparing the Next Generation Gravity Mission (NGGM), scheduled for launch in 2032.

The planned Mass-Change and Geoscience International Constellation (MAGIC) aims to operate these two next-generation missions together, creating a powerful Earth observation system. By combining two pairs of satellites (NGGM and GRACE-C), MAGIC will provide enhanced spatial and temporal resolution for time-varying gravity field measurements, with reduced uncertainty and latency, to address the international user needs as articulated by the International Union of Geodesy and Geophysics (IUGG) and the Global Climate Observing System (GCOS), whilst demonstrating operational capabilities relevant to Copernicus. But how can we be certain that MAGIC will work as anticipated? This is where simulations come into play.

Dr. Josefine Wilms, GFZ Helmholtz Centre for Geosciences

Before launching satellites, scientists use computer simulations to predict how well different mission designs will measure Earth's gravity field. For example, as early as the late 1960s, scientists studied the idea of combining high-low satellite-to-satellite tracking (SST) between a Low Earth Orbiter (LEO) and a Global Navigation Satellite System (GNSS) satellite with low-low SST between two LEOs in the same orbit to measure temporal variations in Earth’s gravity field. This concept was already presented at the Williamstown Conference in 1969. But the realization of a GRACE mission was only possible 33 years later with the development of space-qualified Global Positioning System (GPS)Satellite positioning system of the US military that is also available to civilian users. With the help of nominally 24 satellites in three different orbital planes, the 3D position of a receiver can be determined with centimetre accuracy at any poin... receivers, accelerometers for measuring non-gravitational forces, micrometre-precise microwave SST as well as mechanical and thermal ultra-stable satellites.

At the GFZ Helmholtz Centre for Geosciences, researchers run realistic simulations using an in-house developed specialized software called EPOS (Earth Parameter and Orbit System). These simulations help to evaluate how different mission configurations -- such as the number of LEO satellite pairs, changes in orbit parameters such as altitude or inclination or improved instruments -- would affect the spatial/temporal resolution, quality and latency of future Earth gravity field models. 

The process for analyzing MAGIC scenarios involves – like for all other gravity missions - three key steps:

1. Forward Simulation - Scientists create a virtual version of the MAGIC mission. In this step, they simulate how the two pairs of satellites will orbit Earth (e.g., by defining the altitude, inclination or related repeat cycles) and how their onboard instruments (e.g., SST, accelerometers, or GNSS receivers) will measure gravitational changes caused by moving water, ice, and landmasses. These "real-world” observations are based on well-established models of Earth's gravitational and non-gravitational forces.
2. Adding Realism – In space, no measurement is perfect. Therefore, to make the simulation more realistic, we introduce small disturbances, mimicking actual sources of error, such as instrument noise. This step ensures that the simulated data reflect the challenges of real-world observations.
3. Recovery Simulation - The simulated observations of step 2 are then processed as if they were real data, allowing scientists to test how accurately the MAGIC satellites would detect mass changes. Here, we do not use the exact same background models as in Step 1 (e.g., for ocean tides or short-term non-tidal mass variations in the atmosphere and ocean). Instead, we account for the fact that these models are not perfectly known by incorporating alternative models that reflect the current level of uncertainty. By comparing the recovered gravity field model results to the original input gravity field, we can assess the mission's ability to track water movement and climate-related changes in the Earth system.

Realistic simulations are essential to ensure the success of the MAGIC mission. By testing different mission designs and data processing strategies in a controlled, virtual environment, researchers can evaluate how well MAGIC will perform before it is launched. This allows for the identification and mitigation of potential weaknesses, ensuring that the mission provides the highest-quality gravity field data possible.

A key outcome of these simulations is the ability to compare MAGIC’s anticipated performance with that of previous missions. Figure 1 illustrates this improvement: the left panel represents simulated data retrieved from the current GRACE-FO mission, which is limited in spatial and temporal resolution primarily due to its mission design of a single pair flying on a polar orbit. The right panel demonstrates how the combination of a GRACE-like mission (such as GRACE-C) with an inclined NGGM with improved instrumentation (e.g., ca. ten times more precise accelerometers), in combination with optimized processing strategies, significantly enhances the clarity and detail of retrieved mass change signals from MAGIC – and this without any post-processing which is mandatory for GRACE-like missions. The bottom panel represents the "truth signal," which serves as the benchmark for evaluating the accuracy of different mission designs. These results underscore the substantial advancements that MAGIC will offer in monitoring hydrological and climate-related processes.

Through advanced simulation techniques, researchers can refine the MAGIC mission’s design to optimize its capability to track Earth's changing water and ice distributions with unprecedented precision. This preparatory work is crucial for ensuring the mission delivers reliable, high-resolution data, supporting long-term climate studies, natural hazard assessments, and water resource management. By proactively addressing potential challenges through simulations, the scientific community can maximize the impact of MAGIC, providing critical insights for both research and policy-making in the face of global environmental change.

• Flechtner, F. et al. (2016). Was kann vom GRACE-FO Laser Ranging Interferometer für erdwissenschaftliche Anwendungen erwartet werden? In: Cazenave, A., Champollion, N., Benveniste, J., Chen, J. (eds) Remote Sensing and Water Resources. Space Sciences Series des ISSI, Band 55. Springer, Cham. https://doi.org/10.1007/978-3-319-32449-4_11
• Wilms, J. et al. (2025): Optimized gravity field retrieval for the MAGIC mission concept using background model uncertainty information. Journal of Geodesy, 99, 21. https://doi.org/10.1007/s00190-024-01931-5