GRACE-C
GRACE-Continuity is a joint U.S.-German twin-satellite observatory in a low- Earth orbit that will continue the very successful climate data record of the previous GRACE-series missions: the original GRACE, which operated from 2002 to 2017, and GRACE Follow-On (GRACE-FO), which is operating since 2018. It is one element of NASA's Earth System Observatory, a new set of Earth-focused missions to provide key information to guide efforts related to environmental monitoring, natural hazard mitigation, fighting forest fires, and improving real-time agricultural processes. Each uniquely designed satellite in NASA's Earth System Observatory will complement the others, working in tandem to create a 3D, holistic view of Earth, from below the surface to the top of the atmosphere.
GRACE-C will continue the legacy of GRACE-series observations. The mission’s main objective is to extend the measurements of global, month-to-month gravity and mass variations that arise from large-scale environmental, tectonic, and human-related Mass Change processes. These effects include changes in the mass of ice sheets and glaciers, the amount of water in large lakes and rivers, near-surface and underground water storage, and changes in sea level, ocean heat content and ocean currents.
In turn, these observations provide a unique integrated global view of how Earth’s water cycle and energy balance are evolving—measurements that have far-reaching benefits to society, such as providing insights into where global groundwater resources may be shrinking or growing, where dry soils are contributing to drought, and where saturated soils increase liklihoods of flooding. As such, the GRACE-series observations are vital for addressing water resources demands and national security.
Additionally, GRACE-C will also provide unique observations of solid Earth processes, such as glacial isostatic adjustment earthquakes. The GRACE-series data are an essential element in the development of novel terrestrial reference frames that provide an enduring foundation for monitoring changes in the geoid and sea levels, as well as allowing for precision orbit determination for all Earth-orbiting satellites.
The GRACE-C science and applications objectives are based on the 2017 Decadal Survey, where Mass Change is listed as a "Designated Observable." The Committee on the Decadal Survey for Earth Science and Applications from Space (ESAS) of the National Academies of Sciences, Engineering and Medicine (NASEM) released the Decadal Survey, "Thriving on Our Changing Planet: A Decadal Strategy for Earth Observations from Space," in January 2018.
Similar to the two previous GRACE-series missions, the GRACE-C science and applications objectives are achieved by continuously tracking small distance variations between twin satellites flying in formation around Earth in a co-planar polar orbit at an initial altitude of 310 miles (500 km), separated by approximately 137 miles (220 kilometers). These data are combined with observations of non-gravitational forces acting on the satellites and their orientation and position in inertial space to infer Earth’s varying gravity field and mass redistribution. Each satellite carries geodetic quality Global Navigation Satellite System (GNSS) receivers, attitude determination sensors, a laser ranging interferometer (LRI) for satellite distance change measurements, high accuracy accelerometers, and laser retro-reflectors for orbit determination. The LRI was first successfully demonstrated on GRACE-FO, and improves the precision of the distance fluctuation measurements by a factor of more than 20-30 over the previously used microwave instruments on GRACE and GRACE-FO. The LRI on GRACE-C will feature additional redundancy.
Launch of GRACE-C is scheduled for December 2028 from Vandenberg Space Force Base in central California, aboard a SpaceX Falcon 9 launch vehicle. GRACE-C is planned to collect data for at least 5 years.
