Originally published as: Landerer, F. W., Flechtner, F., Save, H., Webb, F. H., Bandikova, T., Bertiger, W. I., Bettadpur, S. V., Byun, S., Dahle, C., Dobslaw, H., Fahnestock, E., Harvey, N., Kang, Z., Kruizinga, G. L. H., Loomis, B. D., McCullough, C., Murböck, M., Nagel, P., Paik, M., Pie, N., Poole, S., Strekalov, D., Tamisiea, M. E., Wang, F., Watkins, M. M., Wen, H., Wiese, D. N., Yuan, D. (2020): Extending the global mass change data record: GRACE Follow‐On instrument and science data performance. ‐ Geophysical Research Letters, 47, 12, e2020GL088306. https://doi.org/10.1029/2020GL088306 RESEARCH LETTER Extending the Global Mass Change Data Record: GRACE 10.1029/2020GL088306 Follow‐On Instrument and Science Data Performance Key Points: Felix W. Landerer1 , Frank M. Flechtner2,5 , Himanshu Save3 , Frank H. Webb1, • GRACE‐FO is extending the 15‐year 1 1 3 1 GRACE record of global monthly Tamara Bandikova , William I. Bertiger , Srinivas V. Bettadpur , Sung Hun Byun , mass change at an equivalent Christoph Dahle2 , Henryk Dobslaw2 , Eugene Fahnestock1, Nate Harvey1, Zhigui Kang3, precision and spatiotemporal Gerhard L. H. Kruizinga1, Bryant D. Loomis4 , Christopher McCullough1 , sampling 2,5 3 1 3 3 • Since its launch in 2018, GRACE‐FO Michael Murböck , Peter Nagel , Meegyeong Paik , Nadege Pie , Steve Poole , has observed large water storage and Dmitry Strekalov1, Mark E. Tamisiea3, Furun Wang3, Michael M. Watkins1, Hui‐Ying Wen1, ice mass changes driven by David N. Wiese1 , and Dah‐Ning Yuan1 interannual climate anomalies • GRACE‐FO's instrument/flight 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA, 2Helmholtz Centre Potsdam GFZ system performance has largely 3 improved over GRACE. The novel German Research Centre for Geosciences, Potsdam, Germany, Center for Space Research, The University of Texas at laser ranging instrument works Austin, Austin, TX, USA, 4Geodesy and Geophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, successfully USA, 5FG Physical Geodesy, Technische Universität Berlin, Berlin, Germany Supporting Information: ‐ ‐ • Supporting Information S1 Abstract Since June, 2018, the Gravity Recovery and Climate Experiment Follow On (GRACE FO) is • Figure S1 extending the 15‐year monthly mass change record of the GRACE mission, which ended in June 2017. • Figure S2 The GRACE‐FO instrument and flight system performance has improved over GRACE. Better attitude • Figure S3 • Figure S4 solutions and enhanced pointing performance result in reduced fuel consumption and gravity range rate • Figure S5 post‐fit residuals. One accelerometer requires additional calibrations due to unexpected measurement noise. • Figure S6 The GRACE‐FO gravity and mass change fields from June 2018 through December 2019 continue the • Figure S7 ‐ • Figure S8 GRACE record at an equivalent precision and spatiotemporal sampling. During this period, GRACE FO • Figure S9 observed large interannual terrestrial water variations associated with excess rainfall (Central US, Middle East), drought (Europe, Australia), and ice melt (Greenland). These observations are consistent with independent mass change estimates, providing high confidence that no intermission biases exist from Correspondence to: GRACE to GRACE‐FO, despite the 11‐month gap. GRACE‐FO has also successfully demonstrated F. W. Landerer, ‐ ‐ [email protected] satellite to satellite laser ranging interferometry. Plain Language Summary Mass change is a fundamental climate system indicator and provides Citation: an integrated global view of how Earth's water cycle and energy balance are evolving. The Gravity Landerer, F. W., Flechtner, F. M., Save, Recovery and Climate Experiment (GRACE) mission monitored mass changes every month from 2002 H., Webb, F. H., Bandikova, T., ‐ ‐ Bertiger, W. I., et al. (2020). Extending through 2017. Since June 2018, GRACE Follow On (GRACE FO) continues this data record, tracking and the global mass change data record: monitoring changes in ice sheets and glaciers, near‐surface and underground water storage, as well as GRACE Follow‐On instrument and changes in sea level and ocean currents. GRACE‐FO instruments have been successfully calibrated and are science data performance. Geophysical Research Letters, 47, e2020GL088306. providing new monthly mass change observations at a consistent spatial resolution and data quality with https://doi.org/10.1029/2020GL088306 GRACE. Since its launch, GRACE‐FO has measured record land water storage changes in 2018 and 2019 in response to extreme heat waves and droughts over Europe and Australia, as well as to extreme rainfall events Received 10 APR 2020 over the United States and Middle East. In the summer of 2019, GRACE‐FO measured record‐level Accepted 20 MAY 2020 Accepted article online 24 MAY 2020 Greenland mass loss rates. A novel laser ranging interferometer was successfully demonstrated on GRACE‐FO, laying the groundwork for improved future satellite gravity observations. 1. Introduction For more than 15 years, the Gravity Recovery and Climate Experiment (GRACE) mission, a collaboration between the National Aeronautics and Space Administration (NASA) and the German Aerospace Centre (DLR), provided monthly observations of changes in ice sheets and glaciers, near‐surface and underground water storage, as well as changes in sea level and ocean currents. Mass change observations are fundamental indicators of large‐scale climate dynamics and provide an integrated global view of how Earth's water cycle and energy balance are evolving (e.g., Reigber et al., 2002; Rodell et al., 2018; Tapley et al., 2004, 2019). The ©2020. American Geophysical Union. All Rights Reserved. importance of mass change observations was underscored in the 2018 Decadal Survey report (NASEM, 2018) LANDERER ET AL. 1of10 Geophysical Research Letters 10.1029/2020GL088306 as one of five “most important” measurements for NASA to obtain in the next decade and the only one for which continuity with the existing record is recommended as a top priority. Prior to the Decadal Survey, NASA's Climate Architecture Report (NASA, 2010) recognized mass change as a critical climate data record and recommended a gap filler between GRACE and a higher‐capability mis- sion, GRACE‐2 (NASA, 2010), recommended in the US 2007 Decadal Survey (NRC, 2007). In response, NASA and GFZ (Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences) jointly pursued the GRACE Follow‐On (GRACE‐FO) mission with the primary objective of continuing and extending global measurements of monthly gravity and surface mass changes from GRACE, as well as continuing observa- tions of atmospheric soundings from Global Positioning System (GPS) radio occultations for the operational provision of vertical atmospheric temperature/humidity profiles to weather services (e.g., Anlauf et al., 2011; Pingel et al., 2010). A secondary objective was to demonstrate for the first time satellite‐to‐satellite laser ran- ging (Abich et al., 2019) as an enabling technology for future GRACE‐like missions and a demonstrator for the Laser Interferometer Space Antenna (LISA). To minimize the possibility of a gap in the data record, a heritage architecture similar to GRACE was chosen, with a mission lifetime of 5 years after launch. This allowed for rapid development with architectural mod- ifications limited to lessons learned from GRACE. The twin GRACE‐FO satellites were built by Airbus Defence and Space in Germany and launched as part of a commercial rideshare agreement with Iridium Inc. from Vandenberg Air Force Base, California, on a Space‐X Falcon 9 rocket on 22 May 2018. The first GRACE‐FO science data were obtained on 27 May 2018, and as of January 2020, 17 monthly gravity fields and corresponding mass change maps have been processed and released (Supporting Information, Figure S1). 2. Measurement and Ground Data System Configuration and Performance The GRACE‐FO measurement concept is similar to that of GRACE: two identical satellites (GF1 and GF2) in a near‐polar orbit, separated along‐track between 170–270 km, at an initial altitude of approximately 491.5 km (Table S1). After drifting apart for 4 days after launch to a separation distance of 216 km, a drift‐stop maneuver was performed and the two satellites were yaw‐rotated by 180° to align the ranging instruments in relative boresight for nominal science ranging operations. The satellite orbits are allowed to naturally evolve and decay, but regular orbit maintenance maneuvers will be performed throughout the mission duration to keep the satellites in relative fine‐pointing alignment and at nominal separation dis- tance between 170 and 270 km. The satellite system performance requirements were adopted from GRACE, which allowed for a minimiza- tion of cost, risk, and schedule. Changes to the original GRACE designs were implemented to mitigate pre- viously identified in‐flight performance issues, either engineering or scientific, to account for hardware obsolescence, or accommodate the laser ranging interferometer (LRI). Each satellite carries attitude sensors via three star cameras (SCAs) and an angular rate‐sensing inertial measurement unit (IMU), as well as one precise accelerometer (ACC) in the satellite's mass center to measure non‐gravitational forces, for example, from solar radiation pressure and drag, as from attitude orbit control system (AOCS) activations via magne- torquers or cold‐gas thrusters. These signals need to be removed from the intersatellite range changes to esti- mate the gravity field.