Felix Landerer, NASA/Jet Propulsion Laboratory, felix.w.landerer@jpl.nasa.gov

Introduction

In October 2023, the annual gathering of the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On Science Team took place in Boulder, CO, hosted at University Corporation for Atmospheric Research’s (UCAR) Center Green campus. The event had 70 in-person participant and an additional 52 online participants – see Photo. G-FO is a U.S.–German collaboration between NASA and the Helmholtz Centre Potsdam GeoForschungsZentrum (GFZ) .

Photo: Pictured here are the in-person attendees of the 2023 GRACE-FO Science Team. Another 52 people participated online.

Image credit: Felix Landerer/JPL

The meeting agenda featured 15-minute presentations over three days, describing new findings from G-FO observations and the combined GRACE and GRACE-FO climate data record that now spans over 21 years (2002–2023). 

The meeting began with the customary G-FO project status session, covering programmatic mission and flight segment technical updates, future mission plans, and descriptions of the latest data released from the GRACE Science Data System (SDS) centers. Subsequent sessions featured more than 53 contributed presentations covering analyses, algorithms, and science results by Science Team members and attendees, totaling 57 oral and 5 poster presentations. Many of the presentations are posted on the GRACE website. While this summary will cover all the content on the agenda of the meeting – it does do so in an exact linear fashion. It begins with a G-FO mission status update, followed by key highlights from the contributed analysis and science presentations.

Status of GRACE Follow-On

Since their launch on May 22, 2018, the twin G-FO satellites have been tracking Earth’s water movements and global surface mass changes that arise from climatic, anthropogenic, and tectonic changes. G-FO also enables new insights into variations of ice sheet and glacier mass, land water storage, as well as changes in sea level and ocean currents. These measurements have important applications and implications for everyday life. The impact of these data is underscored by the publication of over 6000 scientific papers – an average of 5 new publications per week – that have established G/G-FO as a leading Earth Science mission.

In May 2023, G-FO successfully completed its Prime Mission phase that lasted five years after launch. G-FO was among the missions that went through the 2023 NASA Earth Science Senior Review. The NASA project team submitted its response in spring of 2023 to extend mission operations through 2026. The proposal received overall Excellent score, highlighting the unique utility the data provide for Earth Science research and societal applications. However, the G-FO project’s NASA budget will be reduced (compared to the previous baseline) by 15% in fiscal year (FY) 2024 and 24% in FY 2025 and 2026 due to the overall budget constraints that NASA is facing. The G-FO team remains confident in its ability to continue delivering high-value and high-impact science data products – prioritizing science operations management and data latency over data reprocessing campaigns. Both NASA and GFZ had already formally committed to extending their collaboration on G-FO mission operations and data processing through the end of 2026 via a Memorandum of Understanding.

As of December 2023, the G-FO project team has processed and released 62 monthly gravity fields – the most recent being for October 2023 (at the time of this writing). The primary mission objective for G-FO is to provide continuity for the monthly GRACE mass-change observations (2002–2017) via its Microwave Interferometer (MWI) intersatellite range-change observations. G-FO also demonstrated a novel technology demonstration Laser-Ranging Interferometer (LRI) for more accurate satellite-to-satellite ranging observations for future GRACE-like missions. The LRI has been successfully operated in parallel with the MWI for most of the mission, delivering excellent quality data. LRI-based monthly gravity and mass change fields covering the period from mid-2018 to mid-2023 have been made available by the SDS teams for further analysis and study by the science community. 

Programmatic, Mission, and Operations Updates

The meeting began with Frank Flechtner  and Felix Landerer  giving welcoming remarks, followed by detailed assessments of the G-FO mission and operations status from the core SDS centers and flight operations teams.

GRACE Follow-On Project Status

Felix Landerer gave an overview of the G-FO satellites and the science data system performance. He reported that G-FO continues to meet its goal of extending the GRACE mass-change and gravity data record at equivalent precision and spatiotemporal sampling. 

Since the previous STM in October 2022, the overall G-FO science instrument performance has been stable, and the SDS team continued to deliver a gapless monthly data record to users ahead of schedule (on average, within 43 days instead of the 60-day requirement). Improving the data calibrations of the accelerometer measurements – which are noise contaminated on one of the two G-FO spacecraft – remains a core focus of the project SDS team. To this end, an improved calibration approach that reduced data errors by 10–20% has been developed and will be operationalized by the team in the coming months. 

Landerer reported that, as forecasted, the current Solar Cycle 25 has gained in strength through 2023 and will continue to do so through 2024 before subsiding again. The resulting higher non-gravitational forces acting on the satellites need to be properly accounted for in the accelerometer data processing. 

He also noted that small thruster leaks in the satellites cold gas propulsion system have been closely monitored since 2021. To ensure stable data collection and sufficient lifetime margin to achieve continuity with the proposed successor mission GRACE-Continuity, or GRACE-C (which is the new name for the Earth System Observatory Mass Change mission scheduled for launch no earlier than 2028), the G-FO project team, in conjunction with guidance from the satellite manufacturer Airbus and the German Space Operations Center, decided to adjust the operational data collection mode of G-FO to a wide pointing mode – which means that the two spacecraft are allowed to deviate from their relative line-of-sight pointing by up to 2°, whereas the previous pointing angles were 100 times smaller. This operational change necessitates fewer thruster firings, which in turn reduces leaks and improves accelerometer calibrations – and thus leads to better overall science data quality. Due to the wide pointing, the LRI intersatellite ranging data collection has been suspended in this operational mode. However, the LRI instruments are still activated and fully functional. Landerer emphasized that reducing the leak ensures that the GRACE-FO mission will have sufficient fuel to remain operational up until GRACE-C launches.

Despite these operational challenges, Landerer said that the science data delivered by G-FO continues to provide excellent utility and insights into a rapidly changing Earth system. He briefly highlighted a few scientific and decision-support contributions and achievements of G-FO over the last year. These included: 

- Monitoring California Groundwater. G-FO recorded the largest seasonal total water storage gains over California after the multiple atmospheric rivers made landfall during the 2022/2023 winter. Yet, peak water storage in May was below values observed 15–20 years ago – due to long-term, sustained groundwater declines. Going forward, the data will be invaluable to assess groundwater recharge rates and processes.

- Observing Water Cycle Extremes: Droughts and Pluvials. The G-GFO 20-year data record has been analyzed to show the increasing intensity of wet and dry extremes of the global water cycle, which increased as global temperatures rose.

- Tracking Polar Ice Mass Loss. G/G-FO measured net ice mass gains over Antarctica that began around 2021 due to snow accumulation mainly in East Antarctica, which offset the unabated mass loss of the West Antarctic ice sheet.

Subsequent science presentations presented in-depth analyses of these and other findings in the dedicated science sessions, some of which are summarized below. 

Landerer also highlighted the expanding portfolio of open science contributions that the project team is supporting: Jupyter notebooks are part of an expanding GRACE Open Science toolbox with the goal to expand this toolbox with input from the Science Team and user community in the coming years. In addition, easy-to-use browser data portals at JPL and GFZ have been key to expand the science and applications user community that increasingly use the Level-3 and higher data products in decision support contexts (e.g., for drought monitoring and water resources management).

A series of status reports on programmatic G-FO mission operations, science operations, and SDS processing followed the opening presentations. Krzysztof Snopek  reported on the ground and mission operations at the German Space Operations Center (GSOC), which is responsible for G-FO spacecraft operations. All essential flight operations, software updates, and planned calibrations were successfully scheduled and carried out by GSOC. Himanshu Save  provided the science operations assessment. He described the evolving Solar Cycle 25 and its influence on the G-FO spacecraft, the mission’s fuel budget, and adjusted operational procedures and modes (such as the already-mentioned ‘wide’ pointing mode). Christopher McCullough  reviewed the status of G/G-FO Level 1 processing at JPL, detailing additional improvements made in the accelerometer calibrations. The team is using the noisy accelerometer data on one satellite and retrieving improved science information from it.

A representative from each of the G-FO mission SDS centers – which includes JPL, GFZ, CSR, and GSFC – summarized the status of the latest gravity-field and mass change data products , including an overview of background dealiasing models and the GFZ GravIS portal, the updated JPL mascon data product, new data-processing strategies, e.g., via range acceleration , and the status of ancillary Satellite-Laser-Ranging (SLR) data processing and dedicated G/G-FO products .

Following the project team’s status presentations, there was a 30-minute session to answer questions from the science community and discuss in more detail the mission performance, near-term operations and data processing plans, as well as to gather suggestions and feedback from the community. 

Science Presentations

The remainder of the sessions in the meeting were open-submission science sessions, each of which centered around different thematic topics, including: G/G-FO analysis techniques and next generation gravity mission (NGGM) concept studies, and science analysis of mass-transport data in the fields of glaciology, oceanography, hydrology, and solid-Earth physics. As has been the case in previous years, the presenters underscored the value of interdisciplinary and multi-instrument analyses that utilize the unique complementary value of G/G-FO mass-change observations in combination with other remote sensing data (e.g., satellite altimetry or precipitation observations) and in situ data (e.g., surface deformation or ocean temperature profiles). Such hydrogeodetic combinations yield improved spatial and temporal resolutions that enable advances in Earth system process understanding, which increasingly advance societal applications of science results in support of NASA’s programmatic focus on Earth Science to Action, which seeks to “advance and integrate Earth science knowledge to empower humanity to create a more resilient world.”

Section A: GRACE and GRACE-FO Geodesy

The project status reports presented under the previous heading were part of the first section of the agenda (Session A1) as were two additional sessions: Analysis Techniques and Intercomparisons (Session A2) and NGGM and Bridging the Gap (Session A3), which focused on plans, concepts, and technologies being developed for future gravity missions. Highlights from each of these two sessions follow in the next two subsections. 

Analysis Techniques and Inter-comparisons

This session featured 15 presentations by the SDS centers and ST members on progress in instrument data calibrations and novel data processing algorithms and methods, including data-fusion with other observations.

Representatives from G/G-FO processing centers presented updated gravity-field time-series data, which capitalize on improved parameterizations, better instrument error characterizations (e.g., from star cameras, accelerometers, or ranging instruments) and background models (e.g., for tides) for improved monthly mass change data and uncertainty quantification. The highly accurate LRI data provides further opportunities to identify and characterize measurement system errors, which can be exploited for G-FO data processing but is also informative in the development of the future GRACE-C mission. However, it was also shown that several metrics used in identifying gravitational errors are sensitive to the estimated satellite trajectory, and consequently a sufficient understanding of the orbital trajectory is necessary to make accurate adjustments to the gravity field based on satellite observations.

The G/G-FO data products make use of ground-based geodetic observations, such as satellite laser-ranging (SLR) to a network of dedicated SLR satellites, which can be used to extend the G/G-FO interannual data record back to ~1994 – albeit at a much-reduced spatial resolution. Additionally, SLR data provide an important validation and performance assessment opportunity for G/G-FO observations. In that regard one presenter showed results indicating the recent G-FO accelerometer updates have indeed resulted in better gravity and mass change fields. Other speakers discussed the value and potential for improvement that could be achieved by combining G-FO and SLR observations more formally to exploit the data strengths of the different observation types in an optimal way. Such approaches could reduce uncertainties in global ocean and land ice mass changes. Furthermore, deployment of stable, long-term ocean bottom pressure (OBP) recorders in the Arctic Ocean in 2022 has enabled progress on G/G-FO OBP data validation. The data from these OBP recorders are entirely independent of G/G-FO observations and are thus very valuable to assess the satellite data record. An initial comparison between 1.5 years of OBP data and various G-FO OBP products suggest excellent agreement.

The data collected from G/G-FO has a native resolution of about 300 km (~186 mi). By jointly analyzing these G/G-FO data with higher-resolution surface elevation changes from a multimission synthesis of radar and laser satellite altimeters, net mass changes can be effectively downscaled (within a Bayesian framework) to less than 20 km (~12 mi) resolution, which is sufficiently high resolution to resolve individual ice streams in Antarctica that cannot be separated using G/G-FO data alone.

NGGM and Bridging the Gap 

The presenters in this session provided status-update on the GRACE-C mission, a joint project between NASA and the Deutsches Zentrum für Luft- und Raumfahrt (DLR) , as well as on future instrument developments and mission concepts. 

The 2017 NASA Earth Science Decadal Survey Report highlighted mass-transport monitoring through gravity change as one of five designated observables (i.e., top priorities for study) in Earth observations for the next decade in collaboration with international partners. The GRACE-C project successfully passed the NASA/JPL Mission Concept Review in June 2022, and the NASA Key Decision Point B review in September 2023 and is currently in its Phase B project definition phase. GRACE-C will be a single satellite pair based on a fully redundant LRI (as demonstrated on GRACE-FO) in a polar orbit at 500 km (~311 mi) altitude. To avoid a data gap after GFO, a launch date of no later than 2028 is targeted for GRACE-C.

Similarly, GFZ has been conducting model simulation studies to determine the value of adding a second satellite pair, dubbed Next-Generation Gravity Mission (NGGM) in Europe.  The experiments reveal that advanced parameterization techniques for improved de-aliasing of short-term mass variations can significantly reduce data errors and open the possibility for higher spatial and temporal resolution data products and science applications.

The technology demonstration LRI on G-FO has surpassed its performance requirements. With a LRI expected to be the primary instrument for the GRACE-C mission as well as other future GRACE-like missions, development of a new technique is required to provide long-term laser frequency knowledge to provide a scale correction factor to the geodesy measurement. The LRI-team presented updated results of a so-called scale factor measurement technique that allows the accurate determination of the laser frequency on-orbit that can meet the stringent GRACE-C mission requirements. This was achieved with a dual frequency modulation scheme, and a prototype electronics unit has been developed and tested, demonstrating performance better than the expected mission requirements. 

There were also reports on progress in technology development of low-frequency optomechanical accelerometers for geodetic applications. These highly-sensitive, compact, portable – and cost-effective – optomechanical inertial sensors build upon recent advances in optomechanics to measure accelerations with small form factors. The development of a sensor with lower cost, size, weight, and power – yet with GRACE-like performance – is a major achievement as these could be integrated into cost-effective mission designs, spacecraft miniaturization, simplified architectures, as well as for the deployment of constellations of satellite pairs flying at lower altitudes.

Section B: Geophysics and Climate Science

There were five sessions included in this section of the agenda, which are summarized in the subsections below as follows: Hydrology (Session B4), Cryosphere (Session B2), Solid Earth Sciences (Session B1), Oceanography (Session B3), and Multidisciplinary Science (Session B5). 

Hydrology 

This session, with 12 presentations, highlighted advances in hydrology research and applications using G/G-FO data enabled by the unique value of long, uninterrupted mass change climate data record. 

The topic of terrestrial water storage variations in California came up in several presentations, focusing on the see-saw swings between very wet and very dry years and the early impacts on groundwater recharge after the record-breaking snow accumulation during the 2022/2023 winter. The process of groundwater recharge – an important objective in the 2017 Earth Science Decadal Survey – is not well understood because of the challenges in observing infiltration of new water supply into the ground and the effects of rate of input, amount of input, and various aquifer characteristics. By combining observations of precipitation, snow water equivalent, surface water storage, ground surface deformation, and groundwater storage from G/G-FO, recharge behavior can be characterized in a natural experiment where source inputs are effectively not limited, but recharge capacity is limited. Results of studies shown during the meeting reveal that only a fraction of total available potential recharge can enter the aquifer, and that G/G-FO observations allow us to measure the effective aggregated recharge capacity and how it varies with several predictors. Another paper reported that subsurface water increases in California’s Sierra Nevada by 0.6 m (~2
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