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Global Earth Gravity Field Supply Chain
EGV Level: 3 — Global · Physical | Date: May 2026 | Version: 2.0 Reference: EGV White Paper V7.0 (GGOS, January 2026) · Guiding Principles v2.1 · 1st Joint Development Plan
Overview
The Global Earth Gravity Field is a Level 3 Essential Geodetic Variable within the physical subdomain of the GGOS EGV framework. It encompasses Global Gravity Field Models (GGM) — mathematical representations of the Earth's gravitational potential expressed as spherical harmonic series. These models define the geoid: the equipotential surface of the Earth's gravity field that best approximates global mean sea level. The geoid is the physical reference surface against which all height measurements worldwide are ultimately compared and unified. Realising a globally consistent, centimetre-accurate geoid is the prerequisite for a unified global height reference system and is identified in UN General Assembly Resolution 69/266 as a foundational element of sustainable development.
The supply chain producing this EGV is bifurcated at its most fundamental level. Dedicated satellite gravimetry missions — principally GRACE-FO (a joint NASA/GFZ mission), the ESA GOCE mission (which operated from 2009 to 2013), and the earlier CHAMP mission — provide continuous, homogeneous, global coverage of the long-to-medium wavelength components of the gravity field. These missions sense gravity through orbital perturbations and, in GOCE's case, direct gravity gradiometry measurements. They constitute the only practical means of obtaining global gravity coverage with uniform accuracy. Surface gravity observations — terrestrial gravimeter networks, airborne gravity surveys over mountainous and polar terrain, and shipborne campaigns over the ocean — provide the high-frequency, short-wavelength detail that satellite missions cannot resolve. These two data streams must be combined to produce the ultra-high-degree spherical harmonic models that underpin operational geoid modelling. The combination is technically complex, computationally intensive, and depends on institutional capacity concentrated in a very small number of organisations worldwide.
The EGV White Paper V7.0 (GGOS, January 2026) formalises two changes directly relevant to this supply chain. First, the former "Level 0" stratum is redesignated as the Basis level, encompassing Geodetic Infrastructure and Geodetic Standards and Conventions. Second, a new Level 1 EGV — Geodetic Observations — is introduced; it includes satellite gravity gradiometry, gravimetry, and atomic clock frequencies as observable quantities. Upstream of the Level 3 Global Earth Gravity Field, this supply chain therefore draws from Level 1 Geodetic Observations (satellite and surface gravity measurements) and passes through the Level 2 EGV Land and Marine Gravity Data (standardised gravity anomalies). V7.0 also introduces two new Level 3 EGVs — Regional Reference Frames and Regional Gravity Field Model — that depend directly on the outputs of this pipeline. Those downstream EGVs are noted as co-produced variables but are not the focus of this supply chain description.
Co-Produced EGVs
| EGV | Level | Mechanism |
|---|---|---|
| Sea Level and Sea Surface | 3 | Essential for computing Mean Dynamic Topography from satellite altimetry; the geoid provides the reference surface |
| Terrestrial Water Storage | 3 | Time-variable gravity directly observes mass transport — groundwater depletion, aquifer recharge, and river basin hydrological change |
| Ice Sheets | 3 | GRACE-FO monthly gravity solutions directly measure ice mass change in Greenland and Antarctica |
| Regional Reference Frames | 3 | Regional geoid models underpin physical height systems; new in EGV White Paper V7.0 |
| Regional Gravity Field Model | 3 | Regional densification of global gravity models for national and sub-continental surveying applications; new in EGV White Paper V7.0 |
Governance
| Institution | Role | Risk Level |
|---|---|---|
| International Gravity Field Service (IGFS / IAG) | Coordinates the collection, validation, and distribution of gravity data and models under the IAG umbrella | Medium |
| Bureau Gravimétrique International (BGI) | Archives and distributes surface and absolute gravity data on a largely voluntary, in-kind basis | Medium |
| ICGEM (GFZ Potsdam, Germany) | Collects, evaluates, and distributes global gravity field models through a publicly accessible, FAIR-compliant portal | Low |
| COST-G (IAG Combination Service for Time-variable Gravity Fields) | Independently combines monthly time-variable gravity fields from multiple satellite analysis centres | Medium |
| NASA JPL / CSR (University of Texas) / GFZ | Primary GRACE-FO science analysis centres; producers of the definitive monthly Level-2 gravity solutions | High |
Supply Chain
Tier 0 — Gravity Data Acquisition
Data level: Basis → Level 1 EGV (Geodetic Observations — Geodetic Physical Observations) — raw gravity measurements from satellite missions and surface campaigns.
The acquisition layer is bifurcated between space-based and surface-based techniques, each of which resolves a distinct part of the spatial frequency spectrum of the Earth's gravity field. Satellite gravimetry missions — GRACE-FO (2018–present, NASA/GFZ), GOCE (2009–2013, ESA), and CHAMP (2000–2010, GFZ) — sense gravity through precision measurements of inter-satellite range-rate (GRACE-FO), gravity gradients (GOCE), or orbital perturbations (CHAMP). These missions provide the only means of achieving consistent, continuous, globally uniform gravity field sampling at long and medium wavelengths. Their data forms the homogeneous backbone without which a physically meaningful global geoid cannot be realised. The GRACE-FO mission is particularly critical for time-variable gravity: it produces monthly solutions that directly quantify changes in terrestrial water storage, ice mass loss, and ocean bottom pressure at spatial scales of several hundred kilometres.
Surface gravity campaigns complement the satellite record by providing the short-wavelength, high-resolution measurements that satellite missions cannot resolve. Terrestrial gravimeter networks — operated by national mapping agencies, geological surveys, universities, and in some cases commercial entities for resource exploration — form the densest component. Airborne gravimetry is deployed to cover mountainous terrain, remote polar regions, and areas where ground access is impractical. Shipborne surveys contribute to marine coverage, particularly in polar and shallow-water areas not sampled by satellite radar altimetry. Together, these sources are essential for bringing global geoid models to the centimetre accuracy required by practical surveying and vertical datum unification.
The spatial distribution of surface gravity data is profoundly uneven. High-density coverage exists across North America, Western Europe, Australia, and parts of East Asia. Across large areas of Sub-Saharan Africa, parts of South and Central Asia, the Amazon basin, and much of the polar regions, surface gravity campaigns are sparse or entirely absent. This distributional inequity is not merely a quality concern: it is a structural limitation that directly prevents accurate local geoid modelling in precisely those developing regions where improved height reference systems would have the greatest development impact. Data densification in these regions requires coordinated investment in both instrumentation and observation campaign capacity.
The most acute systemic risk at this tier is the absence of a confirmed, funded successor to the GRACE-FO mission. GRACE-FO's nominal operational lifetime and the budgetary timelines of its successor — provisionally designated GRACE-C or its equivalent — leave a credible gap scenario in which time-variable gravity monitoring is interrupted for a period of years. This risk is formally identified as WA3 Gap 6 within the present engagement. An interruption comparable to the 11-month gap between the end of GRACE and the launch of GRACE-FO (2017–2018) would degrade the temporal continuity of all downstream EGVs that depend on monthly gravity solutions, including Terrestrial Water Storage and Ice Sheets. No surface-based network can substitute for the global coverage provided by a satellite gravimetry pair.
Risk: High — Surface gravity coverage is extremely uneven globally; GRACE-FO has no confirmed funded successor mission, creating a satellite gravimetry continuity risk that would degrade time-variable gravity monitoring across multiple downstream EGVs (WA3 Gap 6).
| Capability | Score | Relevant Dimension |
|---|---|---|
| Gravity Data Acquisition and Storage | 2.0 ⚠ | Technology |
| Ground-Based Asset Management | 1.75 ⚠ | Technology |
| Equipment Calibration and Maintenance | 3.3 | Technology |
Tier 1 — Data Archiving
Data level: Level 1 EGV (Geodetic Observations) — calibrated gravity anomalies and Level-1b satellite mission data ingested to archives.
Satellite gravity data flows through dedicated institutional archives operated by the sponsoring space agencies. GRACE-FO Level-1b data and Level-2 monthly solutions are distributed through NASA's Physical Oceanography Distributed Active Archive Centre (PO.DAAC) and the GFZ data portal. ESA's GOCE data products remain accessible through ESA's Earth Online infrastructure. These archives are mature, maintained, and generally FAIR-compliant, reflecting the data management standards of major civil space agencies. Their continuity is nonetheless contingent on sustained national space agency budgets and on the active maintenance of archive infrastructure that was designed for specific mission timescales.
Surface gravity data is collated by the Bureau Gravimétrique International (BGI) on a largely voluntary, in-kind basis. Member nations contribute data according to bilateral agreements and national policies. The result is an archive of highly variable completeness and quality: dense and well-documented for some regions, absent or fragmentary for others. BGI performs quality checking and standardisation, but its capacity to impose consistent data standards across contributing nations is limited by the voluntary nature of the arrangement and the absence of a formal, legally binding data sharing obligation for Member States.
A structural data access gap persists across this tier. A significant fraction of historical terrestrial gravity data — particularly measurements collected during resource exploration activities by oil and gas companies, and gravity surveys conducted by national military or intelligence agencies — remains restricted or classified. This data is commercially or strategically sensitive, and its owners have neither the legal obligation nor the institutional incentive to share it through international channels. The practical consequence is that BGI's archive underrepresents the actual density of historical gravity measurements in several regions, including parts of the Middle East, the former Soviet Union, and areas of active hydrocarbon production. This structural restriction is incompatible with the FAIR data principles mandated under JDP Objective 1.2 and cannot be resolved through technical means alone; it requires sustained diplomatic and policy engagement at the intergovernmental level.
Risk: Medium — Variable quality in surface gravity archives; significant historical data remains restricted or privately siloed, preventing FAIR compliance and limiting the accuracy of geoid models in data-sparse regions.
| Capability | Score | Relevant Dimension |
|---|---|---|
| Data Quality Management | 2.0 ⚠ | Process |
| Metadata Management | 2.0 ⚠ | Data |
| Data Sharing | 3.25 | Data |
| Data Preservation | 2.0 ⚠ | Data |
Tier 2 — Gravity Data Processing and Analysis
Data level: Level 1 → Level 2 EGV (Land and Marine Gravity Data) — standardised gravity anomalies and calibrated satellite-derived solutions.
GRACE-FO Level-1b data is processed into monthly static gravity solutions independently by three primary analysis centres: the Centre for Space Research (CSR) at the University of Texas at Austin, NASA's Jet Propulsion Laboratory (JPL), and the GFZ German Research Centre for Geosciences in Potsdam. Each centre applies its own independently developed algorithms, software, and background model choices. This algorithmic diversity is a recognised strength of the system: independent solutions enable cross-validation and reduce the risk that a systematic error in any single processing chain propagates undetected into the official products. COST-G, operating under the IAG, further combines solutions from these and additional contributing centres into a single, evaluated combined monthly product. This combination architecture provides a degree of resilience that is not present in comparable components of other EGV supply chains.
National geodetic agencies, university research groups, and specialist commercial consultancies process surface, airborne, and shipborne gravimetry data into standardised gravity anomalies — free-air and Bouguer reductions applied to raw observed gravity values referenced to the international gravity formula. This processing work is largely decentralised and depends on the capacity of individual national institutions. The methodological standards, software toolchains, and quality control practices applied vary significantly across agencies, with well-resourced national mapping agencies in Europe, North America, and Australia maintaining high standards, while many developing-country agencies lack the specialist personnel or software licences required to process their own national data to international standards.
The global pool of professionals possessing the theoretical background and practical experience required to perform geoid model development — the combination of gravity anomalies with satellite models — is both limited and, on the evidence of declining academic programme enrolments, actively contracting. The capability maturity assessment scores Training and Education at 1.7, reflecting a Phase 1 critical priority. This is not a near-term risk in the sense that current geoid models will not immediately degrade, but it is a structural risk that compounds over time: as the cohort of practitioners who produced EGM2008 and similar models approaches retirement, the institutional knowledge required to produce their successors is concentrated in an ever-smaller group of individuals. This knowledge is not adequately captured in documentation or reproducible workflows.
Software diversity exists across this tier, with multiple independently developed processing packages in use (including proprietary tools at NGA, GFZ, and TU Munich, and open-source or research codes at various universities). However, key software tools used by national agencies for routine processing may lack succession planning. Where a national agency's processing capability depends on a single specialist or a bespoke tool maintained by one individual, that agency's contribution to the global network is contingent on that individual's continued availability — a key-person risk that is formally identified in the capability maturity model under Geodetic Software and Tools (score: 2.0).
Risk: High — The global pool of professionals capable of performing geoid model development is limited and decreasing; the capacity challenge is particularly acute in developing countries and represents a structural succession risk to the entire production tier.
| Capability | Score | Relevant Dimension |
|---|---|---|
| Gravity Data Processing and Analysis | 2.75 ⚠ | Technology |
| Geodetic Software and Tools | 2.0 ⚠ | Technology |
| Training and Education | 1.7 ⚠ | People |
Tier 3 — Geoid Model Development (Combination)
Data level: Level 3 EGV (Global Earth Gravity Field) — ultra-high-degree spherical harmonic gravity field models combining satellite and surface data.
The combination phase is the central scientific achievement of this supply chain: the merger of the spatially homogeneous but spectrally limited satellite-only gravity models with the high-resolution but spatially heterogeneous surface gravity anomaly datasets. The resulting ultra-high-degree spherical harmonic models — of which the Earth Gravitational Model 2008 (EGM2008, developed by the National Geospatial-Intelligence Agency to degree and order 2190) and its successor candidate XGM2019 (TU Munich/GFZ, degree 2190) are the most operationally significant — encode the gravity field at spatial resolutions approaching five kilometres. These models are used to compute geoid undulations at any point on Earth, providing the physical height reference that underpins all precision surveying, civil engineering, oceanography, and vertical datum work globally.
COST-G, the IAG's Combination Service for Time-variable Gravity Fields, coordinates and produces the combined monthly time-variable gravity solution. It independently receives solutions from CSR, JPL, GFZ, and additional contributing centres, applies a rigorous weighting and combination procedure, and releases a single evaluated combined product. This governance model — multiple independent contributing centres feeding a single coordination service — is the most structurally sound element of the entire gravity supply chain. The diversity of contributing algorithms means that no single organisation's processing failure will disrupt the combined time-variable product.
The production of a high-degree combined static model, by contrast, is a decade-scale institutional undertaking of extraordinary complexity. EGM2008 required the integration of satellite data from GRACE, marine altimetry-derived gravity anomalies, and terrestrial gravity datasets from more than a hundred countries, processed over several years with proprietary algorithms at NGA and with extensive international collaboration. No global geodetic institution other than NGA (USA), GFZ (Germany), and TU Munich (Germany) currently possesses the combination of data access, computational resources, specialist personnel, and institutional mandate required to produce a comparable model. The subsequent generation — dependent on GOCE data for its high-frequency satellite contribution — has advanced the state of the art but has not substantively broadened the institutional base capable of leading such an effort.
This institutional concentration creates a knowledge management risk that is assessed as critical. The Knowledge Management capability is scored at 1.3 — reflecting limited documentation of combination workflows, no centralised knowledge repositories accessible to the broader community, and key dependencies on individuals approaching retirement. Were a leading combination centre to lose institutional mandate or funding, the geodetic community would have no clear succession arrangement to maintain the production of next-generation ultra-high-degree models.
Risk: Medium — Ultra-high-degree model computation is an infrequent, institutionally concentrated undertaking; the knowledge required to produce successor models is concentrated in very few agencies globally and is inadequately documented or transferred.
| Capability | Score | Relevant Dimension |
|---|---|---|
| Geodetic Data Products | 2.8 ⚠ | Data |
| Research and Development Prototyping | 3.0 | Technology |
| Knowledge Management | 1.3 ⚠ | People |
| Research and Development Planning | 1.75 ⚠ | Process |
Tier 4 — Validation and Distribution
Data level: Level 3 EGV (validated Global Earth Gravity Field models) — published gravity field models distributed via ICGEM.
The International Centre for Global Earth Models (ICGEM), hosted and operated by GFZ Potsdam, functions as the definitive global clearinghouse for gravity field model evaluation and distribution. ICGEM maintains a comprehensive catalogue of global and regional gravity field models, provides interactive online calculation services that allow users to compute geoid undulations, gravity anomalies, gravity disturbances, and other gravity field functionals directly from spherical harmonic coefficients at user-specified locations and resolutions, and publishes model comparison and evaluation reports. The service is publicly accessible without registration, conforms to FAIR data principles, and is frequently cited as one of the most effective examples of open data service delivery in the geodetic community.
Validation of gravity models is conducted both internally — through residual analysis against independent surface gravity data and GPS-levelling comparisons — and externally through independent computation by research groups using alternative software implementations. The geodetically relevant quality indicator for a combined global model is the standard deviation of geoid undulation residuals against GPS-levelling benchmarks, a metric that has improved progressively from approximately one metre (EGM96) to approximately 0.2 metres globally (EGM2008) to below 0.1 metres in well-surveyed regions with high-resolution input data. ICGEM publishes these comparisons systematically.
The principal risk at this tier is structural rather than technical. ICGEM's operation is funded through GFZ's institutional budget, which is in turn supported by the German Federal Ministry of Education and Research and the Helmholtz Association. There is no formal multilateral service level agreement governing ICGEM's continued operation, no formal succession arrangement designating an alternative host institution, and no pooled international funding mechanism that would sustain the service in the event of a GFZ budget reduction or institutional restructuring. As with other IAG Services that rely on in-kind contributions from host institutions, this arrangement has functioned well historically but does not meet the standard of formalised operational continuity required under JDP Objective 1.1 and the WA1 Guiding Principles.
Risk: Low — ICGEM provides a robust, FAIR-compliant distribution mechanism; the principal gap is its dependence on GFZ institutional funding with no formal global SLA, succession plan, or pooled funding arrangement.
| Capability | Score | Relevant Dimension |
|---|---|---|
| Geodetic Services | 2.0 ⚠ | Process |
| Data Distribution | Unscored | Data |
| Data Sharing | 3.25 | Data |
Workflow Diagram
JDP Alignment
| Pipeline Element | Gap | JDP Objective |
|---|---|---|
| Surface gravity coverage (Tier 0) | Extremely sparse in developing regions; hinders accurate geoid modelling locally and prevents vertical datum unification in areas of greatest development need | 1.2 — Modernised infrastructure and data sharing |
| GRACE-FO successor (Tier 0) | No confirmed funded successor mission; a multi-year gap in satellite gravimetry would degrade time-variable gravity monitoring across multiple downstream EGVs (WA3 Gap 6) | 1.3 — Formalised national backing for critical capabilities |
| Gravity data access (Tier 1) | Historical data siloed or classified by national security and commercial interests; FAIR non-compliant; not resolvable through technical means alone | 1.2 — FAIR data principles adoption |
| Professional capacity (Tier 2–3) | Declining number of professionals capable of geoid model development; Training and Education scored at 1.7; inadequate succession documentation | 1.4 — Succession planning and capacity building |
| Model distribution at ICGEM (Tier 4) | Effective but hosted voluntarily by GFZ with no formal global operational SLA, no designated successor institution, and no pooled international funding | 1.1 — SLAs and MoUs for critical functions |