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International Terrestrial Reference Frame Supply Chain
EGV Level: 3 — Global · Geometric/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 International Terrestrial Reference Frame (ITRF) is the Terrestrial Reference Frame (TRF) realisation of the Level 3 Essential Geodetic Variable "Global Reference Frames" in the GGOS EGV framework. It is the fundamental foundational product of global geodesy, providing the stable coordinate system upon which satellite navigation, Earth observation, and critical climate science applications — including sea-level monitoring and crustal deformation analysis — are unconditionally dependent. The authority for its governance and designation rests with the International Earth Rotation and Reference Systems Service (IERS), operating under the joint auspices of the International Astronomical Union (IAU) and the International Union of Geodesy and Geophysics (IUGG).
The ITRF is a combined product derived from observations collected by four distinct space geodetic techniques. These are: the Global Navigation Satellite System (GNSS) network, comprising approximately 500 or more continuous tracking stations; Very Long Baseline Interferometry (VLBI), employing approximately 40 radio telescopes globally; Satellite Laser Ranging (SLR), with approximately 40 laser ranging stations; and the Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) system, supported by approximately 60 ground beacons. Each technique contributes unique and non-substitutable geodetic information: SLR defines the origin (geocentre) and contributes to scale; VLBI independently defines scale and the celestial reference frame tie; GNSS and DORIS provide dense, globally distributed station positions. No single technique can substitute for another in the combination.
A critical structural feature of the ITRF supply chain is a circular dependency with the Level 3 EGV "Satellite Orbits." The ITRF is simultaneously a consumer of precise satellite orbit products — which are required to process GNSS station observations — and an essential prerequisite for their production, since orbit determination itself requires a stable reference frame. A sustained failure in either pipeline degrades the other, ultimately undermining the entire reference frame underpinning all geodetic products globally. The ITRF combination process also co-produces Earth Orientation Parameters (EOP), which connect the terrestrial and celestial reference frames and are essential for all satellite operations.
Co-Produced EGVs
| EGV | Level | Mechanism |
|---|---|---|
| Earth Orientation Parameters | 3 | Consistent EOPs are determined as part of the ITRF solution stacking process |
Governance
| Institution | Role | Risk Level |
|---|---|---|
| IERS Directing Board | Strategic control and official designation of ITRF realisations | Low |
| IERS Central Bureau (BKG) | Administrative coordination | Low |
| ITRS Product Center (IGN, France) | Official computation of the ITRF using CATREF software | High |
| DGFI-TUM (Germany) | Independent DTRF realisation for validation | Medium |
| NASA JPL (USA) | Independent JTRF realisation for validation | Medium |
Supply Chain
Tier 0 — Multi-Technique Data Acquisition
Data level: Basis — Geodetic Infrastructure. The four-technique ground network (GNSS, VLBI, SLR, DORIS) constitutes the Geodetic Infrastructure Basis EGV under EGV V7.0. Raw observations at this tier become Level 1 EGV (Geodetic Observations) upon ingestion to agreed standards.
The global multi-technique ground network is the physical foundation upon which the entire ITRF supply chain rests. Approximately 500 or more GNSS continuous tracking stations, 40 VLBI radio telescopes, 40 SLR laser ranging stations, and 60 DORIS ground beacons collectively provide the raw observations from which the ITRF is ultimately derived. These networks are operated by a diverse set of national mapping agencies, space agencies, and research institutes across Member States, functioning largely on a voluntary, best-effort basis with no binding service level agreements governing station uptime, data quality, or delivery cadence. This structural arrangement, while it has produced a scientifically productive international collaboration, is not consistent with the formal operational commitments required of critical geodetic infrastructure.
Geographic disparities in the distribution of observatories represent a persistent systemic concern. Dense station coverage in Europe and North America contrasts sharply with sparse coverage across Africa, the Pacific, and polar regions. This non-uniform distribution introduces regional weaknesses in the combined ITRF solution, particularly in the estimation of geocentre motion and vertical land movement in underserved regions. Aging VLBI and SLR infrastructure compounds this problem: many stations operate with legacy hardware, constrained maintenance budgets, and limited workforce succession planning, creating a slow degradation in the quality and reliability of observations that is difficult to detect without systematic global network monitoring.
A particularly important element at this tier is the conduct of local tie surveys at co-located sites — geodetic monuments where two or more techniques observe from the same physical location. These surveys, which are terrestrial measurements linking the reference points of different instrument types, are the critical connective tissue between the four technique networks. Any error or inconsistency in a local tie measurement propagates directly into the ITRF combination, introducing biases that are practically impossible to separate from genuine geophysical signals. Under EGV framework V7.0, the four-technique ground network is explicitly recognised as a Geodetic Infrastructure Basis EGV — a formal acknowledgement that this distributed physical asset is not merely supporting infrastructure but an EGV-level resource in its own right, warranting commensurate governance and investment.
Risk: Medium — The network operates on best-effort institutional arrangements with significant geographic disparities and aging infrastructure; no formal SLAs govern station uptime or data delivery.
| Capability | Score | Relevant Dimension |
|---|---|---|
| GNSS Data Acquisition and Storage | 3.5 | Technology |
| VLBI Data Acquisition and Storage | 2.75 ⚠ | Technology |
| SLR Data Acquisition and Storage | 3.25 | Technology |
| DORIS Data Acquisition and Storage | 4.0 | Technology |
| Ground-Based Asset Management | 1.75 ⚠ | Technology |
| Equipment Calibration and Maintenance | 3.3 | Technology |
| Local Tie | 2.25 ⚠ | Process |
Tier 1 — Global Archiving
Data level: Level 1 EGV (Geodetic Observations) — standardised observation files (RINEX, SINEX) ingested and mirrored across global data centres.
Raw observations from Tier 0 are ingested, standardised, and stored in a network of global data centres (GDCs). The primary global custodian is the Crustal Dynamics Data Information System (CDDIS), operated by NASA's Goddard Space Flight Center (GSFC). Regional centres including BKG (Germany) and IGN (France) act as additional repositories, and equalization protocols between GDCs enable cross-replication to provide a degree of redundancy. The CDDIS archive is the principal access point for technique services at Tier 2, and its continuity is therefore a prerequisite for the entire downstream supply chain.
A structural concentration risk is present at this tier. CDDIS and the Scripps Institution of Oceanography (SIO) mirror — the two primary data centres supporting the IGS — are both hosted within the United States. This means the global archiving function for the most critical technique (GNSS) effectively spans only a single political jurisdiction. The impacts of the 2018–2019 U.S. government shutdown, during which CDDIS experienced service disruption affecting data access for multiple technique services, demonstrated in operational terms the consequence of this concentration. The current arrangement is inconsistent with Guiding Principle 2 (Political Resilience), which requires that critical supply chain functions not be concentrated within any single political jurisdiction. A longitudinal review of IGS Technical Reports has also documented repeated failures by major GDCs to submit annual operational reports, reflecting a systemic gap in network monitoring and administrative accountability that corresponds directly to the low maturity score observed in the Network Operations capability.
Risk: Medium — High dependency on CDDIS and U.S. institutional stability; the GDC network spans only two political jurisdictions, conflicting with Principle 2 (Political Resilience).
| Capability | Score | Relevant Dimension |
|---|---|---|
| Network Operations | 2.0 ⚠ | Technology |
| Data Quality Management | 2.0 ⚠ | Process |
| Metadata Management | 2.0 ⚠ | Data |
| Data Preservation | 2.0 ⚠ | Data |
Tier 2 — Technique Services and Analysis
Data level: Level 1 → Level 2 (intermediate) — technique-specific combined solutions (SINEX files with station positions, velocities, EOPs) from each technique service.
Archived observation data is retrieved and processed by the four international technique services, each of which coordinates a network of Analysis Centers (ACs) to produce intra-technique combined solutions. The International GNSS Service (IGS) is coordinated by its Central Bureau at the Jet Propulsion Laboratory (JPL) and its Analysis Center Coordinator (ACC) hosted at NASA GSFC and Geoscience Australia (GA); it produces combined GNSS SINEX files. The International VLBI Service for Geodesy and Astrometry (IVS) is coordinated by NASA GSFC; IVS Correlators first process raw VLBI interferometry data into group delays, which are then processed by IVS Analysis Centers to produce VLBI SINEX files. The International Laser Ranging Service (ILRS) is coordinated by NASA GSFC and produces SLR SINEX files. The International DORIS Service (IDS) is coordinated by CNES and produces DORIS SINEX files.
Each technique service employs multiple Analysis Centers using diverse and independently developed software packages — including Bernese, GipsyX, NAPEOS, PANDA, and EPOS.P8 for GNSS — to mitigate algorithmic bias and validate solution consistency through cross-comparison. This software diversity provides a degree of algorithmic resilience at the AC layer. However, this resilience coexists with a well-documented knowledge fragility: the teams maintaining these specialised software packages are typically small academic groups operating with limited succession planning, and in several cases the expertise is concentrated in a single institution or a small number of individuals. The departure of key personnel from these teams would represent a significant operational risk.
VLBI processing carries an additional throughput bottleneck at the correlation stage. Unlike GNSS and DORIS, which process data in near-real time, VLBI correlation is computationally intensive and operationally constrained by the limited number of active correlators and the data volume generated by modern wideband VLBI systems. This bottleneck is a known throughput constraint for the IVS combination and ultimately for the ITRF combination cadence.
Local tie surveys at co-located sites are also a critical input at this tier. The technique-specific SINEX files produced by each service must be linked through the local tie measurements established at co-located observatories before they can be meaningfully combined at Tier 3. Discrepancies between the local tie values used by different services, or inconsistencies between terrestrially surveyed local ties and those implied by the space geodetic data, are among the most technically challenging aspects of the ITRF combination process.
Risk: High — Knowledge fragility and reliance on specific human capital across all four technique services; VLBI correlation bottlenecks remain a throughput constraint.
| Capability | Score | Relevant Dimension |
|---|---|---|
| GNSS Data Processing and Analysis | 3.75 | Technology |
| VLBI Data Processing and Analysis | 3.25 | Technology |
| SLR Data Processing and Analysis | 3.5 | Technology |
| DORIS Data Processing and Analysis | 3.75 | Technology |
| Geodetic Software and Tools | 2.0 ⚠ | Technology |
| Local Tie Processing and Analysis | 2.0 ⚠ | Process |
Tier 3 — ITRS Combination
Data level: Level 3 EGV (Global Reference Frames — ITRF realisation) — the official ITRF, the fundamental global coordinate system.
The four technique-specific combined SINEX files, together with local tie survey data and Earth orientation series, are ingested at this tier by the ITRS Product Center at IGN (France) to produce the unified ITRF. This is the Critical Hub of the entire ITRF supply chain. The computation is performed using CATREF, a proprietary software package developed and maintained in-house at IGN. CATREF implements a rigorous stacking of time series of station positions and EOPs from all four techniques, applying a datum definition in which: the origin is defined by SLR (which provides the most direct and stable connection to the Earth's centre of mass); scale is defined by VLBI and SLR jointly; and orientation is constrained by a no-net-rotation condition relative to the previous ITRF realisation. The simultaneous estimation of technique-specific biases and the adjudication of local tie discrepancies requires expert scientific judgement that cannot be automated without loss of product integrity.
The key-person risk at this tier is acute and formally documented. The core scientific team responsible for ITRF combination at IGN — led by Dr. Zuheir Altamimi — is small and possesses expertise that is highly concentrated and not readily transferable. The combination process requires iterative adjudication of station discontinuities, outlier handling, and local tie weighting decisions that draw on decades of accumulated institutional knowledge. CATREF, as a proprietary in-house software package, is not openly documented or maintained by an independent community, creating a succession risk that extends beyond personnel to the computational toolchain itself. The departure of key individuals without structured knowledge transfer and software documentation would constitute a critical supply chain disruption.
Two independent parallel realisations provide an essential validation function. DGFI-TUM (Germany) independently computes the DTRF using its DOGS-CS software, and NASA JPL (USA) independently computes the JTRF using a Kalman filter approach via GipsyX. These independent solutions are compared against the IGN candidate ITRF to identify systematic biases or anomalies before formal designation. While this redundancy strengthens scientific confidence in the final product, neither DGFI-TUM nor JPL has the formal mandate or operational agreement to serve as a substitute for IGN as the ITRS Product Center; they function as validators, not as operational backups.
Risk: High — Production relies on a small, stable core team at IGN with concentrated expertise; CATREF software is proprietary and in-house; key-person risk is acute.
| Capability | Score | Relevant Dimension |
|---|---|---|
| Reference Frames | 3.0 | Process |
| Geodetic Data Products | 2.8 ⚠ | Data |
| Knowledge Management | 1.3 ⚠ | People |
| Local Tie Processing and Analysis | 2.0 ⚠ | Process |
Tier 4 — Validation, Designation, and Distribution
Data level: Level 3 EGV (validated Global Reference Frame) — the official ITRF realisation formally designated and distributed.
Following production of the candidate ITRF at IGN, a formal validation process is conducted through systematic comparison of the IGN solution with the independently computed DTRF (DGFI-TUM) and JTRF (JPL). This three-way comparison evaluates origin and scale consistency, station position agreement, and EOP series alignment. Discrepancies exceeding agreed thresholds are reviewed and adjudicated before the candidate solution is advanced for formal designation. The IERS Directing Board exercises its formal authority at this stage, officially designating the validated solution as the current ITRF realisation. This designation carries international legal and scientific standing and is the authoritative coordinate reference for all UN Member State geodetic activities.
Distribution of the designated ITRF is accomplished through IERS Product Centers and the CDDIS archive, providing SINEX files containing station positions and velocities, as well as Helmert transformation parameters enabling users to relate earlier ITRF realisations to the current solution. The long-term operational stability of this distribution infrastructure depends on voluntary institutional funding commitments from IGN, NASA, and BKG — the primary host institutions. No centralised funding mechanism or binding multilateral agreement currently governs the continued availability of these services, meaning the distribution infrastructure rests on institutional goodwill and national budget continuity rather than formal SLAs.
Risk: Medium — Technically robust through independent validation redundancy, but long-term stability depends on voluntary institutional funding with no formal SLA.
| Capability | Score | Relevant Dimension |
|---|---|---|
| Data Distribution | Unscored | Data |
| Geodetic Services | 2.0 ⚠ | Process |
| Regulatory Compliance | 3.0 | Process |
Workflow Diagram
JDP Alignment
| Pipeline Element | Gap | JDP Objective |
|---|---|---|
| ITRS Product Center at IGN (Tier 3) | Sole official custodian; proprietary CATREF software; key-person risk around core team | 1.4 — Succession planning for specialised expertise |
| Local tie surveys (Tier 0–2) | Best-effort campaigns; discrepancies propagate into combinations | 1.1 — Formalised operational agreements |
| VLBI/SLR infrastructure (Tier 0) | Aging and degrading networks; threatens scale and origin definition | 1.3 — Formalised national backing for critical capabilities |
| Independent realisations (DTRF, JTRF) (Tier 4) | Maintained voluntarily with no formal funding SLA | 1.1 — SLAs and MoUs for critical functions |
| GDC political distribution (Tier 1) | Only USA and France host GDCs; two political jurisdictions | 1.2 — Political distribution of critical functions |