Global Cosmic Ray Observatory (GCOS)

• Global Cosmic Ray Observatory is a proposed next-generation facility offering a roughly ten-fold increase in exposure to ultra-high-energy particles for charged-particle astronomy.
• Its design envisions a benchmark 60,000 km² area with multi-site arrays employing water-Cherenkov, fluorescence, and radio detectors to achieve high resolution in energy, mass, and direction.
• GCOS aims to clarify cosmic ray source identification and composition while advancing multimessenger astrophysics through innovative calibration techniques and robust detector integration.

The Global Cosmic-Ray Observatory (GCOS) is a proposed post-2030, next-generation observatory for ultra-high-energy cosmic particles—ultra-high-energy cosmic rays (UHECRs), photons, and neutrinos—designed to extend present observational capability by roughly an order of magnitude in exposure while adding stronger event-by-event mass identification and full-sky reach (Hörandel, 2022, Batista, 2023, Fujii, 7 Jul 2025). In the current literature, GCOS is usually described as a multi-site, very large-aperture facility with a benchmark area of about $60{,}000~\mathrm{km}^2$, intended to succeed the Pierre Auger Observatory and the Telescope Array in the era when present-day detectors are expected to wind down (Ahlers et al., 8 Feb 2025, Batista, 2023).

1. Historical emergence and conceptual basis

GCOS emerged from a sequence of international planning exercises rather than from a single finalized design study. A 2022 synopsis reported that more than 200 scientists had gathered to discuss the future of multi-messenger astroparticle physics beyond 2030 and to outline possible scenarios for a Global Cosmic Ray Observatory (Hörandel, 2022). A subsequent workshop report states that, after a successful kick-off meeting in 2021, two workshops in 2022 and 2023 focused mainly on a straw-man detector design and on the science possibilities for astro- and particle physics, with about 100 participants in in-person and hybrid panel discussions (Ahlers et al., 8 Feb 2025).

The central motivation is the rarity of the target particles. The flux above $7\times10^{19}~\mathrm{eV}$ is described as lower than one particle per square kilometer per century, and the highest-energy events above $100~\mathrm{EeV}$ remain too sparse for current observatories to identify sources decisively (Hörandel, 2022). On this basis, GCOS is conceived not as an incremental extension of present arrays, but as a scale change large enough to collect the Pierre Auger Observatory’s 20-year exposure in about one year (Ahlers et al., 8 Feb 2025). Closely related formulations state that a benchmark $60{,}000~\mathrm{km}^2$ GCOS would reach the projected integrated Auger exposure in 2030 within one year and would deliver roughly a ten-fold increase in statistics over a decade (Batista, 2023).

This scale is tied to a specific scientific ambition: the transition from suggestive anisotropy signals and composition trends to what the literature calls charged-particle astronomy. The underlying premise is that, if a sufficiently large sample of high-rigidity events can be isolated, arrival directions may retain enough source information for source catalogs, magnetic backtracking, and multimessenger association studies (Fujii, 7 Jul 2025, Hörandel, 2022).

2. Scientific program

The dominant science case is source identification for UHECRs. The observatory is explicitly proposed to clarify the origin and nature of the highest-energy particles in the Universe and to establish charged-particle astronomy with UHECRs as a new messenger (Fujii, 7 Jul 2025). Because magnetic deflection depends on rigidity $R = E/Z$, GCOS is designed around the combined measurement of direction, energy, and mass-sensitive observables rather than direction alone (Hörandel, 2022, Fujii, 7 Jul 2025).

Large exposure is expected to turn current anisotropy hints into high-significance detections. A science study argues that, if present event rates are extrapolated, the correlations with Centaurus A and with starburst galaxies indicated by Auger data would reach $5\sigma$ within about 1.5 years if the total area exceeds the $60{,}000~\mathrm{km}^2$ benchmark; the correlation with the Perseus–Pisces supercluster reported by TA would reach $5\sigma$ within about 2 years if the northern coverage alone were at least $40{,}000~\mathrm{km}^2$ (Batista, 2023). The same paper identifies improved anisotropy and spectrum measurements, better composition determination, and full-sky coverage as coequal scientific goals.

Spectrum measurements are intended to resolve the structure of the flux suppression in greater detail. The 2025 requirements report identifies the “instep,” the flux suppression, and a possible flux recovery as explicit targets, because those features constrain source acceleration ceilings, GZK-like propagation losses, and more exotic interpretations (Ahlers et al., 8 Feb 2025). A later overview also emphasizes that GCOS could reach sensitivity to particles approaching $\sim 1~\mathrm{ZeV}\equiv10^{21}~\mathrm{eV}$ over about 10 years (Fujii, 7 Jul 2025).

Composition is treated as indispensable rather than auxiliary. The 2025 concept paper gives a mass-resolution goal of $7\times10^{19}~\mathrm{eV}$0 together with an arrival-direction accuracy of $7\times10^{19}~\mathrm{eV}$1 and energy resolution of $7\times10^{19}~\mathrm{eV}$2 (Fujii, 7 Jul 2025). The 2023 science paper states a more ambitious ultimate composition target of $7\times10^{19}~\mathrm{eV}$3, linked to event-level separation of the electromagnetic and muonic shower components and to $7\times10^{19}~\mathrm{eV}$4-level performance (Batista, 2023). The 2025 requirements report translates this into observatory-level targets of energy resolution better than $7\times10^{19}~\mathrm{eV}$5, muon-number resolution better than $7\times10^{19}~\mathrm{eV}$6, $7\times10^{19}~\mathrm{eV}$7 resolution better than $7\times10^{19}~\mathrm{eV}$8, and angular resolution better than $7\times10^{19}~\mathrm{eV}$9 above the high-quality threshold (Ahlers et al., 8 Feb 2025).

Neutral messengers form a second major branch of the science case. UHE photons are described as both astrophysical and exotic-physics probes, with superheavy dark matter (SHDM) singled out repeatedly as a benchmark scenario (Batista, 2023, Fujii, 7 Jul 2025). The 2023 science paper states that, with an area of at least $100~\mathrm{EeV}$0, the number of events detected by GCOS would be about $100~\mathrm{EeV}$1 larger than current datasets, so upper limits on photon-related fluxes would improve correspondingly, allowing some existing SHDM models to be excluded (Batista, 2023). UHE neutrinos are treated in parallel as probes of source evolution, proton fraction, and multimessenger source physics (Batista, 2023, Fujii, 7 Jul 2025).

A third pillar is particle and fundamental physics. The observatory is proposed as a tool for studying hadronic interactions at center-of-mass energies beyond accelerators, including the forward region with pseudorapidity $100~\mathrm{EeV}$2, and for testing Lorentz invariance, shower-development anomalies, and BSM scenarios (Hörandel, 2022, Ahlers et al., 8 Feb 2025). In this sense GCOS is presented simultaneously as an astrophysical survey instrument and as an air-shower laboratory operating above $100~\mathrm{EeV}$3 (Hörandel, 2022).

3. Straw-man design and detector architecture

The 2025 workshop report presents a preliminary straw-man design centered on a giant surface array, supplemented by fluorescence and/or radio systems for energy-scale and mass-scale calibration (Ahlers et al., 8 Feb 2025). The basic configuration is summarized below.

Feature	Straw-man value	Context
Total area	$100~\mathrm{EeV}$4	Top-level requirement
Sites	$100~\mathrm{EeV}$5	One in each hemisphere
Preferred latitudes	$100~\mathrm{EeV}$6	For homogeneous full-sky exposure
Surface spacing	$100~\mathrm{EeV}$7	Triangular grid
Number of surface stations	$100~\mathrm{EeV}$8	For $100~\mathrm{EeV}$9 coverage
Trigger efficiency	$60{,}000~\mathrm{km}^2$0 at $60{,}000~\mathrm{km}^2$1	Surface-array requirement
High-quality threshold	$60{,}000~\mathrm{km}^2$2	Reconstruction target regime
Angular resolution	$60{,}000~\mathrm{km}^2$3	Observatory target
Energy resolution	$60{,}000~\mathrm{km}^2$4	Observatory target
Muon-number resolution	$60{,}000~\mathrm{km}^2$5	Observatory target
$60{,}000~\mathrm{km}^2$6 resolution	$60{,}000~\mathrm{km}^2$7	Observatory target

The preferred surface technology in the current straw-man is a layered water-Cherenkov detector (WCD), chosen because it is robust, wide-angle, compatible with autonomous field operation, and potentially capable of separating the electromagnetic and muonic shower components at station level (Ahlers et al., 8 Feb 2025). The underlying response model, studied in performance simulations of layered WCDs, is written as

$60{,}000~\mathrm{km}^2$8

$60{,}000~\mathrm{km}^2$9

where $R = E/Z$0 and $R = E/Z$1 are the upper- and lower-layer signals, $R = E/Z$2 is the electromagnetic contribution, and $R = E/Z$3 is the muonic contribution (Billoir et al., 24 Aug 2025). For the original Auger-layered design, the same study quotes simulation-based values $R = E/Z$4 and $R = E/Z$5 and reports previously established muon-number relative resolutions of $R = E/Z$6–$R = E/Z$7 per station (Billoir et al., 24 Aug 2025).

Station spacing is driven by efficiency requirements. The layered-WCD study computes that a spacing of about $R = E/Z$8 is required for almost all air showers above $R = E/Z$9 to trigger the array, whereas a $5\sigma$0 spacing yields full efficiency only at about $5\sigma$1 (Billoir et al., 24 Aug 2025). This is why the 2025 straw-man uses $5\sigma$2 triangular spacing and arrives at $5\sigma$3 stations for $5\sigma$4 (Ahlers et al., 8 Feb 2025).

Radio detection is treated as a major complement because it measures the electromagnetic shower component with high duty cycle. The 2023 science review states that radio detectors can measure the energy of the electromagnetic component with roughly $5\sigma$5 precision, can achieve $5\sigma$6, and can reach angular resolution better than $5\sigma$7 (Batista, 2023). The 2025 requirements report stresses radio especially for inclined showers and proposes attaching radio antennas to particle stations, with the nominal frequency range starting above $5\sigma$8 (Ahlers et al., 8 Feb 2025).

Fluorescence detection remains essential because it provides calorimetric energy measurement and direct longitudinal information. The design space explicitly includes FAST, CRAFFT, and LEFT-like low-cost FD concepts (Ahlers et al., 8 Feb 2025) as well as FAST, CRAFFT, and a scaled-down “cyclops” version of MACHETE in the earlier science review (Batista, 2023). The role of FD in the current straw-man is calibration of the particle-detector energy and mass scales rather than full-duty-cycle operation over the whole array (Ahlers et al., 8 Feb 2025).

The architecture has evolved across successive documents. The 2022 workshop synopsis discussed two-site implementations near $5\sigma$9 and ground-array areas of about $60{,}000~\mathrm{km}^2$0–$60{,}000~\mathrm{km}^2$1 (Hörandel, 2022). The 2025 requirements report prefers $60{,}000~\mathrm{km}^2$2 and $60{,}000~\mathrm{km}^2$3 (Ahlers et al., 8 Feb 2025), while a 2025 overview describes GCOS more generally as a worldwide network of more than two sites with total effective coverage of $60{,}000~\mathrm{km}^2$4 (Fujii, 7 Jul 2025). This indicates an evolving, not yet frozen, global layout.

4. Calibration, reconstruction, and observatory operations

GCOS is conceived as a high-duty-cycle autonomous system, but its calibration strategy is explicitly hybrid. The 2025 requirements report states that FD and/or RD are needed to calibrate the particle-detector absolute energy scale to better than $60{,}000~\mathrm{km}^2$5 and to calibrate the mass or $60{,}000~\mathrm{km}^2$6 scale (Ahlers et al., 8 Feb 2025). In practice, this makes the observatory less a single detector than a calibrated ensemble of detector subsystems.

Fluorescence operation imposes atmospheric requirements familiar from Auger and TA. A directly relevant precedent is the adoption of the Global Data Assimilation System (GDAS), which provides a new atmospheric dataset every 3 hours on 23 constant pressure levels from $60{,}000~\mathrm{km}^2$7 to $60{,}000~\mathrm{km}^2$8 plus surface values over a global $60{,}000~\mathrm{km}^2$9 latitude–longitude grid (Will, 2014). GDAS is already used in standard air-shower reconstruction at the Pierre Auger Observatory, and the corresponding study argues that such global atmospheric products can reduce dependence on costly site-by-site radiosonde programs while improving reconstruction consistency across multiple sites (Will, 2014). For a multi-site GCOS, this provides an existing atmospheric backbone rather than a speculative one.

The operational benchmark for large hybrid arrays remains Auger. Auger’s design—1660 water-Cherenkov detector stations spread over $5\sigma$0 overlooked by 24 fluorescence telescopes, with GPS timing synchronization within $5\sigma$1 RMS and an exposure exceeding $5\sigma$2—is repeatedly treated in the GCOS literature as the immediate large-array predecessor (Collaboration, 2015). The relevance is architectural: modular autonomous stations, explicit exposure accounting, hybrid energy transfer, and integrated atmospheric monitoring are already demonstrated at one-site scale (Collaboration, 2015).

The 2025 GCOS report adds a more forward-looking DAQ picture. It suggests a next-neighbour communication system in which nearby stations exchange trigger-relevant information, form local coincidences, and send summarized event data onward, instead of transmitting all raw waveforms centrally (Ahlers et al., 8 Feb 2025). The same report mentions the use of Galileo GPS-system signals as a route to sub-nanosecond synchronization for demanding radio modes (Ahlers et al., 8 Feb 2025). These are preliminary design ideas, but they show that triggering, timing, and communications are being treated as first-order observatory constraints rather than as secondary implementation details.

Layered-WCD operation has also moved beyond concept-level only. Two prototype tanks deployed at the Pierre Auger Observatory site in 2014 have been recording data for more than 10 years, with one recalibration intervention for Clairon Jr. in 2016, an electronics fix for Guapa Guerrera in 2021, and an upgrade of both detectors to new Auger electronics in April 2023 (Billoir et al., 24 Aug 2025). In the lower-layer calibration histogram, the muon peak appears around 135 FADC counts, which provides a clear VEM-like anchor for bottom-layer calibration (Billoir et al., 24 Aug 2025). This long-baseline field experience is significant because GCOS-scale deployment implies industrial numbers of stations and decade-scale maintenance constraints.

5. Global-observatory precursors and related frameworks

GCOS proper is not the first attempt to think globally about cosmic rays. A major precursor is CREDO, the Cosmic-Ray Extremely Distributed Observatory, which was introduced as a global cosmic-ray detection framework for “extended cosmic ray phenomena,” especially super-preshowers and cosmic-ray ensembles (Sushchov et al., 2017). CREDO changes the event concept from isolated air showers to correlated multi-particle arrivals and explicitly contrasts the standard case $5\sigma$3 with the ensemble case $5\sigma$4 (Sushchov et al., 2017). Its basic observables are the spatial and temporal extents $5\sigma$5 and $5\sigma$6, from which four SPS classes A–D are defined (Sushchov et al., 2017).

Architecturally, CREDO is a worldwide federation of heterogeneous detectors: professional observatories, educational arrays, university counters, single detectors, and smartphones (Sushchov et al., 2017). Its distinctive trigger philosophy is to search, in parallel with conventional local coincidence logic, for spatially isolated stations clustered within a small time window (Sushchov et al., 2017). This is not GCOS in the narrow next-generation-UHE-array sense, but it is explicitly presented as highly relevant to GCOS-like thinking because it defines a planet-scale observable rather than a local one (Sushchov et al., 2017).

By 2019, CREDO had 23 institutions from 11 countries, more than 7500 registered smartphone users with at least one detection, about 2.918 million images stored in the database, and a cumulative observing time of 958 years (Gora et al., 2019). These figures show that the global-observatory idea has also been developed along a distributed, open, citizen-science lineage, complementing the large-area UHE array lineage that now dominates GCOS proper (Gora et al., 2019).

Other existing observatories also function as prototype nodes or methodological benchmarks for parts of a GCOS-like ecosystem. HAWC demonstrates continuous, high-duty-cycle directional cosmic-ray monitoring, Moon-shadow calibration, anisotropy mapping, and heliospheric transient detection in a single site (Collaboration et al., 2013). IceCube combines a $5\sigma$7 in-ice detector with the $5\sigma$8 IceTop surface array to create a three-dimensional cosmic-ray detector, illustrating the value of simultaneous surface electromagnetic and deep high-energy muon measurements for composition studies (Kolanoski, 2012). These are not candidate GCOS baselines by themselves, but they define proven design patterns: hybridization, continuous monitoring, and multi-component shower measurement.

6. Open problems, broader interpretations, and significance

GCOS remains a proposal, and the literature is explicit about unresolved issues. The final detector choice is still open among water-Cherenkov, fluorescence, radio, or mixed designs; site distribution and deployment architecture are not fixed; calibration and atmospheric-monitoring strategies are still being optimized at system level; and the cost, manufacturability, and long-term operation of $5\sigma$9–$40{,}000~\mathrm{km}^2$0 stations remain dominant engineering constraints (Fujii, 7 Jul 2025, Ahlers et al., 8 Feb 2025). The 2025 workshop report is clear that many numbers currently function as straw-man requirements or trade-study anchors rather than as frozen specifications (Ahlers et al., 8 Feb 2025).

A central scientific uncertainty is the composition of the highest-energy events. The more favorable case for source identification assumes a substantial light-mass subset, since lower-$40{,}000~\mathrm{km}^2$1 primaries are less deflected and preserve more directional information (Fujii, 7 Jul 2025). A more challenging case is also acknowledged in the 2022 workshop synopsis: if the composition at the highest energies is heavy and magnetic deflections remain large, then even a greatly enlarged observatory will require stronger event-level mass tagging and more sophisticated magnetic modeling to make source identification practical (Hörandel, 2022).

The broader significance of GCOS extends beyond UHECR astronomy in a narrow sense. A 2026 review argues that cosmic rays have become a multi-domain Earth observation tool spanning atmospheric processes, geosciences, and urban science, and identifies interoperability requirements such as OGC web services, ISO 19115 metadata, CF-compliant variable names, and CityGML-based 3D representations (Bilin et al., 27 Jan 2026). This does not define the current GCOS baseline, which remains centered on UHE particles, but it suggests a plausible broader observatory interpretation in which cosmic-ray infrastructures are coupled to hydrology, atmospheric sensing, subsurface imaging, and GIS-based Earth-system workflows (Bilin et al., 27 Jan 2026).

In the stricter astroparticle-physics sense, GCOS occupies a distinctive position in the post-Auger/TA roadmap. It is proposed as a successor facility large enough to resolve source classes, spectrum structure, composition, UHE photons, UHE neutrinos, and hadronic anomalies within one globally calibrated framework (Batista, 2023, Ahlers et al., 8 Feb 2025). Its encyclopedic significance lies precisely in that synthesis: GCOS is not merely a larger air-shower array, but the current name for an attempt to combine full-sky exposure, composition-sensitive hybrid detection, and multi-messenger ambition at the scale required by the rarity of the highest-energy particles in nature.