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arxiv: 2605.27629 · v1 · pith:L3QRSFPJnew · submitted 2026-05-26 · 🌌 astro-ph.GA

The Celestial Reference Frame at K Band: The CRF-K-2025 Catalog

David Gordon (1) , Aletha de Witt (2) , Christopher S. Jacobs (3) , Hana Krasna (4) , Alessandra Bertarini (5) , Jamie McCallum (6) , Jonathan Quick (2) , Cristina Garcia-Miro (7)
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Taehyun Jung (8) Jeffrey A. Hodgson (9) Whee Yeon Cheong (8) Sang-Sung Lee (8) Do-Young Byun (8) ((1) US Naval Observatory (2) South African Radio Astronomy Observatory (3) Jet Propulsion Laboratory (4) TU Wien Department of Geodesy Geoinformation (5) German Space Agency within DLR (6) University of Tasmania (7) Observatorio Astronomico Nacional IGN Spain (8) Korea Astronomy Space Science Institute (9) Sejong University Korea)
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Pith reviewed 2026-06-29 15:16 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords celestial reference frameVLBIK bandquasarsICRFradio astrometryextragalactic sources
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The pith

Updated K-band celestial reference frame catalogs 1317 quasars with median uncertainties of 60 microarcseconds in right ascension.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper constructs CRF-K-2025, an updated celestial reference frame at 24 GHz drawn from 3.5 million VLBI observations spanning 211 epochs between 2002 and 2025. Positions are reported for 1317 compact extragalactic sources distributed over the full sky. The new catalog adds 493 sources relative to the prior K-band version and improves median precision by roughly 25 percent, reaching levels comparable to the standard S/X-band frame. At K band the sources display less extended emission than at lower frequencies, which the authors argue supports greater long-term stability of the reference frame.

Core claim

The central claim is that VLBI observations at K band have yielded the CRF-K-2025 catalog containing precise angular coordinates for 1317 compact extragalactic radio sources, with median scaled uncertainties of 60 micro-arcseconds in right ascension and 104 micro-arcseconds in declination, achieved through increased observational volume and sensitivity.

What carries the argument

The CRF-K-2025 catalog of source positions derived from 3.5 million VLBI delay measurements at 24 GHz across multiple station networks.

If this is right

  • The catalog supplies 493 more sources than ICRF3-K while maintaining full-sky coverage.
  • Median precision improves by approximately 25 percent over the previous K-band catalog and reaches parity with ICRF3-SX.
  • Reduced extended emission at K band relative to S/X bands supports the possibility of a more stable long-term reference frame.
  • The frame can serve as a complementary or alternative realization to the standard S/X-band ICRF3 for applications sensitive to source structure.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Continued accumulation of K-band observations could further tighten the uncertainties if the same station networks remain available.
  • The reduced source structure at higher frequency may lessen systematic errors when the frame is used for spacecraft navigation or geodetic measurements.
  • Cross-comparison of CRF-K-2025 positions with optical or Gaia-based frames could quantify any frequency-dependent offsets in quasar positions.

Load-bearing premise

The VLBI observations produce positions free of significant unmodeled systematic errors and the sources remain sufficiently compact at 24 GHz that the stated uncertainties are not biased by time-variable structure.

What would settle it

Finding position differences larger than the quoted uncertainties when CRF-K-2025 is compared against independent high-precision measurements at other frequencies or detecting substantial time-variable extended structure in a large fraction of the sources at 24 GHz.

Figures

Figures reproduced from arXiv: 2605.27629 by (2) South African Radio Astronomy Observatory, (3) Jet Propulsion Laboratory, (4) TU Wien, (5) German Space Agency within DLR, (6) University of Tasmania, (7) Observatorio Astronomico Nacional, (8) Korea Astronomy, (9) Sejong University, Alessandra Bertarini (5), Aletha de Witt (2), Christopher S. Jacobs (3), Cristina Garcia-Miro (7), David Gordon (1), Department of Geodesy, Do-Young Byun (8) ((1) US Naval Observatory, Geoinformation, Hana Krasna (4), IGN, Jamie McCallum (6), Jeffrey A. Hodgson (9), Jonathan Quick (2), Korea), Sang-Sung Lee (8), Space Science Institute, Spain, Taehyun Jung (8), Whee Yeon Cheong (8).

Figure 1
Figure 1. Figure 1: Distribution of the 268 K band sources from the ten 24-hour VLBA sessions of Lanyi et al. (2010). The dotted lines show the ecliptic and the galactic plane. 2. OBSERVATIONS Building a modern celestial reference frame with VLBI is a major undertaking typically requiring many years of observations. We inherited 15 K band VLBA sessions from earlier programs. This included the 10 24-hr ses￾sions from Lanyi et … view at source ↗
Figure 2
Figure 2. Figure 2 [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Distribution of the 1317 sources of the CRF-K-2025 catalog. The dotted lines show the ecliptic and the galactic plane [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: The distribution of scaled uncertainties averaged in 3◦ declination bins for CRF-K-2025 (plus signs) and the estimated noise floor values (connected lines). observations, over 7 times as much data as was used for ICRF3-K. Data from the southern hemisphere networks has increased by a factor of 2.3 since ICRF3-K, but is still only a very small fraction of the total data, resulting in an asymmetric distributi… view at source ↗
read the original abstract

We present an updated K band (24 GHz) celestial reference frame (CRF) constructed from 3.5 million Very Long Baseline Interferometry (VLBI) observations collected during 211 observing epochs between May 2002 and December 2025 using the Very Long Baseline Array (VLBA), the HARTRAO-HOBART26 baseline, the HARTRAO-YEBES40M baseline, and the Korean VLBI Network (KVN) augmented with several other VLBI stations. We have successfully observed and determined precise angular coordinates for 1317 compact extragalactic radio sources, essentially quasars, covering the full sky. This updated K band catalog is designated as CRF-K-2025. The precision of CRF-K-2025 is characterized by median scaled uncertainties of 60 and 104 micro-arc-seconds in right ascension and declination, respectively. The increase in number of observations and sensitivity over earlier K band campaigns has resulted in a catalog with 493 additional sources and a precision approximately 25% better than the ICRF3-K catalog, and similar to the ICRF3-SX catalog. At K band, these quasar radio sources generally show less extended emission than at lower frequencies and thus can potentially provide a more stable long term celestial reference frame than at the standard S/X (2.3/8.4 GHz) observing bands of ICRF3-SX.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The manuscript presents CRF-K-2025, an updated K-band (24 GHz) celestial reference frame catalog constructed from 3.5 million VLBI observations across 211 epochs (May 2002–December 2025) using the VLBA, HARTRAO-HOBART26, HARTRAO-YEBES40M, KVN, and additional stations. It reports positions for 1317 compact extragalactic sources covering the full sky, with median scaled uncertainties of 60 μas in right ascension and 104 μas in declination. The work claims 493 additional sources and approximately 25% better precision than ICRF3-K (comparable to ICRF3-SX), attributing potential long-term stability advantages to reduced extended emission at K band relative to S/X frequencies.

Significance. If the positions prove unbiased, the catalog would strengthen high-frequency reference frames by increasing source density and precision while potentially improving long-term stability for astrometric and geodetic applications. The scale of the dataset (3.5 million observations) and the net gain of 493 sources represent clear strengths in observational scope.

major comments (3)
  1. [Abstract and §3] Abstract and §3 (results): The assertion of 'approximately 25% better' precision than ICRF3-K is not supported by a direct statistical comparison (e.g., ratio of median uncertainties or Kolmogorov-Smirnov test on overlapping sources); the quoted medians alone do not establish the improvement factor without showing the underlying distributions.
  2. [§2] §2 (observations and data reduction): No description is given of the delay modeling, baseline-dependent calibration, or weighting scheme applied to the heterogeneous network (VLBA + HARTRAO-HOBART26 + HARTRAO-YEBES40M + KVN); without these details the claim that the scaled uncertainties of 60/104 μas are free of significant systematics cannot be evaluated.
  3. [§4] §4 (source properties and discussion): The statement that sources 'generally show less extended emission' at K band is unsupported by any quantitative metrics such as structure indices, closure-phase rms, or multi-epoch position repeatability for the 1317 sources; this directly affects the central claim of improved long-term stability.
minor comments (2)
  1. [Introduction] The manuscript should cite the specific ICRF3-K and ICRF3-SX reference papers when comparing catalogs.
  2. [Catalog description] The catalog table or supplementary material should report the number of observations and epochs per source to allow readers to assess individual reliability.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the constructive comments on our manuscript. We address each major point below and indicate the revisions planned for the next version.

read point-by-point responses

  1. Referee: [Abstract and §3] Abstract and §3 (results): The assertion of 'approximately 25% better' precision than ICRF3-K is not supported by a direct statistical comparison (e.g., ratio of median uncertainties or Kolmogorov-Smirnov test on overlapping sources); the quoted medians alone do not establish the improvement factor without showing the underlying distributions.

    Authors: We agree that a direct statistical comparison is needed to support the claim. In the revised manuscript we will add, for the 824 sources in common with ICRF3-K, the ratio of median uncertainties, histograms of the uncertainty distributions, and the result of a Kolmogorov-Smirnov test. revision: yes

  2. Referee: [§2] §2 (observations and data reduction): No description is given of the delay modeling, baseline-dependent calibration, or weighting scheme applied to the heterogeneous network (VLBA + HARTRAO-HOBART26 + HARTRAO-YEBES40M + KVN); without these details the claim that the scaled uncertainties of 60/104 μas are free of significant systematics cannot be evaluated.

    Authors: We will expand §2 to include the specific delay modeling (including tropospheric and ionospheric corrections), the baseline-dependent amplitude and phase calibration steps, and the weighting scheme applied to the heterogeneous array. These details were omitted for brevity but are available in the processing pipeline documentation. revision: yes

  3. Referee: [§4] §4 (source properties and discussion): The statement that sources 'generally show less extended emission' at K band is unsupported by any quantitative metrics such as structure indices, closure-phase rms, or multi-epoch position repeatability for the 1317 sources; this directly affects the central claim of improved long-term stability.

    Authors: The statement reflects the well-documented frequency dependence of VLBI source structure. We will add citations to key papers that quantify this effect and will include, where available from our multi-epoch data, a summary of position repeatability for a subset of sources. A full structure-index analysis for all 1317 sources is beyond the scope of the present catalog paper but will be noted as a limitation. revision: partial

Circularity Check

0 steps flagged

No significant circularity; empirical catalog from direct VLBI measurements

full rationale

The paper compiles angular coordinates for 1317 sources from 3.5 million VLBI observations across multiple networks and epochs. No derivation chain, model equations, or parameter fitting is described that reduces by construction to its own inputs. The reported median uncertainties (60/104 μas) and comparison to ICRF3-K are direct statistical summaries of the observed data set, not predictions or self-referential fits. Self-citations, if present, are not load-bearing for the central catalog claim, which rests on the external VLBI data collection itself.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard VLBI domain assumptions about source compactness and observation accuracy; no free parameters or invented entities are introduced in the abstract.

axioms (1)
  • domain assumption VLBI observations from the listed networks can determine precise angular coordinates of compact extragalactic radio sources at 24 GHz
    Invoked as the basis for constructing the catalog positions and uncertainties.

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