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Deriving a VGOS-Band Flux Density Catalog from Real Geodetic VLBI Observations

Chakraborty, Shilpi

Publisher
Zenodo
DOI
10.5281/zenodo.17848776
Repository
zenodo.org
Language
en

Abstract

Efficient scheduling in the VLBI Global Observing System (VGOS) requires accurate knowledge of source flux densities at 3–11 GHz. However, the lack of such data leads to fixed 30-second scan durations, often resulting in sub-optimal use of observing time and limited scan counts per session. To address this, we present a method to estimate flux density per projected baseline length bin (in 1000 Km interval) using real VGOS observations, accounting for intrinsic source structure while minimizing system noise effects. From these baseline bin flux estimates, we compute the integration time needed to achieve a target signal-to-noise ratio (SNR) of 15 per bin. A dedicated pipeline has been developed to dynamically determine observation duration using either individual SEFDs, enabling flexible and accurate flux-based scheduling. Comparative analysis with SKED’s power-law flux model reveals its limitations in capturing detailed source structure, especially at higher frequencies. Applied to one of 19 VGOS sessions, our approach increased the total number of observations by 24%, highlighting its potential to enhance scan efficiency, improve geodetic precision, and support better classification of compact versus extended sources

Full text

“Deriving a VGOS-Band Flux Density Catalog from Real Geodetic VLBI Observations” Co-authors Lucia McCallum Tiege McCarthy B. Nagarajan Onkar Dikshit Presented by: Shilpi Chakraborty [email protected] [email protected] Geoinformatics Indian Institute of Technology Kanpur, India 23-10-2025 1 10th International VLBI Technology Workshop (IVTW) Outline ➢ Role of flux density in VGOS observations ➢ VGOS session data ➢ Methodology ➢ Results Proposed flux density catalog Practical usability of methods Verification of proposed Catalog 23-10-2025 2 Role of Flux Density in VGOS observations VGOS Goal: To maximize the VGOS observational efficiency but an optimal observing time for sources is required at VGOS frequencies (3–11 GHz). Challenge: Lack of source flux density information at VGOS frequencies. Result: scan durations often fixed at 30 seconds which limits the number of scans in a session, sky coverage and resource utilization. Schartner et al (2025), introduced variable scan durations, currently ranging between 10 and 30 seconds at VGOS frequencies. Proposed work: In our study, we propose a new methodology to derive a flux density catalog using real VGOS data as well as theoretical information about source characteristics and assess the practical reliability of the estimation methods. 23-10-2025 3 Study Set up and VGOS data for analysis 23-10-2025 4 Gs (G) GGAO12M K2 (H) KOKEE12M Mg (M) MACGO12M Ow (T) ONSA13SW Wf (E) WESTFORD Ws (V) WETTZ13S Yj (Y) RAEGYEB Oe (S) ONSA13NE Is(I) ISHIOKA •24-hour session, 19 sessions •VGOS four bands : Band A: 3.0 -3.5 GHz Band B: 5.2-5.7 GHz Band C: 6.36.8 GHz Band D: 10.2 – 10.7 GHz •Total Channel – 32 (4*8 channel) •Band width per channel – 32 MHz 23-10-2025 5 23-10-2025 6 23-10-2025 7 Methodology for deriving flux density using VGOS real observations 23-10-2025 8 VgosDB data Extract parameters : source, baseline, visibility amplitude & phase, SNR per baseline, UTC time, scans, RA/Dec, station positions (ECEF), and session date & time. Project baselines into uv plane. Compute radial projected baseline length. 1. Calculate the band-wise flux density using real VGOS observations SNR per band from channel fringe visibilities (Corey B., IVS memo, 2022) 2. Calculate the band-wise flux density by power law model from SKED catalogue (S/X band) S ∝ 𝜗−𝛼 Calculating Flux density in two ways 23-10-2025 9 23-10-2025 16 Verification 23-10-2025 17 23-10-2025 18 Summary ✓Developed a comprehensive methodology to estimate flux density of VGOS sources, incorporating system observational uncertainties. Applied weighted averaging using the inverse of noise variance, enhancing Signal-to-noise ratio (SNR). ✓Comparison analysis of both datasets indicates that power-law models are insufficient for capturing the spectral and structural complexities of extended sources. Require more adaptable or piecewise models for accurate representation. ✓Good agreement observed between VGOS and Schartner catalogues for compact features at long baselines and high frequencies but shows discrepancies at short baselines. ✓This work was submitted to PASA. ✓ The next step is to implement this method operationally that is, to ensure the flux catalog is continuously updated with information from the most recent VGOS sessions, similar to how the legacy S/X flux catalog is regularly updated on a monthly basis. 23-10-2025 19 Thank you 23-10-2025 20