Structure Effects for 3417 Celestial Reference Frame Radio Sources

Abstract

Geodetic/astrometric very long baseline interferometry (VLBI) has been routinely observing using various global networks for 40 yr, and it has produced more than 10 million baseline group delay, phase, and amplitude observables. These group delay observables are analyzed worldwide for geodetic and astrometric applications, for instance, to create the International Celestial Reference Frame (ICRF). The phase and amplitude observables are used in this paper, by means of closure analysis, to study intrinsic source structures and their evolution over time. The closure amplitude rms, CARMS, indicating how far away a source is from being compact in terms of morphology, is calculated for each individual source. The overall structure-effect magnitudes for 3417 ICRF radio sources are quantified. CARMS values larger than 0.3 suggest significant source structures and those larger than 0.4 indicate very extended source structures. The 30 most frequently observed sources, which constitute 40% of current geodetic VLBI observables, are studied in detail. The quality of ICRF sources for astrometry is evaluated by examining the CARMS values. It is confirmed that sources with CARMS values larger than 0.30 can contribute residual errors of about 15 ps to geodetic VLBI data analysis and those with the CARMS values larger than 0.4 generally can contribute more than 20 ps. We recommend CARMS values as an indicator of the astrometric quality for the ICRF sources and the continuous monitoring of the ICRF sources to update CARMS values with new VLBI observations as they become available.

Intercontinental comparison of optical atomic clocks through very long baseline interferometry

“…In Medicina, a 600 m long underground fibre is used, with frequency shifts less than 8 × 10 −17 in a day. In our experiment, we observed excess delay residuals for 3 of the 35 observed sources caused by the source structure effect 46,47 . The increased residuals are accounted for in the fit procedure but a careful selection of radio sources will help to avoid systematic effects in future clock comparisons.…”

Observable quality assessment of broadband very long baseline interferometry system

“…Thus, while the natural weighting statistics are appropriate for evaluating the properties of the delay/δTEC observables, the uniform weighting statistics can be useful for identifying sources with systematic errors, such as those due to source structure. Furthermore, the SNRs of VGOS observations are typically very high, for instance, the median SNR for the CONT17 VGOS observations is ∼ 90; uniform weighting should be used to investigate the systematic error levels, especially if these systematic errors are significantly larger than the ran- In order to further investigate the random measurement noise level in VGOS sessions, we adopted the closure amplitude RMS (CARMS) values based on the basic weighting scheme 7 from Table 2 in Xu et al (2019) to identify the sources with minimum structure in these VGOS sessions. For the definition of CARMS, please consult equations (2)-( 4) and ( 6)-( 8) in Xu et al (2019).…”

“…Furthermore, the SNRs of VGOS observations are typically very high, for instance, the median SNR for the CONT17 VGOS observations is ∼ 90; uniform weighting should be used to investigate the systematic error levels, especially if these systematic errors are significantly larger than the ran- In order to further investigate the random measurement noise level in VGOS sessions, we adopted the closure amplitude RMS (CARMS) values based on the basic weighting scheme 7 from Table 2 in Xu et al (2019) to identify the sources with minimum structure in these VGOS sessions. For the definition of CARMS, please consult equations (2)-( 4) and ( 6)-( 8) in Xu et al (2019). The CARMS value of each individual source was calculated using all the available closure amplitudes for X-band only from historical VLBI observations from 1980 to Aug. 2018 (no VGOS broadband observations are included).…”

“…As we did for the historical S/X VLBI observations (Xu et al 2019), it is beneficial to show a few closure plots for several sources with different magnitudes of structure effects as examples to understand those effects in the broadband VLBI system. Fig.…”

A broadband VLBI system using transportable stations for geodesy and metrology: an alternative approach to the VGOS concept

“…The influence of radio source structure on group delays has been investigated through closure delays by Xu et al (2016Xu et al ( , 2019. This provides delay components related to the radio source while eliminating the contribution from the local observation sites.…”