Imaging VGOS Observations and Investigating Source Structure Effects

Mei-ling, XU; Savolainen, Tuomas; Zubko, Nataliya; Poutanen, Markku; Lunz, Susanne; Schuh, Harald; Wang, Guang L.

doi:10.1029/2020jb021238

Cited by 12 publications

(15 citation statements)

References 39 publications

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“…Evidence for another critical error source in the VGOS system is the presence of discrete jumps in the closure delays and closure TECs, for instance with a delay offset of about 310 ps or integer multiples of that. The likely cause is found to be source-structure-induced phase changes across the four bands (Xu et al 2021a).…”

Section: Conclusion and Discussionmentioning

confidence: 94%

Observable quality assessment of broadband very long baseline interferometry system

Mei-ling

Anderson

Heinkelmann

et al. 2021

J Geod

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The next-generation, broadband geodetic very long baseline interferometry system, named VGOS, is developing its global network, and VGOS networks with a small size of 3–7 stations have already made broadband observations from 2017 to 2019. We made quality assessments for two kinds of observables in the 21 VGOS sessions currently available: group delay and differential total electron content ($$\delta $$ δ TEC). Our study reveals that the random measurement noise of VGOS group delays is at the level of less than 2 ps ($$1\,\hbox {ps}\,=\,10^{-12}$$ 1 ps = 10 - 12 s), while the contributions from systematic error sources, mainly source structure related, are at the level of 20 ps. Due to the significant improvement in measurement noise, source structure effects with relatively small magnitudes that are not overwhelming in the S/X VLBI system, for instance 10 ps, are clearly visible in VGOS observations. Another critical error source in VGOS observations is discrete delay jumps, for instance, a systematic offset of about 310 ps or integer multiples of that. The predominant causative factor is found to be related to source structure. The measurement noise level of $$\delta $$ δ TEC observables is about 0.07 TECU, but the systematic effects are five times larger than that. A strong correlation between group delay and $$\delta $$ δ TEC observables is discovered with a trend of 40 ps/TECU for observations with large structure effects; there is a second trend in the range 60–70 ps/TECU when the measurement noise is dominant.

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Section: Conclusion and Discussionmentioning

confidence: 94%

Observable quality assessment of broadband very long baseline interferometry system

Mei-ling

Anderson

Heinkelmann

et al. 2021

J Geod

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“…The main challenge is that the images for modeling structure effects have to be registered over time for each source in order to maintain a stable CRF at high accuracy. The next generation of geodetic VLBI, known as the VLBI Global Observing System (VGOS) (Niell et al 2007;Petrachenko et al 2009), requires the images of each source at the four different bands to be registered in the range of 3.0-14.0 GHz (Xu et al 2021b). Otherwise, only the relative structure effects can be reduced, and the misalignment of the images at different epochs or at different frequency bands due to core shift, discussed in the next section, inevitably leads to source position variations.…”

Section: Radio Source Structurementioning

confidence: 99%

Evidence of theGaia–VLBI position differences being related to radio source structure

Mei-ling

Lunz

Anderson

et al. 2021

A&A

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Context. We report the relationship between the Gaia–VLBI position differences and the magnitudes of source structure effects in VLBI observations. Aims. Because the Gaia–VLBI position differences are statistically significant for a considerable number of common sources, we discuss and attempt to explain these position differences based on VLBI observations and available source images at centimeter wavelengths. Methods. Based on the derived closure amplitude root mean square (CARMS), which quantifies the magnitudes of source structure effects in the VLBI observations used for building the third realization of the International Celestial Reference Frame, the arc lengths and normalized arc lengths of the position differences are examined in detail. The radio-jet directions and the directions of the Gaia–VLBI position differences are investigated for a small sample of sources. Results. Both the arc lengths and normalized arc lengths of the Gaia and VLBI positions are found to increase with the CARMS values. The majority of the sources with statistically significant position differences are associated with the sources having extended structure. Radio source structure is the one of the major factors of these position differences, and it can be the dominant factor for a number of sources. The vectors of the Gaia and VLBI position differences are parallel to the radio-jet directions, which is confirmed via stronger evidence.

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“…Otherwise, a misalignment of the images at the four bands will introduce a change in the position estimate from the VGOS observables with these corrections applied. This has been demonstrated by Xu et al (2021d). The exact formulas of the impact of the misalignment are given above as Eq.…”

Section: Aligning the Images At The Four Bandsmentioning

confidence: 90%

“…Moreover, these sources tend to have compact cores with extended but weaker jets. To investigate these general cases, we use the images constructed from actual VGOS observations by: 1. deriving the closure images by using the method in Xu et al (2021d); 2. calibrating VGOS observations based on the closure images; 3. performing model fitting by using difmap.…”

Section: Changes Of Source Positions Over Frequencymentioning

confidence: 99%

Impacts of the image alignment over frequency for VLBI Global Observing System

Xu,

Savolainen,

Anderson

et al. 2021

Preprint

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Aims. The VLBI Global Observing System, which is the next generation of geodetic VLBI and is called VGOS, observes simultaneously at four frequency bands in the range 3.0-10.7 GHz (expected to be extended to 14 GHz). Because source structure changes with frequency, we aim to study the source position estimates from the observations of this new VLBI system. Methods. Based on an ideal point source model, simulations are made to determine the relation between the source positions as determined by VGOS observations and the locations of the radio emission at the four bands. Results. We obtained the source positions as determined by VGOS observations as a function of the source positions at the four frequency bands for both group and phase delays. The results reveal that if the location of the radio emission at one band is offset with respect to that at the other bands, the position estimate can be shifted to the opposite direction and even by more than three times that offset. Conclusions. The VGOS source positions will be very variable with time and very imprecise in the sense of relating to the locations of the radio emission at the four bands, if the effects of source structure are not modeled. The image alignment over frequency is essential in order to model the effects of source structure in VGOS observations, which is the only way to mitigate these strong frequency-dependent impacts on VGOS source positions. Key words. reference systems / astrometry / galaxies: active / galaxies: jets / radio continuum: galaxies 2 https://ivscc.gsfc.nasa.gov/index.html

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Imaging VGOS Observations and Investigating Source Structure Effects

Cited by 12 publications

References 39 publications

Observable quality assessment of broadband very long baseline interferometry system

Observable quality assessment of broadband very long baseline interferometry system

Evidence of theGaia–VLBI position differences being related to radio source structure

Impacts of the image alignment over frequency for VLBI Global Observing System

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