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.

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.

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.

Images of radio sources at the four bands were derived directly from VGOS broadband observations.• Source structure effects in VGOS observations were modeled and verified.• The alignments of the images at the various frequency bands were revealed to be very important in correcting source structure effects in VGOS observations.

Over the last decades the Greenland Ice Sheet (GrIS) has undergone a substantial shrinking of its mass. The mass loss is dominated by an accelerating discharge of outlet glaciers in south-east, west, and north-west Greenland (Khan et al., 2015). After glaciers in north-east Greenland had a longer period of relative stability they have shown an increase in dynamic thinning (Khan et al., 2014) and surface melt water runoff (Noël et al., 2019) since the early 2000s. To deal with this area in detail one has to take a closer look at the North-East Greenland Ice Stream (NEGIS). Accounting for about 16% of the entire area of the ice sheet (Khan et al., 2014), NEGIS originates near the summit and splits into the three main branches, Nioghalvfjerdsbrae (NG), Zachariae Isstrøm (ZI), and Storstrømmen (SN) (Figure 1). At present, the mass loss of the GrIS equals about 0.7 mm yr −1 of global sea-level rise (Forsberg et al., 2017;Shepherd et al., 2019; WCRP Global Sea Level Budget Group, 2018). The share of NG and ZI contribute less than 5% to this rise (Mouginot et al., 2019). However, in the consequence of ocean warming (Schaffer et al., 2020) the front of ZI will likely retreat another 30 km over the next decades and if, moreover, frontal melt rates exceed 6 m d −1 , ZI alone might contribute 16 mm to global mean sea level by 2,100 (Choi et al., 2017). Hence, it is essential to monitor ice-surface elevation and mass balance in order to detect and understand any substantial changes.Three main approaches are commonly used in order to infer the mass balance of ice sheets, namely the massbudget method, the gravity-change approach and the altimetry method. Results of the three methods were recently intercompared by Shepherd et al. (2019) andBamber et al. (2018). However, the majority of previous studies applied each method separately (e.g., for satellite altimetry: Hurkmans et al., 2014;