Observable quality assessment of broadband very long baseline
  interferometry system
  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 ($\delta$TEC). Our study reveals that the random measurement noise of
VGOS group delays is at the level of less than 2 ps (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
$\delta$TEC observables is about 0.07 TECU, but the systematic effects are five
times larger than that. A strong correlation between group delay and
$\delta$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 ps/TECU to 70 ps/TECU when the measurement noise is dominant.
