  We performed a unique Venus observation campaign to measure the disk
brightness of Venus over a broad range of wavelengths in August and September
2020. The primary goal of the campaign is to investigate the absorption
properties of the unknown absorber in the clouds. The secondary goal is to
extract a disk mean SO$_2$ gas abundance, whose absorption spectral feature is
entangled with that of the unknown absorber at the ultraviolet (UV)
wavelengths. A total of 3 spacecraft and 6 ground-based telescopes participated
in this campaign, covering the 52 to 1700~nm wavelength range. After careful
evaluation of the observational data, we focused on the data sets acquired by 4
facilities. We accomplished our primary goal by analyzing the reflectivity
spectrum of the Venus disk over the 283-800 nm wavelengths. Considerable
absorption is present in the 350-450 nm range, for which we retrieved the
corresponding optical depth by the unknown absorber. The result shows a
consistent wavelength dependence of the relative optical depth with that at low
latitudes during the Venus flyby by MESSENGER in 2007 (P\'erez-Hoyos et al.
2018), which was expected because the overall disk reflectivity is dominated by
low latitudes. Last, we summarize the experience obtained during this first
campaign that should enable us to accomplish our second goal in future
campaigns.
