The global distribution of radio telescopes and networks participating in routine K-band Celestial Reference Frame (K-CRF) observations. Currently, the Very Long Baseline Array (VLBA) sessions, consisting of ten 25-m antennas (VLBA-BR, HN, FD, KP, LA, MK, NL, OV, PT, SC), account for the majority of K-CRF observations, contributing approximately 92% of the data through the U.S. Naval Observatory's 50% timeshare allocation. Weekly K-band observations commenced in September 2023, utilizing a diverse network of telescopes, including the Korean VLBI Network (KVN) 22-meter antennas (Ky, Ku, Kt), the 26-meter Hartebeesthoek (HartRAO) antenna (Hh) located in South Africa, the 22-meter Mopra antenna (Mp) in Australia, the 20-meter Sejong geodetic antenna (Kv) in Korea, the 26-meter Hobart antenna (Ho) in Australia, and the 40-meter Yebes antenna (Yb) in Spain. Additionally, the Tianma 64-meter antenna (T6) in China and the Tidbinbilla 70-meter antenna (DSS43/Ti) in Australia have previously participated in selected K-band sessions.

A visual representation of the global distribution of telescopes equipped with K-band receivers, either currently operational or planning for their deployment. Telescopes that have actively participated in K-CRF observations are highlighted in blue, showcasing their integral role in the K-CRF network. Additionally, other telescopes within the European VLBI Network (EVN) equipped with K-band receivers are depicted in green, signifying their readiness for K-CRF endeavours. Notably, observations conducted by Charlot et al., have already seen the utilisation of the EVN at K-band for geodetic and imaging observations (Gomez et al., 2021, Charlot et al., 2023). Red markers represent antennas belonging to the Long Baseline Array (LBA) that are equipped with K-band receivers, These antennas may potentially become valuable additions to the expanding K-CRF initiative in the future. Furthermore, antennas with potential K-band capability on the horizon are portrayed in orange, signifying their potential forthcoming participation in K-band observations.