As of August 2023, the K-band Celestial Reference Frame (K-CRF) consists of 1187 relatively uniformly distributed sources  (see sky distribution of K-band sources) - comparable to the number of regularly observed sources at S/X-band - derived from 153 observing sessions and more than 2.3 million observations. For sources overlapping with the S/X-band frame the median precision of the K-band frame is now comparable to the standard S/X-band frame. These observations are comprised of ongoing sessions using the Very Long Baseline Array (VLBA) in the United States, and  observations involving the Korean VLBI Network (KVN) antennas, the Hartebeesthoek (HartRAO) 26-m antenna in South Africa, the Mopra 22-m antenna in Australia, the Sejong 20-m geodetic antenna in Korea, the Hobart 26-m antenna in Australia, and the Yebes 40-m in Spain (see K-band VLBI network map).

The use of K-band was motivated by the fact that the sources are generally intrinsically more compact at higher frequencies as shown in recent near-simultaneous images at S, X, K, and Q-band (de Witt et al., 2023), as well as by the factor of three improvement in interferometer resolution relative to the historically standard S/X-band. For these reasons, astrometric VLBI observations of reference sources at higher radio frequencies should permit the construction of a more accurate and more stable reference frame, which will be advantageous in tying the VLBI frame to optical frames such as Gaia. The K-CRF global solution and timeseries results are available from USNO: https://crf.usno.navy.mil/quarterly-vlbi-solution, and the raw K-band data is available in the NRAO archive (both Mark4 and idifits databases): https://data.nrao.edu

Under our K-band Astrometry and CRF VLBI Program we aim to continually diversify and strengthen the K-CRF network geometry and observation quality, and continually improve the astrometric accuracy of our frame (e.g. Krásná et al., 2023). In 2018 the K-CRF  was adopted as part of the ICRF3 (Charlot et al., 2020) and it will also be a contributor to the next generation ICRF, which is likely to be a fully unified multi-waveband frame (Charlot et al., 2023), also incorporating the optical realisation by Gaia.