The K-Band (24 GHz) Celestial Reference Frame Determined from Very Long Baseline Interferometry Sessions Conducted Over the Past 20 Years

The third realization of the International Celestial Reference Frame (ICRF3) was adopted in August 2018 and includes positions of extragalactic objects at three frequencies: 8.4 GHz, 24 GHz, and 32 GHz. In this paper, we present an update of the celestial reference frame estimated from Very Long Baseline Interferometry measurements at K-band (24 GHz) including data until June 2022. The observations of the radio sources are conducted with the Very Long Baseline Array from the U.S. territory and in the southern hemisphere with single baseline observations between the HartRAO 26m antenna in South Africa and the Hobart 26m in Australia. The data set starts in May 2002 and currently consists of more than 120 24h observing sessions performed over the past 20 years. Since the publication of ICRF3, the additional observations of the sources during the last four years allow maintenance of the celestial reference frame and more than 200 additional radio sources ensure an expansion of the frame. We determine the updated K-band solution (K-CRF) with two independent analysis software packages (VieVS and Calc/Solve) and describe the differences in the solution strategy. We compare the updated K-CRF to ICRF3-K using the so-called vector spherical harmonics to provide information about systematic differences between two astrometric catalogs.