Abstract: The X/S band(8.4/2.3GHz) Celestial Reference Frame is the largest of the three components of the ICRF3 released in 2018. In the nearly six years since, the X/S band data set has increased by ~25% to 5707 sources, based on ~7100 VLBI sessions and ~18.4millionobservations—a 40% increase since ICRF3. Observations using the VLBA have been key to densifying the frame. In particular, the density of sources near the ecliptic has been greatly improved making the frame more useful for spacecraft navigation. The latest solutions have median formal precisions of 107 μas in α cosδ and 189 μas in δ. Median number of sessions per source is 6, but with a much higher number for sources used regularly in geodesy. The spherical harmonic distortions seen in the recent X/S-band CRF vs. ICRF3-X/S with the largest terms: Y-rotation -13 +- 2 μas, quadrupole 2,0 Mag 8.1 +- 1.8 µas and the quadrupole 2,0 electric term of 11.5 +- 2.2 µas. We note that the X/S-band frame is dominated by the all northern geometry of the VLBA. This results in some weakness in sources from the equator to -40 deg declination. Future improvements include the potential for increasing the VLBA data rates from 2 Gbps to 4 Gbps.

Copyright©2024. All Rights Reserved. We acknowledge use of the Very Long Baseline Array under the US Naval Observatory's time allocation. This work supports USNO's ongoing research into the celestial reference frame and geodesy. The VLBA is operated by the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).