The X/Ka-band (8.4/32 GHz) 2024a Celestial Reference Frame C.S. Jacobs1, S. Horiuchi2, D. Firre3, Y. Murata4, H. Takeuchi4, T. Uchimura4 13th International VLBI Service General Meeting, Tsukuba, Japan, 4-9 March 2024 (1) Jet Propulsion Laboratory, California Institute of Technology (2) Canberra DSCC/NASA, CSIRO, Australia, (3) European Space Agency, Darmstadt, German (4) JAXA, Sagamihara Kanagawa, 252-5210, Japan Abstract: The X/Ka-band (8.4/32 GHz) Celestial Reference Frame became one of three components of the ICRF-3 in 2018. In the five years since, the X/Ka data set has increased by about 93% as well as adding the much needed north-south geometry from Japan to Australia. The latest solutions have 686 sources with median formal precisions of 43 μas in α cosδ and 62 μas in δ. The large spherical harmonic distortions seen in the ICRF3-X/Ka are greatly improved with the Z-dipole term reduced from 314 μas to a statistically insignificant -49 +- 40 µas and with the quadrupole 2,0 magnetic term of 186 +- 15 µas and a quadrupole 2,0 electric term of 80 +- 24 µas. We note that the X/Ka frame is derived from a limited geometry of only five observing sites of which the two DSN baselines dominate with 85% of the total data thus creating a susceptibility of this frame to geometric distortions. The prospects for future improvements are bright and we expect the distortions to be reduced as more data from the ESA Malargüe and JAXA Misasa stations are added. Fig. 1 NASA-ESA-JAXA Ka-band network. The addition of Argentina & Japan adds 6 baselines & Full Sky coverage. For d = +45 to +90 deg coverage is from California-=Spain and Japan-Spain baselines. For d -45 to -90 deg, coverage is only from the Australia-Argentina baseline. Three sites are susceptible to earthquakes. V. Conclusions: The X/Ka-band CRF has 686 sources covering the full sky and is making rapid improvements in the precision. The median precision is 43 / 62 µas in acosd / d. Spherical harmonic differences vs. ICRF3-S/X are <= 190 µas. wRMS scatter vs. Gaia is ~230 µas. Improving accuracy depends on controlling systematics via increased observations using a North-South baseline geometry. Acknowledgements: Copyright © 20234 All Rights Reserved. This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). Fig. 4: Schematic of Active Galactic Nuclei (Marscher, 2006, Krichbaum, 1999, Wehrle, 2010) X-band 3.6 cm S-band 13 cm Ka-band 0.9 cm W-band 0.3 cm Water:1.3 cm/ 22 GHz O2 line: 0.5 cm/ 60 GHz L-band 19-24 cm Fig. 5: The radio “window” is transparent compared to most of the spectrum (credit: NASA) Ka-band (32 GHz) is in the saddle point between H20 (22 GHz) and O2 (60 GHz) lines. Fig. 7: Dec precision: Median s is 62 µas for 686 sources. Median = 150 µas for Dec < -45 deg. I. High Frequency Radio Frames: As radio frequencies increase, sources tend to be more core dominated as the extended structure in the jets tends to fade away with increasing frequency (fig. 3,4). The spatial offset of the emissions from the AGN engine due to opacity effects (“core shift”) is reduced as frequency increases. Advantages of Ka-band compared to S/X-band: • More compact, stable sources (Fig. 3,4) • Reduced opacity effects: “core shift” • Ionosphere & solar plasma effects reduced by 15X. Disadvantages of Ka-band: • More weather sensitive (fig. 5) • Shorter coherence times • Weaker sources, many resolved • Antenna pointing is more difficult,. • Combined effect is lower sensitivity, But increasing data rates are rapidly compensating. We have increased JPL operations to 2.0 Gbps. Executive Summary: Celestial angular coordinates (a,d) are derived from VLBI measurements at 8.4/ 32 GHz (36/ 9 mm) of Active Galactic Nuclei. Agreement with S/X is at the part per billion level. X/Ka has reduced astrophysical systematics vs. S/X. Fig. 2. Antennas of combined NASA-ESA-JAXA X/