  Context. We present a celestial reference frame (CRF) based on the
combination of independent, multifrequency radio source position catalogs using
nearly 40 years of Very Long Baseline Interferometry observations at the
standard geodetic frequencies at SX band and about 15 years of observations at
higher frequencies (K and XKa). The final catalog contains 4617 sources. Aims.
We produce a multifrequency catalog of radio source positions with full
variance-covariance information across all radio source positions of all input
catalogs. Methods. We combined three catalogs, one observed at 8 GHz (X band),
one at 24 GHz (K band) and one at 32 GHz (Ka band). Rather than only using the
radio source positions, we developed a new, rigorous combination approach by
carrying over the full covariance information through the process of adding
normal equation systems. Special validation routines were used to characterize
the random and systematic errors between the input reference frames and the
combined catalog. Results. The resulting CRF contains precise positions of 4617
compact radio astronomical objects, 4536 measured at 8 Ghz, 824 sources also
observed at 24 GHz, and 674 at 32 GHz. The frame is aligned with ICRF3 within 3
$\mu$as and shows an average positional uncertainty of 0.1 mas in right
ascension and declination. No significant deformations can be identified.
Comparisons with Gaia-CRF remain inconclusive, nonetheless significant
differences between all frames can be attested.
