Extending the ICRF to Higher Radio Frequencies: Astrometry at 24 and 43 GHz

A program of survey observations using the Very Long Baseline Array (VLBA) at K-band (24 GHz) and Q-band (43 GHz) was begun in 2002 as a first step toward the goal of extending the ICRF to frequencies above 8.4 GHz. Each session covers the full range of right ascension and declinations down to ≈ −30 deg. Analysis of the first three sessions has produced a full sky catalog of 108 sources with median K-band formal position uncertainties of 210 and 280 μas in α cos(δ) and δ, respectively. For Q-band the corresponding results are 292 and 404 μas. Group delay residuals were excellent at ≈ 20 ps weighted RMS. An external comparison of the K-band frame to the S/X-band ICRF shows systematic errors at about 2 times larger than the formal precision. These systematic differences include a zonal trend vs. declination. We suspect that these errors are due in part to the North-South geometric weakness of the array.