Establishing Celestial Reference Frames at Different Ranges of Wavelengths

The current fundamental celestial reference frame is the second realization of the International Celestial Reference Frame (ICRF2) derived from dual frequency VLBI observations at 13/3.6 cm. The ICRF2 catalog includes approximately 3000 compact radio sources, largely quasars, of which 295 defining sources establish the coordinate axes with an accuracy of approximately 10 microarcseconds. More limited catalogs have been developed at 1.2 cm using the VLBA (approximately 300 sources) and at 3.6/0.9 cm using the Deep Space Network (approximately 500 sources), primarily to support spacecraft navigation. Anticipating the prospective Gaia optical catalogue, a set of approximately 400 radio weak but optically bright (V less than or equal to 18) objects is being studied at 13/3.6 cm for the radio-optical frame tie using the high sensitivity of the EVN + VLBA. The Gaia QSO catalog currently has approximately 100,000 defining objects whose morphology and variability are being tabulated. The characteristics, limitations and future development of the various wavelength catalogs will be discussed along with the anticipated radio-optical frame transfer.