The United States Naval Observatory (USNO) operates as an International VLBI Service for Geodesy and Astrometry (IVS) Analysis Center for Source Structure. The charter of the Analysis Center for Source Structure is to provide products directly related to the IVS determination of the “definition and maintenance of the celestial reference frame.” These include radio frequency images of International Celestial Reference Frame (ICRF) sources, intrinsic structure models derived from the radio images, and an assessment of the astrometric quality of the ICRF sources based on their intrinsic structure. The primary product is the Fundamental Reference Image Data Archive (FRIDA), an archive of VLBI images of thousands of compact extragalactic radio sources. FRIDA contains VLBI images at S-band (~2.3 GHz), X-band (~8.6 GHz), K-band (~24 GHz), and Q-band (~43 GHz).

The ICRF is made up of positions of distant quasars determined using very long baseline interferometry at radio frequencies. If these sources are point-like, then their positions can be determined to an accuracy of less than 0.05 miliarcseconds. However, if a source has some structure, two bright components for example, then the determined position of the source will likely lie somewhere between them, and the range of possible positions, or the error of the source position, becomes large. Ideally sources in the ICRF will be point-like, and their positional errors will be small.

One way to determine source structure is to image a source. As part of our duty as an analysis center for Source Structure, USNO makes images of the quasars that comprise the ICRF and provides those images to the community. We also engage in research to improve the ICRF and minimize the impact source structure has on the ICRF.