`Gravitational noise` interferes with determining the coordinates of distant sources
(PHYSorg.com Space & Earth)

Our galaxy`s gravitational field limits the accuracy of astrometric observations of distant objects. This is most apparent for objects that are obscured behind the central regions of the galaxy and the galactic plane, where the deviation can be up to several dozen microarcseconds. And more importantly, the effect of this gravitational `noise` cannot be removed. This means that beyond a certain point, it will no longer be possible to improve the accuracy of determining the position of reference objects that are used to define the coordinates of all other sources.