This answer to Do antennae on the ISS have to constantly move to maintain data links? discusses some of the challenges precisely pointing an antenna mounted to the ISS exterior. While the ISSs orbit and attitude are generally known quite well and probably available those are global parameters and the ISS structure will flex under differential heating as the direction of the Sun is continuously changing and frequently eclipsed:
All pretty simple in principle, complicated by the gimbal's own internal dynamics (including friction), and the significant amount of flexibility and vibration of the ISS (including time-varying thermal distortion, which is what drives the need to search for TDRS at the start of each tracking session, as the ISS "bananas" in different directions as it heats up and cools down).
Question: How much does the ISS banana? How much does it bend and flex and twist under the constantly changing conditions of solar heating?
Just for illustration; three points along a structure might nominally fall on a line, but as it bends they could make two line segments that deviate from collinearity by say 0.01 degrees maximum, or an extreme location's position may deviate from its nominal location by 1 cm maximum for example.