UTC is non-unique because of leap seconds, so TAI + lat + long + altitude is actually required. I work on software for astronomy, and that quadruplet is what’s

UTC is non-unique because of leap seconds, so TAI + lat + long + altitude is actually required. I work on software for astronomy, and that quadruplet is what’s used to describe an observing location. You can actually get a little in trouble because of changes in the shape of the earth over time, so latitude and longitude and altitude need to be treated as time-dependent values, which matters once you are accounting for relativistic effects.