Compute this satellite's GCRS position and velocity at time `t`.

def _at(self, t):
        """Compute this satellite's GCRS position and velocity at time `t`."""
        rITRF, vITRF, error = self.ITRF_position_velocity_error(t)
        rGCRS, vGCRS = ITRF_to_GCRS2(t, rITRF, vITRF)
        return rGCRS, vGCRS, rGCRS, error