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