&quot;XCN&quot;, &quot;XLT+S&quot;, &quot;XCN+S&quot; is
      selected, spkezr_c computes the position of the target body T at
      epoch et+lt, where `lt' is the one-way light time. `lt' is the
      solution of the light-time equation

                | T(et+lt) - O(et) |
         lt = ------------------------                            (3)
                          c

      Subtracting the geometric position of the observer, O(et),
      gives the position of the target body relative to the
      obser