It looks OK to me:

    (define (time-to-move from-pos to-pos)
      ;; Calculates the total time to move between two positions,
      ;; assuming rotation and ascension can occur simultaneously.
      (max (time-to-rotate (position-azimuth   from-pos)
                           (position-azimuth   to-pos))
           (time-to-ascend (position-elevation from-pos)
                           (position-elevation to-pos))))