Complex sentence: PI control yielded sustained yaw (angular error) of ± 2 °, while adding D yielded yaw of ± 1/6 °, better than most helmsmen could achieve.
Split into simpler sentences: PI control yielded sustained yaw (angular error) of ± 2 °. Adding the D element yielded a yaw error of ± 1/6 °, better than most helmsmen could achieve.