The Legends of Dune prequel trilogy (2002-2004) by Brian Herbert and Kevin J. Anderson establish that the Spacing Guild first start using Guild Navigators because the travel technique of spacefolding is not safe; only about nine out of every ten heighliners make it to their final destination without Navigators [REF].
In the Brian Herbert/Anderson Prelude to Dune prequel trilogy (1999-2001), some of the previously-unexplained process of becoming a Navigator is revealed through the story of D'murr Pilru [REF].