Although Mission Control normally performed all the actual navigation calculations, it was necessary to have a crew member adept at navigation so that the crew could return to Earth in case communication with Mission Control was lost.
ɔːlðˈoʊ mˈɪʃən kəntɹˈoʊl nˈoːɹməli pɚfˈɔːɹmd ˈɔːl ðɪ ˈæktʃuːəl nˌævɪɡˈeɪʃən kˌælkjʊlˈeɪʃənz, ɪt wʌz nˈɛsᵻsɚɹi tə hæv ɐ kɹˈuː mˈɛmbɚɹ ɐdˈɛpt æt nˌævɪɡˈeɪʃən sˌoʊ ðætðə kɹˈuː kʊd ɹᵻtˈɜːn tʊ ˈɜːθ ɪn kˈeɪs kəmjˌuːnɪkˈeɪʃən wɪð mˈɪʃən kəntɹˈoʊl wʌz lˈɔst.