For those who would like to explore this subject in more detail, it was initially developed by the German Astronomer Karl Stumpff following the second world war. He later worked as a consultant at the Goddard Space Flight Center in the early 1960's. An English translation of his original report is available from the NASA Technical Report Server [1].
These functions were further developed at the MIT Instrumentation Laboratory for use in the Apollo Guidance Computer. This work was published by Richard Battin originally in Astronautical Guidance, which is no longer in print, and later on in his textbook [2].
These functions were further developed at the MIT Instrumentation Laboratory for use in the Apollo Guidance Computer. This work was published by Richard Battin originally in Astronautical Guidance, which is no longer in print, and later on in his textbook [2].
[1] https://ntrs.nasa.gov/citations/19630000922 [2] https://www.amazon.com/Introduction-Mathematics-Methods-Astr...