https://www.thaitechnics.com/nav/vor.html

https://www.centennialofflight.net/essay/Government_Role/navigation/POL13.htm

https://www.aopa.org/news-and-media/all-news/1997/october/pilot/four-course-radio-ranges

https://www.britannica.com/technology/navigation-technology/Radio-navigation

​https://radiojove.gsfc.nasa.gov/education/educ/radio/tran-rec/exerc/iono.htm

​https://www.casa.gov.au/sites/default/files/non-directional-beacons-associated-automatic-direction-finding.pdf#:~:text=The%20ground%20station%20(NDB)%20radiates,will%20fly%20over%20the%20NDB.

https://www.gps.gov/systems/gps/