Fill in each blank with NO MORE THAN THREE words from the text.To use aircraft as a __________________________, a navigation system is a must need. Today's most popuplar air navigation system is the Very High Frequency ________________________________, or VOR. First developed in the US, it is a _____________________________radio navigation system consisting of many fixed __________________________ and a receiver unit on board. The _______________________it uses ranges from 108 to 117.95 MHz, and its maximum range is 200nm __________________ and 25nm in the terminal area.