An aircraft once airborne sets course to the destination on a predetermined path designed with reference to the ground equipment like VOR (VHF Omni Range), NDB (Non- Directional Beacon) and DME (Distance Measuring Equipment). A pilot when flying is effectively blindfolded - he tunes to these ground installations using onboard instruments, to fly on the desired route.Sector Controllers in Area Control Centre In absence of a Radar, the Controller on ground is also effectively blindfolded , in the sense that he cannot see the aircraft physically, he controls and guides them using mental image that he would create and update continuously, and thereby separates aircraft with other potential traffic present in its route .