Which is a necessary condition for VOR navigational equipment to be effective at high altitudes?

For VOR (VHF Omni-directional Range) navigational equipment to be effective at high altitudes, it is essential to have DME (Distance Measuring Equipment) or suitable RNAV (Area Navigation) equipment. At higher altitudes, the range and accuracy of navigation aids become critical due to the increased distance from the ground-based VOR stations. DME provides essential distance information, allowing pilots to determine their position accurately in conjunction with VOR signals, especially over extended distances where VOR coverage may be less reliable.

Using suitable RNAV systems enhances the effectiveness of VOR by allowing for a more accurate and flexible navigation method, including ways to integrate data from multiple navigational sources. This combination ensures that aircraft can navigate safely and effectively in the more challenging conditions presented at higher altitudes.