What is the method of navigation which allows an aircraft to operate on any desired flight path within the coverage of ground or space-based NAVAIDs?

The method of navigation that allows an aircraft to operate on any desired flight path within the coverage of ground or space-based navigational aids (NAVAIDs) is known as RNAV, which stands for Area Navigation. RNAV enables aircraft to fly on any route within a defined airspace, rather than being restricted to routes that are directly linked to ground-based navigational aids like VOR stations.

RNAV systems use a combination of onboard navigation components, combined with input from various NAVAIDs and, in some cases, satellite systems. This flexibility allows for more efficient air traffic flow and optimized flight paths. Consequently, RNAV contributes significantly to enhancing airspace capacity and improving operational efficiency.

In contrast, other navigation methods listed have more limitations. VOR navigation relies on the use of VOR stations, which defines specific routes or airways, while ILS (Instrument Landing System) is focused on providing guidance for aircraft during the landing phase, typically to a specific runway at an airport. HLD navigation does not specifically refer to a widely recognized method of flight navigation, making RNAV the clear choice when discussing flexible route planning and navigation capabilities.