The solution was to fly lower: since radar operates in line-of-sight, the curvature of the Earth renders low-flying aircraft invisible beyond a certain range, the radar horizon. In practice, trees, hills, valleys and any other obstructions reduce this range even more, making a ground-based interception extremely difficult. The Canberra was designed for medium- to high-altitude flight and was not suitable for continuous terrain-hugging flight; this would require a completely different aircraft. Low-level strike aircraft, or "interdictors", grew into a new class of their own during the late 1950s. They generally featured high wing loading to reduce the effects of turbulence and cross-wind, some form of high-performance navigational radar to allow very low flight at high speeds, and large fuel loads to offset the higher fuel use at low altitudes.