Aperture Synthesis for Ionospherically Propagated HF Radio Signals

A high-frequency (HF) aperture was synthesized by receiving ionospherically propagated signals in an airplane. The signals used the one-hop F lower ray mode to propagate a distance of 2600 km and were processed to yield equivalent aperture lengths of 10 km and 70 km. It was shown that apertures of up to 10 km in length could sometimes achieve their ideal free-space directivity without use of ionospheric compensation.