Ionospheric ducting of radio signals in the ORBIS CAL experiment

ORBIS CAL is the mnemonic for Orbiting Radio Beacon Ionospheric Satellite for Calibration, which is an experiment in ionospheric ducting of satellite beacon signals that was performed March 19–24, 1969. Propagation ranges of 20,000 kilometers were recorded frequently at day and night hours at Thule and at Sagamore Hill, Massachusetts. The results of this experiment are compared with those of earlier ORBIS attempts, and the probable propagation mechanisms are discussed. Rough path-loss calculations indicate that received levels exceed those of free-space propagation by about 5 decibels.