A Doppler technique for measuring an aircraft´s velocity from the spectrum of its HF transmissions

Unmodulated HF signals transmitted by a distant aircraft over the ocean showed a Doppler-spread component of total power 15-25 dB less than the normal coherent component. By interpreting the spread component as caused by nonspecular scatter from the ocean, both the aircraft ground speed and the velocity component in the direction of the observer can be derived without prior knowledge of the exact frequency of transmission. The results using this technique agree with values determined from the aircraft log within a median difference of six percent.