Sweep-Frequency Oblique Ionospheric Sounding at Medium Frequencies

An experimental ionospheric sounder has been constructed for vertical-incidence or oblique sounding in the frequency range 600 kHz to 2 MHz. The equipment employs an FM linear frequency sweep transmission generated by direct synthesis from a frequency standard; an identical sweep is used for demodulation at the receiver. Sounding records, made at night over a 2000-km path, exhibit 10-μs resolution and show no interference from broadcast stations. Radiated power ranged from 25 to 250 watts. The advantage of good time delay resolution, together with suppression of interfering signals, makes possible ionospheric sounding of greatly improved quality in the MF range.