A Low-Power Vertical-Incidence Ionosonde

Vertical-incidence (VI) ionospheric measurements, traditionally performed with 5- to 50-kW ionosondes, may be obtained alternatively using FM CW transmissions of much lower peak power. The FM signal must be interrupted periodically to permit both transmitting and receiving with a single antenna. An ionosonde of this type has been constructed and operated. The monostatic FM sounder radiates a peak power of 8 W and sweeps the frequency range of 0.5 to 30 MHz either linearly or logarithmically. Records from the FM ionosonde are comparable in time delay resolution, S/N, and overall quality to conventional pulse ionograms; the effects of interference from other HF users is somewhat less conspicuous on the FM sounder records. It is suggested that this advantage, plus the benefits of lower power operation, may make possible VI measurements under circumstances where a conventional pulse ionosonde is impractical.