Science & tech · 1973
What can changes in an aircraft's radio signal reveal about movement in the polar ?
Interpreted as reflections from moving ionospheric irregularities, discrete frequency anomalies suggested that these irregularities often moved at 0.5–1.0 km/s. Their preferred direction was roughly toward the geographic east-to-south quadrant, centered on geomagnetic east, although the measurements were relatively insensitive to geomagnetic north–south motion and low velocities.
Details
This observational radio-propagation study recorded the frequency spectrum of a continuous-wave (CW), high-frequency (HF) radio signal transmitted from an aircraft over a one-hop path within the polar cap. Recordings covered about 50 nighttime hours during August 1967.
The records showed two general types of disturbance. One was diffuse spectral broadening: it was always present to some extent, even under quiet conditions, but on some occasions increased by as much as a factor of 2. The other was discrete frequency anomalies, recorded at about two per hour. These generally appeared above the carrier frequency of about 9 MHz and decreased in frequency with a median slope of −2.2 Hz min−1.
The discrete anomalies were most satisfactorily interpreted as Doppler-shifted reflections from moving ionospheric irregularities. On that interpretation, the observations indicated that the irregularities often moved at 0.5–1.0 km sec−1, with a preferred direction roughly toward the quadrant from geographic east to south, centered on geomagnetic east.
However, the measurements were relatively insensitive to geomagnetic north–south movements and to low velocities.
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