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Like all radars, SHARE determines the position and motion of distant objects by
bouncing beams of radio waves off them. In this case, the distant objects are
ripples in the density of electrons in the ionosphere and the ripples are carried along
with the background plasma flow. The time delay between a transmitted and received radio
pulse tells us how far away the ripples are from the radar. The frequency difference between
the pulses tells us how fast the irregularities (and hence the plasma) are moving away
from or towards the radar (the Doppler effect). The SHARE radar operates in a manner similar to air surveillance radars. A narrow beam of high-frequency (HF; 8-20MHz) radio waves is formed which is then swept through 16 pointing directions to scan 4 million square kilometres of the polar ionosphere every 2 minutes. |
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