8.2 Effects in the ionosphere
Satellite navigation systems such as the U.S. Global Positioning System (GPS) or the European Galileo
operate by transmitting radio waves from the spacecraft to ground-based receivers. Signals from several
satellites are then used to calculate the observing position to high accuracy. As the signals propagate
through the ionosphere, they are refracted and slowed especially when they traverse regions of intense
auroral currents. In addition, ionospheric scintillations can cause loss of signal lock and thereby lose the
positioning capability. While many of the malfunctions associated with energetic particles can be
counteracted by engineering the spacecraft with more shielding and redundancy in the systems, this is one
where accurate specification of the ionospheric conditions and modeling their effects on the signal
propagation is the only way the high accuracy of the positioning information can be maintained with
single-frequency receivers.
The ionosphere is heavily utilized as a transmitter of radio-frequency communication signals. As every
radio amateur knows, the radio wave communication is significantly influenced by the ionospheric
properties and especially the auroral currents. High frequency (HF) radio wave communications are
most affected, as they utilize reflection from the ionosphere to carry the signal to distances
beyond the local horizon. At times of high auroral activity, the signal can be even completely
absorbed making the HF radio propagation impossible. Furthermore, the telecommunication
systems increasingly utilize ultra-high-frequency (UHF) bands to transmit signal to satellites to
be relayed to other locations. Also this frequency range is vulnerable to the auroral currents
in the ionosphere, and can be degraded or even completely lost during times of high activity
(Lanzerotti, 2001b).
