Interference with Communication and Navigation

Radiowave communication around the world depends on the bouncing of signals between the conducting Earth and the ionized layer (ionosphere) that surrounds the Earth in the upper atmosphere, principally from about 90 to 600 kilometers (56 to 375 miles). The ionization represents positively and negatively charged particles that are usually produced from neutral air molecules by the arrival of the Sun's strong ultraviolet radiation. During a mag-

Satellite Signal Bounces Off Dome
FIGURE 2.22 ► Long-distance radiowave signals bounce off the ionosphere; higher-frequency satellite signals pass through the ionosphere. Both communications routes can suffer when the ionosphere is disturbed by a geomagnetic storm.

netic storm, the ionosphere can become so disturbed that radiowave signals are scattered, and then degraded or lost. In recent years, the developed countries have become more dependent on satellite signals for communication. These signals, although at higher frequencies than radiowaves (and usually unaffected by the ionosphere), also can be scattered as they try to pass from the satellite through a disturbed ionosphere to the ground receiver. Worldwide telephone, fax, and pager service signals all have been scrambled or removed from effective operation during intense magnetic storms because of the satellite dysfunctions and signal transmission problems (Figure 2.22).

At high-latitude locations when auroral displays (Plate 5) accompany the geomagnetic storms, the local ionosphere is often greatly modified and becomes dominated by strong electric currents. At such times, high-latitude radiowave communication can be completely blacked-out, and at lower latitudes the quality of radio communications can be considerably decreased. The disturbance currents at the auroral and polar latitudes can even make a simple compass needle at the Earth's surface vary noticeably from its usual pointing direction.

Degraded Gps Accuracy
FIGURE 2.23 ► A geomagnetic storm can degrade the location accuracy provided by the Global Positioning System (GPS) satellites circling at 20,200 km (12,625 mi) above the Earth.

For navigation purposes, the United States maintains a family of 24 satellites in orbit at 20,200 km (about 12,600 miles) above the Earth (Figure 2.23). These clock-synchronized, radiowave-signal transmitter satellites, together with the ground receivers, constitute a Global Positioning System (GPS). The GPS allows users to find their time, latitude, longitude, and altitude at any spot on Earth via a reception from just four of the satellites. The GPS is used by a variety of people—scientists, ship captains, hikers, fisherman, hunters, and the U.S. Defense Department which funded the system. Soon U.S. commercial airplanes will be relying on GPS systems for flight directions and airport traffic control. Severe geomagnetic storms can cause ionospheric effects that interfere with the reception of the satellite signals on Earth. Such disturbances, in some situations, can produce position errors of up to several tens of meters.

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