Satellites perform a wide range of functions from their lofty perches. Some are exotic, such as probing the echoes of the big bang at the beginning of the universe. Others are more mundane, such as relaying television programs or phone calls. You might even be receiving Internet traffic that has been routed through a satellite.
Perhaps the most followed by ground observers are the manned satellites such as the Space Shuttles and the Mir Space Station. Because of their large size and low orbits, they can appear very bright in the sky. Local news media often carry predictions of passes of these objects.
Transmitting communications signals represents one of the the earliest and most popular uses for satellites. Early examples such as Echo were passive devices, large metal-coated inflatable balloons off of which radio signals were bounced from one location on the Earth to another. Another interesting passive example was the United States' project West Ford launched in 1963. This consisted of a cluster of metal needles acting as dipole antennae, reflecting signals from the Earth. Today's communications satellites are far more sophisticated. Broadcast satellites relay television programming directly to home subscribers. The new Iridium satellites will expand the range and capabilities of mobile telephony. A reassuring network of Search and Rescue satellites listens for transmissions from the Emergency Position Indicating Radio Beacons used by boaters, Emergency Locator Transmitters used in aircraft, and Personal Locator Beacons used by individuals in remote areas. There are even satellites devoted to amateur radio transmission.
Much discussion has centered around the placing of military weapons in orbit. However, the vast majority of military satellites in orbit are used for communications or surveillance purposes. Surveillance or spy satellites may use optical imagery, radar, or radio signal interception to keep tabs on what is happening below. Some of these devices are highly maneuverable, allowing them to change their orbits in order to focus on a specific target. This capability makes these satellites especially challenging and popular subjects for satellite trackers. Certainly among the favorites of satellite watchers are the "Flying Triangle" NOSS clusters. Flying in formations of three satellites together, the Flying Triangles intercept transmissions from ships at sea.
Satellites have been used as navigation beacons since the launch of Transit-1B in 1960. With the widespread public use of GPS (Global Positioning System) receivers today, modern navigation satellites such as those in the NAVSTAR GPS system have achieved a large degree of public recognition.
Many satellites perform scientific duties. Some perform astronomical research or monitor the space environment. But many focus their attention here on Earth. They monitor our weather, making life-saving forecasts possible. Others measure the drift of continents, the temperature of oceans, the health of crops, or the migrations of wildlife. As our technology advances, the number of uses to which satellites are put continues to grow.