Introduction to Space Exploration - Application Satellites

Application satellites are spacecraft put into Earth orbit to serve as tools of earth science or for navigation or communication purposes. Although they are not really space explorers, they would not be possible without the technology of the space age.

On April 1, 1960, NASA launched the first successful meteorological satellite named TIROS 1 (Television Infrared Observation Satellite). The spacecraft was equipped with television cameras in order to film cloud cover around the Earth. This is an example of an active satellite (one that collects data or performs some other activity and transmits signals back to Earth).

Over the next few decades weather satellites grew increasingly more sophisticated in their capabilities. The primary systems used following TIROS were named NIMBUS, TOS (TIROS Operational Satellite), ITOS (Improved TOS), SMS (Synchronous Meteorological Satellite), NOAA (National Oceanic and Atmospheric Administration), and GOES (Geostationary Operational Environmental Satellite).

Other application satellites perform various duties for earth scientists, such as mapping oceans and land masses or measuring the heat and moisture content of Earth's surface. Famous examples include Landsat 7, Seasat, and the HCMM (Heat Capacity Mapping Mission).

On August 12, 1960, NASA launched its first communications satellite named ECHO 1. It was a large metallic sphere that reflected radio signals. This is an example of what is called a passive satellite. ECHO 1 did not collect data or emit transmissions, it simply reflected radio signals sent from one location on Earth to another location on Earth. Other famous communications satellite series include Relay, Telstar, ATS (Applications Technology Satellite), CTS (Communications Technology Satellite), Comsat, Marisat, Galaxy, and Syncom.

NASA maintains a whole series of communication satellites in Earth orbit that allow the agency to communicate with astronauts and relay data to robotic spacecraft during their missions. They are called Tracking and Data Relay Satellites.

Many communications satellites are placed in Earth orbit 22,241 miles from the planet's surface. At this distance they are anchored in place by Earth's gravity and in synch with the Earth's revolution rate. In other words, they move around the Earth at the same speed that the Earth is revolving around its axis. This is called a geosynchronous orbit. Satellites placed in geosynchronous orbit even with Earth's equator appear from Earth to hover in space at the exact same location all the time. This is called a geostationary orbit.

FIGURE 1.1
Terrestrial navigation system

Navigation is the act of determining one's position relative to other locations. Before the invention of satellites navigational signals were transmitted by land-based systems using antenna. (See Figure 1.1.) These antenna sent low-frequency radio signals that traveled along the surface of Earth or reflected off the ionosphere to reach their target receptor. The ionosphere is a layer of atmosphere that begins about thirty miles above the Earth's surface. Atmospheric gases undergo electrical and chemical changes within the ionosphere. This is what gives it reflective properties.

During the 1970s the U.S. military developed a space-based navigational system called the Global Positioning System (GPS). This system relies on satellites in Earth orbit to handle signal transmissions as shown in Figure 1.2. During the 1980s GPS was made available for international civil use. Over the next two decades it became one of the most popular navigational tools in the world.