The communication systems transmit and receive three different kinds of information: sounds (such as Bob Ballard's voice), pictures (such as live pictures of Jerry Wellington underwater at Conch Reef), and data (statistics in the form of charts and graphs). The next step is to send this information from one point to another. Of the many ways this can be accomplished, three are particularly important to the success of the JASON broadcasts:
Hard wires are metal or fiber-optic cables that connect two points. Metal cables carry electrical pulses, and fiber-optic cables carry light pulses. All of us have seen copper wire such as that used in electrical cords for home appliances. Fiber-optic cables are much thinner-in fact, only one-fifth as thick as a thin spaghetti strand. And they can carry more information than 300,000 pairs of 14-gauge copper wire! In addition, unlike metal, fiber-optic cables are immune to water corrosion and interference from radio waves and other forms of radiation. Metal system uses electricity and fiber optic system uses light.
Over-the-air broadcast technology, such as the systems used in television, radio, and cellular telephones, converts audio, video, and data signals into electromagnetic waves that are transmitted over a large area. (That's why it's called a "broadcast.") The electromagnetic waves are received by individual radio or television antennas and converted back into their original forms. In this system, the signal is broadcast over a wide area, and many receivers can pick up the signal at the same time.
Line-of-sight systems, such as satellite and microwave communication systems, use parabolic (dish-shaped) transmitters and receivers to focus, send, and receive signals. The transmitters focus audio, video, and data signals toward a "feed" horn at the end of a bundle of antennas in the center of the "dish." Each of the antennas is about as thick as a pencil and less than 2.5 centimeters (about 1 in) long. After the signal travels along the antennas and reaches the feed horn, it is sent through the air to a receiving parabolic dish.
In a line-of-sight system, the signals must not encounter any physical obstruction-buildings, trees, hills, etc.-as they travel from the transmitter to the receiver. (That's why it's called "line-of-sight.") Despite this handicap, these systems can still be used to transmit signals over great distances. Microwave systems on Earth transmit signals as far as 48 kilometers, or 30 mi. Communication satellite systems can send signals over 36,000 kilometers, or 22,300 mi. This is because the satellite systems have stronger transmitters, and their receivers are placed in space. Signals sent from Earth into space encounter few obstructions and can therefore travel much greater distances than signals traveling between two points on Earth. Once the signals reach the satellite they can be re-transmitted back to a different location on Earth.
By combining these systems in different ways, engineers can build a network that can simultaneously transmit scientific data, video images, and audio. For example, during a typical JASON broadcast, Dr. Jerry Wellington might be describing an experiment investigating relationships between temperature and coral growth. Underwater cameras and microphones pick up and transmit the video and audio signals of Dr. Wellington. Microwave systems then beam the signals from his location to the JASON production facility. From there, satellite systems re-transmit the signals to Primary Interactive Network Sites (PINS) and JASON Network sites around the world. At the same time, Dr. Wellington's computer is recording data from his temperature probe. The temperature probe readings are sent via modem, telephone lines, and fiber-optic networks over the Internet, where they can be accessed by Internet users at PINS, in homes, and in classrooms around the world.
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Gene Carl Feldman
(gene@seawifs.gsfc.nasa.gov)
(301) 286-9428
Todd Carlo Viola, JASON Foundation for Education (todd@jason.org)
Revised: 17 Oct 1995