How Do Communications Satellites Relay Television Images?
The whole concept of satellite transmissions centers around radio signals. Radio signals can carry sound and images thanks to communication satellites, which act as relay stations that receive signals from one place and send them to another. They're designed to transmit a large number of television programs, and telephone transmissions as well.
The majority of communications satellites are positioned at high altitudes of 22,238 miles, and move according to a geosynchronous orbit. This type of orbit enables the device to move along with the Earth's rotations which is why satellite dishes only need to be positioned once, as the satellite is moving with the Earth. The high-altitude devices typically work as self-contained stations, but there are some low-orbit networks made up of multiple satellites that work together.
The radio signals used to relay television images originate at ground stations. These stations, also known as uplink stations, transmit high-frequency signals to their respective satellites. The satellites then send these signals back to other ground stations on Earth. These are called downlinks since the signal is being transmitted from the device down to Earth.
The ground stations which receive the signal are located in the satellites' coverage areas, also know as footprint areas. Technological improvements have brought about the ability to position these devices at lower altitudes. These are know as medium earth orbit satellites, with altitudes of 8,000 feet. They work in groups, or networks, and are able to transmit stronger signals because they're closer to the Earth.
Yet another type of network --the low earth orbit satellites--sit a mere 500 to 1,000 miles from Earth. Because these bands are so much closer, their signals can carry more information, at faster rates, meaning improved signal strengths with minimal delay times.
Ground stations are also broadcast centers that handle the actual converting of the radio signals into television images. Through the use of encoding devices, signals are made into digital video streams, or files. These streams are then formatted for display on a typical television receiver. As there are large volumes of information contained in these streams, they must be compressed in such a way that allows for fast transmission. To do this, each file is broken down into frames containing video images.
Each image is then transmitted according to one of three techniques--intraframe, predicted frames, or bi-directional frames. Intraframes contain the entire picture, whereas predicted frames are partial images that get "filled in" based on image data that was sent in previous frames(intraframes). Bidirectional images rely on both intraframe and predicted frame transmissions for image reconstruction.
