Welcome to the first title in Understanding Satellite Communications, a Five-Part DVD based training series on satellite communication. In this program, Essential Satellite Communications we will explain and illustrate the following:
- The Satellite Circuit; the pathway of all satellite communications.
- Satellite Orbits, Spacecraft Design, as related to the satellite circuit.
- Two Varieties of Satellites: Bent Pipe & Processing
- Satellite Frequency Bands: Where satellite transmissions exist in the electro-magnetic spectrum.
- Microwaves, and why they are used in satellite transmissions.
- Frequency Reuse and Coding.
- Information Flow, Bandwidth, and Noise Reduction as related to Power
Levels in the satellite circuit.
- Digital Technology in satellite communications.
- Television and Transmission Standards
- Scrambling Techniques
- Future Technological Advances in satellite communications
The Satellite Circuit
The satellite circuit has three essential parts; an uplink earth-based antenna, a transponder aboard the space-based satellite, and a downlink earth station antenna. Once an electronic signal is created, that is, be it an image, a sound, a data bit, or whatever, and that signal is to be transmitted via a satellite circuit, it begins traveling at the uplink antenna. This uplink antenna sends the signal directly to and is received by the transponder aboard an orbiting satellite. And lastly, the orbiting satellite re-broadcasts these signals as a lower frequency band from its downward- facing antennas, to receive antennas located throughout a broad geographic area.
In the DVD, there is an illustration showing a satellite broadcasting toward North America. The area on the earth, reachable by the satellite’s downward beam, is called the satellite footprint. Receivers in both the United States and Canada can receive signals sent earthbound from this satellite. People, be they in the Southern United States or in Alaska, can receive the same signal with the same clarity and fidelity since they are both within this satellite’s footprint. In the next illustration, many European countries can receive the same signal from this satellite. Satellites can cover vast territories with their footprints.
On the video DVD, the viewer is shown a TV satellite that’s broadcasting to Western Europe. The downlink signals are received by a large number of antennas. Signals are bounced off the surface of the antenna into the feed, and then electronically taken through a cable to a satellite receiver indoors. The antenna can be moved by an actuator or positioner to point at different satellites that are within its view.
A typical C-band uplink antenna ranges in size from as small as 3 Meters to up to 10 or 15 meters in diameter. Ku-band uplinks, especially Ku-band flyaway uplinks used for transportable news broadcasting, can be as small as 1 or 2 Meters in diameter.
Radio waves travel at the speed of light and geostationary satellites are about 40,000 kilometers above the equator, so the time it takes the signal to travel from the uplink antenna to the satellite, and back to the earth, is about ¼ of a second. This would be the transmission time or delay if the uplink and the downlink earth stations are relatively close to, or directly below the satellite’s position above the equator. As the earth station is farther from directly below the satellite, in any direction, the time for the signal to pass from the uplink to the downlink stations increases, because the distance increases from the site to the satellite.
In typical broadcast applications, like broadcast television, this time delay is not too critical. For some applications where delay makes a bigger difference, this delay prompts satellite system designers to consider orbits other than geostationary where the earth stations can be closer to the satellites, thus reducing the delay.
Satellite Orbits, Spacecraft Design, as Related to the Satellite Circuit
There are many Geosynchronous orbits, meaning the orbit is “synchronized” to the rotation of the earth, but only one Geostationary orbit. Both Geostationary & Geosynchronous orbits allow objects in their orbits to rotate in sync with the earth’s rotation. But the Geostationary orbit, the most desired orbit for long-life, large-satellites, is directly above the equator. The Geostationary orbit requires some on-orbit adjustment, but it eliminates tracking requirements for earth stations. It is therefore the ideal orbit for least expensive operation, long duration and maximum coverage.
Even Geostationary satellites don’t stay perfectly still relative to the earth. They appear to make a small figure 8 pattern as viewed from the earth. This wandering on orbit can increase transmission times as much as 20 milliseconds. Signal refraction as caused by rain attenuation can also increase transmission time, since the rain causes the signals to slightly bend and scatter as they travel upward and downward. Rain attenuation can increase the transmission times by a few more milliseconds. These delay factors must be addressed in some critical applications.
There are many large earth station antennas that are employed around the world. In the DVD, a 10 meter antenna is shown supported by a stable base, with tracking in both the azimuth and the elevation planes. This type of antenna is called a “Cassegrain” design. Once again, signals are bounced on the antenna primary reflector, the big dish that makes up most of the antenna, to the hyperbolic sub reflector and then down into the feed in the center of the antenna. The signal then flows to the electronics behind the antenna and into the earth station.
In the uplink mode, the signals go in the reverse direction. They come from high power amplifiers located in the earth station, out to the feedhorn. From there they are focused on the hyperbolic sub reflector, bounced back to the antenna primary reflector surface, and then there they’re transmitted in a very narrow beam to a satellite. The uplink signals are received by a satellite in orbit.
In the DVD, the viewer is shown a body-stabilized satellite with its large solar rays deployed. The satellite antennas face towards the earth. The satellite processes the uplink signals and then downlinks them through a shaped beam to the earth below.
In our next blog, we shall address the two principal varieties of satellites (bent-pipe & processing sats) and the all important examination of frequency bands.
Thank you for joining us.