Satellite TVRO Part 1
Aug 13, 2009 8:34 AM, By Russell Brown
Faults
Not all satellites make it up to their orbital slot without a hitch. Sometimes the rockets malfunction causing the satellite to not make it all the way to its geostationary orbit altitude. This is what happened to satellite AMC 14 in March 2008 when one of its rocket stages shut down early. SES Americom considers the satellite a total loss.
Then there’s Astra 5A, which had been working since November 1997 and should have had a few more years of service. But in January of this year (2009), it suffered a sudden failure that could not be corrected from the ground. Another satellite was moved into its place to take over its communication duties until a new satellite can be launched.
To control the satellite, its antennas must point toward Earth. If the satellite moves out of alignment and it loses communications with its control center, there is the possibility that the satellite could collide with another. When the satellite’s antenna is not pointed at Earth, NASA’s Jet Propulsion Laboratory has a very large dish antenna can be aimed at the satellite, and, with enough power, a signal can be sent to the satellite to get it back under control.
Transponders
Communication satellites are capable of receiving and retransmitting several channels at once. The most common number of channels or transponders is 24, and they can use different frequency segments or bands to expand that number. The signal sent up to the satellite (uplink) is always higher in frequency than the signal sent back down (downlink). This keeps the two signals from interfering with one another.
Each communications satellite uses a band or bands of frequencies to receive and send signals to and from Earth. The bands used are:
- L–band: 1Ghz-2GHz, used by Mobile Service Satellites (MSS) and after downconversion from the LNB to the satellite receiver
- S-band: 2GHz-4GHz, used by MSS, NASA and deep space research
- C-band: 4GHz-8GHz, used by Fixed Service Satellites (FSS)
- X-band: 8GHz-12.5GHz, used by FSS and in terrestrial imaging, e.g. military and meteorological satellites
- Ku-band: 12GHz-18GHz, video satellite service use 11.7GHz-12.7GHz and is also called FSS, while DBS uses 12.2GHz-12.7GHz
- K-band: 18GHz-26.5GHz, used by FSS and Broadcast Service Satellites (BSS)
- Ka-band: 26.5GHz-40GHz, used by FSS and DBS
The most common bands used by broadcasters are the C-band and Ku-band. The C-band uses frequencies employed for microwave communications on Earth, so its power is limited to prevent interference. The Ku-band uses higher frequencies that will not cause interference, but can be blocked by heavy rain at times.
Figure 5: Anatomy of a satellite
A signal uplinked to a transponder is received at the main receive antenna and then connected to a filter, for a particular transponder channel, followed by a preamplifier and then to a downconverter (to place it on the correct downlink frequency). The signal is fed to a high-power amplifier, a filter and then to the transmit antenna. All communication satellites carry spare amplifiers that can be switched in to replace any failed ones to extend the life of the satellite. (See Figure 5.)
Transponder frequencies are listed by their center frequency; most C-band transponders have 36Mhz of bandwidth, while Ku-band transponders have bandwidths of 24Mhz and some transponders have bandwidths of up to 54Mhz. C-band frequencies for U.S. domestic satellites are fairly standard, but Ku-band frequencies can vary from satellite to satellite. Satellites covering different parts of the world use different uplink and downlink frequencies, as does transoceanic satellite service.
A modern communications satellite will normally carry 24 C-band transponders and 24 Ku-band transponders. When only analog transmissions were used, that limited a satellite to a maximum of 48 channels that could be broadcast. Today, all of that has changed.
Acknowledgments
Ed Johnson of San Francisco International Gateway Teleport and Frank Foge of CALSAT assisted with this tutorial.
Next Time
The next "Transition to Digital" tutorial will discuss expanding the number of channels a satellite can handle.
Continue reading part two of the Satellite TVRO series.
Chart giveaway
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