Two Varieties of Satellites: Bent Pipe & Processing
There are basically two types of satellites. Bent Pipe & Processing Satellites.
The most common type uses a form of “bent pipe” communications. In a bent pipe type of satellite, the uplink signals are received at very low power in one frequency band. They are then simply amplified and transmitted back down to the earth at a higher power in another frequency band. This amplification and frequency conversion takes place in what is called a satellite “transponder”. The advantages of this type of satellite are that they are relatively easy-to- build, you don’t need to know what types of signals will be used ahead of time, and they are general purpose satellites that can carry any type of traffic.
The other type of satellite is a “processing” satellite. Processing satellites receive the weak uplink signals in one band, then process the signal for separation of the information to different locations. The processed information is then down linked at higher power in another band—sometimes in very small spot beams intended for small regions of the earth.
Most satellites today are “bent-pipe” satellites but several “processing” satellites are being proposed for Ka-band applications where spot beams are more practical.
Satellite Frequency Bands
Several frequency bands have been allocated for use in satellite communications around the world. These same bands are also used to a certain extent for mobile satellite systems, educational uses, radio determination and other terrestrial communications. These signals are relatively weak due to the long distances they have to travel from the uplink stations. Because of this, careful coordination between satellite and terrestrial transmitters is essential.
The frequency bands used in satellite communications are:
- the P-band or UHF, that ranges from 200 to 400 MHz
- L-band which is about 1.53 to 1.7 GHz; S-Band which covers from about 2 to 2.5 GHz
- C-Band which covers about 3.5 to 6.5 GHz
- X-Band covering 7.25 GHz to 8.5 GHz
- Ku-band covering about 10.5 to 14.5 GHz
- Direct Broadcast Satellite Ku-Band covering 17.3 to 18.3 GHz
- Ka-band, which until recently was an experimental band, from 17.70 to 31.0 GHz.
Satellite communications function very effectively around one GHz, because at this frequency, the atmosphere attenuates the signal very little, and the signal isn’t reflected by the ionosphere as lower frequencies are. However, the ITU has decided that the lower frequency bands should be allocated for land use, especially for less developed countries, because they’re suited for low cost, small, light weight, and aerodynamically shaped antennas. As a result, most satellite communications use microwave frequencies.
Microwaves in Satellite Transmissions
Why are microwaves used in satellite communications? We’ve already examined some of the reasons for this, and let’s examine them in somewhat more detail now.
Microwave frequencies begin around 1 GHz. Satellite microwave frequencies are usually in the C and Ku-band, for commercial applications. The military primarily uses the S and X-band. Some wireless cable applications also operate in the S-Band. C-band is also heavily used for terrestrial line-of-sight telephone applications. To avoid interference with these terrestrial microwave links, C-Band satellites have to limit their downlink power levels.
One of the advantages of Ku-band, is that in most parts of the world, it is allocated exclusively to satellite communications, so power levels can be higher. The Direct Broadcast Satellite Ku-Band is allocated exclusively to satellite communications. Satellites operating in the Ku-band are able to transmit thousands of watts of downlink power. As we go higher and higher in the bands, the atmospheric affects become greater, but there are fewer users sharing the same frequency bands. These are some of the advantages in using the Ku and Ka-bands.
First of all, microwaves are higher frequency signals than “short wave” or other forms of radio. The amount of information a radio signal can carry is proportional to its bandwidth. Since there is more microwave bandwidth available, more signals can be transmitted with similar amounts of information on each signal, and thus, more total information can be transmitted.
The entire radio spectrum, up through VHF television, is less than 250 MHz. However, in the range of just C-Band satellite frequency allocations, there is more than 500 MHz available in the uplink range, and another 500 MHz in the downlink. Because of this, satellites have access to wider band communications. They have access to more frequency space.
In addition, microwaves have an interesting property when it comes to the use of real-world antennas. At lower frequencies, antennas must be much larger to be more directional and focus the same amount of energy. In general, antennas must be substantially larger than the wavelength of the radiation they are sending in order to be directional. So for example, when you’re sending radio waves of 50 MHz, even a 6-Meter diameter antenna (about the wavelength) would not be very directional. Energy spreads around the antenna in general, unless the antenna is designed specifically to try to focus it.
Parabolic antennas, as commonly used in satellite communications, work by focusing the radio frequency energy into the antenna feed. However, microwave parabolic antennas are much larger than the typical size of the wavelength used. C-band wavelength is 7.5 centimeters, or a little under 3”, and the typical C-band antenna is 2 to 10 meters in diameter. A Ka-band wavelength is less than a third that size, and the typical Ka-band antenna is 1 to 2 meters wide. As a result, because the waves can be focused in fairly narrow beams, uplink earth stations can target satellites spaced fairly closely together. The same thing happens on the downlink. In fact, with Direct Broadcast Satellites, which operate in the Ku & KA-band, the signals are so strong the normal limitation on downlink antenna size is to reduce interference from adjacent satellites, instead of getting enough receive signal. If we use substantially lower frequencies, the size of the antennas would be prohibitive and this wouldn’t be possible.
In the satellite earth station, terrestrial signals are multiplexed, or added together, and then passed to the uplink circuitry. They’re first modulated onto the carrier wave, then upconverted to the appropriate frequency, 6 GHz in the case of C-band, approximately 14 GHz in the case of Ku-band. Then a high power amplifier takes these signals, increases their power, and passes them into the uplink antenna, where they’re subsequently broadcast to a satellite.
The reverse occurs at the downlink. Downlink signals are received by the satellite receive antenna on the ground. They are then fed by the feed horn and low noise amplifier into the downlink equipment chain. There, the signals are further downconverted, de-modulated, the carrier wave is striped off creating the base band signal, and that signal is demultiplexed for transmission on terrestrial circuits. The uplink and downlink are essentially the same process operating in reverse.
The directionality of antennas of higher frequencies will be touched on in a later blog, when we cover frequency re-use. As frequencies go up, especially Ku-band and Ka-bands, the waves can be sent to smaller localized regions. These small regions can then use the same frequency at the same time, and possibly up to as many as 20 different times at Ka-band. By doing this each region can use the same frequency, at the same time, but have different information imposed upon the radio wave.
Also, microwaves are used for a very basic reason. At frequencies below about 30 MHz, the radiation emitted by the earth is reflected by the ionosphere back to the earth; very little of this radiation ever makes it into space. This effect decreases until at about half a Gigahertz, where pretty much all the radiation passes through the atmosphere and the ionosphere into space. In fact, about one-half to about two Gigahertz is probably the optimal frequency band for satellite communications, in terms of energy propagation. Also, as the frequency increases, atmospheric disturbances that sometimes plague lower frequency transmissions, like short wave radio, disappear. Short wave radio is dramatically affected by things like sunspots and solar radiation storms. These have little or no effect on microwave signals, although particles sent from the sun have physically damaged some satellites.
In our next blog, we shall address frequency reuse and coding used in satellite communications
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