19-07-2012, 04:06 PM
Satellite major component : transponder
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ABSTRACT
This technical report deals with the satellite major component transponder and satellite orbits in a clear and systematic manner . A contemporary approach is adopted throughout the project. The fundamentals of satellite are introduced and explained concisely. There are detailed discussions on satellite, satellite orbits , and on satellite major component : transponder. Suitable figures and examples are provided alongside the text to facilitate an easier understanding of the subject matter.
1.Introduction
1.1Introduction to Satellite
A satellite is a natural object that goes around (orbits) a planet. An artificial satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon.Satellites are used for a large number of purposes. Common types include military and civilian Earth observation satellites, communications satellites, navigation satellites, weather satellites, and research satellites. Space stations and human spacecraft in orbit are also satellites. Satellite orbits vary greatly, depending on the purpose of the satellite, and are classified in a number of ways. Well-known (overlapping) classes include low Earth orbit, polar orbit, and geostationary orbit.
1.2Components of a Satellite
There are 3 major components in a satellite, they are :
(i) Transponder and antenna system
- Thetransponder is a high – frequency radio receiver, a frequency down-converter and a power amplifier, which is used to transmit the downlink signal. The antenna system contains the antennas and the mechanism to position them correctly. Once properly in place, they will generally function trouble-free fro the life of the satellite.
(ii) Power Package- It is a power supply to the satellite. The satellite must be powered either from a battery or a solar energy system. In case of communications satellites in the Clarke orbit, a combination of battery power and solar energy is used. A solar cell system supplies the power to run the electronics and change the batteries during the sunlight cycle and battery furnishes the energy during the eclipse.
(iii) Control and information system & rocket thruster system - The control and information system and the rocket thruster system are called the station keeping system. The function of the station keeping system is to keep the satellite in the correct orbit with the antennas pointed in the exact direction desired.
1.3Transponder overview
The term Transponder, which is the short form of Transmitter-responder and sometimes abbreviated to XPDR, XPNDR or TPDR, is found in telecommunication applications. This is a device that receives signals from the Earth station, amplifies it and sends them for processing. The processed data is re-transmitted by the transponder back to the Earth station, in a different frequency other than that of the received one. As discussed in the past articles, a separation in frequencies between the received and the transmitted signals are maintained in-order to provide enough margins for a possible interference. A Transponder is also used for transmitting reply messages in response of an electronic interrogation.
Transponders are the individual communication channels of a satellite, each being a transreceiver or a repeater. With digital signaling, several video and audio channels may be multiplexed and compressed and made to travel through a single transponder on a single wide band carrier. We have discussed the matter about frequency bands in one of the earlier articles. We have also explained what is meant by 'frequency', which is measured in Hertz (Hz).
The communication signals are radio waves carried by what is termed as a 'carrier frequency'. In order to examine what a carrier frequency is - the method of how a carrier frequency is technically utilised in communication needs to be briefly discussed.
Basically, there are four types of techniques by which signals are transmitted and these are - amplitude modulated (AM) signals, frequency modulated (FM) signals, or phase modulated (PM) signals. This is briefly discussed below.
Let us suppose that the signal, as illustrated in Figure (1), needs to me transmitted using Amplitude Modulation (AM) technique. In this case a carrier signal is generated. A carrier frequency is a constant frequency which carries the generated signal. Here, the signal generated is modulating the amplitude of the carrier signal and this is the signal which is transmitted.
In case of Frequency Modulation (FM), the carrier frequency is modulated by the frequency of the signal being transmitted., unlike AM. You will notice a variation in the frequency of the carrier signal in proportion to the frequency of the signal to be transmitted.
As for Phase Modulation, the information signal is transmitted by the instantaneous variation of the phase of the carrier signal by the signal to be transmitted. This principle is not very much used, unlike its most popular counterpart, frequency modulation (FM). This is because of the reason, that it tends to require more complex receiving hardware and there can be ambiguity problems with determining whether, for example, the signal has 0° phase or 180° phase.
Transponders are transmitters and responders of satellites. The job of a transponder is to receive the signal from the earth station, which gets modulated, amplified and re-transmitted as an up-linked signal. The Transponder is a part of the payload of a telecommunication satellite and there could be 20 to 30 of them in such a satellite. The carrier signals, discussed above, are received by the transponders at very low power, owing to the distance that it needs to travel from the respective Earth stations to the satellite, placed about 35,000 miles above Earth. These signals need to be amplified to a good extent and then re-transmitted to the designated Earth stations. It has a set of high power amplifiers, where each of these amplifiers works on different frequencies.
The combination of the devices, which transmit and receive, along-with the amplifier system, is known as the Transponder. This equipment includes the high power amplifier and electronic filters at the input and output of the amplifiers.. These filter devices filter the carrier frequencies from the ones reaching the transponder and isolates those meant for processing by the other transponders. It is the central element in a satellite which maintains end-to-end link between the Earth stations and the satellite.
In such communication scenario, the up-link from the Earth stations to the satellite has its contribution in distorted signals reaching the satellite as will the down-link have, while transmitting signals from the satellite to the Earth stations. Some of these distortions are not co-related. In order to correct these distortions, the individual contributions must be known. The process of up-link and down-link signal conditioning is quite complex and is briefly discussed here. In order to understand this simplified presentation, there are certain terms which need to be explained.
Modulator - The carrier signal is modulated by the communication signal or the signal which needs to be transmitted. This is done by an electronics system called 'Modulator'. In the context of this discussion, it is enough to understand that the communication signal modulates the carrier signal and then this composite signal is processed for transmission.
De-modulator - As the composite modulated signal gets transmitted, it is received in the same fashion as it was transmitted. We ignore here any distortion or electrical noise that has got associated with the signal on its path either to the satellite or the Earth station. Now, as it reaches the receiver, the embedded communication signal needs to be recovered from the composite or modulated carrier signal received. The process of singling out this real communication signal from the one received is done through a De-modulator, which is just opposite to what the modulator does.
Multiplexers - The cost of implementing several channels of each data source is not only very high but also an inconvenience. The solution to this is a Multiplexer, which combines several incoming signals and produced one output, synchronised in such a manner that a particular input signal is available at the output at a given synchronised moment. This would mean that a multiplexer merges the incoming channels into one channel. Multiplexers can be of two types - analog and digital.
De-multiplexer - At the output end of the multiplexer, where a single channel comprises of the several incoming signals, a De-multiplexer, splits the single data stream into the original multiple signals, performing an exact opposite task of a multiplexer.
Data Compression - Data Compression is the process of encoding electronic information using fewer binary digits or other information bearing units. The volume of data in that information is greatly reduced compared to the un-coded representation. Usually, as in the case of any communication technology, data compression only works when the receiving station understands this encoding and accordingly is able to decode the incoming signal.
Let us now consider a scenario, where a certain Earth station is transmitting information to the satellite. Accordingly, the satellite re-transmits the signal to another earth station where the signal, where it is designated. The signal being transmitted goes through a whole lot of process to reach the satellite and a similar process is followed to transmit the signal to the designated Earth station by the satellite.
LNA - LNA stands for Low Noise amplifier. This is a special type of amplifier having application in communication systems. It is located quite close to the antenna. Using a LNA, the noise is substantially reduced by the gain of the amplifier, while the electrical noise from the amplifier itself is injected into the signal being processed. Very simply put, the 'gain' of an amplifier is its ability to increase the magnitude of an input signal.
In order to understand the working of the transmitter and receiver application, reference is made to the above, where the typical terms have been briefly explained.
The digital input signal goes through 'Compression' of digital data which is 'Multiplexed' and then 'Modulated'. The modulated signal goes through the 'Up-converter' and a 'Filter' and the pure digital information signal is then consequently amplified by a high power amplifier 'HPA' and transmitted to the satellite. The signal needs to be adequately amplified in order to travel about 35,000 miles above the Earth and reach the transponders of the satellite system.
The signal reaching the satellite has by then become weak and is amplified through a low noise amplifier (LNA) down-converted through a 'Down-converter'. The signal goes through filtration of unwanted electrical noises and passes through a 'Limiter'. A limiter is some kind of a circuit that defines the level of the signal that is desirable. A limiter is a circuit that allows signals below a set value to pass unaffected. The signal coming out of the limiter is then amplified by 'A' again as a preparation to transmit it back to the designated Earth station, It is again filtered removing any electrical noise that the signal would have inherited through processing. This is then presented to the transponder dish to be re-transmitted back to the designated Earth Station.
In receiving the signal, it goes through a so called opposite processing. The signal reaching the station is weak and is passed through a low noise amplifier (LNA) and down-converted, demodulated and then decompressed. The output being the pure re-transmitted digital signal is then received by the ground station.
A transponder is therefore is a broadband radio frequency channel and has application in amplifying one or more carrier signals and can deliver data rates in the range of 50 to 150 Mbps. Careful consideration to the design aspects of these transponders are required in achieving these high data rates
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2.Literature review
2.1 Satellite
A satellite is something that goes around and around the earth or another planet. Some satellites are natural, like the moon, which is a natural satellite of the earth.Other satellites are made by scientists and technologists to go around the earth. Some satellites send and receive television signals. The signal is sent from a station on the earth's surface.The satellite receives the signal and rebroadcasts it to other places on the earth. With the right number of satellites in space, one television program can be seen all over the world.Some satellites send and receive telephone, fax, and computer communications. Satellites make it possible to communicate by telephone, fax, Internet, or computer with anyone in the world.Other satellites observe the world's weather, feeding weather information into giant computer programs that help scientists know what the weather will be. The weather reporters on your TV news program get their information from those scientists.Some satellites take very accurate pictures of the earth's surface, sending back images that tell scientists about changes that are going on around the world and about crops, water, and other resources.
2.2The transponder of the satellite
The transponder is a high frequency radio receiver, a frequency down-converter and a power amplifier, which is used to transmit the down link.in a simple way, a transponder is a repeater, although it is a bit different of that we know in terrestrial applications. Or, A transponder receives and transmits radio signals at a prescribed frequency range. After receiving the signal a transponder will at the same time broadcast the signal at a different frequency. The term is a combination of the words transmitter and responder. Transponders are used in satellite communications and in location, identification and navigation systems.When we speak about ham satellites, we are generally speaking about repeatersin the space. It is not easy to build this spacecrafts and it is much more complex to do them reliable. They use solar panels as energy source that they accumulate in batteries to work when they are in the earth shadow, they have complex electronic circuits to stabilize its rotation and to keep the constant direction to the earth, to send telemetry, to be remote-controlled, etc. All this things must live together in a very limited space because these spacecrafts have the size of a 20 litres dustbin we have in our home. If we tried to build a transponder with the uplink and downlink in the same band, we will need an enormous resonant cavity as happens in the terrestrial repeaters to avoid making insensitive the reception. However to build a duplexer to different bands is easier. Some transponders repeat only a frequency, they are usually of FM but others can repeat a slice between 40 KHz and 250 KHz, these are normally SSB or CW modulated. We can define the “mode” of the satellites according to the uplink/downlink band.
Every transponder has a similar pair of frequencies because there must be an uplink and a downlink; the satellite does not modify the bandwidth or modulation of the signal, but merely translates its center frequency and amplifies it to the appropriate power for the downlink.
A single transponder can relay as much as 90 Mbps, if a suitable antenna, power amplifier and modulation equipment is used on the ground. Normally, a transponder is used to carry several VSAT networks or up to 12 digital TV channels.
2.3 How a transponder works
The analogical FM one channel and two bands transponders are the most widespread and its design is similar to a terrestrial repeater. They consist of a receiver and a transmitter, the exchange of information between the uplink and the downlink is made in the audio stage, speaking in a simple way, they work as if the speaker of the receiver was connected to the microphone of the transmitter. Until some years the transponders were transmitting full time, so to save energy some installed a watch, and others do it by means of software, then they transmit only at agreed time or when they are over some latitudes. But recently the SO-50 has incorporated a subtone in the receiver plus a watch, so when the satellite receives this subtone thetransponder is opened over 10 minutes after which it is turned off until it receives a new subtone.