Satellite Communication - The Transponder System Part V

John Dulaney
 


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Commercial companies like Hughes, Aerospatiale, and Lockheed Martin have been designing satellites to continually keep up with the growing demand for satellite phones, TVs and data services. These companies have achieved this at the expense of greater spacecraft size and power. This expense is particularly true for geostationary orbiting satellites, which requires larger transmission power and high gain dish antenna diameters, to overcome the tremendous signal loss, during its path through the 35,000 km from Earth to the satellite and vice-versa.

To begin with, satellites designs are at first cocenptualised, which provides a clear planning regarding, the purpose of the satellite and to examine the systems necessary to complete its mission. What follows next is the preliminary design and basic design stages. During the preliminary stage, a model is constructed for experimentation to test if the new technologies incorporated in the satellite can be realised during this basic design stage. An engineering model is made after this, and intense testing of all the parameters is made in-order to establish the design viability.

There are quite a few shapes that satellites are constructed. There are the Spherical ones, cylindrical shapes, box shapes, and the multi-mission module system, where the two parts, the bus and the mission equipment, are divided into upper and lower sections. The bus system carries many sub-systems, such as, the telemetry tracking and command system, the electrical power system, attitude control system, propulsion system, structural system and thermal control system. The shapes of any satellite are decided on the basis of the size and the weight carried by the satellite.

In discussing the working of a satellite, relating to send/receive of information from the Earth station to the satellite and vice-versa, we had, in the previous artcles, looked at the basic principle of the antenna system deployed in such applications. There is another section in a satellite, which provides the same functional part of this send/receive process. This component is called a transponder.

The Transponder

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. The process is shown in Figure (1). 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. Figure (2) illustrates the example.

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. Figure (3) illustrates an example.

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 - Figures (1), (2) and (3) represents the various techniques used in signal transmission. The figures represent a carrier signal, the communication signal and lastly the modulated carrier signal. 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. A simple diagram of a multiplexer is illustrated in Figure (4).

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. A simple diagram is presented in Figure (5).

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.

Figure (5) Satellite Transponder Communication Technique

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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