1. What is satellite and how does a communication satellite differ from a communication relay?
Satellites go round the globe in 24 hours so they seem to us on earth to keep station some 36000 km vertically above the same point on the earth’s surface. A Satellite is a solid object which revolves around some heavenly body due to the effect of gravitational forces which are mutual in nature.
We can categorize satellites in two types, namely Passive Satellites and Active satellites. Passive satellites are not like active satellites. Even a moon can be a passive satellite. Thus passive satellites are relay stations in space. A passive satellite can be further subdivided into two types, namely Natural satellites and artificial satellites. A moon is a natural satellite of earth. But spherical balloon with metal coated plastic serve as artificial satellites.
Active satellites are complicated structures having processing equipment called Transponder which is very vital for functioning of the satellite. These transponders serve dual purpose i.e. provides amplification of the incoming signal and performs the frequency translation of the incoming signal to avoid interference between the two signals.
- For communication satellites the range is much higher than that of communication relay. Communication Satellite can cover up to several thousand kilometers.
- For communication satellite the uplink and the downlink frequency is the same. But for communication satellites the uplink and the downlink frequencies are different in order to avoid interference.
- The satellite relays are inherently wide-area broadcast, i.e. the point-to- multipoint whereas all the terrestrial relays are point-to-point.
- The satellite circuits can be installed rapidly but communication circuit can not be installed rapidly.
- The mobile communication can be easily achieved by satellite communications as it has a unique degree of flexibility in interconnecting mobile vehicles.
2. Explain the basic differences between an active and passive satellite system. Discuss their merits and demerits.
The basic differences between a passive and an active communication satellite arise from the fact whether the communication relay involves passive reflection or an active electronics system.
In the passive satellite system the ground transmitting system beams power at the reflector.
In active satellite, the satellite receives a fraction of energy beamed toward it by the ground transmitting system and receive power is amplified by active electronics means usually in conjunction with frequency shifting.
Their merits are-
- Comparison of communication capability of active and passive satellite is the amount of power radiated toward the receiving ground stations by the satellite.
- Passive systems may have significant advantages for military systems because of the relative invulnerability of the satellites.
Their demerits are-
- Actually the communication capability of active systems with directional antennas rapidly becomes much greater than that of the passive systems as the altitude is increased.
- However, the principal the modern communication satellites are active satellite systems.
3. Prove that for covering the globe three communication satellites would be sufficient.
Three satellites spaced 120 degrees apart could the whole world with some overlap provided that messages could be relayed between satellites and thus reliable communication between any two points in the world was possible.
4. What is the difference between a geostationary satellite and a low altitude satellite? Can a low altitude satellite be also used for communication purposes? If not why?
There are a few problems with these geostationary satellites, namely the time delay, in connection with telephony and the establishment of the synchronous orbit. But a low altitude satellite is well below the high intensity inner radiation belt and these satellites overcome these problem. This is the main difference between a geostationary satellite and a low altitude satellite.
Equatorial low Earth orbits (ELEO) are a subset of LEO. These orbits, with low inclination to the Equator, allow rapid revisit times and have the lowest delta-v requirement of any orbit. Orbits with a high inclination angle are usually called polar orbits.
The LEO environment is becoming congested with space debris which has caused a growing concern in recent years, since collisions at orbital velocities can be highly damaging or dangerous and can produce even more space debris in the process, called the Kessler Syndrome.
5. List various frequency bands being used in satellite communication. Compare the advantages and disadvantages of different bands considering the effects of propagation media.
Table: Frequency Bands for Satellite Communication
|Band||Downward bands MHz||Uplink Bands MHz|
|Uhf- Military||250-270 (Approx)||292-312|
Satellite communication systems started in C band, with an allocation of 500 MHz shared with terrestrial microwave links. As the GEO orbit filled up with satellites operating at C band, satellites were built for the next available frequency band, Ku band. There is a continuing demand for ever more spectrums to allow satellites to provide new services, with high speed access to the Internet forcing a move to ka band and even higher frequencies.
6. Give the reason as to why the uplink frequency is different than the down link frequency. Also mention the reasons for keeping uplink frequency higher than the downlink frequency.
Satellites have what is called ‘translation frequency’ that defines the relationship between the uplink and downlink frequencies. In traditional ‘bent pipe’ satellites, there is usually one translation frequency that defines what you have to add or subtract to the uplink frequency to see what the downlink frequency is.
In more modern digital or DSP based satellites, the translation frequency can be very different for many different frequencies, due to the satellite’s complex and flexible payload.
The bottom line is that these translation frequencies are available from the satellite operator.
The reasons uplink frequency higher than the downlink frequency:
1) Distance: To prevent interference you can move the transmitters far away from the receivers.
2) Power: You can decrease the power of the transmitters and decrease the receive sensitivity of the receivers.
3) Directivity: Use very directional antennas that don’t have high extraneous emissions.
4) Barrier: Man made or natural barriers such as walls, mountains, trees, metal buildings, etc block unwanted signals.
5) Frequency: Use separate frequencies that are far enough in range from each other to eliminate interference.
7. What are the elements of satellite communication system? Explain each with a suitable block diagram.
The elements of satellite communication system are as bellows-
- Earth Station
- Terrestrial System
Satellite Communication Systems is as below :
It is simply the communication of the satellite in space with large number of earth stations on the ground. Users are the ones who generate baseband signals, which is processed at the earth station and then transmitted to the satellite through dish antennas. Now the user is connected to the earth station via some telephone switch or some dedicated link. The satellite receives the uplink frequency and the transponder present inside the satellite does the processing function and frequency down conversion in order to transmit the downlink signal at different frequency.
Fig: General structure of a satellite communication system
The earth station then receives the signal from the satellite through parabolic dish antenna and processes it to get back the baseband signal. This baseband signal is then transmitted to the respective user via dedicated link or other terrestrial system. Previously satellite communication system used large sized parabolic antennas with diameters around 30 meters because of the very faint and weak signals received. But nowadays satellites have become much stronger, bigger and powerful due to which antennas used have become automatically smaller in size.
Thus the earth station antennas are now not large in size as the antennas used in olden days. A satellite communication system operates and works in the millimeter and microwave wave frequency bands from 1 Ghz to 50 Ghz. There are various frequency bands utilized by satellites but the most recognized of them is the uplink frequency of 6 Ghz and the downlink frequency of 4 Ghz. Actually the uplink frequency band is 5.725 to 7.075 Ghz and the actual downlink frequency band is from 3.4 to 4.8 Ghz. The major components of a Satellite Communication system is spacecraft and one or more earth earths.
8. List various advantages and disadvantages of satellite communication system. Give the reasons that optical fibers in spite of being high bandwidth channel, satellite communication has an edge over it.
The advantages of satellite communication over terrestrial communication system are:
- The coverage area of a satellite greatly exceeds that of a terrestrial system.
- Transmission cost of a satellite is independent of the distance from the center of the coverage area.
- Satellite to Satellite communication is very precise.
- Higher Bandwidths are available for use.
The disadvantages of satellite communication system are:
- Launching satellites into orbit is costly.
- Satellite bandwidth is gradually becoming used up.
- There is a larger propagation delay in satellite communication than in terrestrial communication.
However, if needed repair is nearly impossible after launching the satellite and also the equipments are subject to extreme environmental stresses. As compared to the fiber optic cable, the satellite communication has the advantages that the quality of transmitted signal and the locations of stations sending and receiving information are independent of distance.
9. What are the advantages of a communication system that uses digital signal transmission? Explain as to the digital satellite communication has reduced size of earth station?
Advantages of a communication system that uses digital signal transmission:
- The ease and efficiency of multiplexing multiple signal or handling digital messages in packets for convenient switching
- The relative insensitivity of digital circuits to retransmission noise, commonly a problem with analog system
- potential for extremely low error rates and high fidelity through error detection and corrections
- communication privacy
- the flexibility of digital hardware implementation
The digital satellite communication has reduced size of earth station:
In digital transmission, analog signals that are transmitted digitally can share channels with digital data, since all digital signals are handled in the same way and their content is immaterial. Thus a digital satellite like can carry a mix of telephone and data signals that varies with traffic demand.
Digital transmission has several advantages over analog transmission:
1. Analog circuits require amplifiers, and each amplifier adds distortion and noise to the signal.
2. In contrast, digital amplifiers regenerate an exact signal, eliminating cumulative errors. An incoming (analog) signal is sampled, its value is determined, and the node then generates a new signal from the bit value; the incoming signal is discarded. With analog circuits, intermediate nodes amplify the incoming signal, noise and all.
3. Voice, data, video, etc. can all by carried by digital circuits. What about carrying digital signals over analog circuit? The modem example shows the deputies in carrying digital over analog.
10. What are Modem and Codec? Both are A\D and D\A converters but what makes the difference between the two? Mention the areas of their applications.
A modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio.
The most familiar example is a voice band modem that turns the digital data of a personal computer into modulated electrical signals in the voice frequency range of a telephone channel. These signals can be transmitted over telephone lines and demodulated by another modem at the receiver side to recover the digital data.
Similarly the equipment responsible for carrying out coding and decoding is termed codec. These two devices are widely used in digital satellite communication. Modem is used as an interface between analog and digital systems. These play an important role in computer communication networks and ISDN systems. Codec’s are used in digital television systems and normally consists of a pair of A\D converter and D\A converter. it is a kind of box digital device as shown in fig:-
Fig: Basic Block Diagram of Codec
The areas of their applications:
1. Digital modulation formats that have high Doppler immunity are typically used
2. Waveform complexity tends to be low, typically binary phase shift keying
3. Error correction varies mission to mission, but is typically much stronger than most landline modems