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Course Code : CS- 09
Course Title : Computer Networks
Assignment Number : MCA(5)-09/TMA/05

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Questions 1: Answer the following questions as applied to communication satellite:

Explain the following items:

Answer.

> Transponder: A satellite transponder is a circuit on a satellite that receives, modulates, amplifies and re-transmits an uplinked signal. There can be 20 -30 transponders on a single satellite. Typical bandwidths of a transponder are 27, 33, 36, 54 and 72 MHz.

Carrier signals are received by the satellite at a very low power levels because of the long distance traversed by the radiowaves. The satellite needs to significantly boost the power level of these signals before they are re-transmitted back to the earth to ensure that they are detectable by an earth-based receiver. This is achieved using a set of high power amplifiers on-board the satellite, where each amplifier operates over a defined frequency range.

The combination of equipment required to amplify carriers within a given frequency range is commonly referred to as a transponder. This equipment includes the high power amplifier (HPA) itself, as well as filters at the input and at the output of the amplifier to isolate the desired carriers from the carriers processed by other transponders. The frequency extent over which the amplifier operates is usually referred to as the transponder's usable bandwidth.

> Bent pipe: The bent pipe satellite is essentially a space borne repeater. In a bent-pipe system the satellite is used to relay communication between the end-user equipment and a ground station that is part of the terrestrial infrastructure. The terrestrial infrastructure, rather than satellite-to-satellite communications links, provides the connection to the destination network or end-user.

Bent-pipe satellites don't perform any additional functions like multiplexing, switching or routing. All waveform processing intelligence, like rain fade mitigation or data rate adjustment, is performed by the ground station terminal equipment. This bent-pipe approach is much less complex, less costly, and is less susceptible to obsolescence than the on-board processing approach.

Representative Offerings: WildBlue, Astra-Net, iPStar. Capacity: 30 Gbps

Table - Ka-band (bent-pipe) Economics

Assumptions

Ka-band Bent Pipe

System Cost ($M)

$700

Satellite Life (Yrs)

15

Satellite Capacity (Gbps)

7

Return-link service speed (Mbps)

1.5

Subscriber Rev per Month

$50

ISP and Customer Service cost/mo/sub

$12

Subscriber Acquisition Cost

$450

Customer life (avg) in years

4

Analysis (per subscriber) per year

 

Annual Revenues

$600

ISP & customer service costs

$144

Annual Gross Margin

$456

Subscriber Acq Cost

$113

Cash Flow/Yr

$344

Subs Necessary for Break Even

2,037,846

Cost/Mbps/Mo (all services included)

$739

 

>Geostationary satellite: A geosynchronous satellite is a satellite whose orbital speed equals the Earth's rotational speed. If such a satellite's orbit lies over the equator, it is called a geostationary satellite. The orbits are known as geosynchronous orbit and geostationary orbit. A geostationary (GEO=geosynchronous) orbit is one in which the satellite is always in the same position with respect to the rotating Earth. The satellite orbits at an elevation of approximately 35,790 km because that produces an orbital period (time for one orbit) equal to the period of rotation of the Earth (23 hrs, 56 mins, 4.09 secs). By orbiting at the same rate, in the same direction as Earth, the satellite appears stationary (synchronous with respect to the rotation of the Earth).

Geostationary satellites provide a "big picture" view, enabling coverage of weather events. This is especially useful for monitoring severe local storms and tropical cyclones.

Because a geostationary orbit must be in the same plane as the Earth's rotation, that is the equatorial plane, it provides distorted images of the polar regions with poor spatial resolution. Geostationary satellites appear to hover over one spot above the equator. Receiving and transmitting antennae on the earth do not need to track such a satellite. These antennae can be fixed in place and are much less expensive than tracking antennae. These satellites have revolutionized global communications, television broadcasting and weather forecasting, and have a number of important defense and intelligence applications.

> Band: To prevent total chaos in the sky, there have been international agreements about who may use which orbit slots and frequencies. The main commercial bands are listed below:

Band

Frequencies

Downlink (Ghz)

Uplink (Ghz)

Problems

C

4/6

3.7-4.2

5.925-6.425

Terrestrial interference

Ku

11/14

11.7-12.2

14.0-14.5

Rain

Ka

20/30

17.7-21.7

27.5-30.5

Rain; equipment cost

The C band was the first to be designated for commercial satellite traffic. For a full duplex connection 1 channel each way is required. These bands are already overcrowded because they are used by the common carriers for terrestrial microwave links. The next highest band available to commercial telecommunication carriers is the Ku band. This band is not yet congested, and at these frequencies satellites can be spaced as close as 1 degree. However, another problem exists: rain. Water is an excellent absorber of these short microwaves.

Bandwidth has also been allocated in the Ka band for commercial satellite traffic, but the equipment needed to use them is still expensive. In addition to these commercial bands, many government and military bands also exists.

> Station keeping: The station keeping of satellites, in both geostationary and low-earth orbits, has primarily been ground based, involving the Control Center personnel in all phases of operations, including orbit maintenance and station keeping. Hence, the primary computational burden has been on the ground computers, which provide both the off-line functions of orbit determination and maneuver planning, as well as the on-line functions of commanding and telemetry processing.

Current geostationary satellites operations have evolved so as to take advantage of the stationary nature of the satellite position relative to the ground stations. The favorable geometry provides a continuous window for ranging, tracking, and commanding, thereby minimizing the computational burden on the on-board processors. The low-earth orbit satellites, on the other hand, always required on-board processing capability to provide some limited autonomy in navigation. This need was driven by the fact that these satellites had only intermittent ground station contacts of relatively short duration. In spite of this higher level of autonomy for low-earth orbit satellites, however, both LEO and GEO satellite orbits have been primarily maintained using the ground/human system.

While station keeping of individual satellites only require a certain orbit to be maintained (providing latitude and longitude control for GEOs), stationkeeping of multiple satellites in a constellation (or co-locating in one geostationary slot) has the added requirement of maintaining relative phasing between the orbits in order to provide adequate physical separation between satellites and to maintain the desired distribution within a constellation.

 

b) Describe the operation of VSAT.

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