satellite positions

8/4/2019 Satellite Positions

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Satellite Positions

All the consumer satellites are located in the same geo-stationary orbit 36,000 km above theequator. Their positions vary from east to west in accordance with international agreements.

These agreements about orbital positions allow several satellites to be placed in the same

location, so that TV viewers can receive a greater choice of programs with a fixed dish antenna.Also, when a satellite needs to be replaced (the average lifetime of a satellite is about 15 years)the replacement satellite can be put in the same position, so that when the first one 'dies' and falls

back to earth, the next one is already in place and continues to broadcast the same stations.

Consumer satellites are located above the equator, in different positions from east to west.

The positions of the satellites are controlled by international agreements drawn up by the IRFB(International Radio Frequencies Board). The IRFB also coordinates the frequencies used for

satellite broadcasting, to prevent interference which would be caused by two or more satellitesusing the same frequency. The transmission frequencies used by consumer satellites are in the

KU-band, which roughly stretches from 10 to 17 GHz. The range within the KU-band that isactually used by consumer satellites is between 10.7 GHz and 12.75 GHz.

10.7 - 11.7 GHzFSS+MPS11.7 - 12.5 DBS

12.5 - 12.75 FSS (telecommunications)

Transmission Process



Signals are sent up to the satellite from the earth's surface. The transmission station is called anuplink station. The transmission takes place via frequency modulation (FM). The advantage of

FM is that there are no problems regarding the frequency and dynamic range that needs to betransmitted, plus, FM is less sensitive to interference than AM. For practical reasons,

conventional TV stations broadcast in AM (called earth or terrestrial TV).

The outgoing transmission takes place at a very high frequency of 14,000 MHz (= 14 Gigahertz).To avoid any interference, the incoming signal (downlink) is transmitted at a frequency between

10 and 12 GHz. This is the so-called KU band, which covers the area from 10.7-12.75 GHz. Thedownlink signal is sent to earth in a focused beam, via a parabolic antenna, that looks quite

similar to a receiving dish antenna. From there, it can be picked up by private antenna, sharedantenna installations and cable companies.

Footprint

Consumer satellites use a concentrated beam to give a stronger signal over a smaller land area.

The area over which the signals can be received is called the footprint of a satellite. Footprintdiagrams show the area of coverage, including the antenna size which is needed for goodreception in the central and outlying areas. Under normal conditions, good reception within the

footprint area is possible for as much as 99.9% of the time. However, exceptional weather conditions can have an adverse effect on reception quality for short periods.

T he footprint diagram shows the area of coverage and the required antenna sizes in the central

and outlying areas.

LNC

The signals received by the dish antenna are transferred to a frequency converter called the LNC

(Low Noise Converter), which is placed in the focal point of the dish antenna. The LNC is alsocalled the LNB (Low Noise Block converter). The LNC converts the incoming signal to a lower

frequency in the area between 950 and 2150 MHz, and then amplifies the signal before it is sentto the satellite tuner. Due to the very weak signal levels, it is of vital importance that the

amplification takes place free of noise. During the amplification of the frequencies, allfrequencies will be amplified, including noise. An important performance parameter of the LNC

is therefore its noise factor. The lower the noise factor, the better the picture quality. For goodreception and image results, the quality of the LNC and the satellite tuner are of vital importance.



A Low Noise Converter (Low Noise Block Converter) placed in the focal point of the dishantenna.

Polarization

Polarization is a way to give transmission signals a specific direction. It makes the be

Polarization

Polarization is a way to give transmission signals a specific direction. It makes the beam more

concentrated. Signals transmitted by satellite can be polarized in one of four different ways:linear (horizontal or vertical) or circular (left-hand or right-hand). FSS satellites use horizontal

and vertical polarization, whereas DBS satellites use left- and right-hand circular polarization. Touse the channels that are available for satellite broadcast as efficiently as possible, both

horizontal and vertical polarization (and left- and right-hand circular polarization) can be applied

simultaneously per channel or frequency. In such cases the frequency of one of the two isslightly altered, to prevent possible interference. Horizontal and vertical transmissions willtherefore not interfere with each another because they are differently polarized. This means twice

as many programs can be transmitted per satellite. Consequently, via one and (almost) the samefrequency the satellite can broadcast both a horizontal and a vertical polarized signal (H and V),

or a left- and right-hand circular polarized signal (LH and RH).



TV signals transmitted by satellites can be polarized in four different ways: (1) vertical, (2)horizontal, (3) left-hand circular and (4) right-hand circular.

Types of Polarizers

In order to select either a horizontal, vertical, right- or left-hand circular signal, the LNC must be provided with a polarizer. There are three types of polarizers: mechanical, ferrite/magnetic and

electrically controlled polarizers.

The mechanical polarizer is a small pulse-controlled motor which rotates a metal probe betweenthe horizontal and vertical polarization directions. This system offers high switching precision,

with low signal loss. It gives wide-band reception, covering all the different frequency bands. Byadding a small circular depolarizer, the polarizer can be modified to also receive circular

polarized signals.

The ferrite-magnetic polarizer has no moving parts and gives effectively instantaneous

switching, combined with low signal losses. Channels need to be pre-programmed. By addingthe small circular depolarizer, this type of polarizer can also be modified to receive circular

polarized signals.

The 14/18V electrically controlled polarizer is integrated within the LNC, and requires noadditional connection other than to the LNC over a coax cable.

The Satellite Tuner



Signals come in to the satellite tuner via the LNC. A satellite tuner next to the TV tuner isrequired for satellite reception. Normal TV tuners can only handle signals between 47 to 870

MHz, whereas satellite transmission takes place between 950 and 2150 MHz. TV sets cannotgenerate specific LNC control signals, nor handle polarization switching. Furthermore, TV

tuners cannot process the audio signals from the satellite. Some TV sets and VCRs have satellite

tuners built in. In addition to a satellite tuner, one may also need an additional antenna positioner (in case of a polar mount dish), a descrambler box and a smart card reader in order to receiveencoded transmissions, all of which can be built into the satellite tuner.

All satellite tuners are equipped with a special connection for the existing antenna or cable,

which makes replugging unnecessary if you want to switch from conventional to satellite TV andvice versa.

Scrambling and Conditional Access

Not all signals picked up by a dish antenna are suitable for viewing. For several reasons TV

signals can be scrambled or given conditional access and can only be watched with the help of adecoder or descrambler. These reasons might be that:

y Programs are financed by viewer subscription rather than advertising revenues.y Programs are meant for a selected audience.

y Programs to be broadcast have been acquired with copyright clearance for specific geographical areas only.

There is a distinction between scrambling and conditional access, although for the viewer

without a decoder the result is the same: unclear video and/or audio signals. Scrambling is the jumbling up of a picture and/or a sound channel to make it impossible to watch or listen to a

program without a decoder. Conditional access is a form of encoding to protect information witha scrambled signal that tells the decoder how to decode it. Scrambling is therefore applied to the

picture, whereas conditional access is applied to the control signal. Scrambled signals requireadditional decoder boxes or a smart card reader for access.

The Dish Antenna

Types of Dish Antennae

There are a number of dish antenna types. The first and simplest is theP

rime Feed Focus dish,which is a parabolic dish with the LNC mounted centrally at the focus. Because the LNC is

mounted centrally, it means that a lot of the incoming signals are blocked by the LNC. Itsefficiency of 50% is low compared with the other types. The Prime Feed Focus dishes are mainly

used for antennae with diameters over 1.4 meters. Because of its relatively larger surface, the parabolic antenna is less sensitive to small directional deviations and there is a better chance of

receiving signals outside the normal footprint. On the other hand, rain and snow can easilycollect in the dish and could interfere with the signal.



The C band is a name given to certain portions of the electromagnetic spectrum, includingwavelengths of microwaves that are used for long-distance radio telecommunications. The IEEE

C-band - and its slight variations - contains frequency ranges that are used for many satellitecommunications transmissions, some Wi-Fi devices, some cordless telephones, and some

weather radar systems. For satellite communications, the microwave frequencies of the C-band

perform better under adverse weather conditions in comparison with K u band (11.2 GHz to14.5 GHz) microwave frequencies, which are used by another large set of communicationsatellites.

[1]The adverse weather conditions, collectively referred to as rain fade, all have to do

with moisture in the air, including rain and snow.

Contents

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Why uplink frequency is greater than downlink frequency

A satellite is a repeater in space. It must receive on one frequency, and transmit on another. When it

comes to transmitting, it is more efficient to generate high power at a LOWER frequency.

On the other hand, the higher the frequency, the narrower the beam width from the same diameter

antenna. This becomes important in a crowded geostationary orbit with satellites orbiting only a few

degrees apart. If you increase frequency and the diameter of the dish antenna on earth remains the

same, the effective radiated power of the signal increases because the beam width decreases (becomes

more focused).

As a final reason to support a higher input frequency for satellite transmission, when the space program

first began, the transmit klystron tubes and power transistors available were so inefficient that theyrequired massive amounts of input power to generate a small amount of power output. This power

could easily be supplied on earth, but hard to come by and store in space when the bird is in the shadow

of the earth, and the solar panels are not charging the spacecraft's batteries.

Conclusion: Using a higher receive frequency and a lower transmit frequency will reduce the power

requirements for a communications satellite, and allow closer satellite spacing and lower earth station

transmit power with a smaller antenna.

Answer

This is because the ground temperature of the earth which contributes to the thermal noise is large and

so the frequency is increased to increase the signal to noise ratio. A more conplicated explanation

involves Boltzmann'a constant and a very complicated equation.

At higher frequency attenuation is more hence power will be required for signal transmission to ensure

that it reaches the destination with the required minimum power.Higher power requirements will

involve use of high power amplifiers with high ratings and heat sinks.This will increase weight and power

supply ratings of the equipments.This does not make any difference to the ground station.But for the



satellite this will result in higher power consumption, which will reduce its efficiency. Hence the ground

station uses high power equipments to generate high frequency which can reach the satellite

satellite positions

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