15703 earth stations

8/3/2019 15703 Earth Stations

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TOPICS:

Types of earth stations

Environmental effects on antenna

Antenna Pointing

Antenna mountings


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INTRODUCTIONy The first communications satellite, Telestar, was not in

geostationary orbit. Thus, the huge antennas had to track thesatellite from horizon to horizon in less than hour to maintaincommunications through it.

y To track a satellite, a dish is required which greatly increases thecost of the dish. The faster the satellite moves across the sky, there

will be more need of tracking. Thus, more expensive the dishwill be.

y Thus, to make the dishes cheaper and simpler, geostationarysatellites have been invented.

y This led to the fundamental economic law of satellitecommunications: Every dollar a satellite costs get divided by thenumber of users on the ground, while every dollar a terminal costsget multiplied by the number of users. This leads to moreexpensive satellites and less expensive satellite terminals.


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15.1 Typesofearthstations.One way: BYTHEDIAMETEROF THEDISH:

y Micro-terminals or mini-terminals, used for DBS andVSAT services, diameter: 0.5m to 3 or 4m.

y Small dishes: diameter 3 to 7 meters.

y Medium-sized dishes: 7-15 m in diameter.

y Large dishes: 15-30 m in diameter.

Second way: FIXED ORTRANSPORTABLE

Third Way

:ACCORDINGTOTHEIRUSAGE.

Single purpose

Gateway stations

teleports


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yMajor Characteristics: They provide a single type of link with a satellite. Such stationsmay be only uplinking (transmitting), downlinking ( receiving) or both.y Receive Only:

y Size: vary from a few-inch-long rod on the roof of a car, to dishes or flat-plate panels from a few-inch-long rod on the roof of a car to dishes or f lat plate panels half a meter to a meter in size, up to severalmeter wide dishes for TVRO and cable head ends.

y Examples:y TVRO( Television receive only)y Direct Broad cast Services(DBS)y Digital Audio radio from satellite (DARS)y Reception by Cable-television head-end stationsy Receiving a Corporate Teleconference.

y Transmit Only:y Examples:

y For remote telemetry services such as monitoring pipeline flow, utility stations or even readingelectrical meters from satellite.

y VSAT (Very small aperture Terminals) used for point of sale systems for retail firms, hotels and

gasoline stations.y Two way corporate Television.

y With the advent of the Inmarsat system came the first direct satellite-to-end userterminals, first on ships. With improvement in technology to both satellites andterminals>> Brief case sized individual terminals that are self-contained withantenna, transmitter, receiver, telephone and data equipmentall in one small package.

y Hand held satellite telephone services from such firms as Iridium and Global Star areavailable. Again these are singlepurpose terminals.

y Smaller data only or navigation systems such as OmniTRACS, are also single companylow-speed data systems direct to the end user.

15.1.1


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15.1.2 Gate waystations

y What are Gate way earthstations?y They are the interface points between terrestrial networks and

satellites.y Also serve as transit and relay points between the satellites.

y Such Earth stations consist of dozens of dishes, and connected toterrestrial networks by a variety of transmission technologies including

wires, coaxial cables, waveguides, optical fibers and microwave towers.y MAJOR ACTIVITY: Signal Processing

y At one earth station many terrestrial signals are arrivingsimultaneously .y Some are analog, digital, some are telephone calls, television signals and data

streams. They arrive in different formats, different levels of multiplexing, differentstandards.

y And these need to be converted, demultiplexed, deformatted and converted toother standards appropriate to the satellite system. Thus, gate way station focuson signal manipulations ratherthan only uplinking and downlinking .

y Further they have to be sent on different destinations all around theglobe, so some need to be sent to one satellite and others to othersatellites.


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Gate waystations(Contd>)

y Some stations are part of, and sometimes owned by,the satellite system owner. Some are independent.

Antennas linking with a particular system must bedesigned , manufactured and operated in conformanceto standards provided by satellite fleet owner andcome in a range of sizes and capabilities called stationtypes.

y Type approved means the earthstations that areworking properly with specific earth station.


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y They are typically operated by firms not specifically owned by or part of

specific satellite system.y Why? Teleports are a way of aggregating a market for satellite

telecommunications and avoiding the difficulties of linking to satellites fromcongested urban sites.y They were used for firms that could not justify the expense of putting in their

own dishes, because of: i) there is a blockage by hills, towers etc.. And no lineof sight path exists from the office and satellite.

yThey are usually located at the edges o

fthe cities so as to minimize theinterference from other earth stations, microwave towers and similar

interfering signals.y How they are provided?

y Some point with in near by urban business center will be set up as acommunications hub for users with in that area.

y If a company in the city needs links to satellites, it need only get its signal tothe hub, from where it is then relayed by fiber or microwave tower to theteleport outside the city.

y Size and Capacity: Teleports range in size and capacity from a few dishes todozens of dishes that range in size from fractions of a meter to many meters indiameter to accommodate a wide range of demands from users.

y Value Added Services: Teleports earn a lot of money from value addedservices such as format conversion, encryption, production and post-production, turn around services and other tasks.


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. Environmentaleffectson Antennasy Antennas are subject to various environmental conditions depending on their location.y Sucheffects: temperature, solar radiation, wind loading and seismic effects.y Protection from these effects: In extreme climates, particularly in polar areas, they may be

enclosed in radomes made of radio-transparent material to protect from the elements.

y WIND>>Major environmental factor for unprotected dishes is theWIND.y The dish surface is a huge sail and can be subjected to tons of force in strong wind conditions.y In a huge enough wind, the dish can be destroyed. At lower wind speeds, this dish may point

slightly away from the satellite. As in lower and medium winds, the main effect is to vibrate the dishand cause it to point slightly away from the satellite. If a dish wobbles in wind, this reduces thesignal to and from the satellite, thus increases the interference transmitted and received.

y Thus, Stability of dish is very important. The larger the dish and the higher frequency band, thesmaller the beam-width of the dish, so more accurate the pointing must be.

y The dishes are rated for pointing accuracy during operations in wind conditions. Ratingparameters: Operating Temperature, Operating Wind Load, Survival Wind load, Seismic forces,Rain, Solar radiation , Atmosphere.

y RAIN>> In tropics, where rain is frequent and dishes are pointed almostverticallyy So, some provision must be there to allow rain to flow out of the dish. This may be as simple as using a

perforated dish surface, or may be made more complex with actual drains.

y SNOW>>In latitudes with real winters, snow can accumulateon the surface of the dish.y One effect of this is to alter the reflectivity properties of the dish and thus signal quality. It can

cause mechanical failure of the dish or mounting.y To protect from snow: Dishes in cold are equipped with deicing methods,: Examples :

y Blowing hot air over the dish,y shining infrared lights on it ory embedding heating pads in the dish itself.


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15.3 Antenna Pointingy Except for Omni-directional Antennas (as they radiate the signal in all

directions), an earth station must be pointed at the satellite to which it islinking.

y In case of NGSO satellites :Terminals communicating with NGSO satellitesuse omni-directional antennas, So Pointing is not an issue.

y But in case ofGSO satellites: The dish must be oriented so that it faces thesatelliteand is held there accurately.

y GSO Arc: A space sculptor proposed the idea of placing 360 brightly litsatellites spaced every degree of arc around the Clarkes degree. This idea wasnot adopted. If that idea would have been adopted and if you went outside atnight, what would you see?(See figure from book)y From a site in the northern hemisphere, you would see the first bright point of

light on the horizon just south of due east. The line of lights would rise upwardand toward the right -toward the south-reaching a maximum angle above thehorizon due south of you and declining to the right-westward-reaching thehorizon again a few degrees south of due west. This is called as GSO arc.

y If we were at the equator, then line of lights would pass directly overhead.

y Ifat poles, they would pass below the horizon and be invisible.y Thus, maximum angle, you can see from a GSO satellite=90- latitude at Earth

stationy Declination: Because GSO satellites are only 36,000 km and not at infinity .So,

they really appear a few degrees lower in the sky than this. This smallpermanent equatorial tilt is called declination. It is a function of yourlatitude.


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Declination Correction: At the equator, the declination correction is zero degrees.y At latitude of 40 degrees, it appears that a satellite is 43.7 degrees. Declination

correction is 6.3 degree.

y At Poles: GSO satellites can never actually be seen from either pole, they are belowhorizon above latitudes about 82 degrees.

y During Installation, the declination correction is made.

Since all GSO satellites are over the earths equator, their LATITUDE is zero. So, wecan look up a satellites orbital slot Longitude.

IMPORTANT: Two numbers toPOINT ADISH:

AZIMUTH ELEVATION

(DIAGRAMSON next Slide)

1) AZIMUTH: The compass direction towards the satellite. Azimuth can bemeasured in following ways:

y True Azimuth:y It has zero degree at north and goes clockwise to 90 degree at east, 180 degree at south,

around to 270 degree at west and up to 360 degree, which is same as 0 degree. (i.e. north

again).y So, if earth station is in northern hemisphere , a GSO satellites true azimuth will always

be a number between 90 degree and 270 degree.

y Relative Azimuth:y In the northern hemisphere, the zero point for measurement is due south.y In the southern hemisphere it is due north.y The value will always be less than 90 degree.

ANTENNA POINTING (CONTD>>>)


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Diagramsfor Azimuthand Elevation


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Antenna Pointing(CONTD>>)

2) ELEVATION: The satellites angle above the horizon: Elevation is a number of degrees whose value lies between 0

degree (i.e. at the horizon) and 90(i.e. directly overhead atthe zenith). Elevation is important as it determines howmuch air passes through on its way to and from the satellite.

The azimuth is the angular distance measured along thehorizon.

The elevation is the angular distance measured above thehorizon.

NOTE: Uplinks to Satellites closer than 5 degree to the horizonare strongly discouraged because of the increasedprobability of earth stations transmission interfering withterrestrials receivers.


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ANTENNA POINTING (CONT>>)POINTING FORInclined orbits:y REQUIREMENT: Some earth stations must be designed to

work with slightly inclined orbits. Such a satellite swingsslightly above and below the line of equator in the sky. Therefore,both the elevation and azimuth of the satellite slowly change byseveral degrees during each day.

y So, antenna mountings should have the capability to track suchsatellites accurately.

y Example: Figure-eight diurnal path of the satellite is notoriented vertically, but is tilted.

y TO POINT/TRACK:y Thus, any tracking antenna must move slightly both side-to-side

and up-and down.y Tracking is usually done automatically by having circuits

monitoring signal strength and adjusting the pointing of thedish to maximize it.>> IT increases the complexity and cost of theearth station.

y For a medium-sized earth station, it can add tens of thousands ofdollars to the cost of the station.


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15.4 Antenna Mountingsy Antenna Mounting: Support Structure for antenna can be

either fixed or moveable.y Fixed mounting:

y that holds the dish pointed firmly at the satellite.y They vary widely in complexityy must be able to hold the dish accurately even under the effects of wind.y They include such things as small dishes strapped to a chimney, attached

metal poles in the ground, bolted to a windowsill or just propped upagainst a building.

y Larger dishes have better mounts that are better at resisting the greatereffects of wind and are easier to adjust.

y Antenna mounts installed on a roof can be classified as: Penetrating (ifbolted on roof )/non-penetrating(if they are held down simply byweights of concrete blocks)

y Moveable mountings:y Have the ability to aim the dish anywhere along all or part of the

Clarkes arc (GSOArc)y 2 ways: 1) Elevation-Azimuth mounts.

2) Equatorial mounts.


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1) Elevation-Azimuth Mounts

y works like the familiar camera tripod.

y It has Two Axes of rotation: That can move the dish up

and down in elevation and side-to-side in azimuth.They are sometimes called as x-y mounts OR Elevation-AzimuthMounts.

y Design Variations: Can have the horizontal axiscontained with in the vertical axis or vice-versa.>>Operationally similar and work equally well.

y For infrequent repointing: the shift may be made bymanual movement.

y For frequent Repointing: Each axis is connected to a

motor and gear-train that is controlled from the stationscontrol room. Changes around both the azimuth andelevation axes are needed when shifting the view from onesatellite to another, since the GSO arc crosses the sky at anangle.


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) Equatorial Mounts

y These mountings take the advantage of the fact that all

GSO satellites are over the equator.y One Axis of Rotation: This mount has Polar Axis which is

exactly aligned parallel to the earths axis.

y If dish is attached perpendicular to this axis and adjustedfor declination, it will automatically point along the Clarkearc in the sky.

y Advantage: Thus, when moving from one satellite toanother, turning about one axis, making the pointing a bitsimpler.

yDisadvantage: If the dish is to be used with an inclined-orbit satellite, it must have the ability to nod up anddown several degrees perpendicular to the GSO arc. Butthis adds a slight complication and increased expenseto the dish.


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y THANKS..

15703 earth stations

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