Satellite Sub System

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Satellite Subsystems

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SATELLITE SUBSYSTEMS

A satellite communications system can be broadlydivided into two segments

1.space segment

2.ground segment

The space segment - include the satellites and theground facilities needed to eep the satellitesoperational !tracking, telemetry, and command !TT"#$$.

%&n many networs common practice is to employ aground station solely for the purpose of TT"#.


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#lassification of e'uipment

carried aboard the satellite

1. The payload - the e'uipment used to provide the service forwhich the satellite has been launched.

2. The bus- the vehicle which carries the payload but also tothe various (ubsystems.

). The transponder - the e'uipment which provides theconnecting lin between the satellite*s transmit and receiveantennas. one of the main sections of the payload.

+. The antenna subsystem


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The various subsystems are

1$power

2$thermal control

)$attitude control+$orbital control---- (tation ,eeping

$command and telemetry functions


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Power supply

%The primary electrical power -solar cells

%&ndividual cells can generate only small amounts of power

%arrays of cells in series-parallel connection-large amount of

power.

%or ample ( )34 satellite5 (401 satellite !manufactured byughes (pace and #ommunications #ompany$

%6uring launch phase outer cylinder generates electrical power%At the beginning of life- 9+0 7 dc power%At the end of 10 years- may drop to 340 7

%6uring eclipse5 power -8two nicel-cadmium !i-#d$ long-life batteries-:)0 7%At the end of life5 battery recharge time ; 14 h


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The ( )34 spacecraft is a spin-stabili


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% The solar-sail type is more economical than the cylindricaltype

% To maintain service during an eclipse -8storage batteries

% i-#d batteries continue to be used as in the ughes ()34 satellite

% nicel-hydrogen !i-2$ batteries offer significantimprovement in power-weight ratio.

%

i-2 batteries are used in the ughes ( 401 !e.g.5 the(AT>?- and Ani-2 satellites$5 the &ntelsat series with&T=(AT @& 5 &T=(AT @&& satellites.


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Thermal Control

(atellites are subect to large thermal gradients

1.receiving the sun*s radiation on one side while the other side facesinto space

2.Thermal radiation from the earth and the earth*s albedo Albedo B the fraction of the radiation falling on earth which is reflected

Albedo can be significant for =C satellites " negligible for DCsatellites.

).'uipment in the satellite

The most important consideration is that the satellite*s e'uipment

should operate as nearly as possible in a stable temperatureenvironment.


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Methods to provide stable temperature environment

1.Thermal blanets and shields - to provide insulation.

2.Eadiation mirrors - to remove heat from the communications payload.

%The mirrored drums surround the communications e'uipment

%Frovide good radiation paths- the generated heat to escape into the surroundingspace.

%

advantage of spinning satellites compared with body-stabili


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Attitude Control

1.What is attitude ?

attitude – satellite orientation in space

2.Why it is necessary?

Attitude control is necessary to ensure that directional antennas point inthe proper directions

or eample5 &n the case of earth environmental satellites5 the earth-sensing instruments must cover the re'uired regions of the earth5 whichalso re'uires attitude control

3.What are the forces affect attitude?A number of forces referred to as disturbance torques

some eamples are the gravitational fields of the earth and the moon5solar radiation5 and meteorite impacts.


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6. Where this attitude control takes place?

Gsually5 the attitude-control process taes place aboardthe satellite.

control signals can also be transmitted from earth basedon attitude data obtained from the satellite.

7. What is an attitude maneuver?

&t is a tas performed when a shift in attitude is desired

The control signals needed to achieve this maneuver may

be transmitted from an (

8. What are the types of attitude maneuver?

Fassive and active


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Passive attitude control

The use of mechanisms which stabili


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Active attitude control

o overall stabili


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Spin Stabilization

(pin stabili


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Nutation- a form of wobbling5 can occur due todisturbance tor'ues and/or from misalignment or

unbalance of the control ets

utation dampers- energy absorbers to damp outnutation

&f an omni directional antenna used!&T=(AT & and &&satellites $ then the antenna5 which points along the pitch

ais5 also rotates with the satellite.

&f a directional antenna used!communications satellites$the antenna must be despun5 giving rise to a dual-spinconstruction.

An electric motor drive is used for despinning the antennasubsystem.


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Gtili


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(peed and tor'ue control of the motor is eercisedthrough the currents fed to the stator.

T=6&? manufactures momentum wheels ranging insi


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STATION KEEPING

1. What is Station eepin!?

maintaining a satellite in its correct orbital position

2. Why Station eepin! is re"uiredH

The orbit of the satellite is changed due toi$ #ffects of a nonspherical earth

I arth is of shape oblate spheroid

I affects mean motion of satellite

ii $ $he oblateness of the earth

produces two rotations of the orbital plane.a$ regression of the nodes- the nodes appear to slide

along the e'uator. i.e %5 the right ascension of theascending node5 shifts its position.


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b$ rotation of apsides in the orbital plane

%&' changes

)$ the e'uatorial ellipticity of the earth geostationary

satellites to drift slowly along the orbit5 to one of twostable points5 at 3J and 10J7. !lon!itude chan!e$

+$ the main perturbing forces as gravitational pull of the sunand the moon will cause a drift in satellite latitude

(latitude chan!e). &nclination changes at a rate of about0.:J/year


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*) #ast+'est station+keepin! maneuvers

To prevent shift in longitude change

an oppositely directed velocity component is imparted to thesatellite -ets pulsed once every 2 or ) wees

(atellites in the 4/+-D< band must be ept within KL0.1Jof the designated longitude5 and in the 1+/12-D< band5KLwithin 0.0J.

Crbital correction - by command from the TT"# earth station

7 and ( station-eeping maneuvers are usually carried outusing the same thrusters as are used for attitude control


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**) north+south station+keepin! maneuvers

To prevent shift in latitude change

Fulse the ets at the appropriate time to mae theinclination to


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(atellite altitude show variations of about K-0.1 percentof the nominal geostationary height.

or Deo orbits 5 a rectangle bo limits of 1+3 m isproduced.


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Satellite Sub System

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