satellite technologies-eirp calc

7/24/2019 Satellite Technologies-eirp Calc

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Satellite Technologies

The surprise launch of Russia's Sputnik , the world’s firstartificial satellite, in 1957 prompted an explosion of interest

in the possibilities of satellite technology !part from the

military implications of its possible de"elopment as a

weapons platform it also alerted engineers to the potential of

using the technology for peaceful applications including the

possibility that Arthur C. Clarke's dream of world#wide

communications based on Geostationary Satellites might at

last be realised $ut first some technical issues had to be

resol"ed

%putni& 1 was a ( in )5* cm+ diameter polished metal sphere, weighing 1*( lbs )*(- &g+ at launch,

containing a one .att radio transmitter, powered by two %il"er#/inc batteries, transmitting on 0 and 0

23 through four external radio antennas 4t did not ha"e a recei"er ! third battery powered the

temperature regulation system

Tra"elling at 1*,000 mph )9,000 &ph or *,100 ms+, it circled the 6arth once e"ery 9- minutes emitting

beeping radio signals from its near omnidirectional antennas at as it went

4t ser"ed no useful purpose, but was a spectacular demonstration of the %o"iet capability in space

%ee Sputnik History

The Challenges (Old and New)

! satellite communications lin& pro"ides line of sight transmission of signals between a transmitter and a

remote recei"er or recei"ers on the ground "ia a transponder mounted in a satellite orbiting high abo"e the6arth such that it can be seen by both the transmitter and the recei"er 8reating such a lin& reuired

mastering a series of technologies which were new and radical at the time %pace was an un&nown frontier

%atellites had to be placed into a precisely controlled orbits :nce in place there was no possibility of

maintenance Roc&et power, guidance and control were still in their infancy when this new communications

re"olution was launched %ome of the rele"ant technologies are outlined and explained here

%ee the Communications Satellites page for descriptions of how Telstar Syncom !ntelsat, "olniya and

ATS satellites rose to this challenge

Or#its and Communications

Sputnik $

;ublic <omain

http://www.mpoweruk.com/history.htm#clarke


http://www.mpoweruk.com/history.htm#sputnik

http://www.mpoweruk.com/satellite_communications.htm

http://www.mpoweruk.com/satellite_communications.htm#telstar


http://www.mpoweruk.com/satellite_communications.htm#syncom

http://www.mpoweruk.com/satellite_communications.htm#intelsat1

http://www.mpoweruk.com/satellite_communications.htm#molniya_satellite


http://www.mpoweruk.com/satellite_communications.htm#ats6

http://www.mpoweruk.com/history.htm#sputnik




http://www.mpoweruk.com/satellite_communications.htm#intelsat1






! Geostationary %arth Or#it (G%O) is an

orbit in which the position in s&y of the

orbiting ob=ect remains the same so that it

appears motionless to a stationary obser"er

on 6arth To achie"e this, the orbit needs to

be circular and stationed directly o"er the

euator, with an orbital period eual to the

6arth's rotational period of one sidereal day

and following the direction as the earth's

rotation at an altitude of ,(-7 miles

)(5,7*-1( &ms+ abo"e the 6arth

The "elocity of a satellite orbiting at this

altitude is -,*77* mph )11,0-*7*&ph+ and

this "elocity must be precisely maintained

for the satellite to appear geostationary )!

sidereal day is the time scale based on the6arth's rate of rotation measured relati"e to

the fixed stars and is eual to ( hours, 5-

minutes and 091- seconds+

! Geosynchronous Or#it is also an

orbit with the same period as 6arth's

rotation, in other words it is

synchronous with 6arth's rotation, but

the plane of the orbit can ha"e anyinclination between 0 and 90 degrees

with respect to the euatorial plane

and the orbit may be elliptical rather

than circular To an obser"er on the

ground the orbiting ob=ect appears to

mo"e >orth and %outh in the s&y in

an elongated 'figure of eight' centred

on a fixed longitude, following the

same tra=ectory e"ery day and

passing any particular point at exactly

the same time e"ery day ! steerableantenna may be reuired to maintain

acceptable communications at the

limits of these apparent oscillations

?or satellite communications the ad"antage of the geostationary orbit is that the satellite can be

accessed by means of a fixed antenna and it does not need a large steerable antenna on the ground to

trac& the satellite for optimum signal reception 4n addition, because of the "ery high altitude of their

orbits, geostationary satellites may ha"e a "ery wide signal &ootprint co"ering up to @ of the

6arth's surface, with the potential to pro"ide line of sight communications across oceans and

between continents 4n practical systems, reliable communications are not possible at the limits of

this footprint but a single geostationary satellite can howe"er pro"ide continuous ser"ice, which can

be accessed by fixed antennas, to subscribers in up to (@ of the 6arth's surface Thus they are ideal

The oon, at an altitude of 0,000 miles )(*-,000 &ms+

ta&es a month to orbit the 6arth

http://www.mpoweruk.com/satellites.htm#footprint

http://www.mpoweruk.com/satellites.htm#footprint



for pro"iding low cost tele"ision broadcasting ser"ices as well as for monitoring the en"ironment and

the weather

<isad"antages compared with Aow 6arth :rbit )A6:+ satellites are that orbiting at a higher altitude,

they need more powerful launch "ehicles to put them in place and the communications system needs

higher power transmitters and more sensiti"e recei"ers because of the increased path loss

Beostationary satellites also ha"e poor signal co"erage in the polar regions %ee 'ook Angles which

explains why

?or simplicity, the satellite should be launched into a geostationary orbit directly from a launch site

on the euator but this is not always possible 4n such cases when the satellite is launched from sites

in higher latitudes, assuming it is launched at synchronous speed, it will enter a geosynchronous and

possibly elliptical orbit because of the inclination of the plane of the orbit ?urther or#italmanoeures will be reuired to mo"e the satellite into a geostationary orbit

%ince there must be a reasonable space between satellites to a"oid collisions but more importantly to

a"oid harmful radio#freuency interference during operations there can only be a limited number of

orbital slots a"ailable for B6: satellites and there are hundreds of commercial and go"ernment

satellites "ying for allocation of these slots and the freuency allocations that go with them

'ow %arth Or#it ('%O) satellites can be launched directly into the desired orbits and don't need the

complex orbital manoeu"res reuired by B6: satellites to place them in position They also reuire

less energy to place them into orbit and they can use less powerful amplifiers for successful

transmission of communications 2owe"er the potential atmospheric drag, limits the lowest practical

orbital altitude to about 1*0 miles )(00 &m+

$ecause of their lower orbits, A6: satellites are able to distinguish details of the 6arth's surface

much more clearly as they are not so far away so they are ideal 6arth obser"ation, remote sensingand sur"eillance ?or the same reason, the two way signal transmission delay is much lower than the

transmission delay in B6: systems at only to * milliseconds per hop depending on the position of

the satellite

A6: satellites howe"er must tra"el at a much higher angular speeds to remain in orbit since they

need a greater centrifugal force to balance the higher gra"itational force experienced at the lower

altitude Thus they are non#geosynchronous and will orbit the earth se"eral times per day

8ommunications will therefore be intermittent since the satellites will only be "isible to obser"ers on

the ground for short period each time they pass o"erhead Trac&ing such fast mo"ing satellites also

reuires highly manoeu"rable light weight antennas, and many of them, to pro"ide wide area radio

co"erage

!nother problem with communications satellites in orbits lower than geosynchronous is that a

greater number of satellites are reuired to sustain uninterrupted transmissions .hereas a single

B6: satellite can co"er ( percent of 6arth's surface, indi"idual A6: and 6: satellites co"er only

between and 0 percent This means that a fleet of satellites, &nown as a constellation, is

reuired to pro"ide a global communications networ& with continuous co"erage

$ecause of their relati"e simplicity and lower cost, A6: satellites are still used for many

communications applications %atellite telephone systems such as !ridium use A6: satellites

because their lower orbits permit the use of relati"ely low power, low sensiti"ity telephone handsets

The !nternational Space Station (!SS) and the Hu##le telescope are both in A6: orbits, the 4%% at

-0 miles )0 &ms+ and 2ubble at (7 miles )559 &ms+

http://www.mpoweruk.com/satellites.htm#path

http://www.mpoweruk.com/satellites.htm#look_angle


http://www.mpoweruk.com/satellites.htm#manoeuvres








"edium %arth Or#its ("%O) range in altitude from 1,00 miles ),000 &ms+ up to the

geosynchronous orbit at ,(- miles )(5,7*- &ms+ which includes part of the lower and all of the

upper *an Allen radiation #elts ;ractical orbits therefore a"oid these regions

!s with all satellites in non#geosynchronous orbits, 6: satellites are only "isible intermittently by

obser"ers on the ground The higher the orbit, the greater the footprint

Typical 6: applications are

na"igation, communications, and

geodetic space en"ironment science

The most common altitude is =ust

abo"e the upper Can !llen belt at

around 1,55 miles )0,00

&ilometres+, which yields an orbital

period of 1 hours, and is used for

many national na"igation systemssuch as the D% the Glo#al+ositioning System (G+S)

obile "oice communications tend

to occupy orbits below the upper Can

!llen belt at altitudes below *000

miles )1(,000 &ms+

Highly %lliptical Or#its (H%O)

26: orbits, first proposed by $ritish

engineer $ill Hilton, allow the

satellite footprint to be concentrated

on specific regions of the 6arth The

orbit of the Russian "olniyasatellites for example which pro"ide

telephony and TC ser"ices o"er

Russia is designed so that each

satellite spends the great ma=ority of

its time o"er the far northernlatitudes .ith a period of 1 hours

the satellite is a"ailable for operation

o"er the targeted region for eight

hours e"ery second re"olution 4n this

way a constellation of three olniya

satellites, plus one spare, can pro"ide

uninterrupted co"erage

The "olniya Or#it

%ignal le"els recei"ed from

geostationary satellites diminish the further the distance the ground stations are from the euator so

"olniya Or#it

"olniya Satellite Ground Track

;ublic <omain

http://www.mpoweruk.com/satellites.htm#hazards

http://www.mpoweruk.com/satellites.htm#81252892


http://www.mpoweruk.com/history.htm#molniya

http://www.mpoweruk.com/satellites.htm#hazards



http://www.mpoweruk.com/history.htm#molniya



that communications to high latitude regions by geostationary satellites may be difficult or

impossible )%ee 'ook Angles for an explanation+

To pro"ide acceptable signal co"erage in high latitudes such as 8anada and Russia whose land

masses are mostly between latitudes of 50 and 70 degrees >orth reuires "ery high satellite

transmitter powers or alternati"e satellite orbits which place the satellite directly o"er the country

The 2ighly 6lliptical :rbit )26:+ specified for Russia's olniya satellite, now called the olniya

:rbit, was designed to pro"ide this second solution

The olniya orbit was inclined at -( degrees to the euator and semi#synchronous ma&ing a

complete re"olution of the 6arth e"ery 1 hours synchronised with the 6arth's rotation 4ts perigee in

the southern hemisphere was around (10miles )500 &ms+ and its apogee in the northern hemisphere

was around ,*50 miles )0,000 &ms+

4n practice this means that the satellite ma&es two orbits per day during each of which it mo"es >orth

and %outh speeding "ery uic&ly through its perigee o"er the oceans of the southern hemisphere, butslowly ho"ering around its apogee o"er the northern hemisphere obeying ,epler-s Second 'aw('aw o& %ual Areas) for its highly elliptical orbit <uring this time howe"er the 6arth is rotating,

so the satellite as seen from the 6arth appears to be mo"ing eastwards :n the first 1 hour orbit the

satellite ho"ers for about eight hours o"er 8anada and the D%! and during the following orbit it

ho"ers for eight hours o"er Russia %ee the olniya Bround Trac& diagram opposite

%ome would say that this allows the satellite to spy on the D%! during the day and to download the

information gathered to Russia during the night, but there's nothing to stop !mericans doing

something similar

olniya's main purpose howe"er was to pro"ide tele"ision and telephony ser"ices across Russia and

into the !rctic polar region 4ts high apogee enables it to pro"ide wide co"erage with a single antenna

but a disad"antage of the olniya orbit is that it is not geostationary so steerable antennas were

reuired to send and recei"e the signals howe"er this is mitigated somewhat by olniya's slow speed

through the apogee which puts less demand on the ground station antenna positioning systems

Twenty four hour continuous national co"erage could be pro"ided to a networ& of ground stations by

three satellites each spending eight hours o"er the country This was at least better than the option of

using a larger constellation of A6: satellites which needed fast acting steerable antennas to follow

them

olniya orbits also had the ad"antage of reuiring less roc&et power to launch the satellite into the

26: orbit than to get it into a geostationary orbit

%ee more about the "olniya Satellite

+olar Or#its

%atellites in these orbits fly o"er the 6arth from pole to pole in an orbit perpendicular to the

euatorial plane This orbit is most commonly used in surface mapping and obser"ation satellites

since it allows the orbiting satellite to ta&e ad"antage of the earth's rotation below to obser"e the


http://www.mpoweruk.com/satellites.htm#kepler










entire surface of the 6arth as it passes below any of the pictures of the 6arth's surface in

applications such as Boogle 6arth come from satellites in polar orbits

Or#ital "athematics

.hen a mo"ing satellite, natural or artificial, enters the gra"itational field of a "ery large ob=ect suchas a planet or star, its momentum will &eep it mo"ing and in the "acuum of space there will be no

drag to slow it down so it will &eep mo"ing at the same "elocity 4ts direction will howe"er change

due to the influence of the gra"itational field causing its path to cur"e towards the large ob=ect .hen

the centrifugal force acting on the satellite, due to the tangential "elocity of its cur"ed path, =ust

matches the gra"itational pull of the large ob=ect the satellite will enter a stable orbit around the

larger ob=ect 4f the "elocity is too low, the satellite will fall into the large ob=ect 4f it is too high, it

will fly off into space

The Centri&ugal &orce /c acting on a body or satellite in angular motion is gi"en by0

/c 1 m23r 1 mr42

whereE

m is the mass of the satellite

r is the distance between the centre of motion )the 6arth+ and the centre of the satellite

is the tangential "elocity of the satellite

4 is the angular "elocity of the satellite

The Graitational &orce /g acting between two bodies, one of which is the 6arth, is gi"en byE

/g 1 G"m3r2

whereE

G is the uni"ersal gra"itational constant

" is the mass of the 6arth


r is the distance between the centres of the masses

.hen a satellite is in a steady orbit aroung the 6arth, the centrifugal force actiing on it =ust balancesthe gra"ittational force acting on it This occurs whenE

m23r 1 G"m3r2



The euations describing the satellite's speed and orbital period are deri"ed from this relationship

,epler-s 'aws

,epler-s /irst 'aw ('aw o& Or#its)

!ll planets mo"e in elliptical orbits, with the

%un at one focus

%ee diagram opposite

,epler-s Second 'aw ('aw o& Areas)

The line between a planet and the %un sweeps out eual areas in eual times as the planet tra"els around its

elliptical orbit

%ee diagram opposite

,epler-s Third 'aw ('aw o& +eriods) gi"es the orbital period T of a body orbiting an other in a

circular or elliptical orbit asE

T 1 256 (r7 3 G")

.here r is the semi#ma=or axis or radius of the orbit

.hen the mass of the orbiting body is negligible compared to the mass of the 6arth, the orbital speed

*o is gi"en byE

*o 8 6(G" 3 r)

.here r is the distance between the centre of the masses of the 6arth and the satellite

4n other wordsE The higher the altitude, the longer the orbital period and the slower the orbital speed

%ee more about ,epler and the uestionable scientific ethics he used to arri"e at these laws

Or#ital "anoeures

http://www.mpoweruk.com/history.htm#kepler

http://www.mpoweruk.com/history.htm#kepler



+arking Or#it

4t is not always possible to launch a space

"ehicle directly into its desired orbit The

launch site may be in an incon"enient

location with respect to the orbit or the

launch window may be "ery short, a few

minutes or e"en seconds 4n such cases the

"ehicle may be launched into a temporary

orbit called a par&ing orbit which pro"ides

more options for realising the ultimate orbit

Dsing a par&ing orbit can extend the

launch window by se"eral hours by

increasing the possible range of

locations from which to initiate the

next propulsion stage 4t also enables

the spacecraft to reach a higher

perigee by firing the second stageafter it has reached a higher point in the par&ing orbit which will raise its perigee in the new orbit

?or manned space missions the par&ing orbit pro"ides an opportunity to chec& that all systems are

wor&ing satisfactorily before proceeding to the next critical stage

Trans&er Or#it

The transfer orbit is the orbit used to brea& out of the par&ing orbit and brea& into the

geosynchronous or geostationary orbit The notion of using an elliptical orbit to transfer between twocircular orbits in the same plane but with different altitudes was originally concei"ed by Berman

scientist .alter Hohmann in 195 and published in his boo& Die Erreichbarkeit der Himmelskörper

)The Accessibility of Celestial Bodies+ and the manoeu"re was subseuently named for him

The Hohmann trans&er uses two roc&et engine impulses, one to mo"e the spacecraft onto the

transfer orbit and a second to mo"e off it into a new orbit The first impulse increases the speed and

energy of the spacecraft propelling it into a larger elliptical orbit with its apogee lying on the desired

new orbit The second impulse ta&es place at the apogee and accelerates the spacecraft once more

this time widening the new orbit into a circular path 4t does not in"ol"e any changes in the plane of

the orbit

The inclination of the transfer orbit is the angle between the spacecraft's orbit plane and the 6arth's

euatorial plane and is determined by the latitude of the launch site and the launch a3imuth

)direction+To obtain a geostationary orbit the inclination and eccentricity must both be reduced to

3ero

4n the case of launching a satellite such as the Syncom 2 into a geosynchronous orbit, the launch

"ehicle first stage puts the spacecraft into the par&ing orbit aligned with its launch a3imuth and

direction corresponding to the (( degrees latitude of the launch site The second stage puts it into the

transfer orbit with its apogee corresponding to the geosynchronous altitude after which the satellite

separates from the spacecraft Then the satellite's on board apogee &ic& motor pushes the satellite

into the circular geosynchronous orbit still aligned with the plane satellite's launch and par&ing orbits

at (( degrees inclination to the euator





$ut a geostationary

satellite such as

Syncom 7 also

launched from a

latitude of (( degrees,

needs to change its

orbital plane to align

it with the euator in

order to enter a

geostationary orbit

This is accomplished

by controlling the

roc&et's second stage

yaw which reduces

the angle of

inclination of the orbit

before separation

from the satellite and

by controlling thesatellite's attitude and

hence the direction of

its apogee &ic& motor

after separation when

it executes its roc&et

burns in order to tilt

its orbital plane

dri"ing it into the

desired 3ero degrees

inclination from the

euator %ee Syncom7 in9ection example

Or#its and Solar+ower

The Satellite +osition # Fust

li&e the 6arth, satellites

experience day and night,

except that, rotating typically

at 0 r pm the satellite's

day is "ery short lasting only

05 seconds !lso =ust li&e

the 6arth, as the satellite

rotates, one side will always be illuminated by the %un, except during periods of terrestrial eclipse when the

satellite passes through the 6arth's shadow, while the opposite side is in dar&ness ?or a geostationary

satellite, eclipses happen once e"ery day but only during the period around the "ernal and autumnal

euinoxes when the %un appears to be directly o"er the euator)%ee diagrams opposite+

$ecause the 6arth's orbit is tilted at (5 degrees, as it mo"es in its year long trip around the %un, the %unappears to mo"e north during the summer months towards its position at the summer solstice when it is

abo"e the Tropic of 8ancer !s it mo"es north, its shadow mo"es south so that it no longer co"ers the

satellite which is in the euatorial plane of the 6arth %imilarly when the %un appears to mo"e south to the

Tropic of 8apricorn for the winter, its shadow mo"es north also lea"ing the satellite in sunshine The

The satellite is eclipsed by the 6arth once per day in the period around the

"ernal and autumnal euinoxes when the %un is abo"e the euator

The rest the year the %un is abo"e or below the 6arth's orbital plane and the

satellite recei"es uninterrupted sunlight

<uring most of the year, the %un is abo"e or below the orbital plane of the

6arth and the satellite so that the satellite recei"es uninterrupted sunlight

The satellite is only eclipsed by the 6arth during the period around the

euinoxes when the %un is in the orbital plane of the 6arth

http://www.mpoweruk.com/satellite_communications.htm#injection







satellite itself being fixed in relation to the 6arth, and so tilted with respect to the %un, experiences

the same apparent north and south mo"ement of the %un about the satellite's euatorial plane, thus

changing the angle of incidence of the %un's rays

The result of all of these mo"ements is that a satellite in geostationary orbit experiences ** short

terrestrial eclipses per year, occurring around the "ernal and autumnal euinoxes with a maximum

duration of 70 minutes diminishing to 3ero o"er a few days as the %un progresses towards its summer

and winter solstices The %un is thus "isible to the satellite for about 99@ of the time

The !ncident Solar %nergy # The %un's radiant energy le"el, or irradiant impinging e"ery second

on a perpendicular plane outside the 6arth's atmosphere amounts to about 1(-7 .atts per suare

metre and is &nown as the solar constant The con"ersion efficiency of early solar cells in producing

usable electrical power from this energy was only about *@

aximum power will only be generated from the solar cells pointing directly towards the %un

otherwise the output will be proportional to the cosine of the angle of incidence of the %un's rays on

the cells

?or a cylindrical, spin#stabilised satellite with solar cells mounted around its circumference, and its

axis parallel to the 6arth's axis, power will only be generated from the side of the cylinder facing the

%un and the output will fall off towards the edges of the cur"ed surface as the solar cells present a

different, diminishing angle towards the %un reaching 3ero when the rays are tangential to the

satellite

The power output will also "ary with time during the year as the 6arth mo"es around the %un,

pea&ing during the euinoxes when the %un is directly o"er the euator, )except for short daily

interruptions due to the terrestrial eclipses+, and diminishing towards the solstices when the %un's

angle of incidence is =ust o"er --5 degrees

%atellites using three#axis stabilisation do not suffer from this problem because their flat solar arrays

can be steered to be always normal to the %un's rays so that all of their solar cells are pointing

towards the %un thus maximising the incident solar energy

$atteries will be reuired to maintain the power during the eclipses

%ee more about Solar +ower

On #oard power

The satellite has to carry enough fuel for manoeu"ring it into its synchronous orbit and for station

&eeping and attitude control once it is in place 4t also needs to be able to capture enough solar

energy to power the on board electronics for the transponder and its telemetry and control once it is

operational

The associated weight penalty puts a limit on the useful lifetime of the satellite unless the solar panels are large enough to pro"ide the total operational energy reuirements of the satellite and its

control systems once it has been placed in the desired orbit %imilarly, the allowable weight and finite

lifetime of batteries which may be used to store energy also limit the satellite's lifetime

http://www.mpoweruk.com/solar_power.htm

http://www.mpoweruk.com/solar_power.htm



Satellite si:e and weight

The dimensions and weight are limited by what the launching roc&et can accommodate This in turn

places se"ere restrictions on the performance capability of the satellite The a"ailable on board power

is limited, as is the power output of the transmitter The si3es of the antennas are limited so that

signal strengths transmitted and recei"ed by the satellite are both "ery low %ee typical example inthe 'ink ;udget below

6arly satellites were tiny, considering the amount of technology the were able to cram on board

'aunching the satellite into or#it

The first ma=or challenge was to design a space "ehicle powerful enough and accurate enough to

launch a hea"y payload into a geostationary orbit as en"isaged by 8lar&e ilitary roc&et

programmes initiated after .orld .ar 44 were beginning to deli"er this capability 4n the D%! the<elta rocket, originally deigned as a ballistic missile, was adapted for this purpose Ai&e any pure

ballistic missile howe"er it did not ha"e the capability to ma&e the necessary changes in its orbit to

steer its payload from its launch tra=ectory into a geostationary orbit %uch manoeu"rability had to be

built into the satellite itself by pro"iding it with an independent means of propulsion and directional

control %ee example Syncom Or#ital !n9ection !ll this added to its weight and complexity

,eeping it on station

Betting a satellite into a desired orbit is only half of the =ob Geeping it there is the other half

:b=ects orbiting the 6arth are sub=ect to forces such as solar radiation pressure, )often called solar

wind+, the "arying strength of the 6arth's magnetic field and the "arying gra"itational forces due to

the satellite's changing position with respect to the %un and the oon and the fact that the 6arth is

not a perfect sphere These forces can cause a lateral or precession motion of the orbital plane of the

satellite causing it to drift from its desired position and orientation

.ith the absence of any atmosphere in the "acuum of space, the slightest force applied to the satellite

will set it in motion and since there's no resistance to slow it down it =ust &eeps tumbling and drifting

further and further away from its prescribed orbit and attitude

%atellites therefore need to be euipped with some method of mechanical station keeping for

ma&ing corrections to the orbit and for attitude control to &eep the antennas pointing towards the

6arth and the solar cells pointing towards the %un, together with some form of energy supply to

ma&e the necessary corrections when reuired Fust as the tiniest of forces can send the satellite off

trac&, it only needs eually tiny forces to bring it bac& Bas =et thrusters are often employed for this

purpose and it is the capacity and consumption of the propellants they use which ultimately limit the

acti"e life of the satellite



http://www.mpoweruk.com/figs/gyros.htm#precession







agram sourceE 6lectropaediaE %atellite image sourceE >!%!,

Attitude Control

8ontrolling a spacecraft's attitude

reuires sensors to measure its

current orientation or attitude, a

control system which calculates thede"iation from its desired orientation

and determines the forces needed to

reduce the de"iation to 3ero and

actuators to apply the necessary

forces to re#orient the "ehicle to the

desired attitude The actuators are

normally part of the stabilisation

system and may be gas thrusters or

momentum wheels

o Attitude Sensing #y =adio/reuency !nter&erometer



.hen two electromagnetic wa"es with the same freuency combine, the resulting pattern is determined by

the phase difference between the two wa"es .a"es that are in phase will undergo constructi"e interference

or reinforcement while wa"es that are out of phase will undergo destructi"e interference or cancelling This

property can be used to determine the phase difference or the delay between two wa"es coming from the

same source

The diagram opposite shows a single radio wa"e from a distant ground station impinging on two

antennas attached to a satellite on a plane which is inclined with respect to the direction of the wa"e

The signal arri"ing at the left anntena will be delayed with the delay T depending on the angle >

between the plane of the antennas and the plane of the wa"efront The comparator gi"es an output

depending on the phase difference between the signals from the two antennas The magnitude of the

delay or phase difference between the signals can be determined by inserting a &nown )"ariable+

delay into the non#delayed signal to dri"e the error signal to 3ero, thus bringing the two signals from

the two antennas into phase %ince the distance between the antennas is &nown, the tilt angle between

the satellite body and the direction of the radio wa"e can be determined

The signal delay depends on the freuency or wa"elength of the radio wa"e ?or a -0 B23 )8#$and+

telemetry signal, the wa"elength will be around 50 millimetres enabling accurate determination of

the angle of inclination or attitude of the satellite The greater the distance between the antenna pairs,the greater the accuracy

The error signal may be transmitted to ground control to manage the satellite's attitude or in could be

used in an on board control system which is programmed to &eep the signals from the two antennas

in phase

Radio freuency interferometry can unfortunately only be used with pairs of antennas whose

distance from the source may be different Thus it can only be used to monitor two of the three

orthogonal axes of a geostationary satellite, namely pitch and roll, but not its yaw This is because theinterferometry antennas must be attached to the surface of the satellite and directed towards the 6arth

from whence the radio signal is transmitted .hen the satellite rolls, the surface on which the

antennas are mounted appears from the 6arth to tilt forward and bac& in ele"ation .hen the satellite

pitches, the surface appears to tilt right and left in a3imuth as the satellite increases or decreases its

altitude $ut the satellite's yaw axis is pointed towards the centre of the 6arth and when the satellite

executes a yaw, changing the inclination of its orbit or its latitude, the surface of the satellite facing

the 6arth appears to rotate about its centre staying normal to the direction of the radio signal so that

there is no differential delay between the signals recei"ed by pairs of antennas on the surface :ther

methods such a star trac&ing )see next+ must be used to determine the yaw



o Attitude Sensing #y "eanso& Star Tracking

The star trac&er uses a camera, with a star

map pro=ected on to its focal plane to trac&

the image of a reference na"igation star such

as ;olaris, the ;ole %tar The na"igation

target star should be at the centre of the star

map, and on the optical axis of the camera

The camera is mounted on the

satellite in such a way that, when the

satellite's attitude is correctly

oriented, the optical axis of the

camera will be aligned with the target

na"igation star and the optical image

of this star will be centred directly

o"er the reference image of the

na"igation star on the star map ! photo#multipier is used to increase

the intensity of the "ery wea& light

recei"ed from the stars

4f the orientation of the satellite

changes, the image of the target star

will de"iate from its central position

on the star map )%ee diagram opposite+ This angular error between the camera's optical axis and a

line to the target star is detected by electronically scanning the camera's field of "iew and generating

H and I error signals proportional to the angular error The error signals thus generated are used to

correct the orientation of the spacecraft so that the target star is centred once more on the startrac&er's optical axis

4n general, star trac&ers are the most accurate of attitude sensors, achie"ing accuracies to the arc#

second range 2owe"er star sensors are hea"y, expensi"e, and reuire more power than most other

attitude sensors 4n addition, they reuire on board computing power to scan the images and carry

out pattern recognition to identify the target star followed by calculations of the angular error and

implementation of the control actions needed to re#orient the satellite

To a"oid interference from the %un, star trac&er cameras are usually fitted with %un shades and,where possible, target stars are chosen so that the camera will be mounted on the side of the satellite

in the %un's shadow

o %arth Sensing

A simple though less accurate method of determining a spacecraft's attitude is by sensing the direction of theEarth's horizon. Infrared bolometer (radiometer) detectors, which measure the power of incidentelectromagnetic radiation by measuring its heating effect on a temperature dependent electrical resistance,

can determine the position of the hori3on by detecting the difference in the intensity of radiationcoming from the 6arth =ust below the hori3on and the radiation coming from space =ust abo"e the

hori3on


http://www.mpoweruk.com/history.htm#bolometer






Sta#ilisation

o Spin?Sta#ilisation is a simple and effecti"e method of &eeping a satellite's attitude, that is the

orientation in space of its spin axis, pointed in a certain direction ! spacecraft spinning on its

axis resists perturbing forces in the same way that a spinning gyroscope or a top does so that

its attitude )but not its position+ remains fixed in space !ccording to 2ughes' engineers, spin#stabilisation is the method that nature prefers

!nother ad"antage of spin#stabilisation is that in space, once the satellite is spinning there are

no frictional forces to slow it down so that it will &eep spinning indefinitely

This is basically an open loop system in which the satellite maintains its initial attitude

without further ad=ustment during its life and was the method used by Telstar The system

can howe"er be adapted as part of an automatic )closed loop+ attitude control system %uch a

system reuires a sensor to determine the actual attitude of the satellite which is then

compared with a reference attitude, )the desired attitude+, to generate an error signal which is

used in a feedbac& system to cause an actuator to mo"e the satellite in such a way as toreduce the error to 3ero %ee example Syncom Attitude Control

There are howe"er some inherent inefficiencies associated with this method of stabilisation

since only some of the solar cells can be illuminated by the %un at any one instant as the

satellite rotates !t the same time the satellite needs omnidirectional antennas so that at least

some of the antenna's beam is always pointing towards the 6arth as the satellite rotates This

lea"es most of the radio wa"e energy wastefully radiated into space :"ercoming this

problem reuires complicated systems to de?spin the antennas allowing the use of higher

gain structures which can be &ept in a fixed direction pointing towards the 6arth

%atellites and gyroscopes also

suffer from nutation or

coning, that is the tendency of

the spinning body to nod or

wobble around its spin axis

%pin stabilised satellites

usually incorporate some

form of hydraulic or

mechanical damping to

reduce this effect

o Three a@is sta#ilisation, also

called #ody sta#ilisation, does not

reuire the gyroscopic rotation of the

satellite body for stability 4nstead it

&eeps the satellite body in a fixed attitude, allowing the solar energy capture and radio transmission and

reception to be optimised independently

There are two basic forms of gyroscopic three axis stabilisation E

"omentum wheels, similar to gyroscopes, which spin in one direction only

http://www.mpoweruk.com/motorcontrols.htm#open


http://www.mpoweruk.com/motorcontrols.htm#closed

http://www.mpoweruk.com/satellite_communications.htm#syncom_control

http://www.mpoweruk.com/satellite_communications.htm#de_spun


http://www.mpoweruk.com/motorcontrols.htm#open


http://www.mpoweruk.com/motorcontrols.htm#closed

http://www.mpoweruk.com/satellite_communications.htm#syncom_control




=eaction wheels which can spin in both directions

These wheels are mounted in three orthogonal directions corresponding to the yaw, roll and pitch of

the satellite body and pro"ide a stabilised inertial platform

!ccelerating or decelerating any of the wheels by means of electric motors or gas =et thrusters

increases its angular momentum in that direction by an amount which is proportional to the applied

motor or =et torue and this in turn creates an eual and opposite torue on the satellite body causing

it to rotate in the opposite direction about the axis of the wheel %lowing the wheel brings the satellite

body bac& again Thus angular momentum can be traded bac& and forth between the spacecraft and

the wheels

Thrusters are still reuired for lateral mo"ement

%ee benefits made possible by Three A@is Sta#ilisation

=eaction Control Thrusters are an alternati"e method pro"iding three axis stabilisation !ttitude

correction can be implemented by three small gas thrusters, mounted on three orthogonal axes of the

satellite, which nudge the satellite bac& into position 4t may be simpler but less precise than the

reaction wheel stabilisation methods and possibly unsuitable for some optical applications or

experiments may be affected by the e=ected gas particles

.ith this method of stabilisation the shape of the satellite body and appendages is no longer

important %ub#systems can be accommodated in any con"eniently shaped box %e"eral antennas andsolar cell arrays can be deployed and pointed in the different directions, optimised for the

application

o Graity Gradient Sta#ilisation was explored by the D% <epartment of <efence in a 19-7

<:<B6 )<epartment of <efence Bra"ity 6xperiment+

4n 19-7 the D% <:< carried out a successful experiment to test the feasibility of Graity?Gradient Sta#ilisation, also &nown as Tidal Sta#ilisation for spacecraft or satellite station

&eeping Ai&e three axis stabilisation it does not reuire the gyroscopic rotation of the satellite

body for stability 4t is howe"er a passi"e system which uses the 6arth's gra"itational pull to

&eep the satellite in a stable attitude

!cti"e stabilisation by means of gyroscopic action thrusters or reaction and momentumwheels reuires the use of propellants or electricaal energy to &eep the satellite on station and

the finite capacity of the satellite to carry these propellants sets a limit to its acti"e life

Bra"ity gradient stabilisation howe"er does not need propellants 4t relies instead on the

satellite's mass distribution within the 6arth's gra"itational field and the balance between thegra"itational and centrifugal forces acting on it to &eep the satellite aligned in the desired

orientation %ee the following diagram






Graity Gradient Sta#ilisation

Graitational &orce /g 1 G"m3r2

G is the uni"ersal gra"itational constant

" is the mass of the 6arth


r is the distance between the centres of the

masses

Centri&ugal &orce /c 1 m23r 1 mr42


r is the distance between the centre of the 6arth

and the centre of the satellite

is the tangential "elocity of the satellite

4 is the angular "elocity of the satellite

! body with an unbalanced mass in free space will tend to line up under the influence of

gra"ity with its hea"iest part closer to the ground so that its axis of minimum moment of

inertia, or its longest dimension, is aligned "ertically, that is radially from the centre of the

6arth $ut because the gra"itational pull of the 6arth decreases according the in"erse#suare

law, at "ery high altitudes and the small si3e of the orbiting body, the difference in the

gra"itational force across the body are minute ma&ing such a system ineffecti"e 4f howe"erthe effecti"e si3e of the body is increased by separating off a small part of it and connecting it

by a long tether to the larger mass of the main part, the effecti"e si3e of the body is increased

and the differential gra"itational force across it will li&ewise be increased creating an

appreciable gra"ity gradient across the body sufficient to &eep it aligned in a fixed direction

The tether is &ept tight because both parts of the body are orbiting at the same angular speed,

but the smaller part is orbiting at a higher radius therefore experiences a greater centrifugal

force 4n practice the smaller part can be designed to accommodate part of the spacecrafts

functionality The example abo"e shows this as telemetry but it could be any other con"enient

function

Dsing a "ariety of retractable booms the <:< experiment explored the possibility ofstabilising a satellite along different axes The mission was a success and pro"ed the

feasibility of achie"ing tri axial gra"ity#gradient stabili3ation at synchronous altitudes using

passi"e and semi passi"e techniues



<espite its feasibility and its fuel sa"ing benefits, gra"ity gradient stabilisation has only

occasionally been adopted in practical systems

TranspondersThis is the payload which communications satellites are designed to carry

Transponders are microwa"e repeaters located at intermediate points in a communications lin& which

are used to compensate for the signal attenuation along the route so as to extend the range of the lin&

They recei"e the "ery wea& signals from a sender at one end of the lin&, amplify them, and re#

transmit them at much higher power to the recei"er at the other end of the lin& The whole purpose of

a communications satellite system is to place a transponder in position, to &eep it there and to &eep it

powered up $ecause of the "ery high launch costs, for satellite systems to be economically =ustified,

the transponder should be able to carry high traffic "olumes including tele"ision channels as well as

do3ens of multiplexed "oice communications and other data lin&s 4t should also be small and light

.hen the first pro=ects were concei"ed there were no solid state de"ices a"ailable which could

pro"ide the high power broadband amplification at the high freuency needed for the repeater and

early transponders used pencil slim "acuum tubes )Traelling ae Tu#es (TT)+ to pro"ide the

necessary amplification

$esides amplification repeaters also perform a freuency shift $ecause of the proximity of a

satellite's high power transmitter to its "ery sensiti"e recei"er, and in many cases the use of the same

antenna for recei"ing and transmitting the signals, the high power signals from the transmitter can

swamp the "ery wea& recei"ed signals causing problems in the recei"er To minimi3e this problem

the transponder contains a conerter which changes the freuency of the recei"ed uplink signals toa different freuency, widely separated from the uplin& freuency, for onward transmission by the

downlink. 4t also incorporates a diple@er which connects both the transmitter and the recei"er to the

same antenna inputoutput port by means of filters which bloc& the transmitter signals and other

forms of interference from lea&ing into the recei"er :ther than freuency con"ersion )heterodyning+

and amplification there was no other on board signal processing on the early satellites %imple

transponders of this type were called #ent pipe transponders

$ecause higher power amplifiers and lower noise amplifiers are more a"ailable on the ground

station, the uplin& is always the higher freuency since it has the higher ;ath Aoss %ee 'ink ;udget

"ultiple Access

odern transponders can carry many different types of communications traffic They can also

recei"e signals from multiple ground stations, combining )multiple@ing+ or splitting )de?multiple@ing+ them for onwards transmission to other multiple ground stations This method, by

which many users share a common satellite resource, is called ultiple !ccess There are se"eral

schemes for accomplishing this, each with its benefits and drawbac&s

o T<"A ? Time <iision "ultiple Access allocates a time slot to the user in a repetiti"e timeframe The signal is digitised and the data bits are stored in a buffer in a compressed time

frame until their allocated time slot comes around when they are transmitted during their

allocated time !t the recei"er end of the lin& the bits are rearranged, spreading them out to

reassemble the original digital signal and con"erted bac& to analogue form The signal

http://www.mpoweruk.com/history.htm#twt


http://www.mpoweruk.com/history.htm#twt




occupies the entire transponder bandwidth, but only during its allocated time slot The rest of

the time the bandwidth is a"ailable to other users <igital signals typically ha"e better noise

immunity than analogue signals

o /<"A ? /reuency <iision "ultiple Access shares the bandwidth between the users, with

each user allocated a uniue, narrower section of the a"ailable bandwidth 4t wor&s with

analogue signals and all users ha"e uninterrupted use of their own narrow freuency band or

channel with all users occupying the a"ailable bandwidth simultaneously, each within their

own narrow channel The sender's signal, called the baseband signal, is freuency shifted into

the allocated freuency band for transmission and the recei"er restores it bac& to the

baseband

o C<"A ? Code <iision "ultiple Access also &nown a Spread Spectrum, modulates the

user's signal with a pseudorandom code so that it occupies the full a"ailable spectrum,

appearing as noise The recei"er uses the same pseudorandom code in an autocorrelator

de"ice which only recognises a signal modulated with the same auto code and thus separates

it from the noise 8<! is more complex but has better noise immunity and pro"ides greater

security than the other two systems

Telemetry and Command

o Telemetry systems monitor the status of the satellite's systems including the functioning of

electronic and propulsion sub#systems and its energy management as well as its attitude and

position in space and pro"ide the capability to transmit this information to a control centre on

the ground

o Command systems use the telemetry inputs in control systems to compare the satellite's

actual status with its desired status and to transmit control signals bac& to the satellite to

operate on board actuators such as switches, solenoids, motors or propulsion =ets to &eep the

satellite operating within its design parameters The control functions include manoeu"ring,

antenna deployment, station &eeping, attitude control, energy management and

communications channel switching

o %pacecraft usually incorporate a ;eacon which sends out a signal which enables it to be

trac&ed by a ground station

They normally use separate, dedicated radio channels and antennas for these functions

'atency or +ropagation <elay

Aatency usually refers to the time it ta&es a bit or pac&et of information to dribble through a local

networ& or signal processing euipment from its input point to its output point 4t is often of the order

of microseconds or somewhat longer for long distance cable connections ?or a satellite networ&

howe"er, the signal paths, or hops, include both the long uplin&s and downlin&s between the ground

and the satellite 8ontrol signals pass up the uplin& and telemetry signals return to the signal

originator down the downlin& 8ommunications signals pass through the satellite and onwards to the

remote recei"er <espite the fact that electromagnetic wa"e carrying the signal tra"els at the speed of

light, the distance across the networ& is so large that the delays are of the order of milliseconds and

thus much longer than the delays normally associated with the signal processing euipment







icrowa"e or radio signals are carried by electromagnetic wa"es and he transmission time delay t between sending and recei"ing a signal is gi"en byE

t 1 <3C

.here < is the length of the signal path and C is the speed of light J 1*-,* miles per second

)(00,000 &mssec+

?or a B6: satellite, the distance from the surface of the 6arth to the satellite is about ,(00 miles

)(-,000 &ms+

$oth communications and satellite control systems include uplin& and downlin& signals so that the

signal path distance < per hop is a minimum of ,-00 miles )7,000 &ms+ depending on the user's

position relati"e to the satellite the corresponding propagation delay is around 0 seconds, but

could be as high as 50 # *0 milliseconds for users who are not directly underneath the satellite

?or one#way signals such intercontinental tele"ision broadcasts, this delay is not particularlyannoying or e"en apparent, but for the satellite's telemetry and control systems the delay could cause

unacceptable errors and special error detection and correction circuits may be needed for safety

reasons The delay is more significant for two#way telephone con"ersations since the effecti"e delay

for the round trip between when one person spea&s and the other responds is essentially double the

basic hop delay at around 50 milliseconds which is definitely noticeable This delay may not be

dangerous but it can be uite annoying and echo cancellers may be needed for high uality speech

transmission

?or A6: satellites the propagation delays between sending and recei"ing information bits or pac&etsare relati"ely low due to the shorter signal paths and amount to between and 10 milliseconds for a

single hop depending on the position of the satellite relati"e to the user This is comparable to the

delays experienced in long#distance cable connections )about 5K10 milliseconds+

?or an 6: satellite orbiting at 5,000 miles )*,000+ &ms the delay will be around 15 milliseconds

per hop

4n practice howe"er delays could be much longer than this if the call needs to be transmitted across

multiple hops which is not unusual with A6: and 6: systems which use multiple satellites in

order to pro"ide continuous co"erage

+ower 'eel Bnits (Conention)

The deci#el )d;+ is a logarithmic unit used to express the ratio between two "alues of a physical

uantity ;ower ratios of , 10 and 100 correspond to ( d$, 10 d$ and 0 d$ respecti"ely 4t is

typically used to express the gain or attenuation of a system or circuit

The d;m is a measure of the signal leel relati"e to 1 milli.att expressed in decibels

The d; is a measure of the signal leel relati"e to 1.att expressed in decibels



Antennas

!ntennas are normally passi"e de"ices Though

they ha"e gain, they do not add any energy to

the signal 4nstead they concentrate the

a"ailable transmitted or recei"ed signal energy

into a preferred direction %ee the diagrams

below which show the radiation patterns of adifferent antennas

The %uialent !sotropic =adiated +ower(%!=+) of an antenna is eual to the product of

the !nput +ower applied to the terminals of the

antenna and the Antenna Gain

%@ample0 ! typical ground station

communications transmitter with an

output power of 100 watts, )0 d;+

feeding through an antenna with a gainof -0 d$ will ha"e an eui"alent

radiated power )64R;+ in the direction

of the antenna main beam of *0 d$. or

100,000,000 .atts

!n !sotropic radiator is an omnidirectional

antenna which radiates eually in all spherical

directions

=adiation +atterns

The simplest and most common radiating element is a

hal& wae dipole whose radiation pattern is a toroidal

shape 4t is formed from two conducting elements such

as wires or metal tubes whose length is one half

wa"elength of the radiating radio wa"e 4t is typically

fed in the centre where the impedance falls to its

lowest such that the antenna consists of the feeder

connected to two uarter wa"elength wires or elements

in line with each other <ipoles can also formed by

radiating slots in the walls of a wa"eguide carrying the

radio freuency signal

ore complex, higher gain antennas may be

constructed from multiple radiating elements so that

their indi"idual radiation patterns reinforce or cancel each other to form the desired composite radiation

pattern !lternati"ely the radiation pattern may be formed by means of a reflector such as a metal parabolic

dish which concentrates the antenna beam from a single radiating element, located at the focus of the

parabola, in the desired direction

Antenna <irectiity is the ratio between the power density the antenna radiates in the direction of

its strongest emission and the power density radiated by an ideal isotropic radiator, radiating the

same total power from the same point

Hal& ae <ipole Antenna =adiation+attern

High Gain +ara#olic Antenna =adiation+attern

(+olar <iagram)

%ourceE 8hristian .olff )odified+

http://www.mpoweruk.com/satellites.htm#decibel




Antenna ;eamwidth by con"ention, is the angle between the half#power )#( d;+ points of the

antenna's main beam )or lobe+ The higher the gain, the narrower the beamwidth

!s a rule of thumb, for a parabolic antenna, the approximate beamwidths gi"en byE

7 d; ;eam width 8 2$ 3 (/D<) in degrees

.here

/ J ?reuency of the signal in B23

< J <iameter of the dish in metres

Thus a 1 B23 )% $and+ signal transmitted by a 10 metre parabolic antenna will ha"e a beamwidth

of 10 degree

Antenna %&&iciency is the ratio between the total power actually radiated by an antenna and the net

power accepted by the antenna from its connected transmitter 4t ta&es into account any impedance

mismatch, the conduction and dielectric losses in the antenna structure and feed circuits and theenergy lost in the sidelobes

Antenna Gain in transmitting mode is the ratio between the actual power deli"ered to a far field

recei"er on the axis of the antenna's main beam and the power which would be deli"ered to the same

recei"er by a hypothetical lossless isotropic antenna located at the same point as the transmitting

antenna

The Gain G of a parabolic dish antenna is gi"en byE

Gain G 1 $E log$E, (5 < 2 3 F )

.hereL

G is the gain o"er an isotropic source in d$

, is the efficiency factor which is generally around 50@ to -0@, ie 05 to 0-

< is the diameter of the parabolic reflector in metres

F is the wa"elength of the signal in metres

Note that the gain ta&es into account the antenna efficiency whereas the directi"ity does not

Bain and directi"ity are often incorrectly used interchangeably

=eciprocity0 ?or high gain antennas designed to carry two#way communications, the antenna gain in

transmitting mode is usually the same as the gain in recei"ing mode for any gi"en freuency This is

&nown as reciprocity 2owe"er in normal operations the transmitter freuency will be offset from

)usually higher than+ the recei"er freuency to a"oid interference between the transmitter and the

recei"er $ecause of this freuency difference the actual gain will be slightly higher in the higher

freuency transmission mode

/igure o& "erit of a recei"ing system is the ratio (G3T) of its gain to its noise temperature whereG is the antenna gain in deci#els at the recei"er freuency, and T is the eui"alent noise temperature

in degrees Gel"in of the antenna plus its R? signal path to the recei"er and the noise temperature of

recei"er itself


http://www.mpoweruk.com/satellites.htm#noise



http://www.mpoweruk.com/satellites.htm#noise




Antenna /ootprint is the geographical area co"ered

by the beam of a satellite antenna, within which

acceptable communications with the satellite are

possible ?rom a geosynchronous orbit, a satellite

antenna with a beamwidth of 17( degrees co"ers the

@ of the surface of the 6arth facing the satellite

from which line of site communications are

theoretically possible %ee Or#its diagram

The practical extent of a satellite's footprint is howe"er

determined by the capability of the system to deli"er

reliable communications at its outer limits The link #udget gi"es an indication of the expected signal

le"els on which these limits are based

The theoretical footprint of a parabolic satellite

antenna on a surface normal to the direction of its transmission beam is typically circular in shapeThe higher the gain of the antenna , the narrower its beam The diameter or extent of the practical

footprint or signal co"erage on the ground depends on the satellite transmitter power, the recei"er

sensiti"ity and the gains of both the satellite transmitting antenna and of the recei"er antenna

:ptimising the footprint in"ol"es se"eral trade#offs

.ith a simple, low gain antenna, much of the satellite's a"ailable transmitted energy is radiated into

space with only a low percentage of it falling on the 6arth 2igher gain antennas directed towards the

6arth can concentrate more of the transmitted energy towards the 6arth with "ery high gain antennas

focusing the energy into a desired small region or footprint

?or a gi"en transmitter power, if the recei"ed signal le"el within the desired region is not sufficient

for acceptable or reliable reception o"er the entire region, there's no point in increasing the

transmitter antenna gain any further as this will =ust reduce the footprint e"en more 4ncreasing the

footprint reuires increasing the transmitter power

!lternati"ely, for a gi"en recei"er sensiti"ity, the use of higher gain )larger+ receiver antennas on the

ground can compensate for the lac& of transmitter power The satellite's effecti"e footprint is

impro"ed because a larger recei"ing antenna can capture and raise the power of lower le"el signals to

the le"el which the recei"er can process The higher the gain of the recei"ing antenna, the larger

footprint from which acceptable signals can be recei"ed The diagram abo"e shows the differentfootprints associated with different domestic recei"er antenna si3es )or gains+ of the Astra $Asatellite system designed for direct broadcast of tele"ision channels in 6urope

/ootprint Shape

>ote that the satellite's antenna pattern is not necessarily circular The cross#sectional pattern of the

antenna beam can be shaped by altering the profile of its reflector dish or the structure of its

transmitting elements to change the shape of the footprint on the ground in order to concentrate the

satellite transmitter's energy on particular geographical areas !lternati"ely se"eral smaller antennas

may be used to achie"e the same effect

Astra $A Satellite Antenna /ootprint

%ourceE %6% !stra )odified+

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.hile increasing the gain of the transmitter antenna may be beneficial in enabling the satellite signal

to be focused on "ery small target areas, for wider co"erage, se"eral transmitting antennas may be

needed, but this in turn reuires more transmitter power

'ook Angles

The loo& angles are the a3imuth and ele"ation angles of a satellite as seen from a ground station

antenna The maximum signal le"el will be recei"ed by a ground station when it is directly under the

satellite, that is, at the same latitude and longitude as the satellite or at the satellite's ground 3ero so

that the ground station's antenna is pointing in a direction perpendicular to the plane of the 6arth at

the at point

4f the satellite is not directly o"er the ground station, the signal recei"ed by the ground station will

decrease as the difference between the latitude and longitude of the ground station and the satellite's

ground 3ero increases This occurs for four reasonsE

o

!s the angles of a3imuth and ele"ation )the loo& angles+ between the satellite and the groundstation decrease from 90 degrees, the distance between transmitter and the recei"er increases

so that the &ree space path loss also increases

o !t the same time the distance between the satellite and the ground station also increases as

the surface of the 6arth cur"es away from the ground 3ero point causing a further increase in

the path loss

o The decrease in the loo& angles also causes the signal path through the lower atmosphere to

increase resulting in greater attenuation of the signal

o !t great distances from the satellite's ground 3ero position the loo& angles will be "eryshallow and signals will be sub=ect to interference or bloc&ing from obstructions such as

mountains, buildings and trees

?or these reasons the signal co"erage by geostationary satellites becomes progressi"ely worse at

higher and lower latitudes becoming unusable in the polar regions

Signals and noise

! &ey limiting factor in determining the performance of a communications lin& is the amount of

noise in the recei"ing system, sometimes called the noise &loor which sets the fundamental lower

limit to the signal le"el necessary for extracting the transmitted message from the noise 4n general

terms, the greater the noise, the greater the signal le"el has to be to a"oid being lost in the noise,

howe"er modern signal processing techniues enable signals to be extracted from well below the

noise le"el The noise comes from two main sources, antenna noise which is the unwanted

bac&ground microwa"e radiation, solar and cosmic rays pic&ed up by the antenna and the thermal,

interference and other impulse noise generated in the recei"er electronic circuits

Noise and ;andwidthE The amount of noise in a communications channel also depends on the

bandwidth of the channel Random noise tends to be spread across a "ery wide spectrum and the broader the channel bandwidth, the more of this noise it will contain

The Thermal Noise +ower N at a gi"en temperature T within a system with bandwidth ; is gi"en

byE







N 1 k #T ;

.hereL

k # is ;olt:mann-s constant 1 $.7$ @ $E ?27 atts 3 kH: 1 ? 22. d; 3 kH:

The Noise Temperature, measured in degrees Gel"in, is a con"enient measure for uantifying the

effect of the noise and it allows the total effect of all the contributors to the noise to be calculatedsimply by adding together the indi"idual temperatures of each contributor 4t is the thermal

eui"alent of the noise source or sources and not necessarily an actual temperature The thermal

noise generated within the recei"ing euipment is the biggest factor and recei"er is often cooled to a

"ery low temperature, close to absolute 3ero, to minimise this noise

The Signal to Noise =atio, )specified in d$+, at any point in a communications lin& is the ratio

between the signal le"el at that point and the le"el of the le"el of the bac&ground noise >ote that

when the signal le"el is below the noise le"el the ratio will be negati"e

Noise /igure and Sensitiity0 The %ensiti"ity of a radio recei"er is the minimum detectable input

signal le"el necessary to obtain a gi"en output signal to noise ratio 4n satellite systems, the measureof recei"er's capability to handle low le"el signals is not usually specified as a signal le"el, but rather

as a noise figure )specified in d$+ which is the amount of noise added to the signal by the recei"ing

antenna and the recei"er electronics The recei"er sensiti"ity, can also be specified as a /igure o&"erit which is the ratio of its gain to noise temperature or G3T where G is the gain and T is the

noise temperature

:ther Noise Sources include interference from other external electrical signals or discharges,

crosstal& which is interference from ad=acent parts of the communications system and

intermodulation noise due to non#linearities in the system's signal processing which cause two or

more freuencies in the signal to create other freuencies which did not exist in the original signal

'ink ;udget

The lin& budget is an aid to specifying the reuired performance of the ma=or components which

ma&e up the communications lin&

The &ey parameters areE

o

+r@, the minimum signal leel that the recei"er can distinguish abo"e bac&ground noise

o The /ree Space +ath 'oss ' between the transmitter and the recei"er 4t is not due to

attenuation of the signal, but to the dispersion or spreading of the signal as it radiates

outwards and is represented by the in"erse suare law which indicates the reduction in

radiated signal strength as the distance increases The path loss is also proportional to the

suare of the freuency

Thus the /ree Space +ath 'oss ' is gi"en byE

' 1 (I 5 d 3 F )2 J (I 5 d & 3 c )2

.here

d is the distance between the transmitter and the recei"er





F is the wa"elength of the signal

& is the freuency of the signal

c is the "elocity of light in a "acuum

>ote that the free space path loss is related to number of wa"elengths tra"ersed

The the &ree space path loss &or a geostationary satellite is on the order o& 2EE d; (or a&actor o& $E2E).

o There are also Attenuation losses A due to atmospheric conditions such as rain which absorb

energy from the radiated signal as well as other miscellaneous efficiency or resisti"e losses in

the system transmission channel

The transmitter power +t@ and the transmitter and recei"er antenna gains Gt@ and Gr@ must be

dimensioned to compensate for the path loss and other losses in the system to ensure that there isadeuate signal strength at the recei"er to reco"er the message from the bac&ground noise

!ll factors specified in logarithmic form )d$+

4n its simplest terms the 'ink ;udget is represented by the following euationE

+r@ 1 +t@ J Gt@ J Gr@ ? ' ? A

>ote that the antenna gain and the path loss are both proportional to the suare of the freuency, )but

with different proportionality factors+ so that increasing the transmitter freuency impro"es the

antenna gain, but also increases the path loss

The signal le"els shown in the following path loss diagram are typical of a satellite lin& The signal

power radiated by the 6arth station is about (0 d$ )1000 times+ greater than the signal power

radiated by the satellite and the 6arth station is able to recei"e wea&er signals )and extract them from

the noise+ with le"els of more than 0 d$ )100 times+ lower than the satellite can handle

>ote that the ground station transmitting antenna gain is different from the recei"ing antenna gain

e"en though this is the same antenna This is because the uplin& freuency is higher than the

downlin& freuency $y contrast the satellite transmitter and recei"er antennas ha"e the same gain

This is because they use different antennas





Ground Stations

$ecause of si3e, weight and power supply restrictions, satellites are typically only euipped with

meager resources but fortunately, the lin& budget abo"e shows that this can be counterbalanced byha"ing "ery well endowed ground stations Thus "ery high power transmitters and "ery sensiti"e

recei"ers feeding through "ery large antennas on the ground compensate for low sensiti"ity recei"ers

and low power transmitters feeding through small antennas on the satellites

The noise figure of a modern satellite ground station recei"er is typically less than 1 d$, whereas the

noise figure for a satellite on board recei"er may be around 10 d$ !t the same time the satellite

transmitter power may be less than 10 .atts, while the power output of its related ground station

could be tens of &ilo.atts

Dnless the satellite is in a perfect geostationary orbit, the ground station antenna must be steerable to

trac& it across the s&y



Ha:ards

4t's tempting to thin& of space as a

benign "acuum, but in reality it can

be a hostile en"ironment

o The *an Allen =adiation

;elt is a region of high energycharged particles mo"ing at

speeds close to the speed of

light encircling the 6arth

which can damage solar cells,

integrated circuits, and

sensors and shorten the life of

a satellite or spacecraft

4t is toroidal in shape and centred along the

earth's magnetic euator with intensity

diminishing towards the poles and extendingfrom the upper atmosphere through the

magnetosphere, or e@osphere 2eld in place

by the 6arth's magnetic field, the particle

field "aries in si3e with solar conditions from

time to time but generally extends from an altitude of about -00 miles to (7,000 miles )1,000 &ms to -0,000

&ms+ 8onsidered as a single belt of "arying intensity, the particles are concentrated roughly into two layers

which o"erlap the A6: and 6: orbits

The inner layer which extends between altitudes of about 1000 and (000 miles )1-00 and

*00 &ms+ contains mainly protons with some electrons and is thought to ha"e been created

by the collisions of cosmic rays with atoms in the upper atmosphere

The outer layer is composed mainly of electrons, which are responsible for the Aurora;orealis in the polar regions, and are belie"ed to ha"e originated from the atmosphere and

from solar wind, the continuous flow of particles emitted by the %un in all directions $oth

radiation belts additionally contain smaller amounts of other nuclei, such as alpha particles

The upper layer is much larger than the inner layer extending between *000 and 1,500 miles

)1(,000 and 0,000 &ms+ and its si3e fluctuates widely as its particle population increases and

decreases as a conseuence of geomagnetic storms triggered by magnetic field and plasma

disturbances produced by the %un 4t has also been claimed that the particles are the result of

testing nuclear weapons

Recently a third radiation #elt was disco"ered using more sensiti"e instruments !

temporary phenomenon, it was a thinner later separated from the inner edge of the outer layer

which later merged bac& into the outer layer The creation and re#absorbing of this third layer

was said to be caused by a mass coronal e=ection from the %un, )! massi"e burst of solar

wind+

The Can !llen belts can pose a se"ere danger to satellites and spacecraft, with ha3ards

ranging from minor communications anomalies to the complete failure of critical systems Tominimise potential problems due to radiation, satellite orbits are designed as far as possible to

a"oid the Can !llen radiation belts and sensiti"e electronic components must be protected by

shielding if their orbit spends significant time in the radiation belts %olar cells howe"er are

*an Allen =adiation ;elts

%ourceE >!%!

The orange coloured regions are toroidal shaped =adiation;elts circling the 6arth

The lines represent the %arth-s "agnetic /ield



particularly "ulnerable to radiation damage since they depend for their operation on capturing

the %un's radiation and are therefore difficult to shield from other radiation sources

4t goes without saying that the Can !llen Radiation $elt is also dangerous to human life

%ee *an Allen History

o Temperature %nironment

%atellites operate in extreme thermal conditions with their surface temperatures ranging from

#150�8 to 150�8 depending on whether the surface is in direct sunlight or in the shade

and its electronic components are "ulnerable to permanent damage at both of these extremes

The threat is compounded because of the possibility of further structural and fatigue problems

due to the high temperature gradient across the satellite body as well as the deep repetiti"e

temperature cycling as the satellite changes its attitude with respect to the %un These latter

two effects howe"er can also be harnessed to pro"ide the means for mitigating the extreme

effects by re#distributing the heat and e"ening out the temperature

o Collision with Space <e#ris

The possibility of a collision with space debris is becoming a real problem for satellites

$esides the presence of micrometeorites, the space around the 6arth is becoming cluttered

with spent roc&et stages, old inacti"e satellites, lost tools and components, fragments from

disintegration of other space structures, erosion, and collisions The issue is especially

problematic in geostationary orbits )B6:+, where the number of a"ailable orbital slots is

limited with many satellites sharing the same orbital path, often clustered o"er the primaryground target footprints

!s of 009, the D% %trategic 8ommand was trac&ing about 19,000 pieces of debris larger

than inches )5 cm+, with a further estimated total of o"er -00,000 pieces smaller than 0

inches )1 cm+ of which (00,000 pieces were circulating below an altitude of 15 miles )00

&m+

These pieces may be small but space =un& is usually tra"elling at relati"e speeds of (0,000

mph or 50,000 &phor more with enormous &inetic energy capable of doing catastrophic

damage

%ee also +ioneering Communications Satellites and G+S Satellite Naigation

http://www.mpoweruk.com/history.htm#van_allen



http://www.mpoweruk.com/history.htm#gps

http://www.mpoweruk.com/history.htm#van_allen


http://www.mpoweruk.com/history.htm#gps

satellite technologies-eirp calc

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