Satellite Technologies The surprise launch of Russia's Sputnik, the world’s first artificial 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 ofusing the technology for peaceful applications including the possibility that Arthur C. Clarke's dream of world#wide communications based on Geostationary Satellitesmight 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 antennas4t did not ha"e a recei"e r! third battery powered the temperature regulation systemTra"elling at 1*,000 mph )9,000 &ph or *,100 ms+, it circl ed the 6arth once e"ery 9-minutes emitting beeping radio signals from its near omnidirectional antennas at as it went4t 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 sighttransmission of signals between a transmitter and a remote recei"er or recei"ers on the ground "ia a transpondermounted in a satellite orbiting high abo"e the 6arth such that it can be seen by both the transmitter and the recei"er8reating 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 maintenanceRoc&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 Satellitespage for descriptions of how Telstar Syncom!ntelsat,"olniyaand AT Ssatellites rose to this challengeOr#its and Communications Sputnik $ ;ublic <omain
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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
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
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
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
m is the mass of the satellite
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
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
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
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
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
.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
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
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
! 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
<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
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
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
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
.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
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
>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
$ecause of si3e, weight and power supply restrictions, satellites are typically only euipped with
meager resources but fortunately, the lin& budget abo"e shows that this can be counterbalanced byha"ing "ery well endowed ground stations Thus "ery high power transmitters and "ery sensiti"e
recei"ers feeding through "ery large antennas on the ground compensate for low sensiti"ity recei"ers
and low power transmitters feeding through small antennas on the satellites
The noise figure of a modern satellite ground station recei"er is typically less than 1 d$, 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
;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