Department of ECE

Lecture Notes
EC1015 - SATELLITE COMMUNICATION
Unit I Overvie of Sate!!ite S"stems# Or$its an% Launc&in' Met&o%s
Communication Sate!!ite(-
A communications satellite (Comsat) is an artificial satellite stationed in space for the purposes of
telecommunications. Modern communications satellites use geostationary orbits, Molniya orbits or
low polar Earth orbits. They are also used for mobile applications such as communications to ships
and planes, for which application of other technologies, such as cable, are impractical or
impossible.
U.. military M!"TA# communications satellite
Ear!" Missions(-
$
The first satellite e%uipped with on&board radio&transmitters was the o'iet putni( $, launched in
$)*+. The first American satellite to relay communications was pro,ect score in $)*-, which used a
tape recorder to store and forward 'oice messages. !t was used to send a Christmas greeting to the
world from .resident Eisenhower.
/AA launched an Echo satellite in $)012 the $11&foot alumini3ed .ET film balloon ser'ed as a
passi'e reflector for radio communications. Courier $4, (built by .hilco) also launched in $)01,
was the world5s first acti'e repeater satellite.
Telstar was the first acti'e, direct relay communications satellite. !t was placed in an elliptical orbit
(completed once e'ery 6 hours and 7+ minutes), rotating at a 8*9 angle abo'e the e%uator.
The first truly geostationary satellite launched in orbit was the yncom 7, launched on August $),
$)08. !t was placed in orbit at $-19 east longitude, o'er the !nternational :ate "ine. !t was used
that same year to relay tele'ision co'erage on the $)08 ummer ;lympics in To(yo to the United
tates, the first tele'ision transmission sent o'er the .acific ;cean.
hortly after yncom 7, !ntelsat !, a(a Early 4ird, was launched on April 0, $)0* and placed in
orbit at 6-9 west longitude. !t was the first geostationary satellite for telecommunications o'er the
Atlantic ;cean.
)eostationar" Sate!!ites(-
A satellite in a geostationary orbit appears to be in a fi<ed position to an earth based obser'er. A
geostationary satellite re'ol'es around the earth at a constant speed once per day o'er the e%uator.
The geostationary atellite is useful for communication applications that uses ground based
antennas, which must be directed toward the satellite, can operate effecti'ely without the need for
e<pensi'e e%uipment to trac( the satellite5s motion.
Lo Eart& Or$itin' Sate!!ites(-
A "ow Earth ;rbit ("E;) typically is a circular orbit about 811 (ilometers abo'e the earth5s
surface and, correspondingly, a period (time to re'ol'e around the earth) of about )1 minutes.
4ecause of their low altitude, these satellites are only 'isible from within a radius of roughly $111
(ilometers from the sub&satellite point. !n addition, satellites in low earth orbit change their
6
position relati'e to the ground position %uic(ly. o e'en for local applications, a large number of
satellites are needed if the mission re%uires uninterrupted connecti'ity.
"ow earth orbiting satellites are less e<pensi'e to position in space than geostationary satellites
and, because of their closer pro<imity to the ground, re%uire lower signal strength.
A group of satellites wor(ing in concert thus is (nown as a satellite constellation. Two such
constellations which were intended for pro'ision for hand held telephony, primarily to remote
areas, were the !ridium and =lobalstar. The !ridium system has 00 satellites.
!t is also possible to offer discontinuous co'erage using a low Earth orbit satellite capable of
storing data recei'ed while passing o'er one part of Earth and transmitting it later while passing
o'er another part. This will be the case with the CACA:E system of Canada5s cassiope
communications satellite.
Lo *o!ar Eart& Or$it Sate!!ites(-
As mentioned, geostationary satellites are constrained to operate abo'e the e%uator. As a
conse%uence, they are not always suitable for pro'iding ser'ices at high latitudes> for at high
latitudes a geostationary satellite may appear low on (or e'en below) the hori3on, affecting
connecti'ity and causing multipathing (interference caused by signals reflecting off the ground into
the ground antenna). The first satellite of Molniya series was launched on April 67, $)0* and was
used for e<perimental transmission of T? signal. The Molniya orbit is highly inclined,
guaranteeing good ele'ation o'er selected positions during the northern portion of the orbit.
(Ele'ation is the e<tent of the satellite5s position abo'e the hori3on. Thus a satellite at the hori3on
has 3ero ele'ation and a satellite directly o'erhead has ele'ation of )1 degrees).
@urthermore, the Molniya orbit is so designed that the satellite spends the great ma,ority of its time
o'er the far northern latitudes, during which its ground footprint mo'es only slightly. !ts period is
one half day, so that the satellite is a'ailable for operation o'er the targeted region for eight hours
e'ery second re'olution. !n this way a constellation of three Molniya satellites (plus in&orbit
spares) can pro'ide uninterrupted co'erage.
Molniya satellites are typically used for telephony and T? ser'ices o'er #ussia. Another
application is to use them for mobile radio systems (e'en at lower latitudes) since cars tra'elling
7
through urban areas need access to satellites at high ele'ation in order to secure good connecti'ity,
e.g. in the presence of tall buildings.
App!ications(-
Te!ep&on"(-
The first and historically the most important application for communication satellites is in
international telephony. @i<ed&point telephones relay calls to an earth station, where they are then
transmitted to a geostationary satellite. An analogous path is then followed on the downlin(. !n
contrast, mobile telephones (to and from ships and airplanes) must be directly connected to
e%uipment to uplin( the signal to the satellite, as well as being able to ensure satellite pointing in
the presence of disturbances, such as wa'es onboard a ship.
Sate!!ite Te!evision an% +a%io(-
Tele'ision became the main mar(et, its demand for simultaneous deli'ery of relati'ely few signals
of large bandwidth to many recei'ers being a more precise match for the capabilities of
geosynchronous comsats. Two satellite types are used for /orth American tele'ision and radio>
:irect 4roadcast atellite (:4), and
@i<ed er'ice atellite (@).
A direct broadcast satellite is a communications satellite that transmits to small :4 satellite
dishes (usually $- to 68 inches in diameter). :irect broadcast satellites generally operate in the
upper portion of the microwa'e Au band. :4 technology is used for :TB&oriented (:irect&To&
Bome) satellite T? ser'ices, such as :irecT?, :!B /etwor(.
@i<ed er'ice atellites use the C band, and the lower portions of the Au bands. They are normally
used for broadcast feeds to and from tele'ision networ(s and local affiliate stations (such as
program feeds for networ( and syndicated programming, li'e shots, and bac(hauls), as well as
being used for distance learning by schools and uni'ersities, business tele'ision (4T?),
?ideoconferencing, and general commercial telecommunications. @ satellites are also used to
distribute national cable channels to cable T? headends.
@ satellites differ from :4 satellites>
@ ha'e a lower #@ power output than the :4.
8
@ re%uires a much larger dish for reception (7 to - feet in diameter for Au band, and
$6 feet on up for C band).
@ use linear polari3ation for each of the transpondersC #@ input and output where as :4
satellites use circular polari3ation.
@ree&to&air satellite T? channels are also usually distributed on @ satellites in the Au band.
Mo$i!e Sate!!ite Tec&no!o'ies(-
!nitially a'ailable for broadcast to stationary T? recei'ers, by 6118 popular mobile direct
broadcast applications made their appearance with that arri'al of two satellite radio systems in the
United tates> irius and DM atellite #adio Boldings. ome manufacturers ha'e also introduced
special antennas for mobile reception of :4 tele'ision. Using =. technology as a reference,
these antennas automatically re&aim to the satellite no matter where or how the 'ehicle (that the
antenna is mounted on) is situated. uch mobile :4 antennas are also used by Eet4lue Airways
for :irecT? which passengers can 'iew on&board on "C: screens mounted in the seats.
Amateur +a%io(-
Amateur radio operators ha'e access to the ;CA# satellites that ha'e been designed specifically
to carry amateur radio traffic. Most such satellites operate as spaceborne repeaters, and are
generally accessed by amateurs e%uipped with UB@ or ?B@ radio e%uipment and highly
directional antennas such as Fagis or dish antennas. :ue to the limitations of ground&based
amateur e%uipment, most amateur satellites are launched into fairly low Earth orbits, and are
designed to deal with only a limited number of brief contacts at any gi'en time. ome satellites
also pro'ide data&forwarding ser'ices using the AD.6* or similar protocols.
Sate!!ite ,roa%$an%(-
!n recent years, satellite communication technology has been used as a means to connect to the
!nternet 'ia broadband data connections. This can be 'ery useful for users who are located in 'ery
remote areas, and cannot access a wireline broadband or dialup connection.
*
-re.uenc" ,an%s for Sate!!ite Communication(-
/&at is C ,an%0
C 4and is the original fre%uency allocation for communications satellites.
C&4and uses 7.+&8.6=B3 for downlin( and *.)6*&0.86*=h3 for uplin(.
The lower fre%uencies used by C 4and perform better under ad'erse weather conditions than the
Au band or Aa band fre%uencies.
C ,an% 1ariants
light 'ariations of C 4and fre%uencies are appro'ed for use in 'arious parts of the world.
,an% T2 -re.uenc" +2 -re.uenc"
E<tended C 4and *.-*1 & 0.86* =B3 7.06* & 8.611 =B3
uper E<tended C&4and *.-*1 & 0.+6* =B3 7.811 & 8.611 =B3
!/AT C&4and 0.+6* & +.16* =B3 8.*11 & 8.-11 =B3
.alapa C&4and 0.86* & 0.+6* =B3 7.811 & 7.+11 =B3
#ussian C&4and *.)+* & 0.8+* =B3 7.0*1 & 8.$*1 =B3
"M! C&4and *.+6*1 & 0.16* =B3 7.+11 & 8.111 =B3
0
C ,an% Dis&es
C 4and re%uires the use of a large dish, usually 0C across. C 4and dishes 'ary between 7C and )C
across, depending upon signal strength.
4ecause C 4and dishes are so much larger than Au and Aa 4and dishes, a C 4and dish is
sometimes referred to in friendly ,est as a 4U: (4ig Ugly :ish).
/&at is 3u $an%0
The Au band (Aurt3&under band) is primarily used for satellite communications, particularly for
editing and broadcasting satellite tele'ision. This band is split into multiple segments bro(en down
into geographical regions, as determined by the !TU (!nternational Telecommunication Union).
The Au band is a portion of the electromagnetic spectrum in the microwa'e range of fre%uencies
ranging from $$.+ to $6.+=B3. (downlin( fre%uencies) and $8 to $8.*=B3 (uplin( fre%uencies).
The most common Au band digital reception format is :?4 (main profile 'ideo format) .'s the
studio profile digital 'ideo format or the full&blown :igicipher !! 8:T? format.
The first commercial tele'ision networ( to e<tensi'ely utili3e the Au 4and for most of its affiliate
feeds was /4C, bac( in $)-7.
The !TU #egion 6 segments co'ering the ma,ority of the Americas are between $$.+ and $6.6
=B3, with o'er 6$ @ /orth American Au&band satellites currently orbiting.
Each re%uires a 1.-&m to $.*&m antenna and carries twel'e to twenty four transponders, of which
consume 61 to $61 watts (per transponder), for clear reception.
The $6.6 to $6.+ =B3 segment of the Au 4and spectrum is allocated to the broadcasting satellite
ser'ice (4). These direct broadcast satellites typically carry $0 to 76 transponders.
Each pro'ides 6+ MB3 in bandwidth, and consumes $11 to 681 watts each, accommodating
recei'er antennas down to 8*1 mm ($- inches ).
+
The !TU #egion $ segments of the Au spectrum represent Africa and Europe ($$.8* to $$.+ =B3
band range and $6.* to $6.+* =B3 band range) is reser'ed for the fi<ed satellite ser'ice (@),
with the uplin( fre%uency range between $8.1 and $8.* =B3).
3u ,an% Difficu!ties
Ghen fre%uencies higher than $1 =B3 are transmitted and recei'ed used in a hea'y rain fall area, a
noticeable degradation occurs, due to the problems caused by and proportional to the amount of
rain fall (commonly (nown as (nown as Hrain fadeH).
This problem can be combatted, howe'er, by deploying an appropriate lin( budget strategy when
designing the satellite networ(, and allocating a higher power consumption to o'ercome rain fade
loss. !n terms of end&'iewer T? reception,
it ta(es hea'y rainfalls in e<cess of $11 mm per hour to ha'e a noticeable effect.
The higher fre%uency spectrum of the Au band is particularly susceptible to signal degradation&
considerably more so than C band satellite fre%uency spectrum, though the Au band is less
'ulnerable to rain fade than the Aa band fre%uency spectrum.
A similar phenomena, called Hsnow fadeH (when snow accumulation significantly alters the focal
point of your dish) can also occur during Ginter eason.
Also, the Au band satellites typically re%uire considerably more power to transmit than the C band
satellites. Bowe'er, both Au and Aa band satellite dishes to be smaller ('arying in si3e from 6C to
*C in diameter.)
3u ,an% Sate!!ite Service Don!in4 Usa'e -re.uenc" +an'e
The Au band downlin( uses fre%uencies between $$.+ and $6.+=B3.
The Au band downlin( fre%uencies are further subdi'ided according to their assigned use>
3u ,an% Usa'e Don!in4
@i<ed atellite er'ice $$.+ & $6.6=B3
4roadcast atellite er'ice $6.6 & $6.+=B3
er'ices that can be found on the Au&band include educational networ(s, business networ(s, sports
bac(hauls, tele& conferences, mobile news truc( feeds, international programming, and 'arious
C.C (ingle Channel .er Carrier) transmissions of analog audio, as well as @M audio ser'ices.
!f you already ha'e a operational C&band system in place, you can retrofit it to accept Au band
fre%uencies.
!n order to do so, you will need to obtain a Au&band "/4 as well as a CIAu band feed&horn, plus
some coa< cable for your Au&band "/4.
-
As for the coa< cable recommended& #=&0 is optimal for low loss in the )*1&$8*1 fre%uency
range& what Au&band "/4 processes. Bowe'er, if #=&*) is your only 'iable option, itCll wor( in a
pinch.
3u ,an% Dis& Antenna Compati$i!it"
!if you ha'e a solid dish, you should ha'e no problem con'erting from C band, to Au band.
Bowe'er, with a mesh dish& if the HholesH in the mesh are greater than a %uarter inch, the chances
of computability are not in your fa'or, due to the fact that your dish wonCt reflect Au&band signals
properly.
Therefore, youCll want to strongly consider upgrading to either a solid dish, or a mesh dish in
which the hole si3e under $I8H, and ideally youCll want a dish that is $ piece (or at least 'ery few
pieces)2 as 8 section dish is more optimal than an - section dish.
The fewer the sections, the more accurate your parabola shape is and thereby the more difficult it is
for your dish to become warped (the smaller the number of seams& the better). And insofar as dish
mounts go, the B6B (Bori3on&to&Bori3on) dish mount is more desirable than a polar mount.
This is due to the fact that the Au&band demands that the dish antenna system is well&targeted and
able to closely follow the orbital arc, of which the B6B mount does %uite admirably, as compared
to a polar mount. Also, bear in mind that you will be ad,usting both the a3imuth and ele'ation,
which can be a bit tric(y occasionally.
Importance of Sate!!ite Antenna Dis& *ara$o!a
The parabolic shape of your dish is of critical importance, as warpage causes signal degradation
'ia mis&reflection, seriously down&grading your o'erall system performance. ome tape and string
is all that is re%uired to do a %uic( warpage chec( and some tape.
Anchor a piece of string, stretched as tight as possible, HnorthH to HsouthH across your dish face,
edge to edge. FouCll want to do the same thing again, with another piece of string, only HeastH to
HwestH across the dish face& at )1 degree angles. 4e sure that both strings are tight&
!f the strings come together anywhere but the direct center, then your dish has sustained warp
damage and needs to be bent bac( into proper parabola shape, for optimal performance. !f they
connect in the center of your dish, li(ely that your dish is not warped.
o therefore, youCll want to use either the tri&supports or %uad supports , as they will greatly assist
in (eeping your Au&band feed&horn highly stable, e'en in high winds.
Ghen your button&hoo( feed mo'ing in the wind, your Au&band reception can can easily drop out.
4y putting guy&wires on the button&hoo( feed, youCll create the much&needed support, in the e'ent
you are not able to obtain a tri support or %uad support.
)
/&at is 3a $an%0
The Aa band uplin( uses fre%uencies between 6+.*=B3 and 7$=h3 and the downlin( uses
fre%uencies between $-.7 and $-.-=h3 and between $).+ and 61.6=h3.
Aa band dishes can be much smaller than C band dishes. Aa band dishes 'ary from 6C to *C in
diameter.
Aa band satellites typically transmit with much more power than C band satellites.
The higher fre%uencies of Aa band are significantly more 'ulnerable to signal %uality problems
caused by rainfall, (nown as rainfade
/&at is L $an%0
" band is a fe%uency range between 7)1MB3 and $.**=B3 which is used for satellite
communications and for terrestrial communications between satellite e%uipment.
The high fre%uencies utili3ed by C band, Au band, and Aa band would suffer from high signal loss
when transported o'er a copper coa< cable such as an !ntra&@acility "in(.
$1
An "/4 is used to con'ert these higher fre%uency bands to " band, which can be transmitted o'er
the !@" and processed by the !:U.
ome satellites transmit on " band, such as =. satellites
/&at is S $an%0
band is a fre%uency range from appro<imately $.** to *.6=B3 which is used for :igital Audio
#adio atellite (:A#) satellite radio systems such as irius atellite #adio and DM atellite
#adio.
band is also used by some weather and communications satellites.
In%ian Sate!!ites
Sl.No. Satellite
Launch
Date
Achievements
1. Aryabhata 19.04.1975
First Inian satellite. !rovie technolo"ical e#$erience in
builin" an o$eratin" a satellite system. Launche by
%ussian launch vehicle Intercosmos.
&. 'has(ara)I 07.0*.1979
First e#$erimental remote sensin" satellite. +arrie ,- an
micro.ave cameras. Launche by %ussian launch vehicle
Intercosmos.
/. 'has(ara)II &0.11.1901
Secon e#$erimental remote sensin" satellite similar to
'has(ara)1. !rovie e#$erience in builin" an o$eratin" a
remote sensin" satellite system on an en)to)en basis.
Launche by %ussian launch vehicle Intercosmos.
4.
Ariane !assen"er
!ayloa 1#$eriment
2A!!L13
19.0*.1901
First e#$erimental communication satellite. !rovie
e#$erience in builin" an o$eratin" a three)a#is stabilise
communication satellite. Launche by the 1uro$ean Ariane.
5.
%ohini ,echnolo"y
!ayloa 2%,!3
10.00.1979
Intene 4or measurin" in)4li"ht $er4ormance o4 4irst
e#$erimental 4li"ht o4 SL-)/5 the 4irst Inian launch vehicle.
+oul not be $lace in orbit.
*. %ohini 2%S)13 10.07.1900
6se 4or measurin" in)4li"ht $er4ormance o4 secon
e#$erimental launch o4 SL-)/.
7. %ohini 2%S)D13 /1.05.1901
6se 4or conuctin" some remote sensin" technolo"y
stuies usin" a lanmar( sensor $ayloa. Launche by the
4irst evelo$mental launch o4 SL-)/
0. %ohini 2%S)D&3 17.04.190/
Ientical to %S)D1. Launche by the secon evelo$mental
launch o4 SL-)/.
9.
Stretche %ohini
Satellite Series 2S%7SS)
13
&4.0/.1907
+arrie $ayloa 4or launch vehicle $er4ormance monitorin"
an 4or 8amma %ay astronomy. +oul not be $lace in orbit.
10. Stretche %ohini 1/.07.1900 +arrie remote sensin" $ayloa o4 8erman s$ace a"ency in
$$
Satellite Series 2S%7SS)
&3
aition to 8amma %ay astronomy $ayloa. +oul not be
$lace in orbit.
11.
Stretche %ohini
Satellite Series 2S%7SS)
+3
&0.05.199&
Launche by thir evelo$mental 4li"ht o4 ASL-. +arrie
8amma %ay astronomy an aeronomy $ayloa.
1&.
Stretche %ohini
Satellite Series 2S%7SS)
+&3
04.05.1994
Launche by 4ourth evelo$mental 4li"ht o4 ASL-. Ientical to
S%7SS)+. Still in service.
Inian National Satellite System 2INSA,3
1/. INSA,)1A 10.04.190&
First o$erational multi)$ur$ose communication an
meteorolo"y satellite $rocure 4rom 6SA. 9or(e only 4or si#
months. Launche by 6S Delta launch vehicle.
14. INSA,)1' /0.00.190/
Ientical to INSA,)1A. Serve 4or more than esi"n li4e o4
seven years. Launche by 6S S$ace Shuttle.
15. INSA,)1+ &1.07.1900
Same as INSA,)1A. Serve 4or only one an a hal4 years.
Launche by 1uro$ean Ariane launch vehicle.
1*. INSA,)1D 1&.0*.1990
Ientical to INSA,)1A. Launche by 6S Delta launch vehicle.
Still in service.
17. INSA,)&A 10.07.199&
First satellite in the secon)"eneration Inian)built INSA,)&
series. :as enhance ca$ability than INSA,)1 series.
Launche by 1uro$ean Ariane launch vehicle. Still in service.
10. INSA,)&' &/.07.199/
Secon satellite in INSA,)& series. Ientical to INSA,)&A.
Launche by 1uro$ean Ariane launch vehicle. Still in service.
19. INSA,)&+ 07.1&.1995
:as aitional ca$abilities such as mobile satellite service5
business communication an television outreach beyon
Inian bounaries. Launche by 1uro$ean launch vehicle. In
service.
&0. INSA,)&D 04.0*.1997
Same as INSA,)&+. Launche by 1uro$ean launch vehicle
Ariane. Ino$erable since 7ct 45 97 ue to $o.er bus anomaly.
&1. INSA,)&D,
;anuary
1990
!rocure in orbit 4rom A%A'SA,
&&. INSA,)&1 0/.04.1999
<ulti$ur$ose communication = meteorolo"ical satellite
launche by Ariane.
&/. INSA,)/' &&.0/.&000
<ulti$ur$ose communication ) business communication5
evelo$mental communication an mobile communication
$ur$ose.
&4. 8SA,)1 10.04.&001
1#$erimental Satellite 4or the 4irst evelo$mental 4li"ht o4
8eo)synchronous Satellite Launch -ehicle5 8SL-)D1.
&5. INSA,)/+ &4.01.&00&
,o au"ment the e#istin" INSA, ca$acity 4or communication
an broacastin"5 besies $roviin" continuity o4 the
services o4 INSA,)&+.
&*. >AL!ANA)1 1&.09.&00&
<1,SA, .as the 4irst e#clusive meteorolo"ical satellite built
by IS%7 name a4ter >al$ana +ha.la.
$6
&7. INSA,)/A 10.04.&00/
<ulti$ur$ose Satellite 4or communication an broacastin"5
besies $roviin" meteorolo"ical services alon" .ith INSA,)
&1 an >AL!ANA)1.
&0. 8SA,)& 00.05.&00/
1#$erimental Satellite 4or the secon evelo$mental test 4li"ht
o4 Inia?s )eos"nc&ronous Sate!!ite Launc& 1e&ic!e# )SL1
&9. INSA,)/1 &0.09.&00/
1#clusive communication satellite to au"ment the e#istin"
INSA, System.
/0. 1D6SA, &0.09.&004 Inia@s 4irst e#clusive eucational satellite.
/1. :A<SA, 05.05.&005
<icrosatellite 4or $roviin" satellite base Amateur %aio
Services to the national as .ell as the international
community 2:A<s3.
/&. INSA,)4A &&.1&.&005
,he most avance satellite 4or Direct)to):ome television
broacastin" services.
//. INSA,)4+ 10.07.&00*
State)o4)the)art communication satellite ) coul not be $lace
in orbit.
/4. INSA,)4' 1&.0/.&007
An ientical satellite to INSA,)4A 4urther au"ment the INSA,
ca$acity 4or Direct),o):ome 2D,:3 television services an
other communications.
/5. INSA,)4+% 0&.09.&007
Desi"ne to $rovie Direct),o)home 2D,:3 television
services5 -ieo !icture ,ransmission 2-!,3 an Di"ital
Satellite Ne.s 8atherin" 2DSN835 ientical to INSA,) 4+ .
Inian %emote Sensin" Satellite 2I%S3
/*. I%S)1A 17.0/.1900
First o$erational remote sensin" satellite. Launche by a
%ussian -osto(.
/7. I%S)1' &9.00.1991
Same as I%S)1A. Launche by a %ussian Launch vehicle5
-osto(. Still in service.
/0. I%S)11 &0.09.199/
+arrie remote sensin" $ayloas. +oul not be $lace in
orbit.
/9. I%S)!& 15.10.1994
+arrie remote sensin" $ayloa. Launche by secon
evelo$mental 4li"ht o4 !SL-.
40. I%S)1+ &0.1&.1995
+arries avance remote sensin" cameras. Launche by
%ussian <olniya launch vehicle. Still in service.
41. I%S)!/ &1.0/.199*
+arries remote sensin" $ayloa an an A)ray astronomy
$ayloa. Launche by thir evelo$mental 4li"ht o4 !SL-. Still
in service.
4&. I%S)1D &9.09.1997
Same as I%S)1+. Launche by Inia?s !SL- service. In
service.
4/. I%S)!4 7ceansat &*.05.1999
+arries an 7cean +olour <onitor 27+<3 an a <ulti)4reBuency
Scannin" <icro.ave %aiometer 2<S<%35 Launche by
Inia@s !SL-)+&5
44.
,echnolo"y 1#$eriment
Satellite 2,1S3
&&.10.&001
,echnolo"y 1#$eriment Satellite Launche by !SL-)+/ .
45. I%S)!* %esourcesat)1 17.10.&00/
Launche by !SL- ) +55 carries three camera5 names5 LISS)45
LISS)/ an A.iFS
4*. +A%,7SA, )1 05.05.&005 Launche by !SL-)+*5 carries t.o $anchromatic cameras )
$7
!AN 24ore3 an !AN 2a4t3 ) .ith &.5 meter resolution. ,he cam
mounte .ith a tilt o4 C&* e" an )5 e" alon" the trac( to
$rovie stereo ima"es.
47. +A%,7SA, ) & 10.01.&007
Launche by !SL-)+75 it is an avance remote sensin"
satellite carryin" a $anchromatic camera ca$able o4 $roviin"
scene s$eci4ic s$ot ima"eries.
40. S%1 ) 1 10.01.&007
Launche by !SL-)+75 S$ace ca$sule %ecovery 1#$eriment
2S%1)135 intene to emonstrate the technolo"y o4 an
orbitin" $lat4orm 4or $er4ormin" e#$eriments in micro"ravity
conitions. S%1)1 .as recovere success4ully a4ter 1& ays
over 'ay o4 'en"al.
49. +A%,7SA,)&A &0.04.&000 Ientical to +A%,7SA, ) &5 launche by !SL-)+9
50. I<S)1 &0.04.&000
Launche by !SL-)+9 alon" .ith +A%,7SA,)&A an other
1i"ht Nanosatellites
Keplar's Laws of Planetary Motion
Keplar devised three laws which describe the motions of the planets.
Keplar's First Law
Bodies move around the sun in elliptical orbits, with the sun at one focus. The other focus is empty.
An ellipse is basically a squashed circle. All bodies orbit in an ellipse, although some are more elliptical
than others.
The Earth's average distance from the sun in 1! million "m. #owever, at perihelion
1
it is 1$% million "m
from the sun, and at aphelion
&
, 1& million "m. T#e amount which an ellipse deviates from a perfect
circle can be measured by 'eccentricity'. The Earth has an orbital eccentricity of !.!1' which is relatively
circlular. (luto has a much more eccentric orbit, with an eccentricity of !.&, with perihelion and
apthelion of $$!! and '$!! million "m respectively.
)f you're loo"ing for loads of fun, the easiest way to construct an ellipse is by ta"ing two drawing pins,
stic"ing them into a piece of paper, wrapping a loose piece of string around them, and then using moving
a pencil around the loop, "eeping it taught at all times. *ith this method the pins represent the two
foci.
Keplar's +irst ,aw is significant in that most ancient astronomers believed that the planets moved in
circular orbits.
Keplars Second Law
The radius vector sweeps out equal areas in equal times.
This states that the line -oining the planet to the sun sweeps the same area in equal times. This means,
given Keplar's +irst ,aw, that planets orbit quic"est when they are nearest the sun and the radius vector
is smaller, than when they are furthest from the sun.
Keplar's Third Law
The time period squared is directly proportional to the distance cubed.
$8
This neat relationship was discovered by Keplar before .ewton wor"ed out what gravity was. Therefore,
Keplar was unable to give a proof. #owever/
Proof:
+udge 1/ Assume the planets have circular orbits
The planets orbit e0periencing a centripetal force towards the 1un/
+c 2 mv
&
3r
*here +c is the centripetal force, m is the planet's mass, r is the planet's distance from the 1un
This centripetal force is provided by the gravitational force of the 1un/
+g 2 45m3r
&
*here +g is the gravitational force from the 1un, 4 is the 6niversal gravitational constant and 5 is the
mass of the 1un.
+g 2 +c
27 45m3r
&
2 mv
&
3r
8ancelling m/
453r
&
2 v
&
3r 99:1;
)f the planet moves in a circular orbit, then the distance it moves in a circle is s 2 &<r, and velocity in a
circle is distance over time 27 v 2 &<r3T
27 v
&
2 $<
&
r
&
3T
&
1ub into eqn :1; and cancel r's/
453r
&
2 $<
&
r3T
&
5ultiply both sides by T
&
r
&
/
45T
&
2 $<
&
r
=
27 T
&
2 $<
&
345 0 r
=
1ince $<
&
345 is a constant for any central body >eg, the 1un?
27 T
&
r
=
1o he was right, after all.
1
The point in the Earth's orbit when it is closest to the sun >helion from helios meaning the sun?
&
*hen the Earth's furthest from the sun
Or$ita! E!ements
@igure$
$*
@igure6
Stan%ar% Or$ita! E!ements( 5Sun or$itin' o$6ect7
8 Ar'ument of *eri&e!ion
8 Eccentricit"
8 Inc!ination
8 Lon'itu%e of t&e Ascen%in' No%e
8 Semi-ma6or a9is of or$it
8 Time of peri&e!ion passa'e
Stan%ar% Or$ita! E!ements( 5Eart& or$itin' o$6ect7
(#efer to the e<planations below)
8 Ar'ument of *eri'ee
8 Eccentricit"
8 Inc!ination
8 Lon'itu%e of t&e Ascen%in' No%e
8 *erio%
$0
8 Semi-ma6or a9is of or$it
8 Time of peri'ee passa'e
T&ese e!ements are usua!!"(
(#efer to the e<planations below)
8 Ar'ument of *eri'ee
8 Eccentricit"
8 Epoc&
8 Inc!ination
8 Mean Anoma!"
8 Mean Motion
8 +i'&t Ascension of t&e Ascen%in' No%e
Definitions(
J Ar'ument of Latitu%e (not shown)> The geographic latitude of an Earth orbiting satellite at a
specific time (the Epoch), e<pressed as an angle measured from the celestial e%uator northward.
J Ascen%in' No%e (A/ in @igure 6)> The point in a satelliteCs orbit where it crosses the plane of
the celestial e%uator (or ecliptic for a sun orbiting ob,ect) going north.
J Ar'ument of *eri'ee 5*eri&e!ion7> ( in @igure 6) > The angle between the ascending node and
perigee (or perihelion for sun orbiting satellites), measured counter cloc(wise along the plane of
the orbit.
J Apo'ee 5Ap&e!ion7 (@igure $)> .oint in orbit when the satellite is farthest from the Earth (sun).
J Ce!estia! E.uator( The plane of the EarthCs e%uator pro,ected onto the celestial sphere. The
celestial e%uator is tilted 67.* degrees in relation to the plane of the EarthCs orbit (the ecliptic). The
ecliptic and the celestial e%uator cross at two points, the 'ernal e%uino< and the autumnal e%uino<.
J Ce!estia! Sp&ere> A imaginary sphere surrounding the Earth, at some arbitrary great distance,
upon which the stars are considered to be fi<ed for the purpose of position measurement. Although
it is the Earth that rotates, it appears to an obser'er on the Earth that the Celestial phere re'ol'es
around the Earth in one (sidereal) day.
J Eccentricit"# e > Balf of the distance between the foci of an ellipse di'ided by the semi&ma,or
a<is. Thin( of it as a measure of how Hout of roundH an ellipse is. An eccentricity of 1 would be a
circle.
$+
J Ec!iptic( The plane of the EarthCs orbit around the sun. The ecliptic is the apparent path of the
sun across the celestial sphere o'er the period of one year.
J Ec!iptic Latitu%e( The angle between the position of an astronomical body at the time of interest
and the plane of the ecliptic.
J Ec!iptic Lon'itu%e( The angle of an astronomical body from the 'ernal e%uino<, measured
EAT along the ecliptic.
J Epoc&( The specific time at which the position of a satellite is specified.
J )eo'rap&ic Lon'itu%e of t&e Ascen%in' No%e (not shown)> The geographic longitude EAT
of the .rime Meridian where the orbit of an Earth&orbiting satellite crosses the celestial e%uator.
:o not confuse with "ongitude of the Ascending /ode.
J Inc!ination, (i in @igure 6)> The angle between the plane of the orbit and the plane of the
celestial e%uator for Earth orbiting satellites (or the plane of the ecliptic for sun orbiting satellites).
J Lon'itu%e of t&e Ascen%in' No%e# ( in @igure 6)> The angle between the 'ernal e%uino< and
the ascending node, measured counter&cloc(wise.
J Lon'itu%e of *eri'ee (.erihelion) The angle between the 'ernal e%uino< and perigee (or perihelion) measured in
the direction of the ob,ect5s motion. !t is e%ual to the sum of the Argument of .erigee and the "ongitude of the
Ascending /ode ( K in figure 6).
J Mean Anoma!"> (Compare with True Anomaly) The angle that a satellite would ha'e mo'ed
since last passing perigee (or perihelion), assuming that the satellite mo'ed at a constant speed in a
orbit on a circle of the same area as the actual orbital ellipse. E%ual to the True Anomaly at perigee
and apogee only for elliptical orbits, or at all times for circular orbits.
J Mean Motion( The reciprocal of the .eriod, e<pressed in re'olutions per day
J Meri%ian( An imaginary line on the surface of the Earth running from the north pole to the south
pole through any gi'en point on the Earth. Also, an imaginary line on the celestial sphere running
from the /orth Celestial .ole to the outh Celestial pole directly o'er any gi'en point on the
Earth. These definitions are essentially the same, one line goes under you feet, one goes o'er your
head. The .rime Meridian is the meridian the runs through =reenwich, England (1 degrees
longitude).
8 O$!i.uit" of t&e Ec!iptic( The angle between the celestial e%uator and the ecliptic.
J *eri'ee 5*eri&e!ion7 (@igure $)> The point in an orbit when the satellite is closest to the Earth
(sun).
J *erio%> The time it ta(es the satellite to complete one orbit.
J +i'&t Ascension( A measure of the angle between the 'ernal e%uino< and a gi'en astronomical
ob,ect (star, planet, or satellite), as seen from the Earth. !n astronomy, #ight Ascension (#A) is
e<pressed in units of time. The #A is the time that elapses between the transit of the 'ernal
e%uino< across any gi'en meridian and the transit of the gi'en ob,ect across the same meridian,
$-
e<pressed in a 68 hour format. #ight Ascension can also be e<pressed as the angle between the
'ernal e%uino< and the ob,ect, measured EAT of the 'ernal e%uino< along the celestial e%uator.
J +i'&t Ascension of t&e Ascen%in' No%e 5 in @igure 6)> Another term for "ongitude of the
Ascending /ode, !t is the angle of the ascending node measured EAT of the 'ernal e%uino< along
the celestial e%uator.
J Semi-Ma6or A9is (a in @igure $)> The half of the longer of the two a<es of the orbital ellipse.
J Semi-Minor A9is (b in @igure $)> The half of the shorter of the two a<es of the orbital ellipse.
J Si%erea! Da"( A sidereal day is the amount of time it ta(es the Earth to rotate once on it a<is
relati'e to the stars. A mean sidereal day is e%ual to 1.))+6+ mean solar days, or 67 hours, *0
minutes, 8.$ seconds. The mean solar day and the mean sidereal day differ due to the fact the Earth
is orbiting the sun in 70*.6866 mean solar days, resulting in the sun mo'ing slightly across the
celestial sphere during one solar day (68 hours)
J Time of *eri'ee 5*eri&e!ion7 *assa'e( The time at which a satellite last passed perigee (or
perihelion).
J True Anoma!" ( in @igure $)> The actual angle that a satellite has mo'ed since last passing
perigee (or perihelion).
J 1erna! E.uino9( ;ne of two points where the ecliptic crosses the celestial e%uator, the other
being the Autumnal E%uino<. The ?ernal E%uino< is the point where the ecliptic crosses the
celestial e%uator with the sun passing from south to north. Unfortunately for students of astronomy,
the same term, ?ernal E%uino<, is used to describe both the .;!/T on the celestial sphere where
the crossing occurs (its meaning throughout these e<planations), A/: the M;ME/T !/ T!ME
when the crossing occurs (the first moment of spring). Ghich is the intended meaning in any gi'en
sentence must be determined by the conte<t on the statement.
Unit II )eostationar" Or$it : Space Se'ment
)eostationar" ;rbit
A 'eostationar" orbit is one in which a satellite orbits the earth at e<actly the same speed as the
earth turns and at the same latitude, specifically 3ero, the latitude of the e%uator. A satellite orbiting
in a geostationary orbit appears to be ho'ering in the same spot in the s(y, and is directly o'er the
same patch of ground at all times.
A 'eos"nc&ronous orbit is one in which the satellite is synchroni3ed with the earthCs rotation, but
the orbit is tilted with respect to the plane of the e%uator. A satellite in a geosynchronous orbit will
wander up and down in latitude, although it will stay o'er the same line of longitude. Although the
terms CgeostationaryC and CgeosynchronousC are sometimes used interchangeably, they are not the
same technically2 geostationary orbit is a subset of all possible geosynchronous orbits.
The person most widely credited with de'eloping the concept of geostationary orbits is noted
science fiction author Arthur C. Clar(e (!slands in the (y, ChildhoodCs End, #ende3'ous with
$)
#ama, and the mo'ie 611$> a pace ;dyssey). ;thers had earlier pointed out that bodies tra'eling
a certain distance abo'e the earth on the e%uatorial plane would remain motionless with respect to
the earthCs surface. 4ut Clar(e published an article in $)8*Cs Gireless Gorld that made the leap
from the =ermansC roc(et research to suggest permanent manmade satellites that could ser'e as
communication relays.
=eostationary ob,ects in orbit must be at a certain distance abo'e the earth2 any closer and the orbit
would decay, and farther out they would escape the earthCs gra'ity altogether. This distance is
7*,+-0 (ilometers (66,670 miles) from the surface.
The first geosynchrous satellite was orbited in $)07, and the first geostationary one the following
year. ince the only geostationary orbit is in a plane with the e%uator at 7*,+-0 (ilometers, there is
only one circle around the world where these conditions obtain. This means that geostationary Creal
estateC is finite. Ghile satellites are in no danger of bumping in to one another yet, they must be
spaced around the circle so that their fre%uencies do not interfere with the functioning of their
nearest neighbors.
)eostationar" Sate!!ites
There are 6 (inds of manmade satellites in the hea'ens abo'e> ;ne (ind of satellite ;#4!T the
earth once or twice a day, and the other (ind is called a communications satellite and it is .A#AE:
in a TAT!;/A#F position 66,711 miles (7*,)11 (m) abo'e the e%uator of the TAT!;/A#F
earth.
A type of the orbiting satellite includes the space shuttle and the international space station which
(eep a low earth orbit ("E;) to a'oid the deadly ?an Allen radiation belts.
The most prominent satellites in medium earth orbit (ME;) are the satellites which comprise the
=";4A" .;!T!;/!/= FTEM or =. as it is called.
The =lobal .ositioning ystem
The global positioning system was de'eloped by the U.. military and then opened to ci'ilian use.
!t is used today to trac( planes, ships, trains, cars or literally anything that mo'es. Anyone can buy
a recei'er and trac( their e<act location by using a =. recei'er.
=. satellites orbit at a height of about
$6,111 miles ($),711 (m) and orbit the

About 68 =. satellites orbit the earth e'ery $6
hours.
61
earth once e'ery $6 hours.
These satellites are tra'eling around the earth at speeds of about +,111 mph ($$,611 (ph). =.
satellites are powered by solar energy. They ha'e bac(up batteries onboard to (eep them running
in the e'ent of a solar eclipse, when thereCs no solar power. mall roc(et boosters on each satellite
(eep them flying in the correct path. The satellites ha'e a lifetime of about $1 years until all their
fuel runs out.
)eostationar" Sate!!ites
=eostationary or communications satellites are .A#AE: in space 66,711 miles (7*,)11 (m) abo'e
the e%uator of the TAT!;/A#F earth. =eostationary satellites are used for weather forecasting,
satellite T?, satellite radio and most other types of global communications.
@ig A
@ig 4
@ig A Communications satellite in a stationary position or slot high abo'e the earth.
@ig 4 atellite dish or recei'er installed on a house. These dishes point to a geostationary satellite
At e<actly 66,711 miles abo'e the e%uator, the force of gra'ity is cancelled by the centrifugal force
of the rotating uni'erse. This is the ideal spot to par( a stationary satellite.
6$

At e<actly 66,111 miles (7*,)11 (m) abo'e the
e%uator, the earthCs force of gra'ity is canceled
by the centrifugal force of the rotating uni'erse.
This is the ideal location to par( a stationary
satellite. The signal to the satellite is 'ery, 'ery
precise and any mo'ement of the satellite would
cause a loss of the signal.
un outages affect a geostationary satellite
=eostationary satellites are fantastic means of communication e<cept for one little problem called
U/ ;UTA=E. These sun outages happen during March and eptember when the sun passes the
e%uator. Bere is a %uote from the boo( Satellite Technology>
HThe ele'ated temperature of the sun causes it to transmit a high&le'el electrical noise signal to
recei'ing systems whene'er it passes behind the satellite and comes within the beams of the
recei'er antennas. The increase in noise is so se'ere that a signal outage usually results. The length
and number of the outages depends on the latitude of the earth station and the diameter of the
antenna. At an a'erage latitude of 819 in the continental United tates, and a $1&meter antenna, the
outages occur o'er 0 days with a ma<imum duration of - minutes each day. Gith a less directional
7&meter antenna, the outages occur o'er $* days, with a ma<imum duration of 68
minutes.H(Satellite Technology, p. $7).
This is ob'iously 'ery embarrassing to the heliocentric people because the sun is not supposed to
mo'e. The sun does move howe'er, and twice a year it is o'er the e%uator.
66
The sun mo'es across the e%uator twice a year gi'ing us the 'ernal (spring) and fall (autumnal)
e%uino<es.
J 6 times each year the sun passes the e%uator as it ma(es it north&south spiral.
J At that time, the sun lies on the celestial e%uator. The word e%uino< refers to the fact that,
on this day, the night is e%ual to the day> each is twel'e hours long. The sun is directly abo'e the
e%uator, so its rays fall 'ertically down.
J Unfortunately the stationary satellites eclipses the sun and that causes electrical noise or
interference to the broadcasting signals.
The Eesuits forgot to change the dictionaryLL
;b'iously the Eesuits forgot to change the definition of the word EMU!/;D in the English
dictionary because it still gi'es the true scientific definition of the word with the sun M;?!/=
across the e%uator 6 times each year>
HEither of the two times during a year when the sun crosses the celestial e%uator and when the
length of day and night are appro<imately e%ual2 the 'ernal e%uino< or the autumnal
e%uino<.H(Webster's Third New International Dictionary).
.anAmatCs :escription of sun outagesLL
:escription
.anAmatCs commercial communications satellites are geostationary, and therefore ha'e orbits that
lie near the e%uatorial plane. :uring the spring and fall e%uino<es, the sun also passes close to this
plane. As seen from the ground, the sun seems to pass behind the satellites once per day. :uring
the time when both the satellite and the sun are in the ground stationCs field of 'iew, the #@ noise
energy from the sun can o'erpower the signal from the satellite. !t is this loss or degradation of
communications traffic from the satellite that is referred to as sun fade, sun transit or sun outage
(see diagram).
67
The duration of the sun outage depends on se'eral things such as> the beam width or field of 'iew
of the recei'ing ground antenna, the apparent radius of the sun as seen from the Earth (about
1.6*9), the #@ energy gi'en off by the sun, the transmitter power of the satellite, the gain and I/
performance of the ground station recei'e e%uipment, along with other factors. All this can affect
whether a ground station will e<perience a complete loss of signal or only a tolerable degradation
in signal %uality. The e<act point at which sun outage begins and ends is difficult to determine
since it is a gradual transition. The gain of an antenna falls off sharply outside the 7d4 beam width,
but it does not immediately go to 3ero. Therefore, if the sun is ,ust outside the antennaCs beam
width, it can still contribute noise and degrade system performance. This ma(es it difficult to
define e<actly what conditions constitute a sun outage.
Bow the program wor(s
To aid with sun outage predictions, a parameter called outage angle is defined for the ground
station. ;utage angle is defined as the ma<imum separation angle (measured from the ground
station antenna) between the satellite and the sunCs center, that results in a sun outage. !n other
words, if the separation between the satellite and sun is less than the specified outage angle, then
the station is said to be e<periencing a sun outage. ;therwise, the station is not e<periencing a sun
outage (see diagram).
tationary satellites need 'ery small motors to (eep them in their assigned slotLL
According to the heliocentric theory, the earth is mo'ing at about $,111 mph at the e%uator. !f the
geostationary satellites were mo'ing, they would ha'e to mo'e at a speed of about +,111 mph to
maintain a stationary orbit abo'e a fi<ed point on the earth. That is about the same speed as the
=. satellites that orbit the earth twice a day. Bowe'er, =. satellites are e%uipped with a roc(et
engine to maintain their orbit.
68
=eostationary satellite diagram.
Clic( on image to enlarge.

!mage of a =. satellite. mall roc(et boosters on
each satellite (eep it flying in the correct path.
The satellites ha'e a lifetime of about $1 years
until all their fuel runs out.
Spin an% T&ree-A9is Sta$i!i;ation

Spin and Three-Axis Stabilization
Credits - NASA
6*
Credits - NASA
pin stabili3ation and three&a<is stabili3ation are two methods that are used to orient satellites.
Gith spin stabili3ation, the entire spacecraft rotates around its own 'ertical a<is, spinning li(e a
top. This (eeps the spacecraftCs orientation in space under control. The ad'antage of spin
stabili3ation is that it is a 'ery simple way to (eep the spacecraft pointed in a certain direction. The
spinning spacecraft resists perturbing forces, which tend to be small in space, ,ust li(e a gyroscope
or a top. :esigners of early satellites used spin&stabili3ation for their satellites, which most often
ha'e a cylinder shape and rotate at one re'olution e'ery second. A disad'antage to this type of
stabili3ation is that the satellite cannot use large solar arrays to obtain power from the un. Thus, it
re%uires large amounts of battery power. Another disad'antage of spin stabili3ation is that the
instruments or antennas also must perform NdespinO maneu'ers so that antennas or optical
instruments point at their desired targets. pin stabili3ation was used for /AACs .ioneer $1 and
$$ spacecraft, the "unar .rospector, and the =alileo Eupiter orbiter.

Gith three&a<is stabili3ation, satellites ha'e small spinning wheels, called reaction wheels or
momentum wheels, that rotate so as to (eep the satellite in the desired orientation in relation to the
Earth and the un. !f satellite sensors detect that the satellite is mo'ing away from the proper
orientation, the spinning wheels speed up or slow down to return the satellite to its correct position.
ome spacecraft may also use small propulsion&system thrusters to continually nudge the
spacecraft bac( and forth to (eep it within a range of allowed positions. ?oyagers $ and 6 stay in
position using 7&a<is stabili3ation. An ad'antage of 7&a<is stabili3ation is that optical instruments
and antennas can point at desired targets without ha'ing to perform NdespinO maneu'ers.

60
Station-4eepin' in LEO
tation&(eeping is necessary for ob,ects such as the !nternational pace tation, and for satellites
for which a precise (nowledge of their orbital position is necessary, e.g. earth obser'ation
satellites. The !nternational pace tation has an operational altitude abo'e Earth between 771 and
8$1 (m. :ue to atmospheric drag, the space station is constantly losing orbital energy. !n order to
compensate for this loss, which would e'entually lead to a reentry of the station, it is being
reboosted to a higher orbit from time to time. The chosen orbital altitude is a trade&off between the
delta&' needed to reboost the station and the delta&' needed to send payloads and people to the
station. The upper limitation of orbit altitude is due to the constraints imposed by the oyu3
spacecraft. ;n 6* April 611-, the Automated Transfer ?ehicle HEules ?erneH raised the orbit of the
! for the first time, thereby pro'ing its ability to replace (and outperform) the oyu3 at this tas(.
Station-4eepin' in )EO
;nce a satellite has reached geostationary orbit, it seems natural that it should remain there. "ife,
of course, is not so simple because orbital perturbations cause the satellite to drift.
!nclined orbital planes
The principal correction re%uired is to compensate for /orth&outh drift. The geostationary plane
(abo'e the e%uator) is not aligned to the EarthCs orbit round the un (ecliptic) or the MoonCs orbit
round the Earth, so the gra'itational pull of the un and Moon drags satellites off the plane.
Uncorrected, this would cause the inclination of the orbit to increase by appro<imately one degree
per year. The a'erage annual 'elocity change needed to correct this effect is about *1 mIs, which
can represent )*P of the total station&(eeping propellant budget.
;ther drift pressures are also significant if uncorrected. East&Gest drift occurs because the e%uator
is not perfectly circular, so satellites drift slowly towards one of two longitudinal stable points.
olar radiation pressure, caused by the transfer of momentum from the un5s light and infrared
radiation, periodically flattens and disturbs the orientation of the orbit. ;ther factors, such as local
irregularities in the gra'itational field, also contribute less systematically to drift pressures.
:ue to luni&solar perturbations and the ellipticity of the Earth e%uator, an ob,ect placed in a =E;
without any station&(eeping would not stay there. !t would start building up inclination at an initial
rate of about 1.-* degrees per year. After 60.* years the ob,ect would ha'e an inclination of $*
degrees, decreasing bac( to 3ero after another 60.* years. Therefore, a lot of energy has to be
de'oted to maneu'ers that compensate this tendency. This part of the =E; station&(eeping is
called /orth&outh control. The ellipticity of the Earth e%uator is causing an East&Gest drift if the
satellite is not placed in one of the stable (+* degrees longitude east, $1* degrees longitude west)
or unstable ($* degrees longitude west, $0* degrees longitude east) e%uilibrium points.
/e'ertheless, this part of =E; station&(eeping, called East&Gest control re%uires significantly less
amount of fuel than /orth&outh control. Therefore, in some cases aging satellites are only East&
Gest controlled. This would still guarantee that the satellite is always 'isible to a steerable antenna.
Ta(ing into consideration the relati'ely long periods of operation of modern =E; satellites (about
$* years) the delta&' e<pended o'er such a period can be substantial (about 80 mIs per year). !t is
6+
therefore crucial for =E; satellites to ha'e the most fuel&efficient propulsion system. ome
modern satellites are therefore employing a high specific impulse system li(e plasma or ion
thrusters.
TT:C Su$s"stem
The TTQC ubsystem contains #adio @re%uency (#@) components, wor(ing in &band, that
pro'ides the necessary functions to ensure atellite access from the =round tation for
commanding and telemetry data transmission. The TTQC ubsystem includes>
Two &band Transponders2
Two &band antennas2
;ne #adio @re%uency :istribution Unit (#@:U).
The Transponders are connected through the #@:U and #@ coa<ial cables to the two antennas that
pro'ide full spherical co'erage with an o'erlap of at least ten degrees.
The nominal operation scenario foresees that the #ecei'er sections of both Transponders are
always switched on.
:epending on the atellite attitude during the =round tation contact, only the Transmitter section
of the Transponder connected to the ground&lin(ed antenna is switched on.
;ne Transponder failure can be reco'ered through a cross coupling in the #@:U to allow the
connection of the still wor(ing Transponder with both the antennas.

Unit III EA+T< SE)MENT : S*ACE LIN3
4A!C C;M.;/E/T ;@ ATE""!TE C;MMU/!CAT!;/
6-
E'ery communications satellite in its simplest form (whether low earth or geosynchronous)
in'ol'es the transmission of information from an originating ground station to the satellite (the
uplin(), followed by a retransmission of the information from the satellite bac( to the ground (the
downlin(). The downlin( may either be to a select number of ground stations or it may be
broadcast to e'eryone in a large area. Bence the satellite must ha'e a recei'er and a recei'e
antenna, a transmitter and a transmit antenna, some method for connecting the uplin( to the
downlin( for retransmission, and prime electrical power to run all of the electronics. The e<act
nature of these components will differ, depending on the orbit and the system architecture, but
e'ery communications satellite must ha'e these basic components. This is illustrated in the
drawing below.
Transmitters>&
The amount of power which a satellite transmitter needs to send out depends a great deal on
whether it is in low earth orbit or in geosynchronous orbit. This is a result of the fact that the
geosynchronous satellite is at an altitude of 66,711 miles, while the low earth satellite is only a few
hundred miles. The geosynchronous satellite is nearly $11 times as far away as the low earth
satellite. Ge can show fairly easily that this means the higher satellite would need almost $1,111
times as much power as the low&orbiting one, if e'erything else were the same. (@ortunately, of
course, we change some other things so that we donCt need $1,111 times as much power.)
@or either geosynchronous or low earth satellites, the power put out by the satellite transmitter is
really puny compared to that of a terrestrial radio station. Four fa'orite roc( station probably
boasts of ha'ing many (ilowatts of power. 4y contrast, a 611 watt transmitter would be 'ery
strong for a satellite.
6)
Antennas>&
;ne of the biggest differences between a low earth satellite and a geosynchronous satellite is in
their antennas. As mentioned earlier, the geosynchronous satellite would re%uire nearly $1,111
times more transmitter power, if all other components were the same. ;ne of the most
straightforward ways to ma(e up the difference, howe'er, is through antenna design. ?irtually all
antennas in use today radiate energy preferentially in some direction.
4y doubling the diameter of a reflector antenna (a big HdishH) will reduce the area of the beam spot
to one fourth of what it would be with a smaller reflector. Ge describe this in terms of the gain of
the antenna. =ain simply tells us how much more power will fall on $ s%uare centimeter (or s%uare
meter or s%uare mile) with this antenna than would fall on that same s%uare centimeter (or s%uare
meter or s%uare mile) if the transmitter power were spread uniformly (isotropically) o'er all
directions. The larger antenna described abo'e would ha'e four times the gain of the smaller one.
This is one of the primary ways that the geosynchronous satellite ma(es up for the apparently
larger transmitter power which it re%uires.
;ne other big difference between the geosynchronous antenna and the low earth antenna is the
difficulty of meeting the re%uirement that the satellite antennas always be HpointedH at the earth.
@or the geosynchronous satellite, of course, it is relati'ely easy. As seen from the earth station, the
satellite ne'er appears to mo'e any significant distance. As seen from the satellite, the earth
station ne'er appears to mo'e. Ge only need to maintain the orientation of the satellite. The low
earth orbiting satellite, on the other hand, as seen from the ground is continuously mo'ing.
"i(ewise, the earth station, as seen from the satellite is a mo'ing target. As a result, both the earth
station and the satellite need some sort of trac(ing capability which will allow its antennas to
follow the target during the time that it is 'isible. The only alternati'e is to ma(e that antenna
beam so wide that the intended recei'er (or transmitter) is always within it. ;f course, ma(ing the
beam spot larger decreases the antenna gain as the a'ailable power is spread o'er a larger area,
which in turn increases the amount of power which the transmitter must pro'ide.
Transpon%ers(-
71
A transponder is an electronic de'ice that produces a response when it recei'es a radio&fre%uency
interrogation.
An ;ntario Bighway 81+ toll transponder
!n telecommunication, the term transponder (short&for Transmitter&responder and sometimes
abbre'iated to D.:#, D./:# or T.:#) has the following meanings>
An automatic de'ice that recei'es, amplifies, and retransmits a signal on a different
fre%uency (see also broadcast translator).
An automatic de'ice that transmits a predetermined message in response to a predefined
recei'ed signal.
A recei'er transmitter that will generate a reply signal upon proper electronic interrogation.
A communications satellite5s channels are called transponders, because each is a separate
transcei'er or repeater. Gith digital 'ideo data compression and multiple<ing, se'eral 'ideo and
audio channels may tra'el through a single transponder on a single wideband carrier. ;riginal
analog 'ideo only has one channel per transponder, with subcarriers for audio and automatic
transmission identification ser'ice AT!. /on&multiple<ed radio stations can also tra'el in single
channel per carrier (C.C) mode, with multiple carriers (analog or digital) per transponder. This
allows each station to transmit directly to the satellite, rather than paying for a whole transponder,
or using landlines to send it to an earth station for multiple<ing with other stations.
.ower =eneration>&
The satellite must generate all of its own power. @or a communications satellite, that power usually
is generated by large solar panels co'ered with solar cells. These con'ert sunlight into electricity.
ince there is a practical limit to the how big a solar panel can be, there is also a practical limit to
the amount of power which can generated. !n addition, unfortunately, transmitters are not 'ery
7$
good at con'erting input power to radiated power so that $111 watts of power into the transmitter
will probably result in only $11 or $*1 watts of power being radiated.
Ge say that transmitters are only $1 or $*P efficient. !n practice the solar cells on the most
HpowerfulH satellites generate only a few thousand watts of electrical power. atellites must also be
prepared for those periods when the sun is not 'isible, usually because the earth is passing between
the satellite and the sun. This re%uires that the satellite ha'e batteries on board which can supply
the re%uired power for the necessary time and then recharge by the time of the ne<t period of
eclipse.
Sate!!ite Lin4(-
A radio lin( between a transmitting Earth station and a recei'ing Earth station through one
satellite. A satellite lin( comprises one uplin( and one downlin(.
Eart& station( -
A station located either on the EarthCs surface or within the ma,or portion of the EarthCs
atmosphere and intended for communication>
Gith one or more space stations2 or
Gith one or more stations of the same (ind by means of one or more reflecting satellites or
other ob,ects in space.
/&at is up!in40
Uplin( is the signal path from an earth station to a satellite.
The opposite of uplin( is downlin(. :ownlin( is the signal path from the satellite toward the earth.
Up!in4 -re.uencies
Sate!!ite ,an% Up!in4 -re.uenc"
C 4and *.)6* & 0.86* =B3
Au 4and $8 & $8.* =B3
Aa 4and 6+.* & 7$ =B3
Up!in4 5U=L7(-
The portion of a communications lin( used for the transmission of signals from an earth terminal
to a satellite or to an airborne platform.
76
/&at is %on!in40
:ownlin( is the signal path from a satellite towards the earth.
The opposite of downlin( is uplin(. Uplin( is the signal path from an earth station towards the
satellite.
Don!in4 -re.uencies
Sate!!ite ,an% Don!in4 -re.uenc"
C 4and 7.+ & 8.6 =B3
Au 4and $$.+ & $6.+ =B3
Aa 4and $-.7 & 61.6 =B3
Don!in4 5D=L7(-
$> A data lin( from a satellite or other spacecraft to a terrestrial terminal.
6. A data lin( from an airborne platform to a ground&based terminal.
+outers(-
A router is a de'ice that determines the proper path for data to tra'el between different networ(s,
and forwards data pac(ets to the ne<t de'ice along this path.
They connect networ(s together2 a "A/ to a GA/ for e<ample, to access the !nternet. ome units,
li(e the Cisco $-11 (pictured), are a'ailable in both wired and wireless models.
#outers operate in two different planes>
Control .lane, in which the router learns the outgoing interface that is most appropriate
for forwarding specific pac(ets to specific destinations.
77
@orwarding .lane, which is responsible for the actual process of sending a pac(et
recei'ed on a logical interface to an outbound logical interface.
To understand the role of a router, understand that it does not, in a networ( of any real comple<ity,
ta(e you directly to the destination. !nstead, your information will pass through a series of routers
and intermediate subnets, each getting you one HhopH closer to the destination, until you reach the
router that connects to the subnet that contains your final destination.
@or the pure !nternet .rotocol (!.) forwarding function, router design tries to minimi3e the state
information (ept on indi'idual pac(ets. #outers do maintain state on routes, but not pac(ets. ;nce
a pac(et is forwarded, the router should retain no more than statistical information about it. !t is the
sending and recei'ing endpoint that (eeps information on such things as error or missing pac(ets.
Mo%ems(-
A modem (modulator&demodulator) is a de'ice or program that enables a computer to transmit data
o'er, for e<ample, telephone or cable lines. Computer information is stored digitally, whereas
information transmitted o'er telephone lines is transmitted in the form of analog wa'es. A modem
con'erts between these two forms.
A :" Modem
The most familiar e<ample is a 'oice band modem that turns the digital R$s and 1s5 of personal
computer into sounds that can be transmitted o'er telephone lines of .lain ;ld Telephone ystems
(.;T), and once recei'ed on the other side, con'ert those $s and 1s bac( into a form used by a
U4, erial, or /etwor( connection.
78
Modems are generally classified by the amount of data they can send in a gi'en time, normally
measured in bits per second, or NbpsO. @ortunately, there is one standard interface for connecting
e<ternal modems to computers called #& 676. Conse%uently any e<ternal modem can be
attached to any computer that has an #&676 port, which almost all personal computers ha'e.
There are also modems that come as an e<pansion board that you can insert into a 'acant
e<pansion slot. These are sometimes called onboard or internal modems.
4its .er econd (bps)
?oiceI:ata
Auto&Answer
:ata compression
@lash memory
@a< capability
,its per Secon%(-
Bow fast the modem can transmit and recei'e data. At slow rates, modems are measured in terms
of baud rates. The slowest rate is 711 baud (about 6* cps). At higher speeds, modems are measured
in terms of bits per second (bps). The fastest modems run at *+,011 bps, although they can achie'e
e'en higher data transfer rates by compressing the data. ;b'iously, the faster the transmission rate,
the faster you can send and recei'e data. /ote, howe'er, that you cannot recei'e data any faster
than it is being sent.
1oice=Data(-
Many modems support a switch to change between 'oice and data modes. !n data mode, the
modem acts li(e a regular modem. !n 'oice mode, the modem acts li(e a regular telephone.
Modems that support a 'oiceIdata switch ha'e a built&in loudspea(er and microphone for 'oice
communication.
Auto Anser(-
An auto&answer modem enables your computer to recei'e calls in your absence. This is only
necessary if you are offering some type of computer ser'ice that people can call in to use.
7*
Data Compression(-
ome modems perform data compression, which enables them to send data at faster rates.
Bowe'er, the modem at the recei'ing end must be able to decompress the data.
-!as& Memor"(-
ome modems come with flash memory rather than con'entional #;M, which means that the
communications protocols can be easily updated if necessary.
-a9 Capa$i!it"(-
Most modern modems are fa< modems, which mean that they can send and recei'e
Unit IV SATELLITE ACCESS
ACCESS TEC<NI?UES
Mu!tip!e Access Tec&ni.ues(-
Multiple access techni%ues allow interconnection among large number of earth stations terminals
simultaneously 'ia satellite.
Alternati'ely with multiple access techni%ues any one earth station can communicate with all other
earth stations using the same satellite.
$) Time :i'ision Multiple Access ( T:MA)
6) @re%uency :i'ision Multiple Access (@:MA)
7) Code :i'ision Multiple Access (C:MA)
TDMA(-
!n T:MA many earth stations in the satellite communications networ( use a single carrier for
transmission 'ia the satellite transponder on a time di'ision basis. The earth stations transmit
traffic bursts in a periodic time frame which is termed as T:MA frame.
The earth stations during their traffic transmission ha'e the access to the entire bandwidth of the
transmission.
-DMA(-
The terminology Nmultiple accessO indicates how the radio spectrum resource is intended to be
used> by enabling more than one communications signal to pass within a particular band2 and the
Nfre%uency di'isionO indicates how the sharing is accomplished> by allocating indi'idual
fre%uencies for each communications signal within the band.
70
!n an @:MA scheme, the gi'en #adio @re%uency (#@) bandwidth is di'ided into ad,acent
fre%uency segments. Each segment is pro'ided with bandwidth to enable an associated
communications signal to pass through a transmission en'ironment with an acceptable le'el of
interference from communications signals in ad,acent fre%uency segments.
CDMA(-
C:MA is a form of multiple<ing and a method of multiple access to a physical medium such as a
radio channel, where different users use the medium at the same time than(s to using different
code se%uences.
!n C:MA the whole bandwidth of the transponder is used all the time and signals from the users
are encoded so that information from an indi'idual transmitter can be detected and reco'ered only
by properly synchroni3ed recei'ing station that (nows the code being used.

SOME OT<E+ ACCESS TEC<NI?UES
DAMA(-
:emand Assigned Multiple Access (:AMA) is a technology used to assign a bandwidth to clients
which donCt need to use it constantly. :AMA systems %uic(ly and transparently assign
communication lin(s or circuits based on re%uests issued from user terminals to a networ( control
system. Ghen the circuit is no longer in use, the channels are immediately returned to the central
pool, for reuse by others. !t allows utili3ing of one channel (fre%uency band, timeslot, etc.) by
many users at different times. This technology is mainly used by small clients, as opposed to
.AMA (.ermanently Assigned Multiple Access). 4y using :AMA technology the amount of users
that can use a limited pool of circuits can be greatly increased.
*AMA(-
.re assigned Multiple Access (.AMA) is a technology used to assign a bandwidth to clients which
need to use it constantly. The channel remains allocated to the client e'en when not in use. This
technology is used by big clients as oppose to :AMA.
7+
IN-O+MATION E2C<AN)E
Data E9c&an'e(-
Data Transmission(-
:ata is transmitted in digital form through router that determines the proper path for data to tra'el
between the networ(s, and forwards data pac(ets to the modem along this path. Modem con'erts
this digital form of data into analog form. The fre%uency of this signal is then increased with the
help of up con'erter. The power le'el of the signal is then amplified by the high power amplifier
SB.AT and then sent to the antenna for the transmission.
Data +eception(-
The data is recei'ed by the antenna and then passes through the low noise amplifier
That amplifies the wea( signal recei'ed by the antenna. This amplifies signal is then passed through
the down con'erter that decreases the fre%uency of the signal.
/ow this analog signal is then con'erted to digital signal by the modem. This signal is then routed to
the destination computer by the router.
Computer +outer MODEM U*
Converter
<*A
LNA Don
Converter
MODEM +outer
Computer
7-
1oice E9c&an'e(-
1oice Transmission(-
?oice is transmitted through router that determines the proper path for data to tra'el between the
networ(s, and forwards data pac(ets to the modem along this path. Modem con'erts this digital
form of data into analog form. The fre%uency of this signal is then increased with the help of up
con'erter. The power le'el of the signal is then amplified by the high power amplifier SB.AT and
then sent to the antenna for the transmission.
1oice +eception(-
The 'oice signal is recei'ed by the antenna and then passes through the low noise amplifier that
amplifies the wea( signal recei'ed by the antenna. This amplifies signal is then passed through the
down con'erter that decreases the fre%uency of the signal.
/ow this analog signal is then con'erted to digital signal by the modem. This signal is then routed to
the destination by the e<change.
Up Converter(-
*&one E9c&an'e MU2 U*
Converter
<*A
Mo%em
LNA Don
Converter
MODEM
*&one
E9c&an'e
7)
The up con'erter contains fre%uency, translating circuits, which con'ert +1 MB3 input signal to
signal in the fre%uency, range of *.-* =B3 to 0.86* =B3. The up con'erter has nominal gain of $*
d4, with the nominal power being 1 d4m. The up con'erter contains filters for suppression of local
oscillator lea( and spurious products. E%uali3ers compensate for group delay is reduced by the
filters and (eep amplitude response within specifications.
<i'& *oer Amp!ifier 5<*A7(-
The high power amplifier amplifies the #@ output signal from the up con'erter to the re%uired
power le'el for transmission to the satellite. Amplifiers for satellite 'ideo applications are typically
si3ed in the range from $watt to 7watt. Amplifiers in the $ to $1 watt ranges a'ailable are solid
state configuration. Tra'eling wa'e tube (TGT) amplifiers are a'ailable in configuration up to
appro<imately +*1 watt. @or power le'els abo'e +*1 Gatts (lystron tube amplifiers are used. The
B.A usually contains 4.@ to re,ect harmonics and power sampling circuits for monitoring the
output transmit power and the reflected power from the antenna. They ha'e also the pro'ision for
increase of power from minimum to ma<imum 'alue. Con'entional tube fails to operate
satisfactorily abo'e 711 MB3 mainly due to transmit time effect. UB@ tubes try to o'ercome the
transmit time effect by reducing the tube dimensions.
Don Converter(-
The down con'erter contains fre%uency translating circuit which con'erts fc MB3 input signal to
+1 MB3 signal. The down con'erter contains compensate for group delay introduced by the filters
and (eep amplitude response within specifications.
Lo Noise Amp!ifier 5LNA7(-
The low noise amplifier pro'ides high gain and low noise to establish high system =ITe. =ITe ratio
is a figure of merit used to represent the %uality of a satellite recei'er or an earth station. Total gain
= becomes the sum of antenna gain =a and "/A gain =lna. Te is an effecti'e noise temperature at
the input of "/A.
A transponder (also T.:#, T#, D./:#, D.:#) is an electronic de'ice used in wireless
communications, the word itself is shorthand for transmitter&responder.
This de'ice is primarily used as a re&transmitter due to the fact that it recei'es a particular signal
from a particular source, then it amplifies (strengthens) the signal before sending it to a predefined
location. Transponders ha'e an abnormally large number of applications in our daily li'es. ome
of the most common uses are> satellite tele'ision, satellite telephony, air traffic control and in
automobiles. They are also embedded in cars to open gates automatically. Ge shall loo( at some of
these applications later. @irst of all it is important to mention that transponders are of two general
'arieties which are acti'e transponders and passi'e transponders.
81
Acti'e transponder> These de'ices as the name implies, continually emit radio signals which are
trac(ed and monitored. These can also be automatic de'ices which strengthen the recei'ed signals
and relay them to another location.
These de'ices are so fre%uently used that we often fail to recogni3e them. @or e<ample, how do
you thin( lap times of /ACA# and formula one cars are monitored so accuratelyU Gell the
answer lies in the transponders which cars ha'e embedded in them. Each car has a uni%ue !: code
which is transmitted as the car mo'es. A special cable loop is dug into the ground at the start&finish
lines. o when the cars 3oom by the finish line, their !:s are recorded along with their lap times.
These times are automatically displayed on the position board along with split times, laps
remaining and so on.
Another important use of acti'e transponders is in satellite communications. /ormally there are
hundreds of thousands of tiny transponders embedded in one satellite. These recei'e an incoming
signal o'er a range of fre%uencies (band), measured in hert3 and megahert3 and retransmit these
signals on a different band simultaneously. The incoming signal originating from a point on the
earth (e.g. A broadcaster), is called the uplin( and the outgoing signal bac( to the earth is called the
downlin(. The logic behind using satellites for this purpose is simple & as radio signals cannot
cur'e along the cur'ature of the earth, they are sent in a straight line up and recei'ed down in a
straight line. This reduces time of signal deli'ery and increases range.
/ow we come to the passi'e transponder which although not as acti'e as their counterparts still
play a 'ery important role. These transponders contain information which is used to identify
particular ob,ects. @or e<ample passi'e transponders are sometimes embedded in our credit cards
and on magnetic labels in large stores. These are paired with acti'e transponders which amplify
and transcribe the information.

Unit V DI+ECT ,+OADCAST SATELLITE SE+1ICES
Direct $roa%cast sate!!ite
Direct $roa%cast sate!!ite (:4) is a term used to refer to satellite tele'ision broadcasts intended
for home reception, also referred to as direct-to-home signals. The e<pression direct-to-home or
:TB was, initially, meant to distinguish the transmissions directly intended for home 'iewers from
cable tele'ision distribution ser'ices that sometimes carried on the same satellite. The term
predates :4 satellites and is often used in reference to ser'ices carried by lower power satellites
which re%uired larger dishes ($.+M diameter or greater) for reception. !n Europe, the e<pression
was common prior to the launch of AT#A&$ in $)-- as there were two mar(ets> the :TB mar(et
which re%uired the larger dishes and the :4 (AT#A) mar(et which re%uired smaller (1.)M
dishes). As higher powered satellites li(e AT#A came into operation, the acronym :4 gradually
supplanted it.
The term :4 now co'ers both analog and digital tele'ision and radio reception, and is often
e<tended to other ser'ices pro'ided by modern digital tele'ision systems, including 'ideo&on&
demand and interacti'e features. A H:4 ser'iceH usually refers to either a commercial ser'ice, or
a group of free channels a'ailable from one orbital position targeting one country.
8$
Termino!o'" confusion
!n certain regions of the world, especially in /orth America, :4 is used to refer to pro'iders of
subscription satellite pac(ages, and has become applied to the entire e%uipment chain in'ol'ed.
Gith modern satellite pro'iders in the United tates using high power Au&band transmissions
using circular polari3ation, which result in small dishes, and digital compression (hence bringing in
an alternati'e term, Di'ita! Sate!!ite S"stem, itself li(ely connected to the proprietary encoding
system used by :irecT?, :igital atellite er'ice), :4 is often misused to refer to these. :4
systems are often dri'en by pay tele'ision pro'iders, which dri'es further confusion. Additionally,
in some areas it is used to refer to specific segments of the Au&band, normally $6.6 to $6.+ =B3, as
this bandwidth is often referred to as :4 or one of its synonyms. !n comparison, European HAu
bandH :4 systems can drop as low as $1.+ =B3.
Adding to the naming comple<ity, the !TUCs original fre%uency allocation plan for Europe, the
o'iet Union and /orthern Africa from $)++ introduced a concept of e<tremely high power spot&
beam broadcasting (see E(ran satellite) which they termed :4, although only a handful of the
participating countries e'en went as far as to launch satellites under this plan, e'en fewer operated
anything resembling a :4 ser'ice.
Commercia! D,S services
The first commercial :4 ser'ice, (y Tele'ision plc (now 4(y4), was launched in $)-). (y
T? started as a four&channel free&to&air analogue ser'ice on the Astra $A satellite, ser'ing the
United Aingdom and #epublic of !reland. 4y $))$, (y had changed to a conditional access pay
model, and launched a digital ser'ice, (y :igital, in $))-, with analogue transmission ceasing in
611$. ince the :4 nomenclature is rarely used in the UA or !reland, the popularity of (yCs
ser'ice has caused the terms HminidishH and Hdigibo<H to be applied to products other than (yCs
hardware. 4(y4 is controlled by /ews Corporation.
.rimetar began transmitting an analog ser'ice to /orth America in $))$, and was ,oined by
:irecT? =roupCs :irecT? (then owned by =M Bughes Electronics), in $))8. At the time,
:irecT?Cs introduction was the most successful consumer electronics debut in American history.
Although .rimetar transitioned to a digital system in $))8, it was ultimately unable to compete
with :irecT?, which re%uired a smaller satellite dish and could deli'er more programming.
:irecT? e'entually purchased .rimetar in $))) and migrated all .rimetar subscribers to
:irecT? e%uipment. !n 6117, /ews Corporation purchased a controlling interest in :irecT?Cs
parent company, Bughes Electronics, and renamed the company :irecT? =roup.
!n $))0, EchotarCs :ish /etwor( went online in the United tates and, as :irecT?Cs primary
competitor, achie'ed similar success. Alphatar also launched but soon went under. Astro was
launched, using its direct broadcast satellite system.
:ominion ?ideo atellite !nc.Cs (y Angel also went online in the United tates in $))0 with its
:4 ser'ice geared toward the faith and family mar(et. !t has since grown from si< to 70 T? and
radio channels of family entertainment, Christian&inspirational programming and 68&hour news.
:ominion, under its former corporate name ?ideo atellite ystems !nc., was actually the second
86
from among the first nine companies to apply to the @CC for a high&power :4 license in $)-$
and is the sole sur'i'ing :4 pioneer from that first round of forward&thin(ing applicants. (y
Angel, although a separate and independent :4 ser'ice, uses the satellites, transmission facilities,
Q recei'ing e%uipment used for :ish /etwor( through an agreement with Echostar. 4ecause of
this, (y Angel subscribers also ha'e the option of subscribing to :ish /etwor(Cs channels as well.
!n 6117, Echotar attempted to purchase :irecT?, but the U.. :epartment of Eustice denied the
purchase based on anti&competiti'e concerns.
-ree D,S services
=ermany is li(ely the leader in free&to&air :4, with appro<imately 81 analogue and $11 digital
channels broadcast from the E Astra $ position at $).6E. These are not mar(eted as a :4
ser'ice, but are recei'ed in appro<imately $6 million homes, as well as in any home using the
=erman commercial :4 system, Premiere.
The United Aingdom has appro<imately )1 free&to&air digital channels, for which a promotional
and mar(eting plan is being de'ised by the 44C and !T?, to be sold as H@reesatH. !t is intended to
pro'ide a multi&channel ser'ice for areas which cannot recei'e @ree'iew, and e'entually replace
their networ( of UB@ repeaters in these areas
!ndiaCs national broadcaster, :oordarshan, promotes a free&to&air :4 pac(age as H:: :irect
.lusH, which is pro'ided as in&fill for the countryCs terrestrial transmission networ(.
Ghile originally launched as bac(haul for their digital terrestrial tele'ision ser'ice, a large number
of @rench channels are free&to&air on *G, and ha'e recently been announced as being official in&fill
for the :TT networ(.
!n /orth America (UA, Canada and Me<ico) there are o'er -1 @TA digital channels a'ailable on
!ntelsat Americas *, the ma,ority of them are ethnic or religious. ;ther popular @TA satellites
include AMC&8, AMC&0, =ala<y $1# and atMe< *. A company called =lorytar promotes @TA
religious broadcasters on !A&* and AMC&8.
@orward Error Correction (@EC) is a type of error correction which impro'es on simple error
detection schemes by enabling the recei'er to correct errors once they are detected. This reduces
the need for retransmissions.
@EC wor(s by adding chec( bits to the outgoing data stream. Adding more chec( bits reduces the
amount of a'ailable bandwidth, but also enables the recei'er to correct for more errors.
@orward Error Correction is particularly well suited for satellite transmissions, where bandwidth is
reasonable but latency is significant.
87
-orar% Error Correction vs> ,ac4ar% Error Correction
@orward Error Correction protocols impose a greater bandwidth o'erhead than bac(ward error
correction protocols, but are able to reco'er from errors more %uic(ly and with significantly fewer
retransmissions.
)!o$a! *ositionin' S"stem
=. is the =lobal .ositioning ystem . =. uses satellite technology to enable a terrestrial
terminal to determine its position on the Earth in latitude and longitude.
=. recei'ers do this by measuring the signals from three or more satellites simultaneously and
determining their position using the timing of these signals.
=. operates using trilateration. Trilateration is the process of determining the position of an
un(nown point by measuring the lengths of the sides of an imaginary triangle between the
un(nown point and two or more (nown points.
!n the =. system, the two (nown points are pro'ided by two =. satellites. These satellites
constantly transmit an identifying signal.
The =. recei'er measures the distance to each =. satellite by measuring the time each signal
too( to tra'el between the =. satellite and the =. recei'er.
The formula for this is>
Distance = Velocity * Time
?elocity of the =. signal is the speed of light, appro<imately 711,111 AmIs.
=. transmissions occur on a fre%uency of $*+*.86 and $66+.01 Mh3. 4oth of these fre%uencies
are within the " 4and.
)*S <istor"
=. was originally de'eloped for the U.. military, but is now pro'ided as a public ser'ice for
people all o'er the world by the U.. go'ernment.
:eployment of the =. system began on 66 @ebruary $)+- with the launch of the first 4loc( !
/a'star =. satellite. !nitial ;perating Capability was declared in :ecember of $))7 with 68
operational =. satellites in orbit. @ull ;perational Capability was declared in Eune of $))*.
88
=. was de'eloped by the U.. military to help soldiers locate their positions. Ci'ilian access to
the =. system was guaranteed by .resident #eagan as a response to the communist Chinese
shooting down of Aorean Airline @light AA"&11+. .resident #eagan hoped that =. technology
would help to pre'ent such a tragedy from happening again.
)*S Arc&itecture
The =. system is di'ided into three segments>
The pace egment
The Control egment
The User egment
T&e Space Se'ment
=. uses twenty&one operational satellites, with an additional three satellites in orbit as redundant
bac(up.
=. uses /A?TA# satellites manufactured by #oc(well !nternational. Each /A?TA# satellite
is appro<imately * meters wide (with solar panels e<tended) and weighs appro<imately )11Ag.
=. satellites orbit the earth at an altitude of appro<imately 61,611Am.
Each =. satellite has an orbital period of $$ hours and *- minutes. This means that each =.
satellite orbits the Earth twice each day.
These twenty&four satellites orbit in si< orbital planes, or paths. This means that four =. satellites
operate in each orbital plane.
Each of these si< orbital planes is spaced si<ty degrees apart. All of these orbital planes are
inclined fifty&fi'e degrees from the E%uator.
T&e Contro! Se'ment
The Master Control tation (MC) of the =. system is operated at chrie'er Air @orce 4ase in
Colorado prings, Colorado. The United tates Air @orce maintains redundant Master Control
tations in #oc('ille, Maryland and unny'ale, California.
The Air @orce also maintains monitoring stations in Colorado prings, Bawaii, The Ascension
!slands, :iego =arcia, and Awa,alein.
Communications with the space segment are conducted through ground antennas in the Ascension
!slands, :iego =arcia, and Awa,alein.
T&e User Se'ment
The =. user segment is any person with a =. recei'er.
8*
?AT is an abbre'iation for a ?ery mall Aperture Terminal. !t is basically a two&way satellite
ground station with a less than 7 meters tall (most of them are about 1.+* m to $.6 m tall) dish
antenna stationed. The transmission rates of ?ATs are usually from 'ery low and up to 8 MbitIs.
These ?ATsC primary ,ob is accessing the satellites in the geosynchronous orbit and relaying data
from terminals in earth to other terminals and hubs. They will often transmit narrowband data,
such as the transactions of credit cards, polling, #@!: (radio fre%uency identification ) data, and
CA:A (uper'isory Control and :ata Ac%uisition), or broadband data, such as satellite !nternet,
?o!., and 'ideos. Bowe'er, the ?AT technology is also used for 'arious types of
communications.
E%uatorial Communications first used the spread spectrum technology to commerciali3e the
?ATs, which were at the time C band (0 =B3) recei'e only systems. This commerciali3ation led
to o'er 71,111 sales of the 01 cm antenna systems in the early $)-1s. E%uatorial Communications
sold about $1,111 more units from $)-8 to $)-* by de'eloping a C band (8 and 0 =B3) two way
system with $ m < 1.* m dimensions.
!n $)-*, the current worldCs most used ?ATs, the Au band ($6 to $8 =B3) was co&de'eloped by
chlumberger ;ilfield #esearch and Bughes Aerospace. !t is primarily used to pro'ide portable
networ( connection for e<ploration units, particularly doing oil field drilling.
Imp!ementations of 1SAT
Currently, the largest ?AT networ( consists of o'er $6,111 sites and is administered by pacenet
and MC! for the U .ostal er'ice (U.). Galgreens .harmacy, :ollar =eneral, C?, #iteaid,
Gal&Mart, FumL 4rands (such as Taco 4ell, .i33a But, "ong Eohn il'erCs, and other fast food
chains), =TEC, =!, and !ntralot also utili3es large ?AT networ(s. Many huge car corporations
such as @ord and =eneral Motors also utili3es the ?AT technology, such as transmitting and
recei'ing sales figures and orders, along with announcing international communications, ser'ice
bulletins, and for distance learning courses. An e<ample of this is the H@ordtar /etwor(.H
Two way satellite !nternet pro'iders also use the ?AT technology. Companies li(e tar4and,
Gild4lue, and Bughes/et in the United tates and at"yn<, 4luestream, and Technologie
atelitarne in Europe, and many other broadband ser'ices around the world in rural areas where
high speed !nternet connections cannot be pro'ided use it too. A statistic from :ecember 6118
showed that o'er a million ?ATs were in place.
1SAT Confi'urations
Most of the current ?AT networ(s use a topology>
tar topology> This topology uses a central uplin( site (eg. /etwor( operations center (/;C)), which
transports the data to and from each of the ?AT terminals using satellites
Mesh topology> !n this configuration, each ?AT terminal will relay data o'er to another terminal through the
satellite, acting as a hub, which also minimi3es the need for an uplin( site
tar K Mesh topology> This combination can be achie'ed (as some ?AT networ(s do) by ha'ing multiple
centrali3ed uplin( sites connected together in a multi&star topology which is in a bigger mesh topology. This
topology does not cost so much in maintaining the networ( while also lessening the amount of data that needs
to be relayed through one or more central uplin( sites in the networ(.
80
1SAT@s Stren't&s
?AT technology has many ad'antages, which is the reason why it is used so widely today. ;ne is
a'ailability. The ser'ice can basically be deployed anywhere around the world. Also, the ?AT is
di'erse in that it offers a completely independent wireless lin( from the local infrastructure, which
is a good bac(up for potential disasters. !ts deployability is also %uite ama3ing as the ?AT
ser'ices can be setup in a matter of minutes. The strength and the speed of the ?AT connection
being homogenous anywhere within the boundaries is also a big plus. /ot to forget, the connection
is %uite secure as they are pri'ate layer&6 networ(s o'er the air. The pricing is also affordable, as
the networ(s themsel'es do not ha'e to pay a lot, as the broadcast download scheme (eg. :?4&)
allows them to ser'e the same content to thousands of locations at once without any additional
costs. "ast but not least, most of the ?AT systems today use onboard acceleration of protocols
(eg. TC., BTT.), which allows them to deli'ery high %uality connections regardless of the latency.
1SAT Dra$ac4s
As with e'erything, ?AT also has its downsides. @irstly, because the ?AT technology utili3es
the satellites in geosynchronous orbit, it ta(es a minimum latency of about *11 milliseconds e'ery
trip around. Therefore, it is not the ideal technology to use with protocols that re%uire a constant
bac( and forth transmission, such as online games. Also, surprisingly, the en'ironment can play a
role in slowing down the ?ATs. Although not as bad as one way T? systems li(e :irecT? and
:!B /etwor(, the ?AT still can ha'e a dim signal, as it still relies on the antenna si3e, the
transmitterCs power, and the fre%uency band. "ast but not least, although not that big of a concern,
installation can be a problem as ?AT ser'ices re%uire an outdoor antenna that has a clear 'iew of
the s(y. An aw(ward roof, such as with s(yscraper designs, can become problematic.
+ADA+SAT
#A:A#AT is an ad'anced Earth obser'ation satellite pro,ect de'eloped by Canada to monitor
en'ironmental change and to support resource sustainablility. #A:A#AT was launched on 8 /o'
$))* and is designed for a fi'e&year lifetime.
#A:A#AT uses ynthetic Aperture #adar (A#), an acti'e microwa'e sensor, allowing 68 hour
data collection independent of weather conditions and illumination. The A# sensor uses a *.0 cm
wa'elength which is (nown as C-$an%, has a BB polari3ation (hori3ont transmit, hori3on re'ei'e)
and has selecti'e 'iewing angles that allow a wide range of terrain conditions, applications and
ground co'erage re%uirements to be accommodated.!maging modes for #A:A#AT include @ine,
tandard, Gide, canA# (narrow and wide), and E<tended 4eam (high and low incidence
angles).
8+
+ADA+SAT *rocessin' Leve!s
C#!. supports the following #A:A#AT processing le'els.
Si'na! Data 5+A/7 & ignal :ata cannot be 'iewed as an image. !t is an unprocessed matri< of time delays
that has been repac(aged to fit into standard CE; format. Clients will re%uire A# processing capabilities
to use ignal :ata. All beam mdoes can pro'ide ignal :ata.
*at& Ima'e 5S)-7 & .ath !mage products are recommended for indi'iduals and organisations e<perienced in
image processing. .ath !mage product is aligned parallel to the satelliteCs orbit path. "atitude and longitude
positional information has been added to represent the first, mid, and last pi<el positions of each line of data.
:ata from all beam modes can be processed to this product.
*at& Ima'e Coarse 5S)C7 & .ath !mage Coarse is similar to .ath !mage, e<cept that the image is bloc(
a'eraged by factor of 6,7,8,* or 0. :ata from all beam modes can be processed to this product.
Sin'!e Loo4 Comp!e9 5SLC7 & ingle "oo( Comple< data retains the phase and amplitude information of the
original A# data. ingle "oo( Comple< product data is stored in slant range, and is corrected for satellite
reception errors, includes latitudeIlongitude positional information. !n addition, ingle "oo( Comple< data
retains the optimum resolution a'ailable for each beam mode. This product is suitable for interferometric
processing. :ata from all beam modes, e<cept canA#, can be processed to this product.
Map Ima'e 5SS)7 & Map !mage product is oriented with Hnorth upH and is corrected to a user&re%uested map
pro,ection. The positional accuracy of Map !mage processing depends on the terrain relief and the beam
mode. :ata from all beam modes, with the e<ception of canA# can be processed to this product.
8-
C+IS*@s +ADA+SAT *rocessin' Leve! Avai!a$i!it"
,eam Mo%e
*at& Ima'e
5S)-7
*at& Ima'e Coarse
5S)C7
Map Ima'e
5SS)7
Si'na! Data
5+A/7
Sin'!e Loo4 Comp!e9
5SLC7
@ine
tandard
Gide
canA# /arrow /ot A'ailable /ot A'ailable
canA# Gide /ot A'ailable /ot A'ailable
E<tended Bigh
E<tended "ow
Me%ia
:igital products are a'ailable on C:&#;M, -mm :ata Cartridge, or )&Trac( CCT.
-ormat
All products are produced in #A:A#AT CE; format.
-i!m an% *rints
:igital data can be produced as a film (negati'e or positi'e) or prints
+ADA+SAT Data *rocessin' Time
Near-+ea! TIme 5N+T7 & :igital products are processed within hours of reception.
+us& & :igital products are processed within 8- hours of reception.
+e'u!ar & :igital products are processed within $1 wor(ing days of reception.
;rbcomm
;rbcomm is a commercial 'enture to pro'ide global messaging ser'ices using a constellation of 60
low&Earth orbiting satellites. The planned system is designed to handle up to * million messages
from users utili3ing small, portable terminals to transmit and recei'e messages directly to the
satellites. The first two satellites of the constellation e<perienced communications problems after
launch, but were reco'ered and placed into operational status. The nominal 60 satellite
constellation will be deployed by $))+, with the potential for an additional - satellite plane and
two more polar orbiters depending on demands for increased co'erage. The 'ehicles will be
controlled from a single control center located in :ulles, ?irginia. The cost per satellite has been
estimated at V$.6 million. A small forerunner 'ehicle, ;rbcomm&D, was launched in $))$ as a
feasibility demonstration. This 'ehicle had a different design than the operational 'ehicles and will
not be included in the operational system.
Spacecraft
Circular dis( shaped spacecraft. Circular panels hinge from each side after launch to e<pose solar
cells. These panels articulate in $&a<is to trac( the sun and pro'ide $01G. :eployed spacecraft
8)
measures 7.0 m feet from end to end with 6.7 m span across the circular dis(s. ?B@ telemetry at
*+.0 (bps. ;n&board =. na'igation and timing system. $8 'olt power system. =ra'ity gradient
stabili3ation pro'ides * degrees control with magnetic tor%uers for damping. Cold gas (nitrogen)
propulsion system.
*a"!oa%
Each spacecraft carries $+ data processors and se'en antennas. :esigned to handle *1,111
messages per hour. "ong boom is a 6.0 meter ?B@IUB@ gateway antenna. #ecei'e> 6811 bps at
$8- & $8).) MB3. Transmit> 8-11 bps at $7+ & $7- MB3 and 811.1* & 811.$* MB3. The system
uses D.811 (CC!TT $)--) addressing. Message si3e is 0 to 6*1 bytes typical (no ma<imum).
Countr" of Ori'in United tates
Customer=User ;rbcomm !nc. (subsy. of ;C)
Manufacturer5s7 ;rbital ciences
Si;e 4us> $.1* m diameter < 1.$+ m thic(
Or$it /ominal constellation> 6 .olar (@ $, 6)> +-* (m circular I 68 !nclined> 7 planes
with - e%uidistantly spaced satellites in each plane, +-1 (m circular, 8* deg
inclination & Augmented constellation> 6 more .olar K additional - satellite
plane
Desi'n Life 8 years
Launc& -acts
Name Int@! Desi'> Date Site 1e&ic!e Or$it Mass54'7
Notes
;rbcomm D $))$&1*1C +I$+I)$ Aourou Ariane 8 "E; 66
tore and forward communication
;D. $ $))7&11)A 6I)I)7 EMC .egasus "E; $*
E<perimental spacecraft
;rbcomm @M$ $))*&1$+A 8I7I)* GMC .egasus "E; 81
Commercial communications testbed
;rbcomm @M6 $))*&1$+4 8I7I)* GMC .egasus "E; 81
Commercial communications testbed
*1