Department of ECE Lecture Notes EC1015 - SATELLITE 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 plan planes es,, for for whic which h appl applic icat atio ion n of other other tech techno nolo logi gies es,, such such as cabl cable, e, are are impr imprac acti tical cal or impossible.
U.. military M!"TA# communications satellite
$
Ear!" Missions(The first satellite e%uipped with on&board radio radio&&transmitters transmitters was was the o'iet putni $, launched in $*+. The first American satellite to relay communications was proect score in $*$*-,, which used a tape recorder to store and forward 'oice forward 'oice messages. !t was used to send a Christmas greeting to the world from resident Eisenhower . /AA launched /AA launched an Echo satellite in $01 $0122 the $11&foot $11&foot alumini3ed alumini3ed ET film balloon film balloon ser'ed as a passi'e reflector for radio communications. Courier $4, (built by hilco hilco)) also launched in $01, was the world5s first acti'e repeater satellite. Telstar 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 longitude,, o'er the !nternational :ate "ine. "ine. !t was used that same year to relay tele'ision co'erage on the $08 ummer ;lympics in ;lympics in Toyo Toyo to to the United tates,, the first tele'ision transmission sent o'er the acific ;cean. tates hortly after yncom 7, 7, !ntelsat !, !, aa Early 4ird, was launched on April 0, 0, $0* and placed in orbit at 6-9 west longitude. !t was the first geostationary satellite for telecommunications o'er the Atlantic ;cean. ;cean.
)eostationar" Sate!!ites(A satellite in a geostationary orbit appears orbit appears to be in a fi
Lo Eart& Or$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 6
Ear!" Missions(The first satellite e%uipped with on&board radio radio&&transmitters transmitters was was the o'iet putni $, launched in $*+. The first American satellite to relay communications was proect score in $*$*-,, which used a tape recorder to store and forward 'oice forward 'oice messages. !t was used to send a Christmas greeting to the world from resident Eisenhower . /AA launched /AA launched an Echo satellite in $01 $0122 the $11&foot $11&foot alumini3ed alumini3ed ET film balloon film balloon ser'ed as a passi'e reflector for radio communications. Courier $4, (built by hilco hilco)) also launched in $01, was the world5s first acti'e repeater satellite. Telstar 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 longitude,, o'er the !nternational :ate "ine. "ine. !t was used that same year to relay tele'ision co'erage on the $08 ummer ;lympics in ;lympics in Toyo Toyo to to the United tates,, the first tele'ision transmission sent o'er the acific ;cean. tates hortly after yncom 7, 7, !ntelsat !, !, aa Early 4ird, was launched on April 0, 0, $0* and placed in orbit at 6-9 west longitude. !t was the first geostationary satellite for telecommunications o'er the Atlantic ;cean. ;cean.
)eostationar" Sate!!ites(A satellite in a geostationary orbit appears orbit appears to be in a fi
Lo Eart& Or$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 $111 6
ilome ilometer terss from from the sub sub&sa &satel tellit litee point. point. !n additio addition, n, satell satellite itess in low earth earth orbit orbit change change their their position relati'e to the ground position %uicly. o e'en for local applications, a large number of satellites are needed if the mission re%uires uninterrupted connecti'ity. "ow earth orbiting satellites are less e
Lo *o!ar Eart& Or$it Sate!!ites(As menti mentioned oned,, geosta geostatio tionary nary satell satellit ites es are constr constrain ained ed to operate operate abo'e abo'e the e%uator 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 Molniya series series was launched on April 67, $0* $0* and and was used used for for e
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 first and histor historica ically lly the mos mostt import important ant applic applicati ation on for commun communica icatio tion n satell satellite itess is in international telephony telephony.. @i
Tele'ision became the main maret, its demand for simultaneous deli'ery of relati'ely few signals of la large bandwidth to many many rece recei' i'er erss being being a mo more re prec precis isee matc match h for for the the capab capabil ilit itie iess of geosynchronous comsats. Two satellite types are used for /orth American tele'ision and radio> •
:irect 4roadcast atellite (:4), and
•
@i
A direct broadcast satellite 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 u band. band. :4 technology is used for :TB&oriented (:irect&To& ( :irect&To& Bome)) satellite T? ser'ices, such as :irecT? Bome :irecT?,, :!B /etwor . @i
•
@ ha'e a lower #@ power output than the :4. 8
•
@ re%uires a much larger dish for reception (7 to - feet in diameter for u band, and $6 feet on up for C band).
•
@ use linear polari3ation for polari3ation for each of the transponders #@ input and output where as :4 satellites use circular polari3ation. polari3ation.
@ree&to&air satellite @ree&to&air satellite T? channels are also usually distributed on @ satellites in the u band. Mo$i!e Sate!!ite Tec&no!o'ies(-
!nitia !nitially lly a'aila a'ailable ble for broadc broadcast ast to statio stationar nary y T? recei' recei'ers ers,, by 6118 pop popula ularr mob mobile ile direct 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 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 et4lue Airways for :irecT? which passengers can 'iew on&board on "C: screens mounted in the seats. Amateur +a%io(-
Amateur radio operators radio operators ha'e access to the ;CA# satellites that ha'e been designed specifically to carry carry amateu amateurr radio radio traff traffic. ic. Mos Mostt such such satell satellite itess operat operatee as spacebo spaceborne rne repeaters repeaters,, and are gener general ally ly acces accesse sed d by amat amateu eurs rs e%ui e%uipp pped ed with with UB@ or ?B@ radio radio e%uipm e%uipment ent and highly highly direct direction ional al antennas such such as Fagis or dish dish antenn antennas. as. :ue to the limitat limitation ionss of ground& ground&bas based ed 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* AD.6* or or similar protocols. Sate!!ite ,roa%$an%(-
!n recent years, satellite communication technology has been used as a means to connect to the !nternet 'ia broadband data connections. This can be 'ery useful for users who are located in 'ery remote areas, and cannot access a wireline broadband wireline broadband or dialup connection.
*
re.uenc" ,an%s for Sate!!ite Communication(-
/&at is C ,an% C 4and is the original fre%uency allocation for communications satellites. C&4and uses 7.+&8.6=B3 for downlin and *.6*&0.86*=h3 for uplin . The lower fre%uencies used by C 4and perform better under ad'erse weather conditions than the u band or a band fre%uencies.
C ,an% ariants light 'ariations of C 4and fre%uencies are appro'ed for use in 'arious parts of the world. ,an%
E
T2 re.uenc"
+2 re.uenc"
*.-*1 & 0.86* =B3
7.06* & 8.611 =B3
uper E
7.811 & 8.611 =B3
!/AT C&4and
0.+6* & +.16* =B3
8.*11 & 8.-11 =B3
alapa C&4and
0.86* & 0.+6* =B3
7.811 & 7.+11 =B3
#ussian C&4and
*.+* & 0.8+* =B3
7.0*1 & 8.$*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 0 across. C 4and dishes 'ary between 7 and across, depending upon signal strength. 4ecause C 4and dishes are so much larger than u and a 4and dishes, a C 4and dish is sometimes referred to in friendly est as a 4U: (4ig Ugly :ish).
/&at is 3u $an% The u band (urt3&under band) is primarily used for satellite communications, particularly for editing and broadcasting satellite tele'ision. This band is split into multiple segments broen down into geographical regions, as determined by the !TU (!nternational Telecommunication Union). The u band is a portion of the electromagnetic spectrum in the microwa'e range of fre%uencies ranging from $$.+ to $6.+=B3. (downlin fre%uencies) and $8 to $8.*=B3 (uplin fre%uencies). The most common u band digital reception format is :?4 (main profile 'ideo format) .'s the studio profile digital 'ideo format or the full&blown :igicipher !! 8:T? format. The first commercial tele'ision networ to e
The !TU #egion $ segments of the u spectrum represent Africa and Europe ($$.8* to $$.+ =B3 band range and $6.* to $6.+* =B3 band range) is reser'ed for the fi
3u ,an% Difficu!ties Ghen fre%uencies higher than $1 =B3 are transmitted and recei'ed used in a hea'y rain fall area, a noticeable degradation occurs, due to the problems caused by and proportional to the amount of rain fall (commonly nown as nown as Hrain fadeH). This problem can be combatted, howe'er, by deploying an appropriate lin budget strategy when designing the satellite networ, and allocating a higher power consumption to o'ercome rain fade loss. !n terms of end&'iewer T? reception, it taes hea'y rainfalls in e
The higher fre%uency spectrum of the u band is particularly susceptible to signal degradation& considerably more so than C band satellite fre%uency spectrum, though the u band is less 'ulnerable to rain fade than the a band fre%uency spectrum. A similar phenomena, called Hsnow fadeH (when snow accumulation significantly alters the focal point of your dish) can also occur during Ginter eason. Also, the u band satellites typically re%uire considerably more power to transmit than the C band satellites. Bowe'er, both u and a band satellite dishes to be smaller ('arying in si3e from 6 to * in diameter.)
3u ,an% Sate!!ite Service Don!in4 Usa'e re.uenc" +an'e The u band downlin uses fre%uencies between $$.+ and $6.+=B3. The u band downlin fre%uencies are further subdi'ided according to their assigned use> 3u ,an% Usa'e
Don!in4
@i
$$.+ & $6.6=B3
4roadcast atellite er'ice $6.6 & $6.+=B3
er'ices that can be found on the u&band include educational networs, business networs, sports bachauls, tele& conferences, mobile news truc feeds, international programming, and 'arious CC (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 u band fre%uencies. !n order to do so, you will need to obtain a u&band "/4 as well as a CIu band feed&horn, plus some coa< cable for your u&band "/4.
-
As for the coa< cable recommended& #=&0 is optimal for low loss in the *1&$8*1 fre%uency range& what u&band "/4 processes. Bowe'er, if #=&* is your only 'iable option, itll 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 u 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 wont reflect u&band signals properly. Therefore, youll want to strongly consider upgrading to either a solid dish, or a mesh dish in which the hole si3e under $I8H, and ideally youll 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 u&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 adusting both the a3imuth and ele'ation, which can be a bit tricy 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. Foull 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, liely that your dish is not warped. o therefore, youll want to use either the tri&supports or %uad supports , as they will greatly assist in eeping your u&band feed&horn highly stable, e'en in high winds. Ghen your button&hoo feed mo'ing in the wind, your u&band reception can can easily drop out. 4y putting guy&wires on the button&hoo feed, youll 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% The a 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. a band dishes can be much smaller than C band dishes. a band dishes 'ary from 6 to * in diameter. a band satellites typically transmit with much more power than C band satellites. The higher fre%uencies of a band are significantly more 'ulnerable to signal %uality problems caused by rainfall, nown as rainfade
/&at is L $an% " band is a fe%uency range between 71MB3 and $.**=B3 which is used for satellite communications and for terrestrial communications between satellite e%uipment. The high fre%uencies utili3ed by C band, u band, and a 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% band is a fre%uency range from appro
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 microave cameras. Launche by %ussian launch vehicle Intercosmos.
&0.11.191
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.
19.0*.191
First e#$erimental communication satellite. !rovie e#$erience in builin" an o$eratin" a three)a#is stabilise communication satellite. Launche by the uro$ean Ariane.
/.
'has(ara)II
4.
Ariane !assen"er !ayloa #$eriment 2A!!L3
5.
%ohini ,echnolo"y !ayloa 2%,!3
10.0.1979
Intene or measurin" in)li"ht $erormance o irst e#$erimental li"ht o SL-)/ the irst Inian launch vehicle. +oul not be $lace in orbit.
*.
%ohini 2%S)13
1.07.190
6se or measurin" in)li"ht e#$erimental launch o SL-)/.
$erormance
o
secon
7.
%ohini 2%S)D13
/1.05.191
6se or conuctin" some remote sensin" technolo"y stuies usin" a lanmar( sensor $ayloa. Launche by the irst evelo$mental launch o SL-)/
.
%ohini 2%S)D&3
17.04.19/
Ientical to %S)D1. Launche by the secon evelo$mental launch o SL-)/.
9.
Stretche %ohini Satellite Series 2S%SS) &4.0/.197 13
+arrie $ayloa or launch vehicle $erormance monitorin" an or 8amma %ay astronomy. +oul not be $lace in orbit.
Stretche %ohini 1/.07.19 Satellite Series 2S%SS)
+arrie remote sensin" $ayloa o 8erman s$ace a"ency in aition to 8amma %ay astronomy $ayloa. +oul not be
10.
$$
11.
1&.
&3
$lace in orbit.
Stretche %ohini Satellite Series 2S%SS) &0.05.199& +3
Launche by thir evelo$mental li"ht o ASL-. +arrie 8amma %ay astronomy an aeronomy $ayloa.
Stretche %ohini Satellite Series 2S%SS) 04.05.1994 +&3
Launche by ourth evelo$mental li"ht o ASL-. Ientical to S%SS)+. Still in service.
Inian National Satellite System 2INSA,3 1/.
INSA,)1A
10.04.19&
First o$erational multi)$ur$ose communication an meteorolo"y satellite $rocure rom 6SA. or(e only or si# months. Launche by 6S Delta launch vehicle.
14.
INSA,)1'
/0.0.19/
Ientical to INSA,)1A. Serve or more than esi"n lie o seven years. Launche by 6S S$ace Shuttle.
15.
INSA,)1+
&1.07.19
Same as INSA,)1A. Serve or only one an a hal years. Launche by uro$ean Ariane launch vehicle.
1*.
INSA,)1D
1&.0*.1990
Ientical to INSA,)1A. Launche by 6S Delta launch vehicle. Still in service.
17.
INSA,)&A
10.07.199&
First satellite in the secon)"eneration Inian)built INSA,)& series. :as enhance ca$ability than INSA,)1 series. Launche by uro$ean Ariane launch vehicle. Still in service.
1.
INSA,)&'
&/.07.199/
Secon satellite in INSA,)& series. Ientical to INSA,)&A. Launche by uro$ean Ariane launch vehicle. Still in service.
19.
INSA,)&+
07.1&.1995
:as aitional ca$abilities such as mobile satellite service business communication an television outreach beyon Inian bounaries. Launche by uro$ean launch vehicle. In service.
&0.
INSA,)&D
04.0*.1997
Same as INSA,)&+. Launche by uro$ean launch vehicle Ariane. Ino$erable since ct 4 97 ue to $oer bus anomaly.
&1.
INSA,)&D,
;anuary 199
!rocure in orbit rom A%A'SA,
&&.
INSA,)&
0/.04.1999
=
meteorolo"ical
satellite
&/.
INSA,)/'
&&.0/.&000
&4.
8SA,)1
1.04.&001
#$erimental Satellite or the irst evelo$mental li"ht o 8eo)synchronous Satellite Launch -ehicle 8SL-)D1.
&5.
INSA,)/+
&4.01.&00&
,o au"ment the e#istin" INSA, ca$acity or communication an broacastin" besies $roviin" continuity o the services o INSA,)&+.
&*.
>AL!ANA)1
1&.09.&00&
<,SA, as the irst e#clusive meteorolo"ical satellite built by IS% name ater >al$ana +hala.
&7.
INSA,)/A
10.04.&00/
besies $roviin" meteorolo"ical services alon" ith INSA,) & an >AL!ANA)1. &.
8SA,)&
0.05.&00/
#$erimental Satellite or the secon evelo$mental test li"ht o Inia?s )eos"nc&ronous Sate!!ite Launc& e&ic!e# )SL
&9.
INSA,)/
&.09.&00/
#clusive communication satellite to au"ment the e#istin" INSA, System.
/0.
D6SA,
&0.09.&004
Inia@s irst e#clusive eucational satellite.
/1.
:A
05.05.&005
/&.
INSA,)4A
&&.1&.&005
,he most avance satellite or Direct)to):ome television broacastin" services.
//.
INSA,)4+
10.07.&00*
State)o)the)art communication satellite ) coul not be $lace in orbit.
1&.0/.&007
An ientical satellite to INSA,)4A urther au"ment the INSA, ca$acity or Direct),o):ome 2D,:3 television services an other communications.
0&.09.&007
Desi"ne to $rovie Direct),o)home 2D,:3 television services -ieo !icture ,ransmission 2-!,3 an Di"ital Satellite Nes 8atherin" 2DSN83 ientical to INSA,) 4+ .
/4.
/5.
INSA,)4'
INSA,)4+%
Inian %emote Sensin" Satellite 2I%S3 /*.
I%S)1A
17.0/.19
First o$erational remote sensin" satellite. Launche by a %ussian -osto(.
/7.
I%S)1'
&9.0.1991
Same as I%S)1A. Launche by a %ussian Launch vehicle -osto(. Still in service.
/.
I%S)1
&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 li"ht o !SL-.
40.
I%S)1+
&.1&.1995
+arries avance remote sensin" cameras. Launche by %ussian
41.
I%S)!/
&1.0/.199*
+arries remote sensin" $ayloa an an )ray astronomy $ayloa. Launche by thir evelo$mental li"ht o !SL-. Still in service.
4&.
I%S)1D
&9.09.1997
Same as I%S)1+. Launche by Inia?s !SL- service. In service. +arries an cean +olour
4/.
I%S)!4 ceansat
&*.05.1999
44.
,echnolo"y #$eriment Satellite 2,S3
&&.10.&001
45.
I%S)!* %esourcesat)1
17.10.&00/
4*.
+A%,SA, )1
05.05.&005
,echnolo"y #$eriment Satellite Launche by !SL-)+/ . Launche by !SL- ) +5 carries three camera names LISS)4 LISS)/ an AiFS Launche by !SL-)+* carries to $anchromatic cameras ) !AN 2ore3 an !AN 2at3 ) ith &.5 meter resolution. ,he cam $7
mounte ith a tilt o C&* e" an )5 e" alon" the trac( to $rovie stereo ima"es. 47.
+A%,SA, ) &
10.01.&007
Launche by !SL-)+7 it is an avance remote sensin" satellite carryin" a $anchromatic camera ca$able o $roviin" scene s$eciic s$ot ima"eries.
4.
S% ) 1
10.01.&007
Launche by !SL-)+7 S$ace ca$sule %ecovery #$eriment 2S%)13 intene to emonstrate the technolo"y o an orbitin" $latorm or $erormin" e#$eriments in micro"ravity conitions. S%)1 as recovere successully ater 1& ays over 'ay o 'en"al.
49.
+A%,SA,)&A
&.04.&00
Ientical to +A%,SA, ) & launche by !SL-)+9
50.
I
&.04.&00
Launche by !SL-)+9 alon" ith +A%,SA,)&A an other i"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 perihelion1 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 t hey 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 un/ +c 2 mv&3r *here +c is the centripetal force, m is the planet's mass, r is the planet's distance from the un This centripetal force is provided by t he gravitational force of the un/ +g 2 45m3r& *here +g is the gravitational force from the un, 4 is the 6niversal gravitational constant and 5 is the mass of the un. +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 i t moves in a circle is s 2 &eg, the un? 27 T&∝ r= o 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( Sun 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( Eart& or$itin' o$6ect7 (#efer to the e
$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
J Ar'ument of Latitu%e (not shown)> The geographic latitude of an Earth orbiting satellite at a specific time (the Epoch), e The point in a satellites orbit where it crosses the plane of the celestial e%uator (or ecliptic for a sun orbiting obect) going north. J Ar'ument of *eri'ee *eri&e!ion7 > (ω in @igure 6) > The angle between the ascending node and perigee (or perihelion for sun orbiting satellites), measured counter clocwise along the plane of the orbit. J Apo'ee Ap&e!ion7 (@igure $)> oint in orbit when the satellite is farthest from the Earth (sun). J Ce!estia! E.uator( The plane of the Earths e%uator proected onto the celestial sphere. The celestial e%uator is tilted 67.* degrees in relation to the plane of the Earths 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 Balf of the distance between the foci of an ellipse di'ided by the semi&maor a
J Ec!iptic( The plane of the Earths 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# (Ω in @igure 6)> The angle between the 'ernal e%uino< and the ascending node, measured counter&clocwise. 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 obect5s motion. !t is e%ual to the sum of the Argument of erigee and the "ongitude of the Ascending /ode (Ω K ω 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
J *eri'ee *eri&e!ion7 (@igure $)> The point in an orbit when the satellite is closest to the Earth (sun). J *erio%> The time it taes 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 obect (star, planet, or satellite), as seen from the Earth. !n astronomy, #ight Ascension (#A) is e
e 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 The half of the shorter of the two a The actual angle that a satellite has mo'ed since last passing perigee (or perihelion). J erna! 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
Unit II
)eostationar" Or$it : Space Se'ment
)eostationar" ;rbit A 'eostationar" orbit is one in which a satellite orbits the earth at e
#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 earths surface. 4ut Clare published an article in $8*s Gireless Gorld that made the leap from the =ermans rocet research to suggest permanent manmade satellites that could ser'e as communication relays. =eostationary obects 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 earths 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 real estate 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#E: 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
= 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 bacup batteries onboard to eep them running in the e'ent of a solar eclipse, when theres no solar power. mall rocet boosters on each satellite eep them flying in the correct path. The satellites ha'e a lifetime of about $1 years until all their fuel runs out.
)eostationar" Sate!!ites =eostationary or communications satellites are A#E: in space 66,711 miles (7*,11 m) abo'e the e%uator of the TAT!;/A#F earth. =eostationary satellites are used for weather forecasting, satellite T?, satellite radio and most other types of global communications.
@ig A @ig 4 @ig A Communications satellite in a stationary position or slot high abo'e the earth. @ig 4 atellite dish or recei'er installed on a house. These dishes point to a geostationary satellite At e
6$
At e 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
66
The sun mo'es across the e%uator twice a year gi'ing us the 'ernal (spring) and fall (autumnal) e%uino
6 times each year the sun passes the e%uator as it maes 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 esuits forgot to change the dictionaryLL ;b'iously the esuits forgot to change the definition of the word EU!/;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
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
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
68
!mage of a = satellite. mall rocet boosters on each satellite eep it flying in the correct path. The satellites ha'e a lifetime of about $1 years until all their fuel runs out. =eostationary satellite diagram. Clic on image to enlarge.
Spin an% T&ree-A9is Sta$i!i;ation
Spi nandThr eeAx i sSt abi l i z at i on Cr e di t s-NASA
6*
Cr e di t s-NASA
pin stabili3ation and three&a
60
Station-4eepin' in LEO tation&eeping is necessary for obects 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$1 m. :ue to atmospheric drag, the space station is constantly losing orbital energy. !n order to compensate for this loss, which would e'entually lead to a reentry of the station, it is being reboosted to a higher orbit from time to time. The chosen orbital altitude is a trade&off between the delta&' needed to reboost the station and the delta&' needed to send payloads and people to the station. The upper limitation of orbit altitude is due to the constraints imposed by the oyu3 spacecraft. ;n 6* April 611-, the Automated Transfer ?ehicle Hules ?erneH raised the orbit of the ! for the first time, thereby pro'ing its ability to replace (and outperform) the oyu3 at this tas.
Station-4eepin' in )EO ;nce a satellite has reached geostationary orbit, it seems natural that it should remain there. "ife, of course, is not so simple because orbital perturbations cause the satellite to drift.
!nclined orbital planes The principal correction re%uired is to compensate for /orth&outh drift. The geostationary plane (abo'e the e%uator) is not aligned to the Earths orbit round the un (ecliptic) or the Moons orbit round the Earth, so the gra'itational pull of the un and Moon drags satellites off the plane. Uncorrected, this would cause the inclination of the orbit to increase by appro
therefore crucial for =E; satellites to ha'e the most fuel&efficient propulsion system. ome modern satellites are therefore employing a high specific impulse system lie plasma or ion thrusters.
TT:C Su$s"stem The TTQC ubsystem contains #adio @re%uency (#@) components, woring 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
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
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 dont 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 mae 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 maes 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. "iewise, 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 tracing capability which will allow its antennas to follow the target during the time that it is 'isible. The only alternati'e is to mae that antenna beam so wide that the intended recei'er (or transmitter) is always within it. ;f course, maing 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
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
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 Earths surface or within the maor portion of the Earths 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 obects in space.
/&at is up!in4 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
u 4and
$8 & $8.* =B3
a 4and
6+.* & 7$ =B3
Up!in4 U=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!in4 :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
u 4and
$$.+ & $6.+ =B3
a 4and
$-.7 & 61.6 =B3
Don!in4 D=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 networs, and forwards data pacets to the ne
They connect networs together2 a "A/ to a GA/ for e
#outers operate in two different planes> •
Control lane, in which the router learns the outgoing interface that is most appropriate for forwarding specific pacets to specific destinations. 77
•
@orwarding lane, which is responsible for the actual process of sending a pacet 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
Mo%ems(A modem (modulator&demodulator) is a de'ice or program that enables a computer to transmit data o'er, for e
A :" Modem
The most familiar e
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
#& 676. Conse%uently any e
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
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. oice=Data(-
Many modems support a switch to change between 'oice and data modes. !n data mode, the modem acts lie a regular modem. !n 'oice mode, the modem acts lie a regular telephone. Modems that support a 'oiceIdata switch ha'e a built&in loudspeaer 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
$) 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 adacent 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 adacent fre%uency segments. CDMA(-
C:MA is a form of multiple
SOME OT
:emand Assigned Multiple Access (:AMA) is a technology used to assign a bandwidth to clients which dont need to use it constantly. :AMA systems %uicly and transparently assign communication lins 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+
INO+MATION E2C
Computer
+outer
MODEM
U* Converter
<*A
:ata is transmitted in digital form through router that determines the proper path for data to tra'el between the networs, and forwards data pacets 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 SBA and then sent to the antenna for the transmission.
Data +eception(-
LNA
Don Converter
MODEM
+outer
Computer
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.
7-
/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.
oice E9c&an'e( oice Transmission(-
*&one
E9c&an'e
MU2
Mo%em
U* Converter
<*A
?oice is transmitted through router that determines the proper path for data to tra'el between the networs, and forwards data pacets 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 SBA and then sent to the antenna for the transmission. oice +eception(-
LNA
Don Converter
MODEM
E9c&an'e
*&one
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.
7
/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
Up Converter(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.
Don Converter(The down con'erter contains fre%uency translating circuit which con'erts f c 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 LNA7(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 81
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. Acti'e transponder> These de'ices as the name implies, continually emit radio signals which are traced 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
Unit V
DI+ECT ,+OADCAST SATELLITE SE+ICES
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 the :TB maret which re%uired the larger dishes and the :4 (AT#A) maret which re%uired smaller (1.M dishes). As higher powered satellites lie AT#A came into operation, the acronym :4 gradually supplanted it. 8$
The term :4 now co'ers both analog and digital tele'ision and radio reception, and is often e
Termino!o'" confusion !n certain regions of the world, especially in /orth America, :4 is used to refer to pro'iders of subscription satellite pacages, 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 u&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 liely 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 u&band, normally $6.6 to $6.+ =B3, as this bandwidth is often referred to as :4 or one of its synonyms. !n comparison, European Hu bandH :4 systems can drop as low as $1.+ =B3. Adding to the naming comple
Commercia! D,S services The first commercial :4 ser'ice, y Tele'ision plc (now 4y4), 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 ingdom 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 U or !reland, the popularity of ys ser'ice has caused the terms HminidishH and Hdigibo
:ominion ?ideo atellite !nc.s y Angel also went online in the United tates in $0 with its :4 ser'ice geared toward the faith and family maret. !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 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&thining 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 /etwors channels as well. !n 6117, Echotar attempted to purchase :irecT?, but the U.. :epartment of ustice denied the purchase based on anti&competiti'e concerns.
ree D,S services =ermany is liely the leader in free&to&air :4, with appro
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orar% Error Correction vs> ,ac4ar% Error Correction @orward Error Correction protocols impose a greater bandwidth o'erhead than bacward error correction protocols, but are able to reco'er from errors more %uicly 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 unnown point by measuring the lengths of the sides of an imaginary triangle between the unnown 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
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= 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 orean Airline @light A"&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 redundan t bacup. = uses /A?TA# satellites manufactured by #ocwell !nternational. Each /A?TA# satellite is appro
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 waalein. Communications with the space segment are conducted through ground antennas in the Ascension !slands, :iego =arcia, and waalein.
T&e User Se'ment The = user segment is any person with a = recei'er.
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?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 ?ATs 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 worlds most used ?ATs, the u 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
Imp!ementations of SAT 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 ohn il'ers, and other fast food chains), =TEC, =!, and !ntralot also utili3es large ?AT networs. 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
SAT Confi'urations Most of the current ?AT networs 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 networs 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.
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SAT@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 bacup 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 networs o'er the air. The pricing is also affordable, as the networs 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.
SAT Dra$ac4s As with e'erything, ?AT also has its downsides. @irstly, because the ?AT technology utili3es the satellites in geosynchronous orbit, it taes 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 lie :irecT? and :!B /etwor, the ?AT still can ha'e a dim signal, as it still relies on the antenna si3e, the transmitters 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 sy. An awward roof, such as with syscraper designs, can become problematic.
+ADA+SAT #A:A#AT is an ad'anced Earth obser'ation satellite proect 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
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+ADA+SAT *rocessin' Leve!s C#! supports the following #A:A#AT processing le'els. Si'na! Data +A/7 & ignal :ata cannot be 'iewed as an image. !t is an unprocessed matri< of time delays • that has been repacaged 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 S)7 & ath !mage products are recommended for indi'iduals and organisations e
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C+IS*@s +ADA+SAT *rocessin' Leve! Avai!a$i!it"
,eam Mo%e
*at& Ima'e *at& Ima'e Coarse S)7 S)C7
Map Ima'e SS)7
Si'na! Data +A/7
Sin'!e Loo4 Comp!e9 SLC7
@ine tandard Gide canA# /arrow
/ot A'ailable
/ot A'ailable
canA# Gide
/ot A'ailable
/ot A'ailable
E
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 N+T7 & :igital products are processed within hours of reception. +us& & :igital products are processed wit hin 8- hours of reception. +e'u!ar & :igital products are processed wit hin $1 woring 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