A report on communication satellites

Advance Satellite Communication

A satellite is a physical object that orbits, or rotates about some celestial body.

Satellites occur in nature, and our own solar system is a perfect example. Theearth and other planets are satellites rotating about the sun. The moon is a satelliteto the earth. A balance between the inertia of the rotating satellite at highspeed and the gravitational pull of the orbited body keeps the satellite in place.

Satellites are launched and orbited for a variety of purposes. The most commonapplication is communication in which the satellite is used as a repeater. Inthis chapter, we introduce satellite concepts and discuss how satellites are identifiedand explained. We summarize the operation of a satellite ground stationand review typical satellite applications, with particular emphasis on the GlobalPositioning System, a worldwide satellite-based navigational system.

Satellite Orbits:

The ability to launch a satellite and keep it in orbit depends upon following well-known physical and mathematical laws that are referred to collectively as orbital dynamics. Inthis section we introduce these principles before discussing the physical components ofa satellite and how it isused in various communication applications.


Principles of Satellite Orbits and Positioning:

If a satellite were launched vertically from the earth and then released. It would fall backto earth because of gravity. For the satellite to go into orbit around the earth, it musthave some forward motion. For that reason, when the satellite is launched, it is givenboth vertical and forward motion. The forward motion produces inertia, which tends tokeep the satellite moving in a straight line. However, gravity tends to pull the satellitetoward the earth. The inertia of the satellite is equalized by U1e earth's gravitational pull.The satellite constantly changes its direction from a straight line to a curved line to rotateabout the earth.

If a satellite's velocity is too high, the satellite will overcome the earth's pull andgo out into space. It takes an escape velocity of approximately 25.000 mi/h to cause aspacecraft to break the gravitational pull of the earth. At lower speeds, gravity constantly pulls the satellite toward the earth. The goal is to give the satellite acceleration and speedthat will exactly balance the gravitational pull.

The closer the satellite is to earth, the stronger the effect of the earth'sgravitationalpull. So in low orbits, the satellite must travel faster to avoid falling back to earth. Thelowest practical earth orbit b approximately 100 mi. At this height, the satellite's speedmust be about 17.500 mil/h to keep the satellite in orbit. At this speed, the satellite orbitsthe earth in approximately 1 and 1/2 h. Communication satellites are usually much farther fromearth. A typical distance is 22,300 mi. A satellite need travel only about 6800 mi/h tostay in orbit at that distance. At this speed, the satellite rotates about the earth in approximately24 h, the earth’s own rotational time.

A satellite rotates in an orbit that forms a plane passing through the center of gravityof the earth called the geocenter. In addition, the direction of satelliterotation may be either in the same direction as the earth's rotation or against the directionof earth's rotation. In the former case, the orbit is said to be posigrade and in thelatter case, retrograde. Most orbits are posigrade. In a circular orbit, the speed of rotationis constant. However, in an elliptical orbit, the speed changes depending upon theheight of the satellite above the earth. Naturally the speed of the satellite is greater whenit is close to the earth than when it is far away.

Satellite Height.

In a circular orbit, the height is simply the distance of the satellitefrom the earth. However, in geometric calculations, the height is really the distancebetween the center of the earth and the satellite. In other words, that distance includes the radius of the earth, which is generally considered to be about 3960 mi (or 6373 km).A satellite that is 5000 mi above the earth in circular orbit is 3960 + 5000 or 8960mifrom the center of the earth. When the satellite is in an elliptical orbit, the center of the earth is one of the focal points of the ellipse. In this case, the distance of the satellite fromthe earth varies according to its position. Typically the two points of greatest interest are the highest point above the earth-the apogee- and the lowest point-the perigee. Theapogee and perigee distances typically are measured from the geocenter of the earth.


Satellite Speed. As indicated earlier, the speed varies according to the distance ofthe satellite from the earth. For a circular orbit the speed isconstant, but for an elliptical orbit the speed varies according to the height. Low earth satellites of about 100 miin height have a speed in the neighbourhood of 17,500 mi/h. Very high satellites such ascommunication satellites, which are approximately 22.300 mi out, rotate much moreslowly, a typical speed of such a satellite being in the neighbourhood of 6800 mi/h.

Satellite Period. The period is the time it takes for a satellite to complete one orbit. It is also called the sidereal period. A sidereal orbit uses some external fixed or apparentlymotionless object such as the sun or star for reference in determining a siderealperiod. The reason for using n fixed reference point is that while the satellite is rotating about the earth, the earth itself is rotating.

Communication satellites are not originators of information to be transmitted. Althoughsome other types of satellites generate the information to be transmitted. Communicationsatellites do not. Instead, these satellites are relay stations for earth sources. If a transmittingstation cannot communicate directly with one or more receiving stations becauseof line-of-sight restrictions, a satellite can be used. The transmitting station sends the informationto the satellite, which in turn retransmits it to the receiving stations. The satellitein this application is what is generally known as a repeater.

Repeaters and Transponders

An earth stationtransmits information to the satellite. The satellite contains a receiver that picks up thetransmitted signal, amplifies it, and translates it on another frequency. The signal on thenew frequency is then retransmitted to the receiving stations on earth. The original signalbeing transmitted from the earth station to the satellite is called the uplink, and the retransmittedsignal from the satellite to the receiving stations is called the downlink. Usuallythe downlink frequency is lower than the uplink frequency. A typical uplink frequency is6 GHz. and a common downlink frequency is 4 GHz.The transmitter-receiver combination in the satellite is known as a transponde1:The basic functions of a transponder are amplification and frequency translation. The reason for frequency translation is that the transponder cannot transmit andreceive on the same frequency. The transmitter's strong signal would overload, or "desensitize” the receiver and block out the very small uplink signal, thereby prohibiting any communication.Widely spaced transmit and receive frequencies prevent interference.Transponders are also wide-bandwidth units so that they can receive and retransmitmore than one signal. Any earth station signal within the receiver's bandwidth will beamplified, translated, and retransmitted on a different frequency.Although the typical transponder has a wide bandwidth, it is used with only oneuplink or downlink signal to minimize interference and improve communicationreliability. To be economically


feasible, a satellite must be capable of handling several channels. As aresult,most satellites contain multiple transponders, each operating at a different frequency. A typical communication satellite has 24 channels. 12 verticallypolarized and 12 horizontally polarized. Each transponder represents an individual communicationchannel. Various multiple-access schemes arc used so that each channel cancarry multiple information transmissions.

Frequency Allocations:

Most communication satellites operate in the microwavefrequency spectrum. However, there are some exceptions. For example,many militarysatellites operate in the 200- to 400-VHF/UHF range. Also, the amateur radio OSCARsatellites operate in the VHF/UHF range. VHF, UHF and microwave signals penetratethe ionosphere with little or no attenuation and are not refracted to earth, as are lowerfrequencysignals in the 3- to 30-MHz range.The microwave spectrum is divided up into frequency bands that have been allocatedto satellites as well as other communication services such as radar. These frequencybands are generally designated by a letter of the alphabet. One of the most widely used satellite communication bands is the C band. Theuplinkfrequencies are 5.925 to 6.425 GHz. In any general discussion of the C band,theuplink is generally said to be 6 GHz. The downlink is in the 3.7- to 4.2-GHz range. Butagain, in any general discussion of the C band, the downlink is nominally said to be4 GHz. Occasionally, the C band is referred to by the designation 6/4 GHz, where theuplink frequency is given first.

Satellite Architecture and Organization:


Single Conversion Transponder

Double Conversion Transponder

Regenerative Repeater

All satellite communication systems consist of two basic parts, the satellite or spacecraftand two or more earth stations. The satellite performs the function of a radio repeater orrelay station. Two or more earth stations may communicate with one another through thesatellite rather than directly point-to-point on the earth.Satellites vary in size from about I ft3 for a small LEO satellite to more than 20 ftlong. The largest satellites are roughly the size of the trailer on an 18-wheeler. Weightranges from about 100 lb for the smaller satellites to nearly 10.000 lb for the largest.The heart of a communication satellite is the communication subsystem. This is aset of transponders that receive the uplink signals and retransmit them to earth. Atransponder is a repeater that implements a wideband communication channel that cancarry many simultaneous communication transmissions.The transponders are supported by a variety of additional ''housekeeping•· subsystems.These include the power subsystem, the telemetry tracking and commandsubsystems, the antennas, and the propulsion and attitude stabilisation subsystems. These are essential to the self-sustaining nature of the satellite. The solar panels supply the electric power for the spacecraft. They drive regulatorsthat distribute de power to all other subsystems. And they charge the batteriesthat operate the satellite during eclipse periods. And ac-to-dc converters and dc-to-acinverters are used to supply special voltages to some subsystems. The communication subsystem consists of multiple transponders. These receive theuplink signals, amplify them, translate them in frequency, and amplify them again forretransmission as downlink signals. The transponders share an antenna subsystem forboth reception and transmission.The telemetry, tracking, and command (TT&C) subsystem monitors on-board conditionssuch astemperature and battery voltage and transmits this data back to a groundstation for analysis. The ground station may then issue orders to the satellite by transmittinga signal to the command subsystem, which then is used to control many spacecraftfunctions such as firing the jet thrusters.


The jet thrusters and the apogee kick motor (AKM)are part of the propulsion subsystem.They are controlled by commands from the ground.The attitude control subsystem provides stabilization in orbit and senses changes inorientation. It tires the jet thrusters to perform attitude adjusm1ent and station-keepingmaneuvers that keep the satellite in its assigned orbital position.

Multichannel Subsystem:

There are two basic multichannel architechtures:

Broadband multiple –channel repeater and multichannel(fully channelized) system

The above diagram shows a broadband multiple channel repeater. The receiver antenna is connected to the LNA.Wide bandwidth tined circuits are used so that the entire 500Mhz bandwidth received and amplified. A mixer translates all incoming signals into their equivalent lower down-link frequencies.A wideband amplifier following the mixer amplifies this entire spectrum. The channelisation process occurs in the remainder of the transponder. For example, in a 12 channel satellite, 12 bandpass filters each centred on one of the 12 channels is used to separate all the various received signals. The bandpass filters separate out the unwanted mixer output signals and retain only the difference signals. Then individual HPAs are used to increase the signal level. These are usually TWTs. The output of each TWT amplifier is again filtered to minimise harmonics and intermodulation distortion problems. These filters are usually part of a larger assembly known as a multiplexer or a combiner. In any case, it is a waveguide-cavity resonator assembly that filters and combines all the signals for application to a single antenna.


Another transponder architecture used in communication satellites uses individual narrowband input channels instead of the single wideband input described previously.

The receiver antenna feeds a demultiplexer, a waveguide assembly that seperates a single wideband input into equal feeds for separate channels. Separate LNAs and bandpass filters centered on the designated channels are used at the inputs. The filters sort out the input signals, and LNAs provide the desired amount of gain. Although each bandpass filter contributes to the noise level, this is offset by the narrowband operation.Further HPAs and BPFs are used for the purpose of amplification(by TWT) and removing the noise respectively. The output bandpass filters are part of a multiplexer combiner unit which creates one feed for the transmitting antenna.


Power Subsystem

Today virtually every satellite uses solar panels for its basic power source. Solar panels are large arrays of photocells connected in various series and parallel circuits to createa powerful source of direct current. Early solar panels could generate hundreds of wattsof power. Today huge solar panels are capable of generating many kilowatts. A keyrequirement is that the solar panels always be pointed toward the sun. There are twobasic satellite configurations. In cylindrical satellites, the solar cells surround the entireunit, and therefore some portion of them is always exposed to sunlight. In body-stabilised, or three-axis satellites, individual solar panels are manipulated with variouscontrols to ensure that they are correctly oriented with respect to the sun.Solar panels generate a direct current that is used to operate the various componentsof the satellite. However, the de power is typically used to charge secondary batteriesthat act us a buffer. When a satellite goes into an eclipse or when the solar panels arenot properly positioned, the batteries take over temporarily and keep the satellite operating.The batteries are not large enough to power the satellite for a long time: they areused as a backup system for eclipses, initial satellite orientation and stabilization, oremergency conditions.The basic de voltage from the solar panel is conditioned in various ways. For example,it is typically passed through voltage regulator circuits before being used to powerindividual electronic circuits. Occasionally, voltages higher than those produced by thesolar panels must also be generated. For example, the TWT amplifiers in most communicationtransponders require thousands of volts for proper operation. Special dc-to-dc converters are used to translate the lower de voltage of the solar panels to the higher dcvoltage required by the TWTs.


Satellite Applications

Surveillance Satellites

Another application of satellites is in surveillance or observation. From their vantagepoint high in the sky. Satellites can look at the earth and transmit what they see to groundstations for a wide variety of purposes. For example, military satellites are used to performreconnaissance. On-board cameras take photographs that can later be ejected fromthe satellite and brought back to earth for recovery. TV cameras can take pictures andsend them back to earth as electric signals. Infrared sensors detect heat sources. Smallradars can profile earth features.Intelligence satellites collect information about enemies and potential enemies. Theypermit monitoring for the purpose of proving other countries' compliance with nucleartest ban and missile stockpile treaties.There are many different kinds of observation satellites. One special type is themeteorological. or weather, satellite. These satellites photograph cloud cover and send backto earth pictures that are used for determining and predicting the weather. Geodetic satellitesphotograph the earth for the purpose of creating more accurate and more detailed maps.

Navigation Satellites

A second applications area is navigation. Electronic systems have been used for years toprovide accurate position information to ships, airplanes, and land-based vehicles.Loran and Omega are well-known systems used in marine navigation.

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A report on communication satellites