1.

1 Introduction

Satellite communications are the outcome of research in the area of communications and

space technologies whose objective is to achieve ever-increasing ranges and capacities

with the lowest possible costs. The World War II stimulated the expansion of two very

distinct technologies – missiles and microwaves. The expertise eventually gained in the

combined use of these two techniques opened up the era of satellite communications. The

service provided in this way usefully complements that previously provided exclusively

by terrestrial networks using radio and cables.

The space era started in 1957 with the launching of the first artificial satellite

(Sputnik). Subsequent years have been marked by various experiments including the

following: Christmas greetings from President Eisenhower broadcast by Score (1958),

the reflecting satellite ECHO (1960), store-and-forward transmission by the Courier

satellite (1960), powered relay satellites (Telstar and Relay in 1962), and the first

geostationary satellite Syncom (1963). In 1965, the first commercial geostationary

satellite Intelsat I (or Early Bird) inaugurated the long series of Intelsats; in the same

year, the first Soviet communications satellite of the Molniya series was launched.

Satellites are somewhat closer to the moon than the earth is (or rather, can be),

and might well have the sun blocked from them. Furthermore, they can move somewhat

more than the Earth, and thus have the potential for more eclipses. Since the shadow

effects are very important during the motion of the satellite around the earth, it must be

taken into account when the perturbation problems are treated.

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1.2 Problem Statement

A phase in which the satellite doesn't receive the sunlight to recharge itself through the

solar cells that in turn reduces the power of the satellite to transmit/receive signals from

the earth station.

2.2 Objectives of the Project

1. To know the satellite communication system.

2. To get information about satellite orbits.

3. To know the satellite eclipse of the sun and its calculations.

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2.1 Satellites

A satellite is a body that passes around some other body in a mathematically foreseeable

path which is often called an orbit. A communication satellite is nothing but a microwave

repeater station in space that plays a significant role in telecommunications, radio, and

television along with internet applications.

2.1.1 Satellite Communication

Satellite communication is the technique of conveying data from one place to another

using a communication satellite in the earth’s orbit. A communication satellite is a mock

or artificial satellite which is responsible for transmitting a signal using a transponder by

creating a channel between the transmitter and receiver which are located in two entirely

different locations on earth.

2.1.2 Useful of Satellite Communications

A communications satellite is an artificial satellite that relays and amplifies radio

telecommunication signals via a transponder; it creates a communication channel

between a source transmitter and a receiver at different locations on earth.

Communications satellites are used for television, telephone, radio, internet, and military

applications.

The high frequency radio waves used for telecommunications links travel by line

of sight and so are obstructed by the curve of the earth. The purpose of communications

satellites is to relay the signal around the curve of the earth allowing communication

between widely separated geographical points. Communications satellites use a wide

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range of radio and microwave frequencies. To avoid signal interference, international

organizations have regulations for which frequency ranges or "bands" certain

organizations are allowed to use. This allocation of bands minimizes the risk of signal

interference.

2.1.3 Advantages, Disadvantages and Applications of Satellite Communication

Table 2.1 illustrates the advantages, disadvantages and applications of satellite

communication.

Table2.1: The advantages, disadvantages and applications of satellite

communication.

Advantages Disadvantages Applications

 The introductory

costs, for example

 Flexibility. segment and  In radio telecom.

installation costs are

excessively high.

 Ease in putting in  Congestion of  In TV broadcasting, for example

new circuits. frequencies. Direct to Home (DTH).

 Distances are  In internet applications, for

effortlessly taken example giving internet connection

 Interference and
care of and for transferring, Global System
proliferation
expense doesn’t Position (GSP) applications, and

make a difference internet surfing.

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 Table2.1: The advantages, disadvantages and applications of satellite

communication.

 Broadcasting

conceivable ____  For voice correspondences.

outcomes

 Each side of the  For innovative work, in numerous

____
earth is secured regions.

 User can control

____  In military applications and routes.
the system

2.1.4 Types of Satellites and applications

 Communications satellite.

 Remote sensing satellite.

 Navigation satellite.

 Geocentric orbit type satellites – Low Earth Orbit (LEO), Medium Earth Orbit

(MEO), High Earth Orbit (HEO).

 Global positioning system (GPS).

 Geostationary Satellites (GEOs).

 Drone satellite.

 Ground satellite.

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2.2 Satellite Communications System

The satellite system is composed of a space segment, a control segment, and a ground

segment: The space segment contains one or several active and spare satellites organized

into a constellation. The control segment consists of all ground facilities for the control

and monitoring of the satellites, also named Tracking, Telemetry, and Command (TT&C)

stations, and for the management of the traffic and the associated resources on board the

satellite for communication networks. The ground segment consists of all the traffic earth

stations. Depending on the type of service considered, these stations can be of different

size, from a few centimeters to tens of meters. Figure 2.1 gives an overview of a satellite

communication system and illustrates its interfacing with terrestrial entities.

Figure 2.1 Satellite communications system interfacing with terrestrial entities

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2.3 Communications Links

A link between transmitting equipment and receiving equipment consists of a radio or

optical modulated carrier. The performance of the transmitting equipment is measured by

its Effective Isotropic Radiated Power (EIRP), which is the power fed to the antenna

multiplied by the gain of the antenna in the considered direction. The performance of the

receiving equipment is measured by G/T, the ratio of the antenna receive gain G, in the

considered direction and the system noise temperature T; G/T is called the receiver’s

figure of merit. The types of link shown in Figure 1.1 are:

- Uplinks from the earth stations to the satellites

- Downlinks from the satellites to the earth stations

- Inter satellite links between the satellites.

Uplinks and downlinks consist of radio frequency modulated carriers, while Inter

Satellite Links (ISLs) can be either radio frequency or optical. Some large-capacity data-

relay satellites also use optical links with their ground stations. Carriers are modulated by

baseband signals conveying information for communications purposes.

2.4 Types of Orbit

The orbit is the trajectory followed by the satellite. The trajectory is within a plane and

shaped like an ellipse with a maximum extension at the apogee and a minimum at the

perigee. The satellite moves more slowly in its trajectory as the distance from the earth

increases, according to the laws of physics. The most favorable orbits are as follows:

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- Elliptical orbits inclined at an angle of with respect to the equatorial plane. This

type of orbit is particularly stable with respect to irregularities in terrestrial

gravitational potential and, owing to its inclination, enables the satellite to cover

regions of high latitude for a large fraction of the orbital period as it passes to the

apogee. This type of orbit has been adopted by Russia for the satellites of the Molniya

system with a period of 12 hours. Figure 2.2 shows the geometry of the orbit. The

satellite remains above the regions located under the apogee for a time interval on the

order of eight hours. Continuous coverage can be ensured with three phased satellites

in different orbits. Several studies relate to elliptical orbits with a period of 24 hours

(Tundra orbits) or a multiple of 24 hours. These orbits are particularly useful for

satellite systems for communication with mobiles, where the masking effects caused

by surrounding obstacles such as buildings and trees and multiple-path effects are

pronounced at low elevation angles (less than ). In fact, inclined elliptic orbits can

provide the possibility of links at medium latitudes when the satellite is close to the

apogee with elevation angles close to ; these favorable conditions cannot be

provided at the same latitudes by geostationary satellites. In the late 1980s, the

European Space Agency (ESA) studied the use of elliptical highly inclined orbits

HEOs for Digital Audio Broadcasting (DAB) and mobile communications in the

framework of its Archimedes program. The concept became reality at the end of the

1990s with the Sirius system delivering satellite digital audio radio services to millions

of subscribers (mainly automobiles) in the United States using three satellites in HEO

Tundra-like orbits (AKT-08). Both Molnya and Tundra orbits provide users with

higher elevation angles than Geostationary Earth Orbit (GEO) orbit at high latitude.

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 Figure 2.2 The orbit of a Molniya satellite.

- Circular LEOs: The altitude of the satellite is constant and equal to several hundreds

of kilometers. The period is on the order of one and a half hours. With near

inclination, this type of orbit guarantees worldwide long-term coverage as a result of

the combined motion of the satellite and earth rotation, as shown in Figure 1.7. This is

the reason for choosing this type of orbit for observation satellites (for example, the

SPOT satellite: altitude 830 km, orbit inclination , period 101 minutes). One can

envisage the establishment of store-and-forward communications if the satellite is

equipped with a means of storing information. A constellation of several tens of

satellites in low-altitude (e.g. IRIDIUM with 66 satellites at 780 km) circular orbits

can provide worldwide real-time communication as shown in Figure 2.3. Non-polar

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 orbits with less than inclination can also be envisaged. For instance, the

GLOBALSTAR constellation incorporates 48 satellites at 1414 km with orbit

inclination.

Figure 2.3 Circular polar low earth orbit (LEO)

- Circular MEOs, also called Intermediate Circular Orbits (ICOs), have an altitude of

about 10000 km and an inclination of about . The period is six hours. With

constellations of about 10 – 15 satellites, continuous coverage of the world is

guaranteed, allowing worldwide real-time communications. A planned system of this

kind was the ICO system (which emerged from Project 21 of International

Telecommunications Satellite (INMARSAT) organisation but was not implemented)

with a constellation of 10 satellites in two planes at inclination. O3b is a special

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 case of a MEO circular orbit satellite constellation with altitude at 8063 km and 20

satellites. Each satellite has 12 steerable Ka band antennas of which 2 are for gateways

and 10 are for user terminals.

- Circular orbits with zero inclination (equatorial orbits): The most popular is the

geostationary satellite orbit; the satellite orbits around the earth in the equatorial plane

according to the earth’s rotation at an altitude of 35786 km. The period is equal to that

of the rotation of the earth. The satellite thus appears as a point fixed in the sky and

ensures continuous operation as a radio relay in real time for the area of visibility of

the satellite (43% of the earth’s surface).

2.5 The Geostationary Satellite

The geostationary satellite is certainly the most popular. At the present time there are

around 600 geostationary satellites in operation within the f the whole orbital arc.

Some parts of this orbital arc, however, tend to be highly congested (for example, above

the American continent and Europe). Figure 2.4 illustrates the satellite orbit altitudes

(LEO/MEO/GEO) and coverage areas.

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 Figure 2.4 Illustration of orbit altitudes and coverage

2.6 Keplerian Orbits

These orbits are named after Johannes Kepler (a German mathematician, astronomer, and

astrology, 27 December 1571–15 November 1630), who established, at the start of the

seventeenth century, that the trajectories of planets around the sun were ellipses and not

combinations of circular movements as had been thought since the time of Pythagoras (a

Greek philosophy, around 570–4950 BC. Keplerian movement is the relative movement

of two point bodies under the sole influence of their Newtonian attractions.

2.7 The earth’s orbit

In the Keplerian hypotheses, the earth is assumed to be a spherical and homogeneous

body. The real earth differs from this primarily by a flattening at the poles. The terrestrial

surface is equivalent, on a first approximation, to that of an ellipsoid of revolution about

the line of the poles whose parameters depend on the model chosen. The International

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Astronomical Unit (AU) has, since 1976, recommended a value of 6378.144 km mean

Equatorial Radius (ER) for the semi-major axis and for the oblate-ness

A = (a - b)/a Eq.(1)

The value 1/298.257 (b is the semi-minor axis).

2.7.1 Motion of the earth about the sun

The earth rotates around the sun as shown in Figure 2.5 with a period of approximately

365.25 days following an ellipse of eccentricity 0.01673 and semi-major axis 149597870

km, which defines the AU of distance. Around 2 January, the earth is nearest to the sun

(the perihelion), while around 5 July it is at its aphelion (around 152100000 km). The

plane of the orbit is called the plane of the ecliptic. The plane of the ecliptic makes an

angle of (the obliquity of the ecliptic, which decreases around 47 per century)

with the mean equatorial plane.

Figure 2.5 The earth rotates around the sun

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The apparent movement of the sun around the earth with respect to the equatorial plane is

represented by a variation of the declination of the sun. The declination varies during the

year between +23.44∘ (at the summer solstice) and -23.44∘ (at the winter solstice). The

declination is zero at the equinoxes. The direction of the sun at the spring equinox defines

the vernal point or 𝛾 point on the celestial sphere (the geocentric sphere of infinite

radius). The sun passes through it from the southern hemisphere to the northern

hemisphere, and the declination is zero becoming positive. The relation between the

declination of the sun 𝛿 and the date is obtained by considering the apparent movement of

the sun about the earth in an orbit of ellipticity e equal to 0.01673, inclined at the equator

with obliquity 𝜀.

sin 𝛿 = sin 𝜀 sin u Eq.(2)

With sin 𝜀 = sin 23.44∘ = 0.39795 and u, the nodal elongation of the sun, equal to

the sum of the true anomaly of the sun and the argument of the perigee 𝜔SUN. The

argument of the perigee of the orbit representing the apparent movement of the sun about

the earth remains more or less constant through the years if the precession of the

equinoxes is neglected and has a value around 280∘. Figure 2.6 shows the apparent

movement of the sun about the earth.

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 Figure 2.6 Apparent movement of the sun about the earth

2.8 Satellite Eclipses of the Sun

An eclipse of the sun occurs for a satellite when it passes into the conical shadow region

of the earth or moon. The occurrence and duration of these eclipses depend on the

characteristics of the satellite orbit. The consequences of the eclipse on the satellite are of

two types. On the one hand, the electrical power supplies system of the satellite, which

includes photovoltaic cells to convert solar energy into electrical energy, must make use

of an alternative energy source. On the other hand, as the satellite is no longer heated by

the sun, the thermal equilibrium of the satellite is greatly modified and the temperature

tends to decrease rapidly.

2.8.1 Satellite Eclipse definitions

A satellite is said to be in eclipse when the earth or moon prevents sunlight from

reaching it. If the earth’s equatorial plane coincides with the plane of earth’s orbit around

sun, the geostationary orbit will be eclipsed by the earth. This is called the earth eclipse

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of satellite. For a geostationary satellite, the solar eclipse due to earth occurs during two

periods that begin 23 days before equinox and ends 23 days after equinox. Because

during equinox (autumn and spring) the sun, earth and the satellite are in the same plane.

Solar eclipses are important as they affect the working of the satellite because during

eclipse satellite receives no power from its solar panels and it has to operate on its

onboard standby batteries which reduce satellite life. All these situations the eclipse

happened and depicted in Figure 2.7.

Figure 2.7 Satellite eclipse of the sun

2.8.2 Satellite Eclipses of the Sun by the Moon

The orbit of the moon around the earth, with a semi-major axis of 384400 km and a

period of 27 days, has an inclination of with respect to the ecliptic. The Right

Ascension of the Ascending Node (RAAN) on the ecliptic is also affected by a precession

in the retrograde direction of period 18.6 years. The relative movement of an artificial

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earth satellite and the natural satellite is thus complex, and determination of the dates at

which the artificial satellite is aligned with the sun-moon direction cannot easily be

formulated for the general case.

Eclipses by the moon are infrequent, are most often of short duration, and most

often do not totally obscure the solar disc. They do not generally constrain the satellite

design and operation unless they precede or follow an eclipse of the sun by the earth that

extends the total time during which the satellite is in the dark.

In addition to eclipses due to the earth, the solar disc as seen by a geostationary

satellite can be partially or totally obscured by the moon. Compared with those due to the

earth, eclipses due to the moon are of irregular occurrence and extent. The number of

eclipses per year due to the moon for a given orbital position varies from zero to four

with a mean of two. Eclipses can occur twice in a period of 24 hours. The duration of an

eclipse varies from several minutes to more than two hours with a mean of around 40

minutes (CCIR-Rep 802).

2.8.3 Satellite Eclipses of the Sun by the Earth

The sun’s rays are assumed to be parallel, and this corresponds to a sun assumed to be a

point at infinite distance. The relationship between the declination 𝛿 of the sun and the

latitude l of the satellite for there to be an eclipse is as follows refers to Figure 2.6

−𝛿 − arc sin (RE / r) < latitude of the satellite < −𝛿 + arc sin (RE ∕ r) Eq.(3)

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 The center of the eclipse corresponds to a value of the nodal angular elongation u

of the satellite (equal to the sum of the argument of the perigee 𝜔 and the true anomaly v

of the satellite) that fulfills:

𝛼SUN + 𝜋 =Ω+ arc tan (tan u cos i) Eq.(4)

Where: 𝛼SUN is the right ascension of the sun and Ω is the RAAN of the satellite

orbit. The duration of the eclipse varies as a function of the distance r and the inclination i

of the satellite orbit with respect to the declination of the sun. The longest durations are

observed when the declination of the sun is equal to the inclination of the orbit.

Knowledge of the duration and periodicity of eclipses is important in the case of

satellites that use solar cells as a source of energy. Furthermore, an eclipse causes a

thermal shock that should be taken into account in the design of the satellite.

Duration of an eclipse: The movement of the earth around the sun is represented in

Figure 2.5. Figure 2.8 shows the apparent movement of the sun with respect to the

equatorial plane. The orbit of the satellite is perpendicular to the plane of the figure. At

the solstices, the satellite is always illuminated; but in the vicinity of the equinoxes, it

could pass in the earth’s shadow. Considering, as a first approximation, that the sun is a

point at infinity, this shadow is a cylinder that is tangential to the earth. On the day of the

equinox, the eclipse has a maximum duration dmax determined from Figure 2.9 such that:

dmax = (17.4∘∕360∘) × (23 h × 60 min + 56 min) = 69.4 min Eq.(5)

In reality, the sun has an apparent diameter of . As seen from the earth, and there is a

cone of shadow where the eclipse is total and a region of penumbra where the eclipse is

partial. The penumbra has a width equal to the apparent diameter of the sun: that is .

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In its orbit, the satellite moves in four minutes; also, the total duration of the eclipse is

equal to 71.5 minutes, of which 2 minutes are penumbra at the start and finish.

Figure 2.8 Apparent movement of the sun with respect to the orbit of geostationary

satellites

Figure 2.9 Eclipses at the equinoxes

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2.9 Satellite Eclipse Affects

Darkness does not affect geostationary satellites; however they have been affected by

lack of solar energy coming from sun since they have a large amount of equipment

attached to the satellite for functioning which cannot rely on batteries for such long

period of time during eclipse.

Satellite failure is more at such times when satellite enters into eclipse (sudden

switch to no solar power region) and when it moves out of eclipse (suddenly large

amount of solar power is bombarded on satellite) as this creates thermal stress on

satellite. Eclipse caused by moon occurs when moon passes in front of sun but that is less

important as it takes place for short duration (twice in every 24 hours for an average of

few minutes).

2.10 Way to Avoid Eclipse during Satellite Lifetime

Satellite longitudes which are west rather than east of the earth station are most desirable.

When satellite longitude is east of the earth station, the satellite enters eclipse during

daylight and early morning hours of the earth station. This can be undesirable if the

satellite has to operate on reduced battery power when satellite longitude is west of the

earth station, eclipse does not occur until the earth station is in darkness when usage is

likely to be low. Figure 2.10 shows the position of the satellite eclipse.

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 Figure 2.10 The position of the satellite eclipse

2.11 Conclusion

In this report we discussed the satellite communication and its system, the different

orbits, the geostationary satellites, Keplerian orbits, the earth’s orbit and its motion

around the sun, satellite eclipses of the sun (definition, eclipse by the moon and earth),

satellite eclipse affects and way to avoid eclipse during satellite lifetime.

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 REFRENCES

1. Gerard Maral, [Satellite Communications Systems], sixth edition, Zhili Sun, Michel

Bousquet.

2. https://www.vedantu.com/physics/satellite-communication

3. https://www.ques10.com/p/2938/what-is-earth-eclipse-of-satelliteare-there-any--1/

4. https://www.ict-pulse.com/2016/09/satellite-communication-pros-cons/

5. https://www.satellite-calculations.com/Satellite/satellite_suneclipse_info.htm

6. https://space.stackexchange.com/questions/22691/how-does-a-solar-eclipse-affect-

satellites

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