roddy chap 1 to 3

8/6/2019 Roddy Chap 1 to 3

http://slidepdf.com/reader/full/roddy-chap-1-to-3 1/48

Satellite Communications

Text book:

Satellite Communications, 4th ed.

Dennis RoddyMcGraw-Hill International Ed.



1.1 Introduction

Features offered by satellite communications

� large areas of the earth are visible from the satellite, thus the

satellite can form the star point of a communications net linking

together many users simultaneously, users who may be widely

separated geographically

�Provide communications links to remote communities

�Remote sensing detection of pollution, weather conditions,

search and rescue operations.



1.2 Frequency allocations

� International Telecommunication Union

(ITU) coordination and planning

� World divided into three regions:

± Region 1: Europe, Africa, formerly Soviet

Union, Mongolia

± Region 2: North and South America, Greenland

± Region 3: Asia (excluding region 1), Australia,

south west Pacific



Frequency band designations in common use for

satellite service



1.3 Intelsat

� International Telecommunications Satellite

�Created in 1964, now has 140 member countries, >40

investing entities

�Geostationary orbit orbits earth`s equitorial plane.

�Atlantic ocean Region (AOR), Indian Ocean Region

(IOR), Pacific Ocean Region.

�Latest INTELSAT IX satellites wider range of servicesuch as internet, Direct to home TV, telemedicine, tele-

education, interactive video and multimedia



Satellite Coverage Maps

Source: http://www.intelsat.com



Coverage maps: Footprints



1.4 U.S DOMSAT (Domestic Satellites)�Provide various telecommunication service within a

country

�In U.S.A all domsats in geostationary orbit

�Direct-to-home TV service can be classified as high

power, medium power, low power



1.5 Polar orbiting satellites

� Orbit the earth such a way as to cover the north and south polar regions

� A satellite in a polar orbit passes above or nearly above both poles of the planet (or other celestial body) on eachrevolution. It therefore has an inclination of (or very closeto) 90 degrees to the equator.

� Since the satellite has a fixed orbital plane perpendicular to

the planet's rotation, it will pass over a region with adifferent longitude on each of its orbits.

� Polar orbits are often used for earth-mapping-, earthobservation, as well as some weather satellites.



� in U.S.A, the National Oceanic and Atmospheric

Administration (NOAA) operates a weather satellite system,

geostationary operational environmental satellites

(GEOS) and

polar operational environmental satellites (PEOS)



2.0 Orbits and Launching Methods� Johannes Kepler (1571 ± 1630) derive empirically three

laws describing planetary motion.

� Kepler ¶s laws apply quite generally to any two bodies in

space which interact through gravitation.

� The more massive of the two bodies is referred to as the

primary, the other, the secondary, or satellite.



The center of mass of the two-body system, termed the

barycenter, is always centered on one of the foci

2.2 Kepler ¶s first law

states that the path followed by a satellite around the

primary will be an ellipse. An ellipse has two focal pointsshown as F1 and F2



� In our specific case, because of the enormous difference

between the masses of the earth and the satellite, the center of mass coincides with the center of the earth, which is therefore

always at one of the foci.

� The semimajor axis of the ellipse is denoted by a, and thesemiminor axis, by b. The eccentricity e is given by

a

bae

22 !

For an elliptical orbit, 0 < e < 1. When e = 0, the orbit

becomes circular .



2.4 Kepler ¶s Third Law

states that the square of the periodic time of orbitis proportional to the cube of the mean distance between

the two bodies.

The mean distance is equal to the semimajor axis a.

For the artificial satellites orbiting the earth, Kepler ¶s third

law can be written in the form

2

3

n

aQ

!

a = semimajor axis (meters)

n = mean motion of the satellite (radians per second)

Q = earth¶s geocentric gravitational constant.

= 3.986005 v 1014 m3/sec2

« (2.2)



Eqt (2.2) applies only to ideal situation satellite

orbiting a perfectly spherical earth of uniform mass,with no pertubing forces acting, such as atmospheric

drag.

Section 2.8 will take account of the earth`s oblateness

and atmospheric drag.

With n in radians per second, the orbital period in

seconds is given by

(2.4)n

P T 2!

This shows that there is a fixed relationship between

period and size



Chapter 3: Radio Wave Propagation

3.1 Introduction

A signal traveling between an earth station and a

satellite must pass through the earth¶s atmosphere,

including the ionosphere.This introduce certain impairments, summarized in

Table 4.1. (Refer text book, page 93)



3.2 Atmospheric Losses

Losses occur in the earth¶s atmosphere as a result of

energy absorption by the atmospheric gases. These

losses are treated quite separately from those which

result from adverse weather conditions, which of course are also atmospheric losses.

To distinguish between these, the weather-related

losses are referred to as atmospheric attenuation and

the absorption losses simply as atmospheric

absorption.The atmospheric absorption loss varies with

frequency, as shown in Fig. 4.2. (Refer text book,

page 94)



Two absorption peaks will be observed:1. at a frequency of 22.3 GHz, resulting from

resonance absorption in water vapor (H2O), and

2. at 60 GHz, resulting from resonance absorption

in oxygen (O2).

At other frequencies, the absorption is quite low.



All these effects decrease as frequency increases.

Only the polarization rotation and scintillationeffects are of major concern for satellite communications.

Ionospheric scintillations

� are variations in the amplitude, phase, polarization, or

angle of arrival of radio waves.

� Caused by irregularities in the ionosphere which changes

with time.

� Effect of scintillations is fading of the signal.

Severe fades may last up to several minutes.



Polarization rotation:

� porduce rotation of the polarization of a signal

(F ar ad ay rot ation)

�When linearly polarized wave traverses in the

ionosphere, free electrons in the ionosphere are sets in

motion a force is experienced, which shift the

polarization of the wave.

�Inversely proportional to frequency squared.

� not a problem for frequencies above 10 GHz.



3.4 Rain Attenuation

Rain attenuation is a function of rain rate.

Rain rate, R p = the rate at which rainwater would

accumulate in a rain gauge situated at the ground in

the region of interest (e. g., at an earth station).

The rain rate is measured in millimeters per hour.

Of interest is the percentage of time that specified

values are exceeded. The time percentage is usuallythat of a year; for example, a rain rate of 0.001 percent

means that the rain rate would be exceeded for 0.001

percent of a year, or about 5.3 min during any one

year.



The specific attenuation E is

kmdBaRb

p /!E « (4.2)

where a and b depend on frequency and polarization.

Values for a and b are available in tabular form in anumber of publications. (eg, Table 4.2, pg 95)

Once the specific attenuation is found, the total

attenuation is determined as:

dB L A E! « (4.3)

where,

L = effective path length of the signal through the rain.



Because the rain density is unlikely to be uniform

over the actual path length, an effective path lengthmust be used rather than the actual (geometric)

length.

Figure 4.3 shows the geometry of the situation.

Figure 4.3: Path length through rain



The geometric, or slant, path length is shown as LS . This

depends on the antenna angle of elevation U and the rainheight hR , which is the height at which freezing occurs.

Figure 4.4 shows curves for hR for different climatic

zones.

Method 1: maritime climates

Method 2: Tropical climates

Method 3: continental climates

Figure 4.4: Rain height as a function of earth station latitude for

different climatic zones



For small angles of elevation (E l < 10°), the determination

of LS is complicated by earth curvature.For E l u 10°, a flat earth approximation may be used.

From Fig. 4.3 it is seen that

El

hh L

o R

S sin

!

« (4.4)

The effective path length is given in terms of the slant

length by

pS r L L ! « (4.5)

where r p is a reduction factor which is a function of the

percentage time p and LG, the horizontal projection of LS .

R efer Table 4.3, page 97, for values of reduction factors, r p



This chapter describes how the link-power budget

calculations are made. These calculations basically

relate two quantities, the transmit power and the

receive power, and show in detail how the differencebetween these two powers is accounted for.

Link budget calculations are usually made using

decibel or decilog quantities. These are explained in

App. G.

Chapter 4: The Space Link

4.1 Introduction



4.2 Equivalent Isotropic Radiated Power

A key parameter in link budget calculations is the

equivalent isotropic radiated power, conventionally

denoted as EIRP.

The Maximum power flux density at some distance r from a transmitting antenna of gain G is

24 r

GP S M

T ] ! « (12.1)



An isotropic radiator with an input power equal to GPS

would produce the same flux density. Hence thisproduct is referred to as the equivalent isotropic

radiated power, or

S P EI R P ! « (12.2)

EIRP is often expressed in decibels relative to one

watt, or dBW. Let PS be in watts; then

? A ? A ? A dBW G P E I RP S ! « (12.3)

where [PS] is also in dBW and [G] is in dB.



The isotropic gain for a paraboloidal antenna is

2472.10 fDG L! « (12.4)

Where,

f

L

is the carrier frequency

is the reflector diameter in m

is the aperture efficiency



4.3 Transmission Losses

The [EIRP] is the power input to one end of the

transmission link, and the problem is to find the power

received at the other end.

Losses will occur along the way, some of which areconstant. Other losses can only be estimated from

statistical data, and some of these are dependent on

weather conditions, especially on rainfall.



The first step in the calculations is to determine the

losses for clear weather, or clear-sky, conditions.These calculations take into account the losses,

including those calculated on a statistical basis, which

do not vary significantly with time. Losses which are

weather-related, and other losses which fluctuate with

time, are then allowed for by introducing appropriatefade margins into the transmission equation.



4.3.1 Free Space Transmission

Spreading of the signal is space causes power loss.

The power flux density at the receiving antenna is

2

4 r

EI R P M

T

] ! « (12.6)

The power delivered to a matched receiver is this

power flux density multiplied by the effective aperture of

the receiving antenna. The received power is therefore:

e ff M R A P ] !

« (12.7)

T

P

T 44

2

2

RG

r

E I RP !

2

4))(( ¹

º

¸©ª

¨!

r G EI R P R

T

P



where

r = distance, or range, between the transmit and receiveantennas

GR = isotropic power gain of the receiving antenna. The

subscript R is used to identify the receiving antenna.

In decibel notation, equation (12.7) becomes

? A ? A ? A2

4log10 ¹

º

¸©ª

¨!

P

T r G EI R P P R R

« (12.8)

Free space loss is given by

? A2

4log10 ¹

º

¸©ª

¨!

P

T r FS L « (12.9)

f r log20log204.32 ! « (12.10)



Equation (12.8) can then be written as

? A ? A ? A ? A F S LG E I RP P R R ! « (12.8)

The received power [PR] will be in dBW when the[EIRP] is in dBW and [FSL] in dB.

Equation (12.9) is applicable to both the uplink and

the downlink of a satellite circuit



4.3.2 Feeder Losses

Losses occur in the connection between the

receive antenna and the receiver proper, such as in

the connecting waveguides, filters, and couplers.

These will be denoted by RFL, or [RFL] dB, for

receiver feeder losses.The [RFL] values are added to [FSL] in Eq. (12.11).

Similar losses occur in the filters, couplers, and

waveguides connecting the transmit antenna to the

high-power amplifier (HPA) output. However,

provided that the EIRP is stated, Eq. (12.11)

can be used without knowing the transmitter feeder

losses. These are needed only when it is desired

to relate EIRP to the HPA output



4.3.3 Antenna misalignment losses

When a satellite link is established, the ideal situationis to have the earth station and satellite antennas

aligned for maximum gain, as shown in Fig. 12.1 a.

Figure 12.1: (a) aligned for maximum gain, (b) earth-station

antenna misaligned



There are two possible sources of off-axis loss, one

at the satellite and one at the earth station.

The off-axis loss at the earth station is referred to as the

antenna pointing loss, which are usually only a few

tenths of a decibel.

Losses may also result at the antenna from misalignment

of the polarization direction. The polarization

misalignment losses are usually small, and is assumed

that the antenna misalignment losses, denoted by [AML],include both pointing and polarization losses resulting

from antenna misalignment.



Antenna misalignment losses have to be estimated

from statistical data, based on the errors actually

observed for a large number of earth stations.

Separate antenna misalignment losses for the uplink

and the downlink must be taken into account.



4.3.4 Fixed atmospheric and ionospheric losses

Atmospheric gases result in losses by absorption.

These losses usually amount to a fraction of a

decibel, and decibel value will be denoted by [AA].

Table 12.1 shows values of atmospheric absorptionlosses and Satellite pointing loss in Canada.



4.4 The Link-Power Budget Estimation

Losses for clear sky conditions are

? A ? A ? A ? A ? A ? A P L AA AM L R

F L

FS L LO

SS E S

!

« (12.12)

The decibel equation for the received power is

? A ? A ? A ? A LOSS E S G E I RP P

R R! « (12.13)



where

[PR] = received power, dBW

[EIRP] = equivalent isotropic radiated power, dBW

[FSL] = free-space spreading loss, dB

[RFL] = receiver feeder loss, dB

[AML] = antenna misalignment loss, dB

[AA] = atmospheric absorption loss, dB

[PL] = polarization mismatch loss, dB

roddy chap 1 to 3

Documents