Satellite Communications

Reference book: Satellite Communications, 3rd 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

• Within regions, frequency bands are allocated to various satellite services:

•Fixed satellite service (FSS)•Telephone networks, television signals to cable

•Broadcasting satellite service (BSS)•Direct broadcast to home Astro is a subscription-based direct broadcast satellite (DBS) or direct-to-home satellite television and radio service in Malaysia and Brunei

•Mobile satellite service •Land mobile, maritime mobile, aeronautical mobile

•Navigational satellite service•Global positioning system

•Meteorological satellite service

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 service such 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 each revolution. It therefore has an inclination of (or very close to) 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 a different longitude on each of its orbits.

• Polar orbits are often used for earth-mapping-, earth observation- and reconnaissance satellites, as well as some weather satellites.

The orbit of a near polar satellite as viewed from a point rotating with the Earth.

• 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 points shown 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 the semiminor axis, by b. The eccentricity e is given by

abae

22

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

2.3 Kepler`s Second Law

Kepler’s second law states that, for equal time intervals, a satellite will sweep out equal areas in its orbital plane, focused at the barycenter.

Thus the farther the satellite from earth, the longer it takes to travel a given distance

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

23

na

a = semimajor axis (meters)n = mean motion of the satellite (radians per second) = earth’s geocentric gravitational constant. = 3.986005 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 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), and2. at 60 GHz, resulting from resonance absorption

in oxygen (O2).

At other frequencies, the absorption is quite low.

3.3 Ionospheric Effects

Radio waves traveling between satellites and earth stations must pass through the ionosphere. The ionosphere has been ionized, mainly by solar radiation.The free electrons in the ionosphere are not uniformly distributed but form in layers. Clouds of electrons may travel through the ionosphere and give rise to fluctuations in the signal.

The effects include scintillation, absorption, variation inthe direction of arrival, propagation delay, dispersion, frequency change, and polarization rotation.

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 (Faraday rotation)

•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, Rp = 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 specifiedvalues are exceeded. The time percentage is usually that 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 is

kmdBaR bp / … (4.2)

where a and b depend on frequency and polarization. Values for a and b are available in tabular form in a number of publications. (eg, Table 4.2, pg 95)Once the specific attenuation is found, the total attenuation is determined as:

dB LA … (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 length must 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 and the rain height hR, which is the height at which freezing occurs. Figure 4.4 shows curves for hR for different climaticzones.

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 (El < 10°), the determination of LS is complicated by earth curvature.For El 10°, a flat earth approximation may be used.From Fig. 4.3 it is seen that

ElhhL oR

S sin

… (4.4)

The effective path length is given in terms of the slant length by

pS rLL … (4.5)

where rp is a reduction factor which is a function of the percentage time p and LG, the horizontal projection of LS.Refer Table 4.3, page 97, for values of reduction factors, rp

From Fig. 4.3 the horizontal projection is seen to be

ElLL SG cos … (4.6)

Putting all the factors into one equation, we obtain the rain attenuation (decibels) as:

dB rLaRA pSb

pp … (4.7)

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 rGPS

M … (12.1)

An isotropic radiator with an input power equal to GPS would produce the same flux density. Hence this product is referred to as the equivalent 　 isotropic radiated power, or

SGPEIRP … (12.2)

EIRP is often expressed in decibels relative to one watt, or dBW. Let PS be in watts; then

dBW GPEIRP 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 … (12.4)

Where,

f

D

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 are constant. 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 appropriate fade 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

24 rEIRP

M … (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:

effMR AP

… (12.7)

44

2

2RG

rEIRP

2

4))((

rGEIRP R

wherer = distance, or range, between the transmit and receive antennas 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

24log10

rGEIRPP RR … (12.8)

Free space loss is given by

24log10

rFSL … (12.9)

fr log20log204.32 … (12.10)

Equation (12.8) can then be written as

FSLGEIRPP RR … (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 andthe 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 lossesWhen a satellite link is established, the ideal situation is 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, oneat 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 adecibel, and decibel value will be denoted by [AA].Table 12.1 shows values of atmospheric absorption losses and Satellite pointing loss in Canada.

4.4 The Link-Power Budget Estimation

Losses for clear sky conditions are

PLAAAMLRFLFSLLOSSES … (12.12)

The decibel equation for the received power is

LOSSESGEIRPP RR … (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