EEE 545:SATELLITE

COMMUNICATIONS

Danson Njue

Satellite speed and attitudes (1) Altitude (Km) Orbital speed (Km/s)

200 7.8

500 7.6

1,000 7.4

5,000 5.9

10,000 4.9

20,200 (Semisynchronous) 3.9

35,800 (Geosynchronous) 3.1

• The speed needed to keep a satellite in orbit does not depend on the mass of the

satellite

• Once a satellite has been accelerated up to orbital speed by a rocket, it does not need to

be continually powered to stay in orbit - Newton’s first law of motion, which states that in

the absence of forces such as friction and air resistance, an object at rest will stay at rest

and an object in motion will stay in motion with same speed and in the same direction.

• As a result, once put in motion by a rocket, a satellite will stay in motion, with the Earth’s

gravity bending its path from a straight line into an orbit

• As such, satellites can stay in orbit for long periods of time, since they do not need to

carry large amounts of fuel to keep them moving. It also means that once in orbit, other

objects (debris) will stay in orbit essentially indefinitely, unless they are at low enough

altitudes that atmospheric drag slows them over time and they fall to Earth.

Speed and altitude of satellites in circular orbits

Orbital periods of satellite

• Orbital Period: Time it takes for the satellite to travel around the Earth once (complete

one orbit)

• As the altitude of the orbit increases, the satellite moves more slowly and must travel

farther on each orbit, hence the period increases with the altitude of the orbit

Altitude (Km) Orbital period (Minutes)

200 88.3

500 94.4

1,000 104.9

5,000 201.1

10,000 347.4

20,200 (Semisynchronous) 718.3 (12 hours)

35,800 (Geosynchronous) 1436.2 (24 hours)

Orientation of the Plane of the Orbit • A satellite’s orbit always lies in a plane, and that plane passes through the center of the

Earth and the description of a satellite’s orbit requires specifying the orientation of this

orbital plane

• Equatorial orbit: Occurs when the plane of the orbit includes the Earth’s equator

• Polar orbit: When the inclination angle is 90 degrees, the orbital plane contains the

Earth’s axis and the orbit passes over the Earth’s poles

• Inclination angle: Angle at which the plane of the orbit lies with respect to the Earth’s

equatorial plane

Circular and elliptical orbits

Circular Orbits:

• For a satellite travelling in a circular orbit at an altitude h and speed V, the centifugal

force equals the gravitational force on the satellite.

Where m is the mass of the satellite, G is the gravitational constant, Me is the mass of the

Earth (GMe=3.99 x 1014 m3/s2) and Re is the average radius of the Earth (6.370 Km)

• The speed of the satellite can be expressed as follows;

• If we let r be the distance from the satellite to the center of the Earth, so that

Then V can be written as;

NB; The mass of the satellite does not appear in the above equation

Circular Orbits:

• The period can be found by dividing the distance the satellite travels in one

orbit (in this case, the circumference of a circle with radius h + Re) by the

speed of satellite, V. The period Pcirc of a circular orbit is therefore given by;

Elliptical Orbits:

Assignment 1

• Derive equations for the speed, V and the period, Pcirc of

a satellite travelling in an elliptical orbit

Escape Velocity • If an object is launched from Earth with a speed of 11.2 km/s or greater, the

Earth’s gravity is not strong enough to keep it in orbit and it will escape into

deep space. This speed is called the escape velocity

• The escape speed is the minimum speed necessary for an object to escape

permanently from a gravitational field.

SATELLITE LINK

DESIGN Satellite Link Design and Link Budget

Calculations

Satellite link

• The satellite link is the most basic microwave

communications link since a line-of-sight (LOS) path

typically exists between the Earth and space.

• The LOS exists in some form of an imaginary line

extending between the transmitting or receiving Earth

station and the satellite antenna and passing through the

atmosphere as opposed to the ground.

• As such, such a link is governed by free-space

propagation with only limited variation with respect to time

due to various constituents of the atmosphere.

Satellite link- Attenuation

• Free-space attenuation is determined by the inverse

square law;

• The power received, Pr is inversely proportional to the square of the

distance, d.

• Other effects that produce a significant amount of signal

degradation and time variation in a satellite link include;-

• Rain

• Terrain effects such as absorption by trees and walls

• Other impairments produced by unstable conditions of the air and

ionosphere.

Satellite link- parameters

• Important parameters of interest during design of a

satellite link include;-

• Performance of the satellite

• The configuration and performance of the uplink and downlink

Earth stations

• The impact of the propagation medium in the frequency band of

interest.

• Efficient transfer of user information across the relevant interfaces

at the Earth stations, involving such issues as the precise nature of

this information, data protocol, timing, and the telecommunications

interface standards that apply to the service.

Satellite link- Elements

Satellite link- Elements

• Baseband The basic direct output signal in an intermediate frequency obtained directly from a source (TV camera, satellite television receiver, or video tape recorder).

Baseband signals needs to be modulated to convert them to a format which a terminal equipment can be tuned to (e.g VHF or UHF television channels which the television set can be tuned to receive).

• Carrier The carrier is an analog signal which is modulated by manipulating its amplitude (making it louder or softer) or its frequency (shifting it up or down) in relation to the incoming signal. Satellite carriers operating in the analog mode are usually frequency modulated.

Satellite link- Elements • LNB (LOW NOISE BLOCK DOWN CONVERTER)

A device mounted in the dish, designed to amplify the satellite signals and convert them from a high frequency to a lower frequency. LNB can be controlled to receive signals with different polarization. The television signals can then be carried by a double-shielded aerial cable to the satellite receiver while retaining their high quality. A universal LNB is the present standard version, which can handle the entire frequency range from 10.7 to 12.75 GHz and receive signals with both vertical and horizontal polarization.

• Demodulator A satellite receiver circuit which extracts or "demodulates" the "wanted "signals from the received carrier.

• Decoder

A box which, normally together with a viewing card, makes it possible to view encrypted transmissions. If the transmissions are digital, the decoder is usually integrated in the receiver.

• Modulation The process of manipulating the frequency or amplitude of a carrier in relation to an incoming video, voice or data signal.

• Modulator A device which modulates a carrier. Modulators are found as components in broadcasting transmitters and in satellite transponders.

Satellite link- Elements

• Most transponders operate on a "bent pipe" principle,

referring to the sending back of what goes into the conduit

with only amplification and a shift from uplink to downlink

frequency, as opposed to a 'regenerative' system

whereby the signal is modified to its original format

through re-modulation.

• Bidirectional (duplex) communication occurs with a

separate transmission from each Earth station.

• Due to the analog nature of the radio frequency link, each

element contributes a gain or loss to the link and may add

noise and interference as well.

Satellite link- Elements

• Carrier to Noise Ratio (C/N) The ratio of the received carrier power and the noise power in a given bandwidth, expressed in dB. This figure is directly related to G/T and S/N; and in a video signal the higher the C/N, the better the received picture.

• G/T A figure of merit of an antenna and low noise amplifier combination expressed in dB. "G" is the net gain of the system and "T" is the noise temperature of the system. The higher the number, the better the system.

• The link budget analysis can predict if the link will work with satisfactory

quality based on the specifications of the ground and space components.

• Any uncertainty can be covered by providing an appropriate amount of link margin, which is over and above the C/N needed to deal with propagation effects and nonlinearity in the Earth stations and satellite repeater.

Satellite link- Design process

• Define Requirements for each link

• Design Each Link

• Select frequency

• Select modulation & coding

• Apply antenna size & beam width constraints

• Estimate atmospheric, rain attenuation

• Estimate received noise, interference power

• Calculate required antenna gain & transmitter power

• Size the Payload – Payload antenna configuration, size &

mass – Estimate transmitter mass & power – Estimate

payload mass & power

Satellite link - propagation factors

Atmospheric losses • Different types of atmospheric losses can perturb radio

wave transmission in satellite systems:

• Atmospheric absorption;

• Atmospheric attenuation;

• Traveling ionospheric disturbances.

Atmospheric absorption

• Energy absorption by atmospheric gases, which varies with the frequency of the radio waves.

• Two absorption peaks are observed: • 22.3 GHz from resonance

absorption in water vapour (H2O)

• 60 GHz from resonance absorption in oxygen (O2)

Atmospheric attenuation

• Rain is the main cause of atmospheric attenuation (hail,

ice and snow have little effect on attenuation because of

their low water content).

• Total attenuation from rain can be determined by:

• A = L [dB]

• where [dB/km] is called the specific attenuation

• where L [km] is the effective path length of the signal through the

rain; note that this differs from the geometric path length due to

fluctuations in the rain density.

Traveling Ionospheric Disturbances

• Traveling ionospheric disturbances are clouds of

electrons in the ionosphere that provoke radio signal

fluctuations.

• The disturbances of major concern are:

• Scintillation;

• Polarisation rotation.

• Scintillations are variations in the amplitude, phase,

polarisation, or angle of arrival of radio waves, caused

by irregularities in the ionosphere which change over

time. The main effect of scintillations is fading of the

signal.

Illustration of the various propagation loss mechanisms on

a typical earth-space path

Refractive effects

(tropospheric

scintillation) cause

signal loss.

The absorptive effects of

the atmospheric

constituents cause an

increase in sky noise to be

observed by the receiver

The ionosphere can cause the electric

vector of signals passing through it to

rotate away from their original

polarization direction, hence causing

signal depolarization. the sun (a very “hot”

microwave and

millimeter wave

source of incoherent

energy), an increased

noise contribution

results which may

cause the C/N to drop

below the

demodulator

threshold.

The ionosphere has its principal impact on

signals at frequencies well below 10 GHz while

the other effects noted in the figure above

become increasingly strong as the frequency

of the signal goes above 10 GHz

Atmospheric propagation impairments Propagation impairment Physical cause Prime importance

Attenuation and sky noise

increase

Atmospheric gases, clouds, rain Frequencies above 10GHz

Signal depolarization Rain, ice crystals Dual-polarization systems at C and Ku

bands (depends on system

configuration)

Refraction, atmospheric multi-

path

Atmospheric gases Communication and tracking at low

elevation angles

Signal scintillations Tropospheric and ionospheric

refractivity fluctuations

Tropospheric at frequencies above 10

GHz and low elevation angles;

ionospheric at frequencies below 10

GHz

Reflection multipath, blockage Earth’s surface, objects on surface Mobile satellite services

Propagation delays, variations Troposphere, ionosphere Precise timing and location systems;

time division multiple ac-

cess (TDMA) systems

Intersystem interference Ducting, scatter, diffraction Mainly C band; rain scatter may be

significant at higher frequencies

Link-Power Budget Formula

• Link-power budget calculations take into account all the gains and losses from the transmitter, through the medium to the receiver in a telecommunication system.

• It also takes into account the attenuation of the transmitted signal due to propagation and the loss or gain due to the antenna.

• The decibel equation for the received power is:

[PR] = [EIRP] + [GR] - [LOSSES]

Where:

[PR] = received power in dBW

[EIRP] = equivalent isotropic radiated power in dBW

[GR] = receiver antenna gain in dB

[LOSSES] = total link loss in dB

dBW = 10 log10(P/(1 W)), where P is an arbitrary power in watts, is a unit for the measurement of the strength of a signal relative to one watt.

Link Budget parameters

• Transmitter power at the antenna

• Antenna gain compared to isotropic radiator (dBi)

• EIRP

• Free space path loss

• System noise temperature

• Figure of merit for receiving system

• Carrier to thermal noise ratio

• Carrier to noise ratio

Equivalent Isotropic Radiated Power

(EIRP) • An isotropic radiator is one that radiates equally in all directions.

• The power amplifier in the transmitter is shown as generating PT watts.

• A feeder connects this to the antenna, and the net power reaching the antenna will be PT minus the losses in the feeder cable, i.e. PS.

• The power will be further reduced by losses in the antenna such that the power radiated will be PRAD < PT.

Antenna Gain

• We need directive antennas to direct the power in the

wanted direction.

• Gain of antenna is the increase in power in a given

direction compared to isotropic antenna.

4/

)()(

0P

PG

• P() is variation of power with angle.

• G() is gain at the direction .

• P0 is total power transmitted.

Link-Power Budget Formula – Other Variables • Link-Power Budget Formula for the received power [PR]:

[PR] = [EIRP] + [GR] - [LOSSES]

• The equivalent isotropic radiated power [EIRP] is given by:

[EIRP] = [PS] + [G] dBW, where:

[PS] is the transmit power in dBW and [G] is the transmitting antenna gain in dB

• [GR] is the receiver antenna gain in dB

• [LOSSES] = [FSL] + [RFL] + [AML] + [AA] + [PL], where:

[FSL] = free-space spreading loss in dB = PT/PR (in watts)

[RFL] = receiver feeder loss in dB

[AML] = antenna misalignment loss in dB

[AA] = atmospheric absorption loss in dB

[PL] = polarisation mismatch loss in dB

• The major source of loss in any ground-satellite link is the free-space spreading loss.

• The above formula assumes an idealized case

Complete formulation

• Other effects accounted for in the transmission equation are as follows: • La = Losses due to attenuation in atmosphere

• Lta = Losses associated with transmitting antenna

• Lra = Losses associated with receiving antenna

• Lpol = Losses due to polarization mismatch

• Lother = (any other known loss - as much detail as available)

• Lr = additional Losses at receiver (after receiving antenna)

rotherpolrataap

rttr

LLLLLLL

GGPP

Complete formulation

• Some intermediate variables are also defined:

Pt =Pout /Lt EIRP = Pt Gt Where: • Pt = Power into antenna • Lt = Loss between power source and antenna • EIRP = effective isotropic radiated power

rotherpolrataapt

rtout

rotherpolrataap

r

rotherpolrataap

rttr

LLLLLLLL

GGP

LLLLLLL

GEIRP

LLLLLLL

GGPP

x

There are many ways to express the link budget

transmission equation. The user has to pick the

one most suitable to each need.

Link Power Budget

• The transmission formula can be written in dB as:

• This form of the equation is easily handled as a spreadsheet (additions and subtractions!!)

• The calculation of received signal based on transmitted power and all losses and gains involved until the receiver is called “Link Power Budget”, or “Link Budget”.

• The received power Pr is commonly referred to as

“Carrier Power”, C.

rrotherrapolaptar LGLLLLLLEIRPP

Link Power Budget

Transmission:

+ Power

- Transmission Losses

(cables & connectors)

+ Antenna Gain

EIRP Tx

- Antenna Pointing Loss

- Free Space Loss

- Atmospheric Loss

(gaseous, clouds, rain)

- Rx Antenna Pointing Loss

Rx

Reception:

+ Antenna gain

- Reception Losses

(cables & connectors)

-Other losses

Pr

All link factors are accounted for

as additions and subtractions

Easy Steps to a Good Link Power Budget

• First, draw a sketch of the link path • Doesn’t have to be artistic quality

• Helps you find the stuff you might forget

• Next, think carefully about the system of interest • Include all significant effects in the link power budget

• Note and justify which common effects are insignificant here

• Roll-up large sections of the link power budget • TXd power, TX ant. gain, Path loss, RX ant. gain, RX losses

• Show all components for these calculations in the detailed budget

• Use the rolled-up results in build a link overview

• Comment the link budget • always use units on parameters (dBi, W, Hz ...)

• Describe any unusual elements (eg. loss caused by H20 on radome)

Simple Link Power Budget Parameter Value Totals Units Parameter Value Totals Units

Frequency 11.75 GHz

Transmitter Receive Antenna

Transmitter Power 40.00 dBm Random Loss 0.50 dB

Modulation Loss 3.00 dB Diameter 1.5 m

Transmission Line

Loss

0.75 dB Aperture Efficiency 0.6 none

Transmitted Power 36.25 dBm Gain 43.10 dBi

Polarization Loss 0.20 dB

Transmit Antenna Effective RX Ant.

Gain

42.40 dB

Diameter 0.5 m

Aperture Efficiency 0.55 none Received Power -98.54 dBm

Transmit Antenna

Gain

33.18 dBi

Slant Path Summary

Satellite Altitude 35,786 km Transmitted Power 36.25 dBm

Elevation Angle 14.5 degrees Transmit Anntenna

Gain

33.18 dBi

Slant Range 41,602 km EIRP 69.43 dBmi

Free-space Path Loss 206.22 dB Path Loss 210.37 dB

Gaseous Loss 0.65 dB Effective RX

Antenna Gain

42.4 dBi

Rain Loss (allocated) 3.50 dB Received Power -98.54 dBm

Path Loss 210.37 dB

Why calculate Link Budgets?

• System performance tied to operation thresholds.

• Operation thresholds Cmin tell the minimum power that should be received at the demodulator in order for communications to work properly.

• Operation thresholds depend on: • Modulation scheme being used.

• Desired communication quality.

• Coding gain.

• Additional overheads.

• Channel Bandwidth.

• Thermal Noise power.

Closing the Link

• We need to calculate the Link Budget in order to verify if we are “closing the link”.

Pr >= Cmin Link Closed

Pr < Cmin Link not closed

• Usually, we obtain the “Link Margin”, which tells how tight we are in closing the link:

Margin = Pr – Cmin

• Equivalently:

Margin > 0 Link Closed

Margin < 0 Link not closed

Carrier to Noise Ratios

• C/N: Carrier/noise power in RX bandwidth (dB) • Allows simple calculation of margin if:

• Receiver bandwidth is known

• Required C/N is known for desired signal type

• C/No: Carrier/noise power density. (dBHz) • Allows simple calculation of allowable RX bandwidth if

required C/N is known for desired signal type

System Figure of Merit

• G/Ts: RX antenna gain/system temperature • Also called the System Figure of Merit, G/Ts

• Easily describes the sensitivity of a receive system

• Must be used with caution: • Some (most) vendors measure G/Ts under ideal conditions only

• G/Ts degrades for most systems when rain loss increases

• This is caused by the increase in the sky noise component

• This is in addition to the loss of received power flux density

System Noise Power

• System noise is mainly caused by thermal noise sources • External to RX system

• Transmitted noise on link

• Scene noise observed by antenna

• Internal to RX system

• The power available from thermal noise is: where k = Boltzmann’s constant = 1.38x10-23 J/K Ts is the effective system noise temperature, and

B is the effective system bandwidth

(dBW) BkTN s

Noise factor/figure

• TX: signal is far larger than noise hence the effect of noise causes limited problem.

• RX: signal and noise have similar values which causes a major problem.

• Noise Factor/Figure is a means of measuring the noise added by a device.

• Na: noise added by the device.

• G: device gain.

• F: is the so-called noise factor.

• NF(dB) = 10 · log10 (F) is the noise figure.

• This quantity is relative to the input noise level

• The standard reference is kT0, with T0 = 290oK.