Satellite Communications

Dr. Samy Elmokadem

Topics of Presentation

Applications

IntroLunching

How itworks

FrequencyBands

Satellites

Objectives

• Provide a broad overview of the status of digital satellite communications.

• Discuss main physical, architectural and networking issues of satellite systems.

• Provide in-depth understanding of modern modulation, coding and multiple access schemes.

• Review the state of the art in open research areas such as speech and video coding, satellite networking, internet over satellite and satellite personal communications.

• Highlight trends and future directions of satellite communication

Overview• Satellite technology has progressed tremendously over

the last 50 years since Arthur C. Clarke first proposed its idea in 1945 in his article in Wireless World.

• Today, satellite systems can provide a variety of services including broadband communications, audio/video distribution networks, maritime navigation, worldwide customer service and support as well as military command and control.

• Satellite systems are also expected to play an important role in the emerging 4G global infrastructure providing the wide area coverage necessary for the realization of the “Optimally Connected Anywhere, Anytime” vision that drives the growth of modern telecom industry.

Overview• Satellite technology has progressed tremendously over

the last 50 years since Arthur C. Clarke first proposed its idea in 1945 in his article in Wireless World.

• Today, satellite systems can provide a variety of services including broadband communications, audio/video distribution networks, maritime navigation, worldwide customer service and support as well as military command and control.

• Satellite systems are also expected to play an important role in the emerging 4G global infrastructure providing the wide area coverage necessary for the realization of the “Optimally Connected Anywhere, Anytime” vision that drives the growth of modern telecom industry.

Satellite Components

• Satellite Subsystems– Telemetry, Tracking, and Control– Electrical Power and Thermal Control– Attitude Control– Communication Subsystems

• Link Budget• Modulation Techniques• Coding and Error Correction• Networking (service provisioning, multimedia

constraints and QoS)• Multiple Access and On-board Processing• Applications (Internet, Mobile computing)

Classification of Satellite Orbits• Circular or elliptical orbit

– Circular with center at earth’s center – Elliptical with one foci at earth’s center

• Orbit around earth in different planes– Equatorial orbit above earth’s equator– Polar orbit passes over both poles– Other orbits referred to as inclined orbits

• Altitude of satellites– Geostationary orbit (GEO)– Medium earth orbit (MEO)– Low earth orbit (LEO)

Satellite Orbits

• Equatorial

• Inclined

• Polar

Here’s the Math…• Gravity depends on the mass of the earth, the mass of the

satellite, and the distance between the center of the earth and the satellite

• For a satellite traveling in a circle, the speed of the satellite and the radius of the circle determine the force (of gravity) needed to maintain the orbit

• The radius of the orbit is also the distance from the center of the earth.

• For each orbit the amount of gravity available is therefore fixed

• That in turn means that the speed at which the satellite travels is determined by the orbit

Let’s look in a Physics Book…

• From what we have deduced so far, there has to be an equation that relates the orbit and the speed of the satellite:

T 2r3

4 1014

T is the time for one full revolution around the orbit, in seconds

r is the radius of the orbit, in meters, including the radius of the earth (6.38x106m).

R^3=mu/n^2

N=2pi/T

The Most Common Example

• “Height” of the orbit = 22,300 mile

• That is 36,000km = 3.6x107m

• The radius of the orbit is3.6x107m + 6.38x106m = 4.2x107m

• Put that into the formula and …

The Geosynchronous Orbit

• The answer is T = 86,000 sec (rounded)

• 86,000 sec = 1,433 min = 24hours (rounded)

• The satellite needs 1 day to complete an orbit

• Since the earth turns once per day, the satellite moves with the surface of the earth.

Assignment

• How long does a Low Earth Orbit Satellite need for one orbit at a height of 200miles = 322km = 3.22x105m

• Do this:– Add the radius of the earth, 6.38x106m– Compute T from the formula– Change T to minutes or hours

T 2r3

4 1014

Basics

• Satellites in circular orbits– attractive force Fg = m g (R/r)²– centrifugal force Fc = m r ²– m: mass of the satellite– R: radius of the earth (R = 6370 km)– r: distance to the center of the earth– g: acceleration of gravity (g = 9.81 m/s²)– : angular velocity ( = 2 f, f: rotation frequency)

• Stable orbit– Fg = Fc

32

2

)2( f

gRr

Basics• elliptical or circular orbits• complete rotation time depends on distance satellite-earth• inclination: angle between orbit and equator• elevation: angle between satellite and horizon• LOS (Line of Sight) to the satellite necessary for connection

high elevation needed, less absorption due to e.g. buildings• Uplink: connection base station - satellite• Downlink: connection satellite - base station• typically separated frequencies for uplink and downlink

– transponder used for sending/receiving and shifting of frequencies

– transparent transponder: only shift of frequencies– regenerative transponder: additionally signal regeneration

Inclination

inclination

satellite orbit

perigee

plane of satellite orbit

equatorial plane

Elevation

Elevation:angle between center of satellite beam and surface

minimal elevation:elevation needed at leastto communicate with the satellite

footprint

• Four different types of satellite orbits can be identified depending on the shape and diameter of the orbit:

• GEO: geostationary orbit, ca. 36000 km above earth surface

• LEO (Low Earth Orbit): ca. 500 - 1500 km• MEO (Medium Earth Orbit) or ICO (Intermediate

Circular Orbit): ca. 6000 - 20000 km• HEO (Highly Elliptical Orbit) elliptical orbits

Orbits I

Orbits II

earth

km35768

10000

1000

LEO (Globalstar,

Irdium)

HEO

inner and outer VanAllen belts

MEO (ICO)

GEO (Inmarsat)

Van-Allen-Belts:ionized particles2000 - 6000 km and15000 - 30000 kmabove earth surface

Geostationary satellites

• Orbit 35,786 km distance to earth surface, orbit in equatorial plane (inclination 0°)

complete rotation exactly one day, satellite is synchronous to earth rotation

• fix antenna positions, no adjusting necessary• satellites typically have a large footprint (up to 34% of

earth surface!), therefore difficult to reuse frequencies • bad elevations in areas with latitude above 60° due to

fixed position above the equator• high transmit power needed• high latency due to long distance (ca. 275 ms) not useful for global coverage for small mobile

phones and data transmission, typically used for radio and TV transmission

LEO systems• Orbit ca. 500 - 1500 km above earth surface• visibility of a satellite ca. 10 - 40 minutes• global radio coverage possible • latency comparable with terrestrial long distance

connections, ca. 5 - 10 ms• smaller footprints, better frequency reuse• but now handover necessary from one satellite to another • many satellites necessary for global coverage • more complex systems due to moving satellites

• Examples: • Iridium (start 1998, 66 satellites)

– Bankruptcy in 2000, deal with US DoD (free use, saving from “deorbiting”)

• Globalstar (start 1999, 48 satellites)– Not many customers (2001: 44000), low stand-by times for mobiles

MEO systems• Orbit ca. 5000 - 12000 km above earth surface

• comparison with LEO systems:

• slower moving satellites

• less satellites needed

• simpler system design

• for many connections no hand-over needed

• higher latency, ca. 70 - 80 ms

• higher sending power needed

• special antennas for small footprints needed

• Example:

• ICO (Intermediate Circular Orbit, Inmarsat) start ca. 2000

– Bankruptcy, planned joint ventures with Teledesic, Ellipso – cancelled again, start planned for 2003

Satellite Communications

Lunching Satellites

• How does a satellite stay in it’s orbit?

Satellite Systems Applications

What are Communication Satellites?

• A satellite is an object that orbits another large object like planet.

• A communication satellite is a staion in space that is used for telecommuncation, radio and television signals.

• The first satellite with radio transmitter was in 1957.

Geostationary orbits

• What are them? Geostationary orbits is fixed position to an earth-based observer.

• When was the first use? The first truly geostationary sateliite was the SYNCOM3 in 1964.

• Why they are important in communications? - The antennas in the ground don’t need equipment to track the satellite. - Lower cost & complixity.

• Disadvantages? - Not always suitable for providing services at high latitudes. - Molniya satellite was introduced as a solution.

Frequency Bands

• Three common bands:1) C-Band.

2) KU-Band.

3) KA-Band.

• Most common are C-Band & KU-Band.

• C-Band occupy 4 to 8 GHz frequency:- Low frequency.- Large antenna (2-3 meters).

• KU-Band occupy 11 to 17 GHz:- Large frequency.- Small antenna (18-inches!)

Applications

• Telephony - Fixed points< earth station> Satellite> earth station> fixed points.

• Televesion & Radio - e.g. Direct broadcast satellite (DBS) & Fixed service satellite (FFS).

• Mobile satellite technology - Special antenna called mobile satellite antenna. - No matter where or how this antenna is mounted on.

Applications

• Amateur radio - Access to OSCAR satellite. - Low earth orbits.

• Internet - High Speed. - Useful for far away places.

• Military - Uses geostationary satellites. - Example: The Defense Satellite Communications System (DSCS).

Disadvantages

• The antenna noise due to energy - Unwanted radiation sources (stars – galaxies - …etc). - Worsen S/N ratio.

• Atmosphere behaves as a resistive medium - Supplies noise power to the antenna.

• Meteors - Have to be programmed to avoid any rock or any harmful thing. - Rules of orbits.

• Expensive - only for governments or large organizations.

Communication Satellite

• A Communication Satellite can be looked upon as a large microwave repeater

• It contains several transponders which listens to some portion of spectrum, amplifies the incoming signal and broadcasts it in another frequency to avoid interference with incoming signals.

Motivation to use Satellites

Source: Union of Concerned Scientists [www.ucsusa.org]

Satellite Missions

Satellite Microwave Transmission

• Satellites can relay signals over a long distance

• Geostationary Satellites– Remain above the equator at a height of

about 22300 miles (geosynchronous orbits)– Travel around the earth in exactly the

same time, the earth takes to rotate

Satellite System Elements

Space Segment

• Satellite Launching Phase• Transfer Orbit Phase• Deployment• Operation

– TT&C - Tracking Telemetry and Command Station

– SSC - Satellite Control Center, a.k.a.:• OCC - Operations Control Center• SCF - Satellite Control Facility

• Retirement Phase

Ground Segment

• Collection of facilities, Users and Applications

• Earth Station = Satellite Communication Station (Fixed or Mobile)

Satellite Uplink and Downlink• Downlink

– The link from a satellite down to one or more ground stations or receivers

• Uplink– The link from a ground station up to a satellite.

• Some companies sell uplink and downlink services to – television stations, corporations, and to other

telecommunication carriers. – A company can specialize in providing uplinks,

downlinks, or both.

Satellite Uplink and Downlink

Source: Cryptome [Cryptome.org]

When using a satellite for long When using a satellite for long distance communications, the distance communications, the satellite acts as a repeater.satellite acts as a repeater.

An earth station transmits the signal An earth station transmits the signal up to the satellite (uplink), which in up to the satellite (uplink), which in turn retransmits it to the receiving turn retransmits it to the receiving earth station (downlink).earth station (downlink).

Different frequencies are used for Different frequencies are used for uplink/downlink.uplink/downlink.

Satellite Communication

Satellite Transmission Links

• Earth stations Communicate by sending signals to the satellite on an uplink

• The satellite then repeats those signals on a downlink

• The broadcast nature of downlink makes it attractive for services such as the distribution of TV programs

Direct to User Services

One way Service (Broadcasting)One way Service (Broadcasting) Two way Service (Communication)Two way Service (Communication)

Satellite Signals

• Used to transmit signals and data over long distances– Weather forecasting– Television broadcasting– Internet communication– Global Positioning Systems

Satellite Transmission Bands

Frequency Band Downlink Uplink

C 3,700-4,200 MHz 5,925-6,425 MHz

Ku 11.7-12.2 GHz 14.0-14.5 GHz

Ka 17.7-21.2 GHz 27.5-31.0 GHz

Types of Satellite Orbits

• Based on the inclination, i, over the equatorial plane:– Equatorial Orbits above Earth’s equator (i=0°)– Polar Orbits pass over both poles (i=90°)– Other orbits called inclined orbits (0°<i<90°)

• Based on Eccentricity– Circular with centre at the earth’s centre– Elliptical with one foci at earth’s centre

Types of Satellite based Networks

• Based on the Satellite Altitude– GEO – Geostationary Orbits

• 36000 Km = 22300 Miles, equatorial, High latency

– MEO – Medium Earth Orbits• High bandwidth, High power, High latency

– LEO – Low Earth Orbits• Low power, Low latency, More Satellites, Small Footprint

– VSAT• Very Small Aperture Satellites

– Private WANs

Source: Federation of American Scientists [www.fas.org]

Satellite Orbits

Geosynchronous Orbit (GEO): Geosynchronous Orbit (GEO): 36,000 km above Earth, includes 36,000 km above Earth, includes commercial and military commercial and military communications satellites, satellites communications satellites, satellites providing early warning of ballistic providing early warning of ballistic missile launch.missile launch.

Medium Earth Orbit (MEO): from Medium Earth Orbit (MEO): from 5000 to 15000 km, they include 5000 to 15000 km, they include navigation satellites (GPS, Galileo, navigation satellites (GPS, Galileo, Glonass).Glonass).

Low Earth Orbit (LEO): from 500 to Low Earth Orbit (LEO): from 500 to 1000 km above Earth, includes 1000 km above Earth, includes military intelligence satellites, military intelligence satellites, weather satellites.weather satellites.

Satellite Orbits

GEO - Geostationary Orbit• In the equatorial plane

• Orbital Period = 23 h 56 m 4.091 s = 1 sidereal day*

• Satellite appears to be stationary over any point on equator:– Earth Rotates at same speed as Satellite– Radius of Orbit r = Orbital Height + Radius of Earth– Avg. Radius of Earth = 6378.14 Km

• 3 Satellites can cover the earth (120° apart)

NGSO - Non Geostationary Orbits

• Orbit should avoid Van Allen radiation belts:– Region of charged

particles that can cause damage to satellite

– Occur at • ~2000-4000 km and • ~13000-25000 km

LEO - Low Earth Orbits

• Circular or inclined orbit with < 1400 km altitude– Satellite travels across sky from horizon to horizon in

5 - 15 minutes => needs handoff– Earth stations must track satellite or have Omni

directional antennas– Large constellation of satellites is needed for

continuous communication (66 satellites needed to cover earth)

– Requires complex architecture– Requires tracking at ground

HEO - Highly Elliptical Orbits

• HEOs (i = 63.4°) are suitable to provide coverage at high latitudes (including North Pole in the northern hemisphere)

• Depending on selected orbit (e.g. Molniya, Tundra, etc.) two or three satellites are sufficient for continuous time coverage of the service area.

• All traffic must be periodically transferred from the “setting” satellite to the “rising” satellite (Satellite Handover)

Why Satellites remain in Orbits

Advantages of Satellite Communication

• Can reach over large geographical area• Flexible (if transparent transponders) • Easy to install new circuits • Circuit costs independent of distance • Broadcast possibilities • Temporary applications (restoration) • Niche applications • Mobile applications (especially "fill-in") • Terrestrial network "by-pass" • Provision of service to remote or underdeveloped areas • User has control over own network • 1-for-N multipoint standby possibilities

Disadvantages of Satellite Communication

• Large up front capital costs (space segment and launch)

• Terrestrial break even distance expanding (now approx. size of Europe)

• Interference and propagation delay

• Congestion of frequencies and orbits

When to use Satellites• When the unique features of satellite communications make it

attractive • When the costs are lower than terrestrial routing • When it is the only solution

• Examples:– Communications to ships and aircraft (especially safety

communications) – TV services - contribution links, direct to cable head, direct to

home– Data services - private networks – Overload traffic – Delaying terrestrial investments – 1 for N diversity – Special events

When to use Terrestrial• PSTN - satellite is becoming increasingly

uneconomic for most trunk telephony routes • but, there are still good reasons to use satellites for

telephony such as: thin routes, diversity, very long distance traffic and remote locations.

• Land mobile/personal communications - in urban areas of developed countries new terrestrial infrastructure is likely to dominate (e.g. GSM, etc.)

• but, satellite can provide fill-in as terrestrial networks are implemented, also provide similar services in rural areas and underdeveloped countries

Frequency Bands Allocated to the FSS

• Frequency bands are allocated to different services at World Radio-communication Conferences (WRCs).

• Allocations are set out in Article S5 of the ITU Radio Regulations.

• It is important to note that (with a few exceptions) bands are generally allocated to more than one radio services.

• CONSTRAINTS – Bands have traditionally been divided into “commercial" and

"government/military" bands, although this is not reflected in the Radio Regulations and is becoming less clear-cut as "commercial" operators move to utilize "government" bands.

Earth’s atmosphere

Source: All about GPS [www.kowoma.de]

Atmospheric Losses

• Different types of atmospheric losses can disturb 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 (for 90º elevation angle):– 22.3 GHz from resonance absorption in water

vapour (H2O)– 60 GHz from resonance absorption in oxygen

(O2)

• For other elevation angles:– [AA] = [AA]90 cosec

Source: Satellite Communications, Dennis Roddy, McGraw-Hill

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, and can be

calculated from specific attenuation coefficients in tabular form that can be found in a number of publications

– 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.