Communication Satellites

Geostationary Satellites ; Medium-Earth Orbit Satellites; Low-Earth Orbit Satellites; Satellites versus Fiber

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Communication Satellites

• In the 1950s and early 1960s – attempts for communication systems by bouncing signals off metalized weather balloons• received signals - too weak to be of

any practical use• Communication satellites can be

regarded as big microwave repeaters in the sky

Communication Satellites• Contain several transponders – device which

listens to some portion of the spectrum, amplifies the incoming signal, and then rebroadcasts it at another frequency to avoid interference with the incoming signal

• The downward beams can be• - broad, covering a substantial fraction of the

earth's surface, or • - narrow, covering an area only hundreds of

kilometers in diameter - bent pipe mode

Communication Satellites• The higher the satellite, the longer the orbital

period• Near the surface of the earth, the period is about

90 minutes• low-orbit satellites pass out of view fairly quickly,

so many of them are needed to provide continuous coverage

• At an altitude of about 35,800 km, the period is 24 hours

• At an altitude of 384,000 km, the period is about one month, proof - the moon

Communication Satellites

• Another issue is the presence of the Van Allen belts - layers of highly charged particles trapped by the earth's magnetic field

• Any satellite flying within them would be destroyed fairly quickly by the highly-energetic charged particles trapped there by the earth's magnetic field

• Hence there are three regions in which satellites can be placed safely - illustrated in the following figure

Communication Satellites

Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of

satellites needed for global coverage.

Communication Satellites

Geostationary Satellites

• In 1945, the science fiction writer Arthur C. Clarke calculated that a satellite at an altitude of 35,800 km in a circular equatorial orbit would appear to remain motionless in the sky, so it would not need to be tracked

• Clarke described a complete communication system that used these (manned) geostationary satellites, including the orbits, solar panels, radio frequencies, and launch procedures

Geostationary Satellites

• The first artificial communication satellite, Telstar, was launched in July 1962

• Since then, communication satellites have become a multibillion dollar business and the only aspect of outer space that has become highly profitable

• Often called GEO (Geostationary Earth Orbit) satellites

Geostationary Satellites• Satellites in geostationary orbit must all occupy a single

ring above the Equator and must be spaced apart to avoid harmful radio-frequency interference during operations

• Limited number of orbital "slots" available, thus only a limited number of satellites can be operated in geostationary orbit

• Conflict between different countries wishing access to the same orbital slots - countries near the same longitude (east-west position of a point on the Earth's surface) but differing latitudes and also conflict on radio frequencies (L, S bands – crowded, C, Ku, Ka bands – with problems-rain…)

• Disputes are addressed through the International Telecommunication Union's allocation mechanism

Geostationary Satellites• Modern satellites can weigh up to 4000 kg and consume

several kilowatts of electric power produced by the solar panels

• The effects of solar, lunar, and planetary gravity tend to move them away from their assigned orbit slots and orientations, an effect countered by on-board rocket motors – with them fine-tuning activity is achieved - station keeping.

• When the fuel for the motors runs out (in about 10 years), the satellite drifts and tumbles helplessly - must be turned off

• Eventually, the orbit decays and the satellite reenters the atmosphere and burns up or occasionally crashes to earth

Geostationary Satellites

• A modern satellite has around 40 transponders, each with an 80-MHz bandwidth

• Each transponder operates as a bent pipe (narrow down link)

• Earliest satellites had static division of channels -the bandwidth was split up into fixed frequency bands

• With modern satellites each transponder beam is divided into time slots, with various users taking turns

Geostationary Satellites

• Each downward beam can be focused on a small geographical area, so multiple upward and downward transmissions can take place simultaneously

• Typically, the spot beams are elliptically shaped, and can be as small as a few hundred km in diameter

• A communication satellite for the United States typically has one wide beam for the contiguous 48 states, plus spot beams for Alaska and Hawaii

VSAT systems• VSAT (Very Small Aperture Terminal) – a satellite

communication system that serves home and business users for data, voice, and video signals

• Tiny terminals have 1-meter or smaller antennas (versus 10 m for a standard GEO antenna) and can put out about 1 watt of power.

• A VSAT end user has a box that interfaces between the user's computer and an outside antenna with a transceiver.

• The tranceiver receives /sends a signal to a satellite transponder

VSAT systems

• The satellite sends and receives signals from an earth station computer that acts as a hub for the system.

• Each end user is interconnected with the hub station via the satellite in a star topology. For one end user to communicate with another, each transmission has to first go to the hub station which retransmits it via the satellite to the other end user's VSAT

VSAT systems• VSAT is used both by home users (who sign up

with a large service) and by private companies that operate or lease their own VSAT systems

• VSAT offers a number of advantages over terrestrial alternatives - companies can have total control of their own communication system without dependence on other companies

• Business and home users also get higher speed reception than if using ordinary telephone service or ISDN

• VSATs have great potential in rural areas

VSATs using a hub

Satellites vs terrestrial • The long round-trip distance introduces a

substantial delay for GEO satellites• Depending on the distance between the user and

the ground station, and the elevation of the satellite above the horizon, the end-to-end transit time is between 250 and 300 msec - typically 270 msec (540 msec for a VSAT system with a hub)

• Terrestrial microwave links have a propagation delay of roughly 3 µsec/km, and coaxial cable or fiber optic links have a delay of approximately 5 µsec/km

Satellites vs terrestrial • Satellites are inherently broadcast media - it does

not cost more to send a message to thousands of stations within a transponder's footprint than it does to send to one

• Useful property in some cases - a satellite can broadcast popular Web pages to the caches of a large number of computers spread over a wide area

• Security and privacy – poor - everybody can hear everything

• When security is required, encryption is needed

Satellites vs terrestrial

• The cost of transmitting a message is independent of the distance traversed for satellites

• A call across the ocean costs no more to service than a call across the street

• Satellites also have excellent error rates and can be deployed almost instantly

Medium-Earth Orbit Satellites• The MEO (Medium-Earth Orbit) satellites are

between the two Van Allen belts• Takes something like 6 hours to circle the earth• As MEO are lower than the GEOs (typically

16,000 km above Earth), they have a smaller footprint on the ground and require less powerful transmitters to reach them

• Currently they are not used for telecommunications but in navigation systems

Medium-Earth Orbit Satellites• The 24 GPS (Global Positioning System) satellites

orbiting at about 18,000 km are American (DoD) but free to use by anyone

• Russia is using Glonass – 24 satellites orbiting at 19,100 km

• European Union works on Galileo – 30 satellite MEO navigation system orbiting at 23200 km - global navigation satellite system (GNSS) (2019)

• China is building Compass – 35 satellites – 5 GEO and 30 MEO at 21,100 km (10 in use and offering services now) (2020)

Low-Earth Orbit Satellites• A Low Earth Orbit (LEO) is generally defined as an orbit below

an altitude of approximately 2,000 kilometers and above 160 kilometers – almost all human spaceflights have taken place in LEO

• (-) large numbers of LEO are needed for a complete system due to their rapid motion (26,000 to 27,000 km per hour)

• (-) most spacecraft and artificial satellites have rapidly decaying orbits due to the drag created by air molecules in the thermosphere, the last layer of the Earth’s atmosphere

• (+) satellites are very close to the earth, so the ground stations do not need much power

• (+) the round-trip delay is only a few milliseconds

Low-Earth Orbit Satellites

• Because these LEO orbits are not geostationary, a network (or "constellation") of satellites is required to provide continuous coverage

• LEOs are used for a variety of civil, scientific and military roles including Earth observation, radar, optical, telecoms and demonstrator

• Some use as many as 66 satellites (Iridium – satellite phones)

Low-Earth Orbit Satellites• The Iridium satellite constellation - group of satellites

providing voice and data coverage to satellite phones, pagers and integrated transceivers over Earth's entire surface

• Originally developed in 1992, services started in 1998, bankrupted in August 1999 and subsequently restarted in March 2001 (513 000 subscribers in 2011)

• Satellites are at a height of approximately 781 km• Customers include the maritime, aviation, and oil

exploration industries, as well as people traveling in parts of the world lacking a telecommunications infrastructure (e.g., deserts, mountains, jungles, and some Third World countries)

Iridium

• The Iridium satellites are in circular polar orbits

• They are arranged in north-south necklaces, with one satellite every 32 degrees of latitude

• With six satellite necklaces, the entire earth is covered

• Each satellite can support up to 1100 concurrent phone calls and weighs about 680 kg

Low-Earth Orbit SatellitesIridium

(a) The Iridium satellites from six necklaces around the earth.

(b) 1628 moving cells cover the earth.

Iridium• Each satellite has a maximum of 48 cells (spot beams), with

a total of 1628 cells over the surface of the earth (figure in previous slide)

• Each satellite has a capacity of 3840 channels, or 253,440 in all

• Some of these are used for paging and navigation, while others are used for data and voice

• An interesting property of Iridium is that communication between distant customers takes place in space, with one satellite relaying data to the next one

• A caller at the North Pole contacts a satellite directly overhead - the call is relayed via other satellites and finally sent down to the callee at the South Pole.

Iridium - Globalstar

(a) Relaying in space.(b) Relaying on the ground.

Globalstar• Globalstar - based on 48 LEO satellites but uses a

different switching scheme than that of Iridium (relays calls from satellite to satellite, which requires sophisticated switching equipment in the satellites)

• Globalstar uses a traditional bent-pipe design - a call originating at the North Pole is sent back to earth and picked up by the large ground station

• The call is then routed via a terrestrial network to the ground station nearest the callee and delivered by a bent-pipe connection as shown

• Advantage - puts much of the complexity on the ground, where it is easier to manage (412300 subscribers in 2010)

Teledesic• Teledesic was as high-speed, wireless, switched global

network -an IP-based system designed to support data, voice and video at the same quality level as fiber-based terrestrial systems - a global "Internet-in-the-Sky“ providing broadband and Internet access

• Originally (1995) planning 840 active satellites with in-orbit spares at an altitude of 700 km,

• Scaled (1997) to 288 active satellites at 1400 km• Officially suspended its satellite construction work on

October 1, 2002• Funding from Microsoft (investing US$30 million for an

8.5% stake), Craig McCaw, Bill Gates, Paul Allen, and Saudi prince Alwaleed bin Talal, and for achieving allocation on the Ka-band frequency spectrum for non-geostationary services

Highly elliptical orbit• HEO – an elliptic orbit with a low-altitude (about

1,000 kilometres) perigee and a high-altitude (over 35,786 kilometres) apogee

• Advantage - long dwell times at a point in the sky during the approach to, and descent from, apogee - phenomenon known as apogee dwell (acts like GEO) (3-4 hours can receive signal with 0.6 m antenna)

• Used by Sirius Satellite Radio (operating in North America), the Molniya communication satelites (at least 3) (Russia),

Satellites versus Fiber

• A single fiber has more potential bandwidth than all the satellites ever launched but this bandwidth is not available to most users

• The fibers that are installed are used within the telephone system to handle many long distance calls at once and not provide individual users with high bandwidth

• With satellites, a user has an antenna on the roof of the building and completely bypasses the telephone system to get high bandwidth

Satellites versus Fiber• Many people nowadays want to communicate while

jogging, driving, sailing, and flying• Terrestrial fiber optic links are of no use to them, but

satellite links potentially are• A combination of cellular radio and fiber will do an

adequate job for most users (except for those airborne or at sea)

• A message sent by satellite can be received by thousands of ground stations at once – i.e. an organization transmits a stream of stock, bond, or commodity prices to thousands of dealers - a satellite system may be cheaper than simulating broadcasting on the ground

Satellites versus Fiber• Places with hostile terrain or a poorly developed

terrestrial infrastructure - Indonesia, for example, has its own satellite for domestic telephone traffic

• In areas where obtaining the right of way for laying fiber is difficult or unduly expensive

• Where rapid deployment is critical, as in military communication systems in time of war, satellites win easily

• Seems that mainstream communication of the future will be terrestrial fiber optics combined with cellular radio, but for some specialized uses, satellites are better