BASICS OF SATELLITE

Value of Satellite Systems


• Value of satellite systems grows with widely distributed networks and mobility of users
• Satellite systems perform most effectively when:
• interconnecting wide distributed networks,
• providing broadcasting services over very wide areas such as a country, region, or entire hemisphere
• providing connectivity for the “last mile” in cases where fiber networks are simply not available for interactive services.
• providing mobile wideband and narrow band communications
• satellites are best and most reliable form of communications in the case of natural disasters or terrorist attacks - fiber networks or even terrestrial wireless can be disrupted by tsunamis, earthquakes, etc..
Satellite-Fiber Comparison


“Typical” Fixed Satellite Network
Applications
• Credit Card Validation
• ATM/Pay at the Pump
• Inventory Control
• Store Monitoring
• Electronic Pricing
• Training Videos
• In-Store Audio
• Broadband Internet Access
• Distance Learning

Low Earth Orbit (LEO)
Characteristics of Low-Earth Orbit (LEO) Systems
- Low latency or transmission delay
- Higher look angle (especially in high-latitude regions)
- Less path loss or beam spreading
- Easier to achieve high levels of frequency re-use
- Easier to operate to low-power/low-gain ground antennas

Challenges of Low-Earth Orbit (LEO) Systems
- Larger number of satellites (50 to 70 satellites). Thus higher launch costs to deploy, build, and operate.
- Harder to deploy, track and operate. There is higher TTC&M costs even with cross links.
- Shorter in-orbit lifetime due to orbital degradation
Medium Earth Orbit (MEO)
Characteristics of Medium-Earth Orbit (MEO) Systems
• Less latency and delay than GEO (but greater than LEO)
• Improved look angle to ground receivers in higher latitudes
• Fewer satellites to deploy and operate and cheaper TTC&M systems than LEO (but more expensive than with GEO)
• Longer in-orbit lifetime than LEO systems (but less than GEO)


Challenges of Medium-Earth Orbit (MEO) Systems
• More satellites to deploy than GEO (10 to 18 vs. 3 to 4)
• Ground antennas are generally more expensive and complex because of the need to track satellites. Or, one must use lower-gain, quasi-omni antennas.
• Increased exposure to Van Allen Belt radiation
Components
Bus
Power Subsystem
Telemetry and Command Subsystem
Attitude and Control Subsystem
Propulsion Subsystem
Payload
Communications Subsystem
Transponders
Transponders
• The transponder is the “brains” of the satellite - provides the connection between the satellite’s receive and transmit antennas.
• Satellites can have 12 to 96 transponders plus spares, depending on the size of the satellite.
• A transponder bandwidth can frequently be 36 MHz, 54 MHz, or 72 MHz or it can be even wider.

• A transponders function is to
• Receive the signal, (Signal is one trillion times weaker then when transmitted)
• Filter out noise,
• Shift the frequency to a down link frequency (to avoid interference w/uplink)
• Amplify for retransmission to ground
Frequency Efficiency
• The vital resource in satellite communications is spectrum.
• As the demand for satellite services has grown, the solution has been;
• To space satellites closer together,
• Allocate new spectrum in higher bands,
• Make satellite transmissions more efficient so that more bits/Hz can be transmitted, and
• To find ways to re-use allocated spectrum such as through geographic separation into separated cells or beams or through polarization separation

• Today the satellites systems transmit more efficiently than ever before but interference is now a bigger problem - there is a basic trade off;
• The higher the frequency the more spectrum that is available
• But, the higher the frequency the more problems with interference from other users terrestrial, unlicensed, etc.
Satellite Frequencies
There are specific frequency ranges used by commercial satellites.
L-band (Mobile Satellite Services)
1.0 – 2.0 GHz
S-band (MSS, DARS – XM, Sirius)
1.55 – 3.9 GHz
C-band (FSS, VSAT)
3.7 – 6.2 GHz
X-Band (Military/Satellite Imagery)
8.0 – 12.0 GHz
Ku-band (FSS, DBS, VSAT)
11.7–14.5 GHz
Ka-band (FSS “broadband” and inter-satellite links)
17.7 - 21.2GHz and 27.5 – 31 GHz
Satellite Power Systems
• Main source of power is solar cell panels - new solar cells are increasingly efficient

• The solar cell system is backed up by battery system that provides energy during solar eclipses and other periods of outages.

• Typical power levels of 2 to 5 KWs for Fixed Satellite Systems and 10 to 12 KWs for Mobile and Broadcast Satellite Systems.

Batteries
• latest battery technology is represented by Lithium Ion systems that can provide a greater power density for longer periods of time and survive a greater depth of discharge
Satellite Technologies of the Future
• Satellites in general are becoming more capable, with higher power and larger aperture antennas to promote frequency re-use and creating highly capable “super- computers-in-the-sky”

• With electronically formed beams, the beam patterns can be re-formed on command to respond to needs at different times of day, or of changing requirements that emerge in response to market demand

• Future Technologies include:
• Advanced Phased Array Antennas
• Dynamic Beam Management
• Advanced Antenna Systems
• More Efficient Power Systems
• Turbo-coding
• Advanced Modems
• New materials for Light weight antennas (Inflatable Antennas)

Satellite Services & Applications
Voice/Video/Data Communications
• Rural Telephony
• News Gathering/Distribution
• Internet Trunking
• Corporate VSAT Networks
• Tele-Medicine
• Distance-Learning
• Mobile Telephony
• Videoconferencing
• Business Television
• Broadcast and Cable Relay
• VOIP & Multi-media over IP
GPS/Navigation
• Position Location
• Timing
• Search and Rescue
• Mapping
• Fleet Management
• Security & Database Access
• Emergency Services
Direct-To-Consumer
• Broadband IP
• DTH/DBS Television
• Digital Audio Radio
• Interactive Entertainment & Games
• Video & Data to handhelds
Remote Sensing
• Pipeline Monitoring
• Infrastructure Planning
• Forest Fire Prevention
• Urban Planning
• Flood and Storm watches
• Air Pollution Management
• Geo-spatial Services
Fixed Satellite Services
• FSS Industry
• Geosynchronous Spacecraft
• ~22,000 miles in orbit

• C, Ku and Ka Frequencies

• Terrestrial Infrastructures
• Teleports
• TT&C Centers
• Service Platforms
• Fiber Interconnects

• Diverse market-base
• Media Distribution
• Telecom Infrastructure
• Enterprise Networks
• Government Networks & Apps

• ~ 250 operational commercial GEO satellites in use today
• 59 to be launched over next 3 years
Mobile Satellite Industry
• Mobile Satellite Services (MSS) refer to services to mobile user terminals
• Use a mix of orbit types
• Geosynchronous (GEO)
• Non-geosynchronous (LEO and MEO)
• System sizes range from (1) GEO satellite to (66) LEO Satellites
• Some use Inter-Satellite Links (ISLs)
• Use a mix of frequencies
• Mostly L-Band / Some S-Band, UHF/VHF
• Feeder links and some services use C, Ka, and Ku-Band
• Applications
• Aeronautical
• Maritime
• Land
Ground Antennas
The size of the antenna depends on the satellite frequency band used, the data rate, and whether the service is bidirectional or receive only
Higher data rates require larger antennas and/or higher power
Higher transmit capability (EIRP) of the satellite allows the antenna size to be reduced
The use of spot beams instead of global beams improves VSAT link performance
Receive-only antennas can be substantially smaller
Ground Equipment Trends
• Overall systems costs have decreased because of the explosion of low cost user terminals that can now receive video via hand-held units.
• Omni directional antennas
• Smaller, lighter, cheaper
• More powerful, faster
• Pocket, notebook, rugged
• Application specific terminals, embedded modems

Phones
• Voice, Asynchronous Data and Packet Data
• Smaller (antenna and battery in particular)

Satellite Communication

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.


Lunching Satellites
How does a satellite stay in it’s orbit?

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!)

How Does a Satellite Work?
Consider the light bulb example

Applications
• Telephony - Fixed points<> 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.
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.
Satellites remain the best utilization used for communications due to their speed and other advantages

Orbits

• Several types
• LEOs - Low earth orbit
• MEOs - Medium earth orbit
• GEOs - Geostationary earth orbit

GEOs
• Originally proposed by Arthur C. Clarke
• Circular orbits above the equator
• Angular separation about 2 degrees - allows 180 satellites
• Orbital height above the earth about 23000 miles/35000km
• Round trip time to satellite about 0.24 seconds
• GEO satellites require more power for communications
• The signal to noise ratio for GEOs is worse because of the distances involved
• A few GEOs can cover most of the surface of the earth
• Note that polar regions cannot be “seen” by GEOs
• Since they appear stationary, GEOs do not require tracking
• GEOs are good for broadcasting to wide areas

Early experiments
• US Navy bounced messages off the moon
• ECHO 1 “balloon” satellite - passive
• ECHO 2 - 2nd passive satellite
• All subsequent satellites used active communications
ECHO 1
Early satellites
• Relay
– 4000 miles orbit
• Telstar
– Allowed live transmission across the Atlantic
• Syncom 2
– First Geosynchronous satellite





TELSTAR

SYNCOM 2 Major problems for satellites

• Positioning in orbit
• Stability
• Power
• Communications
• Harsh environment
Positioning
• This can be achieved by several methods
• One method is to use small rocket motors
• These use fuel - over half of the weight of most satellites is made up of fuel
• Often it is the fuel availability which determines the lifetime of a satellite
• Commercial life of a satellite typically 10-15 years
Stability
• It is vital that satellites are stabilised
– to ensure that solar panels are aligned properly
– to ensure that communications antennae are aligned properly
• Early satellites used spin stabilisation
– Either this required an inefficient omni-directional aerial
– Or antennae were precisely counter-rotated in order to provide stable communications
• Modern satellites use reaction wheel stabilisation - a form of gyroscopic stabilisation Other methods of stabilisation are also possible
• including:
– eddy currrent stabilisation
– (forces act on the satellite as it moves through the earth’s magnetic field)
Reaction wheel stabilisation
• Heavy wheels which rotate at high speed - often in groups of 4.
• 3 are orthogonal, and the 4th (spare) is a backup at an angle to the others
• Driven by electric motors - as they speed up or slow down the satellite rotates
• If the speed of the wheels is inappropriate, rocket motors must be used to stabilise the satellite - which uses fuel
Power
• Modern satellites use a variety of power means
• Solar panels are now quite efficient, so solar power is used to generate electricity
• Batteries are needed as sometimes the satellites are behind the earth - this happens about half the time for a LEO satellite
• Nuclear power has been used - but not recommended
Harsh Environment
• Satellite components need to be specially “hardened”
• Circuits which work on the ground will fail very rapidly in space
• Temperature is also a problem - so satellites use electric heaters to keep circuits and other vital parts warmed up - they also need to control the temperature carefully
Alignment
• There are a number of components which need alignment
– Solar panels
– Antennae
• These have to point at different parts of the sky at different times, so the problem is not trivial
Antennae alignment
• A parabolic dish can be used which is pointing in the correct general direction
• Different feeder “horns” can be used to direct outgoing beams more precisely
• Similarly for incoming beams
• A modern satellite should be capable of at least 50 differently directed beams
Satellite - satellite communication
• It is also possible for satellites to communicate with other satellites
• Communication can be by microwave or by optical laser
LEOs
• Low earth orbit satellites - say between 100 - 1500 miles
• Signal to noise should be better with LEOs
• Shorter delays - between 1 - 10 ms typical
• Because LEOs move relative to the earth, they require tracking
Orbits
• Circular orbits are simplest
• Inclined orbits are useful for coverage of equatorial regions
• Elliptical orbits can be used to give quasi stationary behaviour viewed from earth
– using 3 or 4 satellites
• Orbit changes can be used to extend the life of satellites
Communication frequencies
• Microwave band terminology
– L band 800 MHz - 2 GHz
– S band 2-3 GHz
– C band 3-6 GHz
– X band 7-9 GHz
– Ku band 10-17 GHz
– Ka band 18-22 GHz
Rain fade
• Above 10 GHz rain and other disturbances can have a severe effect on reception
• This can be countered by using larger receiver dishes so moderate rain will have less effect
• In severe rainstorms reception can be lost
• In some countries sandstorms can also be a problem

Ku band assignments

BASICS OF SATELLITE

1. Benefits of satellite communications

A. How do satellites deliver vital communications around the world?
Because of their universal and multi-point nature, satellite-based solutions can provide a flexible and cost-effective answer to support:
Fixed or wireless voice and data communications
Enterprise networking
Financial transactions
Internet linkages
Satellite video transmission and distribution networks
In every case, Intelsat solutions provide for the delivery of vital information, news, sports and entertainment to every corner of the globe, no matter how remote.

B: What are the key benefits of satellites?

Satellite communications have distinct benefits over terrestrial alternatives:

UNIVERSAL: Satellite communications are available virtually everywhere. A small constellation of satellites can cover the Earth's entire surface. And even the reach of a single satellite is far more extensive than what any terrestrial network can achieve.

VERSATILE: Satellites can support all of today's communications needs - transactional and multimedia applications, video, voice, cellular networks, entertainment and breaking news.
Bring broadband to the last mile of residences and businesses.

Overcome regulatory issues that make alternative carriers dependent on incumbents.
Deliver a communications infrastructure to areas where terrestrial alternatives are unavailable, unreliable or simply too expensive.

RELIABLE: Satellite is a proven medium for supporting a company's communications needs. Whereas terrestrial IP networks are often a mix of different networks and topologies, with different level of congestion and latency. Satellite networks are extremely predictable allowing constant and uniform quality of service to hundreds of locations, regardless of geography.

SEAMLESS: Satellite's inherent strength as a broadcast medium makes it ideal for the simultaneous distribution of bandwidth-intensive information to hundreds or thousands of locations.

FAST: Unlike most terrestrial alternatives, satellite networks can be rolled out quickly and inexpensively to hundreds or thousands of locations, connecting cities or remote locations across a large landmass, where copper or fiber is cost prohibitive. Since satellite networks can be set up quickly, companies can be fast-to-market with new services.

EXPANDABLE: Satellite networks are easily scalable, allowing users to expand their communications networks and their available bandwidth easily. In coordination with local vendors, expanding a network on the ground requires the ordering of new terminal components and the commissioning of increased bandwidth at each site.

FLEXIBLE: Satellites can be easily integrated to complement, augment or extend any communications network, helping overcome geographical barriers, terrestrial network limitations and other constraining infrastructure issues.

2. How satellite communications work
A. What is a communications satellite and how does it work?
A communications satellite is a radio relay station in orbit above the earth that receives, amplifies, and redirects analog and digital signals carried on a specific radio frequency.
In addition to communications satellites, there are other types of satellites:

Weather satellites: These satellites provide meteorologists with scientific data to predict weather conditions and are equipped with advanced instruments

Earth observation satellites: These satellites allow scientists to gather valuable data about the earth's ecosystem Navigation satellites: Using GPS technology these satellites are able to provide a person's exact location on Earth to within a few meters

B. What are the different kinds of orbits?
An orbit is the path that a satellite follows as it revolves around Earth. In terms of commercial satellites, there are three main categories of orbits:

Geosynchronous Orbit (GEO): 35,786 km above the earth
Orbiting at the height of 22,282 miles above the equator (35,786 km), the satellite travels in the same direction and at the same speed as the Earth's rotation on its axis, taking 24 hours to complete a full trip around the globe. Thus, as long as a satellite is positioned over the equator in an assigned orbital location, it will appear to be "stationary" with respect to a specific location on the Earth.
A single geostationary satellite can view approximately one third of the Earth's surface. If three satellites are placed at the proper longitude, the height of this orbit allows almost all of the Earth's surface to be covered by the satellites.

Medium Earth Orbit (MEO): 8,000-20,000 km above the earth
These orbits are primarily reserved for communications satellites that cover the North and South Pole
Unlike the circular orbit of the geostationary satellites, MEO's are placed in an elliptical (oval-shaped) orbit

Low Earth Orbit (LEO): 500-2,000 km above the earth
These orbits are much closer to the Earth, requiring satellites to travel at a very high speed in order to avoid being pulled out of orbit by Earth's gravity
At LEO, a satellite can circle the Earth in approximately one and a half hours

GEO vs. MEO vs. LEO
Most communications satellites in use today for commercial purposes are placed in the geostationary orbit, because of the following advantages:
One satellite can cover almost 1/3 of Earth's surface, offering a reach far more extensive than what any terrestrial network can achieve
Communications require the use of fixed antennas. Since geosynchronous satellites remain stationary over the same orbital location, users can point their satellite dishes in the right direction, without costly tracking activities, making communications reliable and secure
GEO satellites are proven, reliable and secure - with a lifespan of 10-15 years
For a more comprehensive understanding of satellite advantages, see benefits of satellite.
C. Satellite architecture
Communications data passes through a satellite using a signal path known as a transponder. Typically satellites have between 24 and 72 transponders. A single transponder is capable of handling up to 155 million bits of information per second. With this immense capacity, today's communication satellites are an ideal medium for transmitting and receiving almost any kind of content - from simple voice or data to the most complex and bandwidth-intensive video, audio and Internet content.

Diagrammatic Representation of a Satellite

D. Orbital location and footprint

The location of a geostationary satellite is referred to as its orbital location. International satellites are normally measured in terms of longitudinal degrees East (° E) from the Prime Meridian of 0° (for example, Intelsat's IS-805 satellite is currently located at 304.5° E).
The geographic area of the Earth's surface over which a satellite can transmit to, or receive from, is called the satellite's "footprint." The footprint can be tailored to include beams with different frequencies and power levels.

E. Frequency bands and beams
Satellites transmit information within radio frequency bands. The frequency bands most used by satellite communications companies are called C-band and the higher Ku-band. Over the next several years, the use of a higher frequency band known as Ka-band is expected to increase. Modern satellites are designed to focus on different ranges of frequency bands and different power levels at particular geographic areas. These focus areas are called beams. Intelsat offers four beam types:
Global: covering almost 1/3 of Earth's surface
Hemi: covering almost 1/6 of Earth's surface
Zone: covering a large landmass area
Spot: covering a specific geographic area

F. What is installed on the ground?
All communications with a geostationary satellite require using an earth station or antenna. Earth Stations may be either fixed (installed at a specific location) or mobile for uses such as Satellite News Gathering (SNG) or maritime applications. Antennas range in size, from large telecommunications carrier dishes of 4.5 to 15 meters in diameter, to VSAT antennas which can be as small as under one meter, designed to support services such as Direct to Home TV (DTH) and rural telephony.

The antenna, itself, will generally be connected to equipment indoors called an indoor unit (IDU), which then connects either to the actual communications devices being used, to a Local Area Network (LAN), or to additional terrestrial network infrastructure.

G. Network topologies
Depending on the application, satellites can be used with different ground network designs or network topologies. At its simplest, satellite can support one-direction or two-direction links between two earth stations (called respectively simplex transmission and duplex transmission). More complex communications needs can also be addressed with more sophisticated network topologies, such as star and mesh.

The following examples show some of the options available to customers for configuring their satellite networks:
Simplex Transmission

Applications for simplex services include broadcast transmissions such as:

TV and video services
Radio services

Point-to-Point Duplex Transmission

Applications for duplex services include:
Voice Telephony transport
Data and IP transport (especially in asymmetric configurations)
Corporate networks
TV and Broadcast program contribution and distribution

Point-to-Multipoint Transmission

(May be simplex or duplex, symmetric or asymmetric).
Applications for point-to-multipoint services include:
Corporate networks, including VSAT services and business television
Video and broadcast distribution, including Direct-to-Home Internet services

Mobile Antenna Service
Applications for mobile antenna services include:
Satellite News Gathering
Special Event Backhaul and Broadcasting
Maritime services

Star Network Applications for Star Networks include:
Corporate Networks
Distance Learning
Mesh Network
Applications for Mesh Networks includ
National and International Telephony and Data networks
Rural Telephony

SATELLITES

A communications satellite is a specialized wireless receiver/transmitter — receiving radio waves from one location and transmitting them to another (also known as a “bent pipe”) — that is launched by a rocket and placed in orbit around the earth. Today, there are hundreds of commercial satellites in operation around the world. Those satellites are used for such diverse purposes as wide-area network communications, weather forecasting, television broadcasting, amateur radio communications, Internet access and the Global Positioning System.
Satellites have many important uses, not just communications. Most modern weather reports rely on satellite information. Global Positioning systems work because of a linked set of satellites. Scientific studies of our planet, the atmosphere and the universe all rely on satellites.

ORBITS
There are three areas for satellite orbits:
GEO: Geostationary Earth Orbit
MEO: Medium Earth Orbit
LEO: Low Earth Orbit

GEO satellites orbit the earth directly over the equator, approximately 35 400 km (22 000 miles) up. At that altitude, one complete trip (orbit) around the earth takes 24 hours. Thus, the satellite remains over the same spot on the surface of the earth (geo) at all times, and stays fixed in the sky (stationary) from any point on the surface from which it can be "seen."

MEO is defined simply as the area between LEO and GEO. The primary satellite systems there are the GPS (Global Positioning System) satellite constellations.

LEO is between 200 and 1400 km above the earth. Satellites in LEO rapidly circle the earth and are typically in range of one location for only 90 minutes. Their main advantage is how close they are, providing shorter delays for faster communications. However, for consistent communications they require a constellation of satellites so that communications can be maintained as one satellite moves out of range and another moves within range of the ground station. LEO satellites are less expensive to build, typically less powerful, and have a shorter average life span.

COMMUNICATIONS SATELLITES
Most communications satellites are in GEO. A single geostationary satellite can cover as much as 40 percent of the earth's surface; so, in theory, three such satellites can provide global coverage. To ensure accurate and strong coverage of a specific region, continent or country, the transponders are often “shaped” to focus transmission and increase signal strength for a service area.

A satellite’s job in the communications network is to serve as a repeater. That is, it receives a signal from one location and rebroadcasts it so another station can receive the signal. Reception and retransmission are accomplished by a transponder. A single transponder on a geostationary satellite is capable of handling approximately 5,000 simultaneous voice or data channels. A typical satellite has 32 transponders.

Transponders each work on a specific radio frequency wavelength, or “band.” Satellite communications work on three primary bands: C, Ku and Ka. C was the first band used and, as a longer wavelength, requires a larger antenna. Ku is the band used by most current VSAT systems. Ka is a new band allocation that isn’t yet in wide use. Of the three, it has the smallest wavelength and can use the smallest antenna.

Because of attenuation and business competition, there are far more than three GEO satellites. Satellites of similar frequency can be as close as 3 degrees apart without causing interference. Since there are 360 degrees in a circle, that means 120 satellites of a specific frequency can be placed in GEO orbits.

The combination of individual transponder volumes and the number of transponders in orbit means today's communication satellites are an ideal medium for transmitting and receiving almost any kind of content, from simple data to the most complex and bandwidth-intensive video, audio and data content.

How to Communicate Across Satellite Networks
In our satellite basics discussion, we’ve now described what communication satellites are, but how does a satellite network work? A primary method for providing Wide Area Networks (WANs) to governments and businesses via satellites is something called a VSAT network.
WHAT IS A VSAT? A Very Small Aperture Terminal (VSAT) is a device, known as an earth station, that is used to receive satellite transmissions. The "very small" component of the VSAT acronym refers to the size of the VSAT dish antenna, typically about 0.55-1.2 m (2 to 4 feet ) in diameter, that is mounted on a roof or wall, or is placed on the ground. That size is appropriate for Ku band communications which, as mentioned in “What is Satellite Communications”, is most used for current system

The antenna, along with the attached low-noise block converter or LNB (which amplifies the received satellite signals) and the transmitter (which sends signals) make up the VSAT outdoor unit (ODU), one of the two components of a VSAT.

The second component of the VSAT is the indoor unit (IDU). The indoor unit is a small desktop appliance that converts between satellite analog communications and appropriate protocols for local devices such as telephones, computer networks, PCs, TVs, kiosks, etc. On top of basic conversion routines, IDUs can also contain value-added functionality such as security, network acceleration and other features. The indoor unit is connected to the outdoor unit with a pair of cables.

The key advantage of a VSAT earth station, versus a typical terrestrial network connection, is that VSATs are not limited by the reach of buried cable. A VSAT earth station can be placed anywhere — as long as it has an unobstructed view of the satellit. VSATs are capable of sending and receiving all sorts of video, data and audio content at the same high speed regardless of their distance from terrestrial switching offices and infrastructure.

HOW DOES A VSAT NETWORK WORK?
A VSAT network has three components:
A central hub (also called a master earth station).
The satellite.
A virtually unlimited number of VSAT earth stations in various locations - across a country or continent.

Content typically originates at the hub. It is also where equipment and software used to control the satellite network are located. The hub is usually connected to the main communications network, whether a big city PSTN, a company’s central computing network or the internet backbone.

The most obvious component of the hub is a very large, 4,5-11 m (15-36 ft ), antenna. The indoor components include multiple devices that control two-way communications through the antenna, conversions between satellite and terrestrial protocols, and other technical matters. A Network Management System (NMS) server controls the operation of all the devices, as well as allocates communications priorities to applications depending on customer defined rules for quality of service.

The VSATs, as already described, are the devices used in remote locations to provide communications to the central site through the network hub.

In the simplest configuration, outbound information (from the hub to the VSATs) is sent up to the communications satellite's transponder, which receives it, amplifies it and beams it back to earth for reception by the remote VSATs. The remote VSATs send information inbound (from the VSATs to the hub) via the same satellite transponder to the hub station.

This arrangement, where all network communication passes through the network's hub processor, is called a "star" configuration, with the hub station at the center of the star. One major advantage of this configuration is that there is virtually no limit on the number of remote VSATs that can be connected to the hub.

NETWORK TOPOLOGIES
As just described, the star topology is the simplest way to configure a satellite network. However, it has one issue that affects performance. Remember that a satellite in geostationary orbit is 35 400 km above the earth. That means transmission time matters. Because of the distance, sending a bit from one location through a satellite to another location (a single “hop”) averages around a quarter of a second. If communications is going from one VSAT to another, the star topology requires two hops, with a half second delay.

That time delay doesn’t typically matter when sending data between two computers, to update a database for instance. In addition, the star topology allows VSATs to use smaller antennas and lower power transmitters, since they’re communicating only with the large hub antenna.
However, star topology delay can become noticeable for voice communications. Therefore, the star topology is best when communications is primarily between a central system and remote locations in a single hop, or when VSAT to VSAT communications doesn’t require immediate response.

Mesh topologies provide the ability for VSATs to communicate directly to other VSATs, minimizing delay for distributed communications. That means, for example, a telephone conversation between people on VSAT connected telephones only has a single hop, a delay not noticeable to most people. Mesh IP supports single hop performance for computer applications such as client/server software which require frequent two-way connectivity between computers at remote locations. The trade-off is the requirement for a larger antenna and a higher powered transmitter at the VSAT, both of which increase costs.

Multi-Star topologies provide a mix of star and mesh solutions, where the hub sends information to the VSATs, but VSATs are also capable of direct, mesh connections. That provides the capability, for instance, of a VoIP phone on one VSAT to communicate directly with a PSTN network accessible via a second VSAT. In another example, a corporate server for an international company can send database updates from the hub to national headquarters behind one VSAT, which can then forward the information on to regional offices.

Because of the trade-offs in price and performance, cost-benefit analysis mustalways be performed in order to understand the appropriate topology necessary for each site in order to design the appropriate network topology for your needs.

Why?
Satellite Basics has described what a communications satellite is, how it works, how to create a satellite network, and who is in the industry, but the most important question still remains. Why use a satellite network?
That decision is answered differently in the two critical communications environments due to a location’s existing telephony and telecommunications infrastructure.
Satellite communications can complement existing terrestrial infrastructure, providing competitive advantages such as:
Ubiquitous availability.
Terrestrial-free network.
Reliability.
Multi-cast content distribution.
Security & privacy.
Satellite communications provides advanced communications infrastructure to regions that do not have adequate terrestrial infrastructure through:
Superior economics.
Rapid deployment & installation.
Flexibility & expandability.
Ubiquitous Availability
Satellite communications is the only telephony and broadband wide-area network technology that is available everywhere. All that's needed is a clear view of the sky. For multi-national and multi-regional enterprises, that means uniform service levels for all locations. Managing the communication network is also simpler and less expensive, since there is one point of contact for all locations, versus multiple local service providers and problematic demarcations between them.
Terrestrial-Free Network
Satellite networks are independent of terrestrial disruptions, making them an excellent solution for communication backups for terrestrial networks. They are also useful for disaster recovery and quick-deploy solutions, enabling communication services to be quickly restored.
Reliability
Satellite networks have far fewer potential points of failure than terrestrial solutions, and built-in redundancy at almost every level, satellite networks provide unmatched reliability. Typical satellite networks can assure 99.95% availability to all sites, regardless of location.
Terrestrial networks have multiple potential points of failure. To add to the challenge, terrestrial network providers may not be able to fix outages without third-party carriers or work crews. Both telephony and modern broadband networks are challenged to provide this level of nationwide reliability.
Multi-Cast Content Distribution
Satellite's inherent strengths as a broadcast medium makes VSAT networks ideal for the distribution of bandwidth-intensive information — data, video or audio — to large numbers of remote locations.
A terrestrial network requires sending separate and identical messages to all recipients, consuming valuable bandwidth and server resources. Satellite multicast technology eliminates duplicate transmissions and maximizes the efficiency of existing servers and networks, freeing valuable bandwidth. There’s no extra overhead at the hub or in the satellite. The only added cost for each recipient is the VSAT. That translates to significant cost savings.
Security And Privacy
VSAT networks are more secure and private than alternative terrestrial technologies, making them the technology of choice for governmental agencies, military and enterprises that require high levels of security. IP traffic across the internet crosses many computers, providing multiple points of access to private traffic. Satellite communications does not travel across third party terrestrial computers, minimizing chances of unapproved access. Traffic can be encrypted using the most advanced algorithms to ensure that data will not be compromised.
Superior Economics
Satellite networks are much less costly to deploy, maintain and operate in many cases than terrestrial network technologies. Terrestrial networks require heavy infrastructure, whether they are telephony networks (based on copper wiring, fiber optic cables, radio or microwave towers), or broadband data networks (such as Frame Relay, DSL, ISDN and cable). In remote areas, building such networks is often prohibitively expensive. VSATs are not only significantly less expensive to deploy, VSATs built for low power consumption can run without electrical power infrastructure, using simple solar panels.
Rapid Deployment & Installation
Satellite networks can be rolled out to hundreds or thousands of locations in a fraction of the time required for a comparable terrestrial network. VSAT installation requires only a single vendor, so multi-vendor coordination is not needed. An installation can usually be completed in a matter of hours, no matter where the site is located, meaning complete network deployment can be accomplished in a matter of weeks, rather than months.
Satellite network installation and deployment are quick and simple.
Flexibility & Expandability
VSAT technology has an unmatched ability to support a wide range of devices and applications. Single platforms can provide voice, fax, data and video communications. Data networks can support advanced functions including IP multicasting.
VSATs are software upgradeable, adding new capabilities usually does not require a technician at the remote locations. The modular design of VSAT systems also allows for maximum scalability and fast upgrades — with an ease and simplicity not possible with terrestrial networks.