Satellite Communications

IntroductionOrbitsFrequency Allocations for Satellite ServicesThe Space SegmentThe Earth SegmentSatellite Services

Introduction

Satellite:An object in an orbit is called a satellite. A satellite

can be natural, like the Moon, or human-made. Satellites can travel around planets or around stars such as our Sun.

Station on the earth transmits signal to satellite is called uplink,while from satellite to earth station is called downlink.

Types of Orbits

Geosynchronous OrbitsPolar OrbitsSun-Synchronous Orbits

Geosynchronous Orbits

A geosychronous orbit (GEO) is a circular, low orbit about Earth having a period of 23 hours 56 minutes 4 seconds--that is, the same amount of time it takes for the Earth to turn, so as the Earth spins, the satellite moves in time with it. Geosynchronous means "in time with the Earth."

When a satellite in geosynchronous orbit stays above the same spot on Earth, it is called geostationary.

Polar Orbits

A Polar orbit is a particular type of Low Earth Orbit. The only difference is that a satellite in polar orbit travels a north-south direction, rather than the more common east-west direction.

Polar orbits are useful for viewing the planet's surface. As a satellite orbits in a north-south direction, Earth spins beneath it in an east-west direction. As a result, a satellite in polar orbit can eventually scan the the entire surface.

For this reason, satellites that monitor the global environment, like remote sensing satellites and certain weather satellites, are almost always in polar orbit. No other orbit gives such thorough coverage of Earth.

Sun-Synchronous Orbits

This orbit is a special case of the polar orbit (which is a special coordinated LEO orbit). In a sun-synchronous (SS) orbit, (also called a helio-synchronous orbit), the satellite passes over the same part of the earth at roughly the same local time each day.

Sun-sychronous Orbits

This can make communication and various forms of data collection very convenient. For example, a satellite in an SS orbit could measure the air quality of Ottawa, Canada, at noon each and every day. In an SS orbit the angle between the orbital plane and sun remains constant, i.e. a constant node-to-sun angle, and therefore the satellite passage over a certain area occurs at the same time of the day each day.

All sun-synchronous orbits are polar orbits but not all polar orbits are sun-synchronous orbits.

Frequency Allocations for Satellite Services

The process of Frequency Allocations for Satellite Services is carried out under the auspices of ITU.

To facilitate frequency planning the world is divide into three regions

Region 1: Europe, Africa and MangoliaRegion 2: North and South America and

GreenlandRegion 3: Asia, Australia and the South-west

Pacific

Direct Broadcast Satellite Services Ku Band (12/14 GHz)

Fixed Satellite Services C Band(4/6 GHz)

Mobile, Navigational and Weather Satellites VHF (0.1 to 0.3 GHz)

The Space Segment

The space segment of a satellite consists of:Power SupplyAttitude ControlStation KeepingThermal ControlTT&C SubsystemTranspondersThe Antenna Subsystem

Geostationary Orbit

A satellite in a Geostationary Orbit appears to be stationary with respect to the earth, hence the name geostationary.

Three conditions are required for an orbit to be geostationary:

1. The Satellite must travel eastward at the same rotational speed as the earth.

2. The orbit must be circular.3. The inclination of the orbit must be zero.

TYPES OF SATELLITE SYSTEM

Satellite orbits are also classified based on their heights above the earth:

Lower earth orbit ( LEO)

Medium earth orbit (MEO)

Geostationary (GEO)

All the satellite system designs are differentiated by the type of the orbit in which the satellite is deployed.

LOW EARTHORBIT(LEO)

LEO systems fly about 500 to 1,500 kilometers above the Earth. Atypical LEO satellite takes less than two hours to orbit the Earth,and are only visible for 15 to 20 minutes each pass.

Low earth orbiting satellites are less expensive to launch into orbit as distance from the earth is less.

Don't require as high signal strength and give less time delay.

A large number of satellites are needed if the mission requires uninterrupted connectivity.

MEDIUM EARTH ORBIT (MEO)

A MEO satellite is in orbit somewhere between 8,000 km and18,000 km above the earth's surface.

MEO satellites are similar to LEO satellites in functionality.

MEO satellites are visible for much longer periods of time than LEOsatellites, usually between 2 to 8 hours.

MEO satellites have a larger coverage area than LEO satellites,hence there are less number of satellite required as compared to LEO.

A MEO satellite's distance gives it a longer time delay and weakersignal than a LEO satellite, but not as bad as a GEO satellite.

GEOSTATIONARY ORBIT (GEO)

These satellites are in orbit 35,863 km above the earth's surfacealong the equator.

Objects in Geostationary orbit revolve around the earth at the samespeed as the earth rotates. This means GEO satellites remain in thesame position relative to the surface of earth.

Advantages :A GEO satellite's distance from earth gives it a

large coverage area,almost a fourth of the earth s surface.

GEO satellites have a 24 hour view of a particular area. These factors make it ideal for satellite broadcast and other multipoint applications.

GEO systems are less complicated to maintain because their fixed location in the sky requires relatively little tracking capability in ground equipment.

Disadvantages :

As compared to the previous method because launching of the satellite into their orbit is too expensive.

At large distance from the earth hence there is problem with signal strength which is very low, due to this large antenna is required for reception of the signal.

GEO has more distant orbit so it subject to transmission delays.

Satellite Services

Satellite Mobile ServicesVSATsRadarsatsGlobal Positioning Satellite System

VSAT

VSAT stands for “Very Small Aperture Terminal” and refers to receive/transmit terminals installed at dispersed sites connecting to a central hub via satellite using small diameter antenna dishes (0.6 to 3.8 meter). Generally, these systems operate in the Ku-band and C-band frequencies.

Types of Operations

Bi-Directional Operation - The dish both sends (uplinks) and receives (downlinks) information.

Receive-Only Operation - The dish receives (downlinks) information only.

Components of VSAT

A VSAT consists of two components: an ODU (Outdoor Unit) and an IDU (Indoor Unit).

The ODU is made up of the dish, the BUC (Boost Up Converter) and the LNB (for receive only systems) or LNA (for bi-directional systems) which amplify the signal received by the unit.

On the other hand, IDU part consists of the satellite router or modem that offer the interface for the connection to the network.

VSATs commonly have 1.2 meter, 1.8 meter, or 2.4 meter dish antennas (versus 10 meter for a standard GEO antenna), however, any dish 3.8 meters or smaller is technically termed a VSAT.

Operation of a typical VSAT system

A VSAT system consists of a satellite transponder, central hub or a master earth station, and remote VSATs. The VSAT terminal has the capability to receive as well as transmit signals via the satellite to other VSATs in the network. Depending on the access technology used the signals are either sent via satellite to a central hub, which is also a monitoring centre, or the signals are sent directly to VSATs with the hub being used for monitoring and control.

In many VSAT systems,typically the ground stations do not have enough power to communicate directly with one another via the satellite. In this case a special ground station “hub”, with a large antenna is used to relay traffic between VSATs.

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

The transceiver receives or sends a signal to a satellite transponder. The satellite sends and receives signals from an earth station computer that acts as a hub for the system.

VSAT Network Configurations

VSAT networks can be arranged in: point to pointStarMeshstar/meshand broadcast configurations.

Point-to-point

Star Network

Applications of VSAT

Broadband direct to the home. e.g. Downloading MP3 audio to audio players.Broadband direct small business, office etc, sharing local use with many PCs.Internet access from on board ship, commercial shipping communications.Hotel chains, hotel internet cafes. Airlines, travel agents, booking systems.Airport air traffic control, flight data.Financial institutions - Banks, ATM machines.

Internet Service Providers. POP, VoIP, Cafe. Rural telephony, data, videophone. Military, data transfer, voice, temporary fixed

and mobile VSAT. Environmental monitoring, weather stations,

seismic monitoring.