Satellite Communications Chapter 7

8/11/2019 Satellite Communications Chapter 7

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GPS Satellite Signal


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GPS Receiver Block Diagram


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LOW EARTH ORBIT

AND

NON-GEO-STATIONARY SATELLITE SYSTEMS


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Orbits of Satellites

Equatorial Inclined

Polar


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Categories of Satellites


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Categories of Satellites


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Frequency bands for Satellite Communication


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LOW-EARTH ORBIT (LEO)

LEO systems fly about 1,000 kilometers above the

Earth (between 400 miles and 1,600 miles) A typical LEO satellite takes less than two hours to

orbit the Earth, which means that a single satelliteis "in view" of ground equipment for a only a fewminutes.

As a consequence, if a transmission takes morethan the few minutes that any one satellite is inview, a LEO system must "hand off" betweensatellites in order to complete the transmission.

In general, this can be accomplished by constantlyrelaying signals between the satellite and variousground stations, or by communicating between thesatellites themselves using "inter-satellite links."


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Types of LEOs

There are two types of LEO systems

Big LEOs

Little LEOs


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Little LEO satellites Very small

Use very little bandwidth for communications.

Operates under 1 GHz

Their size and bandwidth usage limits the amount of traffic

the system can carry at any given time.

Little LEO systems support services that require short

messaging and occasional low-bandwidth data transport, such

as paging, fleet tracking and remote monitoring of stationary

monitors


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Big LEO systems are designed to carry voice traffic as

well as data. Operates in the range from 1 GHz to 3 GHz.

They are the technology behind "satellite phones" or"global mobile personal communications system"(GMPCS) services now being developed and launched.

Examples of Big LEO systems include Iridium, Globalstar

and the regional Constellation and ECO-8 systems.

Big LEO satellites


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LEO Advantages The transmission delay associated with LEO systems is the

lowest of all of the systems.

In addition, because the signals to and from the satellitesneed to travel a relatively short distance, LEOs can operatewith much smaller user equipment (e.g., antennae) than cansystems using a higher orbit.

LEO systems are expected to cost less to implement than theother satellite systems.

In addition, a system of LEO satellites is designed to maximizethe ability of ground equipment to "see" a satellite at anytime, which can overcome the difficulties caused byobstructions such as trees and buildings.


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LEO Disadvantages The small coverage area of a LEO satellite means that a LEO

system must coordinate the flight paths and communicationshand-offs a large number of satellites at once, making theLEOs dependent on highly complex and sophisticated controland switching systems.

LEO satellites have a shorter life span than other systemsmentioned here. There are two reasons for this:

first, the lower LEO orbit is more subject to thegravitational pull of the Earth

and second, the frequent transmission rates necessaryin LEO systems mean that LEO satellites generally have ashorter battery life than others.


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MEDIUM-EARTH ORBIT (MEO) MEO systems operate at about 10,000 kilometers

(between 1,500 and 6,500 miles) above the Earth,which is lower than the GEO orbit and higher thanmost LEO orbits.

The MEO orbit is a compromise between the LEOand GEO orbits.

Compared to LEOs, the more distant orbit requiresfewer satellites to provide coverage than LEOsbecause each satellite may be in view of anyparticular location for several hours.

Compared to GEOs, MEOs can operate effectivelywith smaller, mobile equipment and with lesslatency


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Typically, MEO constellations have 10 to 17 satellitesdistributed over two or three orbital planes.

Most planned MEO systems will offer phone servicessimilar to the Big LEOs.

In fact, before the MEO designation came into wide use,MEO systems were considered Big LEOs. Examples of MEO systems include GPS, ICO Global

Communications

MEDIUM-EARTH ORBIT (MEO)


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MEO Advantages

MEO systems will require far fewer satellites than LEOs,reducing overall system complexity and cost, while stillrequiring fewer technological fixes to eliminate signal delaythan GEOs.

MEO systems larger capacity relative to LEOs may enablethem to be more flexible in meeting shifting market demandfor either voice or data services.


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MEO Disadvantages

MEO satellites, like LEOs, have a much shorter life expectancythan GEOs, requiring more frequent launches to maintain thesystem over time.

MEO systems, as well as some Big LEOs, targeted at the voice

communications market may have a disadvantage whencompared with cellular and other terrestrial wirelessnetworks.

A satellite signal is inherently weaker and is more subject tointerference than those of terrestrial systems, thus requiring alarger antenna than a traditional mobile phone.


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Delay and Throughput considerations


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Delay considerationsLEO

The time delay for a signal passing between LEO user1 and LEO user2 inthe same instantaneous coverage is 5.4 ms (2.7 ms up and 2.7 ms down)

and go and return delay between two users is twice this at 10.8 ms

MEO The time delay for a signal passing between MEO user1 and MEO user2 in

the same instantaneous coverage is 69 ms (34.5 ms up and 34.5 ms down)

and go and return delay between two users is twice this at 138 ms

GEO The time delay for a signal passing between GEO user1 and GEO user2 in

the same instantaneous coverage is 238.6 ms (119.3 ms up and 119.3 msdown)

and go and return delay between two users is twice this at 477.2 ms


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System considerations

There are four important factors that influence thedesign of any satellite system

1. Incremental Growth

2. Interim Operations

3. Replenishment options

4. End-to-end system implementation


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Operational NGSO constellation designs

There are seven satellite constellation designs

1. Ellipso

2. Globalstar

3. New ICO

4. Iridium

5. Orbcomm

6. Skybridge

7. Teledesic

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Satellite Communications Chapter 7