dharma chapt 11 sistema satelital 2002
TRANSCRIPT
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1Copyright 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Chapter 11
Satellite Systems
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2Copyright 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Outline
Introduction
Types of Satellite Characteristic of Satellite Systems
Satellite System Infrastructures
Call Setup GPS
Limitations of GPS
Beneficiaries of GPS
Applications of GPS
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3Copyright 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Introduction
Satellites being above the earth can cover a
larger area.
The information to be transmitted from amobile user should be correctly received bya satellite and forwarded to one of the earthstations (ESs).
This puts the limitation that onlyLine ofSight(LOS) communication is possible.
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Application Areas of Satellite System
Traditionally
Meteorological satellites Radio and TV broadcast satellites
Military satellites
Satellites for navigation and localization (e.g GPS) Telecommunications
Global telephone connections
Backbone for global networks Connections for communication in remote places
Global mobile communication
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Types of Satellite Systems
Four different types of satellite orbits have
been identified depending on the shape anddiameter of each orbit: GEO (Geostationary Earth Orbit) at 36,000 kms above
earths surface. LEO (Low Earth Orbit) at 500-1500 kms above earths
surface.
MEO (Medium Earth Orbit) or ICO (Intermediate
Circular Orbit) at 6000-20000 kms above earths
surface.
HEO (Highly Elliptical Orbit).
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Orbits of Different Satellites
35,768 km
10,000 km
1,000 km
HEO
MEO (ICO)
GEO (Inmarsat)
LEO (Globalstar, Irdium)
Earth
Orbits of different satellites
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7Copyright 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Earth-satellite Parameters for a Stable Orbiting Path
R
Satellite (mass = m)
r
Satellite Orbit
Earth
g = gravitational acceleration
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Earth-Satellite Parameters (Contd)
The orbits could be elliptical or circular.
Rotation time depends on the distance between the satellite and theearth.
For satellites following circular orbits, applying Newtonsgravitational law:
Fg(attractive force) = mg (R/r)2
Fc (centrifugal force) = mr2
= 2f
Where, m= mass of the satellite
g= gravitational acceleration (9.81 m/s2)
R= radius of the earth (6,370 kms)
r= distance of the satellite to the center of
earth
= angular velocity of satellite
f = rotational frequency
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Earth-Satellite Parameters (Contd)
For the orbit of the satellite to be stable, we
need to equate the two forces.
Thus,
( )3
2
2
2 f
gR
=
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Inclination
Inclination
Satellite orbit
Perigee
Plane of satellite orbit
Equatorial plane
The plane of the satellite orbit with respect to earth
Apogee
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11Copyright 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Footprint
The area inside the circle is considered to be
an isoflux area and this constant intensity
area is taken as the footprint of the beam.
A satellite consists of several illuminatedbeams. These beams can be seen as cells of
the conventional wireless system.
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Elevation and Footprint
Elevation:
Angle between center of satellite beam
and surface of the earth.
Minimal elevation:
Elevation needed to at least
communicate with the satellite.
The elevation angle between the satellite
beam and the surface of earth has an impact
on the illuminated area (footprint)
Footp
rint
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Intensity Level of the Footprint of GEO Satellites
Km
0
dB
-5 -10 -20 -50Isoflux
Area
-2,000 -1,000 1,000 2,0000
0Km
-2,000
-1,000
1,000
2,000
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Satellite Beam Geometry
#1
R
#2
#3
#4
#5
#6
#7
0 dB
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Satellite Communication
Satellite
R
R
sh
Rsin
Rcos
MS
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Satellite Communication
The figure shows the paths taken for
communication from a MS to the satellite.
The time delay is a function of various parameters
and is given by:
where, R = radius of the earth
h = orbital altitude = satellite elevation angle
c = speed of light
( )
+== sincos1 222
RRhRcc
s
Delay
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Variation of Delay in MS as A Function of
Elevation Angle
Delay(ms)
34
36
3840
42
44
46
48
50
52
54
Distance(km)
10000
10500
11000
11500
12000
12500
13000
13500
14000
0 10 20 30 40 50 60 70 80 90
Elevation angle (degrees)
14500
15000
15500
16000
16500
Variation of delay in MS as a function of elevation angle
when the satellite is at an elevation of 10,355 kms
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Different Frequency Bands
The satellites operate in different frequencies for
the uplink and the downlink
2.483-2.501.610-1.625LIS
27.5-30.517.7-21.7Ka
14.0-14.511.7-12.2Ku
5.925-6.4253.7-4.2C
Downlink (GHz)Uplink (GHz)Band
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Different Frequency Bands (Contd)
C band frequencies have been used in the first
generation satellites and has become overcrowdedbecause of terrestrial microwave networks
employing these frequencies.
Ku and Ka bands are becoming more popular,even though they suffer from higher attenuation
due to rain
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Transmission Power Characteristics
Satellites receive signals at very low power levels (lessthan 100 picowatts) which is one to two orders ofmagnitude lower than the terrestrial receivers (1 to 100micro watts)
The received power is determined by four parameters:
Transmitting Power
Gain of transmitting antenna Distance between transmitter and receiver
Gain of receiving antenna
The atmospheric conditions cause attenuation to the
transmitted signal and the loss is given by:L = (4rf/c)2
where ris the distance,fis the carrier frequency
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Atmospheric Attenuation
Elevation of the satellite operating in 4-6 GHz
Attenuation
ofthesignalin%
5 10 20 30 40 50
10
20
30
40
50
Fog Absorption
Rain Absorption
Atmospheric
Absorption
Atmospheric Attenuation as a function of the elevation angle
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Characteristics of Satellite Systems
Satellites weigh around 2,500 kgs.
The GEO satellites are at an altitude of 35,768 kmswhich orbit in equatorial plane with 0 degreeinclination.
They complete exactly one rotation per day.
The antennas are at fixed positions and use anuplink band of 1,634.5-1,660.5 MHz and downlinkin the range of 1,530-1,559 MHz.
Ku band frequencies (11 GHz and 13 GHz) areemployed for connection between the BS and thesatellites.
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Characteristics of Satellite Systems (Contd)
A satellite typically has a large footprint 34% of earths surfaceis covered. Therefore it is difficult to reuse frequencies.
There is a high latency (about 275 ms) due to global coverage ofmobile phones.
LEO satellites are divided into little and big satellites.
Little LEO satellites are smaller in size, in the frequency range148-150.05 MHz (uplink) and 137-138 MHz (downlink).Theysupport only low bit rates (1 kb/s) for two way messaging.
Big LEO satellites have adequate power and bandwidth to providevarious global mobile services like data transmission, paging etc.
Big LEO satellites transmit in the frequency range of 1,610-
1,626.5 MHz (uplink) and 2,483.5-2,500 MHz (downlink).
It orbits around 500-1,500 kms above the earths surface.
The latency is around 5-10 ms and the satellite is visible for 10-40min
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A Typical Satellite System
Base station
or gateway
Inter Satellite Link
(ISL)
Mobile UserLink (MUL) Gateway Link
(GWL)
Footprint
Small cells
(spotbeams)
User data
PSTNISDNMobile phone
systems
GWL
MUL
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Satellite System Infrastructure
Once contact has been established between a MSand a satellite using a LOS beam, the rest of the
world can be accessed using the underlying wiredbackbone network.
The satellites are controlled by the BS located at the
surface of the earth which serves as a gateway. Inter satellite links can be used to relay information
from one satellite to another, but they are stillcontrolled by the ground BS.
The illuminated area of a satellite beam, called thefootprint, is the area where a mobile user cancommunicate with the satellite.
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Satellite System Infrastructure (Contd)
There are losses in free space and also due toatmospheric absorption of the satellite beams.
Rain also causes attenuation to signal strengthwhen 12-14 GHz and 20-30 GHz bands are usedto avoid orbital congestion.
The satellites beam may be temporarily blockeddue to flying objects or the terrain of the earthssurface.
Therefore a concept known as diversity is usedto transmit the same message through more thanone satellite.
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Satellite Path Diversity
Satellite 2Satellite 1
Coverage AngleCoverage Angle
Delay 2
Delay 1
Coverage Area 1 Coverage Area 2
MSES
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Satellite Path Diversity (Contd)
Idea behind diversity is to provide a mechanism that combines
two or more correlated information signals.
These signals have uncorrected noise and/or fadingcharacteristics.
A combination of the two signals improves the signal quality.
The receiving end has the flexibility to select one of the bettersignals received while the other is lost due to temporary LOS
problem, or attenuated because of excessive absorption in the
atmosphere.
The net effect of diversity is to utilize twice the bandwidth and
therefore it is desirable to employ this in as small a fraction of
time as possible.
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29Copyright 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Satellite Path Diversity (Contd)
The use of diversity can be initiated by either the MS orBS located on the earth.
The use of a satellite path diversity is done due to thefollowing conditions:
Elevation Angle: Higher elevation angle decreases shadowingproblems. So, one approach is to initiate path diversity when the
elevation angle is less than some predefined threshold value. Signal Quality: If the average signal level quality fades beyond
some threshold, then this could force the use of path diversity.
Stand-by option: A channel could be selected and reserved as a
stand by option, when obstruction of the primary channel occurs.Several MSs can share the same stand-by channel.
Emergency Handoff: Whenever the connection of an MS with asatellite is lost, the MS tries to have an emergency handoff.
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Channel Model for MS
The channel of a satellite system is usuallyrepresented by a two state Markov model.
The MS in the good state has Rician fading.
A bad or shadowed state indicatesRayleigh/Lognormal fading.
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Channel Model for the MS
Good State
(LOS)
Bad State
(shadowed)
Rician Rayleigh/Lognormal
PGG
PBG
PGB
PBB
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Satellite System Architecture
MSC
/VLR
MSC
/VLR
Gateway
EIR
AUC
HLR
SUMR
Earth Station (ES)
Earth Station (ES)
PSTN
Public fixed
and mobile
networks
Mobile Link
Satellites
Mobile Station
(MS)
Gateway
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Call Setup
ES is the heart of the overall system control.
ES performs functions similar to the Base Station Subsystem
(BSS) of a cellular wireless system.
ES keeps track of all MSs, located in the area and controls the
allocation and de-allocation of radio resources.
The MSC/VLR are important parts of the ES, providingfunctions similar to those of a cellular network.
The HLR-VLR pair supports the basic process of Mobility
management.
The Satellite User Mapping Register (SUMR) in ES lists the
positions of all satellites and indicate the satellite assigned to
each MS.
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Call Setup (Contd)
These ESs are also connected to the PSTN and ATM backboneso that calls to regular household phones can also beestablished.
For an incoming call, the gateway helps to reach the closest ES,that in turn, using the HLR/VLR pair indicates the satelliteserving the most recently known location of the MS.
The satellite employs a paging channel to inform the MS about
an incoming call and the radio resource to use for the uplinkconnection.
For a call originated from a MS, the MS accesses the sharedcontrol channel of an overhead satellite and the satellite in turn
informs the ES for authentication of the user/MS.
The ES then allocates a traffic channel to the MS via thesatellite and also informs the gateway about additional controlinformation.
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Call Setup (Contd)
Similar to cellular systems whenever a MS moves to a
new area served by another satellite, then the MS has togo through the registration process.
The only difference is the use of ES in all intermediate
steps.
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System Timings for the Satellite
At Satellite 1
16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3 5 6 7 8 9
12 13 14 15 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3 5
TX 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3 5 6 7 8 9
12 13 14 15 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3 5RX
Frame i
Frame i + n
Framej
Framej + mAt Satellite 2
RX
TX
4 12 6 14At MS, scheme 1
At MS, scheme 2 4 6 1412
RX Slot 1 TX Slot 1 RX Slot 2 TX Slot 2
RX Slot 1 RX Slot 2 TX Slot
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System Timings for the Satellite (Contd)
Scheme-1 employs half of the 16 burst half rate while the
second half is for the TDMA frame of satellite 2.
Diversity is employed in scheme-2 and the TDMA frame is
split into 3 parts, the first 2 for reception from satellites 1
and 2 and the third for the communication with the satellitewhich has the best signal.
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Types of Handoff
Intra Satellite handoff: Handoff from one spot beam to anotherdue to relative movement of the MS with respect to the satellitesas MS needs to be in the footprint area to be able to
communicate with a satellite.
Inter Satellite handoff: As the MS is mobile and most satellitesare not geo-synchronous, the beam path may change
periodically. Therefore there could be a handoff from one
satellite to another under control of ES. ES handoff: This may happen because of frequency
rearrangement occurring due to traffic balancing in neighboringbeams. There could be situations where a satellite control may
change from one ES to another.This may cause handoff at ESlevel.
Inter System Handoff: Handoff from a satellite network to aterrestrial cellular network.
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39Copyright 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Global Positioning System (GPS)
Used in applications such as military targeting, navigation,
tracking down stolen vehicles, guiding civilians to the
nearest hospital, exact location of the callers for E-911
emergency.
GPS system consists of a network of 24 orbiting satellites
called NAVSTAR placed in 6 different orbital paths with4 satellites in orbital plane.
The orbital period of these satellites is 12 hours.
The first GPS satellite was launched in Feb. 1978. Each satellite is expected to last approx. 7.5 years.
GPS Nominal Constellation of 24 Satellites
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GPS Nominal Constellation of 24 Satellites
in 6 Orbital Planes [2]
GPS Master Control and Monitor Station
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GPS Master Control and Monitor Station
Network [2]
Falcon AFB
Colorado Springs
Master Control
Monitor Station
Hawaii
Monitor Station
Ascension Island
Monitor Station
Diego Garcia
Monitor Station
Kwajalein
Monitor Station
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42Copyright 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
The Triangulation Technique [2002 IEEE]
These two points indicate the location
Satellite A Satellite B
Satellite C
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GPS
GPS is based on the Triangulation Technique.
Consider the GPS receiver (MS) to be placed at a point on an imaginary
sphere of radius equal to the distance between Satellite A and the
receiver on the ground.
The GPS receiver MS is also a point on another imaginary sphere with a
second satellite B at its center.
The GPS receiver is somewhere on the circle formed by the intersection
of 2 spheres.
Then with the measurement of distance from a third satellite C the
position of the receiver is narrowed down to just 2 points on the circle.
One of these points is imaginary and is eliminated.
Therefore the distance measured from 3 satellites is sufficient to
determine the position of the GPS receiver on earth.
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44Copyright 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
GPS (Contd)
The GPS signal consists of a Pseudo-Random Code
(PRN), ephemeris and navigation data.
The ephemeris data corrects errors caused by gravitational
pulls from the moon and sun on the satellites.
The navigation data is the information about the located
position of the GPS receiver. The pseudo-random code identifies which satellite is
transmitting.
Satellites are referred to by their PRN ranging from 1-32.
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Limitations of GPS
Distance measurements may vary as the values of signal
speed vary in atmosphere.
Effects of Multi-path fading and shadowing are significant.
In GPS, multi-path fading occurs when the signal bounces
off a building or terrain.
Propagation delay due to atmospheric condition affects
accuracy.
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Beneficiaries of GPS
GPS has become important for nearly all military operations
and weapons systems.
It is used on satellites to obtain highly accurate orbit data andto control spacecraft orientation.
GPS can be used everywhere except where it is impossible to
receive the signal such as inside most buildings, in caves and
other subterranean locations.
There are airborne, land and sea based applications of GPS.
Anyone who needs to keep track of where he/she is and needs
to find his/her way to a specified location, or know whatdirection and how fast they are going can utilize the GPS
service.
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Some Applications of GPS
Application AreaUser Group
Reduces setup time at survey site and offers
precise measurements
Surveyors
NavigationRecreational boaters and
commercial fishermen
NavigationGeneral aviation and
commercial aircrafts
Checking positions along the way and making
sure that they meet in the middle
Building the English
channel
Maneuvering in extreme conditions and
navigating planes, ships, etc.
U.S. Military
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Some Applications of GPS (Contd)
Application AreaUser Group
Display of maps in moving cars
that can be used to plan a trip
Automobile manufactures
Determine location of car, truck,
or ambulance closest to the
accident site
Emergency vehicles
Monitor locations at all timesFleet vehicles, public
transportation systems, delivery
trucks, and courier services
Emergency roadside assistanceAutomobile services
Keeping track of where they are
and finding a specific location
Recreational users (Hikers,
hunters, mountain bikers, etc.)