EC 723 Satellite Communication Systems

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EC 723Satellite Communication Systems

• Mohamed Khedr
• http//webmail.aast.edu/khedr

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Grades
Load Percentage Date
Midterm Exam 30 Week of 3 December 2007
Final Exam 30
Participation 10
Report and presentation 30 Starting week 11th

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Textbook and website

• Textbook non specific
• Website http//webmail.aast.edu/khedr

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Syllabus
Week 1 Overview
Week 2 Orbits and constellations GEO, MEO and LEO
Week 3 Satellite space segment, Propagation and satellite links , channel modelling
Week 4 Satellite Communications Techniques
Week 5 Satellite error correction Techniques
Week 6 Multiple Access I
Week 7 Multiple access II
Week 8 Satellite in networks I
Week 9 INTELSAT systems , VSAT networks, GPS
Week 10 GEO, MEO and LEO mobile communications INMARSAT systems, Iridium , Globalstar, Odyssey
Week 11 Presentations
Week 12 Presentations
Week 13 Presentations
Week 14 Presentations
Week 15 Presentations

• Tentatively

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Satellite Components

• Satellite Subsystems
• Telemetry, Tracking, and Control
• Electrical Power and Thermal Control
• Attitude Control
• Communication Subsystems
• Link Budget
• Modulation Techniques
• Coding and Error Correction
• Networking (service provisioning, multimedia
  constraints and QoS)
• Multiple Access and On-board Processing
• Applications (Internet, Mobile computing)

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Classification of Satellite Orbits

• Circular or elliptical orbit
• Circular with center at earths center
• Elliptical with one foci at earths center
• Orbit around earth in different planes
• Equatorial orbit above earths equator
• Polar orbit passes over both poles
• Other orbits referred to as inclined orbits
• Altitude of satellites
• Geostationary orbit (GEO)
• Medium earth orbit (MEO)
• Low earth orbit (LEO)

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Satellite Orbits

• Equatorial
• Inclined
• Polar

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Heres the Math

• Gravity depends on the mass of the earth, the
  mass of the satellite, and the distance between
  the center of the earth and the satellite
• For a satellite traveling in a circle, the speed
  of the satellite and the radius of the circle
  determine the force (of gravity) needed to
  maintain the orbit
• The radius of the orbit is also the distance from
  the center of the earth.
• For each orbit the amount of gravity available is
  therefore fixed
• That in turn means that the speed at which the
  satellite travels is determined by the orbit

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Lets look in a Physics Book

• From what we have deduced so far, there has to be
  an equation that relates the orbit and the speed
  of the satellite

R3mu/n2
N2pi/T
T is the time for one full revolution around the
orbit, in seconds r is the radius of the orbit,
in meters, including the radius of the earth
(6.38x106m).
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The Most Common Example

• Height of the orbit 22,300 mile
• That is 36,000km 3.6x107m
• The radius of the orbit is
• 3.6x107m 6.38x106m 4.2x107m
• Put that into the formula and

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The Geosynchronous Orbit

• The answer is T 86,000 sec (rounded)
• 86,000 sec 1,433 min 24hours (rounded)
• The satellite needs 1 day to complete an orbit
• Since the earth turns once per day, the satellite
  moves with the surface of the earth.

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Assignment

• How long does a Low Earth Orbit Satellite need
  for one orbit at a height of 200miles 322km
  3.22x105m
• Do this
• Add the radius of the earth, 6.38x106m
• Compute T from the formula
• Change T to minutes or hours

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Answer

• r6.7x106 m
• r33.01x1020 m3
• T2p x 868 sec
• T54,500 sec 90.8 min 1.51 hours

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Classical satellite systems
Inter Satellite Link (ISL)
Mobile User Link (MUL)
MUL
Gateway Link (GWL)
GWL
small cells (spotbeams)
base station or gateway
footprint
GSM
PSTN
ISDN
User data
PSTN Public Switched Telephone Network
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Basics

• Satellites in circular orbits
• attractive force Fg m g (R/r)²
• centrifugal force Fc m r ?²
• m mass of the satellite
• R radius of the earth (R 6370 km)
• r distance to the center of the earth
• g acceleration of gravity (g 9.81 m/s²)
• ? angular velocity (? 2 ? f, f rotation
  frequency)
• Stable orbit
• Fg Fc

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Satellite period and orbits
Velocity Km/sec
satellite period h
12
24
velocity x1000 km/h
10
20
8
16
6
12
4
8
2
4
synchronous distance 35,786 km
10
20
30
40 x106 m
radius
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Basics

• elliptical or circular orbits
• complete rotation time depends on distance
  satellite-earth
• inclination angle between orbit and equator
• elevation angle between satellite and horizon
• LOS (Line of Sight) to the satellite necessary
  for connection
• ? high elevation needed, less absorption due to
  e.g. buildings
• Uplink connection base station - satellite
• Downlink connection satellite - base station
• typically separated frequencies for uplink and
  downlink
• transponder used for sending/receiving and
  shifting of frequencies
• transparent transponder only shift of
  frequencies
• regenerative transponder additionally signal
  regeneration

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Inclination
plane of satellite orbit
satellite orbit
perigee
d
inclination d
equatorial plane
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Elevation
Elevation angle e between center of satellite
beam and surface
minimal elevation elevation needed at least to
communicate with the satellite
e
footprint
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Orbits I

• Four different types of satellite orbits can be
  identified depending on the shape and diameter of
  the orbit
• GEO geostationary orbit, ca. 36000 km above
  earth surface
• LEO (Low Earth Orbit) ca. 500 - 1500 km
• MEO (Medium Earth Orbit) or ICO (Intermediate
  Circular Orbit) ca. 6000 - 20000 km
• HEO (Highly Elliptical Orbit) elliptical orbits

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Orbits II
GEO (Inmarsat)
HEO
MEO (ICO)
LEO (Globalstar,Irdium)
inner and outer Van Allen belts
earth
Van-Allen-Belts ionized particles 2000 - 6000 km
and 15000 - 30000 km above earth surface
1000
10000
35768
km
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Geostationary satellites

• Orbit 35,786 km distance to earth surface, orbit
  in equatorial plane (inclination 0)
• ? complete rotation exactly one day, satellite
  is synchronous to earth rotation
• fix antenna positions, no adjusting necessary
• satellites typically have a large footprint (up
  to 34 of earth surface!), therefore difficult to
  reuse frequencies
• bad elevations in areas with latitude above 60
  due to fixed position above the equator
• high transmit power needed
• high latency due to long distance (ca. 275 ms)
• ? not useful for global coverage for small
  mobile phones and data transmission, typically
  used for radio and TV transmission

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LEO systems

• Orbit ca. 500 - 1500 km above earth surface
• visibility of a satellite ca. 10 - 40 minutes
• global radio coverage possible
• latency comparable with terrestrial long distance
  connections, ca. 5 - 10 ms
• smaller footprints, better frequency reuse
• but now handover necessary from one satellite to
  another
• many satellites necessary for global coverage
• more complex systems due to moving satellites
• Examples
• Iridium (start 1998, 66 satellites)
• Bankruptcy in 2000, deal with US DoD (free use,
  saving from deorbiting)
• Globalstar (start 1999, 48 satellites)
• Not many customers (2001 44000), low stand-by
  times for mobiles

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MEO systems

• Orbit ca. 5000 - 12000 km above earth surface
• comparison with LEO systems
• slower moving satellites
• less satellites needed
• simpler system design
• for many connections no hand-over needed
• higher latency, ca. 70 - 80 ms
• higher sending power needed
• special antennas for small footprints needed
• Example
• ICO (Intermediate Circular Orbit, Inmarsat) start
  ca. 2000
• Bankruptcy, planned joint ventures with
  Teledesic, Ellipso cancelled again, start
  planned for 2003

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Routing

• One solution inter satellite links (ISL)
• reduced number of gateways needed
• forward connections or data packets within the
  satellite network as long as possible
• only one uplink and one downlink per direction
  needed for the connection of two mobile phones
• Problems
• more complex focusing of antennas between
  satellites
• high system complexity due to moving routers
• higher fuel consumption
• thus shorter lifetime
• Iridium and Teledesic planned with ISL
• Other systems use gateways and additionally
  terrestrial networks

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Localization of mobile stations

• Mechanisms similar to GSM
• Gateways maintain registers with user data
• HLR (Home Location Register) static user data
• VLR (Visitor Location Register) (last known)
  location of the mobile station
• SUMR (Satellite User Mapping Register)
• satellite assigned to a mobile station
• positions of all satellites
• Registration of mobile stations
• Localization of the mobile station via the
  satellites position
• requesting user data from HLR
• updating VLR and SUMR
• Calling a mobile station
• localization using HLR/VLR similar to GSM
• connection setup using the appropriate satellite

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Handover in satellite systems

• Several additional situations for handover in
  satellite systems compared to cellular
  terrestrial mobile phone networks caused by the
  movement of the satellites
• Intra satellite handover
• handover from one spot beam to another
• mobile station still in the footprint of the
  satellite, but in another cell
• Inter satellite handover
• handover from one satellite to another satellite
• mobile station leaves the footprint of one
  satellite
• Gateway handover
• Handover from one gateway to another
• mobile station still in the footprint of a
  satellite, but gateway leaves the footprint
• Inter system handover
• Handover from the satellite network to a
  terrestrial cellular network
• mobile station can reach a terrestrial network
  again which might be cheaper, has a lower latency
  etc.

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Overview of LEO/MEO systems
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Keplers First Law

• The path followed by a satellite around the
  primary will be an ellipse.
• An ellipse has two focal points shown as F1 and
  F2.
• The center of mass of the two-body system, termed
  the barycenter, is always centered on one of the
  foci.
• In our specific case, because of the enormous
  difference between the masses of the earth and
  the satellite, the center of mass coincides with
  the center of the earth, which is therefore
  always at one of the foci.
• The semimajor axis of the ellipse is denoted by
  a, and the semiminor axis, by b. The eccentricity
  e is given by

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Keplers Second Law

• For equal time intervals, a satellite will sweep
  out equal areas in its orbital plane, focused at
  the barycenter.

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Keplers Third Law

• The square of the periodic time of orbit is
  proportional to the cube of the mean distance
  between the two bodies.
• The mean distance is equal to the semimajor axis
  a. For the satellites orbiting the earth,
  Keplers third law can be written in the form
• where n is the mean motion of the satellite in
  radians per second and is the earths geocentric
  gravitational constant. With a in meters, its
  value is

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Definition of terms for earth-orbiting satellite

• Apogee The point farthest from earth. Apogee
  height is shown as ha in Fig
• Perigee The point of closest approach to earth.
  The perigee height is shown as hp
• Line of apsides The line joining the perigee and
  apogee through the center of the earth.
• Ascending node The point where the orbit crosses
  the equatorial plane going from south to north.
• Descending node The point where the orbit crosses
  the equatorial plane going from north to south.
• Line of nodes The line joining the ascending and
  descending nodes through the center of the earth.
• Inclination The angle between the orbital plane
  and the earths equatorial plane. It is measured
  at the ascending node from the equator to the
  orbit, going from east to north. The inclination
  is shown as i in Fig.
• Mean anomaly M gives an average value of the
  angular position of the satellite with reference
  to the perigee.
• True anomaly is the angle from perigee to the
  satellite position, measured at the earths
  center. This gives the true angular position of
  the satellite in the orbit as a function of time.

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Definition of terms for earth-orbiting satellite

• Prograde orbit An orbit in which the satellite
  moves in the same direction as the earths
  rotation. The inclination of a prograde orbit
  always lies between 0 and 90.
• Retrograde orbit An orbit in which the satellite
  moves in a direction counter to the earths
  rotation. The inclination of a retrograde orbit
  always lies between 90 and 180.
• Argument of perigee The angle from ascending node
  to perigee, measured in the orbital plane at the
  earths center, in the direction of satellite
  motion.
• Right ascension of the ascending node To define
  completely the position of the orbit in space,
  the position of the ascending node is specified.
  However, because the earth spins, while the
  orbital plane remains stationary the longitude of
  the ascending node is not fixed, and it cannot be
  used as an absolute reference. For the practical
  determination of an orbit, the longitude and time
  of crossing of the ascending node are frequently
  used. However, for an absolute measurement, a
  fixed reference in space is required. The
  reference chosen is the first point of Aries,
  otherwise known as the vernal, or spring,
  equinox. The vernal equinox occurs when the sun
  crosses the equator going from south to north,
  and an imaginary line drawn from this equatorial
  crossing through the center of the sun points to
  the first point of Aries (symbol ). This is the
  line of Aries.

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Six Orbital Elements

• Earth-orbiting artificial satellites are defined
  by six orbital elements referred to as the
  keplerian element set.
• The semimajor axis a.
• The eccentricity e
• give the shape of the ellipse.
• A third, the mean anomaly M, gives the position
  of the satellite in its orbit at a reference time
  known as the epoch.
• A fourth, the argument of perigee ? , gives the
  rotation of the orbits perigee point relative to
  the orbits line of nodes in the earths
  equatorial plane.
• The inclination I
• The right ascension of the ascending node ?
• Relate the orbital planes position to the earth.

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NASA
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Gravitational force is inversely proportional to
the square of the distance between the centers of
gravity of the satellite and the planet the
satellite is orbiting, in this case the earth.
The gravitational force inward (FIN, the
centripetal force) is directed toward the center
of gravity of the earth. The kinetic energy of
the satellite (FOUT, the centrifugal force) is
directed opposite to the gravitational force.
Kinetic energy is proportional to the square of
the velocity of the satellite. When these inward
and outward forces are balanced, the satellite
moves around the earth in a free fall
trajectory the satellites orbit.
Forces acting on a satellite in a stable orbit
around the earth.
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The initial coordinate system that could be used
to describe the relationship between the earth
and a satellite. A Cartesian coordinate system
with the geographical axes of the earth as the
principal axis is the simplest coordinate system
to set up. The rotational axis of the earth is
about the axis cz, where c is the center of the
earth and cz passes through the geographic north
pole. Axes cx, cy, and cz are mutually
orthogonal axes, with cx and cy passing through
the earths geographic equator. The vector r
locates the moving satellite with respect to the
center of the earth.
Cartesian coordinate system
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In this coordinate system, the orbital plane of
the satellite is used as the reference plane. The
orthogonal axes, x0 and y0 lie in the orbital
plane. The third axis, z0, is perpendicular to
the orbital plane. The geographical z-axis of the
earth (which passes through the true North Pole
and the center of the earth, c) does not lie in
the same direction as the z0 axis except for
satellite orbits that are exactly in the plane of
the geographical equator.
The orbital plane coordinate system.
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The plane of the orbit coincides with the plane
of the paper. The axis z0 is straight out of the
paper from the center of the earth, and is normal
to the plane of the satellites orbit. The
satellites position is described in terms of the
radius from the center of the earth r0 and the
angle this radius makes with the x0 axis, Fo.
Polar coordinate system in the plane of the
satellites orbit.
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A satellite is in orbit about the planet earth,
E. The orbit is an ellipse with a relatively
high eccentricity, that is, it is far from being
circular. Two shaded portions of the elliptical
plane in which the orbit moves, one is close to
the earth and encloses the perigee while the
other is far from the earth and encloses the
apogee. The perigee is the point of closest
approach to the earth while the apogee is the
point in the orbit that is furthest from the
earth. While close to perigee, the satellite
moves in the orbit between times t1 and t2 and
sweeps out an area denoted by A12. While close
to apogee, the satellite moves in the orbit
between times t3 and sweeps out an area denoted
by A34. If t1 t2 t3 t4, then A12 A34.
Keplers second law of planetary motion.
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The orbit as it appears in the orbital plane.The
point O is the center of the earth and the point
C is the center of the ellipse. The two centers
do not coincide unless the eccentricity, e, of
the ellipse is zero (i.e., the ellipse becomes a
circle and a b). The dimensions a and b are
the semimajor and semiminor axes of the orbital
ellipse, respectively.
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Point O is the center of the earth and point C is
both the center of the orbital ellipse and the
center of the circumscribed circle. The
satellite location in the orbital plane
coordinate system is specified by (x0, y0). A
vertical line through the satellite intersects
the circumscribed circle at point A. The
eccentric anomaly E is the angle from the x0 axis
to the line joining C and A.
The circumscribed circle and the eccentric
anomaly E.
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This geocentric system differs from that shown in
Figure 2.1 only in that the xi axis points to the
first point of Aries. The first point of Aries is
the direction of a line from the center of the
earth through the center of the sun at the vernal
equinox (about March 21 in the Northern
Hemisphere), the instant when the subsolar point
crosses the equator from south to north. In the
above system, an object may be located by its
right ascension RA and its declination ?.
The geocentric equatorial system.
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Locating the orbit in the geocentric equatorial
system. The satellite penetrates the equatorial
plane (while moving in the positive z direction)
at the ascending node. The right ascension of
the ascending node is ? and the inclination i is
the angle between the equatorial plane and the
orbital plane. Angle ?, measured in the orbital
plane, locates the perigee with respect to the
equatorial plane.
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The elevation angle is measured upward from the
local horizontal at the earth station and the
azimuth angle is measured from the true north in
an eastward direction to the projection of the
satellite path onto the local horizontal plane.
The definition of elevation (EI) and azimuth (Az).
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The line joining the satellite and the center of
the earth, C, passes through the surface of the
earth and point Sub, the subsatellite point. The
satellite is directly overhead at this point and
so an observer at the subsatellite point would
see the satellite at zenith (i.e., at an
elevation angle of 90). The pointing direction
from the satellite to the subsatellite point is
the nadir direction from the satellite. If the
beam from the satellite antenna is to be pointed
at a location on the earth that is not at the
subsatellite point, the pointing direction is
defined by the angle away from nadir. In
general, two off-nadir angles are given the
number of degrees north (or south) from nadir
and the number of degrees east (or west) from
nadir. East, west, north, and south directions
are those defined by the geography of the earth.
Zenith and nadir pointing directions.
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The geometry of elevation angle calculation. The
plane of the paper is the plane defined by the
center of the earth, the satellite, and the earth
station. The central angle is ?. The elevation
angle EI is measured upward from the local
horizontal at the earth station.
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The satellite is said to be visible from the
earth station if the elevation angle EI is
positive. This requires that the orbital radius
rs be greater than the ratio re/cos(?), where re
is the radius of the earth and ? is the central
angle.
The geometry of the visibility calculation.
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During the equinox periods around the March 21
and September 3, the geostationary plane is in
the shadow of the earth on the far side of the
earth from the sun. As the satellite moves around
the geostationary orbit, it will pass through the
shadow and undergo an eclipse period. The length
of the eclipse period will vary from a few
minutes to over an hour (see Figure 2.22),
depending on how close the plane of the
geostationary orbit is with respect to the center
of the shadow thrown by the earth.
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Dates and duration of eclipses. (Source Martin,
Communications Satellite Systems, Prentice Hall
1978.)
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Schematic of sun outage conditions. During the
equinox periods, not only does the earths shadow
cause eclipse periods to occur for geostationary
satellites, during the sunlit portion of the
orbit, there will be periods when the sun appears
to be directly behind the satellite. At the
frequencies used by communications satellites (4
to 50 GHz), the sun appears as a hot noise
source. The effective temperature of the sun at
these frequencies is on the order of 10,000 K.
The precise temperature observed by the earth
station antenna will depend on whether the
beamwidth partially, or completely, encloses the
sun.
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Thank you