Satellite Communications
Part I (Contd)- Orbital Mechanics and
Orbit Classifications, Antenna Look
Angles
Lecturer Madeeha Owais12/26/2008 1NUST-SEECS
Learning Objectives• Sidereal day vs Solar Day
• Satellite Orbits
• Orbit Classifications
• Geo-synchronous vs Geo-stationary Orbit
• Three ways to establish permanent communication links
• Sub-satellite Points
• Antenna Look Angles
• Calculation of Antenna Look Angles for GEO satellites in equatorial plane
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– the square of the orbital period (T) of any satellite/planet is proportional to the cube of the average distance(semi-major axis of its elliptical orbit)from the satellite/planet to the earth/sun. OR
– The square of the time revolution of a planet /satellite divided by the cube of its mean distance from the sun /earth gives a number that is same for all planets/satellites.
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Kepler’s Third Law:Orbital Period
– The period of revolution,T,is referenced to inertial space or the galactic background.
– The orbital period is the time the orbiting body takes to return to the same reference point in space with respect to galactic background.
– Nearly always,the primary body will also be rotating and so the period of revolution of the satellite may be different from that perceived by the observer standing on the surface of the primary body.
– This is most obvious in Geostationary earth orbit.
– Any ideas……How??
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Orbital Period : Important Point to Remember!
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Sidereal Day vs Solar Day
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http://www.dur.ac.uk/john.lucey/users/e2_solsid.html
http://www.astronomynotes.com/nakedeye/s7.htm
Solar vs. Sidereal day-1
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Solar vs. Sidereal day-3
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Solar vs. Sidereal day-2
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Animation Link : http://spot.colorado.edu/~underwod/apas1110/rotate.html
Solar vs. Sidereal day-4
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References for the slides that follow
• Advanced Electronic Communication Systems
by Wayne Tomasi
• Dr N.D Gohar,Lecture Notes Fall 2007
• Internet(links/webpages provided)
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Satellite Orbits
• In physics, an orbit is the gravitationally curved path of one
object around a point or another body, for example the
gravitational orbit of a planet around a star.
http://www.cnes.fr/web/CNES-en/1108-the-beginnings-of-satellite-telecommunications.php
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Orbit Classifications
• Eccentricity classifications
• Altitude classifications
• Synchronous classifications
• Inclination classifications
• Centric classifications
• Special classifications
• Pseudo-orbit classifications
• Other
• Read more at http://en.wikipedia.org/wiki/List_of_orbits
To be focussed for
their relevance to
Communication
Satellites
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Eccentricity Classifications• Circular orbit: An orbit that has an eccentricity of 0 and
whose path traces a circle.
• Elliptic orbit: An orbit with an eccentricity greater than 0 and
less than 1 whose orbit traces the path of an ellipse.
Speed:
Constant in Circular Orbit,
Varies in Elliptical Orbit
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Altitude Classifications
• Low Earth Orbit (LEO): Geocentric orbits ranging in
altitude from 0–2,000 km (0–1,240 miles)
• Medium Earth Orbit (MEO): Geocentric orbits ranging in
altitude from 2,000 km (1,240 miles) to just below
geosynchronous orbit at 35,786 km (22,240 miles). Also
known as an intermediate circular orbit.
• Geosynchronous orbit: The orbit around Earth exactly
matching Earth's sidereal rotation period at the height of
35,786 km .
• High Earth Orbit (HEO): Geocentric orbits above the
altitude of geosynchronous orbit 35,786 km (22,240 miles).
– Highly Elliptical Orbits are generally considered to be a subset of High
Earth Orbits.12/26/2008 16NUST-SEECS
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Van Allen Radiation Belts and
LEO,MEO,HEO and GEO Orbits
Figure Courtesy:http://www.g0mrf.freeserve.co.uk/MEOSAT.htm12/26/2008 18NUST-SEECS
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HEO-Molniya System
•Molniya Satellites have apogee
at about 40,000km
•Perigee at about 400km
•Characterised by an inclination
of +63.4 degrees
• Period of around 12 hours
•Such orbits allowed them to
remain visible to sites in polar
regions for extended periods,
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http://en.wikipedia.org/wiki/Molniya_orbit
Synchronous Classifications• Synchronous orbit:
An orbit where the satellite has an orbital period equal to the
average rotational period (earth's is: 23 hours, 56 minutes, 4.091
seconds) of the body being orbited and in the same direction of
rotation as that body.
• Non-synchronous orbit:
– Orbital Period not equal to the rotational period of the body
being orbited
– non-synchronous satellites rotate around the earth in low altitude
elliptical or circular orbits
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Non-synchronous orbits• Prograde Orbits or Posigrade orbit:
If the satellite is orbiting in the same direction as the Earth’s
rotation(counter clockwise) and at an angular velocity greater
than that of Earth(ωs > ωe)
• Retrograde Orbit:
If the satellite is orbiting in the opposite direction as the Earth’s
rotation or in the same direction with an angular velocity less
than that of Earth(ωs < ωe)
Most non-synchronous satellites revolve around the earth in posigrade
orbit, due to which the position of satellites is continuously changing
w.r.t a fixed position on earth, and thus they can only establish
communications for a few minutes during each pass
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Inclination Classifications
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Inclination Classifications
• Inclined Orbit:
An orbit whose inclination in reference to the equatorial plane
is not 0.
• Polar Orbit:
An orbit that passes above or nearly above both poles of the
planet on each revolution. Therefore it has an inclination of (or
very close to) 90 degrees
• Equatorial Orbit:
A non-inclined orbit with respect to the equator. It is usually a
circular orbit and the angle of inclination is 0˚
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•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
•Angle of Inclination:The angle between the Earth’s equatorial plane and
orbital plane of a satellite measure counter clockwise at the Ascending node.
•Line of Nodes:The line joining the ascending and descending nodes through
the center of earth is called the line of nodes
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Line joining the apogee and the perigee through the
center of the earth is called the Line of Apsides
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Geo-synchronous vs Geostationary
Orbit
Geo-Synchronous Orbit Geo-Stationary Orbit(special case of geosynchronous orbit)
Eccentricity is not zero
and/or….
It must be exactly circular
(i-e have an eccentricity of zero)
…..The inclination is not
zero..but
It must be in the plane of the equator
(i-e have a zero inclination w.r.t
equator)
..Orbital period is correct. It must be at the correct altitude
(i-e have the correct orbital period
which is 23hr 56 min 4.1 sec)
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http://calgary.rasc.ca/geo_orbits.htm12/26/2008 30NUST-SEECS
Geosynchronous inclined orbit
Geostationary satellite
Geostationary equatorial orbit
N
S
i
Geosynchronous satellite
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NOTE:
• In Wayne Tomasi book,geo-synchronous and
geo-stationary satellite have been considered
as one and the same thing(synonymous)
however as we have seen they are not same.
• Consider all his discussion to be relevant for
Geo-stationary satellites.
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Geosynchronous Orbit• Orbital period is the sidereal rotation period of the Earth which is 23 hrs
56 min 4.0909 seconds
• Orbit is an inclined ellipse i.e. the orbital tilt is non-zero
• Orbital height varies (the satellites will have an apogee different from its
perigee)
• Orbital centre point - the centre of the Earth is at one of the two "foci" of
the elliptical orbit
• Since the orbit has some inclination and/or eccentricity, the satellite
would appear to describe a more or less distorted figure-eight
(analemma)in the sky, and would rest above the same spots of the
Earth's surface once per sidereal day.
• There are more orbital planes and positions available to satellites using
this technique
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A satellite in geo-synchronous but non-
geostationary orbit
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http://www.ccrs.nrcan.gc.ca/glossary/index_e.php?id=184
Geostationary Orbit
• Orbital period is the sidereal rotation period of the Earth which is
23 hrs 56 min 4.0909 seconds (the time it takes the Earth to rotate
once on its axis)
• Orbital height is 42,164 km (26,200 miles,above the center of
earth) always rotating exactly with the Earth
• Orbital plane is above the equator
• Orbital position is always above a certain point on the Earth's
equator (i.e. a specific longitude)
• Orbital path is circular
• Orbital centre point is the centre of the Earth
• Orbital tilt is zero
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Geostationary Orbit
• An observer on the ground would not perceive the satellite as
moving and would see it as a fixed point in the sky
• There are a limited number of positions available in this orbit due
to safety and maneuvering limits. Retired satellites are often
pushed slightly away from the precious exact positions.
• Another name for geostationary name is Clark’s Orbit
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Geostationary Satellite
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http://www.ccrs.nrcan.gc.ca/glossary/index_e.php?id=3108
120O
17..3 beamwidth for earth coverage
42,162 km geostationary orbit
radius
Atlantic Ocean (relay station)
Pacific Ocean (relay station)
Indian Ocean (relay station)
264,000 km
88,000 km
36,000 km
(a) Equilateral triangle
• A system of three such Geostationary satellites separated by 120 deg in longitude can cover the whole globe (81 deg south – 81 deg north) except polar regions• Beam BW = 17.3 deg, • Three satellites at the nodes of an Equilateral Triangle with side = 88k km• Availability = 100% to earth stations within their shadow
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• Advantages of geostationary orbits
– - The satellite is almost stationary in respect to a given earth station.
Consequently, expensive tracking equipment is not required at the earth
stations.
– - There is no need to switch from one satellite to another as they orbit
overhead. Consequently,there are no breaks in transmission because of the
switching times.
– - High-altitude geostationary satellites can cover a much larger area of the
earth than their low-altitude counterparts.
– - The effects of Doppler shift are negligible.
• Disadvantages of geostationary orbits
– - The higher altitudes of geostationary satellites introduce much longer
propagation times. The round-trip delay between two earth stations
through a geo satellite is 500 to 600 ms.
– - Geostationary satellites require high transmit powers and more sensitive
receivers because of the longer distances and greater path losses.
- High-precision spaceman ship is required to place a geostationary satellite
into orbit and keep it there. Also, propulsion engines are required on
board the satellites to keep them in their respective orbits.
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The GEO belt(as in 2001)
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Numerical Example
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Formulas for Orbital Velocities
• We saw that for orbital velocity for circular orbit is:
• For elliptical orbit the velocity is given by:
• In above equations, r=distance of satellite from the center of
the earth,a=semi-major axis
• For circular orbit,r=a
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Example:Satellite A is orbiting earth in a near-earth orbit of radius 7000 km. Satellite B is orbiting earth in an elliptical eccentric orbit with apogee and perigee distances of 47,000 and 7,000 km, respectively. Determine the velocities of the two satellites at point X. (µ = 39.8 x 1013 m3/s2)
The velocity of a satellite moving in a circular orbit is constant throughout the orbit as
= √(/r)
Therefore,A = √ [(39.8 x 1013/7000000)] = 7.54 km/s
The velocity of a satellite at any point in an elliptical orbit is given by eqn
B = √ (2/r – 1/a)
Here, r = 7000 km, and a = (47000 + 7000)/2 = 27000 km
Therefore, = √(39.8 x 1013) x (2/7000000 – 1/27000000) = 9.946 km/s12/26/2008 43NUST-SEECS
Thus, Three ways to establish permanent
communication links: • Elliptical orbits. These are highly elongated orbits in which the apogee
(40,000 km) is directly above the ground station, thus providing a useful
coverage period ;
• Constellations, made up of a large number of non-synchronous satellites,
which together provide permanent links and global coverage ;
• Geostationary orbits. These are circular orbits in which the position of the
satellite is fixed with respect to Earth. This is the cheapest and most
effective solution. A satellite orbiting at an altitude of 35,786 kilometres
above the equator has a period of revolution around the Earth the same as
the Earth's period of rotation, so it’s ground track is a point on the equator.
Three geostationary satellites, correctly positioned, can cover the entire
surface of the globe between the latitudes of +80° and –80°.
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Sub-satellite Point(SSP)• The location of a satellite is generally specified in terms of
latitude and longitude similar to the way the location of a point
on Earth is described.
• Since satellite is orbiting many miles above the earth,it has no
latitude and longitude
• Thus, its location is identified by a point on the earth directly
below the satellite.
• Sub-satellite points and earth station locations are specified
using standard latitude and longitude co-ordinates.
• SSP of GEO Satellite
Falls on the Equator and has 0 deg latitude
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Antenna Look Angles
• To optimize the performance of sat comm. system, the
direction of maximum gain of antenna(boresight) must be
pointed directly at the satellite
• For earth station antenna alignment, Two Angles need to be
determined and fixed:
• Azimuth Angle
• Elevation Angle
• Jointly referred as Antenna Look Angles.
• For GEO satellite, look angles of Earth Station only need to
be adjusted once as satellite will remain in given position
permanently, except for minor variations
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Elevation Angle(EA)• The vertical angle formed between the direction of travel of
EM wave radiated from an earth station antenna pointing
directly toward a satellite and the horizontal plane.
Earth
Angle of elevation
SatelliteEarth station antenna
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Elevation Angle(EA)
• Smaller the EA,the greater the distance a propagated wave
must pass through Earth’s atmosphere.
• Longer the distance travelled, greater is its deterioration due to
absorption and contamination by noise.
• Generally,5˚ is considered as the minimum acceptable EA
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Elevation Angle (EA) Effect on Satellite Received Signal Strength due to Heavy
Rain, Thick Fog, and Atmospheric Absorption
0º 5º 10º 20º 30º 40º 50º
10
20
30
40
50
0
0.5
1.0
1.5
2.0
2.5
3.0
Signal
power
lost (%)
Power
loss
(dB)
Elevation angle
Absorption by heavy rain
Absorption by thick fog
Atmospheric absorption
(a) 6/4 GHz band
Elevation angle
(b) 14/12 GHz band
0º 5º 10º 20º 30º 40º 50º
50
80
90
99.5
99.9
0
5
10
15
20
25
30
Signal
power
lost (%)
Power
loss
(dB)
Absorption by heavy rain
Absorption by thick fog
Atmospheric absorption
99
Figure 3.4.3.1-2 Attenuation due to atmospheric attenuation
Severe deterioration at Higher Frequencies(>10GHz)
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Azimuth Angle• It is defined as the horizontal pointing angle of the Earth
Station.It is measured eastward(clockwise)
• Azimuth is the horizontal angular distance from a reference
direction,either southern or northern most point of the horizon
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Figure shows Azimuth angle referenced to due north (0˚) and due south (180˚)
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Calculation of Look Angles for Satellites in
Equatorial Plane• Angle of Elevation and azimuth angle both depend on the
latitude of the earth station and the longitude of both the earth
station and the orbiting satellite.
• For a Geo-stationary satellite in equatorial plane,the procedure
to calculate antenna look angles is:
– From a map,determine the longitude and latitude of the earth station
– From the Table A, determine the longitude of the satellite of interest.
– Calculate the difference,in degrees(ΔL),between the longitude of the
satellite and the longitude of the earth station.
– From Figure 1,determine Azimuth Angle
– From Figure 2,determine the Elevation Angle
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Table A-SSP’s of some current Sats
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Figure 1
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Figure 2
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