Satellite Comm-Lec II

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.


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??


Orbital Period : Important Point to Remember!


Sidereal Day vs Solar Day


12/26/2008 NUST-SEECS 7

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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Animation Link : http://spot.colorado.edu/~underwod/apas1110/rotate.html


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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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Documents

Satellite Comm-Lec II