satellite basics 1

8/7/2019 Satellite Basics 1

1/31

MCSMCS

Satellite Communication

System

Chapter -1

Introduction to Satellite Communications


2/31

Introduction to Satellite CommunicationsSystem Elements

Main Features

HistoryOverview and Basic concepts of Satellite Communications

Spectrum Allocation

Satellite Systems Applications

System Design ConsiderationsCurrent Developments and Future Trends

14 February, 2011 Satellite Communication System 2


3/31

SatellitesMan made machine which Revolves around the Earth in a

fixed orbit and is capable of Reflecting radio frequencies /

signals



4/31

Satellite System Elements


Space Segment

Satellite

TT&C Ground Station

Ground Segment

EarthStations

Coverage Region

SCC


5/31

Main FeaturesLarge area coverage Foot print area

Coverage regardless of political boundaries / divides

Inaccessible / remote areaLarge channel capacity / broadband

Higher data rates

Variety of services

GPSTelephone

Data

Video etc



6/31

14 February, 2011Satellite CommunicationSystem 6

REGION 2 REGION 1 REGION 3


7/31

TYPES OF ORBITS



8/31

Main orbit types


LEO 500 -1000 km

GEO 36,000 km

MEO 5,000 15,000 km


9/31

Useful Orbit-1

GEOSTATIONARY ORBITIn the equatorial plane

Orbital Period = 23 hrs 56 min 4.091 secs

= One Sidereal Day (defined as one complete

rotation relative to the fixed stars)

Satellite appears to be stationary over a point on the equator

Radius of orbit r = 42,164.57 km


NOTE: Radius = orbital height + radius of the earth

Average radius of earth = 6,378.14 km


10/31

USEFUL ORBITS 2:Low Earth Orbit (>250 km); T 92 minutes

Polar (Low Earth) Orbit; earth rotates about 23o each orbit;

useful for surveillance

Sun Synchronous Orbit

Example

Tiros(Television Infrared Observation Satellite)

NOAA (National Oceanic and Atmospheric Administration) satellites

used for search and rescue operations)

8-hour and 12-hour orbits

Molniya orbit (Highly Elliptical Orbit (HEO); T 11h 38 min;

highly eccentric orbit; inclination 63.4 degrees



11/31

Molniya view of the earth(Apogee remains over the northern hemisphere)


Period 12h(11h 58m 2s)

Semi Major Axis 26556 Km

Inclination 63.4o

Eccentricity 0.6- 0.75

Perigee altitude a(1-e)-Re

Apogee altitude a(1+e)-Re


12/31

Molniya Variants (HEOs)

Satellite Communication System14 February, 2011 12

Period 24h(23h 56m 4s)

Semi Major Axis 42164 Km

Inclination 63.4o

Eccentricity 0.25 0.4

Perigee altitude a(1-e)-Re

Apogee altitude a(1+e)-Re

Tundra Orbit


13/31

Molniya Variants (HEOs)Tundra Orbit Lies entirely above the Van Allen belts.

The Earth's Van Allen Belts consist of highly energetic ionizedparticles trapped in the Earth's Geomagnetic fields. On the sunwardside of the Earth, the geomagnetic fields are compressed by the SolarWind, on the opposite side of the Earth, the geomagnetic fields extendto 3xRE . As a result, the geomagnetic field forms an elongated cavity,known as the Chapman-Ferraro Cavity, around the Earth. Within thiscavity, reside the Van Allen Radiation Belts. These radiation belts arecomposed of electrons with thousand eV energies, and protons with

million eV energies.The Russian Tundra system, which employs two satellites intwo 24-hour orbits separated by 180 deg around the Earth,with an apogee of 53,622 km and a perigee of 17,951 km.



14/31

Molniya Variants (HEOs)The Molniya orbit crosses the Van Allen belts twice for each

revolution, resulting in a reduction of satellite life due to

impact on electronics

the Russian Molniya system employs three satellites in three

12-hour orbits separated by 120 deg around the Earth, with

an apogee of 39,354 km and a perigee of 1000 km.



15/31

Molniya Variants (HEOs)The LOOPUS orbit.The LOOPUS system employs three satellites in three eight-hour

orbits separated by 120 deg around the Earth, with an apogee of

39,117 km and a perigee of 1238 km.

The ELLIPSO orbit

The ELLIPSO system - Elliptical low orbits for mobile communications

and other optimum system elements

Ellipso 2G: Ellipso enhanced its initial Ellipso system using

anticipated 2 GHz(1.8) to offer two way voice and higher data rates,(up to 64 kbps) for 3G data services. Combining the spectrum bands

in a single sat sys will lead to increased capacity, and array of mobile

and fixed services, including video conferencing, and all at affordable

cost to consumer.



16/31

A Highly Elliptical Orbit (HEO)Perigee at about 500 km

Apogee at 50,000 km.

Angle of Inclination 63.4 deg.

(This inclination value is selected to avoid rotation of the apses; thus,

a line from the Earth's center to the apogee always intersects the

Earth's surface at a latitude of 63.4 deg North).

Orbit period varies from eight to 24 hours.

Owing to the high eccentricity of the orbit, a satellite spendsabout two-third of the orbital period near apogee, during

which it appears to be almost stationary to an observer on

the Earth-a phenomenon known as `apogee dwell').



17/31

A Highly Elliptical Orbit (HEO)During the brief time the satellite is below the local horizon,

a hand-off to another satellite in the same orbit is required in

order to avoid loss of communications.

Free space loss and propagation delay comparable to that of

Geosynchronous satellites.

However, due to the larger movement of the sat in HEO,

larger Doppler shifts are expected.



18/31

Medium-Earth Orbit (MEO)

Altitude 10,000 km

Intermediate Circular Orbit (ICO) (apogee and perigee areequidistant)

Orbital period about seven hours

Dwell time is few hours.Global communications system requires relatively fewsatellites in two to three orbital planes

MEO systems operate similarly to LEO systems.

Hand-over is less frequent,Propagation delay and free space loss are greater.

Examples of MEO (specifically ICO) systems are

Inmarsat-P (10 satellites in 2 inclined planes at 10,355 km)

Odyssey (12 satellites in 3 inclined planes, also at 10,355 km).14 February, 2011 Satellite Communication System 18


19/31

Low-Earth Orbit (LEO)Orbital period 90 minutes to 2 hrsTypical LEO is elliptical / circular,

Altitude less than 2000 km above the surface of the Earth

Radius of footprint ranges between 3000 and 4000 km.Dwell time local horizon 20 minutes.

Global communications system requires a large number ofsatellites (different orbital planes)

Handover on setting LEO. (Subsequent or adjacent)

Large Doppler shift.Atmospheric Drag causes the orbit to gradually deteriorate.Examples

GlobalstarTM (48+8 satellites in 8 orbital planes at 1400 km)

Iridium (66+6 satellites in 6 orbital planes at 780 km).



20/31

Geosynchronous & Geostationary OrbitsAltitude 35,786 Kms above MSL

Orbital Period one sidereal day (1436.1 minutes), 23 hrs 56mins 4.1 sec in length.

The footprint almost 1/3 of the earth's surface (up to 75o

Nand S latitude)

Global coverage with three satellites in orbit. (120o apart)

Dwell time local horizon 23 hrs 56 mins 4.1 sec

Geosynchronous orbit has small non-zero values forinclination and eccentricity, causing the satellite to trace outa small figure eight in the sky.

Round-trip delay approximately 250 millisec

Geostationary orbit: Geosynchronous orbit with zero

inclination and zero eccentricity (equatorial, circular orbit)14 February, 2011 Satellite Communication System 20


21/31

A Polar Orbit

Angle of inclination about 90 deg

intersecting the North and South poles.

Earth rotates underneath fixed polar orbit in space,

coverage area the entire globe, (long periods during which

the satellite is out of view of a particular ground station).

Suitable for a store-and-forward communications systems.



22/31

A Polar OrbitImproved accessibility using two or more satellites in

different polar orbits.

Mostly small LEO systems employ polar or near-polar orbits.

Example

COSPAS-SARSAT

Maritime Search and Rescue system

Uses eight satellites in near polar orbits: four SARSAT satellites

moving in 860 km orbits inclined at 99o (which makes them Sun-synchronous) and four COSPAS satellites moving in 1000 km orbits

inclined at 82o.



23/31

Sun-Synchronous OrbitIn a Sun-synchronous orbit, the angle between the orbital plane and Sunremains constant, resulting in consistent light conditions for the satellite.

This is achieved by careful selection of orbital altitude, eccentricity andinclination, producing a precession of the orbit (node rotation) of

approximately 1

o

eastward each day, equal to the apparent motion of theSun.

This condition can be achieved only for a satellite in a retrograde orbit. Asatellite in Sun-synchronous orbit crosses the equator and each latitudeat the same time each day.

This type of orbit is therefore advantageous for an Earth observationsatellite, since it provides constant lighting conditions.

In a sun-synchronous orbit, the satellite passes over the same part of theEarth at roughly the same local time each day.

This can make communication and various forms of data collection veryconvenient



24/31

Parameters Determining Orbit Size & ShapeParameter Definition

Semimajor Axis Half the distance between the two points in the orbit that are farthest

apart

Apogee/PerigeeAltitude Measured from the "surface" of the Earth (a theoretical sphere with aradius equal to the equatorial radius of the Earth) to the points of

maximum and minimum radius in the orbit

Apogee/Perigee

Radius

Measured from the center of the Earth to the points of maximum and

minimum radius in the orbit

Period The duration of one orbit, based on assumed two-body motion

Mean Motion The number of orbits per solar day (86,400 sec/24 hour), based on

assumed two-body motion

Eccentricity The shape of the ellipse comprising the orbit, ranging between a

perfect circle (eccentricity = 0) and a parabola (eccentricity = 1)



25/31



26/31

Orientation of Orbital Plane in SpaceParameter Definition

Inclination The angle between the orbital plane and the Earth's equatorial

plane (commonly used as a reference plane for Earth satellites)

Right Ascension ofthe Ascending Node The angle in the Earth's equatorial plane measured eastward fromthe vernal equinox to the ascending node of the orbit

Argument of

Perigee

The angle, in the plane of the satellite's orbit, between the

ascending node and the perigee of the orbit, measured in the

direction of the satellite's motion

Longitude of the

Ascending Node

The Earth-fixed longitude of the ascending node

The ascending node (referenced in three of the above definitions) is the point in the

satellite's orbit where it crosses the Earth's equatorial plane going from south to north.



27/31

Satellite Location parametersTo specify the satellite's location within its orbit at epoch

Parameter Definition

True Anomaly

The angle from the eccentricity vector (points toward perigee) to

the satellite position vector, measured in the direction of satellite

motion and in the orbit plane.

Mean Anomaly The angle from the eccentricity vector to a position vector wherethe satellite would be if it were always moving at its angular rate.

Eccentric Anomaly

An angle measured with an origin at the center of an ellipse from

the direction of perigee to a point on a circumscribing circle from

which a line perpendicular to the semimajor axis intersects the

position of the satellite on the ellipse.

Argument ofLatitude

The sum of the True Anomaly and the Argument of Perigee.

Time Past

Ascending NodeThe elapsed time since the last ascending node crossing.

Time Past Perigee The elapsed time since last perigee passage.



28/31

Orbital Velocities and Periods


SystemHeight

(km)

Velocity

(km/s)

Period

h min s

INTELSAT 35,786.43 3.0747 23 56 4.091

ICO-Global 10,255 4.8954 5 55 48.4

Skybridge 1,469 7.1272 1 55 17.8

Iridium 780 7.4624 1 40 27


29/31

LEO, MEO and GEO Orbit Periods


0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 5000 10000 15000 20000 25000 30000 35000 40000

Altitude [km]

Hours


30/31

Minimum Delay for two hops

0.0

50.0

100.0

150.0

200.0

250.0

300.0

0 5000 10000 15000 20000 25000 30000 35000 40000

Altitude [km]

Delay[ms]



31/31

Why do satellites stay moving and in orbit?

F1(Gravitational

Force)

v (velocity)

F2(Inertial-Centrifugal Force)

satellite basics 1

Documents