INTELSAT• International Telecommunications Satellite

• Organization -created in 1964 and 140 member countries • It handles the technical and administrative problems associated with a world wide telecommunications systems.

• July 2001 INTELSAT - private company

• May 2002 the company -providing end-to-end solutions through a network of Teleports, leased fiber, and points of presence (PoPs) around the globe.

Cont.,

• INTELSAT -I was launched (Early Bird satellite) -1965 and provided 480 voice channels.

• The series of satellites were INTELSAT –I,II,III,IV,V and VII.

• The capacity - number of voice channels, increased as well as the design lifetime.

• These satellites are in geostationary orbit, meaning that they appear to be stationary in relation to the earth.

• INTELSAT covers three main regions- AOR, IOR and POR

Evolution of INTELSAT satellites

INTELSAT VI-VII/A seriesINTELSAT VII • February 1997 to June 1998 , lifetime - 14 to 17 years

• The construction is similar to that for the V and VA/VB series

• Capacity - 18,000 two-way telephone circuits and three TV channels;

- up to 90,000 two-way telephone circuits can be achieved with the use of “digital circuit multiplication”.

• VII series has solar sails rather than a cylindrical body

CONT.,• The VII series - service in the POR and also for some of the less demanding services in the AOR

• Antenna beam footprints for the C-band hemispheric cover- age and zone coverage and the spot beam coverage possible with the Ku-band antennas

• In POR, C band (6/4) MHZ antennas and Ku band (14/12) MHZ Antennas are used.• Based on number of satellites, system includes ground

spares and orbiting spares.• In 1992 INTELSAT had 6 satellites in AOR – at degrees east longitude (307,325.5, 332.5, 338.5, 341.5, 359)

• 3 in IOR –at degrees 60,63,66

• 3 in POR – at degrees 174,177 ,180

• Traffic in AOR – 3 times of IOR twice of IOR and POR combined

INTELSAT VII/A

• Capacity - 22,500 two-way telephone circuits and three TV channels; up to 1,12500 two-way tele- phone circuits can be achieved with the use of digital circuit multiplication

• Services - Internet, DTH TV, tele-medicine, tele-education, and interactive video and multimedia.

CONT.,

Cont.,Transponder

• One 36 MHz transponder is capable of carrying about 9000 voice channels, or two analog TV channels, or about eight digital TV channels.

USES

- domestic services within any given country - and regional services between countries - vista application (f-1.4)

U.S. Domsat• Provide various telecommunications services, such as voice, data, and video transmissions, within a country.

• Geostationary orbit

• TV channels, and carrying a large amount of commercial telecommunications traffic.

• Provide a DTH television service

Classification

• High power, medium power and low power• EIRP - 60 dBW for the high-power category • 37 dBW for the low- power category, a difference of 23 dB.

Cont.,

• High-power category, which allows much smaller antennas to be used with the receiver.

• High-power category - DBS service.

• Medium-power category- point-to-point services,

• Low-power category - no official DBS services are provided a wide range of radio and TV programming could be received on this band.

• North American C-band TV broadcasts are now encrypted, or scrambled, to prevent unauthorized access

These satellites are in geostationary orbit• Broadband services- Internet at Ka-band frequencies.

orbital spacing

• U.S. FCC (Federal communications commission) adopted a policy 2° for satellites operating in the 6/4- Ghz (C band) 1.5° for those operating in the 14/12-GHz (Ku band)

• Interference - acceptable interference levels

• Home satellite receivers - 6/4-GHz band - excessive interference at 2° spacing

Cont.,

To cover the north and south polar regions.

RUSSIA -uses the highly elliptical orbits to cover the northern region.

In fig.1.6 only one geostationary orbit, but an infinite number of polar orbits

Orbits are generally LEO- So we call the polar orbiting satellites are LEOSAT.

• Height of the orbit- 800 -900 Km above the earth compared to Geostationary orbit.

• Height of Geostationary orbit-36,000 Km above the earth

Polar orbiting satellites

• Generally Low earth orbit -circular orbit (SUN SYNCHRONOUS

ORBIT)- They cross the equator at the same local time.

• In U.S -weather satellite services – National Polar Orbiting Operational Environmental Satellite System (NPOESS)

• 1. NASA (National Aeronautics and Space and Administration) 2. NOAA (National Oceanic and Atmospheric Administration) – Manages the program for series of Satellites known as Tiros-n- series.• Tiros- Television and infrared observational satellite, mission of NOAA – Environmental monitoring• List of Instruments 1. AVHRR- Advanced Very High Resolution Radiometer. 2. SBUV Mod2 - Solar Backscatter ultraviolet radiometer mod2

Cont.,

3. Tovs- Tiros operational vertical sounder system.

4. SEM- Space Environmental monitor

• Uses: Weather forecasting, environmental data

• Argos data collection system (DCS)- collects environmental data.

• PTT (Platform transmitter terminal)-located at the polar region. • NOAA satellites help locate ships and aircraft in distress. This service is known as SARSAT

• Application : Search and rescue satellites

Cont.,

Cont.,

COSPAS-SARSAT• Combined system of Russian and SARSAT.

• The satellite receives signal from an emergency beacon set off automatically at distress site.

• Satellites moves at velocity relative to the beacon.

• If received frequency >transmitted frequency of the satellite- so satellite approaches the beacon, otherwise recedes from the beacon.

• Whether beacon –east or west of the orbit.

• Doppler shift can’t determined by single pass.

• Two successive passes, Doppler shift can be determined.

Cont.,

Cont.,

Fig. Doppler shift in received frequency on successive passes of the satellite. ELT—emergency locator transmitter.

Kepler’s First law

• It states that the path followed by the satellite around the primary will be an ellipse.

• F1 & F2 – Focal points• The center of mass of the two body system

termed the Barycenter.• 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

Fig. The foci F1 and F2, the semimajor axis a and the semiminor axis b of an ellipse

Cont.,• Eccentricity e= squart(a2 - b2 )/a

• The semimajor axis of the ellipse is denoted by a, and the semiminor axis, by b.

• For an elliptical orbit, 0 < e < 1.

• When e = 0, the orbit becomes circular.

Kepler’s Second law• It states that, for equal time intervals, a satellite

will sweep out equal areas in its orbital plane, focused at the barycenter.

• Assuming the satellite travels distances S1 and S2 meters in 1 s, then the areas A1 and A2 will be equal.

• Average velocity in each case is S1 and S2 m/s

• The satellite takes longer to travel a given distance when it is farther away from earth

Cont.,

Kepler’s Third law• Kepler’s third law states that 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.

• a3 = μ/n2, μ= 3.986005 1014 m3/s2

• where n is the mean motion of the satellite in radians per second and is the earth’s geocentric gravitational constant

• With n in radians per second, the orbital period in seconds is given by P =2n

Definitions of Terms for Earth-orbiting Satellites

• Sub satellite path: This is the path traced out on the earth’s surface directly below the satellite.

• Apogee: The point farthest from earth. Apogee height is shown as ha in Fig. below

• Perigee: The point of closest approach to earth. The perigee height is shown as hp in Fig. below

Apogee height ha, perigee height hp, and inclination i. la is the line of apsides

Cont.,• 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.

Cont.,• Line of nodes: The line joining the

ascending and descending nodes through the center of the earth.

• Inclination:

1. The angle between the orbital plane

and the earth’s equatorial plane.

2. It is measured at the ascending node

from the equator to the orbit, going

from east to north.

Cont.,• Prograde orbit:

1. An orbit in which the satellite moves in the

same direction as the earth’s rotation.

2. The prograde orbit is also known as a direct

orbit.

3. The inclination of a prograde orbit always lies between 0° and 90°.

4.Most satellites are launched in a prograde

orbit.

Fig. Prograde and retrograde orbits

Cont.,• Retrograde orbit:

1.An orbit in which the satellite moves in a direction counter to the earth’s rotation, as shown in Fig.

2.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 earth’s center, in the direction of satellite motion. The argument of perigee is shown as w in Fig.

Fig. The argument of perigee w and the right ascension

of the ascending node Ω.

Cont.,• Right ascension of the ascending node:

1. To define completely the position of the orbit in space, the position of the ascending node is specified.

2. However, because the earth spins, while the orbital plane remains stationary.

3. For absolute measurement, fixed reference –required.

4. Reference chosen – “FIRST POINT OF ARIES”

5. Vernal equinox –sun crosses the equator from S to N.

Cont., • Mean anomaly:

Mean anomaly M gives an average value of the angular position of the satellite with reference to the perigee

• True anomaly:

1. The true anomaly is the angle from perigee to the

satellite position, measured at the earth’s center.

2. This gives the true angular position of the satellite

in the orbit as a function of time.

Orbital Elements-Six Orbital elements

• Six orbital elements referred “keplerian element set”.

1. The semimajor axis a and the eccentricity e –

give the shape of the ellipse.

2. The mean anomaly M0--- gives the

position of the satellite in its orbit at a

reference time known as the epoch.

CONT.,• The argument of perigee ω -- gives the

rotation of the orbit’s perigee point relative to the orbit’s line of nodes in the earth’s equatorial plane.

• The inclination i and the right ascension of the ascending node Ω-gives the orbital plane’s position to the earth

Apogee and Perigee Heights

• Along with orbital elements, the apogee height and perigee height are often required.

• The length of the radius vectors at apogee and perigee can be obtained from the geometry of the ellipse

• ra= a(1+e) ha= ra-RR=earth

• rp = a(1-e) hb=rb-R radious

• The apogee and perigee heights, the radius of the earth must be subtracted from the radii lengths

Orbit Perturbations• Orbit- keplerian orbit ideal-elliptical.

• The earth - uniform spherical mass.

• The only force acting -the centrifugal force resulting from satellite motion balancing the gravitational pull of the earth.

• The gravitational forces of the sun and the moon and atmospheric drag. (negligible effect on low-orbiting satellites)

Cont.,• But they affect – satellites in GEO.

• Atmospheric drag- negligible effect on geostationary satellites, but affect low orbiting earth satellites below about 1000 km.

Effects of a non-spherical earth

• For a spherical earth of uniform mass, Kepler’s third law

• The 0 subscript – this result applies for a perfectly spherical earth of uniform mass

• But earth not perfectly spherical.

• Non-spherical earth- oblate spheroid.

Cont.,• Earth oblateness – mean motion given by

• K1= 66,063.1704 km, negligible effect on the semi major axis a

• The orbital period taking into account the earth’s oblateness - anomalistic period

Cont.,• The oblateness of the earth - produces two rotations

of the orbital plane.

• 1) Regression of the nodes- Equatorial plane,

rotates about the center of the earth.

2) Thus Ω the right ascension of the ascending

node, shifts its position.

Cont.,• If the orbit is prograde , the nodes slide

westward.

• A satellite in prograde orbit moves eastward, and in a retrograde orbit, westward.

• The nodes therefore move in a direction opposite to the direction of satellite motion, hence the term regression of the nodes

Cont.,

• Second effect – Rotation of apsides in the orbital plane.

• Both effect depend on mean motion n,a,e

• Rate of change of Ω with respect to time

Cont.,

• The other major effect produced by the equatorial bulge is a rotation of the line of apsides. This line rotates in the orbital plane.

• The rate of change of argument of perigee

Cont.,• New values for Ω and w at time t

Atmospheric drag• For near-earth satellites, below about 1000

km, the effects of atmospheric drag are significant.

• The drag is greatest at the perigee- to reduce the velocity at this point.

• The result that the satellite does not reach

the same apogee height on successive

revolutions.(a, e reduced)

Cont.,• The change of major axis is

• Mean anomaly is also changed

Inclined Orbits• Satellite in an inclined elliptical orbit.

• The orbital elements are known with

reference to the orbital plane.

• The location of the earth station - in terms of the local geographic coordinates.

• Calculations of satellite position and velocity in space-Represented in rectangular co-ordinates.(azimuth, elevation, angles & range).

Cont.,• Transformations between coordinate systems are

therefore required.

• Determination of the look angles and range involves

1. Orbital elements- in the NASA bulletins.

2. Various measures of time

3. Perifocal coordinate system- based on orbital plane.

4. Geocentric-equatorial coordinate system-based on earth’s equatorial plane.

5. Topocentric-horizon coordinate system- based on observer’s horizon plane.

The two major coordinate transformations

• The satellite position measured in the perifocal system.

• Transformed to the geocentric-horizon

system (satellite position, earth station location).

• The satellite-to-earth station position- transformed to the topocentric-horizon system (look angles and range )

Calendars• Calendar is a time-keeping device.

• Calendar days- units of time.

• year is divided into months, weeks, and days.

• Calendar days - based on the earth’s motion relative to the sun.

• The sun moving relative to the earth.

• Motion is not uniform- the mean sun.

Cont.,• A day measured relative to this mean sun is

termed a mean solar day.

• A tropical year -365.2422 days (take real sun motion)

• The calendar year, also referred to as the civil year.

• After 100 years- 24 days between the calendar year and the tropical year.(to avoid discrepancy

-Julian calendar)

Universal time• Universal time coordinated (UTC) - time used

for all civil time–keeping .

• It is the time reference - by the National Bureau of Standards as a standard for setting clocks.

• The fundamental unit for UTC is the mean solar day.

• In terms of “clock time,” the mean solar day is divided into 24 h, an hour into 60 min, and a minute into 60 s.

• Thus there are 86,400 “clock seconds” in a mean solar day.

Cont.,• Universal time coordinated - equivalent to

Greenwich mean time (GMT), as well as Zulu (Z) time.

• UT in two forms: as a fraction of a day and in degrees.

Julian dates• All events measured by reference time .

• Reference time -provided by the Julian zero time reference.

• The important point - ordinary calendar times are easily converted to Julian dates. (measured on a continuous time scale of Julian days).

• Eg. First determine the day of the year- (day zero) denoted as Jan 0.0

Cont.,• JAN 0 - December 31

• JAN 0.5 - Noon on December 31

• JAN 1.5 – Noon on January 1

JD = JD0,0 + day number + UT day

• Number of methods – calculating the Julian day

Cont.,

Cont.,

Sidereal time• Sidereal time is time measured relative to the

fixed stars .

• one complete rotation of the earth relative to the fixed stars is not a complete rotation relative to the sun.

• The sidereal day is defined as one complete rotation of the earth relative to the fixed stars

Cont.,• 1 sidereal day - 24 sidereal hours

• 1 sidereal hour - 60 sidereal minutes

• 1 sidereal minute - 60 sidereal seconds

• 1 mean solar day = 1.0027379093 mean

sidereal days.

= 24 h 3 m 56.55536 s

sidereal time.

= 86,636.55536 mean

sidereal seconds

Cont.,

Cont.,• 1 mean sidereal day = 0.9972695664 mean

solar days

• = 23 h 56 m 04.09054 s

mean solar time

• = 86,164.09054 mean

solar seconds

• Measurements of longitude on earth surface- use sidereal time

The orbital plane

• In the orbital plane, the position vector r and the velocity vector v specify the motion of the satellite.

• The magnitude of the position vector

• To determining true anomaly - two stages.• First, the mean anomaly M at time t is found.

Cont.,

Cont.,

Here, n is the mean motion .

For NASA elements M0 = n(t0 –T)

T= t0