Chapter 2 Satellite Communications: Satellite Launching

7/28/2019 Chapter 2 Satellite Communications: Satellite Launching

1/36

Chapter 2

Satellite Launching


2/36

Launches and Launch

Vehicle

A satellite cannot be placed into a stable

orbit unless two parameters that are uniquely

coupled together => the velocity and the

orbital height simultaneously correct.

Example:

A Geostationary Satellite:

must be in orbit at a height of 35786.03 km above thesurface of the earth or 42164.17 km from center of the

earth.

Inclination zero degrees.

Velocity of 3074.7 m/s. tangential to the earth.


3/36

The further out from the earth the orbit is, the greater theenergy required from the launch vehicle to reach the orbit.

In any earth satellite launch, the largest fraction of energyexpended by the rocket is used to accelerate the vehicle fromthe rest until it is about 20 miles (32 km) above the earth.

To make efficient use of the fuel, it is common to shed excessmass from the launcher as it moves upward on launch =>staging.

most launch vehicle have multiple stages and as each stageis completed, that portion of the launcher is expended untilthe final stage places the satellite into the desired trajectory.

Launch vehicles may be classified as expendable orreusablelaunch vehicles.

CONTINUED


4/36

Expendable Launch Vehicles (ELV)- typical expendable launchers => US, Atlas-

Certaur and Delta Rockets and European SpaceAgency Ariane Rocket.

Reusable Launch Vehicles (RLV)- also referred as Space Transportation System

(STS).

- Partially reusable.

- The solid rocket booster are recovered and

refurbished for future missions and the shuttlevehicle itself is flown back to earth for refurbishedand reuse.

CONTINUED


5/36

Placing Satellite Into

Geostationary Orbit

There are two way in placing the

satellite into geostationary orbit.

i. Geostationary transfer orbit and AKM

ii. Geostationary transfer orbit with sloworbit rising.


6/36

Geostationary Transfer

Orbit and AKM the initial approach to launching geostationary orbit

satellite was to place the spacecraft with the final rocketstage still attached into LEO.

After been attached into LEO and the orbital elements

are measured, the final stage is reignited and thespacecraft is launched into a geostationary transferorbit (GTO).

The GTO perigee is the original LEO orbit altitude while

the GTO apogee is the GEO altitude. After a few orbitsin GTO while the orbital elements are measured, arocket motor (contained within the satellite itself) isignited at apogee and the GTO is raised until it is acircular, geostationary orbit.


7/36

Since the rocket motor fires at apogee, it is commonlyreferred to as the apogee kick motor (AKM).

AKM => used to circularize the orbit at GEO and toremove any inclination error so that the final orbit of

the satellite is very close to geostationary.

CONTINUED

LEO

GTO

GEO

AKM


8/36

Geostationary Transfer Orbit

With Slow Orbit Rising

in this procedures of launching, the spacecraftthrusters are used to raise the orbit from the GTO toGEO over a number of burns.

Many of the satellite elements are deployed while in

GTO (solar panel).

The satellite has two power levels of thrusters:

- for more powerful orbit raising maneuvers.

- for more on-board maneuvers.

since thruster take many time (hours) of operation toachieved the geostationary orbit, the perigee of theorbit is gradually raised over successive thrusterfirings.


9/36

LEO

GTO

GEO

Successive

Orbit Rising


10/36

Orbital Mechanic

How orbit is achieved,

Describe the motion of an object

orbiting another body,

How sat maneuver in space.

Most sat are placed atleast 250

miles (400km) above earthsurface.


11/36

Newtons law

important to describe the motion of a sat.

The coordinates axes within which the orbit of

the sat can be set

Determine the various forces on the earthsatellite.

s = ut + at2

v2 = u2 + 2at

v = u + at

F = ma


12/36

Two main forces that acting on the sat (in

stable orbit)

Me

Fin

Fout

Fout = (mv2)/ r

Fin = (GMem)/ r2

CONTINUED

i. Centrifugal Force (Fout)

ii. Centripetal Force (Fin)


13/36

Centrifugal Force (Fout)

- Due energy for satellite.- Attempts to fling the satellite into a higher

orbit.

Centripetal Force (Fin)

- due to the gravitational attraction of theearth which the sat is orbiting.

- attempts to pull the sat to the earth.


14/36

Keplers Law

First law

- states that the path followed by a satellitearound the primary will be an ellipse.

- eccentricity and semimajor axis are two of

the orbital parameters specified for thesat orbiting the earth.

- the eccentricity e is given by

e = [(a2 _b2)/a]

- when e = 0, the orbit become circular- when 0 < e < 1 , the orbit become ellipse


15/36

Second Law- states that for equal time intervals, a sat

will sweep out equal areas in its orbitalplane.

- orbit is an ellipse.

- two shaded area of elliptical plane inwhich the orbit moves.

- from figure, sat moves in orbit betn:

t1 t2 and sweep area 1

t3 t4 and sweep area 2

- if t1 t2 = t3 t4 ,

then area 1 =area 2

CONTINUED


16/36

Third Law

- states that the square of the

period time of orbit is proportional

to the cube of the mean distance

betn two bodies.

- the mean distance is equal to

the semimajor axis a.

T2 = (42 a3)/


17/36

Geostationary Orbit

The most widely used orbit.

The sat appears stationary relative to theearth.

Lies in equatorial plane and inclination is

zero. The sat must orbit the earth in the samedirection as the earth spin and at the samespeed.

h

ae

Where:

h = 35786 km above the

equator.

= from earth surface to

geo orbit.

ae

= radius of earth


18/36

Because of the orbit is circular:a = b = ae + h

The periodic time, T for the geo

orbit to the nearest minute is 23h56m in mean solar time. Less 4

min because the differences is

being accounted for the earths

movement around the sun.


19/36

Exercise

1. The earth rotates once per sidereal day of23h 56m 4.09s. Find the radius of Geo inunit km.


20/36

2. The space shuttle orbit at an altitude of

250 km above the earths surface. The

mean earth radius is approximately

6378 km. Using these figures,calculate the period of the shuttle orbit

the earth.

Find also the linear velocity of the

shuttle along the orbit.


21/36

3. A sat is in an elliptical orbit with

perigee of 1000 km and apogee

of 4000 km. using a mean earth

radius of 6378.4 km, find theperiod of the orbit in hours, min

and sec.


22/36

Past Paper Questions

Question 4 (Apr 2006)

Determine the apogee height of the ellipticalorbit satellite when the time period of the

satellite to rotate around the earth is 2 hours18 minutes and 45 seconds. Given theperigee height as 1000km with the earthradius of 6378 km and the Keplers constant is3.986005 105 km3/s2.

Compare mathematically the velocity of thesatellite while its rotating along perigee andapogee. By using the Keplers second law,comment on your answers.


23/36

Question 5 (Nov 2005)

Using Keplers third law, find the period

of the orbit in hours, minutes and

seconds for an elliptical orbit satellite

with an apogee of 4500 km and perigeeof 1500km. Given the earth radius is

6378 km and the Keplers constant is

3.986005 105 km3/s2.


24/36

Question 6

In stable orbit, name two main forces that acton the satellite due to kinetic energy andgravitational attraction.

Derive the equation to prove that the satelliteis stable in orbit when the orbit is situated31200km above the earth surface. Given theKeplers constant is 3.986005 105 km3/s2 .The earth radius of 6378 km and the satellitemass of 200 kg, find the velocity of the

satellite on the orbit.


25/36

Question 7 (Apr 2007)

Find the height of the orbit and

the period of time in hour, minute

and second for the satellite

maneuvers on the orbit withvelocity 3.10 kms-1. Given the

Keplers constant is 3.9860045 x

105 km3s-2and the mean earthsradius is 6378 km.


26/36

Antenna Look Angles

The user must be able to determine the azimuth andelevation angles of the ground stations antenna.

Figure indetermining thelook angles for

a geostationarysatellite.

d

rs

re

Local

horizontal

centre


27/36

Look Angles Equations


28/36

For Geostationary Satellite


29/36

The azimuth look angle (Az) can be foundfrom:

Hemisphere

Station

Position of

Satellite With

Respect toStation

Relation

between Az and

Northern East Az = 180 -

Northern West Az = 180 +

Southern East Az =

Southern West Az = 360 -


30/36

Azimuth Angle Diagrams


31/36

Visibility Test

For a sat to be visible from an e/s, its El must be abovesome min value, which is atleast 0. A +ve elevationangle requires that:

This mean that the max central angular seperationbetn the e/s and the subsat point is limited by:

For a nominal geo orbit, the last equation reduces to: 83.1 for the sat to be visible.


32/36

Exercise 1

1. An e/s situated in Docklands, London needsto calculate the look angle to a geo sat in theIndian Ocean operated by Intelsat. Thedetails of the e/s site and the sat are as

follows:

E/s latitude and longitude are 52.0 N and 0.

Sat longitude (subsat point) is 66.0 E


33/36

Exercise 2(Apr 2007)

Consider an earth station situated atHoustan , Texas has a longitude of99.5 at West and latitude of 29.5 atNorth. The earth station received

signal from Westar V satellite whichsituated at a longitude of 119.5 atWest.

Find the elevation angle and theazimuth angle of the earth stationantenna. Determine whether thesatellite is visible or not.


34/36

Exercise 3(Oct 2006)

An earth station located at 20.5 S and78.4 E is up linking a signal to a 90.5E geostationary satellite. Given theratio of the distances between the

satellite to the earth center and theradius of the earth as 6.6107345,calculate its azimuth, elevation anglesand the distance between the earth

station and the satellite. Verify thevisibility of the satellite from the earthstation.


35/36

Exercise 4(Apr 2006)

An earth station located inHoustan, Texas with the latitudeof 29.5 N and longitude of 95.5

W transmit a signal to the SatcomI located at geosynchronous orbitwith longitude of 135 W.Calculate the elevation angle and

azimuth of the earth station.Determine whether the satellite isvisible from the earth station.


36/36

Exercise 5(Nov 2005)

Consider an earth station located at a

longitude of 80 W and its latitude

located at a position somewhere in

North. The geostationary satellite islocated 120 West and is visible from

the earth station. If the intermediate

angle is 52.55, find the latitude

position of the earth station, theelevation angle and the azimuth angle

of the earth station antenna.

Chapter 2 Satellite Communications: Satellite Launching

Documents

Transcript of Chapter 2 Satellite Communications: Satellite Launching