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Sunday, December 7, 2014

How does one achieve Geosynchronous orbit? Follow GSAT - 16

Here is a running commentary on Orbit Raising Maneuvers as they happen. There is some delay in posting because we have wait for technical details to percolate in open public domain. As soon as some bits are available I work on it and post it here.

The orbit in which the satellite was launched has been plotted in the graphic here. Left figure shows the orbit as seen from North pole. The satellite is at orbit perigee so it is just about 250 Kms from the surface of Earth and that perigee point is ( as seen in the figure on right ) is almost over 55 degrees East longitude which is the home of the satellite when it is in regular orbit. Also note that the orbit is not parellel to equator. It makes an angle of 6 degrees with equator. This ngle is called as 'Inclination' in technical parlance.

So the corrections that will be carried out will have to correct this inclination also to 0 degrees so that the satellite appears stationary from Earth. Now it will ascillate N-S  over equator by +/- 6 degrees.

08dec/0830 IST
As and when we get updates we will post the various steps.
08dec 1430 IST
ISRO has now raised the perigee..  when we get more details we will continue
09Dec2000 IST
As per details given :
2 Orbit raisings have been carried out:  1st 08Nov/0533 IST  LAM fired for 102min and  10 sec,
                                                                2nd 09Dec/1305 IST LAM fired for 36min and  43 sec,
                                                                3rd 10Dec/1115 IST LAM fired for 4 min and 49 sec.
                                                                 Solar Arreys and Antenna 1730 IST on 10th Dec
Some details are available and I am working on it ..  will post 10thDec morning ..  please hold on.

Fixed discussion is below ..  day to day running commentary is above

Geosynchronous Orbit ( we will use GSO as abbreviation ) is an orbit in which the satellite appears to be stationary over a fixed location on Earth.
We know that Earth rotates  on its N-S axis in about 24 hours so it follows that the GSO Satellite should also make one revolution around Earth in that period so that it remains stationary over a place on equator as that place makes one rotation around axis.

Suppose that there is an observer who is standing on equator. ( We are looking to Earth from a point above North pole.  ).  As the Earth rotates on its axis the observer who is fixed on the surface of the Earth also moves with it ( but he does not feel that he is moving ) and makes one revolution ( i.e.  by 360 degrees ) in 24 hours. Now suppose that we place a satellite above his head and make it to make one revolution around Earth in 24 hours then for every degree of rotation of Earth, both the observer and the satellite will move by 1 degree and so the satellite will still be above his head or in other words he will feel that the satellite is steadfastly above his head throughout the rotation of Earth.
Following figure shows four such situations.

A simple calculation using Kepler's laws ( Square of period is proportional to Cube of Radius ) gives a solution that for Earth's gravity the satellite kept at an altitude of 42164 Kms Radius has a revolution period same as that of Earth.
( Try the math and you will find that it does not give 24 hours but 23 Hrs 56 min and 4 sec  ..  why this discrepancy ..  we will cover it later )
But every book says that GSO satellites are 36000 Kms ..  yes it is true...  it is the 'Height' of the satellite from Earth's surface and Earth surface has a radius of about 6380 kms so 42164 kms radius is same as ( 42164 - 6380) kms from surafce = 35784 kms !!!

How does one keep the satellite at that height? Obviuosly using a rocket.
But the GSO satellites are made do do a lot of work and so are very heavy. Complex satellites can weigh nearly 10000 kgs and carriing such satellites all the way upto 36000 kms  directly with  rocket will require a very big rocket.
So scientists play a trick. They use the rocket to carry satellite only upto a few hundred kms  and release it. But during this release they place the satellite in an orbit which has the farthest point of orbit at a much higher altitude from which it returns back. This point is ( again Kepler ..  ) called an apogee.

We illustrate with a practical example..  that of an Indian communications satellite GSAT-16  which is planned to be positioned at 55 deg East longitude.. The satellite was made at ISRO facilities and it arrived in at launchpad on Oct22. Preparations for its launch began on 13th Nov and it was finally intgrated with the rocket Ariane-5 on 25th Nov.  This particular flight of Ariane  rocket is named as VA221.  The rocket carried 2 satellites ..  An American Satellite DIRECT TV - 14  and Indian satellite GSAT-16. The launchpad is located at  Kourou  in French Guiana.
Originally set to launch on 4thDec it was  launched on 6th Dec2014 ( 07Dec/0210 IST ) after one postponement due to bad weather. .  It released DIRECT TV satellite 27Min54Sec after flight ignition and released  GSAT after 32Min20Sec.

Ariane5 carried the satellite to 250 kms and released it in an orbit which has apogee of about 36000 kms above the surface of Earth.
Is it so????????
I have reproduced above the casual statement made by everyone ..  it is a misnomer!!
Let's face the facts:

Arian5  ( ECA version ) rocket was launched from Kourou launchpad in French Guyana. It is a two stage rocket with massive ( 2*Solid+ 1*Liquid ) engines as first stage. ( Refer Wiki Page for deatails ).  It carried 2 satellites to be placed in Geosynchronous Orbit ( GSO ), A US TV satellite DirecTV - 14  to be placed at 99 Deg W and India's GSAT-14 to be placed at 55deg E.

Arian5 ECS is designed to carry two satellites in a flight and both these satellites are placed in a capsule, technically called as a fairing. In case of this flight Arian5 carried this fairing to a height of    about 250 kms and releases it there in a geosynchronous transfer orbit ( GTO ) of 249.5 Kms * 35975 Kms with an inclination of 6 degrees with equator. The fairing ( called ESC  ..  a French acronym ) is now seperated from rocket but because it is already in a GTO, it starts moving in that orbit. Notice here that the satellites are not yet released but are also not attached to the rocket. 
Figure on left below shows the ground track path that the rocket had taken
It started from French Guyna launch site ( just below horizon pointed in top-left corner ) with a place mark and traveled along the diagonal shown by 4 points ESC, GTV, SYLD and GSAT-16.

The rocket actually acheived GTO at  250 Kms  height but it carried the satellites upto 690 Kms and released the capsule ( at point ESC ) 25 minutes from Launch,  with a velocity of 9.3 Kms /sec.

 Figure on right  shows the height aspect with  white lines.  The capsule was on its own with its own Liquid engine  travelling along the GTO. So it continued to rise along the GTO path. It changed attitudes several times in order to release the satellites.

Those maneuvers are shown in next figure.  Figure on right shows various changes in attitude that happened.

1. After the upper stage shutdown the capsule carrying two satellites changed attitude so that the satellites which were in front are now facing the Earth. This trick ensures that the satellite will not hit the capsule when it is released. This is shown in the top portion of figure on right.
DirecTV was released at 27:58 minutes at 1158 Kms ( point DTV in the figures above).

2. Between DirecTV and GSAT-16 there was an intermediate adaptor ( called SYLDA ..  a french acronym ) for mounting DirecTV on top of GSAT-16.
2 minutes after DirecTV was released this adapter  was released, exposing the GSAT-16.

3. Later the capsule again changed its attitude to releases GSAT-16  (32:20 minutes from launch)

4. The remaining part ( basically the engine )  reoriented attitude  and the engine was imparted a spin and is re-ignited so that the fuel is consumed fully. This was necessitated to make it  a passive object, The spin ensures that the debris will not wander out of orbit afterwards.

Total launch mission lasted for 51 minutes including passivization of engine.

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