Flight Question Get to GEO at desired location

[lowerlowerhk]

Getting to geostationary orbit is not difficult, but i can't control where will i be flying above when orbit insertion is completed. How can I do so in Orbiter and how do real satellites do it?

 

[cjp]

I guess you can do it by first entering an 'almost-GEO', with an apogee on the GEO, and a perigee slightly below. With such an orbit, your satellite will drift slowly. When you approach the desired position, you slow down the drift with a prograde burn at apogee.

You can also drift in the opposite direction by having your apogee above the GEO. As you never have an EXACT GEO, you'll probably need both to stay at the desired position. The 'lower perigee' version requires less fuel when you start from a GTO.

 

[zerofay32]

One way it is done is to enter a near circular "parking orbit" in LEO and when you are 180 degrees from the area you want to be over, you then perform a Prograde burn. This will place apogee over the target area. Then all you need to do is circularize your orbit at apogee.

Zerofay32

 

[Turbinator]

The way you would do it in real life is by finding the exact opposite side of the earth, over witch you want to stay. Then once you are on the opposite side, you burn your GEO insertion burn. And set the apogee in the GEO right above the spot you want to hover above. Once you reach the apogee, you raise your perigee to GEO as well, and that's it.

 

[Stinger]

GEO

One way it is done is to enter a near circular "parking orbit" in LEO and when you are 180 degrees from the area you want to be over, you then perform a Prograde burn. This will place apogee over the target area. Then all you need to do is circularize your orbit at apogee.
Zerofay32

Almost right. :tiphat:
But you have to take Earth's rotation into account.

1) Find out your orbital period (T) for the transfer-orbit! I just tried it with PeR=6600 and ApR=42000. -Makes 37780 Seconds for one orbit. -For the transfer from Perigee to Apogee you need half the time (180°):
37780/2=18890 Seconds.

2) Earth rotates once every 86164.09 Seconds (23h 56min 4.09s).
Thus: 360°/86164.09*18890=78.9°
That means your target on Earth's surface moves on 78.9° while your spacecraft does the 180° transfer from LEO to GEO.

3) 180°-78.9°=101.1°
So, do the Prograde-burn 101.1° BEFORE you pass over your target-area. -Burn prograde until ApR=42157. This way your Apogee should be right above your target-longitude. Once you reach Apogee, burn prograde again to lift your Perigee to the same Altitude (or until Ecc=0.0000 ;-) ).

e.g.: Astra1-Satellite (19.2° East) -> Burn at 81.9° West

Cheers

 

[Turbinator]

Isn't it supposte to be [math]180^{\circ} + 78.9^{\circ} = 258.9^{\circ}[/math] ?
Because you are moving prograde relative to Earth's rotation.

If you burn 78.9° earlier, rather than 78.9° later, wouldn't you come 157.8° to short?


In order to have a motionless footprint on the surface of the Earth, true GEO orbit is at exactly 35,786 kilometres above earth sea level, directly above the equator (0° latitude). Orbital speed is at 3.0746 km/s with an orbital period of 86,160 seconds. Ecc of course needs to be 0.






.

 

[Stinger]

Isn't it supposte to be [math]180^{\circ} + 78.9^{\circ} = 258.9^{\circ}[/math] ?
Because you are moving prograde relative to Earth's rotation.

If you burn 78.9° earlier, rather than 78.9° later, wouldn't you come 157.8° to short?

In order to have a motionless footprint on the surface of the Earth, true GEO orbit is at exactly 35,786 kilometres above earth sea level, directly above the equator (0° latitude). Orbital speed is at 3.0746 km/s with an orbital period of 86,160 seconds. Ecc of course needs to be 0.

I don't think so.
Since your ship and your target do move in the same direction. (from West to East)

Imagine a watch.
You want to reach Apoapsis at the 6 o'clock-position.
-With your ship you start the transfer (180°) at the 12 o'clock-position moving counterclockwise to the 6 o'clock-position.
-At the time you start the prograde-burn at the 12 o'clock-position, your target-longitude is at ~ 8:40 o'clock moving also counterclockwise towards 6 o'clock. (78.9° difference)
The time needed for the target-latitude to move from 8:40 down to 6 o'clock is equal to the time the spacecraft needs to get from 12 (counterclockwise) to 6 o'clock.

So you start your burn 101.1° (distance between 12 o'clock and ~8:40) before you would fly over the target-longitude.



Just try it! :thumbup:

I'll try it tomorrow...


Part2: "35,786 kilometres above earth sea level" Right!
35,786 + Earth's radius (6,370km) = 42,156km (SemiMajorAxis)
:hello:


Goodnight!
:zzz:

 

[Turbinator]

I am having a real problem finding a detailed, digital, easy to work with map of the Earth.

[edit]

Boy was I wrong regarding that last point;

Google Earth is by far the best source for this, just enable the Grid View, Ctrl+L. And change the coordinate system to decimal.
It is perfect for this. It's beyond perfect, it's exactly what we need.


I am going to try and place a satelite right above this road, directly above it, straight up.
It's perfect because the road crosses the equator, and is really near the shore, so it's easy to see in Orbiter.

Coordinates:
lat -0.000000°
lon -80.090129°

Opposite side is at:
lon +99.909871°
Funny, it actually happened to also be right next to a shore.

Burn location:
lon +178.909871
Happened to be right above another road, in Congo.

I recommend you use this tool for working with Google Earth coordinates:
Google Earth Position 1.12
http://code.google.com/p/googleearth-autohotkey/

Can someone check my numbers?
Adding and subtracting longitude and latitude gets very confusing after a while.






---------- Post added Feb 3rd, 2010 at 03:49 AM ---------- Previous post was Feb 2nd, 2010 at 08:47 PM ----------


Allright, I finally did it. Using Stinger's calculations and an XR-2 I get very close to the target spot on the equator. However, I have to say that the orbital information available in Orbiter (stock MFD's) makes it extremely hard to keep track of my Equator relative inclination, and my launch azimuth. Because of this I had to rely on the scenario editor to get my orbital elements, in order to get myself in the right orbit. Otherwise using the stock MFD's would always produce a slight equatorial drift, and knock me out of the GEO sweet-spot.

It took me an entire hour and a half, and some 100+ correction burns and infinite fuel enabled. But I finally did it, however I drifted far away from my Ecuador GEO mark, and ended up directly over Congo, in Africa. However, this was due to the many correction burns, because I was essentially flying blind.

God, I love Orbiter. Where else could you do something like this? To place all your theories, tough experiments, and calculations right in to action. Without wasting a single $ billion.

:thumbup:


Here is a screen cap of my final Orbital Elements, both the default Orbit MFD that uses the ecliptic frame of reference, and the scenario editor's equatorial ref, and the surface MFD:

 

[ex-orbinaut]

...God, I love Orbiter. Where else could you do something like this? To place all your theories, tough experiments, and calculations right in to action. Without wasting a single $ billion.

Agreed. Played around with these orbits myself a few months ago, attempting to recreate the Sirius and XM satellite orbits (albeit in a DG and ShuttleA). Now there's a challenge / project for Orbiter.

Real hardcore stuff....

http://www.heavens-above.com/orbit.aspx?satid=26390&lat=0&lng=0&loc=Unspecified&alt=0&tz=CET

http://www.heavens-above.com/orbit.aspx?satid=26761&lat=0&lng=0&loc=Unspecified&alt=0&tz=CET

http://spaceflightnow.com/proton/siriusfm5/