Geostationary Orbit

[RocketScientistsSon]

How do you get a geostationary orbit?

From my understanding this is when a satellite orbits the earth (or Kerbin in this case) in the same position relative to the earth. Thats a horrible explanation I know. So any help or guidance on how to do this would be much appreciated!

Douglas

Edited September 25, 2013 by RocketScientistsSon

[Vilheim]

In order to do this, the spacecraft's angular velocity must be equal to the earth's angular velocity. The orbit at which this works for Kerbin is a circular orbit (or as close as you can get!) at 2,868.75 km above the surface of Kerbin, at a speed of 1009.0 m/s

[Anton P. Nym]

A geostationary orbit is a circular (and equatorial) orbit with a period exactly the same as the length of the planet's day. On Kerbin, that's 6 hours... which you can get with a circular, equatorial orbit of 2868.75km according to the Wiki.

Some planets don't have a geostationary orbit, as their sphere of influence isn't big enough. (Could be due to low mass, slow rotation, or close to a bigger planet.)

-- Steve

[rodion]

A GSO is simply a circular, equatorial orbit at a certain altitude, different for each body, where the orbital and rotational velocities are matched so that the same face of the body being orbited is presented to the spacecraft at all times - so if you drew an imaginary point on the surface of the body, that point would appear to always be in the same place from orbit, whatever time you looked - and similarly, from the ground, the satellite would appear to hang motionless in the sky.

Here's a Wikipedia animation of two bodies in GSO:

The KSP Wiki gives the altitude your spacecraft needs to be at to be in a GSO for all bodies in the game. Some bodies don't have one, though, due to the limited sphere of influence the game gives them.

[Matt_Sharkey]

TL;DR 2 868.75 km

You're on the right track. A Geo-Stationary Orbit is one where the time for one orbit is the same as the time for one sidereal day; 6 hours in Kerbin's case. When your orbit is at 0 degrees inclination (around the equator) then the satellite appear to stay in place with the Planet, however you cannot achieve this at any other spot than the equator. If you think about how you would stay stationary over the north or south pole, this quickly becomes obvious. Each planet is different in their geostationary position; and it depends on 2 variables the mass of the planet (and effectively the gravitational field) and how fast it rotates.

Information on various geostationary orbits can be found on the KSP Wiki I suggest you look here in the future for any other bits of info.

[Meithan]

So, in order to get there, you just perform a standard Hohmann transfer from your initial orbit to one with an altitude of 2868.75 km. The KSP Wiki even has an article with a table listing the required delta-v for the two burns, depending on your starting orbit.

[ArmchairGravy]

I highly recommend putting on RCS thrusters. Trying to get that exact speed without them...

[Meithan]

I highly recommend putting on RCS thrusters. Trying to get that exact speed without them...

I just learned about that last night while spending a good hour trying (and failing) to get the altitude just right with the ship's main engine. RCS thrusters are a tremendous help.

[Arbolus]

Higher orbits have slower orbital speeds. Get high enough and eventually you'll reach a point where the satellite's orbital angular velocity is equal to Kerbin's rotational angular velocity.

[RuBisCO]

Just go to an orbit where your surface speed is zero. Its not as tricky as it sounds the kerbal wiki (and people above) tells you already where it is, but remember you need to have a perfect 0 degree plane alignment as well, use a tiny engine for these very small adjustments, or RCS (someone suggest that already). Mechjeb can get you almost all the way there but no matter what your still going to need to do some manual orbital tuning to get as close to perfection as possible. This is going to require looking at your surface speed and providing thrust prograde, retrograde, north and south pole until it is zero, time warping a bit until the speed is not zero and repeat, be smart about it if you need to do more then a few d/v your doing it wrong.

One thing I like about Kerstationary Orbit is that its a great place to do max speed time warps, the skymap is wizing around and yet the planet below is perfectly still!

[SkyRender]

Some small notes on synchronous orbits in general...

What really matters in a synchronous orbit is your period: the time it takes to go from a given -apsis back to it. It just so happens that a circular orbit of 2,868,750m or so has an orbital period of 6 hours... which is exactly how long a sidereal day is on Kerbin.

The main difference caused by not having perfectly matched apses and 0.0 degrees of inclination is that the planet will appear to wobble, have a back-and-forth rocking motion, and/or draw closer or move further away as you orbit. Generally the land below you will still remain stationary below you as long as your period is monosynchronous (ie. for Kerbin, a 6-hour orbit), but the more off you are from ideal conditions, the worse the wobble, rock, and approach/move away effects will appear. If you watch a time-lapse of our own moon, you'll see that it too is not in a perfectly synchronous orbit and does in fact wobble, rock, and approach/move away over time.

[Astral_Nomad]

Here ya go... a number of video tutorials on it..

Scott Manley to the rescue again.. lol:

[RocketScientistsSon]

Thanks for the help all. Now I gotta start playing around with RCS thursters. I havent given them much look because I have mainly been dealing with smaller rockets and probes. Any guidance on that would be helpful!

Also, is your 'ground speed' the velocity above the navball or is that somewhere else?

[Meithan]

Thanks for the help all. Now I gotta start playing around with RCS thursters. I havent given them much look because I have mainly been dealing with smaller rockets and probes. Any guidance on that would be helpful!

Just put two RCS thruster blocks symmetrically around your ship's center of mass (you can see your ship's center of mass in the Vehicle Assembly Building by clicking on the button in the lower left, below the parts). Then, for precise corrections of your orbit, shutdown your main engine (I've accidently hit SHIFT many times, messing things up), and hit R to activate the RCS. You'll see a green light show up on the nav ball. Now point your ship towards the prograde marker and use the key H to thrust forward using RCS and key N to thrust backward. That way you don't have to turn your ship around, just use those two keys until you get exactly the orbit you want.

Also, is your 'ground speed' the velocity above the navball or is that somewhere else?

Click on the navball's speed reading and you'll see it toggle between "surface" and "orbit". "Surface" indicates your speed relative to the ground (that is, it corrects for Kerbin's rotation).

[Stone Blue]

Click on the navball's speed reading and you'll see it toggle between "surface" and "orbit". "Surface" indicates your speed relative to the ground (that is, it corrects for Kerbin's rotation).

It can show a third speed, also...Especially helpfull for docking. Anytime you have a "target" set, it will also show relative speed between your ship and the target.

[RocketScientistsSon]

so i can only get in to a Kerbostationary orbit at 2868.75 m. Which is when my angular velocity is 0.

Does it matter if its on a polar or equatorial orbit or any in between?

[Anton P. Nym]

so i can only get in to a Kerbostationary orbit at 2868.75 m. Which is when my angular velocity is 0.

Does it matter if its on a polar or equatorial orbit or any in between?

It must be equatorial to be a true geostationary "Clarke orbit".

-- Steve

[RuBisCO]

The further the orbit is from a plane of zero the more the satellite will appear to precess in the sky all the way to the extreme cases of polar orbiting satellites that fly over the same path on the ground once a day. Why not put a satellite in 6 hour geosynch orbit and change its inclination as well as make it non-circular and look down at kerbin at high time warp, record the pattern or trace the path the satellite appears to cover along the ground. I'm going to guess that adding inclination will cause precession along the longitude, while adding inclination will cause precession along the latitude and the two combined could make kerbin's surface appear to move in circles or even figure eights.

Edited September 27, 2013 by RuBisCO
Auto spell check can't be trusted sometime

[Rhino516]

If you have any interest in the math behind calculating geostationary orbits wikipedia has a decent entry. http://en.wikipedia.org/wiki/Geostationary_orbit

[Linear]

...Your orbit just needs to have a 6 hour period.

[Red Iron Crown]

I highly recommend putting on RCS thrusters. Trying to get that exact speed without them...

The ion engines work well for this, too. Don't even need a big solar array as the burns are short enough for batteries to cover it.

...Your orbit just needs to have a 6 hour period.

Not sufficient for geosynchonicity. Orbit must be circular and equatorial as well.

[juvilado]

Thats very interesting, but can anyone tell us any easy formula to calculate the stationary Orbit of any planet given? I dont find it anywhere

[BudgetHedgehog]

Not sufficient for geosynchonicity. Orbit must be circular and equatorial as well.

Well.. if you want a perfect geostationary orbit, yes. Geostationary is a special type of geosynchronous in that it stays above the exact place on the planet (i.e. no inclination or eccentricity). Geosynchronous means that the orbital period is equal to to the sidereal day. But there are other options, like Molniya or Tundra orbits, that work just as well in KSP. When I get RT set up in 0.24, I want to try them.

Edited April 19, 2014 by ObsessedWithKSP

[BudgetHedgehog]

Thats very interesting, but can anyone tell us any easy formula to calculate the stationary Orbit of any planet given? I dont find it anywhere

For absolutely ANY planet imaginable, you'd need to know the sidereal day, gravitational constant and all that. For any planet/moon in KSP, the info is all on the KSP wiki. (EDIT: Side note - some geosynchronous orbits lie outside the bodies SoI so it's not possible and semi-synchronous orbits are needed instead)

Edited April 19, 2014 by ObsessedWithKSP

[Yasmy]

ObsessedWithKSP gave you the correct practical answer. Geosynchronous orbits are in the wiki.

None the less, you asked for a formula, so here goes:

For a circular orbit at radius r, the velocity is v = sqrt(mu/r). The circumference of the orbit is C = 2 pi r.

Thus the period of the orbit is T = C / v = 2 pi r / sqrt(mu/r) = 2 pi sqrt(mu) r^3/2.

This is Kepler's Third Law: The square of the period is proportional to the cube of the semi-major axis. For a circular orbit, the semi-major axis equals the radius.

So find r such that your period T equals the planet's sidereal day: r = (T² / 4 pi² mu)^1/3

Finally, KSP shows you altitude above the nominal radius of the body, r_b. So you have to set your altitude to r - r_b.