Celestia Forums

I'm a Celestia noob (or rather, returning after many years) developing a scifi mod with a planet that has four geosynchronous satellites in equally spaced equatorial orbits.

I've got the objects in my SSC (largely I copied the text from the mod listing satellites orbiting Earth), and I've adjusted so their orbit is correct - but how do I space them out? Essentially I want them in geosynchronous orbits at 0, 90, 180 and 270 degrees of longitude. I think it's to do with the ArgOfPericenter or MeanAnomaly parameters but this is a bit beyond me. Sidereal orbit of the planet is 22 hours.

I have a similar problem with getting binary planets to behave - I have the orbits, but they aren't in the right places in their orbit.

Any suggestions?

There are several ways to do this, assuming the orbits are nearly circular and uninclined. AscendingNode, ArgOfPericenter and MeanAnomaly are angles which orientate the orbit around a central object and you could use every one of them to assign values of 0, 90, 180 and 270°. If you are interested in what these angles define, read: https://en.wikibooks.org/wiki/Celestia/Trajectories#Period or some definition on Wikipedia. It alsways helps to create a cluster of satellites and vary one element at a time to see what they really do (this will only become obvious if you do this with eccentric, inclined orbits).

In your case for the satellites I'd recommend to use the same orbital elements for all of them and distribute them by varying only MeanAnomaly. For binary objects it's always good to take a look at an example within Celestia to see that you need to shift "ArgOfPericenter" by 180° for one of the objects.

For a geostationary orbit, the simplest is to define the orbit with a fixed position relative to the planet.

To do this you must define
- A reference frame centered on the planet
- A fixed position indicating longitude, latitude and altitude of the satellite.

The corresponding instructions in the ssc file are:


There is only one geostationary orbit for a planet.
As it is in the plane of the equator, the latitude is 0
You can specify the longitude you want.

As the geostationary orbit is circular the altitude is constant.
Celestia lets you enter any altitude you want, but to be coherent with gravity, altitude depends on the mass of the planet.

It can be calculated with the following formula:



Where
h = altitude (in meters)
G = gravitational constant = 6.67 10^-11 N.m?.kg
M = mass of the planet (in kg)
R = planet radius (in meters)
T = sideral rotation period of the planet (in seconds)

Altitude must be in km in the ssc file.
Pay attention to units in the formula.