Flight Question Achieving GPS Orbits

[PETC]

Hey Guys,

The things I understand:

1. GPS satellites orbit on six separate planes, all inclined at 55 degrees relative to the equator.

2. The Planes are inclined(?) relative to each other at increments of 60 degrees.

3. The GPS satellites orbit in a circular semi-sidereal 12-hour orbital period.

The things I don't understand:

1. How can an orbital plane be inclined relative to another orbital plane without changing its inclination relative to the equator?

2. How do you set these orbital parameters at launch?

3. If you don't get the launch perfect, where do you burn and in what orientation to rotate the orbital plane?

I made this diagram to help me understand it a little better, but aside from being able to make this in a 3D program, I still am having problems visualizing how to accomplish this goal.

Thanks guys, please let me know if I've forgot to include important pieces of the puzzle that I don't quite understand yet.

-Gary

 

[Urwumpe]

1. You have two angles for defining the orientation of the orbit plane in space. Inclination and Longitude of the ascending node (LAN).

All GPS satellites have the same inclination, but the LAN varies.

2. Yes, by the time when you launch and the direction, in which you launch. LAN is defined by the time, inclination by the latitude of the launch site and the launch azimuth.

3. Like always, where your current orbit plane and your target orbit plane intersect.

 

[PETC]

Thank you, I had the thought that the time of the launch was related to all of this right as I was drifting off to sleep last night thinking about this. I'm glad I was on to something!

 

[csanders]

For imagining, start simple.

Think 2 satellites with an inclination of 90. These satellites will be in polar orbits.

Now imagine one satellite is over the equator at 0 longitude, while the other is over the equator at 90 longitude (both heading north).

Their orbital planes will look something like this:

Two orbits with 90 degree inclination and a 90 degree relative inclination.

Now imagine one satellite is over the equator at 0 longitude, while the other is over the equator at 180 longitude (both heading north). They will share the same plane, but will be going in opposite directions. Two orbits with 90 degree inclination and a 180 degree relative inclination.

 

[PETC]

So then, if I wanted multiple satellites in the same plane (like the real system has) I could essentially find all of the launch windows for each of the six planes and then only launch in those windows to send my desired satellite into my desired plane, right?

 

[csanders]

So then, if I wanted multiple satellites in the same plane (like the real system has) I could essentially find all of the launch windows for each of the six planes and then only launch in those windows to send my desired satellite into my desired plane, right?

Yep.

LaunchMFD is great for that.
[ame="http://orbithangar.com/searchid.php?ID=2802"]Launch MFD - v. 1.5.5 for Orbiter 2010[/ame]

When you press "tgt" (target), you can enter inclination and LAN, so the 6 values would be something like (I'm not sure about their actual LAN, so I will assume they start at 0):

55 0
55 60
55 120
55 180
55 240
55 300

 

[boogabooga]

Careful!
LAN in Launch MFD is in the ECLIPTIC frame only. Probably not what you want.

 

[csanders]

Careful!
LAN in Launch MFD is in the ECLIPTIC frame only. Probably not what you want.

Good point. I didn't consider that. Is there an easy conversion?

 

[Enjo]

Not at my end, but I could add an option for that.

 

[Enjo]

Try the attached version and let me know how it works.
To enter equatorial inclination and LAN, you need to enter one of these examples:

55 0 1
55 60 1
55 120 1
55 180 1
55 240 1
55 300 1

Remember to wait on the pad for the off-plane correction to function, or the LAN target won't be achieved.