### Author Topic: Plane change for satellites of a constellation  (Read 3029 times)

#### guckyfan

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##### Plane change for satellites of a constellation
« on: 06/19/2017 02:55 PM »
This subject confuses me a lot.

I understand inclination changes. But satellites of a constellation not only go to different inclinations but to multiple planes in each inclination. From what I read, people seem to imply that plane changes are difficult and delta-v consuming much like inclination changes. I don't understand why.

My mental picture says a satellite in an elliptic transfer orbit will quickly drift from one plane to another and can reach any inclination by waiting for the right moment to circularize. But discussions seem to imply that each satellite has to be launched not only into its inclination but right into its plane as well, if I understand correctly.

Can anyone explain or point me to a resource that gives a compehensible explanation?

Thanks.

#### envy887

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##### Re: Plane change for satellites of a constellation
« Reply #1 on: 06/19/2017 04:25 PM »
An orbital plane is defined not only by the inclination, but also by the longitude of the ascending node:
https://en.wikipedia.org/wiki/Orbital_plane_(astronomy)

An orbital satellite has angular momentum, which can be represented as a vector normal to the plane. Moving to a different plane with the same inclination (i.e. changing longitude of the ascending node) requires "turning" the plane, which reorients the angular momentum vector, which requires a torque. It's much like trying to reorient the axis of a spinning wheel or gyroscope.

It is possible to drift into different planes by using the torque imparted by the oblateness of the Earth, which causes a precession of the ascending node. This is how sun-synchronous satellites stay in the same orbit relative to the Sun: they use an orbit that precesses exactly once per year.

Precession does vary with orbital period, so a satellite in transfer will indeed be changing planes relative to one in a circular orbit, as you suspected. But it's a very slow process.
« Last Edit: 06/19/2017 04:25 PM by envy887 »

#### guckyfan

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##### Re: Plane change for satellites of a constellation
« Reply #2 on: 06/19/2017 05:54 PM »
I don't doubt that your explanation is correct. This means I have some fundamental flaw in my line of thinking and need to find it.

Thank you.

#### Jim

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##### Re: Plane change for satellites of a constellation
« Reply #3 on: 06/19/2017 06:15 PM »
I don't doubt that your explanation is correct. This means I have some fundamental flaw in my line of thinking and need to find it.

and this is it

My mental picture says a satellite in an elliptic transfer orbit will quickly drift from one plane to another

The orientation of the plane is fixed in inertial space and will require a delta v to move.  It can't be changed by phasing (by using different orbital periods).  You might be thinking in a earth fixed frame of reference.
« Last Edit: 06/19/2017 06:16 PM by Jim »

#### Lars-J

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##### Re: Plane change for satellites of a constellation
« Reply #4 on: 06/20/2017 09:39 PM »
The orientation of the plane is fixed in inertial space and will require a delta v to move.  It can't be changed by phasing (by using different orbital periods).  You might be thinking in a earth fixed frame of reference.

I'm foggy about this too... What you say is what I have always assumed: You launch into one plane and stay there until you perform an plane/inclination change maneuver which requires delta-v.

But then simply lowering or raising your orbit should NOT change your plane, right? Yet people keep mentioning how if you do that your plane will drift apart. Is this just due to perturbations in low LEO caused by Earth's gravitational field not being perfectly spherical (or the moon), or is there no such drift?

#### meberbs

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##### Re: Plane change for satellites of a constellation
« Reply #5 on: 06/20/2017 09:48 PM »
The orientation of the plane is fixed in inertial space and will require a delta v to move.  It can't be changed by phasing (by using different orbital periods).  You might be thinking in a earth fixed frame of reference.

I'm foggy about this too... What you say is what I have always assumed: You launch into one plane and stay there until you perform an plane/inclination change maneuver which requires delta-v.

But then simply lowering or raising your orbit should NOT change your plane, right? Yet people keep mentioning how if you do that your plane will drift apart. Is this just due to perturbations in low LEO caused by Earth's gravitational field not being perfectly spherical (or the moon), or is there no such drift?
The drift is (mostly) due to the Earth's oblateness. It is very slow (see the months for new Iridium satellites that are drifting between planes).

Jim's comment is correctly pointing out that there is no quick drift possible like guckyfan had been picturing, and there would be no drift at all without perturbations.

#### LastStarFighter

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##### Re: Plane change for satellites of a constellation
« Reply #6 on: 06/20/2017 10:28 PM »
This is a good explanation. Includes the equation so you can plug in different orbits and see the effects.

https://en.m.wikipedia.org/wiki/Nodal_precession

#### Danderman

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##### Re: Plane change for satellites of a constellation
« Reply #7 on: 08/15/2017 09:03 PM »

The orientation of the plane is fixed in inertial space and will require a delta v to move.  It can't be changed by phasing (by using different orbital periods).  You might be thinking in a earth fixed frame of reference.

An earth fixed frame of reference is very useful for designing satellite constellations.

And yes, by changing orbital period, it is possible for satellites to move from plane to plane. It just takes a lot of delta-V, or a lot of time.

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