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General Discussion => Q&A Section => Topic started by: Robert Thompson on 11/10/2020 12:26 am

Title: Does ISS compensate for mascons or quadrupole
Post by: Robert Thompson on 11/10/2020 12:26 am

I will have zero idea where I heard this.
I heard that the ISS' orbit is affected by the Earth's quadrupole.
I don't know if effects cancel themselves out, and it's "only interesting", or, if effects require active compensation of some form (in navigation, telemetry, something else).
I'm not asking for the ISS - I'm asking for Hubble.
I am trying to see if a phenomenon that can affect ISS, which is N * Hubble's Mass, can in theory affect Hubble.
... which can affect Hubble's pointing accuracy.
... which is expressly the only thing this is all about, but I'm using ISS as a proxy for the effect.

(And, relatedly, turning over every stone, does ISS register earth mascons in its orbit or telemetry?)
That's it.

Thank you

Title: Re: Does ISS compensate for mascons or quadrupole
Post by: Proponent on 11/10/2020 12:07 pm

As we all know, Earth is not perfectly spherical, and therefore its gravitational field is not perfectly spherical either.  The most obvious departure from spherical symmetry is the equatorial bulge.  The bulge is the principal cause of orbital precession (regression of the nodes), meaning that plane of an orbit tends to rotate around Earth's axis.

Quadrupole moments (there is more than one of them) are a way of quantifying the departure from spherical symmetry.  So, you might say that the quadrupole moment causes precession.

That means that if you want to predict Hubble's or ISS's location, you need to take the quadrupole moment into account.  The further you are from Earth, the less the lack of perfect spherical symmetry matters, so the quadrupole moment has less effect on Hubble's orbit than on ISS's.

If you're planning observations with Hubble, you need to know accurately where it is at any given time, but the quadrupole moment doesn't really affect pointing accuracy itself.

ISS cannot "register" irregularities in Earth's gravitational field with on-board instruments.  As far as ISS is concerned, it is just falling.  However, careful tracking of ISS or other satellites can reveal the irregularities.