ISS orbital "pattern"

[Ripley]

Hi all.
Last year, while sailing in the Mediterranean sea, I remember watching some ISS passes around 10/11 p.m.
Now I'm sailing again in the same places, and NASA app says (to my noobish surprise) that ISS passes occur much later, around 3/4 in the morning.

Which is the "repeatibility pattern", if such a thing exists?

[george7378]

Wherever you are, it alternates between evening - not visible - morning - not visible, and then the pattern repeats. It probably takes about two months to complete.

[Fabri91]

Yes, I've noticed that the sighting opportunities slowly move "earlier", with regard to the time of day.

[boogabooga]

Well, load up an ISS scenario in Orbiter and watch it for a few weeks of sim time. The Map MFD groundtrack will be helpful. That will do a better job explaining it than anyone on the forum.

[garyw]

Quote:

Originally Posted by Ripley

 Which is the "repeatibility pattern", if such a thing exists?

It depends on many things - especially if there is a reboost or orbital phase change for an upcoming mission.

I use http://heavens-above.com/ to display ISS and other satellite passes.

[Urwumpe]

Quote:

Originally Posted by Ripley

 Which is the "repeatibility pattern", if such a thing exists?

First of all, the biggest differences comes from the fact that a sidereal day (time until a distant star is at its apex again) is shorter than a tropical day (time until sun is at its apex again = noon), that we use for time keeping on Earth. The Earth simply needs to rotate 360/365.254° longer everyday for its motion around the sun for one day, compared to a distant reference. That means the ISS is about 10 minutes earlier everyday, if its orbit wouldn't drift but stay fixed in space.

The Orbit plane of the ISS also drifts because of Sun, Moon and Non-spherical gravity. The orbit height changes of the ISS also have their effect on how fast the ISS actually drifts.

So, by average drift in its average orbit height, you can deduce when the ISS will repeat its ground track pattern again: About every 180 days, if I used the right number (8 minutes per day)