[X] My Assistant

[brellis]

It is often mentioned in various reports that a shuttle, Progress or Soyuz, and now Verne crafts boost the height of the ISS orbit, which loses altitude due to atmospheric drag. Does it also drift laterally?

Does the moon affect the shape of the orbit?


Thanks for any insight!

[brellis]

Thanks for that research, ugordan. I understand that the shape of the orbit is affected by irregularities in the shape (and perhaps the density) of the earth.

I'm curious if an effect such as atmospheric drag causes the craft to drift laterally in a random or abnormal way from its prescribed course. For example, if they let something float inside ISS, does it drift laterally due to the fact that ISS itself is actually drifting? I'm doing a quick search for articles myself.

Found a doc from research for NASA's MAXIM Pathifinder project, describing problems with lateral drift when using principles of interferometry in several orbiting scopes. Lateral drift -- all kinds of drift -- in LEO is way too much to allow the precise formation flying needed for synchronizing even two orbital craft.

I found other bits about "random micro-" this and that on studies performed in ISS and other craft.

After several minutes of armchair research, one may conclude that there is a tangible amount of random lateral drift on ISS, but I still wonder how they compensate for it.

I appreciate the insight from UMSFers!


And, many thanks for the pass, Mr. Ellison!

[dvandorn]

After several minutes of armchair research, one may conclude that there is a tangible amount of random lateral drift on ISS, but I still wonder how they compensate for it.

I think the real point is that they really don't compensate for it. So what if the orbit shifts laterally? The major lateral movement is the nodal regression, which is extremely well understood and can be predicted pretty well to the nth degree. Other vectors applied to *any* spacecraft in LEO (be they large and busy with lots of movement and contact, like ISS, or small, compact and quiescent) likely don't add up to even a few percent of the nodal regression effect. And remember, all of those random vectors applied to a spacecraft push it in all sorts of directions, not just those that would "push it sideways."

Of course, pretty much every spacecraft in LEO is being ground-tracked, all the time, so to an extent we don't need the ability to accurately predict the result of random orbital perturbations over periods of months or years. We always have the "ground truth" of the direct tracking to establish the exact orbital parameters of anything in LEO. So it's possible to just crank in the actual location numbers and use those for any positional-sensitive maneuvers (like rendezvous and, perhaps more important, debris avoidance).

Heck, even rendezvous maneuvers are forgiving of errors in estimated location of hundreds of meters. Once you begin your terminal phase, you end up using things like radar and lidar to establish exact values for relative positions and velocities, and any errors can be steered out as the rendezvous maneuvers reach their conclusion.

Final analysis -- there's no need compensate for lateral regression movements of spacecraft; as long as you can observe spacecraft directly, you can always correct your understanding of your position, at almost any time. And there is nothing that requires the ISS (or most any other spacecraft in LEO) to be at an exact location predicted more than a few days before.

What needs compensating, much moreso, is the loss of total orbital energy to friction with the extremely tenuous atmosphere present at 300 or so km. In this case, ISS sees a more profound effect than other spacecraft, since its solar panels present an awful lot of surface area. I know ISS is big and heavy by now, but the ratio of surface area to mass is still probably quite a bit higher than your average satellite in LEO. So, with more drag, it's slowed more quickly. Add to that the fact that the upper atmosphere "breathes," puffing out and snugging back under various circumstances, and you get a much more dynamic situation. Of course, that kind of orbital perturbation is slow enough that a need to give the station a boost can be identified in March and not be executed until November...

-the other Doug