Compensate for Kerbin rotation ?

[Buisson]

Hello !

I have to achieve a mission asking me to take a multi spectral image log above some place on Kerbin. So I am puting myself on the right orbit to be on top of that place. However, as every kerbal knows, Kerbin rotates so, by the time I get on the predicted place on the orbit, I won't be on top of the location anymore.

Is there a way to know how much do I need to compensate my orbit to be on top of the target location ? Maybe a mod or some calculations ? 

Thank you for your help

[Mastikator]

Polar orbit should get you 100% coverage.

[Fluburtur]

You could probably try to go to a geosynch orbit, however a geosynch orbit is always equatorial but given the altitude you should get a lot of coverage, unless you have a specified altitude.

[Vaporized Steel]

Sure you have enough Delta V for course corrections after you put it in orbit right?

First go into a polar orbit. Keep orbiting until your target intersects that of your orbit. If the target happens to be nearer to the equator it might be that the target moves to much because of kerbins rotation by the time you get there.

So when you seem to intersect close to your target, Quiksave.
If you happen to miss your target where the mission system doesn't detect you are near enough, reload and then use a normal or antinormal burn to correct your trajectory in the direction necessary. Best to do this one quarter of a orbit away from the latitude of where you'll meet your target to save as much Delta V as possible.

[Snark]

Moving to Gameplay Questions.

If you'd like to take the math-and-old-fashioned-eyeball route:

Kerbin rotates once every six hours, i.e. 60 degrees per hour.  A ship orbiting in LKO takes roughly half an hour to go around once, so Kerbin will rotate roughly 30 degrees per orbit.

So when you're setting up your orbit, do it so that your orbital track passes east of the target, by roughly 30 degrees of longitude times the fraction-of-an-orbit to get to that spot.

You're unlikely to nail it perfectly, of course, so you'll need to do some corrections along the way, but that'll get you in the ballpark.  As for making those corrections, there are two places that are good to do it.

The first good opportunity is when you're 90 degrees, i.e. a quarter-orbit, before you get to the destination.  (That's because this is the point at which you get the most bang for your buck, i.e. the greatest amount of sideways adjustment to your orbital path over the target for a given amount of dV expended.)  So when you're about 90 degrees out, do a normal/antinormal burn to put your path-over-the-target at around 8 degrees of longitude (or slightly less) east of the target.

The last good opportunity is when you're getting pretty close to the target, so you can think in more Cartesian fashion without worrying about Coriolis effects too much, and just pretend you're not in a rotating reference frame.  When you're getting relatively close to the target, put your navball into surface mode, do a roll until the horizon looks level (sky above, ground below) on the navball), then compare your prograde/retrograde velocity markers to the target marker on the navball.  If it looks like you're pointing right of the target, do an orbit-normal burn to adjust.  If it looks like you're pointing left of the target, do an orbit-antinormal burn.  Once you've got it lined up so that the target marker is directly below (nadirwards) your orbit-prograde marker (again, while the navball is in surface mode), you've got it lined up perfectly and all you have to do is wait until the target marker hits the nadir on the navball.

[Zhetaan]

11 hours ago, Fluburtur said:

You could probably try to go to a geosynch orbit, however a geosynch orbit is always equatorial [snip]

Not to be unnecessarily picky, but you're describing a geostationary orbit.  Geosynchronous orbit always remains at the same longitude; geostationary orbit always remains at the same point.

Snark has the best approach for a direct rendezvous with your target point; there are few problems in KSP that mathematics won't solve.  If you miss your mark, however, then the second-best option is to get the altitude right and go for a circular polar orbit.  In this fashion, you're just about guaranteed to get your objective within three hours (a six hour day plus the orbit going round the back of Kerbin means you make two tracks over the ground).

The only way this won't happen is if your target window is very small and Kerbin rotates too far each time you pass near.  In that case, your problem is orbital resonance:  a half-hour orbit on Kerbin's six-hour day is a simple integer ratio that means you exactly repeat your ground track every twelve orbits (ground track in this sense means a unique orbital path as a projection onto Kerbin's surface; it has nothing whatever to do with rovers or other ground vehicles).  If none of those twelve is close enough, you'll never get where you need to go.  The solution then is to destroy the resonance, which you can do by modifying inclination or eccentricity.  Each of these has a drawback:  changing inclination changes your ground track at the cost of missing certain latitudes; changing eccentricity changes your ground track at the cost of missing certain altitudes.

Edited June 23, 2016 by Zhetaan

[bewing]

I would strongly recommend not doing geostationary, geosynch, or polar. I would also recommend not trying to modify your current orbit to force the intercept to happen early.

The best answer is to create an orbit that is inclined just enough to get you to your target latitude.

An inclined orbit gives you a sine wave of latitudes, above and below the equator. The more inclined the orbit is as you pass over a particular latitude, the less time you will spend at that latitude. So if your orbit is inclined, say, 12 degrees -- then you will spend a lot of time at +11 to 12 degrees and a lot of time at -11 to -12 degrees, and not much time in between. You can try to be really precise about the timing and positioning and all -- but if you miss, you've blown your opportunity for many orbits, so I woudn't recommend that, either.

So, start with an equatorial orbit, and as you are passing above or below the target point, burn normal or antinormal to incline your orbit just enough so that it looks like the highest point in the orbit will be a little higher than the target latitude. This will be 90 degrees ahead of your target -- so your target will be there in 3 orbits = 90 minutes = 90 degrees. And since you will be spending a lot of time over that latitude, you don't need to sweat the timing.

 

[monstah]

Or, since you've opened up for mods, use SCANSat, on the big map: there is an orbital display there which is relative to the surface. Remember what Snark said about the best place to correct your trajectory: 90 degrees before your target. Orbit until you see, on the map, the prediciton for the next orbit getting near your target, and do the final adjustment 90 degrees before

[Buisson]

Hello everyone and thank you for your awesome answers !

I was doing my correction at 90° / 270° from the latitude I wanted to be. That seemed to work. However, I really like when I am doing calculations to get perfectly to where I want to be. That is why I am not a big fan of the eyeball technique. But it shure works !  I am sure I can get a good prediction with KER.

I didn't knew the technique from Snark with the surface mod, I am quite excited to try this !

I will tell you if I manage to find a way to achieve this precision.

 

Thanks again and have fun with this amazing simulator !

Edited June 23, 2016 by Buisson