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Kerbin Orbit projection


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hello guys,

I'm dealing with a couple contracts that demand for some gravitational survey on kerbin at very high altitude (>69K)

I have a probe I use for that needing, set it into 90° orbit at pretty low altitude (around 77K which is fitting all waypoint needings) and just wait. Same approach worked really fine on Mun but on Kerbin I immediately realized the planet rotation is definitely higher compared to the probe orbital time. 

On Mun you basically have just to wait for the waypoint to pass under the orbit track, worst case wait for the next body rotation, but on Kerbin things got extremely more complicated. 

Is there any way to calculate a specific lat/long to match my orbit? Is there any tool that would project my orbit track onto the planet surface taking account of planet rotation?

Do you have other very different ways to face this types of mission?

Thankyou in advance for sharing your opinions at regards, as usual,

 

 

 

 

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1 hour ago, Joe Kerbal said:

Is there any tool that would project my orbit track onto the planet surface taking account of planet rotation?

I know the ScanSat mod does it. I mean, it doesn't draw a dozen orbits forward, just one or so, but there is a ruler tool running along the equator that shows you the cadence of equator passes, so you can estimate whether or not the current orbit will result in passing over the target area at some point, as well as whether that point is close or far in time. The orbit projection and ruler tool update in real time if your engine is running, too.

Though when I last used it, these features were available only to spacecraft which had a scanner mounted on them, so they could call up the scan map (where these things would be drawn). So you'd need to launch a new spacecraft for the job.

Keep in mind however that it's been a good while since I last had it installed, and it only recently had a big update. The capabilities have likely been expanded since I last checked.

 

Edited by Streetwind
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1 hour ago, Joe Kerbal said:

Is there any way to calculate a specific lat/long to match my orbit?

At a minimum, your minimum inclination must be the same as the latitude of the waypoint, or else you'll never reach it.  There are some disadvantages to doing that rather than using a polar orbit:  you'll only touch the waypoint at the highest latitude, whereas a polar orbit gives two chances (one on each side of the planet).

The other thing to remember is that you need to account for orbital resonance.  Kerbin rotates once in six hours, so it rotates at one degree per minute.  Your orbital period is 31 minutes, which is very, very close to a resonance; your orbit track moves across the surface of Kerbin at a rate of one degree per orbit.  Of course, Kerbin itself rotates thirty degrees in that time, so to cover all of the planet, it's going to take a while.

1 hour ago, Joe Kerbal said:

Is there any tool that would project my orbit track onto the planet surface taking account of planet rotation?

There are none I know that will project your track onto the planet, but ScanSat will project it onto a map of the surface, which is of equivalent value.  Furthermore, it will project your equator crossings for the next 100 orbits and give you an idea of whether you're in a repetitive resonance or whether you'll actually cover the whole of the surface.

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If you'll look at this image that I pulled from the ScanSat Readme, you can see the blue and orange tick marks at the equator.  Blue ticks are crossings going north and orange ticks are crossings going south; the spread and even distribution shows that the orbit is not resonant.  If the ticks seem to bunch together into groups of five or six, then that indicates a strong resonance, and that means that you'll only ever cross the equator at those points; in other words, you'll pass over the same ground again and again without going to new territory.  That's great if your waypoint is on your orbital track, because you'll have many chances to get the right encounter and run your tests.  It's terrible otherwise.

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Once you're settled in to a circular orbit at the correct altitude, you can enter into an elliptical orbit, having its periapsis at the altitude and latitude needed for the survey, with a period such that you'll be in the correct spot next orbit.

Say for example the measurement needs to happen at 100°W longitude, 20°S latitude. You notice this orbit that you're crossing 20°S latitude at 0° longitude. So you know Kerbin needs to rotate 100° in the time it takes you to complete some whole number of orbits.

As the commenters above said, Kerbin rotates at 1°/min, so you need to complete an integer number of orbits in 100 minutes. You can then burn prograde at 20°S latitude while you're over 0° longitude so that your new orbit is 100/3, 100/2, or 100/1 minutes long. You can use the orbital period formula to calculate what your new apoapsis would need to be, or you can use the tools built into Kerbal Engineer and fiddle with the maneuver node until the post-burn orbital period is the desired length of time.

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3 hours ago, Joe Kerbal said:

Is there any tool that would project my orbit track onto the planet surface taking account of planet rotation?

Should be no surprise that the mod that allow to project your trajectory like that is called 

 

 

 

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3 hours ago, MrSystems said:

Once you're settled in to a circular orbit at the correct altitude, you can enter into an elliptical orbit, having its periapsis at the altitude and latitude needed for the survey, with a period such that you'll be in the correct spot next orbit.

 

I so love this answer because is fully numerical (and pretty much I need a pair of subraction and multiplication to get the proper answer).

Maybe I'll do a mess attempting but I just love the approach

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