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Am I missing something about Minmus axis?

Kermanzooming
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steuben

steuben

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to quote Kalileo "And yet it moves."  I don't think he really said it, but it makes a good closing line at the end of his trial.

Don't forget Minmus rotates under the satellite. Pics are blocked on this machine... but my rough guess is that the 35-40 degrees you're seeing is the rotation of Minmus relative to the fixed plane of the polar orbit. I'm sure somebody will bring in the math to support it.

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ElWanderer

ElWanderer

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47 minutes ago, steuben said:

to quote Kalileo "And yet it moves."  I don't think he really said it, but it makes a good closing line at the end of his trial.

Don't forget Minmus rotates under the satellite. Pics are blocked on this machine... but my rough guess is that the 35-40 degrees you're seeing is the rotation of Minmus relative to the fixed plane of the polar orbit. I'm sure somebody will bring in the math to support it.

I believe you have identified the cause, yes. In the images, the orbital inclination is 88°, but the orbital period is 5 days. According to the wiki, Minmus takes a little under 2 days to complete one revolution on its axis. :)

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Zophos

Zophos

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@steuben and @ElWanderer  are correct; SCANSat accounts for the rotation of the planet underneath the satellite. It's particularly pronounced at Minmus because the low gravity leads to slow orbits (giving the moon more time to rotate), but you can see the same effect around any body.

I find the information very useful when I'm trying to do precision landings. In the regular map view, it can be hard to tell when you've set your orbital inclination properly to bring you over the target.  But if you watch the SCANSat view, especially the zoom window, you can dial it in within a few meters from three-quarters of an orbit away.

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Pecan

Pecan

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The point of polar orbit for mapping is exactly this - the planet/moon rotates underneath you, presenting a different 'strip' of ground on each orbit.
If that wasn't the case then a polar orbit would be no better than an equatorial one - going over and over the same strip every time.

Be warned - just because the 'optimal' altitude is given for a particular scanner that might not be the optimal altitude for an orbit around every body.  Guess what happens if your polar orbit takes exactly the same time as the planet rotates (a <body>-synchronous but not -stationary orbit)?  You get the same strip again and again.  How about half the rotation period?  Twice?  Avoid harmonics.

Because there is a wide-ish Field Of View (FOV) on the SCANsat instruments you don't have to be hyper-sensitive to this but as you travel the solar system you might just find it becomes significant somewhere.

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Hydrothermalventclam

Hydrothermalventclam

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On 8/2/2016 at 0:45 PM, Kermanzooming said:

Hmmm... And specially when doing the ScanSat hi-res scan, that's optimal at a 800 Km orbit.

Thanks folks!

I usually put my scanners in a 250 km orbit, because that is the optimal altitude for the multi spectral scanner, which I generally also have on my scansats.  Minimus is so small, that even at 250 km, the scanner scans a pretty big swath of ground, and there is much less problems with the planet moving.

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Kermanzooming

Kermanzooming

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11 hours ago, NathanMcGuire30 said:

I usually put my scanners in a 250 km orbit, because that is the optimal altitude for the multi spectral scanner, which I generally also have on my scansats.

Yeah, I was doing the hi-res scan although, that's why I went so high. By the way, do you know how to get a zoomed map of the ore concentration? When I zoom, it's only biome, slope or altitude.

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Hydrothermalventclam

Hydrothermalventclam

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If you use the large map, I think that there is a option to turn on ore viewing, and then you can zoom.  Not really sure.  I always use the stock ore scanner, so I don't use scansat for viewing ore.  I probably should learn though...  Anyway, the scansat page probably has more information.

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Geschosskopf

Geschosskopf

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On 8/2/2016 at 7:06 AM, Kermanzooming said:

Can't understand this; I have a probe scanning Minmus on a nearly perfect polar orbit (see MechJeb window)

It's not being in a circular orbit that matters, it's being in a NON-synchronous orbit.  You're in a synchronous orbit, which means your ship's orbital period is some multiple or even fraction of a Minmus "day".  Therefore, you keep going over the same ground.

To fix this, turn either prograde or retrograde, your choice, then open the SCANBsat Big Map.  Now make a teeny tiny burn (use RCS if you've got it) until the hash marks on the Big Map don't match up with each other along the equator.

FWIW, drawing on earlier work in a post that no longer seems to exist (it was for ISA MapSat, which was an ancient mod before SCANsat), I made a table of good non-synchronous altitudes for the various SCANsat instruments (and the stock M700 set via SCANsat to map resources) at all the bodies in the stock system.  But this table is really not necessary because the altitudes are pretty much the same.  Just get in the ballpark then use the Big Map orbit-tweaking described above get non-synchronous.

"Low" altitude:  250-265km.  Used by low-res radar, biome scanner, and M700 with SCANsat settings.  The M700 with SCANsat settings says it works best at 150km but it works just fine at 260.

"High" altitude:  760-770km.  Used by high-res radar.

NOTE:  Some moons are so small, their SOIs ain't wide enough for 1 or both of these values, so in those cases, get out as far as the SOI allows.

NOTE:  Inclination should usually not be exactly 90^.  With the wide-angle stuff used at "Low" altitude, 86^ is fine and in fact advantageous.  With the high-res radar, you want about 88^.

NOTE:  I didn't include the NBS in the "Low" altitude list because I find it pretty much useless to use in orbit.

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