The Keostationary Orbit Anomaly

[Leszek]

So, when it comes to Keostationary orbits, I know what I am doing.  I get my orbits down to <.01 of a second above 6 hours.  Using a resonating transfer orbit I have been able to put multiple satellites into perfect formations with so little difference in orbital time between them that the formation isn't noticeably distorted even after many game years has passed.  This isn't a problem for me and I have been doing it since version .25.

I did it just last career.

Then I started a new career and put a satellite into KSO just ahead of KSP.  Even though it was just one satellite and not a formation I set it to 6 hours and 0.033 seconds.  This should be just fine for the rest of the career.  One trip to just outside of the Kerbal SOI and back and it is noticeably behind KSC.  I jump to the satellite and it still shows 6 hours and 0.05 seconds. (The time normally fluctuates a bit when you are that precise.)  So I figured I just remembered its position wrong, since I put so many up all the time perhaps I am just confused with another career.  But now with my eye watching it, I can see it lagging farther and farther behind.  I set the orbital time to 5:55 and let it catch up so that it is right over KSC again.  Again I set it to 6 hours and 0.01 seconds.  Again I notice it is lagging behind the KSC after just a few short orbits.

The only mod I added since last time is Pilot Assistant, but I don't see how that would make any difference.

Any ideas what might be causing this anomaly?

[Taki117]

This isn't actually an anomaly, it's call a Analemma.  Basically what's happening is while your orbital period might be fine, your inclination is probably off (This is only a guess as I do not know the exact orbital parameters) also, your AP and Pe could be slightly off from the ideal (despite the orbital period being nearly perfect) what this is going to cause is your satellite to appear to move relative to your target.  because of all these factors the orbit (when observed from the ground) will look like a figure 8 or an infinity symbol.

https://en.wikipedia.org/wiki/Analemma

[FancyMouse]

Nope 6 hours is a solar day, not sidereal rotation period. Sidereal rotation period is the rotation period in Kerbin's frame. Solar day is the period for Sun rotating around the same point on Kerbin. In a typical setting (including Kerbin/Sun system), solar day is longer than sidereal rotation period, because when one completes a sidereal rotation period, Kerbin orbits the Sun a bit, so that the Sun is lagging behind a little bit, and you'll need a little bit extra time to complete a solar day.

So 6 hour orbit is not a synchronous orbit. The wiki now contains the right information about solar day/sidereal rotation period/synchronous orbit altitude. I believe it was a recent change (0.9 or early 1.0) that fixes solar day to be 6 hours.

[ibanix]

1 hour ago, Leszek said:

So, when it comes to Keostationary orbits, I know what I am doing.  I get my orbits down to <.01 of a second above 6 hours.  Using a resonating transfer orbit I have been able to put multiple satellites into perfect formations with so little difference in orbital time between them that the formation isn't noticeably distorted even after many game years has passed.  This isn't a problem for me and I have been doing it since version .25.

I did it just last career.

Then I started a new career and put a satellite into KSO just ahead of KSP.  Even though it was just one satellite and not a formation I set it to 6 hours and 0.033 seconds.  This should be just fine for the rest of the career.  One trip to just outside of the Kerbal SOI and back and it is noticeably behind KSC.  I jump to the satellite and it still shows 6 hours and 0.05 seconds. (The time normally fluctuates a bit when you are that precise.)  So I figured I just remembered its position wrong, since I put so many up all the time perhaps I am just confused with another career.  But now with my eye watching it, I can see it lagging farther and farther behind.  I set the orbital time to 5:55 and let it catch up so that it is right over KSC again.  Again I set it to 6 hours and 0.01 seconds.  Again I notice it is lagging behind the KSC after just a few short orbits.

The only mod I added since last time is Pilot Assistant, but I don't see how that would make any difference.

Any ideas what might be causing this anomaly?

I used ScanStat for quite a while. I was never able to get a perfectly circular orbit that wouldn't "drift" like this while you were watching it. I asked about this some time back and was told that it has to do with rounding error in the engine for extremely small values.

[Leszek]

1 hour ago, FancyMouse said:

Nope 6 hours is a solar day, not sidereal rotation period. Sidereal rotation period is the rotation period in Kerbin's frame. Solar day is the period for Sun rotating around the same point on Kerbin. In a typical setting (including Kerbin/Sun system), solar day is longer than sidereal rotation period, because when one completes a sidereal rotation period, Kerbin orbits the Sun a bit, so that the Sun is lagging behind a little bit, and you'll need a little bit extra time to complete a solar day.

So 6 hour orbit is not a synchronous orbit. The wiki now contains the right information about solar day/sidereal rotation period/synchronous orbit altitude. I believe it was a recent change (0.9 or early 1.0) that fixes solar day to be 6 hours.

Aha, according to the wiki it is now: 5 h 59 m 9.4 s.  That is almost a minute an orbit, enough time to lag behind noticeably in very short order.  I has been a while since I payed attention to the exact position above Kerbin I place the satellites so it makes sense that perhaps I haven't noticed until now.

1 hour ago, Taki117 said:

This isn't actually an anomaly, it's call a Analemma.  Basically what's happening is while your orbital period might be fine, your inclination is probably off (This is only a guess as I do not know the exact orbital parameters) also, your AP and Pe could be slightly off from the ideal (despite the orbital period being nearly perfect) what this is going to cause is your satellite to appear to move relative to your target.  because of all these factors the orbit (when observed from the ground) will look like a figure 8 or an infinity symbol.

https://en.wikipedia.org/wiki/Analemma

That is a good guess, I should have specified that the orbit is nearly circular and equatorial.  While I don't make any effort to get a circular orbit (I just boost to start at 2868.7 and then circularize based on orbital period only.) it is very nearly perfectly circular, and the inclination is the .2 degrees that the launch pad at KSC is off.  I am aware that this means the satellite moves around the sky a small amount because of this, but it is going to be close enough that I call it Keostationary.  I was talking about the satellite drifting out of position over successive orbits so that it is no longer in the position I first placed it.

5 minutes ago, ibanix said:

I used ScanStat for quite a while. I was never able to get a perfectly circular orbit that wouldn't "drift" like this while you were watching it. I asked about this some time back and was told that it has to do with rounding error in the engine for extremely small values.

If you set your orbit by time and be picky enough you can get it so that it doesn't drift noticeably for many game years.  Rounding errors only matter if you are watching the satellite and are not using time compression, otherwise the Satellite is on rails and will be perfectly placed for a long time.  If you go for a perfectly circular orbit instead of an orbit with the right period, then you will never get it right no matter how picky you are.

Edited April 19, 2016 by Leszek

[ibanix]

9 minutes ago, Leszek said:

If you set your orbit by time and be picky enough you can get it so that it doesn't drift noticeably for many game years.  Rounding errors only matter if you are watching the satellite and are not using time compression, otherwise the Satellite is on rails and will be perfectly placed for a long time.

Rails uses different calculations (that is, it doesn't recalculate).

[Leszek]

1 minute ago, ibanix said:

Rails uses different calculations (that is, it doesn't recalculate).

That is correct, but the orbit also doesn't drift any.  It is calculated once and then you follow it.  If you watch the satellite without time compression its position is constantly recalculated, over time errors creep in and the orbit changes.  I don't know if you have ever gone to mun without time compression, but I did once while reading a book, I lost the encounter completely and had to make adjustments to reach the mun.

[mrclucks]

When I was an avid user of remote tech, ID just circulize to roughly the KEO altitude in the wiki( I think it's 2867km) and then use RCS to adjust my orbital to 6hours within a second or two.  It didn't matter to me if there was one satellite over KSC at all times, as long as I had the coverage and they stayed roughly 90 degrees from each other.   

Technically as long as the antennas can communicate, satellites arent in too much of an elliptical orbit, are equidistant apart, and have basically the same period, you will have a solid network for kerbun

[Leszek]

19 hours ago, mrclucks said:

When I was an avid user of remote tech, ID just circulize to roughly the KEO altitude in the wiki( I think it's 2867km) and then use RCS to adjust my orbital to 6hours within a second or two.  It didn't matter to me if there was one satellite over KSC at all times, as long as I had the coverage and they stayed roughly 90 degrees from each other.   

Technically as long as the antennas can communicate, satellites arent in too much of an elliptical orbit, are equidistant apart, and have basically the same period, you will have a solid network for kerbun

That is what I did and I don't care how close it is to over KSC as long as it gets signal.  But if the orbit loses 1 minute every day, then 60 days later you are 1/6th of the way around the planet and after 120 days you are 1/3.  That is one complete lapping of the planet every 360 days.

The correct time is now 5 hours 59 minutes 9.4 seconds and that difference isn't trivial.  I thank FancyMouse for pointing that out.

[KocLobster]

I found this thread pretty interesting because I've never really understood what KSO orbits are or what you'd need one for.

What mod are you using that requires that kind of communication, that requires a satellite to be directly over KSC? I assume that is RemoteTech? I've been thinking of trying that mod but think I'll wait a few weeks until the majority of the mods I use are updated to 1.1.

[Callmedave]

For my early career mode satellites, i would just aim to have them all in the same orbital period, regardless of if it was Keo-synchronous or not. Inclination would differ slightly, but not enough to be a problem.

 

[FancyMouse]

5 hours ago, KocLobster said:

what you'd need one for.

I have one that is useful even for stock - when I need to launch at a particular spot (usually launching into a inclined orbit), I just go to my keostationary satellite, put maneuver node, and get the precise time I need to wait (and even just Warp there). Way less annoying than manual time warping.

You can but you don't have to leverage KSO for RT. I don't.