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Using Gravity Simulator with Systemic (Read 6483 times)
Tony
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Using Gravity Simulator with Systemic
03/08/07 at 01:38:52
 
If you have not seen the Systemic web site, you should check it out.  It's all about the detection of exosolar planets.
http://oklo.org/
 
This is an outline of how to use the latest beta version of Gravity Simulator with Systemic.
Remember, Gravity Simulator runs on Windows only.
 
Download the latest beta version:  http://orbitsimulator.com/gravity/beta/GravitySimulatorBeta07March2007.exe
 
If you already have Gravity Simulator 2.0 installed (the version that's been out for the last 2 years), you will not have to do a complete install.  If you don't have Gravity Simulator 2.0 installed, you should install it first.
 
Place the new file in the same directory as your existing Gravitysimulator.exe.  If you want to place it in a new folder, make a copy of the files resume.gsim and codeoptomizer.dll from your existing Gravitysimulator folder and place it in the new folder.
 
The Systemic output is functional, but I haven't yet made it user friendly.  Here's how to work it:
 
Create your solar system.  Save it in in the directory of your choice.  I'm assuming you know how to do this, but if you don't, let me know.
 
Open the simulation and pause the simulation.
 
Create 1 additional object using "Objects>Create Objects" and call it "Observer" or any other name you like.  Set its mass to 0.  Set its SMA to any value outside the radius of the central star.
 
Use the "Objects > Edit Objects" window to edit the new object.  Place it 10 light years from the central star by putting an 10 in the X position box.  Enter 0 for all other position and velocity vectors.  You don't have to use 10 light years, but it is important that it is far enough away that the central star does not significantly pull the "observer".
 
Open the Systemic Output GUI from the File Menu.
 
Check the box that says "Create data file"
 
Choose a maximum number of data samples, and the sample period.  (for example,  Create 1 data point every 60 days.  Do this 100 times).
 
Enter a number in the error box.  This number is for the 3rd piece of data in each row of the .vels file.  That's all it does.
 
In the checklist box, choose "Observer" or whatever you called it.  Do not choose anything else.
 
Press OK.
 
Unpause the program.
 
In my above example, where I choose 1 data point every 60 days for 100 samples, it will take 6000 days to complete.  So wait 6000 simulated days ~17 years.
 
Look in the same directory where you saved your simulation.  You should see a file with the same filename as your simulation, except .gsim is replaced by .vels
 
 


 
If you have any suggestions to make it more user friendly or any other improvements, let me know.  Some ideas I have are:
 
Automatically generate "observer".  That way you won't have to see a checklist full of stuff you're not supposed to check.
 
Actually add the error from the error box to the outputted velocities.  As it is now, it outputs the true answer with no additional error.
 
semi-random sampling times that might be more like what you'd expect from a real data set, rather than a perfect "1 observation every 60.0 days, etc."
 
 


 
Here's a link to a stand-alone program that creates Gravity Simulator .gsim files from Systemic fits.  Copy all these files into the same folder:
http://orbitsimulator.com/systemic/SystemicAutomator.exe
http://orbitsimulator.com/systemic/block1.dat
http://orbitsimulator.com/systemic/block2.dat
http://orbitsimulator.com/systemic/block3.dat
http://orbitsimulator.com/systemic/block4.dat
http://orbitsimulator.com/systemic/block5.dat
 
 
To use it, open a web page that contains a fit, such as this one:  http://207.111.201.70/php/showfit.php?id=55cancri_3datasets&fn=fc286f02ef6c8 df92f8b756f6b8e6bcef1248d96
 
Hit ctrl+a to highlight the entire web page.  Then hit ctrl+c to copy the entire web page.  Paste this into the text area in the "Systemic to Gravity Simulator" program.  Press the "Save As..." button, and it will create a .gsim file for you.
 

 
In this example, I used one of the available  fits for the 55 Cancri star system.  Here's a screenshot from the simulation:

 
You might also find this a convenient method for creating new solar systems.
 
As you're running your simulations, you can use File > Output file... to create data files readable by Excel as a comma delimited txt file.  Then you can graph your data.  For example, this graph is from the Cancri 55 simulation, showing the periods of planets 3 and 4.  The pattern reveals that they are co-orbital, in a tadpole orbits.  (hint:  make sure your fits have a good JD.  The data output feature doesn't like negative years, only 1 AD and above.)

 
Another new feature in the Beta is the ability to take screen shots.  It can be tricky to use, as Gravity Simulator must be the active window, or you'll get screen shots of whatever the active window is.  Then you can assemble animated GIFs like this hypothetical planet system around Alpha Centauri A.  The Kozai Mechanism is wreaking havoc on the orbits of planets that are not co-planar with the Alpha Cen B.  (further details in this thread: http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1162874858)

 
** edit ** changed -10 to 10 for the distance of observer
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« Last Edit: 03/12/07 at 18:58:25 by Tony »  
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Tony
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Re: Using Gravity Simulator with Systemic
Reply #1 - 03/12/07 at 19:01:21
 
When making your systems in Gravity Simulator, set your longitude of ascending node to 0.  Also, give each object an initial inclination of 0.5 degrees, just so your longitude of ascending node has meaning.
 
When examining your .vels file in Systemic, for Longitude of Periapsis, add 90 degrees to whatever you set for your argument of periapsis in Gravity Simulator.
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frankuitaalst
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Re: Using Gravity Simulator with Systemic
Reply #2 - 03/13/07 at 11:54:42
 
I visited the site of Systemic , downloaded the program and ran it . I wonder what's the purpose in general ... Is my conclusion correct that one ...or more .. have to "play" with the parameters of the planet and once they get a good Chi-Squared they post their results . The data supplied , are they really original data ? Maybe this is a question I should post to the message board of systemic.
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Re: Using Gravity Simulator with Systemic
Reply #3 - 03/13/07 at 17:24:47
 
And a lot of people post their fits with very high Chi2.  I'm guessing that it is because they got a low Chi2 using Keplerian advancement, but the dynamics of the orbits, as reveled by their Runge Kutta method, raises the Chi2.  
 
There are two dropdown lists on the console.  The top one has real data.  The bottom one has synthetic data.  The beta feature of Gravity Simulator lets you create your own synthetic data as well.  And since you already know the answer, you can get very low Chi2s, if you create your system with LAN=0, and for Longitude of Peri in their console, use the value you gave Gravity Simulator for Argument of Peri + 90 degrees.
 
Of course it's more tempting to use real data, as there's a chance you might actually discover a planet hiding in the data.
 
But the purpose of the synthetic data is to see how well people can find the planets hidden in the data.  Since they know the exact parameters of this data, they know how well people did.  Knowing what types of orbits give people difficulties might help them determine what types of orbits are hidden in the real data.
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frankuitaalst
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Re: Using Gravity Simulator with Systemic
Reply #4 - 03/13/07 at 21:45:55
 
Quote from Tony on 03/13/07 at 17:24:47:

But the purpose of the synthetic data is to see how well people can find the planets hidden in the data.  Since they know the exact parameters of this data, they know how well people did.  Knowing what types of orbits give people difficulties might help them determine what types of orbits are hidden in the real data.

I guess that professionals have their own code to extract more or less automatically -ie creating the params in order to get a low Chi2 - the params of the planet(s) out of the given data , is this correct ? or do they also have to play with the params?  
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Re: Using Gravity Simulator with Systemic
Reply #5 - 03/13/07 at 22:31:15
 
These are good questions for their forum.  I imagine they do a bit of both.  It seems like the dynamics of exo-solar systems present quite a challenge.  This is the type of stuff we like to discuss here... precession of nodes, Kozai mechanism, orbit stability, etc.
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Re: Using Gravity Simulator with Systemic
Reply #6 - 01/09/10 at 02:30:06
 
hi everyone.
 
For me it seems that the latest console (v. 1.5.12) only opens .sys files and not .vels files. So any .vels files i output with Gravity Sim is not readable by the console... cry
 
Does anyone know how to make the console read the vels file?
 
 
Thanx
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Tony
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Re: Using Gravity Simulator with Systemic
Reply #7 - 01/10/10 at 17:43:57
 
I haven't played around with the new Systemic console, so I'm not sure.  If I get a chance soon, I'll see if I can make a patch.
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Re: Using Gravity Simulator with Systemic
Reply #8 - 04/29/10 at 22:00:23
 
Dang it, I was just about to do this.  cry
 
Glad I heard from Grumpster first!
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Re: Using Gravity Simulator with Systemic
Reply #9 - 04/30/10 at 20:35:16
 
I think i know how! After the vels is generated, you add them to the "sys" files. make a system file called "name".sys and add the following code to it, obviously replacing it with your vels files.  
 
Code:
Data  {
	RV[] "14Her_1.vels"
	RV[] "14Her_2.vels"
}
"14Her" {    
  Mass   1.00
}
 


 
It worked! I tried it! That's the way around it!  Grin Grin Grin Grin
 
Hmmm... maybe not... I can't find the new one I made, maybe the vels wasn't done being made by GSim... Tony, you need to give an alert when these things are done!
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