Re: [Gmat-users] Setting cartesian states in Moon Fixed coords

[Steven H. <Ste...@na...>]

Hi Steve,

   Can you send your emails in text form when you're using the 
sourceforge mailing list.  Unfortunately it doesn't handle html 
emails...... bummer

   To define a spacecraft state in Moon Fixed coordinates is relatively 
easy and I'll explain it below.  However, there seems to be a small 
numerical issue that causes problems for your applications.  I've 
attached a script with a workaround for the issue.  It's trivial to work 
around the issue while we figure out where the numerical issue lies.

Here's how to set a spacecraft state in moon fixed:

1)  You need to create a MoonFixed coordinate system.  Here is a script 
snippet for it
Create CoordinateSystem MoonFixed;
MoonFixed.Origin = Luna;
MoonFixed.Axes = BodyFixed;

2)  Next, when you define the spacecraft's state vector, you need to set 
the coordinate system to "MoonFixed"
MoonFixedSat.DateFormat = UTCGregorian;
MoonFixedSat.Epoch = 06 Feb 2009 07:30:00.000;
MoonFixedSat.CoordinateSystem = LunaFixed;
MoonFixedSat.DisplayStateType = Cartesian;
MoonFixedSat.X = 40.80379818964957;               
MoonFixedSat.Y = 40.69785376287012;              
MoonFixedSat.Z = -1736.441237786784;                  
MoonFixedSat.VX = 0.2075088635462135     
MoonFixedSat.VY =  0.3163450685464736       
MoonFixedSat.VZ = 2.503942913713620        


For your mission, GMAT is taking the MoonFixed state, converting it to 
MoonMJ2000Eq,   adding in a delta V, and then propagating to the 
collocation piont.  Somewhere in this process, there is some noise being 
introduced that causes about a 50 m position error at the collocation 
point.  This causes problems for the optimizer

THE SOLUTION:
1)  Set the moon fixed state on MoonFixedSat
2)  At the beginning of the mission sequence, set the state for 
ImpactSat using the following lines, where the coordinate system for 
ImpactSat is EarthMJ2000Eq.

ImpactSat.X = MoonFixedSat.EarthMJ2000Eq.X
ImpactSat.Y = MoonFixedSat.EarthMJ2000Eq.Y
ImpactSat.Z = MoonFixedSat.EarthMJ2000Eq.Z
ImpactSat.VX = MoonFixedSat.EarthMJ2000Eq.VX
ImpactSat.VY = MoonFixedSat.EarthMJ2000Eq.VY
ImpactSat.VZ = MoonFixedSat.EarthMJ2000Eq.VZ

I attached a script that does this already if you want to see an example.

Steve (H.)


Steven Cooley wrote:
> Hello,
>
> How do i set the Cartesian state in Moon fixed coords?
>
> Thanks,
>
>
>
> =====================================================================
> D. Steven Cooley                               301 286-6653 (office)
> Flight Dynamics Analysis Branch                301 286-0369 (fax)
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