What frames am I using by default in BSK?

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sjkcarnahan
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Joined: Tue Jun 19, 2018 8:22 pm

What frames am I using by default in BSK?

Post by sjkcarnahan » Mon Jul 16, 2018 4:26 pm

This only really comes into play when creating a simulation with more than 1 planet via Spice.

Spice uses the International Celestial Reference Frame as its default inertial frame and leaves the frame centered at the Solar System Barycenter ('SSB'). Note that internal to Spice, this is referred to as 'J2000' for legacy reasons and because the two frames are pretty close to each other. Within Basilisk, the center of this frame can be changed by setting the "zerobase" of a Spice object. Planet Positions and Velocities are given relative to this zerobase planet, then.

Conveniently, for Earth-based analysis, the ICRF X-Y plane is set to be coplanar with the mean equator. The X-Y axes are the same as most Earth Centered Inertial (ECI) systems, with the X-axis pointing toward the vernal equinox. The Z-axis is along the north pole rotation axis (which I'm not defining precisely here), and the Y-axis completes the right-handed system. Therefore, only a translation is needed to convert from most often-used ECI frames to the spice inertial frame.

For rotating Earth (and other planets, similarly) frames, the BSK interface to Spice uses "IAU_EARTH". This is based on the IAU definitions of planet frames most published in 2009, even though it was updated in 2017. This frame has a +Z along the north pole (rotational), +X toward the prime meridian, and +Y completing the right-hand system. While other, more precise frame definition files are available from Spice, it would take a little digging to learn to get BSK to call them.

Finally, there is the question of what happens when setting a planet isCentralBody = True. Explicitly for the user, this means that spacecraft initial conditions should be given relative to the central body, but will still be output relative to the zero base ('SSB' by default). Under the hood, it means that the BSK dynamics engine is propagating only the relative position and velocity based on central-body-acceleration. The velocity and position are then always updated by the central body position and velocity to produce absolute results. This improves integration accuracy.

Also see: https://naif.jpl.nasa.gov/pub/naif/tool ... rames.html
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adambsk
Site Admin
Posts: 11
Joined: Mon Jun 11, 2018 4:13 pm

Re: What frames am I using by default in BSK?

Post by adambsk » Mon Jul 16, 2018 5:01 pm

Great information thank you!
BSK CS Engineer

sudupa
Posts: 2
Joined: Tue Aug 07, 2018 10:50 pm

Re: What frames am I using by default in BSK?

Post by sudupa » Tue Aug 14, 2018 5:20 pm

Hello,

I was also wondering what the time format in Basilisk was. We've been assuming UTC, but wanted to double check.

Thanks!

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