Help with reference frames

[Sp3rt0ls]

Hello Basilisk community!

I am currently using Basilisk for AOCS simulations of a satellite that we plan to launch as part of a university project.
After a month of deep diving into the examples and documentation, I decided to write a Python script using the core modules to fully define the mission setup, including the orbit, environmental disturbances, attitude control with reaction wheels, and desaturation using magnetorquers.

Since I am working on the satellite’s attitude control, I started wondering which reference frames I am actually using, and whether I am handling them correctly.

I know that I set the inertial J2000 reference frame using the Earth.isCentralBody command, but I still have two open questions:

How is the body reference frame defined in Basilisk? In particular, how are the three spacecraft axes oriented?

We have decided to use a specific onboard computer, which assumes that when the satellite is “at rest” (i.e., with roll, pitch, and yaw angles equal to zero), it is aligned with the orbital reference frame. This orbital frame is defined with the x-axis along the velocity vector, the y-axis along the anti-normal direction of the orbit, and the z-axis pointing nadir. Moreover, this computer uses a 2-1-3 Euler rotation sequence. My questions regarding this are:

2.1) Is it possible in Basilisk to define such an orbital reference frame and ensure that the spacecraft body frame is aligned with it when the spacecraft is at rest?

2.2) How can I simulate the 2-1-3 Euler rotation convention of the onboard computer within Basilisk?

Thanks a lot for your support and for any advice you may have!