gcrf_to_lunar

ssapy.utils.gcrf_to_lunar(r, t, v=None)[source][source]

Transform position and velocity vectors from the GCRF (Geocentric Celestial Reference Frame) to a lunar-centric frame.

This function converts coordinates from the Earth-centered GCRF to a coordinate system centered on the Moon. It uses the Moon’s position and velocity to define the transformation.

Parameters:

r (numpy array): Position vector(s) in the GCRF [x, y, z]. t (numpy array): Time(s) at which the position vector(s) are defined. v (numpy array, optional): Velocity vector(s) in the GCRF [vx, vy, vz]. If not provided, only position is transformed.

Returns:

numpy array or tuple:

  • If v is not provided: Transformed position vector(s) in the lunar-centric frame.

  • If v is provided: A tuple containing:

    • r_lunar (numpy array): Transformed position vector(s) in the lunar-centric frame.

    • v_lunar (numpy array): Transformed velocity vector(s) in the lunar-centric frame.

Notes:

  • The MoonPosition class is used to calculate the Moon’s position at a given time.

  • The Moon’s velocity is approximated using finite differences over a ±5-second interval.

  • The lunar-centric frame is defined with the following axes:

    • x-axis: Points from the Moon toward the Earth (direction of the Moon’s position vector).

    • y-axis: Perpendicular to the Moon’s velocity vector, in the plane of motion.

    • z-axis: Perpendicular to both the x-axis and y-axis (right-hand rule).

  • The transformation matrix R is constructed using these axes and applied to the input position vector(s).