WIP

src/n_E2R_EN.ts

@@ -17,27 +17,30 @@ export function n_E2R_EN(
   // Based on https://github.com/pbrod/nvector/blob/b8afd89a860a4958d499789607aacb4168dcef87/src/nvector/rotation.py#L478
   const [n_e_x, n_e_y, n_e_z] = rotate(R_Ee, n_E);
 
-  // Nz_e = -n_e
+  // The z-axis of N (down) points opposite to n-vector
   const Nz_e_x = -n_e_x;
   const Nz_e_y = -n_e_y;
   const Nz_e_z = -n_e_z;
 
-  // Ny_e_direction = np.cross([[1], [0], [0]], n_e, axis=0)
+  // Find y-axis of N (East)
+  // Remember that N is singular at poles
+  // Ny points perpendicular to the plane formed by n-vector and Earth's spin
+  // axis
   const Ny_e_direction_y = -n_e_z;
   const Ny_e_direction_z = n_e_y;
-
   const Ny_e_direction_norm = Math.hypot(Ny_e_direction_y, Ny_e_direction_z);
   const on_poles = Math.hypot(Ny_e_direction_y, Ny_e_direction_z) === 0;
-
-  // Unit vector
+  // Ny_e_x is always 0, so it's factored out in the following equations
   const Ny_e_y = on_poles ? 1 : Ny_e_direction_y / Ny_e_direction_norm;
   const Ny_e_z = on_poles ? 0 : Ny_e_direction_z / Ny_e_direction_norm;
 
-  // Nx_e = np.cross(Ny_e, Nz_e, axis=0)
+  // Find x-axis of N (North)
   const Nx_e_x = Ny_e_y * Nz_e_z - Ny_e_z * Nz_e_y;
   const Nx_e_y = Ny_e_z * Nz_e_x;
   const Nx_e_z = -Ny_e_y * Nz_e_x;
 
+  // Use each component as a column vector, then multiply by the transpose of
+  // R_Ee to get the rotation matrix R_EN
   return multiplyTransposed(R_Ee, [
     [Nx_e_x, 0, Nz_e_x],
     [Nx_e_y, Ny_e_y, Nz_e_y],