Radar Cross Section is defined in a spherical coordinate system defined with respect to the target’s body frame.
You can define two different coordinate frames:
- Rho from X-axis. The reference axis for the measurement of the RCS is the target's body frame x-axis. The
and 
(Rho and Theta) along with the reference axis are defined as 0° respectively. - Rho from Z-axis. The reference axis for the measurement of the RCS is the target's body from Z-axis. The Rho and Theta both are defined as 0° along the Z-axis of the target's body frame.
The Rho () angle is the angle off the reference axis Z-axis (or X-axis), while the Theta () angle is defined as the rotation around the reference axis starting from the X-axis (or Z-axis).
The range is 0° through 180° for the Rho angle, and -180° through +180° for the Theta angle.
The figure below shows the Rho and Theta angles defined in the target's body frame. The red arrow points to the radar direction (incident/reflected wave for a mono-static radar). The target body X-axis is used as the reference axis. The Rho angle is the angle between the reference axis (body X-axis) and the incident wave (radar) direction.
Theta angle is measured from the body Z-axis to the projection of the incident vector onto the target body YZ plane; i.e., the rotation angle around the reference axis (body X-axis).
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