title: Azimuth and Elevation from one moving object to another, relative to first object's orientation
body: <p>I have two moving objects with position and orientation data (Euler Angles, Quaternions) relative to ECI coordinate frame.  I would like to calculate AZ/EL from what I'm guessing is the "body frame" of the first object.  I have attempted to convert both objects into the body frame through rotation matrices (X-Y-Z and Z-Y-X rotation sequence) and calculate a target vector AZ/EL this way but have not had success.  I've also tried to get body frame positions and calculate the body axis/angles and convert back to Eulers (relative to body frame).  I'm never sure how the coordinate system axes I'm supposedly creating are aligned along my object. </p>
  
  <p>I have found a couple other questions like this answered with Quaternion solutions so that may be the best path to take, however my Quaternion background is weak at best and I fail to see how the computations given result in a target vector.  </p>
  
  <p>Any advice would be much appreciated and I'm happy to share my progress/experiences going forward.</p>