r_E

The radius of the Earth is not uniform over its surface. The Earth more resembles an oblate spheroid than a perfect sphere. Therefore, the equatorial radius of the Earth would not be the most accurate value when determining the satellite's orbit radius (semi-major axis), unless the observer is located right on the equator.

The Earth's radius at the observer's latitude can be found by using the following equation:
 

r_E = [ cos²(l) / a² + sin²(l) / b² ] ^-1/2

r_E = 6367.75 km

PROCEED TO STEP #4