I'm in the process of buying a handheld radio as a backup for when I go flying. According to the owner's manual, I'd need a ground station license to operate it when I'm not actually in an airplane, and to fill out FCC Form 406. But, I can't find any information on a "Form 406" on the FAA's website, and the online application has five different options that seem to be aviation-related ("AA - Aviation Auxiliary Group", "AC - Aircraft", "AF - Aeronautical and Fixed", "CA - Commercial Air-ground Radiotelephone", and "CG - General Aviation Air-ground Radiotelephone"), and I can't figure out which one is applicable.
So, if I wanted to use my shiny new aviation radio on the ground, legally speaking, what license would I need?
(I already have a Restricted Radiotelephone license, if that helps.)