Victim GSO service area on uplink

Tuser asked 3 years ago

On page 92 of Recommendation ITU-R S. 1503-2, “step 3 define the GSO service area as the region enclosed by the contour representing a relative gain of 15dB.”
Is this is relative -15dB gain from the peak gain?
Does Recommendation ITU-R S. 1503-2 antenna uses single feed circular beams of S672-4 for calculations?