I'm curious if anyone here knows what the restricted airspace in Horseshoe Beach, Florida is for. It extends from ground level up to 15,000 feet. It doesn't affect my flight plans, but I just happened to notice it on my new Jacksonville sectional and was wondering what it is restricted for. It is only about 3 nm wide.

Might be seeing the area for the Tethered Aerostat Radar System use.

I think you are right. That would explain the 15,000 ft ceiling and 3 nm wide diameter, and also why it is restricted and not prohibited.