Remove GPS on copter

Hi all,

Newbie here. I want to build cleaning copter. Somehow, near the building, the GPS signal weak. The pressure water really make the copter unstable and drifting. I try to fly at open area which make the GPS signal strong, copter can withstand the pressure and hover steady. I read about optical flow sensor, but it seem to have limited altitude.

My question is, what is your suggestion to increase the GPS signal or replace the GPS for my copter fly steady with water pressure?

Thank you.