Antigravity emitter
From The Stargate Omnipedia
One of four glowing units that maintain the position of a Goa'uld antigravity dry dock above the surface of a planet. The emitters can be disabled from a console within the Ha'tak itself. Two emitters are not enough to maintain the altitude of the ship under construction. If any are disabled, the Ha'tak comes crashing to the planet.
KEY EPISODE
Orpheus - SG-1 blows the controls that keep the antigravity emitter from allowing the Ha'tak to plummet to the ground.