The AG-3 satellite (standing for "Anti-Goa'uld") was a planetary defense platform that represented the ultimate climax of all Goa'uld technology, buried deep in their genetic memory. The system cost more than USD $160 Billion over one year to deploy and was made up of 288 satellites that covered the entire planet Earth. They were powered by heavy liquid naquahdah and used subatomic particles focused into a very fine beam to yield destructive energy. This type of attack was so exact that there was no residual energy.
These satellites also carried deep space sensors that could detect an incoming ship thousands of light-years away. Their weapons were capable of rendering the shields of a Ha'tak class ship ineffective. When more power than one unit could provide was necessary, a group of units would combine to add their destructive potential together. In addition to attacking incoming enemy ships, it was also shown that the system could attack other satellites or objects in orbit as well as targets on the ground.
The system was seen in a dream by Daniel Jackson, but it is unknown whether or not the AG-3 system is feasible or even possible to build. (SG1: "Absolute Power")