The AG-3 satellite (standing for "Anti-Goa'uld") was a planetary defense platform Dr. Daniel Jackson invented during a dream sequence triggered by Shifu. Having been given the genetic memory of the Goa'uld, he devised the system to be the perfect defense against them.
The system cost more than USD $160 billion over one year to deploy and was made up of 288 satellites that formed a complete grid over Earth. They were powered by heavy Liquid Naquadah (possibly Liquid naquadria) and generated subatomic particles focusing them into a very fine beam of destructive energy. The design made the weapon pinpoint-accurate, leaving no residual radiation or damage outside its intended target.
A single one could destroy a Ha'tak with a single shot, while several working in unison could vaporize a city the size of Moscow. They were also equipped with deep space sensors that could detect an incoming ship thousands of light-years away.