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.
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The system cost more than US$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 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.
As this weapon was created only in a dream environment, its true viability is unknown. (SG1: "Absolute Power")