Sky-spy was Dalek technology. It was housed in the Daleks' capital city and protected Skaro. It allowed the Daleks to detect incoming enemy vessels and launch deadly, missile-based counter-attacks.
It was the primary target of the first ADF raid against Skaro . It was destroyed by Joel Shaw, Reb Shavron and Mark Seven. So integral was it to Skarosian defence, its destruction left the city open to easy attack by aerial ADF bombardment. (DAN: Terror Task Force)