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