The most obvious and essential ATC tool is a working radio. Imagine an airport is fogged in, with hard IMC and essentially zero visibility. The tower controller cant see the airplanes. However, using a single radio with his knowledge of the airport layout, and accurate position reports from pilots moving around in the soup, he can still work the traffic. Radar controllers can also rely on pilot position reports to separate traffic in non-radar operations. Again, all they need is a radio and knowledge of their surroundings to build a mental traffic picture.