GRadar refers to GRAvitational-wave] Detection And Ranging, derived from Radar, is fictional detection device that is basically a faster than light radar.
Most of the current detection methods use electro-magnetic waves, which travels at the speed of light, and is not really useful when the distance is too far away, especially in combat. Gradar makes use of gravitational waves in a way similar to radar, which emits radio waves and uses the reflected patterns to detect targets, gradar emits gravitational waves and uses the reflected waves to detect targets.
Similar to passive radar, gradar can be utilized in passive mode and detect the gravitational waves emitted by other active gradar. On the other hand, anything with mass has gravity, and will generate gravitational waves when moving through other gravity wells, thus gradar can also detect these motions. In theory, gravitational warp drives will also generate gravitational waves that gradar can detect.
Tadar Tachyon detection and ranging