A crystal gravfield trap (CGT) was a rare and expensive type of sensor that used a synthetic crystal grid to detect and identify fluctuations in the local gravity field. They could be blocked by the presence of mass, not being able to detect a ship on the far side of a planet due to its mass.
CGTs were the only sensor that could reliably detect a ship with an active cloaking device. When Grand Admiral Thrawn blockaded Coruscant with a number of cloaked asteroids, locating them became a top priority.
The LightStealth-18 Reconnaissance Ship, used by the New Republic during the Yuuzhan Vong War, was equipped with such device as part of its sensor array.