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. Bartokk assassins hired by Groodo the Hutt had access to a CGT sensor and thus were able to detect the cloaked Sith Infiltrator of Darth Maul.
When Grand Admiral Thrawn blockaded Coruscant with a number of cloaked asteroids, locating them became a top priority. The New Republic did not possess any CGTs, and so was required to attempt to steal one from the Imperial base at Bilbringi. Garm Bel Iblis had had one before Battle of Svivren, but it got lost.
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.
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