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.