The AN/APQ-181 is an all-weather, low probability of intercept (LPI) phased array radar system designed by Hughes Aircraft (now Raytheon) for the U.S. Air Force B-2A Spirit bomber aircraft. The system was developed in the mid-1980s and entered service in 1993. The APQ-181 provides a number of precision targeting modes, and also supports terrain-following radar and terrain avoidance. The radar operates in the Ku band (a subset of the J band). The original design uses a TWT-based transmitter with a 2-dimensional passive electronically scanned array (PESA) antenna.