Sometimes considered a contradiction in terms, the AN/APQ-181 is the active radar on the radar-evading B-2 Spirit "stealth" bomber. While low probability of intercept (LPI) is extreme important — and even then, the radar is probably activated only in brief and necessary bursts — it shares some design with the AN/APG-70 multimode radar in the F-15E Strike Eagle.
It operates in the Ku band with wavelengths in the 2-3 cm range. Antennas are passive electronically scanned arrays in the bottom front part of each wing. This wavelength gives very high precision but suffers from atmosphering attenuation. Increased attenuation and smaller sidelobes, however, actually contributes to LPI.
An upgrade in progress converts the antenna to active electronically scanned.
Like the AN/APG-70 and its successor, the AN/APG-63 (V)4, it has many operational modes, including terrain following/terrain avoidance, ground mapping, weather. moving target indicator and navigation. While the B-2 certainly will not fly air-to-air combat as will the Strike Eagle, the radar does have an air-to-air mode for air refueling.
The radar works with a highly classified defensive system, the AN/ZSR-63, which certainly has extensive tactical electronic intelligence capabilities, but also may have an active cancellation mode used to "null out" enemy radar.