The B-21 will have regular stealth against higher-frequency engagement radar and also against lower-frequency surveillance radar. China and Russia have shifted to using a network of low-frequency radar to detect and locate stealth aircraft.
The B-21 will also have thermal management to try to make the aircraft somewhat aligned with the temperature of the surrounding air so as not to create a heat differential for enemy sensors to detect.
It will use new materials that are described as radar-absorbent. The radar’s electronic signal will be absorbed. Properly-completed stealth construction can make a stealth airplane appear like a bird or an insect to enemy radar.
SOURCE – National Interest