The U.S. Air Force is spending about $30 million on each of its B-2 bombers to upgrade its innovative APQ-181 radar systems. There are basically two changes. New radar antennas (the electronic transmitters and receivers) are being installed. The new AESA (active electronically scanned array) radar consists of thousands of tiny radars that can be independently aimed in different directions. This is more powerful, versatile and reliable than the older "dish" (the usual shape, but not the only one) type.
The other reason for the upgrade was the use of a new frequency. The original frequency, selected to be hard to detect, also turned out to be the same one that became widely used for communicating with space satellites. This often caused problems when a B-2 was using its radar near where a ground station was talking to a satellite overhead. The B-2 frequency will eliminate that problem, and still keep the B-2 radar stealthy (as radars go.)
The B-2 radar is used for a wide range of functions, from terrain avoidance to air search and weather detection.