Airborne Multichannel Microwave Radiometer

The Airborne Multichannel Microwave Radiometer (AMMR) measures thermal microwave emission (in degrees Kelvin of brightness temperature) from surface and atmosphere. The up-looking radiometer at 21 and 37 GHz is a component of AMMR that was developed in the 1970's for precipitation measurements from an aircraft. The entire AMMR assembly covers a frequency range of 10-92 GHz. The 21/37 GHz unit has been flown in many types of aircraft during the past three decades in various field campaigns. It was refurbished during the year 2000 and is ready for flight again.

The fixed-beam Dicke radiometer has a beam width of about 6 degrees and is currently programmed with radiometric output every second. The temperature sensitivity is < 0.5 K, and the calibration accuracy is about ±4 K. The calibration is performed on the ground by viewing targets of known brightness (e.g., sky and absorber with known brightness temperature). The unit can be installed in one of the windows of the NASA P-3 aircraft so that it views at an angle of about 15º from zenith. Thus, it is necessary to spiral the aircraft gradually down to region below the freezing level in order to make measurements effectively. Ideally, the aircraft descends at the rate of about 1 km per 5 minutes. The system requires a bottle of N2 gas to keep the wave guides dry during the in-flight operation.