The Second Generation Airborne Precipitation Radar (APR-2) is a dual-frequency (13 GHz and 35 GHz), Doppler, dual-polarization radar system. APR-2 has a downward looking antenna that performs cross track scans, covering a swath that is +/- 25 degrees to each side of the aircraft path. 

Additional features include simultaneous dual-frequency; matched beam operation at 13.4 and 35.6 GHz (the same as GPM Dual-Frequency Precipitation Radar); simultaneous measurement of both "like" and "cross-polarized" signals at both frequencies; doppler operation, and real-time pulse compression (calibrated reflectivity data can be produced for large areas in the field during flight, if necessary).

The APR-2 system includes a real-time pulse compression processor, a fully-functional control and timing unit, and a very compact LO/IF module, all of which could be used in spaceborne applications. The cylindrical reflector antenna and linear feed array for the spaceborne PR-2 have been replaced by traveling wave tube amplifiers (TWTAs), front-end electronics, and an offset parabolic reflector antenna with mechanical scanning. The APR-2 operational geometry is shown in Figure 1 below; it looks downward and scans its beam across-track, with each scan beginning at 25 degrees to the left of nadir and ending at 25 degrees to the right. 

It uses the same scanning antenna reflector as that used for the Airborne Rain Mapping Radar (ARMAR), which consists of a 0.4 m offset reflector antenna with a mechanically scanned flat plate. The APR-2 antenna feed is a dual-frequency feed (13.4 and 35.6 GHz) and the aperture at 35.6 GHz is under-illuminated to provide matched beams at the two frequencies. This choice results in poor Doppler accuracy at Ka-band, but is needed for rain retrieval.