After many years of use of Tropical Rainfall Measurement Mission (TRMM) and CloudSat data, focus groups within the cloud and precipitation science community produced requirements for the next generation of missions. The first draft of Aerosol-Cloud-Ecosystem mission requirements was formalized in 2009, snowfall observation requirements were documented in 2011, and recently the discussion for the definition of a mission concept called the Cloud and Precipitation Process Mission (CaPPM) has been initiated.
Each focus group targets specific science topics, but several common measurement requirements emerged. They are driven by the need to observe the dynamics (hence Doppler velocity), the entire column (hence multi-frequency), measurements close to the surface (hence reduction of surface clutter extent), the 3D structure (hence wide swath), the main microphysical parameters (hence the joint use of multi-frequency, Doppler, and linear depolarization ration), and to improve global coverage (both mapping and frequency; hence, again, wide swath). Addressing these needs will unlock a multitude of improvements in weather and climate modeling.
The Three-band Cloud and Precipitation Radar (3CPR) is a spaceborne instrument concept evolved from the Second Generation Precipitation Radar (PR-2) architecture that meets the measurement requirements described above with a versatile architecture that enables a wide range of instrument performance vs. resource allocation trades. 3CPR enables simultaneous and collocated transmission of the three main radar bands inherited from TRMM/Precipitation Radar, Global Precipitation Mission/Dual-frequency Precipitation Radar, and CloudSat (i.e., Ku-, Ka-, and W-band — 14/35/94 GHz). It also enables cross-track scanning at all bands, and use of advanced waveform generation and signal processing methods to maximize Doppler performance vs. resource demands.
The 3CPR system concept combines a cylindrical-parabolic reflector with a three-band Active Linear Array Feed (ALAF). The development of the ALAF is enabled by novel architectures as well as new technologies in microfabricated antennas and interconnects: gallium nitride (GaN) and silicon germanium (SiGe) monolithic microwave integrated circuits (MMICs). The combination of these technologies within the 3CPR architecture provides new scanning radar capabilities in support of cloud and precipitation science measurements.
The novel part of the 3CPR system is the antenna, which is composed of a singly curved parabolic cylindrical reflector fed by the three-band ALAF. Each ALAF is an electronically scanned phased-array antenna that directs the beam along the non-curved dimension of the reflector. This provides cross-track scanning and 3D sampling of the clouds and precipitation.
Each ALAF consists of scanning array tiles (SATs) with a varying number of elements (depending upon frequency), RF distribution network, up-down converters, power supplies, thermal management, and structure. Each SAT contains micromachined radiators and interconnects along with power amplifiers, low-noise amplifiers, phase shifters, and attenuators. The extremely high degree of integration (with one 2.5-mm space per element for W-band SAT) makes implementation very challenging.
The 3CPR system is the first Ku/Ka/W-band radar system to use a three-frequency linear feed to feed a parabolic-cylindrical reflector. It uses novel all-metal micro-fabricated radiating elements, and a novel interlaced array architecture whereby the transmit and receive arrays are separate arrays interlaced on a 2 × N grid.