Title: A Cloud and Precipitation Radar System Concept for the ACE Mission
Author: Stephen Durden
Organization: Jet Propulsion Laboratory
Co-Authors: S. Tanelli, L. Epp, V. Jamnejad, R. Perez, A. Prata, L. Samoska, E. Long, H. Fang, D. Esteban-Fernandez, C. Lee
In 2007, the Aerosol/Cloud/Ecosystem (ACE) mission was recommended for a NASA launch in the next decade by the NRC Decadal Survey. One of the primary goals of ACE is to reduce the uncertainty in the impact of clouds and aerosols on climate modeling, and the Decadal Survey specifically calls for an ACE cloud radar with 94- and possibly 35-GHz channels for cloud droplet size, glaciation height, and cloud height.
In this presentation, we describe a radar design, called ACERAD, which has 35- and 94-GHz channels, each having Doppler and dual-polarization capabilities. The baseline requirement for range resolution defined for ACERAD at this stage is 250 m, sufficient to reduce the ground clutter contamination problem to below the height of cloud-base of low level clouds and to resolve some of the most important features of cloud systems. Nadir-only pointing allows achievement of the required sensitivity by allowing relatively long integration times (and therefore large number of integrated pulses to reduce signal variability). Lack of scanning in the cross-track, however, represents a weakness for cloud monitoring systems. ACERAD will scan at Ka-band and will be nadir-looking at W-band. The ACERAD concept has an along-track antenna dimension of 5 m, sufficient to reduce the Doppler bandwidth due to platform motion (i.e., to maintain coherence between pulses) and to provide high-quality Doppler information at convection-scale in realistic scenarios.
To allow ACERAD to use a common antenna for both frequencies, a reflector antenna fed by extended interaction klystrons (EIKs) at both frequencies is used. Scanning at Ka-band presents a serious challenge for the antenna. To get a swath of 25-30 km, considered the minimum useful, ACERAD needs to scan at least 2 degrees off nadir; this is at least 20 beamwidths, which is quite large for a typical parabolic reflector. This problem is being solved with a Dragonian design; a systematic procedure for designing Dragonian reflectors with circular aperture was developed by one of the co-authors a number of years ago and has been applied to the ACERAD antenna design. A scaled prototype of the antenna is being completed and will be tested on an antenna range. ACERAD also uses a quasi-optical transmission line at W-band to connect the transmitter to the antenna and antenna to the receiver. This is implemented with mirrors and free-space propagation, rather than waveguide, significantly reducing losses. Conventional waveguide is used at Ka-band, since losses are lower. The challenge in using quasi-optics for ACERAD is that it must accommodate dual-polarized operation. A design for this has been completed and is being laboratory tested.