A new high-altitude airborne millimeter-wave radar for atmospheric research

Atmospheric clouds play an important role in the radiation budget for the earth-atmosphere system. To improve climate models, observations of cloud formation and associated dynamics are required. The recent launch of CloudSat will provide a wealth of information about clouds on a global scale. However, space-borne instruments are unable to provide simultaneous in situ measurements about the observed clouds, and thus, measurements from a combination of in situ sensors and airborne cloud radars are still necessary to study cloud processes.

Airborne millimeter wave radars have been used for atmospheric remote sensing since the early 1990s, but most of the aircraft platforms on which these are borne lack a comprehensive suite of in situ measurement capabilities. The National Center for Atmospheric Research in the United States has recently taken delivery of a modified Gulfstream V aircraft and has installed an array of sensors for atmospheric research. The aircraft is called the High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER) and is funded by the National Science Foundation (NSF). As part of the instrument suite for HIAPER, NSF is funding the development of the HIAPER Cloud Radar (HCR). The HCR will operate at millimeter-wavelength frequencies and will serve the atmospheric science research community by adding millimeter-wave remote sensing capabilities to the HIAPER aircraft. Thus, the HCR measurements in conjunction with other HIAPER instrumentation will provide the most complete picture of cloud physics available to atmospheric scientists. The HCR is scheduled for completion in the latter half of 2008.

The requirements for the HCR were distilled from a survey of the atmospheric research community. Responses from this survey were used in combination with a thorough examination of millimeter wave radar technology, and input from discussions with engineers and scientists at the NASA Jet Propulsion Laboratory, NASA Goddard Space Flight Center, and the University of Wyoming, to specify the a final set of design requirements for the HCR. The most important capabilities identified were spacial coverage, a minimum sensitivity of at least −25 dBz at 10 km, and Doppler and polarimetric capability at two wavelengths. The radar is also to be housed in a 20-inch diameter wing-mounted pod to minimize the space usage in the aircraft cabin.

This paper presents the specific design challenges of the HCR and describes the expected performance of the radar. We describe the technical solutions to operating the radar in a high-altitude environment, and the limitations imposed on the performance of the system by installing the radar in a wing pod. We present the transmitter, calibration network, and receiver design. We also characterize the the expected system performance through metrics such as the minimum detectable signal, the reflectivity and Doppler accuracies, polarimetric purity, an analysis of the transmitted polarimetric vector from a steerable mirror, and the expected accuracy of cloud liquid water measurements. The paper also details the phased approach to implementing the system which starts with a W-band Doppler radar, and through phases, adds pulse compression, polarimetric capability, and a second wavelength (K_a-band) radar to the HCR.