
Aerosol pollution in Northern India and Bangladesh. Source: Wiki Commons.
ACE seeks to study cloud and aerosol height, organic material in surface ocean layers, and aerosol and cloud type properties. Overall, the mission will provide improved climate modeling and better predictability of climate change variability, measure ocean productivity and health, and create air-quality models and forecasts. Results of the mission will help decrease uncertainty in climate predictions. ACE will focus primarily on two phenomena: aerosol-cloud interactions and carbon uptake in oceans.
The ACE mission calls for the use of two radars, ACERad, which represent a significant advancement compared to earlier atmospheric radar technologies. The radars will operate at 94 gigahertz in W-band and 35 gigahertz in Ka-band, and unlike previous missions, have Doppler and polarimetric capabilities. The dual-frequency Doppler measurements will provide valuable information about how particle sizes change with height. Dual-polarization will provide information about the water particle state and shape. The Ka-band radar will scan to provide images for a more comprehensive picture of precipitating systems. The use of these two radars will create stronger vertical profiles for measurement of cloud droplet size, glacier height, and cloud height.

A three-dimensional, detailed structure of Hurricane Sandy measured by the TRMM Precipitation Radar (PR) on Oct. 28, 2012 at 1725 UTC (1:25 PM EDT). Source: NASA.
“There are many tradeoffs between the ACE mission and its radar specification,” said Dr. Gerald Heymsfield, a cloud radar expert and research meteorologist at NASA’s GSFC. “One of the biggest (tradeoffs) being the imaging capability of the radar and its sensitivity. A significant challenge is to obtain high-sensitivity measurements while scanning.”
Imaging capability, for example over convective storm cells, provides context for cloud generation, and improved understanding between precipitation, aerosol and cloud formation. As a trade between measurement sensitivity and imaging capability, the ACE mission calls for the use of a nadir-only pointing W-band radar and a scanning Ka-band radar. Such dual-frequency and dual-mode operation presents challenges in implementing on a single spacecraft. A primary challenge relates to the large antenna size of about 3 meters by 5 meters required for each radar to achieve the sensitivity and spatial resolution to meet the ACE science objectives. The antenna architecture to achieve fixed-beam pointing differs from that needed for scanning, thus, complicating the shared use of the large antenna aperture by the radars.
“It is very challenging to have two frequencies on the radar that will fit in the spacecraft and still have adequate performance,” Heymsfield said.

Candidate antenna architecture for ACE radar. Source: NASA.
A 2011 IIP project aims to develop an antenna technology to support dual-frequency, dual-mode operation from a shared aperture. The principle investigator is Earthzine’s Editor-in-Chief, Paul Racette. Developed in partnership between the NASA Goddard Space Flight Center and Northrop Grumman Electronic Systems, the approach calls for the use of a novel dual-band reflector/reflect array using a W-band and a Ka-band Active Electronically Scanned Array (AESA) feed module. The reflector is a parabolic cylinder that permits scanning at Ka-band using an AESA line feed. A reflector is formed with a frequency selective surface that is reflective at Ka-band and focuses the W-band beam to a point. This approach offers cost advantages over other dual-reflector architectures. The architecture allows wide-swatch scanning of more than 100 kilometers at Ka-band and effectively uses the full antenna aperture to maximize the sensitivity at W-band while using W-band radar technologies also used for CloudSat and EarthCare.

GPM’s DPR. Source: NASA.
(See also “‘Revolutionary’ Space Project to Improve Weather and Climate Forecasting”)
NASA’s CloudSat, flying in tandem with the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), studies the vertical structure of clouds and the role of clouds and aerosols on weather, climate and air quality. In 2006, CALIPSO and CloudSat joined other satellites in the A-Train formation to study the Earth. EarthCare, a global observation mission of clouds, aerosols, and radiation by the European Space Agency (ESA), will use two active instruments including a lidar and W-band radar, and two passive instruments, including a multispectral imager and broadband radiometer, for data collection. The EarthCare mission launch is scheduled for 2014. CloudSat and EarthCare, unlike the future ACE mission, do not have scanning capability and instead focus on high-sensitivity measurements at nadir. The ACE mission plans on using radar technologies that have both capabilities, making it an innovative new radar system.