Aerosols (particulate matter) have a dramatic effect on radiative forcing of the climate, in some cases cooling and in other cases warming. The Fourth Assessment Report of the IPCC estimates that direct radiative forcing due to all aerosols is a cooling of -0.50 W m-2 with absorbing aerosol (black carbon) responsible for a warming of +0.22 W m-2, but the uncertainties associated with these numbers are very large. Better measurements of the optical properties of aerosols, especially absorption coefficient and asymmetry parameter, and their spatial and temporal distribution are required to reduce these uncertainties and improve the ability of models to predict climate change. Aero3X was designed to provide such measurements. It is a light weight (11 kg), compact (0.25 x 0.30 x 0.6 m), and fast (1 Hz sample rate) instrument intended for use on an Unmanned Aerial System (UAS) but suitable for flight on other aircraft and for surface measurements. Aero3X uses an off-axis cavity ring-down technique to measure extinction coefficient and a reciprocal nephelometry technique for measurement of total-, forward- and back-scatter coefficients at wavelengths of 405 nm and 675 nm. Its outstanding precision (0.1 Mm-1) and sensitivity (0.2 Mm- 1) allow the determination of absorption coefficient, single-scattering albedo, estimates of backscatter to extinction ratio and asymmetry parameter at both wavelengths, and Angstrom exponent. Together with its humidification system for measurement of the dependence of aerosol optical properties on relative humidity, these represent a complete set of the aerosol optical properties important to climate and air quality. Aero3X was designed to operate in pollution plumes where NO2 may cause interference with the measurement, therefore, a measurement of NO2 mixing ratio is also made.

The 2D-S Stereo Probe is an optical imaging instrument that obtains stereo cloud particle images and concentrations using linear array shadowing. Two diode laser beams cross at right angles and illuminate two linear 128-photodiode arrays. The lasers are single-mode, temperature-stabilized, fiber-coupled diode lasers operating at 45 mW. The optical paths are arbitrarily labeled the “vertical” and “horizontal” probe channels, but the verticality of each channel actually depends on how the probe is oriented on an aircraft. The imaging optical system is based on a Keplerian telescope design having a (theoretical) primary system magnification of 5X, which results in a theoretical effective size of (42.5 µm + 15 µm)/5 = 11.5 µm. However, actual lenses and arrays have tolerances, so it is preferable to measure the actual effective pixel size by dropping several thousands of glass beads with known diameters through the object plane of the optics system.

Fine depth resolution profiling of the top 100 m of the ice column is achieved with this radar designed to map variations in the snow accumulation rate. When operated from aircraft, it operates from 600 to 900 MHz providing 28-cm depth resolution in ice and when operated on the ground (500 MHz to 2 GHz) a 5.6-cm depth resolution in ice is achieved. This fine depth resolution enables area extensive spatial mapping of the annual accumulation layers.

The Airborne Earth Science Microwave Imaging Radiometer (AESMIR) is a passive microwave airborne imager covering the 6-100 GHz bands that are essential for observing key Earth System elements such as precipitation, snow, soil moisture, ocean winds, sea ice, sea surface temperature, vegetation, etc.

AESMIR’s channels are configured to enable it to simulate various channels on multiple satellite radiometers, including AMSR-E, SSMI, SSMIS, AMSU, ATMS, TMI, GMI, ATMS, & MIS. Programmable scan modes include conical and cross-track scanning. As such, AESMIR can serve as an inter-satellite calibration tool for constellation missions (e.g., GPM) as well as for long-term multi-satellite data series (Climate Data Records).

The most unique/cutting edge feature of the instrument is its coverage of key water cycle microwave bands in a single mechanical package—making efficient & cost-effective use of limited space on research aircraft, and maximizing the possibilities for co-flying with other instruments to provide synergistic science. State-of-the-art calibration, fully-polarimetric (4-Stokes) observations, and the ability to accommodate large/heavy sensors (up to 300 kg) are other features of AESMIR. AESMIR currently flies on the NASA P-3 aircraft.

With these capabilities, AESMIR is an Earth Science facility for new microwave remote sensing discovery, pre-launch algorithm development, and post-launch Calibration/Validation activities, as well as serving as a technology risk reduction testbed for upcoming spaceborne radiometers. In the latter role, AESMIR is already supporting the GPM, Aquarius, and SMAP missions.

2D-STAR is a dual-polarized L-band radiometer that employs aperture synthesis in two dimensions. This airborne instrument is the natural evolution of the Electronically Scanned Thinned Array Radiometer, which employs aperture synthesis only in the across-track dimension, and represents a further step in the development of aperture synthesis for remote sensing applications. 2D-STAR was successfully tested in June 2003 and, then, participated in the SMEX03 and SMEX04 soil moisture experiments.

The 2D-STAR instrument was developed as a research instrument with the flexibility to test options in the evolution of the technology as it existed in ESTAR (synthesis in one dimension, one polarization, and analog processing) to aperture synthesis in two dimensions, dual polarization, and digital processing. The 2D-STAR was designed to fly on a P-3 research aircraft (the NASA Orion P-3B), and to simplify installation, the size was chosen to be similar to that of ESTAR. Several options, such as the choice of the antenna array and number of bits in the digital processor, were made to accommodate potential research rather than efficiency of design.

In 1991, NASA initiated an airborne remote sensing program in conjunction with coordinated surface measurements for determining the mass balance of the Greenland ice sheet, which plays in important role in the rise of global sea level. Starting in 1995, NASA combined various efforts on the mass-balance studies into a coordinated effort called Program in Arctic Regional Climate Assessment (PARCA). The University of Kansas has been participating in this program to make airborne ice thickness measurements using coherent radar depth sounders. Since 1993, the authors have collected a large volume of ice-thickness data over the ice sheet. They have demonstrated that coherent radars can acquire ice thickness and internal structure data over the thickest part of the ice sheet and outlet glaciers located around the ice margin.

The AXCTDs measure the ocean salinity, or saltiness (proportional to conductivity), and temperature, which are necessary 1) for computing ocean density, stability and buoyancy, and 2) for identifying different ocean water masses.

The utility of millimeter-wave radars have been successfully used for cloud sensing and cloud microphysical studies. Studies of the impact of cloud feedbacks on the earth's radiation budget have underscored the importance of having a means of measuring the vertical distribution of clouds. Millimeter-wave radars can provide this information under most conditions, with high resolution, using a relatively compact system.

The Airborne Cloud Radar (ACR) for profiling cloud vertical structure was developed by the Jet Propulsion Laboratory and the University of Massachusetts in 1996. It is a W-band (95 GHz) polarimetric Doppler radar designed as a prototype airborne facility for the development of the 94 GHz Cloud Profiling Radar (CPR) for NASA CloudSat mission.

The ACR is a third-generation millimeter-wave cloud radar. While adopting the well tested techniques used by its predecessors, ACR also has a number of new features including an internal calibration loop, frequency agility, digital I and Q demodulation, digital matched filtering, and a W-band low-noise amplifier.