AIRINTE

Aircraft Integration and Flight Testing of 4STAR

1 October 2010 - 30 September 2011

Lead Scientist: Connor Flynn

Observatory: AAF

The goal of the research effort was to develop and demonstrate an airborne Sun-sky spectroradiometer to provide information on aerosols, clouds, and trace gases extending beyond what can be derived from existing airborne sun photometers such as the 14-Channel NASA Ames Airborne Tracking Sunphotometer (AATS-14) and to improve compactness, modularity, and versatility. The enhanced instrument we used was an airborne Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR-Air) based on the ground prototype (4STAR-Ground) that we have developed and extensively tested over the past several years. This proposal was supplemental to a companion NASA ROSES 2008 proposal which had been selected for funding. The 4STAR instrument concept combines the sun-tracking ability of the current AATS-14 with the sky-scanning ability of the ground-based AERONET sun/sky photometers, while extending both AATS-14 and AERONET measurements by providing full spectral information in 1536 wavelength channels from the UV (210 nm) to the SWIR (1690 nm). The full spectral information was used to improve determination of gas phase atmospheric components which in turn yielded improved aerosol measurements. Sky-scanning measurements enabled retrievals of size distribution modes and also provided information on aerosol type via retrievals of complex refractive index and shape. Cloud property retrievals combining airborne measurements of zenith radiance by 4STAR and upwelling flux by an auxiliary radiometer are better constrained than analogous ground-based techniques. Fast temporal sampling permits clear/cloudy transition studies. Our proposed approach to providing these enhanced capabilities was to link via fiber optics the sun-tracking and sky-scanning optical collectors mounted on the exterior of the aircraft to rack-mounted spectrometers housed within the aircraft cabin. This compact modular design yielded easier integration on a wider range of aircraft while also affording the potential to substitute or add more specialized spectrometers in future versions. To demonstrate the feasibility of this approach we developed and extensively tested a ground-based prototype surpassing critical performance requirements of pointing accuracy, optical throughput and sensitivity, repeatability of fiber optic rotational couplings, and stray light rejection. These tests and our airborne experience with AATS demonstrated that our approach can overcome the challenges of an airborne version (4STAR-Air). To limit costs and increase chances of success, the companion NASA ROSES proposal focused on the limited set of goals comprising the development and demonstration of the airborne prototype 4STAR-Air. Budgetary constraints precluded requesting full support for final hardening and testing of the 4STAR with pressurized aircraft. Therefore, we proposed to integrate the 4STAR into the DOE-supported G1 under support from the DOE ARM Aerial Facility (AAF). This supplemental investment by the DOE AAF highly leveraged the funding provided by both agencies and significantly improved the probability of success. The integration effort included mechanical accommodation as well as electrical and data communication details. In essence, all of the technological challenges we have successfully surpassed with the ground-based instrument were implemented and validated with the airborne system. We conducted flight tests during clear and broken skies to validate the capabilities of the instrument to track the sun under a variety of conditions and to perform programmed sky-scans of diffuse sky radiance. Test flight patterns included spirals as well as level legs at various altitudes. The 4STAR collected direct solar and sky scanning measurements suitable for retrieving aerosol properties with customized versions of AERONET retrieval code.