SIRAS-G, The Spaceborne Infrared Atmospheric Sounder: The Potential for High-Resolution IR Imaging Spectrometry From Geosynchronous Orbit

The Spaceborne Atmospheric Infrared Sounder for Geosynchronous Earth Orbit (SIRAS-G) represents a new approach to infrared atmospheric sounding of the Earth from geosynchronous orbit. SIRAS-G, one of nine proposals selected for development under NASA’s 2002 Instrument Incubator Program, is an instrument development effort for an instrument of less mass and power than heritage sounding instruments that offers enhanced capabilities for the measurement of atmospheric temperature, water vapor profiles, and trace gas column abundances. The SIRAS-G flight instrument concept is designed to measure infrared radiation in 2048 spectral channels with a nominal spectral resolution (∆λ/λ) of 1100. Combined with large area 2-D focal planes, this system simultaneously provides both high-resolution spectral and spatial imaging. In 1999, the SIRAS team built and tested the LWIR (12.0 – 15.4μm) SIRAS spectrometer under NASA’s Instrument Incubator Program (IIP-1999). SIRAS-G builds on this experience with a goal of producing and demonstrating a laboratory prototype instrument. In this paper, we describe planned development activities and potential future scientific instrument applications for this instrument concept. Introduction The Spaceborne Infrared Atmospheric Sounder for Geosynchronous Earth Orbit (SIRAS-G) is an instrument concept designed to provide highly accurate atmospheric temperature and water vapor profile measurements from geosynchronous orbit (GEO) to facilitate weather forecasting, severe storm tracking, and scientific research. The flight instrument concept measures infrared radiation in 2048 channels extending from 3.7μm to 14.8μm with a spectral resolution (λ/∆λ) of 700 to 1100. As currently envisioned, large format 512 x 512 (spatial by spectral), focal plane arrays (FPAs) will provide the maximum information collection given the current state of technology. SIRAS-G employs a wide field-of-view (WFOV) hyperspectral infrared optical system that splits the incoming radiation into four separate grating spectrometer channels. This allows for slow scanning of the scene, increasing dwell time, and improved radiometric sensitivity. Unlike competing technologies, such as Fourier Transform Spectrometers (FTS), the SIRAS-G grating spectrometers employ no moving parts or metrology lasers, leading to improved reliability over mission lifetime. SIRAS-G follows the successful completion of the 1999 NASA-sponsored SIRAS (Spaceborne Infrared Atmospheric Sounder) Instrument Incubator Program (Kampe and Pagano, 2002) which focused on the development of a compact LWIR spectrometer module as a potential follow-on to the Atmospheric Infrared Sounder (AIRS) instrument (Aumann et al., 2001). NASA Instrument Incubator Program SIRAS-G, one of nine, but the only industry-led proposal, selected for the third IIP solicitation in 2002 and is being managed by Ball Aerospace & Technologies Corp. (Ball). The NASA Instrument Incubator Program was established as a mechanism for the development of innovative technology concepts suitable for future space-borne Earth Science Enterprise (ESE) programs and as a means to demonstrate and assess the performance of these instrument concepts in ground, airborne, and engineering model demonstrations. IIP is funded through NASA’s Office of Earth Sciences Technology Office (ESTO). The goals set forth for an IIP program are to (1) develop and demonstrate mission development in less than thirty-six months; (2) develop the technology such that it is suitable for integration in an operational space instrument within eighteen months following the 3-year IIP development; (3) the instrument concepts developed under IIP must reduce instrument and measurement concept risk to allow the concept to be competitive in an Earth Science Enterprise Announcement of Opportunity; and finally, (4) the concepts shall enable new science and/or reduce instrument cost, size, mass and resource use. More information on the Instrument Incubator Program can be found at http://esto.gsfc.nasa.gov/programs/iip/. SIRAS-G Overview The SIRAS-G IIP effort will focus on advancing the SIRAS instrument concept for insertion into future Earth Science missions and on developing an engineering demonstration instrument. While the SIRAS-G hardware demonstration instrument is primarily intended as a laboratory demonstration, it will be built sufficiently robust to be upgradeable to an airborne instrument. As part of this program, a series of engineering studies will be conducted to demonstrate the applicability of SIRAS-G to a variety of critical earth remote sensing needs. SIRAS-G builds on the success of the earlier SIRAS-1999 IIP effort. Here we successfully demonstrated a compact long wavelength infrared (12-15.4μm) grating spectrometer with a spectral resolution (λ/∆λ) of 900 to 1200. In SIRAS-G, we will improve on the SIRAS-1999 spectrometer by increasing the spatial FOV to provide true hyperspectral imaging capability. One of the key benefits offered by SIRAS-G is in its ability to improve the spatial resolution of future sounders while simultaneously providing high spectral resolution. This allows more opportunities for clear sky measurements in the absence of a microwave instrument; a crucial factor in improving the yield of retrieved cloud-free scenes that can be assimilated into Numerical Weather Prediction (NWP) models. As an example, for the current state of the art instrument flying, the Atmospheric Infrared Sounder, currently on NASA’s Earth Observing System (EOS) Aqua satellite, Goldberg et al. (2003) found that only 4.5% of fields observed over oceans exhibited less than 0.6% cloud contamination. This is largely attributable to the relatively large footprint (13.5-km) of AIRS, which is flying in Low Earth Orbit (LEO). SIRAS-G, on the other-hand, is being designed for a 4-km footprint from GEO, and a LEO version of the same instrument concept would yield a 0.5-km footprint. SIRAS-1999 Results In 1999, the NASA JPL-lead SIRAS team undertook the development of an advanced instrument concept as a potential replacement for AIRS. This instrument concept is referred to as SIRAS-1999. The original SIRAS-1999 instrument concept was designed to meet the requirements of AIRS, but in a smaller package and with improved spatial resolution (0.5-km vs. AIRS 13.5-km). This effort focused on the development the SIRAS flight instrument concept suitable for LEO and the development of a hardware demonstration of the SIRAS LLWIR (12-15.4 μm) spectrometer (Spectrometer No. 4 in Table 1). A high-resolution infrared imaging spectrometer was built and tested at cryogenic temperatures in a laboratory environment. A detailed study of the size, mass, and power of a SIRAS-L (Low Earth Orbit) instrument configuration was also performed. Finally, it was demonstrated that the same spectrometer could meet the requirements of a GEO sounder. A system concept was developed that included scanning, passive and active cooling systems, the infrared spectrometers, fore-optics and the focal plane arrays offering a system of comparable performance, yet with considerable size, mass, and power savings. Reductions in subsystem complexity through modular design, the use of standard format FPAs and low-order gratings results in significant cost reductions when compared to AIRS. The flight instrument concept developed in SIRAS-1999 has four spectrometer modules that cover the 3.4 to 15.4 μm spectral region with the spectral bands broken out as shown in Table 1. A barrel scan mirror provides the ground coverage, and an all-reflective fore optic serves to focus the scene energy onto the slit. Scene energy was then split into four separate spectrometer modules via beamsplitters. The requirement for low background required that the spectrometer modules be cryogenically cooled to 140 K, and the focal planes to 60 K. Active cooling of the detectors was proposed for the flight instrument configuration using a splitStirling pulse-tube cooler. The laboratory spectrometer developed on this program is shown in Figure 1. The spectrometer measures 10 x 10 x 14 cm and weighs only 2.03 kg. For the purposes of laboratory measurements, a PV HgCdTe multiplexed detector array was provided on loan from the AIRS program. All hardware development and testing was performed at BALL. The most challenging optical system was the camera. This fast (F/1.7) optical system required near diffraction-limited performance over a wide field-of-view. Table 1. Preliminary Design Parameters for SIRAS Parameter Spectrometer Number