INFRARED GRATING SPECTROMETER

Objective of the flight was to test a prototype satellite solar occultation instrument designed for monitoring the concentrations of ozone (O3), water vapor (H2O), and nitric acid (HNO3) in the stratosphere. The experiment, was developed by a collaboration between the NOAA/National Environmental Satellite, Data and Information Service (NESDIS) that designed and built the prototype grating spectrometer, NASA's Goddard Space Flight Center which provided the gondola, attitude control and stabilization systems, sun-tracking equipment, and telemetry systems and HSS, Inc. that was in charge of the optical design of the spectrometer, including the telescope and grating system.

Final goal of the project was to demonstrate the feasibility of using a multi-detector infrared grating spectrometer, operating in solar occultation mode, as a low-cost, stable, and reliable satellite sensor for long-term atmospheric monitoring. The spectrometer observed solar radiation as it passed tangentially through the atmosphere during sunset, allowing the retrieval of vertical concentration profiles of selected trace gases in the upper stratosphere between 25 and 39 km in altitude.

The apparatus consisted of a Cassegrain telescope with a 0.6-meter focal length and a fixed grating spectrometer equipped with eight discrete infrared detectors. The telescope directed sunlight through an entrance slit into the spectrometer, which had an over/under Ebert layout with a 0.5-meter focal length. Instead of scanning the spectrum, the spectrometer simultaneously measured in eight preselected spectral intervals associated with absorption bands of target gases. Two intervals were centered on the 9.6 µm ozone band, two on the 6.6 µm water vapor band, and one on the 11.3 µm nitric acid band. The remaining intervals were initially intended to detect CO2 and Freon-12, although issues with atmospheric transparency and nitrogen interference limited their usefulness in this flight. Each detector used a thermopile with a built-in filter-window to isolate the desired wavelength, and the optical signal was mechanically chopped and then processed via amplification, synchronous detection, and digitization. The data were transmitted to Earth via a 50 kbps PCM telemetry system.

Sun tracking was essential for the instrument's operation and was achieved through a combination of NASA's gondola-mounted tracking system for high solar elevations and a dedicated two-pair infrared detector system embedded in the telescope for low solar elevations. The field of view of the spectrometer was carefully controlled, and pointing accuracy was maintained to within 3 arcminutes during ground tests.

Balloon launched on: 6/21/1982 at 20:06 utc
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 196.095 m3 (12.70 Microns - Stratofilm) / SF 260.05-050-NSCHR-01
Balloon serial number: W6.925-1-01
Flight identification number: 1291P
End of flight (L for landing time, W for last contact, otherwise termination time): 6/22/1982 at 3:15 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): F 8 h
Landing site: 30 miles S of Brownwood, Texas, US
Payload weight: 489 kgs

The balloon was launched from the National Scientific Balloon Facility, in Palestine, Texas at 20:06 UTC on June 21, 1982. During the flight, the balloon rose to 39 km and remained at that altitude through the sunset period. The payload was recovered near Brownwood, Texas.

The occultation geometry enabled observations of solar radiation through decreasing tangent altitudes, from 39 km down to about 25 km. The instrument collected high-quality transmittance data with very low noise levels and high signal-to-noise ratios. These were then converted into vertical profiles of trace gas concentrations using atmospheric ray-tracing and analytic transmittance models. The ozone profiles retrieved agreed well with simultaneous in-situ ECC-sonde measurements below 32 km and with independent satellite data above that level. Water vapor and nitric acid retrievals showed profiles consistent with previous satellite and in-situ observations, although retrieval uncertainties increased with altitude due to reduced atmospheric absorption and signal strength.

• Balloon-based infrared solar occultation measurements of stratospheric O3, H2O, HNO3, and CF2C12 National Environmental Satellite, Data, and Information Service, 1987
• Data processing algorithms for inferring stratospheric gas concentrations from balloon-based solar occultation data Washington, D.C.: U.S. Dept. of Commerce, National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service
• High-altitude balloon test of satellite solar occultation instrument for monitoring stratospheric O3, H2O and HNO3 Journal of Atmospheric and Oceanic Technology, vol. 1, March 1984, p. 87-100
• National Scientific Balloon Facility Annual Report FY 1982 National Center for Atmospheric Research, February 1983