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 and the University of Denver that provided the Sun tracking 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 instrument was a multi-detector grating spectrometer that detects radiation continuously in eight discrete infrared spectral intervals (channels). Six of the channels were located in the absorption bands of HNO3, CF2C12, 03, and H20 while the other two channels were not used. The detectors were uncooled thermistors as more sensitive detectors were unnecessary because the sun is such an intense source of radiation. Radiation was focussed onto the spectrometer by an f/4.5 Cassegrain telescope with a focal length of 600 mm. The angular field of view was a rectangle 4.5 arc min in azimuth and 8.3 arc min in elevation.

Due to some issues experienced by the instrument during the first two flights in 1982 and 1983, the sun-tracking system was redesigned for this flight. In the first two flights a NASA GSFC balloon-borne gondola was used on which the entire instrument pivoted in azimuth and elevation on two sets of gimbels. For this flight, the University of Denver supplied a biaxial solar tracking system (sun-seeker) that used a servo-controlled plane mirror for elevation and a spherical mirror for focusing solar radiation onto the spectrometer. The system tracked the sun with high accuracy, maintaining the solar image on the spectrometer slit within ±6–8 arc minutes, and typically achieved an average pointing error of just 2 arc minutes during stable flight.

To compensate for payload motion and varying solar intensity, the system included high-torque servo drives with hysteresis clutches and a dual CdS-cell setup that adjusted gain based on solar brightness. These features allowed it to function reliably despite atmospheric scattering and payload rotation, even at high altitudes and during challenging conditions such as low solar angles.

Additionally, the telescope's baffling was increased to reduce scattered radiation, prompted by an unexplained data feature observed in the second flight.

Balloon launched on: 7/5/1985 at 19:50 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 SF-319.18-070-NSCH-03
Balloon serial number: W11.62-1-28
Flight identification number: 1403P
End of flight (L for landing time, W for last contact, otherwise termination time): 7/6/1985 at 2:56 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 7 h
Landing site: 20 miles NW of Ozona, Texas, US

This was the third flight of the instrument. The balloon was launched from the National Scientific Balloon Facility, in Palestine, Texas at 19:50 UTC on July 5, 1985. The balloon reached a float altitude of 39 km in the late afternoon and remained within 1 km of that altitude until the experiment was terminated after sunset. Total flight time was 7 hours and the payload was recovered the next day 20 miles NW of Ozona, Texas.

Generally, the instrument operated reliably and provided data with low noise. However, there were two problems: First, the electronic gain for channel 3 was unstable. As a result, only the data in channel 4 were usable for sounding ozone. Second, transmission of data from the gondola to the ground was temporarily interrupted during the observations between the tangent heights of 38 and 36 km.

As a conclusion, the researchers retrieved stratospheric mixing ratio profiles of ozone, water vapor, nitric acid, and CFC-12 between 12 and 35 km. The ozone results were reliable and matched well with in-situ measurements, supporting the method's use for satellite monitoring. Water vapor results had larger uncertainties and showed discrepancies in altitude dependence. Nitric acid and CFC-12 profiles were consistent with past data, though no concurrent measurements were available. Overall, the experiment improved on previous flights, especially in detecting CFC-12, but the conclusion was that more studies were needed to validate results for all gases.

• Balloon-based infrared solar occultation measurements of stratospheric O3, H2O, HNO3, and CF2C12 Washington, D.C. : 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