A wide field of view Gas Filter Correlation Radiometer (GFCR) has been developed to make solar occultation measurements of the vertical methane distribution in the stratosphere from a sounding rocket platform. The GFCR has demonstrated a 50° solar acceptance angle that allows for a GFCR measurement during every rotation of the payload without active orientation control. The flat surface of a plano-convex ZnSe lens was etched to diffuse the projected image of the sun. By diffusing the incident solar radiation through a wide angle, sufficient radiation could be directed to the collimating GFCR optics even when the optical axis points as far as ± 25° away from the Sun. The system can be configured to measure other gaseous species with spectral bands in the 2 - 6 μm region by simply changing the bandpass filter and the correlation gas. In a laboratory calibration, the optical density of methane in a test cell was varied from 10^-4 to 10^-2 atm·m as the GFCR correlation cell optical density was held at 2.5×10^-3 atm-m. The process showed that measurements with a signal to noise ratio > 30:1 can be expected when the system operates in altitudes from 25 to 40 km. The GFCR performed with a correlation of 99.7% to the prediction of a theoretical model created with the HITRAN database. Sensitivity to gas distributions at other altitudes can be optimized by changing the gas pressure in the correlation cell. The payload featuring the GFCR is scheduled to be launched on an Enhanced Orion sub-orbital sounding rocket from NASA Wallops Flight Facility in April 2003. Future applications include validation and truthing for space-born remote sensing systems.

Date Published: 1 April 2003
PDF: 9 pages
Proc. SPIE 5073, Thermosense XXV, (1 April 2003); doi: 10.1117/12.486309

Published in SPIE Proceedings Vol. 5073:
Thermosense XXV
K. Elliott Cramer; Xavier P. Maldague, Editor(s)