Abstract

High spectral resolution (≈.005 to .05 cm^–1) ground based solar infrared absorption experiments have demonstrated capability to sound atmospheric species. A sample of these includes HCN, C₂H₂, C₂H₀, CH₄, NO₂, and HCl from solar absorption spectra obtained in the 3 to 3.5 jam region. The GLAES concept is aimed at obtaining similar absorption spectra in light that passes through the atmosphere and is reflected from the earths surface back out to the satellite. This would enable the solar absorption technique to be expanded from a limited number of ground base cites to satellite borne global mapping. Spectral resolution of the order a few hundredths cm^-1 is necessary to discriminate against absorption continuua introduced by the reflecting media and the atmosphere, and will enhance identification of the target molecular species absorption lines. The GLAES approach involves tuneable piezo-electric etalons and a CVF to obtain spectra across an interval ≈ 1 cm^-1 any where on the range ≈ 2.5 to 4.5 um, with a scan time of ≈ 0.1 s. Several hundredths cm^-1 resolution can be achieved with a 7.7 mrad field of view for example. The corresponding perpendicular footprint is ≈ 5.6 km from 700 km polar orbit, and ≈ 275 km from geosynchronous. The most effective mode of operation is to use a front end mirror to point at the solar specular point on a body of water (lakes, ocean, etc.) larger than the footprint. At 3.5 μm and 60 deg angle of solar incidence for example the glitter radiance (≈ corrected for continous atmospheric absorption) is of the order 3.5×10^–7 w/cm^–2 sr^–1(cm^–1)^–1. This is considerably larger than 290 K black body radiance ≈ 1.9×10^–8 w/cm^–2sr^–1(cm^–1)^–1, or solar Lambertian (typical terrain) reflected radiance of ≈2.2×10^–8 w/cm^–2sr^–1(cm^–1)^–1. By using the solar glitter and 4 inch aperture/etalon diameters a S/N ≥ 100 per spectral sample can be achieved by using an InSb PV detector cooled to ≈ 65 K. This S/N is more than adequate to detect 1 % absorption lines (which typically include several spectral samples). The detector cooling could be achieved by Stirling cycle refrigerators, or by passive radiation for appropriate satellite operations/accomodation. Modest spectrometer cooling of the order ≈ 160 K is required. No optics cooling is required. A sun synchronous near-polar orbit at altitude ≈ 700 km, with orbital plane ⊥ to the direction to the sun is an interesting candidate for GLAES deployment. It would provide for nearly ideal viewing into ≈ 75 degree solar incident glitter, for nearly continous coverage, and is the best low altitude orbit for passive radiative cooling. More instrument description, simulated solar glitter absorption spectra, and estimates for the retrievability of mixing ratio and/or column density for selected molecular species will be presented.

© 1991 Optical Society of America