Abstract: An apparatus is provided that includes a plurality of reflectors arranged over an area, and a plurality of transceivers. A first of the transceivers is configured to transmit radiation at each of the plurality of reflectors at a first wavelength and a second wavelength, and receive radiation reflected from each of the plurality of reflectors. A second of the plurality of transceivers is configured to transmit radiation at each of the plurality of reflectors at a third wavelength and a fourth wavelength and receive radiation reflected from each of the plurality of reflectors. The apparatus includes a processor configured to calculate a change in concentration of a gas within the area between a first time and a second time based upon signals received from the transceivers.

Abstract: A spectrum is automatically searched to select monochromatic spectral points, monochromatic transmittances or radiances are automatically calculated at the selected monochromatic spectral points, for one or more viewing geometries and for non-homogeneous atmospheres belonging to an ensemble of non-homogeneous atmospheres, and a weighted sum of the monochromatic calculated transmittances or radiances is automatically determined. The individual spectral points are selected so that the weighted sum is representative of calculations of spectrally averaged transmittances or radiances for the viewing geometries and for any non-homogeneous atmosphere belonging to the ensemble, in spectral bands to which the selected spectral points belong. The weighted sum is used for computing transmittances or radiances in emitting or scattering atmospheres.