1. A method for spectral signal detection comprising: scanning fields of view (FOV's) of a panoramic field of regard; capturing radiance in the FOV's; selecting an FOV having the spectrum of interest; recalling n FOV's previous to the FOV having the spectrum of interest; obtaining n estimated background spectra from the n FOV's, respectively; and determining a difference spectrum from each of the n estimated background spectra and the FOV having the spectrum of interest.