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