Identification and Correction of Residual Image in the O₂ A-Band of the Orbiting Carbon Observatory

The detector used for the O 2 A-band (0.76 μm) of the National Aeronautics and Space Administration's Orbiting Carbon Observatory (OCO) employed a HyViSI Hawaii-1RG sensor, operating at 180 K in a rolling read-out mode. During the thermal vacuum testing of the flight instrument, it was discovered that the detector exhibited residual images that lasted for many seconds and were of sufficient magnitude to compromise the mission objectives. Independent testing of flight-spare detectors revealed that the problem was common to all and was not simply a fault of the flight detector. The residual image was found to depend upon even-order derivatives of the spectrum, and its decay was a function of the number of frames rather than time. An empirical model was developed, which represented the measured spectrum in terms of the true spectrum and a history of all previous changes in the spectra. On the basis of the model, an algorithm was devised to correct spectra for the effects of residual image, using a time-marching analysis of a history of previous spectra. The algorithm was tested with spectra acquired during the second thermal vacuum test of OCO and was found to reduce the effect of residual image to almost the noise level of the detector. Numerical simulations indicate that residual image has a negligible impact on retrieved concentrations of O 2 and CO 2 once the spectra have been corrected.