Abstract
A common problem with space-based sensors is sharp spikes in the signal caused by ionizing radiation striking the detectors or analog signal chain. To minimize the spectral corruption caused by these spikes, a new spike correction method has been developed for the Cross-track Infrared Sounder (CrIS) on the Suomi National Polar-orbiting Partnership (SNPP) spacecraft. CrIS is an infrared, Fourier transform spectrometer used to make weather and climate observations. Radiation-induced spikes are a particularly serious problem for Fourier transform spectrometers since a single spike in the interferogram will cause corruption throughout the entire length of the spectrum. For CrIS, a spike large enough to cause a pixel to be flagged as invalid happens every few days. However, a number of smaller spikes that cause corruption but are not sufficiently large to be detected by the present quality control software happen daily.
The CrIS sensor was launched with a simple onboard spike correction method that set any interferogram samples above a given threshold to zero. This method did not take into account interferogram offsets and low frequency drifts. As a result, it caused more errors than it corrected and has been mostly disabled by setting the threshold to a very high value. The new ground-based correction method was subsequently developed to remove interferogram spikes. This algorithm works by modeling a radiation spike then subtracting the modeled spike from the interferogram that contained the spike. The radiation event is much faster than the detector electronic, so the shape of the spike is determined by the electronic response of the electronics. The amplitude and position of the spike is determined from a least-squares fit to the interferogram data. The spikes occur in the raw interferogram, but to reduce bandwidth, the digitized interferograms are processed with a Finite Impulse Response (FIR) filter and decimated before being transmitted to the ground. The filtering and decimating process spreads and alters the appearance of the original spike. However, since the FIR filtering and decimating process is linear, it is possible to perform a least-squares fit between the filtered and decimated interferogram containing a spike and a modeled spike that has also been filtered and decimated. Once the amplitude and position of the spike have been estimated, the effects of the spike can readily be calculated and subtracted from the original interferometer. The method has been very successful in removing spikes and is presently being evaluated for inclusion into the operational CrIS ground processing software.
Removing Radiation-Induced Spikes from Fourier Transform Data
A common problem with space-based sensors is sharp spikes in the signal caused by ionizing radiation striking the detectors or analog signal chain. To minimize the spectral corruption caused by these spikes, a new spike correction method has been developed for the Cross-track Infrared Sounder (CrIS) on the Suomi National Polar-orbiting Partnership (SNPP) spacecraft. CrIS is an infrared, Fourier transform spectrometer used to make weather and climate observations. Radiation-induced spikes are a particularly serious problem for Fourier transform spectrometers since a single spike in the interferogram will cause corruption throughout the entire length of the spectrum. For CrIS, a spike large enough to cause a pixel to be flagged as invalid happens every few days. However, a number of smaller spikes that cause corruption but are not sufficiently large to be detected by the present quality control software happen daily.
The CrIS sensor was launched with a simple onboard spike correction method that set any interferogram samples above a given threshold to zero. This method did not take into account interferogram offsets and low frequency drifts. As a result, it caused more errors than it corrected and has been mostly disabled by setting the threshold to a very high value. The new ground-based correction method was subsequently developed to remove interferogram spikes. This algorithm works by modeling a radiation spike then subtracting the modeled spike from the interferogram that contained the spike. The radiation event is much faster than the detector electronic, so the shape of the spike is determined by the electronic response of the electronics. The amplitude and position of the spike is determined from a least-squares fit to the interferogram data. The spikes occur in the raw interferogram, but to reduce bandwidth, the digitized interferograms are processed with a Finite Impulse Response (FIR) filter and decimated before being transmitted to the ground. The filtering and decimating process spreads and alters the appearance of the original spike. However, since the FIR filtering and decimating process is linear, it is possible to perform a least-squares fit between the filtered and decimated interferogram containing a spike and a modeled spike that has also been filtered and decimated. Once the amplitude and position of the spike have been estimated, the effects of the spike can readily be calculated and subtracted from the original interferometer. The method has been very successful in removing spikes and is presently being evaluated for inclusion into the operational CrIS ground processing software.