Spectrally dependent radiometric measurement errors due to CCD serial pixel-to-pixel readout interference in the Ocean Color Instrument of the NASA PACE mission

The Ocean Color Instrument on NASA’s PACE mission is a 322-887nm hyperspectral imager with 1km x 1km nadir spatial resolution and 5nm spectral resolution utilizing charge-coupled devices (CCDs) operating in Time Delay Integration (TDI) mode where each TDI column represents a different wavelength in 0.625nm increments. After TDI, the charge is moved into serial output pixels and read out. The spatial resolution requires an 8.5MHz readout rate. This only allows 59ns for the CCD reset and video to be asserted and settled before sampling. The response exhibits serial pixel-to-pixel readout interference due to the lack of full settling. Each serial pixel value has a dependence on the value of the preceding pixel value. This leads to a spectrally dependent radiometric measurement error of up to 0.3%. We explain the operation of the detection system, the behavior of the interference, and show the resulting measurement error based on data from both ground testing and on-orbit characterization.