# HYPSO-1 CubeSat: First Images and In-Orbit Characterization
## 3 In-Orbit Payload Performance
### Spatial Destriping  
Stripes in hyperspectral datasets commonly limit their use [48]. One of the major causes of striping in push-broom scanning is differences in the radiometric sensitivity between pixels. Even sensitivity differences of a few percent between pixels can lead to stripes in the collected data. The differences can be systematically mitigated by improving the estimate of the relative radiometric sensitivity. The relative sensitivity differences between the pixels of the HYPSO-1 sensor are estimated over a mostly uniform ocean scene. Clouds and land are excluded from the analysis, and the laboratory radiometric calibration is applied to the data. Because the expected correction is multiplicative, the scene is analyzed after a logarithm is applied. As the scene is uniform, most of the across-track variation is due to stripes. The magnitude of the stripes within each band is determined by computing the difference of each along-track column of pixels from its across-track neighbor, pixel by pixel. The median of the differences is taken to be the magnitude of each stripe, in order to limit the effect of spectral features which may be present in the ocean. The cumulative sensitivity differences are then computed relative to mean intensity, and the mean is removed for each wavelength. The multiplicative correction to the relative sensitivity differences is then the exponential of the cumulative differences (Figure 13). The correction is filtered to remove vertical stripes which originate from the smile artifact [24]. The dust particle moved during launch, as seen in Figure 13, and new calibration coefficients were needed to compensate for this movement. The effect of the multiplicative radiometric correction is evaluated on a second ocean scene. The spectral pixelwise variance between the original and corrected test scene is about 9.1% of the total variance of the former. Principal Component Analysis (PCA) is applied to ocean pixels of the scene, and the different components are inspected for stripes to evaluate the correction. Before the multiplicative correction, stripes are visible starting at the first component, but after the multiplicative correction, they are not visible until after the sixth component. The remaining variance in these higher components is about 3.3% of the total variance, of which stripes are only one part. Therefore, about 11% of the variance was due to stripes prior to the correction and so the multiplicative correction reduces striping by almost a factor of four in this sense. This result will be scene dependent but gives an indication of the effect from the correction. Figure 12: ToA reflectance spectral comparison of HYPSO-1 and RadCalNet calibration site Railroad Valley Playa. The date of each capture is stated in the top left. The HYPSO-1 ToA reflectance was calculated using the Chance+Kurucz solar spectrum and the Thuillier solar spectrum. See Table 7 for information on the geometry. The correction can be applied to all HYPSO-1 images because it is multiplicative rather than additive. It is combined with the standard radiometric calibration for conversion from digital number to ToA radiance. The change in radiometric sensitivity between laboratory calibration and the observing of the test image is likely due to the relocation of dust and small particles within the imaging system arising from vibration during the spacecraft tau, as discussed further in [49]. The scene used to characterize and correct for the striping effect is of the open ocean, a relatively dark target with limited signal to measure. Utilizing clouds may produce better outcomes; however, due to the linear response of the imaging sensor, images easily become oversaturated over bright targets. Although utilizing a uniform ocean scene for spatial destriping is effective for the spectral range of 430 nm to 600 nm, striping effects still appear above 600 nm. The calibration coefficients for each capture should improve with time. A more campaign to achieve a flat radiometric response is planned for the future.