## 2 Data and Methods
### BNU-1 Data  
Since its launch, BNU-1 has acquired over 20,000 images of polar regions. These images were received from the ground stations from the satellite directly. We call these images BNU-1 level-0 images. BNU-1 level-0 images contains the observations in panchromatic (P), blue (B1), green (B2), red (B3), and red-edge spectral (B4) bands which are recorded as digital number (DN) values. The DN images of B1, B2, and B3 bands can be used to compose true RGB color images. Figure 2 shows two RGB color composite images of Amery Ice Shelf, Antarctica Synchronously acquired by the CMOS1 and CMOS2 sensors of BNU-1 on 8 October 2019 (Orbit Number: 200214500). The color misrepresentation issue can be observed as the image appears purplish as it gets closer to the center field of view (FOV) and appears yellowish as it gets farther to the center FOV (Figure 2). A similar issue also appeared in the other BNU-1 level-0 images.  
To address the color misrepresentation issue, several series of the DN values extracted cross the satellite flight direction from P, B1, B2, and B3 bands are shown in Figure 3a. The corresponding BNU-1 image in B1 band is shown in Figure 3b. The image shown in Figure 3b was stitched together using images from the CMOS1 and CMOS2 sensors. The numbers of 1-5056 columns of the image are from the CMOS1 sensor, while the numbers of 5057-10112 columns are from the CMOS2 sensor. As shown in Figure 3a, great consistency is observed between the two multispectral bands (B2 and B3) and the panchromatic band (the black line in Figure 3a). However, the values of the blue (B1) band exhibit abnormal changes in the center FOV and in the off-center FOVs. The B1 band appears to be higher near the center FOV and declines generously towards both the edge areas of the mosaic image, which may cause the color misrepresentation issue shown in Figure 2. It is worth noting that the DN values in the blue band perform even lower than that in the red band in the first 3000 s columns (Figure 3a), which indicates a further correction procedure is required. The red box highlights where the values in the B1 band perform troublesome due to the higher DN values near the center FOV, while the rest of the DN values due to lower column values. Similar DN offsets are observed in the BNU-1 imagery acquired in the North Pole region, but the trend of the column means that the blue band is flipped, since to the scanning mode is switched between both polar regions.  
Figure 2: Two RGB color composite images in Amery Ice Shelf, Antarctica acquired by BNU-1. (**a**,**b**) are acquired by the CMOS1 and CMOS2 sensors, respectively. The bright pixels in the images represent ice and snow surfaces, while the dark pixels represent water bodies.  
After a vast amount of image data reviewing, it was found that the intensity performs good uniformity in the panchromatic bands of both the CMOS1 and CMOS2 sensors that are carried on the BNU-1 satellite, while the obvious unbalance intensity existed over the true color composite images. The intensity of B1 band appeared to be higher surrounding the centerline and declined generously towards both the edge areas of the image. Therefore, it is necessary to have a relative sensor normalization for BNU-1 images to correct radiometric errors before their applications.