The existence of bad pixels on the image sensor where the sun spot may fall will affect the accuracy and even function, while for the multi-aperture sun sensor, with a N �� N imaging point, the distribution of the image sensor area is relatively large, the damage to individual pixels has almost no effect on the system accuracy.From the above analysis we can find that multi-aperture sun sensors are superior to single-aperture sun sensors as far as the accuracy, reliability, and the requirements of the image sensor are concerned, so multi-aperture sun sensors must be the future direction of the technology.2.?Modeling of the Sun Sensor A multi-aperture sun sensor is commonly composed of a piece of mask and a CCD or APS (Active pixel sensor) CMOS detector below at a distance of about several millimeters.
The sun spot is formed at the detector through the aperture, as shown in Figure 1(a). In the Figure 1(a), mc and nc denote the coordinates of the sun spot in the sun sensor body coordinates, l denotes the distance between the sun spot and hole sight point, h denotes the distance between the mask and the focal plane, �� denotes the incidence angle and ��, �� denote the sunray horizontal and azimuth orientation in the sun sensor body coordinates, respectively.The relationships are as follows:��= arctan(lh),��= arctan(mch),��= arctan(nch)(1)l=mc2+nc2(2)tan ��=(tan ��)2+(tan ��)2(3)For a given distance h, the greater the number of apertures the higher the accuracy.
However, the assembly and alignment errors will significantly impact the system accuracy when the location accuracy is of 1/100 pixels, about 0.
005��. Generally, the sub-pixel accuracy of a single sun spot is at the level of 0.05�C0.1 pixels, to obtain the whole sun spot location centroid accuracy of 1/100 pixels, the number of apertures should be more than 25 (1/100 = 0.05/251/2), so the aperture pattern is designed as a 6 �� 6 array in this paper, as shown in Figure 2.Figure Dacomitinib 2.6 �� 6 multi-aperture mask.Assuming Anacetrapib the sun angles are ��,�� respectively and the sun spot centroid is mc, nc their relationship can be expressed by Equation (1).
In fact, the Equation becomes much more complicated because of the refraction which occurs when a sun ray goes through different media sequentially (vacuum-mask glass-vacuum-detector protecting glass-air) before it arrives at the detector, as shown in Figure 3.Figure 3.illustration of the sunray tracking route in the sun sensor.From Figure 3 we can conclude that the greater h the higher accuracy, however, the h must ensure that the image of sunlight in the field of view (FOV) is still within the photosensitive surface of the detector.