Research On Digital Sun Sensor

In the aerospace science field, artificial satellite has been the most important component. Sun sensor determines the coordinates of artificial satellite in sun coordinates system through the sunlight position detection, which is a key component of artificial satellite attitude determination system. Along with the miniaturization of artificial satellite, we also propose new requirement for the artificial satellite device. We need light weight, small size, low power consumption and high accuracy new sun sensor. This would guarantee the normal operation of the satellite, increase satellite payloads, and improve artificial satellite efficiency. At present, it is a new direction to use CMOS Image Sensor as metering equipment of digital sun sensor in the world.CMOS image sensor OV7640 is used as measuring equipment in this digital sun sensor. The OV7640 camera chip is a low voltage CMOS image sensor that provides the full functionality in a single-chip VGA (640×480) camera and image processor. The OV7640 sensor has an image array capable of operating at up to 30 frames per second (fps) in a small footprint package. The OV7640 is controlled through OmniVision's Serial Camera Control Bus (SCCB) interface, which is a simple I~2C interface, with complete user control over image quality, format and output data transfer. All required image processing functions, including exposure control, gamma, white balance, color saturation, hue control etc., are also programmable through the SCCB interface.The sun rays are introduced into sun sensor through a pinhole camera, which form a sun spot on the image sensor. We calculate the sun spot's relative coordinates to the pinhole, and based on simple geometric principle obtain the sun's rays' angle. Then we determine satellite's relative angle to the sun.We use DSP as read data unit and processing unit, as well as CMOS sensor control unit, which is linked with OV7640 SCCB interface and control CMOS settings by simulating the I~2C interface. After anglicizing on the data signals, and DSP internal storage space and DSP instruction cycle, a digital circuit is adding between CMOS image sensor and DSP to reduce CMOS output rate and compress of storage space. Then the data is send to the GPIO DSP modules. When total 75K data of 640×480 (300K) pixels of OV7640 is read into DSP, the data processing begin. All data is stored in a two-dimensional array. 2D array's order of each pixel is the coordinates on the image sensor plane. In data processing, first read data, and then revert it to its original signal. According to an algorithm, calculate the relative coordinates of the sun spot from image signal, then confirm the sun's rays' angle based on the dimensions of sun sensor, which is also the artificial satellite's relative angle to the sun coordinate system.