Star sensor image acquisition system

Star sensor is a high-precision attitude sensor widely used in attitude measurement of satellites, long-range ballistic missiles, and deep space exploration. The standard image sensor used in the previous generation of star sensors was CCD. With the advancement of CMOS active image sensor technology, replacing CCD with small size, low power consumption, and strong anti-interference ability in the image acquisition system of star sensors has become the goal of the development of the next generation of star sensors.

Disadvantages of CCD:

During the widespread application of CCD in star sensors, its shortcomings have gradually emerged. Firstly, the pixel units of CCD are composed of MOS capacitors, and the electronic effects excited by charges are easily affected by space radiation. Therefore, the ability of CCD to resist space radiation is relatively poor; Secondly, the manufacturing process of CCD is complex and cannot be compatible with the manufacturing process of general integrated circuits. Therefore, CCD requires support from more complex peripheral circuits; In addition, CCD requires a variety of power sources, and image charges need to be output in serial order to reach the output terminal. These shortcomings make it impossible for CCD based star sensor application systems to significantly reduce technical indicators such as volume, power consumption, and weight on the existing basis.

Advantages of CMOS:

The CMOS image sensor highly integrates the image sensing part and control circuit into the same chip. Its volume is significantly reduced, and its power consumption is also greatly reduced, which is only one-third to one-tenth of that of CCD. It has many advantages such as light weight, high reliability, high integration, low cost, wide dynamic range, radiation resistance, and no dragging, overcoming the inherent shortcomings of CCD, It fully meets the requirements of star sensors for image sensors.

Star sensors are mainly composed of optical systems, image sensor circuits, and control and data processing circuits. The image sensor part includes a CCD (or APS) image plane component, a driving circuit, a timing signal generator, and a video signal processor; The control and data processing circuit includes hardware and software such as digital signal processors (star image memory, star image address generator, program memory, star catalog memory, CPU) and interface circuits for connectivity analysis, subdivision algorithms, star map recognition, attitude angle calculation, and coordinate conversion. Hardware structure diagram

Fig.1-1 Work diagram of star sensor

The optical system completes the optical imaging of the starry sky, imaging it on the optical focal plane; The image detector is placed on the optical focal plane to complete photoelectric conversion, converting the star image into a video electrical signal output; The video processor completes video processing, including noise reduction (correlated double sampling), bias, and gain adjustment; Finally, perform A/D conversion to output digital images; The timing signal generator and driver provide the working timing for controlling the image sensor detector and video signal processor.

The output digital image is transmitted to the digital signal processor in the control and data processing circuit for star image coordinate extraction, coordinate transformation, star image recognition, attitude calculation, and other processing work to determine the attitude information in the inertial coordinate system. The remote control and telemetry interface receives instructions from the onboard main control computer and provides telemetry data of its own working status to the main control computer. The attitude control system interface is responsible for outputting the attitude information calculated by the star sensor to the attitude control system.

Fig.1-2 Hardware block diagram of star sensor