With the continuous development of aerospace technology, the accuracy of spacecraft attitude measurement is also constantly improving. Star sensors are widely used attitude measurement components in the aerospace field. Due to their high measurement accuracy, high degrees of freedom, strong anti-interference ability, and no accumulated error, star sensors play an important role in the positioning and orbit change of spacecraft.
Traditional star sensor optical systems use the focal length of the optical system to calculate the incident angle of the star point relative to the optical system by detecting the position of the scattered circle on the image plane. The positioning accuracy of the incident light mainly depends on the field of view of the optical system, the number of detector arrays, and the accuracy of the algorithm for determining the centroid of the scattered circle. Usually, each blurry star has an image size of 2 × 2 to 6 × Between 6 pixel areas. In addition, the grayscale information of other pixel regions is also used to calculate the impact of stray light on the star signal. The positioning accuracy of star points is between 1/10 to 1/50 pixels. In order to improve the accuracy of star point positioning, it is usually necessary to reduce the field of view, which can lead to a decrease in the number of reliable stars. Moreover, the defocusing technology of star sensors highly relies on the spectral distribution of starlight, which leads to a decrease in the sensitivity of the star sensor to detect a given star point, limiting the number of star points that can be used for star sensor positioning. The improvement of star positioning accuracy of traditional star sensors can be achieved by introducing focal plane array (FPA). But as the resolution of FPA improves, the weight, volume, and power of the system will also increase, and the amount of data required to be processed will also increase. Moreover, FPA is also susceptible to radiation damage.
Interferometry is one of the ways for star sensors to improve their resolution during star observation. The interferometric star sensor improves the angle detection accuracy of the star sensor by adding interference components to the front end of the traditional optical system without increasing the number of detector arrays or changing the field of view. Due to the difference in light source area between the star target and the sky background, stars can be considered as point light sources, while the sky background is an extended light source. Based on the difference in light source coherence between the two, it is possible to achieve the separation of small angle frequency stellar targets from the sky background at large angle frequency in optical system design and algorithm processing, filter out background noise, and ultimately achieve high magnitude detection of stellar targets under daytime operating conditions.
Compared to traditional star sensors, interferometric star sensors have the following advantages:
(1) It is beneficial for improving the signal-to-noise ratio of stellar observations. The signal-to-noise ratio of stellar observations is related to the exposure time of the system. In interferometry, the exposure time of the system is several tens of milliseconds, and the influence of noise is relatively small.
(2) It is possible to collect star data with a larger amount of data and higher data quality.
(3) Helps improve the resolution and accuracy of imaging systems. By using more precise instruments, the accuracy and resolution of the system can be improved.
(4) It can overcome the performance defects of other traditional star sensors. For example, for a single star, it can solve the problem of gray edges on the star; For binary systems, the integrity of the binary will be preserved during imaging. Therefore, conducting research on the star detection technology of interferometric all-time star sensors for near-ground applications has significant theoretical research significance and application value in the industrial field for improving the navigation accuracy and adaptability to complex environments of all-time star sensors in China.
In 1974, Anthony B. Decou proposed an interferometric star sensor technology scheme based on a spectroscopic prism. The optical structure of the interferometric star sensor based on a splitter prism is shown in Figures 1-8. The optical structure consists of a prism, a reflector, a beam splitter, a telescope, and a detector. Some areas (shaded) on the surface of the prism are covered by opaque objects. After passing through a prism, reflector, and beam splitter, the incident light is divided into two beams with slightly different propagation directions, resulting in linear stripes on the detector plane. The position of the linear stripes on the detector plane is determined by the angle between the incident light wave front and the surface of the first prism, that is, the angle in the direction indicated by the star point.
Figure 1-8 Structure of Interferometric Star Sensor System Based on Splitting Prism
In 2008, Richard A. Hutchin of the Optical Physics Company invented an interferometric star sensor based on the Talbot effect. And in the same year, a interferometric star sensor product was launched. The interferometric star sensor optical system adds interference components to the front end of the traditional optical system. The interference components are composed of two intersecting phase gratings and a wedge-shaped mirror array spaced at a certain distance. After the wedge-shaped mirror array, the imaging optical system, image intensifier, and CCD detector are placed in sequence. The internal structure is shown in Figures 1-9.
Figure 1-9 Structure of interferometric star sensor system based on the Talbot effect
The various indicators of the internal components of the interferometric star sensor and the performance comparison with traditional models of star sensors are shown in Tables 1-1 and 1-2, respectively:
Table 1-1 Various indicators of internal components of star sensors based on the Taber effect
Table 1-2 Comparison between Star Sensors Based on the Taber Effect and Traditional Models
In 2018, Richard A Hutchin proposed an interferometric star sensor design based on biaxial shearing. The interferometric star sensor based on biaxial shearing adopts a mutually perpendicular biaxial structure, which can determine the relative angular position of stars at the image plane. The structure of the dual axis shear interferometric star sensor is shown in Figures 1-10. Figures 1-10 show a biaxial interference tracking device composed of optical aperture, three Wollaston prisms, two pairs of LCPG gratings, imaging lens, detector, and processor. The interferometric star sensor device based on biaxial shearing includes two shearing interferometers, each of which contains two gratings aligned with each other and separated by a gasket. The light incident on the grating is diffracted by the first grating, which provides strong+1 and -1 order diffraction, while the energy of the other orders is very small. Each mode undergoes further diffraction at the grating against the second grating. The resulting 0-order mode (1st order mode from the first grating and -1st order mode from the second grating, and vice versa) will generate an interference pattern that varies with the angle of the incident light at the first grating. This arrangement encodes the position of the light source as the sinusoidal modulation intensity on the focal plane array (FPA).
Figure 1-10 Schematic diagram of the structure of a dual axis shear parametric star sensor
In the research of all day star sensors, foreign research institutions and enterprises have developed star sensors with diverse structures and the ability to detect weak star targets and high-precision pose calculation. At present, the star sensors developed in China cannot fully meet the requirements of near ground operation for all weather conditions. Most research focuses on the simulation of the working environment and detection ability of star sensors, as well as the establishment of link transmission models. Key technologies such as the design of high-precision all weather star sensors and the implementation of star point extraction algorithms for all weather star sensors are still immature. Interference detection is a new technology that can significantly improve the all-weather detection capability of star sensors, and has been confirmed in relevant patents and products abroad. At present, the research on interferometric all time star sensors in China is not yet mature, and there is still a certain gap compared to developed countries.
At present, there are still the following development bottlenecks in the research of interferometric all time star sensors in China:
The development bottlenecks
(1) There is a lack of in-depth research on the characteristics of stellar targets and background radiation. Although there has been a lot of research on the spectral characteristics of stellar targets and backgrounds in China, there is relatively little research on the polarization characteristics of stellar targets and sky backgrounds, as well as the interference characteristics of stellar light sources. An interferometric all day star sensor detection model that combines polarization detection and spectral filtering technology has not yet been established.
(2) There is a lack of theoretical guidance for the system design of interferometric all day star sensors. At present, the development technology of interferometric star sensor products abroad is very mature and has high detection accuracy. There is no manufacturing capability for interferometric all time star sensors in China, and there is a lack of systematic theoretical design methods. Further research is still needed in system indicator demonstration, optical system design, link model construction, and detection capability simulation.
(3) There is relatively little research on star map modeling and simulation around interferometric all day star sensors. Starting from the imaging mechanism of interferometric all-time star sensors, a full link imaging model has not been constructed for signal transmission and conversion from radiometry to grayscale. The research on the imaging and noise characteristics of interferometric star sensors is not in-depth enough, and there is still a lack of effective means for star image recovery and accuracy evaluation. It is urgent to conduct in-depth modeling and simulation research. Compared with traditional star sensors, interferometric all day star sensors have the advantages of high detection accuracy, strong resistance to sky background light, and high detection rate for dim stars. The interferometric all day star sensor model provides a new approach to improving the performance of star sensors in China, so it is urgent to carry out theoretical model construction and simulation work on interferometric all day star sensors.
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TY-Space Technology (Beijing) Ltd. is professional focusing on advanced attitude optical sensors, especially Star Trackers for space industry.
