Firstly, it is necessary to understand the working principle of star sensors and the process of star map recognition, in order to provide a basis for navigation of star map parameters. Secondly, in order to make the parameters of the star points in the navigation star map of the simulated dynamic star simulator more accurate and targeted, it is necessary to understand the basic information that the star points in the navigation star map need to contain before designing the overall dynamic star simulator. At the same time, it is also necessary to understand the relationship between stars and blackbodies, as well as between stellar spectra and color temperature.
The specific working principle of a star sensor is as follows: first, a light mask is used to remove stray light in the cosmic space light environment. Then, the star sensor lens images the position information of stars in the field of view. The detector samples and processes the star map containing star information. Then, based on the star information in the navigation star library, the processor compares the star position information, After extracting the position coordinates of several star points in the star map, the position of the extracted star points in the celestial coordinate system is obtained through star map recognition, and the position vector of the star sensor in the celestial coordinate system is calculated. Finally, based on the position vector of the star points in the star sensor coordinate system and the position vector in the celestial coordinate system, the current star sensor attitude can be calculated and the spacecraft attitude can be controlled.
The process of star map recognition refers to the recognition of star position information within the optical system field of view by star sensors, which corresponds to specific star parameters in the celestial coordinate system. Star map recognition usually includes star tracking mode and all day recognition mode. The star tracking mode is mainly used for local position recognition. All sky star map recognition is the recognition of any position on the entire sky spherical star map. After the star sensor detects the star map in the field of view, the centroid position of the star point is extracted, and then compared with the stars in the navigation star library to determine the specific position of the star map in the celestial coordinate system. As shown in Figure 2 The diagram shown in Figure 1 shows the recognition of the entire sky map.
At present, star sensors mainly complete star map recognition through the extraction of star position information. In this process, the processor of star sensors needs to spend a lot of time comparing star position information, resulting in low star map recognition efficiency of star sensors and a significant impact on spacecraft attitude navigation. In response to this situation, spectral information needs to be added to the star points in the star map to improve the star map recognition efficiency of star sensors. Therefore, when calibrating the star sensor on the ground, the star simulator simulates the navigation of the star map, and the star point information not only includes the position information of the star in the celestial coordinate system and its own magnitude information, but also needs to add spectral information of the star.
The star catalog is the most complete summary and display of the basic information of stars observed in astronomical records. The catalog contains parameters such as the name, sign, magnitude, position in the celestial coordinate system, self motion, and spectral type of stars, as shown in Table 2 The main parameters of some stars in the Ebaco catalog are shown in Figure 1. The commonly used catalogs include the SAO catalog published in 1966, the FK5 basic catalog published in 1984, and the Hipparcos catalog officially published in 1997. The navigation star library is the selection of specific star catalogs as the navigation basis for star sensor star map recognition process. Therefore, the navigation star map simulated by star simulators generally needs to include the specific position, magnitude, and spectral type of stars in the celestial coordinate system. The star catalog used in this article is the Ebacus catalog.
In astronomy, the precise position of stars in the universe is determined through the celestial coordinate system, which is an extended concept based on the celestial sphere. The celestial sphere is not present in the real world, but rather a hypothetical ideal model artificially defined, as shown in Figure 2 2. The celestial sphere is a virtual sphere model with an infinite radius, centered around the Earth.
We can extend our knowledge of geography to astronomical concepts, where the connection between the north and south poles of the Earth is extended to the celestial sphere, corresponding to the south and north celestial poles. The radius of the equator extends from the radius of the Earth to the radius of the celestial sphere, which is the celestial equator.
The celestial coordinate system position of stars in the catalog is usually represented by the equatorial coordinate system. The equatorial coordinate system includes the first equatorial coordinate system and the second equatorial coordinate system. The first equatorial coordinate system, also known as the hour angle coordinate system, starts from the meridian circle in a clockwise direction. The second equatorial coordinate system is calculated counterclockwise from the vernal equinox (March 21). In the catalog data of Tables 2-1, the position information of stars is determined by the joint declination and declination in the first equatorial coordinate system.
A star is a collective term for spherical or quasi spherical celestial bodies that can emit light on their own. In astrophotometry, the concept of magnitude is used to measure the brightness of different stars, which is the amount of radiation energy received from the corresponding star outside the Earth’s atmosphere to measure the brightness of the star. British scholar Herschel once discovered in 1830 using modern measurement methods that the brightness of+1Mv and+6Mv differed by about 100 times, which can be used to deduce that the illuminance ratio of adjacent magnitudes is 2.512. Currently, in international astronomical standards, the illuminance of zero magnitude stars is specified as 2.65 × 10 ^ -6 lx, as the benchmark for calculating specific magnitude illuminance.
When making specific measurements of celestial bodies, due to the spectral response of the measuring instrument itself, the radiation energy received by the celestial body fluctuates within a certain spectral range. Therefore, according to the spectral response range of different measuring instruments, star magnitude can be divided into visual magnitude (Mv), absolute magnitude (Ma), radiation magnitude (Mr.), and thermal magnitude (Mb) UBV magnitude (Mu, Ms, Mv) and instrument magnitude (Mx).
The spectral information of stars includes their motion state, temperature, mass, and chemical composition, so stars can be accurately distinguished through their spectral information. There are numerous stars in space, and their radiation spectra have certain rules, so stars can be classified. The most commonly used star classification method currently is Harvard spectral classification. Harvard Spectral Classification categorizes the spectra of stars into seven categories in order of temperature, from high to low, namely O, B, A, F, G, K, and M. Each category is further divided into ten subtypes: 0,1,2,…, and 9. Table 2.2 shows the spectral types of stars and their corresponding color temperatures. It should be noted that in astronomy, lowercase letters are usually used after stellar spectral types to indicate special meanings in order to distinguish between different physical properties of the same spectral type. For example: e (with emission lines in the spectrum), m (with strong metal lines), n (with blurry spectral lines), s (with sharp spectral lines). Meanwhile, due to the development of astronomical detection devices, more detailed classification is needed for the rapidly increasing number of stars in the catalog. Therefore, based on the Harvard spectral classification, binary spectral classification of stars is obtained by combining the luminosity level classification of stars. Add the Roman numerals I-VII after the Harvard spectral type to represent it, where the higher the numerical value, the wider the spectral line, and the lower the stellar luminosity.
A blackbody is a body that, under any temperature condition, neither reflects nor transmits energy, but completely absorbs energy of any wavelength radiating onto its surface. An ideal blackbody does not exist in known cosmic space, and stars are not strictly blackbodies. However, from the study of stellar spectra in spectroscopy, it can be seen that in the visible and near-infrared bands (300nm~1100nm), the radiation energy of stars can be approximated as blackbody radiation at a certain temperature. In this band, the spectral curve of stars can be replaced by the color temperature curve of the corresponding blackbody, as shown in Tables 2-3, In the spectral wavelength range of 300nm~1100nm, we can simulate M, K, G, F, and some A-type stars, corresponding to a color temperature of 3000K~9000K. When the color of the light emitted by the luminous source matches the color of the blackbody when it radiates energy at a certain temperature, the temperature of the blackbody at this time is called the color temperature of the light source, abbreviated as color temperature (CT), and expressed in absolute temperature scale. As shown in Figure 2 3 are the spectra of M-type, G-type, and A-type stars at color temperatures of 3000K, 5800K, and 10000K, respectively.
According to the quantum theory of blackbody radiation, the Planck formula can characterize the spectral radiation distribution of blackbody,
According to Figure 2.4, it can be seen that blackbody at different color temperatures has different spectral radiation curves. The higher the color temperature, the stronger the energy of blackbody radiation in the ultraviolet band, and each color temperature has a radiation peak. The total radiation energy of blackbody at a specific color temperature within any band range can be calculated through the blackbody spectral radiation curve.
Due to the difficulty in obtaining actual stellar spectral curves and the fact that the stellar spectral range studied in this study falls within the visible and near-infrared bands, the simulated stellar spectral curves in subsequent spectral simulations of navigation star maps are actually their corresponding blackbody color temperature radiation curves.
Send us a message,we will answer your email shortly!
TY-Space Technology (Beijing) Ltd. is professional focusing on advanced attitude optical sensors, especially Star Trackers for space industry.
