Navigation stars are an important basis for star map recognition using star sensors. Scholars such as Kan Daohong have analyzed the basic conditions that navigation stars should possess and the field of view density of navigation stars; Scholars such as Zheng Sheng used a support vector machine based automatic selection algorithm for navigation stars based on dynamic magnitude thresholds to establish a navigation star catalog with a small number and uniform distribution of navigation stars; Scholars such as Chen Yuanzhi used the storage of mid year flat positions to shorten the time for visual position conversion. The literature currently available only focuses on spaceborne missions using a single star catalog. Except for scholars such as Chen Yuanzhi who use the mid year position for epoch conversion, other methods do not mention epoch conversion (the harsh selection conditions in the literature have already avoided positional changes between stellar epochs). Common star catalogs such as the Hipparcos catalog have an epoch of J1991.25, the Tycho-2 catalog has an epoch of J2000.0, and the SAO catalog provides B1950.0 bisection point epoch positions and self values, as well as J2000.0 positions and self values. After the launch of the FY-4 meteorological satellite in 2012, after ten to twenty years, there will be more or less changes in the instantaneous position and catalog position of stars, which can be ignored in the process of star map recognition, However, it should be considered in the high-precision attitude determination process (taking the star with HIP number 57939 as an example, after passing precise epoch from J1991.25 to J2012.0 and comparing its position in the Hipparcos catalog, the declination difference is 0.0292 ° about 105.12 arcseconds, and the declination difference is -0.0335 ° about -120.6 arcseconds). In order to improve the attitude determination accuracy of star sensors, a hypothesis was adopted in the literature, which is that the epoch of the star catalog is the current time and the influence of optical aberration is taken into account. This also explains the importance of precise epoch conversion to some extent.
Based on the advantages of abundant computing and storage resources in ground application systems, a method for optimizing navigation stars is proposed. Starting from the demand for high-precision attitude determination, this method takes the Hipparcos catalog as the center and combines multiple catalogs. For the first time, the position accuracy after precise epoch conversion is used as the basic condition for selecting navigation stars, and concepts such as auxiliary navigation stars and redundant stars are introduced. By assigning different identifiers to the selected constant stars, it is determined whether they participate in the star map recognition and attitude determination process, Established navigation star catalogs for different purposes of star map recognition and attitude determination.
In order to perform precise epoch conversion of stars, it is necessary to know the six astronomical parameters, standard deviations, and correlation coefficients of each star. Few single catalogs can provide all of these data. To this end, multiple catalogs were selected and their results were comprehensively utilized to complete the selection of navigation stars. The basic data for selecting navigation stars mainly comes from four data sources:
(1) Hipparcos catalog:
The Hipparcos catalog provides the parameters of the right ascension, right ascension, parallax, self motion, standard deviation of right ascension and right ascension, standard deviation of parallax, standard deviation of self motion, correlation coefficient, etc. of 118218 stars brighter than VT=11.5 in the entire sky. The epoch and coordinate system used is J1991.25, the International Celestial Reference Coordinate System (ICRS). Hipparcos space observations have made unprecedented progress in optical observations, with the root mean square errors of positions, parallax, and annual motion obtained for stars brighter than 9 magnitude ranging from 0.7 mas to 0.9 mas. The uncertainty of aligning the reference frame of the Hipparcos catalog published in 1997 with the reference frame of the river source at epoch J1991.25 is ± 0.6mas (1 σ)， The remaining rotation rate is 0.25 mas/yr, becoming the main implementation of the International Celestial Reference System (ICRS) in the optical band. Therefore, the selection of navigation stars based on ground application systems will be centered around the Hipparcos catalog under J1991.25
(2) Tycho-2 catalog:
The Tycho-2 catalog was derived from ESA’s Hipparcos satellite observations and incorporates the results of astronomical catalogs and 143 other ground observation catalogs. It provides the average position (right ascension, right ascension) of 2539913 stars in the entire sky at epoch J2000.0, the observation position (right ascension, right ascension) at epoch J1991.25, parallax, right ascension self motion, right ascension self motion, BT and VT magnitude, error, and other parameters. In the Tycho-2 data analysis method, many quite bright Hipparcos and Tycho-1 stars have been discarded. In order to facilitate user use, a supplementary catalog has been provided. The first supplementary catalog contains data on 1758 high-quality stars, while the second supplementary catalog contains 1146 Tycho-1 stars, which are either inaccurate or interfered with by bright stars (all the auxiliary navigation stars proposed in this study are from the first supplementary catalog). The epoch and coordinate system used is J1991.25, the International Celestial Reference Coordinate System (ICRS).
The Tycho-2 catalog data has high accuracy, neglecting the Hipparcos astronomical data system error, at a temperature of 6 ° or greater × At a scale of 6 °, the position system error of Tycho-2 is less than 1mas (milliarcseconds), and the self system error is less than 0.5mas/yr.
(3) Pulkovo visual speedometer:
The Pulkovo visual velocity table [94] contains the weighted average absolute visual velocity of 35493 Hipparcos stars, which are distributed throughout the entire celestial sphere, covering various spectral types and brightness levels, and are within a distance of 500 seconds from the sun. The star catalog mainly provides absolute apparent velocity and its average error, as well as the declination, declination, and stars at epoch J2000.0. The median accuracy of the apparent velocity is 0.7km/s.
(4) SAO catalog:
The SAO catalog [92] provides 258997 stars that are brighter than 11 degrees, with SAO numbers, B1950.0 equinoxes, J2000.0 positions, spectral types, V magnitudes, and cross identification numbers with star catalogs such as HD and DM.
(1) Position accuracy threshold: The position accuracy threshold for selecting a navigation star is when the sum of the standard deviations of the precise epoch of Hipparcos catalog star is converted to J2000.0 epoch time and the flat position (right ascension, right ascension) of Tycho-2 catalog star J2000.0 epoch time is less than a certain value.
(2) Navigation star: Hipparcos catalog stars that meet certain magnitude and position accuracy threshold conditions.
(3) Auxiliary navigation star: In Tycho-2 catalog.dat, the horizontal position is marked with X and supply_ 1. Stars with a magnitude below 6.5 in Dat (epoch J1991.25), after excluding binary stars, use the information in the Hipparcos J1991.25 catalog to perform precise epoch conversion. Non navigation stars with a standard deviation of less than 1 arc-second from the plane position of the star in the J2000.0 system in the SAO catalog are called auxiliary navigation stars.
(4) Redundant stars: Navigation stars that mirror each other with non navigation stars (pseudo stars) under certain tolerance conditions in the star map recognition process. The removal of redundant stars is closely related to the accuracy, field of view size, and star pattern recognition algorithm of the optical system. That is to say, redundant stars are conditional and are redundant navigation stars in a certain field of view of an optical system. However, in another optical system and another field of view, they may not necessarily be redundant navigation stars. Therefore, the removal of redundant navigation stars should be cautious and depend on specific conditions.
(5) Star identification: Assign each star a specific identification and determine whether it will participate in subsequent processing based on the identification.
(1) Firstly, regarding hip_ Process the dm.idx data and extract the hip number information of double and multiple stars;
(2) Process the Hipparcos main star table to extract information on the declination, declination, self motion, parallax, standard deviation, and correlation coefficients of stars with apparent magnitudes greater than 6.5 magnitudes;
(3) Use the results of (1) to “eliminate” double and multiple stars in the Hipparcos navigation catalog. The removal here is not a true deletion, but rather a symbol assigned to the navigation star, with the aim of improving the role of binary and multiple stars in star map recognition and attitude determination in subsequent work, especially in sky areas with sparse stars;
(4) Extract the visual velocity and average error information from the Pulkovo visual velocity table, and add this information to the J1991.25 Hipparcos navigation star catalog;
(5) Perform precise epoch conversion on J1991.25 Hipparcos navigation star, converting it to J2000.0 epoch time, and generate J2000.0 Hipparcos navigation star catalog;
(6) Analyze the declination, declination, self motion, average error information, and magnitude information of the Tycho-2 main star catalog and the J2000.0 Hipparcos navigation star catalog. The position accuracy threshold is set at 1 corner second, resulting in a total of 6072 navigation stars;
(7) Perform precise epoch conversion on 6072 J1991.25 Hipparcos navigation stars, convert them to satellite epoch time, and generate a satellite epoch time navigation star table with a navigation star identifier of 0;
(8) Extract the HD number of stars with magnitudes brighter than 6.5 from the SAO catalog, and search for the HIP number of stars in the J1991.25 Hipparcos catalog using the HD number;
(9) Extract the hip number of stars with a flat position mark of X and an magnitude brighter than 6.5 from the Tycho-2 main star table; From supply_ 1. Extract HIP numbers of stars with magnitudes brighter than 6.5 from dat;
(10) Extract the HIP numbers of common stars in (4), (7), (8), and (9), and use the results of (1) to “eliminate” binary and multiple stars, assigning star identification 2;
(11) Using the information from the J1991.25 Hipparcos catalog for precise epoch conversion, convert to J2000.0, select stars with a position accuracy threshold of 1 corner second compared to the SAO catalog as auxiliary navigation stars, totaling 189 stars, and assign star identifiers of 5;
(12) Use the information in the J1991.25 Hipparcos catalog to perform precise epoch conversion, convert it to satellite epoch time, generate a satellite epoch time auxiliary navigation star table, and add the auxiliary navigation star information to the navigation star data file;
(13) Remove the navigation stars that cause redundant matching from the failed cases of star map recognition, assign the redundant star flag 6, and add the redundant star information to the navigation star data file;
(14) Compare the star positions from the precise epoch to the satellite epoch time with the corresponding positions in the J1991.25 Hipparcos catalog. For stars with a standard deviation of position greater than 1 angular second before and after epoch conversion, assign star identification 3 and add position accuracy information to the navigation star data file.
At this point, the navigation star table (including auxiliary navigation stars) for FY-4 meteorological satellite positioning has been established, which includes all stars that can be observed by star sensors while ensuring the accuracy of star position.
Only by ensuring that there are more than 3 navigation stars in the field of view (FOV) can the star sensor achieve all day autonomous star map recognition.
Expanding the field of view of the star sensor or increasing the detection magnitude of the star sensor can increase the proportion of sky areas with more than three guiding stars in the field of view; The introduction of auxiliary navigation stars not only introduces a large number of bright stars but also improves the coverage of sky regions with more than 3 stars, to some extent compensating for the shortage of sparse polar stars.
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