Research And Implementation Of Attitude Determination For FY-4 Meteorological Satellite Based On Star Tracker

The new generation geostationary meteorological satellite FY-4 is a three-axis, body-stabilized satellite, on which the various remote instruments can be mounted. This highly improved the sounding efficiency and even allowed the primary sensors to"stare"at Earth. To achieve the high accuracy attitude, high precise star trackers will be used. The star trackers incessantly observe the portion of the sky and select the stars in view, then, send the stars to the Star-Identification Process. The process identifies the readings from the guide star catalog and the Attitude-Determination Process determines the exact attitude of the star tracker. According to the mounted attitude matrix of the star tracker, the attitude of the satellite can be determined. At Present, there is a large gap between the star tracker made in China and the international state-of-art star tracker on the aspects of precision, data updating rate and lifetime etc. This will bring a much higher challenge to the Attitude Control System (ACS) of the FY-4 satellite. According to the needs of the three-axis stabilized ACS of FY-4 satellite, this research has developed the Star-Identification algorithm and single-frame Attitude-Determination algorithm for the ground-based application system. Through Star-Identification and Attitude-Determination from the star map stores in the ground, the rate of attitude will be improved, the process of attitude change can be got, which will improve the accuracy of the satellite attitude and image navigation and registration.After the further study on star navigation technology of geostationary meteorological satellite, several algorithms of attitude determination based on star tracker for FY-4 meteorological satellite are researched and implemented. In the sub-catalog research, a new approach to choose guide star is proposed. In which the new concept of threshold of position precision and Additional Guide Star are introduced. In the star pattern recognition algorithm, the triangle-based Star-Identification algorithm , including entire celestial sphere autonomous Star-Identification algorithm and partial celestial sphere Star-Identification algorithm have been improved. In the attitude determination algorithm research aspect, under ideal conditions' Optimized-TRIAD best stars distribution characteristic and ideal conditions' QUEST best star triangle distribution characteristic is summarized. The major research and conclusions are as follows:(1) sub-catalog methodIn order to meet the requirement of high precision of attitude determination and image navigation of FY-4 meteorological satellite, set up a high precision guide star catalog for ground-base is very important. A method of chosen guide star is proposed. This method is based on the J1991.25 Hipparcos catalog and Pulkovo Compilation of Radial Velocities for Hipparcos Stars, through the rigorous treatment of epoch transformation, the star's position of the FY-4 runtime epoch can be got. In order to got the more accurate Guide Star(with star flag 0), the stars with magnitude brighter than 6.5 of J1991.25 Hipparcos catalog were chosen, after the rigorous epoch transformation, the J2000.0 Hipparcos sub-catalog was got, with the help of the information of Tycho-2 star catalog and SAO star catalog at the same epoch, the threshold of position precision is introduced, which can meet the precision of attitude determination in arc-second level. The Additional Guide Star (with star flag 5) is come into use for improving the accuracy and speed of star identification. After the cases analysis of unsuccessful star identification, giving the guide star which always cause false star identification a given star flag(such as 6), which denote that the star is a Redundant Guide Star and can't take part in the star identification processing. Through the introduction of Redundant Guide Star, the accuracy of star identification is improving. The distribution of Additional Guide Stars spread all over the whole sky, even the star sparse area. Moreover, in the 189 Additional Guide Stars, the proportion of star magnitude brighter than 3 reached to 34.4%, which is of great benefit to improve star identification. The attitude error will be introduced by the inaccuracy reference guide star, which is inevitable. The accuracy reference guide star, however, can improve the error of attitude determination.(2) Centroiding and Image SimulationAs far as possible to reduce the influences of the star pattern distortion and atmospheric refraction etc., the central regions of the star maps are centroiding, the positions of the stars in the star maps are output. It's the input of the Star-Identification and attitude determination software. In order to estimate the accuracy of attitude determination algorithm, based on the camera simulator of Kenneth Daniel Diaz, an ideal star camera simulator suitable for this research is constructed. The parameters of the ideal star camera simulator are given, the coordinate transformations are well illustrated and the attitude transformation matrix is derived. Through the coordinate move and coordinate translation, the stars of Hipparcos star catalog were projected from the celestial sphere coordinate system to the image plane. The output of the star camera simulator is the basis of simulated star identification test, the attitude determination algorithm precision analysis and the factors analysis of which impact on star tracker boresight's attitude determination.(3) Improved triangle Star identification algorithmFor the Improved triangle Star identification algorithm, the identify characteristics which stored in the guide star database are the separation angles of the two stars, rather than the guide star triangles, the storage space is saving. For the star tracker, the entire celestial sphere autonomous Star-Identification is the most essential technology. In this study, based on the Christopher T.F. Kuehl's triangle Star-Identification, some improvement had made. With a certain angular and distance tolerance, the separation angle of the observed stars and that of the catalog stars are matched, then, the star triangles are composed in real time. According to the rules of vector rotation, the inconsistent rotation star triangles are removed. After that, put the star triangles with public vertex into star polygon. Because of the false star existence, the biggest polygon is not necessarily the optimum match, this study introduced the concept of voting algorithm to the stars of the matched star triangles, the star polygon who takes stars with high match rate as a right identification, thus removed the false star disturbance.In partial celestial sphere celestial Star-Identification, the initial attitude is known. With a certain angular and distance tolerance, the separation angle of the observed stars and that of the catalog stars are matched, then, the star triangles are composed in real time. According to the rules of vector rotation, the inconsistent rotation star triangles are removed. After that, put the star triangles with public vertex into star polygon. In this research, because of the attitude determination process had already realized, the identified stars of the polygon were send to the attitude determination process, the new attitude was got. After comparing the new attitude with the initial attitude, the accuracy of the Star-Identification is to be ensured.Through Monte-carol simulation, the entire celestial sphere autonomous Star-Identification algorithm and the partial celestial sphere Star-Identification algorithm are tested. For the former algorithm, it is shown that the star identification accuracy is higher than 97.7%, the speed of identify 5 stars in the same field of view is fast than 0.4 second. As far as the identification accuracy and the speed are concerned the latter is better than the former.(4)Attitude-Determination algorithmThe optimized TRIAD algorithm and QUEST algorithm are discussed, these two algorithms each has its good points. If there only 2 high accuracy guide stars are identified, the QUEST algorithm was unable to give the attitude, but the optimized TRIAD algorithm might. When the number of high accuracy guide stars identified is large, the QUEST algorithm has obvious superiority.According to night-sky observation, based on the guide star flag and the star- Identification information, the selection principle of attitude determination algorithm are given. That is: According to the guide star flag, the number of stars identified and the cost-function of two attitude determination algorithm to carry on the final attitude determination, the better attitude are chose as the output.Through Monte-carol simulation, the error of four cases of attitude determination algorithms is studied. The four cases of attitude determination algorithms namely QUEST, optimized TRIAD, Kendiaz and the QUEST₃Stars. It is shown that the QUEST algorithm's attitude error is the most superior, the QUEST₃Stars next, optimized TRIAD third and the Kendiaz algorithm error is the biggest.(5)Attitude accuracy influencing factor analysisThrough the image simulation, several factors such as the accuracy of the stars'location ,the number of stars take part in attitude determination and the distribution of star in the field of view which influencing the attitude accuracy were studied. Some new findings are listed as follows:For the specific FOV size and the CCD size, with the enhancement of star Centroiding accuracy, the precision of three-axis attitude is enhanced. Afterward achieves the marginal value, hereafter with the enhancement of star Centroiding accuracy in any rate, the precision of three-axis attitude will not be improved.From statistics significance, the more stars take part in the attitude determination, the higher attitude precision of star tracker's boresight is got. But speaking of certain star map, needs for concrete analysis, it depends on the attitude algorithm.The distribution of stars in the field of view is of great essential to the attitude determination algorithm, different algorithms have different best (/worst) distributed characteristic. Under ideal conditions, the optimized TRIAD algorithm's best 2-stars distribution basically has the symmetrical distribution with the boresight and distribute on its two sides.Under ideal conditions, the QUEST algorithm's best star triangle distribution basically satisfied the boresight locates nearby the gravity center of star triangle. When the number of stars in FOV is large, select stars satisfy the QUEST algorithm's best distribution to take part in the attitude determination will help improve calculation accuracy.(6) Cross-platform application-oriented R & D systemOn the basis of the above provision, a set of cross platform research and development system with the C language has established. This system can use the real sky observation information to get the brighter star identified from the whole catalog and provides 3-axis pointing knowledge of the boresight relative to an inertial reference frame. It is suitable to PC and UNIX platforms. The night-sky Star maps are used for testing the correctness and reliability.