Geometric voting algorithm for star trackers

We present an algorithm for recovering the orientation (attitude) of a satellite-based camera. The algorithm matches stars in an image taken with the camera to stars in a star catalogue. The algorithm is based on a geometric voting scheme in which a pair of stars in the catalogue votes for a pair of stars in the image if the angular distance between the stars of both pairs is similar. As angular distance is a symmetric relationship, each of the two catalogue stars votes for each of the image stars. The identity of each star in the image is set to the identity of the catalogue star that cast the most votes. Once the identity of the stars is determined, the attitude of the camera is computed using a quaternion-based method. We further present a fast tracking algorithm that estimates the attitude for subsequent images after the first algorithm has terminated successfully. Our method runs in comparable speed to state of the art algorithms but is still more robust than them. The system has been implemented and tested on simulated data and on real sky images.     

Accuracy performance of star trackers - a tutorial

An autonomous star tracker is an avionics instrument used to provide the absolute 3-axis attitude of a spacecraft utilizing star observations. It consists of an electronic camera and associated processing electronics. The processor has the capability to perform star identification utilizing an internal star catalog stored in firmware and to calculate the attitude quaternion autonomously. Relevant parameters and characteristics of an autonomous star tracker are discussed in detail     

Star trackers for attitude determination

One problem comes to all spacecrafts using vector information. That is the problem of determining the attitude. This paper describes how the area of attitude determination instruments has evolved from simple pointing devices into the latest technology, which determines the attitude by utilizing a CCD camera and a powerful microcomputer. The instruments are called star trackers and they are capable of determining the attitude with an accuracy better than 1 arcsecond. The concept of the star tracker is explained. The obtainable accuracy is calculated, the numbers of stars to be included in the star catalogue are discussed and the acquisition of the initial attitude is explained. Finally the commercial market for star trackers is discussed     

Joint sensor registration and track-to-track fusion for distributed trackers

Sensor registration deals with the correction of registration errors and is an inherent problem in all multisensor tracking systems. Traditionally, it is viewed as a least squares or a maximum likelihood problem independent of the fusion problem. We formulate it as a Bayesian estimation problem where sensor registration and track-to-track fusion are treated as joint problems and provide solutions in cases 1) when sensor outputs (i.e., raw data) are available, and 2) when tracker outputs (i.e., tracks) are available. The solution to the latter problem is of particular significance in practical systems as band limited communication links render the transmission of raw data impractical and most of the practical fusion systems have to depend on tracker outputs rather than sensor outputs for fusion. We then show that, under linear Gaussian assumptions, the Bayesian approach leads to a registration solution based on equivalent measurements generated by geographically separated radar trackers. In addition, we show that equivalent measurements are a very effective way of handling sensor registration problem in clutter. Simulation results show that the proposed algorithm adequately estimates the biases, and the resulting central-level trucks are free of registration errors.     

Evaluation of multiple radar target trackers in stressfulenvironments

This paper evaluates the performance of multiple target tracking (MTT) algorithms in real-life stressful radar tracking environments. Real closely spaced maneuver radar data, generated by six F-18 fighters and other targets, were collected jointly by the defence departments of Canada and United States to support this practical MTT algorithm evaluation study. A set of performance metrics was defined here to compare the suboptimal nearest neighbor (SNN), global nearest neighbor (GNN), and various variants of the joint probabilistic data association (JPDA) MTT trackers. Results reveal an interesting result that all these MTT algorithms exhibited very close performance. In addition, the weighted sum approach of the PDA/JPDA trackers which are theoretically effective were observed to perform poorly in tracking closely spaced targets. Overall speaking, the GNN filter based on the Munkres algorithm had the best performance in terms of both tracking performance and robustness     

False alarm effects on estimation in multitarget trackers

An analysis of false alarm effects on tracking filter performance in multitarget track-while-scan radars, using variable correlation gates, is presented. The false alarms considered originate from noise, clutter, and crossing targets. The dimensions of the correlation gates are determined by filter prediction and measurement error variances. Track association is implanted either by means of a distance weighted average of the observations or by the nearest neighbor rule. State estimation is performed by means of a second-order discrete Kalman filter, taking into consideration random target maneuvers. Measurements are made in polar coordinates, while target dynamics are estimated in Cartesian coordinates, resulting in coupled linear filter equations. the effect of false alarms on the observation noise covariance matrix, and hence on state estimation errors, is analyzed. A computer simulation example, implementing radar target tracking with a variable correlation gate in the presence of false alarms, is discussed     

星点坐标辅助的全天区三角形星图识别算法

随着当前星敏感器视场（FOV）的增大,探测能力的提高,一帧图中拍摄到的恒星更多。但是受星敏感器光谱范围的限制及空间环境干扰影响,星等测试精度一般不高于0.2 mV。为了充分发挥当前星敏感器视场和探测能力的优势,并避免星等误差的影响,提高全天区星图识别算法在线应用的适用性,提出了一种星点坐标辅助的全天区三角形星图识别算法。该方法采用"全局初步搜索识别—局部精细匹配验证—最优结果选取"的算法思想。首先,根据星敏感器探测到的极限星等范围构建导航星表,选取亮星构建角距星表,既确保了星表的完备性,又有利于充分利用星敏感器的探测能力。然后,在三角形约束条件下进行角距匹配识别,得到一个或多个导航三角形,在该识别环节提出了非线性矢量法查找星表,既提高了定位精度,又能采用单精度数据类型降低存储空间。最后,提出局部天区星点坐标匹配算法进一步消除冗余匹配,同时又识别出视场内更多的观测星,有利于提高识别率和定姿精度。试验结果表明,与其他一些经典的星图识别算法相比,所提算法在识别率和星表容量方面更有优势。识别率可达99.9%,且随着星等的增加,存储容量增加的最少。所提算法更加适于大视场、高星等敏感范围的星敏感器在线应用。     

Momentum wheel start-up method for HAUSAT-2 ultra-small satellite

This paper addresses a newly proposed start-up method of the HAUSAT-2 satellite momentum wheel. The HAUSAT-2 is a 25 kg class nanosatellite which is stabilized to an earth pointing attitude by 3-axis active control method. A momentum wheel performs two functions. It provides a pitch-axis momentum bias while measuring satellite pitch and roll attitude. Pitch control is accomplished in a conventional way by driving a momentum wheel in response to pitch attitude errors. Precession control and nutation damping are provided by operating the pitch-axis magnetic torquer. A momentum wheel is nominally spinning at a particular rate, and speed changes are made as needed. Spinning the momentum wheel redistributes the angular momentum to the spacecraft body. If the wheel is suddenly spun up to its nominal speed, the large momentum produced together with the existing momentum due to the nominal rotational motions will result in a high disturbance and unstable motion. This simulation study investigates the feasibility and performance of a proposed strategy for starting-up the wheel. A proposed strategy to start-up the wheel shows that its pitch momentum wheel can start-up successfully to its nominal speed from rest, and can be stabilized to nadir pointing within about three orbits.     

涡轮泵启动过程振动故障检测方法

为了更全面地保证液体火箭发动机涡轮泵的安全,提出了一种针对其启动过程的振动故障检测方法,首先对等时间隔采集的振动数据进行重采样,获得等角间隔的振动数据,消除转速变化对振动特征的影响,选择并计算振动特征值,最后采用支持向量回归方法建立振动特征与转速的非线性关系模型.当新的测试数据偏离模型一定限度时,表明涡轮泵工作异常.2.2 s试车数据的检测时间只有0.0470 s,说明模型可用于涡轮泵启动过程故障检测.      

A grid algorithm for autonomous star identification

An autonomous star identification algorithm is described that is simple and requires less computer resources than other such algorithms. In simulations using an 8/spl times/8 degree field of view (FOV), the algorithm identifies the correct section of sky on 99.7% of the sensor orientations where spatial accuracy of the imaged star is 1 pixel (56.25 arc seconds) in standard deviation and the apparent brightness deviates by 0.4 units stellar magnitude. This compares very favorably with other algorithms in the literature.     

Pattern recognition of star constellations for spacecraftapplications

A software system for a star imager for on-line satellite attitude determination is described. The system works with a single standard commercial CCD-camera with a high aperture lens and an onboard star catalogue. It is capable of both an initial course attitude determination without any prior knowledge of the satellite orientation and a high-accuracy attitude determination based on prediction and averaging of several identified star constellations. In the high accuracy mode the star imager aims at an accuracy better than 2 arc sec with a processing time of less than a few seconds. The star imager is developed for the Danish microsatellite Oersted     

Pattern recognition of star constellations for spacecraftapplications

A software system for a star imager for online satellite attitude determination is described. The system works with a single standard commercial CCD camera with a high aperture lens and an onboard star catalog. It is capable of both an initial coarse attitude determination without any prior knowledge of the satellite orientation and a high-accuracy attitude determination based on prediction and averaging of several identified star constellations. In the high-accuracy mode the star image aims at an accuracy better than 2 arc sec with a processing time of less than a few seconds. The star imager has been developed for the Danish Oersted satellite     

某型飞机发动机地面起动故障解析

通过一起发动机地面起动不成功故障分析 ,介绍了逐条排查找出故障原因的方法及排故方法。某次故障发生后 ,通过涡轮起动机、综合调节器、起动自动器、飞机电气系统、燃油泵—调节器检查得知 ,它们的工作参数均满足发动机起动过程中的技术要求 ,但燃油分配器去主燃烧室第 1级集油官的燃油压力在n2 >18%时不符合技术要求。通过对燃油分配器前后的燃油压力分析 ,得知造成发动机起动不成功的根本原因是燃油分配器调整不当。排除飞机起动故障时应综合考虑各种因素 ,一一加以排查和分析 ,排除故障时应由简到繁 ,由外及里进行 ,以减少人力物力和时间的浪费     

An algorithm for astro-inertial navigation using CCD star sensors

A self-contained suite of astro-inertial navigation system is capable of autonomous mission and is operationally reliable. The typical astronavigation system (ANS) makes use of star-trackers, which are expensive and complex. To make the system cost effective and less complex, the star-tracker is replaced by a charge coupled device (CCD)-based star sensor, rigidly mounted on a strapdown inertial measurement unit (SIMU) of the system. This electro-optical star sensor is compact and easy to use with an ANS that utilizes efficient star identification techniques. This paper designs an algorithm that estimates axes misalignment angles of strapdown inertial navigation system (SINS) that makes stars' observations utilizing a CCD star sensor. Mathematical modeling of the suggested scheme was carried out and transformations between different frames were exercised. From the image projection geometry, stars' right ascensions and declinations, relative to the body frame, were estimated. Lastly, from the known stars' position vectors in mathematical platform and reference frames, axes misalignment matrix representing SINS attitude errors can be estimated employing the derived relationship.     

Star recognition algorithm for APS star tracker: oriented triangles

An efficient star pattern recognition algorithm is presented. The purpose of this algorithm is to make sure of the compatibility of the software and the imaging sensor noise level. The new CMOS APS sensors have not currently reached the same accuracy as the former CCD sensors in position as well as in magnitude determination, especially in the dynamic stages. This algorithm allows the system to recognize the star pattern 20% faster than with reference algorithms. No false recognition has been noticed. Used databases have a size 5 to 10 times smaller, depending on other reference algorithms. Oriented triangles are used to compare the measured star pattern with the catalogue stars. The triangle's characterization criteria propose several solutions in a first time. A unique solution is selected by means of identification and validation methods in a second time. First results, presented hereinafter, are very encouraging, and this algorithm may be used in the future APS star trackers. APS star tracker robustness is significantly enhanced by this method during the critical navigation phases     

Precession–nutation correction for star tracker attitude measurement of STECE satellite

Space Technology Experiment and Climate Exploration(STECE) is a small satellite mission of China for space technology experiment and climate exploration. A new test star tracker and one ASTRO 10 star tracker have been loaded on the STECE satellite to test the new star tracker's measurement performance. However,there is no autonomous precession–nutation correction function for the test star tracker,which causes an apparent periodic deflection in the inter-boresight angle between the two star trackers with respect to each other of up to ±500 arcsec,so the precession and nutation effect needs to be considered while assessing the test star tracker. This paper researches on the precession–nutation correction for the test star tracker's attitude measurement and presents a precession–nutation correction method based on attitude quaternion data. The periodic deflection of the inter-boresight angle between the two star trackers has been greatly eliminated after the precession and nutation of the test star tracker's attitude data have been corrected by the proposed method and the validity of the proposed algorithm has been demonstrated. The in-flight accuracy of the test star tracker has been assessed like attitude noise and low-frequency errors after the precession–nutation correction.     

Astrometric study of Barnard's star

Space Science Reviews -     

黎莉莉 充满传奇色彩的明星

岁月悠悠,时光荏苒,在上个世纪三十年代的电影界里明星云集,佳丽如云。然而,在那争艳斗美的美女明星中有一个女明星以漂亮、健壮、结实、体育运动好而博得观众和影迷的喜欢,她的名字就叫黎莉莉,也被影迷称作为"运动明星"。而黎莉莉的一生充满了惊人的传奇色彩。在苦艰的童年中拼搏成名黎莉莉出生在一个极其不平凡的家庭里,她的父母亲都是中共地下党员,尤其是她的父亲钱壮飞曾为保卫中共中央的安全作出了重大的贡献。     

Recursive mode star identification algorithms

Star identification can be accomplished by several different available algorithms that identify the stars observed by a star tracker. However, efficiency and reliability remain key issues and the availability of new active pixel cameras requires new approaches. Two novel algorithms for recursive mode star identification are presented here. The first approach is derived by the spherical polygon search (SP-search) algorithm, it was used to access all the cataloged stars observed by the sensor field-of-view (FOV) and recursively add/remove candidate cataloged stars according to the predicted image motion induced by camera attitude dynamics. Star identification is then accomplished by a star pattern matching technique which identifies the observed stars in the reference catalog. The second method uses star neighborhood information and a catalog neighborhood pointer matrix to access the star catalog. In the recursive star identification process, and under the assumption of "slow" attitude dynamics, only the stars in the neighborhood of previously identified stars are considered for star identification in the succeeding frames. Numerical tests are performed to validate the absolute and relative efficiency of the proposed methods.