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This paper proposes a new optical hybrid navigation method for use in close flybys of small celestial bodies. Two steps are taken to estimate the inertial position and attitude of the spacecraft: 1...

This paper presents an interplanetary optical navigation algorithm based on two spherical celestial bodies. The remarkable characteristic of the method is that key navigation parameters can be estimated depending entirely on known sizes and ephemerides of two celestial bodies, especially positioning is realized through a single image and does not rely on traditional terrestrial radio tracking any more. Actual Earth–Moon group photos captured by China’s Chang’e-5T1 probe were used to verify the effectiveness of the algorithm. From 430,000 km away from the Earth, the camera pointing accuracy reaches 0.01° (one sigma) and the inertial positioning error is less than 200 km, respectively; meanwhile, the cost of the ground control and human resources are greatly reduced. The algorithm is flexible, easy to implement, and can provide reference to interplanetary autonomous navigation in the solar system.

The mystery of the asteroid (4179) Toutatis was revealed by Chang'e-2 spacecraft during a close flyby on December 13, 2012. Optical imaging and navigation of the probe during the flyby were performed entirely under ground-based radio tracking and default sequence built on ground. This paper establishes a set of estimation algorithms of the relative trajectory between Chang'e-2 and Toutatis based on dynamics, optical, and radio constraints that are determined by the unique flyby mode. This study is the first time to precisely reproduce the core process of Chang'e-2's encounter with Toutatis based on several optical images. In addition to constructing a strict photogrammetric model, the shadowing effects caused by the illumination and the deviation of the center-of-mass (COM) from the center-of-figure (COF) in optical images are also considered. The spacecraft trajectory with regard to the COF of the body is estimated using images taken from 120 km or less. The formal one sigma uncertainty is (67, 20, and 11 m) in the principal axes frame of the position error ellipse, and the closest approaching distance between Chang'e-2 and Toutatis's COF is calculated as 1557 ± 11 m, which is more precise than previous results with an uncertainty of hundreds of meters. The spacecraft trajectory with regard to the COM of the body is estimated with an uncertainty of (211, 34, and 17 m), and the corresponding closest distance is estimated as 1451 ± 18 m based on the previously developed shape model of Toutatis. The algorithms and results in this study are important for evaluating the performance of this flyby mission and are also valuable for any similar optical navigation during a close approach. In addition, our results can help in precisely determining the axis of Toutatis and sizes of impact craters, which are critical for understanding the formation and evolution of Toutatis.

The Doppler tracking data of the Chang'e 3 lunar mission is used to constrain the stochastic background of gravitational wave in cosmology within the 1 mHz to 0.05 Hz frequency band. Our result improves on the upper bound on the energy density of the stochastic background of gravitational wave in the 0.02 Hz to 0.05 Hz band obtained by the Apollo missions, with the improvement reaching almost one order of magnitude at around 0.05 Hz. Detailed noise analysis of the Doppler tracking data is also presented, with the prospect that these noise sources will be mitigated in future Chinese deep space missions. A feasibility study is also undertaken to understand the scientific capability of the Chang'e 4 mission, due to be launched in 2018, in relation to the stochastic gravitational wave background around 0.01 Hz. The study indicates that the upper bound on the energy density may be further improved by another order of magnitude from the Chang'e 3 mission, which will fill the gap in the frequency band from 0.02 Hz to 0.1 Hz in the foreseeable future., The paper consists of 28 pages, 16 figures. It has been accepted by the journal: Advances in Space Research

In the paper, a set of algorithms to construct synthetic aperture radar (SAR) matching suitable features are firstly proposed based on the evolutionary synthesis strategy. During the process, on the one hand, the indexes of primary matching suitable features (PMSFs) are designed based on the characteristics of image texture, SAR imaging and SAR matching algorithm, which is a process involving expertise; on the other hand, by designing a synthesized operation expression tree based on PMSFs, a much more flexible expression form of synthesized features is built, which greatly expands the construction space. Then, the genetic algorithm-based optimized searching process is employed to search the synthesized matching suitable feature (SMSF) with the highest efficiency, largely improving the optimized searching efficiency. In addition, the experimental results of the airborne synthetic aperture radar ortho-images of C-band and P-band show that the SMSFs gained via the algorithms can reflect the matching suitability of SAR images accurately and the matching probabilities of selected matching suitable areas of ortho-images could reach 99 ± 0.5%.

On December 13, 2012, China's Chang'e 2 Spacecraft successfully flew past No.4179 Asteroid Toutatis, and acquired a series of clear optical images concerning Toutatis. The data obtained is very important for us to be able to establish feasible methods for deep-space optical navigation. However, as the fly-by process cannot be repeated and deep-space environments cannot be tested using physical platforms on the ground, the problem concerning how to verify the correctness and accuracy of optical positioning algorithms emerges. According to the fly-by strategy of Chang'e 2 to detect Toutatis, in combination with navigation motion relationships between the spacecraft and the asteroid, such as orbit and position, and in light of actual data and problem models, the paper constructs a set of algorithms of ground numerical appraisal for relative positioning accuracy when Chang'e 2 flies past Toutatis. Through three relative positioning algorithms, conducting algorithm testing of ground numerical appraisal, which effectively appraises the positioning algorithm accuracy through numerical methods and clearly explains the sources of random error and system error, This method can be used as the basic means for ground verification of deep-space optical navigation algorithms.

According to the characteristics of lunar orbiter tracking, a novel automotive navigation method based on camera and laser altimeter was proposed. It is aided by historic images and terrain features as to enhance the autonomy of aerospace control. The prototype of the method was studied, and the key technologies was analyzed. Cameras obtain real-time images of the lunar surface, while laser altimeter provides real-time orbital altitude. The altitude data can be introduced to get the resolution of the real-time image, in case camera may bring a series of uncertainties because the orbiter's altitude changes with the lunar gravity anomaly and topography. This is good for the reliability of image matching. The method of solving the position and attitude of the orbiter is given. The sufficiency and advantages of this method are also discussed.

On December 13, 2012, China’s Chang’E II spacecraft successfully flew by the asteroid Toutatis, and obtained a series of clear optical images on Toutatis surface, which is China’s first asteroid detection and also human’s first close optical exploration on the asteroid. These optical images are of great benefit to build an accurate model of Toutatis surface, and to analyze physical properties of the asteroid. However, due to the acquired optical images can only reveal a partial surface of Toutatis, meanwhile influenced by light, one cannot visually determine whether the optical images reveal a complete side of Toutatis or not, in other words, how much part of the side was not photographed because of light shadows. In the paper, based on the Chang-E II flew over strategy, combined with the spacecraft’s orbit, attitude and other navigation information, and through photographic measurement method, the angle between the direction of the sun light and the line of sight of Toutatis’s surface feature points was accurately calculated, namely irradiation angle. Through precise calculation, irradiation angles of Toutatis surface feature points are 36.8°–39.0°, and furtherly, with the help of a pose simulation of a radar-observing-based shape model by NASA, we can get one important conclusion, that is during the fly by, the space borne optical camera got complete imaging for one side of Toutatis’s surface, and there is no wide shadow on the images.

This paper presents a novel navigation scheme which the switch between INS/SAR integration and inertial navigation is available. The FAST feature detector for image registration is employed for SAR/INS integrated navigation. Simulation results show that FAST feature based image registration method is able to meet the requirements of the real time applications. With the proposed navigation scheme, the performance of the integrated navigation has been improved significantly compared with traditional method. It means the effect of SAR aided navigation error has been reduced.

A set of new orbit Measurement method was proposed based on the integration of high-resolution lunar surface imaging and ground-based radio measurement. Through the method, the advantage of stable tracking by radio measurements and the advantage of short-range measurement by onboard camera are expect to be fully integrated. The basic idea of the method is as following. First, combined with orbit constraints of a lunar high resolution imaging satellite, relying on optical imaging information and available reference images on the lunar surface, and based on the visual related theory of multi-view geometry and photogrammetry, the accurate location information of the satellite platform in a moment was calculated. Then, the location information was fused reasonably with satellite speed, range and angle by ground radio measurements. Thereby, a more flexible lunar orbiter orbit determination method than the existing methods was established. Currently, based on real data of “Chang’E II” satellite, visual/radio fusion-based orbit determination model were studied. Transfer relationship of the position and attitude between the satellite and imaging points on lunar surface was derived. Typical aspects of information fusion-based orbit determination were analyzed. Preliminary experimental results verify the feasibility of the proposed method. After parameter calibration, the partial positioning accuracy of “Chang’E II” is better than 100 m, variance limits at 30 m.

In this paper, the processing course of monitoring data recorded by space monitoring cameras on Chang E satellite was analyzed, which mainly includes data transfer and data conversion. Monitoring devices and data formats were illuminated firstly, based on which, the process of data collection, data receiving and data processing was analyzed at length. Key indicators such as data quantity and processing time were estimated based on simulated data. Analysis results of the paper are helpful to making valid control plans in real mission.

In this paper, a set of terrain generation algorithm was proposed to simulate lunar terrain in landing areas. Based on characteristic analysis of lunar surface, simulated terrain was constructed by combination of a basic terrain and an impact terrain. According to the algorithm, roadblocks such as screes and craters were modeled through a parameterized method firstly, then the basic terrain was created through random seeds and the impact terrain was generated based on parameterized shape models of roadblocks. Simulation experiments show that terrains generated by the algorithm accord to general features of real lunar terrain, and simulated results can be used to validate lunar road planning algorithm on the ground.

The performance of wavelet-based multifocus image fusion depends on the number of decomposition levels in the wavelet transform. Too few decomposition levels result in poor spatial quality in the fused images. On the other hand, too many levels induce distortion between the original and the fused images. In general, fusion of images with larger resolution requires a higher number of decomposition levels. In this paper, a multiobjective optimization approach to determine the optimal number of decomposition levels is presented. First the optimization flow of decomposition levels for wavelet-based multifocus image fusion is presented; then the proper evaluation metrics for multifocus image fusion are given, new conditional mutual information is proposed, which can avoid the information overlapping; and finally the algorithm of multiobjective particle swarm optimization using the local best model is designed to search the optimal number of decomposition levels. In the experiments it is found that the optimal decomposition level is not a fixed value, but rather, changes with the characteristics of the original images. The experimental results also show that using the method of multiobjective optimization can effectively obtain the optimal number of decomposition levels.

In this paper, an attempt is made to define the features of SAR employment planning problem for tactical aircraft in military operations. General concepts for use of airborne SAR to find and locate targets are described and formulated into a parameter optimization problem. The utility and constrains of SAR imaging followed by weapon delivery are described, which rely on target characteristics, threat distribution, terrain and attack conditions. Then some approaches to solving the technique problem are described. Although the interests expressed in this paper are motivated by capabilities that might be afforded by manned aircrafts, the concepts are relevant for unmanned air vehicles conducting battle reconnaissance tasks equipped with SAR sensors.

The mysteries of near-Earth asteroid 4179 Toutatis have been more comprehensively unveiled by analyzing the optical images taken during the Chang'e-2 flyby in 2012. Compared with previous works, this paper concentrates on the photogrammetric relation between the Chang'e-2 spacecraft and Toutatis and the imaging shadow effect during the flyby. Accurate models of imaging and optical measurements are developed to study Toutatis's dimensions and rotational state at the time of imaging. As the illumination study shows, the shadowed region perpendicular to the long axis accounts for 27.78% of the Toutatis images, while the long axis of the body is fully captured. With a compensation on the shadow effect, the optical measurements reveal that Toutatis's long axis is 4354 ± 56 m, the maximum length is 4391 ± 56 m, and the spatial orientation described with the angles of direction cosine during the flyby is (12613 ± 029, 12298 ± 021, 12663 ± 046). Furthermore, a new triaxial ellipsoid of 4354 × 1835 × 2216 m and a volume of 7.5158 km3 are proposed based on the previous Toutatis shape model. The effectiveness of the proposed method is validated, since typical features such as the neck and endpoints agree well with the results simultaneously observed by the ground radar. Moreover, it also potentially provides a feasible approach to precisely calculate the spin period of Toutatis.