Arecibo radar observations of Phobos and Deimos

In November 2005, we observed the moons of Mars using the Arecibo 2380-MHz (13-cm) radar, obtaining a result for the OC radar albedo of Phobos (0.056±0.014) consistent with its previously reported radar albedo and implying an upper bound on its near-surface bulk density of . We detected Deimos by radar for the first time, finding its OC radar albedo to be 0.021±0.006, implying an upper bound on its near-surface density of , consistent with a high-porosity regolith. We briefly discuss reasons for these low radar albedos, Deimos' being possibly the lowest of any Solar System body yet observed by radar.     

Cassini图像中受散射光影响的7颗土星近环内卫星的天体测量

卡西尼号探测器携带了光学成像科学子系统(ISS: Imaging Science Subsystem), 在2004--2017年间拍摄了一些土星内卫星的图像. 部分图像中土星内卫星非常靠近土星环, 观测对象受土星环散射光影响导致测量精度差, 甚至无法测量. 由此提出一种适用于这类图像的背景消除算法, 可对受土星光环散射光影响的土星内卫星进行测量. 归算了7颗近环内卫星(土卫十(Janus)、土卫十一(Epimetheus)、土卫十五(Atlas)、土卫十六(Prometheus)、土卫十七(Pandora)、土卫三十二(Methone)和土卫四十九(Anthe))的70张ISS图像, 并与不消除散射光的方法进行了比较. 结果显示, 该方法至少可以提高43%的精度. 和喷气推进实验室的土星卫星历表\lk SAT415相比, 测量得到的赤经和赤纬方向的残差均值分别为0.72km和2.26km, 标准差分别为10.99km和11.36km.     

A method for averaging the perturbing function in Hill’s problem

A modified method for averaging the perturbing function in Hill’s problem is suggested. The averaging is performed in the revolution period of the satellite over the mean anomaly of its motion with a full allowance for a variation in the position of the perturbing body. At its fixed position, the semimajor axis of the satellite orbit during the revolution of the satellite is constant in view of the evolution equations, while the remaining orbital elements undergo secular and long-period perturbations. Therefore, when the motion of the perturbing body is taken into account, the semimajor axis of the satellite orbit undergoes the strongest perturbations. The suggested approach generalizes the averaging method in which only the linear (in time) term is included in the perturbing function. This method requires no expansion in powers of time. The described method is illustrated by calculating the perturbations of the semimajor axes for two distant satellites of Saturn, S/2000 S 1 and S/2000 S5. An approximate analytic solution is compared with the results of numerical integration of the averaged system of equations of motion for these satellites.     

高轨空间碎片的观测模拟

对高轨空间碎片进行了观测模拟。在观测模拟中，分别选取南京和云南两个观测站；采用2种跟踪方式：跟踪恒星和跟踪卫星；对于同一地理经度上的同步卫星进行了观测；提供了发现新目标的方法；并给出了新目标的初轨。这为我国空间碎片的数据库的建立作了有益的尝试，对于卫星和航天器的飞行安全具有特别重要的意义。     

光学卫星常态化相对辐射定标方法研究

相对辐射定标是获得各类光学遥感卫星传感器的高精度观测产品的必备条件,由于受发射震动、在轨空间环境变化等因素以及传感器自身衰减的影响,传感器在轨响应状态会随时间发生衰减和漂移,单一相对辐射定标方法无法保障卫星传感器在特定时间的探元响应一致性。本文以光学卫星传感器全生命周期影像辐射质量保障为目标,主要研究了光学卫星传感器在轨生命周期内高频率、高定标精度的常态化相对辐射定标方法,并总结了当前常用的相对辐射定标精度评估方法,评估了各定标方法的指标以及应用场景。利用珞珈一号01星（LJ1-01）夜光传感器获取的影像进行在轨暗电流、和在轨均匀场地定标实验和验证,珠海一号02组高光谱卫星影像被用来实施和验证统计定标和偏航辐射定标方法,并耦合多种定标方法实现常态化辐射定标。实验结果表明：各种定标方法处理后影像的条纹系数小于0.25%,图像相对标准差均优于3.00%;多种定标方法相互结合的常态化辐射定标方法实现了多种定标方法的优势最大化,完成常见传感器的高精度在轨标定。     

The formation of Ganymede's grooved terrain: Numerical modeling of extensional necking instabilities

Ganymede's grooved terrain likely formed during an epoch of global expansion, when unstable extension of the lithosphere resulted in the development of periodic necking instabilities. Linear, infinitesimal-strain models of extensional necking support this model of groove formation, finding that the fastest growing modes of an instability have wavelengths and growth rates consistent with Ganymede's grooves. However, several questions remain unanswered, including how nonlinearities affect instability growth at large strains, and what role instabilities play in tectonically resurfacing preexisting terrain. To address these questions we numerically model the extension of an icy lithosphere to examine the growth of periodic necking instabilities over a broad range of strain rates and temperature gradients. We explored thermal gradients up to 45 K km⁻¹ and found that, at infinitesimal strain, maximum growth rates occur at high temperature gradients (45 K km⁻¹) and moderate strain rates (10⁻¹³ s⁻¹). Dominant wavelengths range from 1.8 to 16.4 km (post extension). Our infinitesimal growth rates are qualitatively consistent with, but an order of magnitude lower than, previous linearized calculations. When strain exceeds ∼10% growth rates decrease, limiting the total amount of amplification that can result from unstable extension. This fall-off in growth occurs at lower groove amplitudes for high-temperature-gradient, thin-lithosphere simulations than for low-temperature-gradient, thick-lithosphere simulations. At large strains, this shifts the ideal conditions for producing large amplitude grooves from high temperature gradients to more moderate temperature gradients (15 K km⁻¹). We find that the formation of periodic necking instabilities can modify preexisting terrain, replacing semi-random topography up to 100 m in amplitude with periodic ridges and troughs, assisting the tectonic resurfacing process. Despite this success, the small topographic amplification produced by our model presents a formidable challenge to the necking instability mechanism for groove formation. Success of the necking instability mechanism may require rheological weakening or strain localization by faulting, effects not included in our analysis.     

第一代北斗星座卫星对GALILEO卫星导航定位系统增强性能仿真

第一代北斗星座已经于2003年布置完毕，基本实现了中国本土及周边的导航定位功能。中国作为GALILEO卫星导航定位系统开发的重要参与者，研究北斗系统和GALILEO卫星导航定位系统的集成问题十分必要和迫切。本文，就第一代北斗星座对GALILEO卫星导航定位系统的增强性能进行分析，主要对PDOP指标进行了仿真。给出了国内典型城市的PDOP及其一天内的变化曲线。     

Ground-based near infrared spectroscopy of Jupiter's ring and moons

The backscattered reflectivity of Jupiter's ring has been previously measured over distinct visible and near infrared wavelength bands by a number of ground-based and spaceborne instruments. We present spectra of Jupiter's main ring from 2.21-2.46 μm taken with the NIRSPEC spectrometer at the W.M. Keck observatory. At these wavelengths, scattered light from Jupiter is minimal due to the strong absorption of methane in the planet's atmosphere. We find an overall flat spectral slope over this wavelength interval, except for a possible red slope shortward of 2.25 μm. We extended the spectral coverage of the ring to shorter wavelengths by adding a narrow-band image at 1.64 μm, and show results from 2.27-μm images over phase angles of 1.2°-11.0°. Our images at 1.64 and 2.27 μm reveal that the halo contribution is stronger at the shorter wavelength, possibly due to the redder spectrum of the ring parent bodies as compared with the halo dust component. We find no variation in main ring reflectivity over the 1.2°-11.0° phase angle range at 2.27 μm. We use adaptive optics imaging at the longer wavelength L′ band (3.4-4.1 μm) to determine a 2-σ upper limit of 22 m of vertically-integrated I/F. Our observing campaign also produced an L′ image of Callisto, showing a darker leading hemisphere, and a spectrum of Amalthea over the 2.2-2.5 and 2.85-3.03 μm ranges, showing deep 3-μm absorption.