Energy-optimal reconfiguration of satellite formation flying in the presence of uncertainties

In this study, a two-step control methodology is developed for energy-optimal reconfiguration of satellites in formation in the presence of uncertainties or external disturbances. First, based on a linear deterministic system model, an optimal control law is analytically determined such that a satellite maneuvers from an initial state to a final state relative to another satellite. The structure of this optimal solution is predetermined and simply given by a linear combination of the fundamental matrix solutions associated with the original equations of relative motion. Only the coefficients are to be determined to satisfy given initial and final conditions. In the second step, an uncertain nonlinear formation system is considered and a robust adaptive controller is designed to compensate for the effects of uncertainties or disturbances that the formation system may encounter. Although the control strategy is inspired by sliding mode control, it produces smooth control signals, thereby avoiding chattering. Also, an adaptation law is added such that the uncertainty or disturbance effects are effectively and quickly eliminated without a priori information about them. The combination of these two controllers guarantees that the satellite accurately tracks the optimal path in the unknown environment. Numerical simulations demonstrate the effectiveness and accuracy of the proposed two-step control methodology, in which a satellite formation is optimally reconfigured under unknown environmental disturbances.     

Higher order ionospheric error correction in BDS precise orbit determination

The ionospheric error affects the accuracy of the Global Navigation Satellite Systems observation and precise orbit determination. Usually, only the first order ionospheric error is considered, which can be eliminated by the ionospheric-free linear combination observation. But the remaining higher order ionospheric error will affect the accuracy of observations and their applications. In this paper, the influence of the higher order ionospheric error have been studied by using the International Geomagnetic Reference Field 13 and the Global Ionosphere Maps model produced by the Center for Orbit Determination in Europe. Focus on ionospheric error, the experiment of paper at doy 302 in 2019, which show that the second order ionospheric error impacting BeiDou Navigation Satellite System (BDS) B1I and B3I observation is 6.3569 mm and 11.8484 mm, respectively. Whereas, the third order ionospheric error impacting BDS B1I and B3I observation is 0.1734 mm and 0.3977 mm, respectively. Due to the current measurement accuracy of BDS carrier-phase observation can reach 2 mm, the influence of high order ionospheric error on observation should be considered. For BDS precise orbit determination, the orbit overlapping results are indicated that its orbit accuracy can be improved approximately 5 mm with the higher order ionospheric error correction, which is also in agreement with the results of Satellite Laser Ranging in this work.     

Measuring atmospheric density using GPS–LEO tracking data

We present a method to estimate the total neutral atmospheric density from precise orbit determination of Low Earth Orbit (LEO) satellites. We derive the total atmospheric density by determining the drag force acting on the LEOs through centimeter-level reduced-dynamic precise orbit determination (POD) using onboard Global Positioning System (GPS) tracking data. The precision of the estimated drag accelerations is assessed using various metrics, including differences between estimated along-track accelerations from consecutive 30-h POD solutions which overlap by 6 h, comparison of the resulting accelerations with accelerometer measurements, and comparison against an existing atmospheric density model, DTM-2000. We apply the method to GPS tracking data from CHAMP, GRACE, SAC-C, Jason-2, TerraSAR-X and COSMIC satellites, spanning 12 years (2001–2012) and covering orbital heights from 400 km to 1300 km. Errors in the estimates, including those introduced by deficiencies in other modeled forces (such as solar radiation pressure and Earth radiation pressure), are evaluated and the signal and noise levels for each satellite are analyzed. The estimated density data from CHAMP, GRACE, SAC-C and TerraSAR-X are identified as having high signal and low noise levels. These data all have high correlations with anominal atmospheric density model and show common features in relative residuals with respect to the nominal model in related parameter space. On the contrary, the estimated density data from COSMIC and Jason-2 show errors larger than the actual signal at corresponding altitudes thus having little practical value for this study. The results demonstrate that this method is applicable to data from a variety of missions and can provide useful total neutral density measurements for atmospheric study up to altitude as high as 715 km, with precision and resolution between those derived from traditional special orbital perturbation analysis and those obtained from onboard accelerometers.     

Long term changes of altimeter range and geophysical corrections at altimetry calibration sites

Accurate sea level trend determination is fundamentally related to calibration of both the instrument as well as to investigate if there are linear trends in the set of standard geophysical and range corrections applied to the sea level observations. Long term changes in range corrections can leak into the observed sea level record and be interpreted as part of the sea level trend. Particularly if these exhibit anomalous trend close to the satellite calibration sites.     

Detection of volcanic gases and particles by satellite

In this paper the detection of components of volcanic eruption has been carried out investigating, in appropriate bands of the electromagnetic spectrum (6.25, 8.7, 10.8, 12 μm), the values of the brightness temperature. The analysis has been performed in the Thermal Infrared Region (TIR) studying both the absorption–emission and scattering phenomena related to the interactions between electromagnetic radiation and volcanic emissions. The results have been achieved by means of a combined use of numerical simulations, devoted to examining the behaviour of the atmosphere gases and volcanic components, and remotely sensed satellite images. The proposed methodologies allow an estimate of the amount of gaseous and solid components, of the size of the emitted particles, of the height of the volcanic plume and of the distance of the volcanic components from the crater. The processed images come from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) sensor on board the geo-stationary satellite Meteosat Second Generation (MSG) and take into consideration an eruption of the Etna volcano as a study case (1st of April 2012, 04:30 and 05:30 UTC). The procedures are general and may therefore be extended to any other similar case.     

Using the Global Navigation Satellite System and satellite altimetry for combined Global Ionosphere Maps

For deriving global maps of the Total Electron Content (TEC) from space geodetic techniques usually observations from the Global Navigation Satellite System (GNSS) are taken. However, the GNSS stations are inhomogeneously distributed, with large gaps particularly over the sea surface.     

Recovery of the Earth’s gravity field from formation-flying satellites: Temporal aliasing issues

Temporal and mean gravity field models derived from the twin-satellite, leader–follower mission GRACE have provided a new type of information for Earth sciences. In this contribution, we study the potential of various alternative satellite formations for gravity field determination in the post-GRACE era in a simulation environment. In particular, the effects of spherical harmonic truncation and of temporal aliasing in the processing of gravity products from such future formations are investigated.     

编队卫星相对运动描述方法综述

对于近地轨道卫星编队飞行的相对运动理论研究,可以采用的方法包括直角坐标法和 轨道要素法。利用直角坐标法得到的相对运动动力学方程可以用于编队队形控制研究,轨道 要素法能够给出相对运动的运动学描述,便于定量研究摄动影响和进行编队队形设计。分析 了直角坐标法在描述卫星长期编队飞行方面的局限性,综述了利用轨道要素描述编队卫星相 对运动的各种研究方法,包括轨道要素差法、相对轨道要素法和参照轨道要素法等。     

一种基于分组截短PN码的SOFDM信道估计方法

基于PN序列的信道估计方法的特点是计算过程简明,已被广泛应用在地面无线局域网系统中。在卫星正交频分多路复用传输体制(SOFDM)系统中引入传统PN序列信道估计方法的主要问题在于卫星信道的多径延时远大于SOFDM数据符号的持续时间,信道特性比较复杂,使得传统方法的估计精度严重下降。提出了一种改进的方法,根据信道的近似周期特性对数据帧进行合理的等长度分组,并把长PN序列改为短PN码来跟踪信道局部特性的变化,再对分组后的数据子帧分别进行信道估计。对该方法的设计思想进行了理论分析,并通过仿真验证了该方法的估计精度比传统方法至少提高了10倍。     

Proposal for ionospheric tomography technique using a new HF radio source

Measurements of the line integral of the electron density along satellite-to-ground ray paths (i.e. TEC) using differential phase or Doppler of two coherent VHF/UHF signals transmitted from NNSS or GPS satellite networks have been used in the ionospheric tomography for mapping large-scale ionospheric images over region of interest. In this paper, we present theoretical studies of using a new signal source HF frequency in tomographic imaging. The initial phase problem inherent in the phase measurement can be eliminated by measuring Faraday rotations. Relative rotation on two adjacent HF frequencies is used to solve the ambiguity problem. A second-order approximation of the Faraday rotation incorporated with ray-tracing technique improves the reconstruction degradation due to ionospheric refractions. CASSIOPE is a multi-purpose small satellite that receives HF signals from ground radar facilities and it is scheduled for launch in early 2008. Simulations have demonstrated the potential applications of the ionospheric tomography in CASSIOPE/e-POP satellite experiment.