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Telescopes
Published in Daniel Malacara-Hernández, Brian J. Thompson, Fundamentals and Basic Optical Instruments, 2017
Marija Strojnik, Maureen S. Kirk
This observational mode is limited by the facility's physical construction and the different air mass between the early/late and optimum (zenith) observation periods. The air mass is the ratio of the thickness of the atmosphere that is traversed by a ray from an object at an angle θ over that in a zenith (see Figure 11.28). In the case of star position B, the ray passes through the air thickness d, given by d=h/cosθ[km].
Fault Tolerant Attitude Estimation
Published in Chingiz Hajiyev, Halil Ersin Soken, Fault Tolerant Attitude Estimation for Small Satellites, 2020
Chingiz Hajiyev, Halil Ersin Soken
This chapter investigates the adaptation methods for the measurement noise covariance (R) matrix of the attitude filter. As known, space is a severe environment so faults such as abnormal measurements, increase in the background noise etc. are always a potential threat for the onboard attitude sensors. The main issue is such faults degrade the estimation accuracy and may make the attitude estimator diverge in the long term. Hence the filtering algorithm that is used as the attitude estimator must be made robust using an adaptive approach. We are specifically interested in sensor fault cases but the proposed methods can be easily generalized for any other case including but not limited to:Experimental attitude sensors or commercial off-the-shelf (COTS) devices, for which it is difficult to get accurate noise characteristics, are used (this became very common issue together with the popularity of the low-cost small satellite missions).Environmental effects make the sensor noise characteristics gradually change. A common example may be thermal effects on the gyro measurements [1].Randomly delayed measurements are used in the filtering algorithm. This is very common case when the star trackers are used for attitude estimation. Before the sensor measurement is available to the filter, star image sampling, star position determination, star identification and attitude calculation are required. All these processes require handling time and cause a random delay in the star tracker measurements [2, 3].
Electrical Impedance Tomography Using Evolutionary Computing: A Review
Published in D. P. Acharjya, V. Santhi, Bio-Inspired Computing for Image and Video Processing, 2018
Wellington Pinheiro dos Santos, Ricardo Emmanuel de Souza, Reiga Ramalho Ribeiro, Allan Rivalles Souza Feitosa, Valter Augusto de Freitas Barbosa, Victor Luiz Bezerra Arajo da Silva, David Edson Ribeiro, Rafaela Covello de Freitas
For exponential crossover, its star position is chosen between 1,…,D $ 1,\ldots , D $ , and L consecutive elements are taken from the mutant vector Vi,j,G $ V_{i,j,G} $ , meaning that L adjacent elements are changed in exponential variant. Also, the relation between the probability of crossover and PCR $ P_{CR} $ is nonlinear and the deviation from linearity is bigger with the increasing dimension of the problem. Wi,j,G+1=Vi,j,Gfor(j=nD,n+1D…n+L-1D)Xi,j,Gfor all otherj∈[1,D] $$ \begin{aligned} W_{i,j,G+1}= \left\{ \begin{array}{rl} V_{i,j,G}&\text{ for} (j = \left\langle n \right\rangle _D, \left\langle n+1 \right\rangle _D \ldots \left\langle n+ L - 1 \right\rangle _D) \\ X_{i,j,G}&\text{ for} \text{ all} \text{ other} j \in [1,D] \end{array}\right. \end{aligned} $$
Effect of proton beam irradiation on the tracking efficiency of CMOS image sensors
Published in Radiation Effects and Defects in Solids, 2022
Jing Fu, Jie Feng, Yu-Dong Li, Lin Wen, Dong Zhou, Qi Guo
The diagonal distance error was calculated using the direction vector of the star in the inertial system of the celestial sphere and that measured on the star map. Then, the error in the star centroid position was further calculated using the relationship between the standard deviation of the star diagonal distance and that of the star position.