Dynamics of an autonomous spacecraft control system at initial transition to a tracking mode

The problems of autonomous digital control of the information satellites and space robots during their initial transition to a tracking mode, namely in the initial orientation modes, are considered. Autonomous angular guidance and modularly limited vector digital control using a vector of the modified Rodrigues parameters are applying to bring the spacecraft’s orientation from completely arbitrary to the required one. The developed methods, algorithms and simulation results for a mini-satellite in a sun-synchronous orbit are presented.

Steering drive operation in tracking and positional mode

В статье рассматривается система управления электромеханическим рулевым приводом, работающим в двух режимах: позиционирование угла руля с заданной скоростью перекладки и слежение сигнала управления от системы «Авторулевой». Исследовались суда различной длины. Показано, что качество переходных процессов угла поворота руля судна несущественно зависит от осадки и скорости судна. Коэффициент гибкой обратной связи для режима позиционирования угла поворота руля определяется заданием скорости и требуемого угла перекладки. Вычисление коэффициента гибкой обратной связи осуществляется интерполяционным способом на основе данных полученных решением системы уравнений. В статье также приводится вывод данной системы уравнений. Режим слежения реализуется заданием максимальной скоростью перекладки в той же структурной схеме системы управления. Это приводит к унификации системы управления рулевым приводом. Выведены точность и ограничения по скорости и ускорению угла поворота для следящего режима. Показано, что в режим слежения с учетом ограничений выполняется с высокой точностью, что делает возможным реализацию программного управления рулевым приводом. Предполагается использовать данную систему унифицированного управления в судах с беспилотным управлением. The article considers the control system of an electromechanical steering drive operating in two modes: positioning the steering angle with the required speed of shifting and tracking the control signal from the "Autopilot" system. Ships of various lengths were analyzed. It is shown that the quality of the transients of the ship's rudder angle does not significantly depend on the draft and speed of the ship. The coefficient of flexible feedback for the steering angle positioning mode is determined by setting the speed and the required angle of shifting. The flexible feedback coefficient is calculated using an interpolation method based on the data obtained by solving a system of equations. The article also provides a conclusion of this system of equations. The tracking mode is realized by setting the maximum speed of the shift in the same block diagram of the control system. This results in a unified steering control system. The accuracy and limits on the speed and acceleration of the angle of rotation for the tracking mode are derived. It is shown that the tracking mode, taking into account the limitations, is performed with high accuracy, which makes it possible to realize the program control of the steering drive. It is assumed to use this unified control system in ships with drone control.

STRUCTURAL ANALYSIS OF ON-BOARD MULTIMODE AUTO TRACKING SYSTEM

Two variants of two-channel structures of tracking systems has been introduced into consideration, which are sequentially used in two modes of operation of the on-board automatic tracking system. Taking into account the fact of quantization according to the level of signals for measuring the angles of mismatch of each of the channels, it is justified to use not one, but two different structures in the second auto-tracking mode. Using mathematical modeling, the expediency of increasing the number of two-channel structures from two to three, used in a single auto-tracking process according to the criterion of the provided dynamic accuracy, have been demonstrated. This paper presents a two-channel structures investigation for tracking systems operating in tracking mode of maneuverable targets, taking into account various systems of carrier operations. Considered multichannel complex control system (CCS) structure that include precise channel (PC) and coarse channel (CC). CC use the carrier and the flight control system (FCS) to make rude tracking. The PC function implemented by an optoelectronic device (OED) has placed in a traditional angular mechanical suspension, which act with high accuracy. Described three CCS operation modes classified by target mark fall inside or outside direction finder (DF) CC or PC field of view. At the capture mode can be used only information from the CC DF. At the complex control mode becomes possible to use information from both CC DF and PC DF. At the precision mode, it is advisable to use information only from the PC DF. Developed CCS functioning math model to check main points of this work. Math model makes possible to carry out simulation tests for each mode in a separate operation, as well as when they are integrated into a single complex automatic target tracking mode. Has been described efficiency factor as the total time of satisfactory accuracy relative to entire experiment time. On math model modes efficiency has compared to each other. As result precision mode have higher efficiency and capture mode lowest. As conclusion recommended use a precision mode to achieve higher accuracy at auto tracking tasks.

Recent Results from the ALBIOM Project on Biomass Estimates from Sentinel-3 Altimetry Data

<p>The ALtimetry for BIOMass (ALBIOM) project is an ESA-funded Permanent Open Call Project that aims to retrieve forest biomass using Copernicus Sentinel-3 (S3) altimeter data. The overall goal of ALBIOM is to estimate biomass with sufficient accuracy to be able to increase existing satellite data for biomass retrieval, as well as to improve the global mapping and monitoring of this fundamental variable.  </p><p>The project core tasks consist of 1) an analysis of the sensitivity of altimetry backscatter data on land parameters; 2) the development and validation of a Sentinel-3 altimeter backscatter simulator, including the effect of both topography and vegetation  and 3) the development and validation of a machine-learning biomass estimation algorithm.</p><p>Here we present a summary of the results obtained from the project. The sensitivity analysis reveals that both the altimetric waveforms and the corresponding Normalised Radar Cross Sections (NRCSs) can be sensitive to the presence of biomass in the order of 100-400 tons/ha, but they are also influenced by topography and water bodies. Different sensitivities with respect to the different frequencies and resolution modes are observed, highlighting non-linear behaviours of the NRCSs. The use of differential NRCSs, defined as the difference among those calculated over two different bandwidths, was demonstrated to be not necessarily more sensitive to vegetation, as it was instead highlighted by previous studies like [Papa et al., 2003].</p><p>The tracking window often appears partly or completely misplaced, when the tracking mode is in open-loop mode prescribing a predetermined range, and its size is often not long enough when collecting data over land, especially over regions with complex topography. The length and correct positioning of the tracking window over land represent therefore critical aspects for a study like ALBIOM.</p><p>The modelling work has been focused on the development of a merged model approach to simulate altimeter waveforms over vegetated areas. The merging is obtained via the simultaneous use of the modifiedTor Vergata Scattering Model (TOVSM) [Ferrazzoli and Guerriero, 1995, 1996] to simulate the waveform of a flat surface covered by forest vegetation, and the use of the Soil And Vegetation Reflection Simulator (SAVERS) [Pierdicca et al., 2014], originally conceived for GNSS-Reflectometry, and here adapted to the Altimetry system. The simulator developed within ALBIOM shows promising ability to reproduce the general characteristics of the S3 waveforms. The simulations related to forested surfaces present at least two peaks, due to the top of canopy and to the ground, but the presence of topography may introduce other peaks in the waveforms, making the identification of vegetation and topographic effects challenging.</p><p>Initial results on the algorithm development using Artificial Neural Networks (ANN) highlight some promising biomass estimates over specific areas (e.g. Central Africa) but also differences in algorithm performances among different regions. The corrected “ice” backscatter coefficient showed the highest sensitivity to biomass, but its values are often invalid over land, which limits the number of meaningful retrievals. The different altimeter tracking mode of Sentinel-3 over different areas of the globe (i.e., open loop and closed loop) could also be responsible for the differences in results.</p><p> </p><p> </p><p> </p>

New upgrades of Open-Loop Tracking Command (OLTC) tables of nadir altimeters in 2020 and benefits for inland waters users

<p>Fresh water is an essential resource that requires a close monitoring and a constant preservation effort. The evolution of hydrological bodies water level constitutes a key indicator on the available quantity of fresh water in a given region. The limited extent of the in situ networks currently deployed has generated a growing interest in using space borne altimetry as a complementary data source to increase the coverage of emerged fresh water stocks and ensure a more global and continuous monitoring of their water surface height.</p><p>A great effort has been carried out over the past decade to improve altimeters’ capability to acquire quality measurements over inland waters. In particular, the Open-Loop Tracking Command (OLTC), which consists in calibrating the altimeter signal acquisition window with a prior information on the overflown hydrological surface height, represents a major evolution of the tracking function. This tracking mode’s efficiency is such that it is now stated as operational mode for current Sentinel-3 and Jason-3 missions as well as the recently launched Sentinel-6A mission. The improvements brought to onboard tables contents in 2017 (Jason-3), 2018 (Sentinel-3B) and 2019 (Sentinel-3A) enhanced and confirmed the OLTC performances.</p><p>In 2020, the onboard OLTC tables of the Jason-3, Sentinel-3A and Sentinel-3B missions have benefitted from further new major upgrades. The first version of the Sentinel-6A onboard OLTC tables holds the same content as Jason-3. The tracking command defined over Jason-3 and Sentinel-6A repeat cycle now accounts for more than 30,000 hydrological targets which represents five times more targets than in the previous version. For each Sentinel-3, the number of water body surface heights coded into the OLTC has been increased by a factor of 3 to 70,000. This further major step is made possible by the analysis and merging of the most recent digital elevation models (SRTM, MERIT and ALOS/PalSAR) and water bodies databases (HydroLakes, GRaND v1.3, SWBD, GSW). This methodology ensures coherency and consistent standards between all nadir altimetry missions and types of hydrological targets.</p><p>A detailed description of the 2020 upgrades will be given as well as measurements validation results obtained since their upload. An overview of the global validation of Sentinel-6A measurements over hydrological targets will also be presented.</p><p>These 2020 OLTC upgrades constitute a great asset for building a valuable and continuous record of the water surface height of worldwide lakes, rivers, reservoirs and wetlands. In addition, for a continuous improvement of the OLTC tracking mode, the users can check the content of the onboard OLTC tables over hydrological targets for both Sentinel-3 missions on the https://www.altimetry-hydro.eu/ web portal. When relevant, they can correct existing water surface heights or submit new targets.</p>

Overview of methods for controlling the parameters of the tracking mode of a multifunctional radar station

Problem statement. With the development of devices for electronic control of the spatial position of the antenna radiation pattern, it became possible to control the time and duration of probing the radar station of various angular directions. There are many works based on the use of this principle to solve various radar tasks, in particular, to implement the target tracking mode. However, there is no general overview of the developed methods for controlling the parameters of the maintenance mode, the conditions and restrictions introduced in them, control and optimization criteria, as well as the scope of application. Goal. Analysis of the main methods of controlling the parameters of the tracking mode of a multifunctional radar station, including the control criteria used. Results. The analysis of the main methods of controlling the time of probing a target by a radar station, the duration of its probing pulses and signal is carried out. All methods are combined into several groups, the conditions and restrictions introduced during their development are defined. The quality criteria on the basis of which they are synthesized are analyzed. Practical approaches to calculating the parameters of the tracking mode of modern multifunctional radar stations are considered. The directions of further research of the subject area under consideration are formulated. Practical significance. The scientific and methodological apparatus used in calculating the parameters of the tracking mode of multifunctional radar stations is determined. Mathematical equations are given for calculating the duration of the probing signal and the time of probing the target. A set of parameters that must be taken into account when synthesizing control methods is determined. The applicability of the methods in various operating conditions of the radar station is evaluated.