Space-based Optics Research Articles

Sensor scheduling with orbital uncertainty for space target tracking

The orbit determination accuracy of space debris can be remarkably improved when it is observed by two space-based optical (SBO) sensors simultaneously. The key to observation with two SBO sensors is optimizing the pointings of sensors to maximize the length of the double coverage arc. The traditional sensor scheduling method is only based on the mean of the predicted target orbit, and its coverage performance degrades when the error of the predicted target orbit increases. In this paper, the double coverage probability (DCP) is employed as a sensor scheduling criterion with considered the uncertainty of the predicted target orbit. The DCP is obtained by calculating an integral of the probability density function of target position distribution. Furthermore, an approximate expression of the DCP is derived by simplifying the integral region to reduce the computational burden. The simulation results show that the proposed sensor scheduling method based on the DCP can provide a longer double coverage arc and higher orbit determination accuracy than the traditional method.

Review on strategies of space-based optical space situational awareness

Space-based optical (SBO) space surveillance has attracted widespread interest in the last two decades due to its considerable value in space situation awareness (SSA). SBO observation strategy, which is related to the performance of space surveillance, is the top-level design in SSA missions reviewed. The recognized real programs about SBO SAA proposed by the institutions in the U.S., Canada, Europe, etc., are summarized firstly, from which an insight of the development trend of SBO SAA can be obtained. According to the aim of the SBO SSA, the missions can be divided into general surveillance and space object tracking. Thus, there are two major categories for SBO SSA strategies. Existing general surveillance strategies for observing low earth orbit (LEO) objects and beyond-LEO objects are summarized and compared in terms of coverage rate, revisit time, visibility period, and image processing. Then, the SBO space object tracking strategies, which has experienced from tracking an object with a single satellite to tracking an object with multiple satellites cooperatively, are also summarized. Finally, this paper looks into the development trend in the future and points out several problems that challenges the SBO SSA.

Toward space situational awareness: A space-based autonomous optical tracking method for space objects

<p indent=0mm>Space-based optical (SBO) surveillance is one of the important means of space situational awareness. Traditional space-based observational platforms cannot operate autonomously because accurate ephemeris needs to be obtained before determining the states of the observed objects. In this article, the concept of autonomous space-based surveillance is proposed. A space-based optical observation platform can carry out space situational awareness (SSA) autonomously using the obtained angle-only measurements, even if an accureate ephemeris is unknown. In contrast to traditional space object tracking, the states of an observed space object can be determinted by only using the obtained angle-only measurements. In combination with long-arc observational data, an autonomous orbit improvement algorithm based on historical data is presented. The orbit of the original object can be improved by using historical observational data obtained by observing other space objects. Thus, a real-time autonomous tracking strategy with the assistance of additional measurements is presented, which can be performed by tracking multiple space objects using an observational satellite that has a payload of multiple SBO sensors. Finally, the proposed method was demonstrated by simulation cases. Compared with traditional object tracking, the estimation accuracy of autonomous tracking is worse but it will be similar when an observational satellite tracks two objects at the same time, especially if one of them operates in low earth orbit. This paper provides a feasible method for the development of an autonomous space-based SSA platform.

Reinforcing of a Mirror Surface via the Deposition of a Carbon Nanostructure

The problem of reinforcing the mirror surfaces of space-based astronomical optics and their protection against external factors is discussed. To solve this problem, the possibility of the deposition of diamond-like coatings onto them is considered. Using mirrors with Al and Cu coatings as an example, it has been experimentally demonstrated that the pulsed laser deposition of a carbon layer with a thickness of 30 nm onto them leads to an increase in the surface hardness by 25 and 100%, respectively. It has been established that the reinforcing coating has no effect on the shape deviations of mirrors and decrease their surface roughness. In this case, the reflection factor appreciably decreases in the visible region (400–780 nm), whereas its decrease in the infrared region (above 780 nm) is no more than 5%.

Cooperative space object tracking via universal Kalman consensus filter

Cooperative space object tracking using a sensor network plays an important role in space situational awareness and improves the accuracy, robustness, and dependability of space object tracking over a single sensor. However, different sensors may have different sampling rates and may work asynchronously. It is difficult to fuse these asynchronous measurements together and track space objects in time using traditional consensus-based filters, which require synchronous measurements. To overcome this restriction, a universal Kalman consensus filter (UKCF) is proposed. Based on the state transition matrix, each sensor can fuse the received information within a measurement period and track the object over time. Sensors with arbitrary sampling rates and working times can be incorporated into the cooperative tracking system. Optical observations are an efficient way to track space objects, especially medium- or high-orbit objects. The ground-based and space-based optical (SBO) tracking models are established first. Then, a centralized fusion algorithm for an asynchronous sensor network is deduced. Based on this algorithm, the topology for an asynchronous sensor network is established and the UKCF is produced. To demonstrate its performance, cooperative tracking scenarios for a geosynchronous object that use SBO sensors with different working times and use both SBO and ground-based optical sensors with different sampling rates are simulated. As shown in the paper, the UKCF, which has a similar performance relative to the KCF but expands its application range, is more suitable for real cooperative space object tracking.

Particle contamination monitoring in the backscattering light experiment for LISA

Abstract In the context of space-based optics, contamination due to particle deposition on the optics is inevitable and constitutes a critical issue. This gets more challenging for the sensitive heterodyne measurements of the Laser Interferometer Space Antenna (LISA), the space-based gravitational wave observatory to be launched in 2034. Therefore, table-top experiments need to be developed for a better understanding of how micrometer to millimeter sized dust particles, present on optical surfaces, affect LISA measurements. In this work, we present an experimental setup for the simultaneous measurement of the coherent backscattering and the monitoring of particles deposition on the optics to be tested. The results of the first measurements are presented and discussed in this article.

Strategy Design and Sensor Scheduling for Optical Navigation of Low Earth Orbit Satellites

Space-based optical (SBO) sensor is a special kind of star sensor, which can be used in not only the space-based observation for space objects but also satellite navigation, called optical navigation in this paper. The aim of this paper is to design optical navigation strategy for low earth orbit (LEO) satellites, including the selection of beacon objects, and the sensor scheduling for improving navigation accuracy. The processing and observation geometry of space-based optical observation is illustrated first. Based on the characteristics of the space-based optical observation, the distribution of beacon geosynchronous orbit satellites is designed for optical navigation of LEO satellites. And the sensor scheduling criterion for optical navigation is deduced to analyze the characteristics of optical navigation and schedule the navigation mission. It is found that an efficient way to improve the optical navigation accuracy is to add an additional sensor to track other beacon objects, whose line of sight (LOS) should be perpendicular to the original LOS of the sensor as far as possible. Therefore, global positioning system satellites are used as the supplement of beacon objects. Finally, based on the sensor scheduling criterion, navigation for a LEO satellite with two SBO sensors is scheduled, which obtains more accurate results and converges more rapidly.

A new approach of orbit determination for LEO satellites based on optical tracking of GEO satellites

Ability of autonomous orbit determination (OD) for a satellite is receiving increasing interests due to its considerable value in engineering application. A new OD approach for low earth orbit (LEO) satellites based on tracking geosynchronous orbit (GEO) satellites using space-based optical tracking approach is proposed which can improve the autonomy of LEO satellites and reduce their dependence on ground facilities. Based on the information support of Space Surveillance Network (SSN), several GEO satellites are selected as guidance and calibration functions like beacons for LEO satellites equipped with space-based optical (SBO) sensors, as they are called beacon GEO satellites in this paper. LEO satellites can determine their own orbits by tracking these GEO objects without measures of ground facilities, which only need to provide necessary information of beacon objects for them. Unlike traditional space-based observation, long arcs and high-frequency measurements, which are more beneficial to determine orbit precisely, are possibly obtained. This paper focuses on demonstrating the feasibility and performance of proposed approach. To determine orbits, batch least-squares (LS) estimator with Tschauner–Hempel (T–H) equation state-transition matrix to estimate the propagation of initial errors is used. Different scenarios are simulated and results show that tracklet length, measurement frequency, and measurement errors are the main factors that influence the accuracy of OD.

Inversion and reconstruction of the macroscopic photometric characterization model for on-orbit space object

In the field of space object optical situational awareness by space-based optics, the current research focuses on the detecting of long distance point target, the predicting and confirming of target trajectory. It is very important to analyze the on-orbit operation status and basic physical attribution of the space object by remote imaging without any structure or texture information. The analysis method can be used effectively to support the space object status discrimination and the related decision for on-orbit maintenance. In recent years, the number of three-axis stabilization space objects in orbit has increased dramatically. In order to retrieve the optical characteristic parameters of the three-axis stabilization space object surface in a long on-orbit distance, a new method is proposed to reconstruct the macroscopic photometric characterization based on analyzing visible photometric sequence signal. Firstly, based on the principle that solar panel can receive the maximum solar radiation energy, a directing model of solar panel is proposed. Considering the structural characteristics of space object, surface material characteristics, directing characteristics of the solar panels, illumination-observation geometry and the optical system characteristics, the photometric modeling method of the space object oriented to space-based observation is improved. Secondly, the photometric model is equivalent to a two-facet model, then multi-level fusion model of bidirectional reflectance distribution function (BRDF) is used to characterize optical reflection characteristics of complex material surfaces of the space object, and the product of area and reflection corresponding to multi-level BRDF is taken as the parameter of the reconstruction model. Finally, the minimum error between the measured result and reconstruction model of photometric signal is used as the optimization goal, and the linear optimization method is established to realize the inversion of the model parameters. As the space object and the observing satellite are in the same orbit or near orbit condition, the simulating experiment of photometric sequence signal and reconstructing experiment of macroscopic characterization model of optical characteristics are carried out. The simulation result shows that the proposed photometric model can describe the dynamic characteristics of on-orbit space object more comprehensively. And using the on-orbit reconstruction method of the macroscopic photometric model, the photometric signal reconstruction accuracy achieves more than 97% in the near orbit condition. So it is demonstrated that the on-orbit reconstruction method is correct. The method can provide a solution of optical situational awareness for space object on space-based platform, and provides technical support for inversing the attitude and shape of space object.

Feasibility of performing space surveillance tasks with a proposed space-based optical architecture

Under ESA contract an industrial consortium including Aboa Space Research Oy (ASRO), the Astronomical Institute of the University of Bern (AIUB), and the Dutch National Aerospace Laboratory (NLR), proposed the observation concept, developed a suitable sensor architecture, and assessed the performance of a space-based optical (SBO) telescope in 2005. The goal of the SBO study was to analyse how the existing knowledge gap in the space debris population in the millimetre and centimetre regime may be closed by means of a passive optical instrument. The SBO instrument was requested to provide statistical information on the space debris population in terms of number of objects and size distribution. The SBO instrument was considered to be a cost-efficient with 20 cm aperture and 6° field-of-view and having flexible integration requirements. It should be possible to integrate the SBO instrument easily as a secondary payload on satellites launched into low-Earth orbits (LEO), or into geostationary orbit (GEO). Thus the selected mission concept only allowed for fix-mounted telescopes, and the pointing direction could be requested freely. Since 2007 ESA focuses space surveillance and tracking activities in the Space Situational Awareness (SSA) preparatory program. Ground-based radars and optical telescopes are studied for the build-up and maintenance of a catalogue of objects. In this paper we analyse how the proposed SBO architecture could contribute to the space surveillance tasks survey and tracking. We assume that the SBO instrumentation is placed into a circular sun-synchronous orbit at 800 km altitude. We discuss the observation conditions of objects at higher altitude, and select an orbit close to the terminator plane. A pointing of the sensor orthogonal to the orbital plane with optimal elevation slightly in positive direction (0° and +5°) is found optimal for accessing the entire GEO regime within one day, implying a very good coverage of controlled objects in GEO, too. Simulations using ESA’s Program for Radar and Optical Observation Forecasting (PROOF) in the version 2005 and a GEO reference population extracted from DISCOS revealed that the proposed pointing scenario provides low phase angles together with low angular velocities of the objects crossing the field-of-view. Radiometric simulations show that the optimal exposure time is 1–2 s, and that spherical objects in GEO with a diameter of below 1 m can be detected. The GEO population can be covered under proper illumination nearly completely, but seasonal drops of the coverage are possible. Subsequent observations of objects are on average at least every 1.5 days, not exceeding 3 days at maximum. A single observation arc spans 3° to 5° on average. Using a simulation environment that connects PROOF to AIUB’s program system CelMech we verify the consistency of the initial orbit determination for five selected test objects on subsequent days as a function of realistic astrometric noise levels. The initial orbit determination is possible. We define requirements for a correlator process essential for catalogue build-up and maintenance. Each single observation should provide an astrometric accuracy of at least 1”–1.5” so that the initially determined orbits are consistent within a few hundred kilometres for the semi-major axis, 0.01 for the eccentricity, and 0.1° for the inclination.

Wavefront correction of optical beam for large space mirrors using robust control techniques

For an imaging spacecraft to provide higher resolution imaging capability, the satellite will require larger mirrors. Because of the weight and launch constraints for space-based optics systems, the mirrors will have to be segmented and light-weight, resulting in increased flexibility and lower structural frequencies. Consequently, achieving surface accuracy requirements for high resolution imaging with these large aperture light-weight mirrors become a challenging task. This paper investigates the use of adaptive optics for wavefront sensing and surface control of large segmented mirrors. An H ∞ robust control technique is considered for the control of the adaptive optics system with the reduced number of inputs and outputs using singular value decomposition and Zernike polynomials. The robust controller design with the reduced model is validated with a computer simulation for a full plant in the presence of disturbances.

Tilt corrector based on spring-type magnetostrictive actuators

Tilt corrector based on a spring-type magnetostrictive actuator (STMA) was investigated for wavefront tilt correction. The STMA is a very efficient type of macroactuator for application in large ground- and space-based adaptive optics. By means of the STMA it is possible to deliver, in a real-time scale (1 to 10 ms), precision displacements in a range up to 100 μm with a 0.1-μm positioning accuracy. On the basis of experimentally investigated static and dynamic characteristics of STMAs the real-time two-coordinate mirror positioning system for precise optical engineering was developed and investigated. A dynamic range of corrected tilts up to 8 arc mm with a frequency range up to 50 Hz delivered by the developed tilt corrector allowed compensation for natural thermoconvectionally induced tilts with accuracy of 0.1 arc sec.

Laser radar beam steering mirrors

The structural aspects of developing a flat laser radar beam steering mirror for optical discrimination of an array of spaceborne targets at 10.6 p.m are discussed. Results of trade studies to determine feasible design concepts capable of meeting the system performance goals during steering are included. Aperture diameters between 0.5 and 2 m were studied for three principal structures: (1) a thin glass sandwich coupled to adeep backup structure, (2) a full-depth beryllium sandwich, and (3) a full depth graphite epoxy sandwich. The results of computer simulations of the structural performance of each of these concepts are presented, and fabrication efforts in support of these concepts are described. These studies demonstrate that lightweight, dimensionally stable flat mirrors can be fabricated with present technology. Aluminum-clad graphite/epoxy mirrors were successfully manufactured within 0.5 pm rms of a flat surface, and initial test results indicate dimensional stability after several thermal cycles. A low cost, uncomplicated method to manufacture low thermal expansion aluminum-clad graphite/epoxy tubes as backup structural members for large space-based optics was also developed. Further investigations are warranted to determine the dynamic tensile strength of glass, to develop thermal distortion compensation methods for metal mirrors, and to improve the dimensional and temporal stability of clad graphite/ epoxy mirrors.