Spaceborne Optical Systems Research Articles

Automatic Detection of Marine Litter: A General Framework to Leverage Synthetic Data

The spatial and temporal coverage of spaceborne optical imaging systems are well suited for automated marine litter monitoring. However, developing machine learning-based detection and identification algorithms requires large amounts of data. Indeed, when it comes to marine debris, ground validated data is scarce. In this study, we propose a general methodology that leverages synthetic data in order to avoid overfitting and generalizes well. The idea is to utilize realistic models of spaceborne optical image acquisition and marine litter to generate large amounts of data to train the machine learning algorithms. These can then be used to detect marine pollution automatically on real satellite images. The main contribution of our study is showing that algorithms trained on simulated data can be successfully transferred to real-life situations. We present the general components of our framework, our modeling of satellites and marine debris and a proof of concept implementation for macro-plastic detection with Sentinel-2 images. In this case study, we generated a large dataset (more than 16,000 pixels of marine debris) composed of seawater, plastic, and wood and trained a Random Forest classifier on it. This classifier, when tested on real satellite images, successfully discriminates marine litter from seawater, thus proving the effectiveness of our approach and paving the way for machine learning-based marine litter detection with even more representative simulation models.

Compensation for general asymmetric static loads for a complete optical system of freeform mirrors.

Optical systems consisting of freeform metal mirrors are state of the art in optical engineering. The freeform shapes allow for more compact designs and offer more degrees of freedom for aberration correction. The metal components allow for the relocation of the effort from the integration to the fabrication stage. Metal mirrors for spaceborne optical systems experience several loads during orbital commissioning or operation. The present paper focuses on describing general static loads within the optical design to close the iteration loop between optical and mechanical design using this knowledge to investigate how to compensate for load-induced surface shape errors within the optical design. First, this analysis is performed for a two-mirror system with an asymmetric force load step. Second, the compensation for this load is discussed by performing another optical design step or a direct mechanical compensation step at mechanical design level. Since thermal loads usually introduce expansion effects, a third point of the paper is the discussion of thermal loads with some general results, to embed them into the shown formalism. The paper concludes by showing residual optical errors of the compensated optical system and comparing them with the nominal design.

Utilizing Statistical Information for Interval Analysis: A Method for Analyzing the Interval Uncertainty of Line-of-Sight Measurement Error of Space-Borne Observation Platforms

Analyzing the space-borne observation platform's line-of-sight measurement error (SOPLME) is of great significance for the improvement of the space-borne optical tracking system. In this regard, we consider the space-borne observation platform (SOP) as a multi-body system and build both the target imaging model from the inertial space to the optical camera image plane and the line-of-sight measurement model. The relationship between the SOPLME and the satellite's orbit error, attitude error, platform angular vibration error, inner and outer frame rotation error, and position quantization error of target image point is deduced theoretically. However, since there are many error sources and the modeling process is highly nonlinear, the amplification of the output range caused by the “wrapping effect” may occur when using the interval calculation method to analyze the output range of the SOPLME. To solve this problem, we propose a novel method of interval analysis by using statistical information. This method combines the convenience of interval representation with the high accuracy of probability calculation, which can effectively avoid the overestimation problem caused by the inherent “wrapping effect” of the interval calculation method. Firstly, we describe the parameters of each error source with the interval constructed by upper and lower bounds. Secondly, the optimal Latin hypercube sampling (LHS) matrix of error source parameters is designed by the quasi-optimization method. Then we use the Gaussian mixture model (GMM) to approximate the distribution function of the SOPLME. Finally, the output range of the SOPLME is estimated by Taylor expansion. The simulation test verifies the reliability of the model and interval calculation method in this paper. The simulation example shows that the proposed method can get a more reliable error output interval with less sampling amount, which is suitable for analyzing the dynamic process of the output interval of the SOPLME.

Design, additive manufacturing, processing, and characterization of metal mirror made of aluminum silicon alloy for space applications

Metal mirrors are used for spaceborne optical systems, such as telescopes and spectrometers. In addition to the optical performance, the mechanical needs and the mass restrictions are important aspects during the design and manufacturing process. Using the additive manufacturing process, optimized internal lightweight structures are realized to reduce the weight of the system while keeping the mechanical stability. A mass reduction of ≈60.5 % is achieved. Using the aluminum silicon alloy AlSi40, the thermal mismatch of the mirror base body to a necessary electroless nickel-polishing layer is minimized. Based on an exemplary mirror design, the optimization of the interior lightweight structure is described, followed by the manufacturing process from additive manufacturing to diamond turning, plating, and polishing. Finally, the results of surface metrology and light scattering measurements are presented. A final form deviation below 80 nm p . − v . and a roughness of ∼1 nm rms could be demonstrated.

Synergies of Spaceborne Imaging Spectroscopy with Other Remote Sensing Approaches

Imaging spectroscopy (IS), also commonly known as hyperspectral remote sensing, is a powerful remote sensing technique for the monitoring of the Earth’s surface and atmosphere. Pixels in optical hyperspectral images consist of continuous reflectance spectra formed by hundreds of narrow spectral channels, allowing an accurate representation of the surface composition through spectroscopic techniques. However, technical constraints in the definition of imaging spectrometers make spectral coverage and resolution to be usually traded by spatial resolution and swath width, as opposed to optical multispectral (MS) systems typically designed to maximize spatial and/or temporal resolution. This complementarity suggests that a synergistic exploitation of spaceborne IS and MS data would be an optimal way to fulfill those remote sensing applications requiring not only high spatial and temporal resolution data, but also rich spectral information. On the other hand, IS has been shown to yield a strong synergistic potential with non-optical remote sensing methods, such as thermal infrared (TIR) and light detection and ranging (LiDAR). In this contribution we review theoretical and methodological aspects of potential synergies between optical IS and other remote sensing techniques. The focus is put on the evaluation of synergies between spaceborne optical IS and MS systems because of the expected availability of the two types of data in the next years. Short reviews of potential synergies of IS with TIR and LiDAR measurements are also provided.

Design and development of Shack–Hartmann wavefront sensor-based testing of high-resolution optical system for earth observation

The deformation of the optical surfaces of a large aperture high-resolution space-borne optical system induced by earth’s gravity on the ground, which is not present during in-orbit operations, necessitates the evaluation of its performance in terms of wavefront error at various stages of development of the earth observation system. A direct method of evaluation for an optical system at an integrated electro-optical module based on a Shack–Hartmann wavefront sensor (SH WFS) is proposed. Design and analysis of the wavefront sensor that are tailored to meet the requirements of the high-resolution optical system are described. We show that the procedure followed for the development of the SH WFS not only addresses the parameters of the wavefront sensor that are critical to its performance, but also aides in the wavefront sensor alignment and calibration. The performance of the developed SH WFS is demonstrated by testing a simulated telescope which is <italic<in situ</italic< verified in a test configuration using a standard Fizeau interferometer; a close match of the coefficients of Zernike modes between them is established.

초점거리 편의가 지상 정확도에 미치는 영향 비교 연구 - 세가지 라인 스캐너를 대상으로 -

다수의광학위성센서들이선형 배열 형태에 기반을두고설계되었다. 널리 알려진 광학위성센서의 종류는 다음과 같이 경로 평행 방향 1열스캐너(along-track line scanner), 경로 직각 방향 1열 스캐너(across-track linescanner), 3열 스캐너(three- line scanner)로 구분할 수 있다. 이들 센서들을 이용하여 위성 및 항공기에서 지상지물의 정확한 위치정보를 획득하려고 할 때 센서의 외부 표정요소와 내부 표정요소는 매우 중요한 요소들이다. 이들 센서들의 영상생성기하구조는 태양에 의한열영향, 진동, 바람등의 다양한 물리적 현상들에 의하여 시시각각 변동될 가능성이 있기에 내부표정요소의 편의가 지상에 미치는 영향을 분석하는 것은 매우 중요하다. 실제적인 비행경로와 자세정보를 바탕으로 생성된 시뮬레이션 자료를 이용하여 본 연구에서는 각 센서의 초점거리에 편의량을 점진적으로 추가하면서 실험과 분석을 수행하였으며, 또한 비행고도를 고고도와 저고도의 두가지 경우로 비교하였다.실험결과, 경로평행방향 1열 스캐너의 경우에 초점거리편의량의 증가가 지상스캔라인방향(Y 방향)의 오차를 유발하였으며, 경로 직각 방향 1열 스캐너의 경우 스캔라인방향과 수직방향으로 오차가 증가하였고, 3열 스캐너의 경우에는 수직방향으로만 오차가 증가하였다. 이들 실험결과는 향후 새로운 센서개발과 센서캘리브레이션 정확도에 가이드라인을 제공할 수 있다고 본다. Most space-borne optical scanning systems adopt linear arrayconfigurations. The well-knownthree different types of space-borne sensors arealong-track line scanner, across-track linescanner, and three line scanner. To acquire accurate location information of an object on the ground withthose sensors, the exterior and interior orientation parameters are critical factors for both of space-borne and airborne missions. Since the imaging geometry of sensors mightchange time to time due to thermal influence, vibration, and wind, it is very important to analyze the Interior Orientation Parameters (IOP) effects on the ground. The experiments based on synthetic datasets arecarried out while the focal length biases are changing. Also, both high and low altitudes of the imagingsensor were applied. In case with the along-track line scanner, the focal length bias caused errors along the scanline direction. In the other case with the across-track one, the focal length bias caused errors alongthe scan line and vertical directions. Lastly, vertical errors were observed in the case ofthree-line scanner. Those results from this study will be able to provide the guideline for developing new linearsensors, so as for improving the accuracy of laboratory or in-flight sensor calibrations.

Grinding strategies for machining the off-axis aspherical reaction-bonded SiC mirror blank

Demand for large-scale off-axis aspherical mirrors is increasing in next-generation space-borne optical imaging systems. In this paper, a variable-axis single-point grinding strategy is developed for precisely, cost-effectively figuring silicon carbide (SiC) mirror blanks that have high-order high-gradient off-axis aspherical surfaces on a precision five-axis machining center. The grinding strategies also include tool path generation/optimization, feeding direction control and wheel wear reduction/compensation. Applying the developed grinding strategies, by only one grinding cycle, a near-circular 372-mm SiC mirror blank is successfully grounded to 7.8 m in peak to valley, which is comparable to the reported machining accuracy of the BoX ultraprecision grinding machine. Moreover, the wheel need not have to be dressed during the whole grinding cycle due to the rotary ultrasonic grinding method. Therefore, this paper offers an efficient and economic solution for grinding off-axis aspherical and free from surfaces to micron-level accuracy, thus significantly decreasing the subsequent lapping/polishing production cycle time. 2014 中文 Optics Letters.

Radiation-induced transmission degradation of borosilicate crown optical glass from four different manufacturers

Space-borne optical systems must be tolerant to radiation to guarantee that the required system performance is maintained during prolonged mission times. We show that the radiation hardness can be improved by selecting the relevant glass from a group of optical analogs. This conclusion is based on the study of gamma-radiation-induced absorption of crown glasses from group 517640 (glasses with nd1.517 and d64)—BK7 and NBK7 (Schott), S-BSL7 (Ohara), BSC 517642 (Pilkington), and K8 (NITIOM). Our results show that whereas these glasses are optical analogs before irradiation, they behave differently during and after irradiation.

Radiation characteristics of a high-emissivity cylindrical–spherical cavity with obscuration

We have calculated, to first order, the apparent emissivity of the bounding diffuse surfaces of a high-emissivity cylindrical-spherical cavity enclosure. Our calculations indicate that to achieve emissivities close to a perfectly absorbing blackbody cavity along the bounding surfaces of the spherical enclosure, the radius of the sphere must be equal to or greater than a factor of 4 times the cylinder radius R(S) > or = 4R(C). Furthermore, to achieve emissivities approaching a blackbody cavity along the lower bounding surfaces of the cylindrical enclosure, the length of the cylinder must be a factor of 4 times greater than the radius of the cylinder L > or = 4R(C). In addition, we present the mathematical framework necessary to calculate radiant transfer within a cavity enclosure that contains obscuration. These results can be applied to the design of high-emissivity blackbody calibration cavities and to the reduction of stray light in terrestrial and spaceborne optical systems.

An Integrated Acoustic Analysis Capability

To perform the analysis of multidisciplinary systems in the early design phase, an Integrated Modeling of Optical Systems (IMOS) was developed by NASA/JPL. The existing code has Stationary Statistical Energy Analysis (SEA) capability for acoustic analysis. An implementation of Transient SEA is performed to enhance the IMOS acoustic analysis capability. It is benched marked against industry standard vibroacoustic analysis tools VAPEPS and TRANSTAR. The comparisons are excellent. As an example, it is applied to a generic structure subjected to a realistic transient excitation. The enhanced capability can be applied to the study of microdynamics and other random vibration/shock issues related to the high precision space-borne optical systems.

Design of a space borne optical aperture synthesis system for high resolution astronomy

This paper proposes a design for a space borne demonstrator interferometer with imaging capabilities. This demonstrator would be a first step in the development of an operational astronomical instrument using aperture synthesis. Multiple aperture interferometry is a promising technique for high resolution imaging in space. Indeed the use of modular elements based on optical fibres represents an attractive low cost approach. The instrument is based on the use of independent triplets of telescope linked with optical fibres. For each triplet, only the phase closure of the system is measured. This does not require an accurate cophasing of the apertures and leads to realistic stability and control specifications. The complete instrument is then built by adding the desired number of such triplet modules. The detailed design of one module is described starting at level of the receiving telescopes down to the focal plane where the control of the stability of the optical path differences is performed as well as the scientific observations. The global operation of the system will also be described. We also briefly recall the technology validation activities which have shown on the ground the feasibility and performance of all components of such a systems as well as its end to end validation.

A robotic approach to fault-tolerant, precision pointing

Future spaceborne optical interferometers and laser satellite communication systems are two space applications that require a precision pointing function in order to meet mission goals. Spaceborne interferometers provide a promising means to discover Earth-like planets in other solar-like systems. The laser communication systems provide a low-power, low-cost, lightweight means of data relay between ground and space and for deep-space communications to interplanetary probes. Both applications share the need to acquire and track a target. The interferometry application requires pointing errors to be submicroradian while the laser communication application requires microradian-level errors. In order to meet the precision pointing requirements found in these applications, a precision pointing strategy has been developed. The strategy employs a hexapod as the pointing platform to reject vibrations from a noisy spacecraft bus over all frequencies: at low frequency using 2- or 3-axis pointing and at high frequency using 6-axis vibration isolation. The benefits include broadband pointing stability without a high-bandwidth pointing sensor or destabilizing excitation of high-frequency structural modes, as well as tolerance to failures. This article outlines this approach to pointing.

Shape estimation from incomplete measurements: a neural-net approach

Accurate estimation of the shape of precision space structures is the key to the success of spaceborne optical systems. A combined simulated annealing and neural-network approach is proposed whereby one can infer the current deformed state of the structure from a limited number of on-board measurements. The approach is especially effective when most of the computations must be done on-ground or off-line, and only minimal calculations are allowed for near real-time on-board processing. It is shown that the performance of the network and its ability to estimate the shape accurately is highly dependent upon the off-line training and tuning of the model to a specific family of expected disturbances. Details of the methodology and results of numerical simulations are given for various on-board estimation scenarios.

Inflatable, space-rigidized support structures for large spaceborne optical interferometer systems

The realization of structural supports for kilometric optical arrays requires innovative approaches to comply with both the transportation and installation requirements and the operational needs for such a delicate application. Inflatable, space-rigidized structure technology has been used to define a number of configurations of 100m nominal extension. Evaluation of a number of system aspects, basing on current-technology parameter values, has shown that such structures are not only of low mass (0.8 – 1.8 kg/m baseline) and efficient packaging (all can be launched by an ARIANE 5) but also provide a thermally stable backbone, with sufficient stiffness to make attitude control easily manageable and offering the promise of good separation between the equipment and the sensors at the critical high frequencies, where backbone stability is mandatory to make astronomical observations possible.

Design of Highly Stable Optical Support Structure

As spaceborne optical systems increase in diameter to achieve improved resolution, the stability requirements imposed on structures approach values which were unthinkable only several years ago. To achieve the capabilities of these apertures optical path errors must not exceed a specific fraction of the wavelength of light, and this fraction, typically x/20 rms in the focal plane, is independent of system size. Thus, from a percentage error basis, large optical support structures represent a far more formidable development task than do smaller systems. The Large Space Telescope (LST) sponsored by MSFC/NASA is a case in point. The Optical Telescope Assembly (OTA) is shown (Fig. 1) installed in the LST spacecraft. The vertex-to-vertex spacing of the primary and secondary mirror is 193 inches. To achieve satisfactory optical performance, this spacing must be maintained constant to a precision of -±11.1 for observation periods up to 10 hours. During this time it may be necessary to alter the spacecraft attitude with respect to the sun, which would change the temperature levels and gradients within the structures. It is believed that by exploiting the use of graphite-epoxy in a novel manner, the stringent alignment require-ments can be satisfied with a nominally passive structure.