Aperture Space Research Articles

Active optics system prototype applied on a passive large aperture off-axis LEO space telescope

A large aperture space telescope with a passive structure with high stiffness and thermal stability can maintain a good image quality in a large span of disturbance. However, for the proposed more stringent requirements, it is beneficial to add active optics to the conventional design. In these telescopes, the off-axis design provides more design variables than the coaxis system and also provides greater adjusting ability for the active optics. We introduce an active optics system prototype deployed on an off-axis space telescope working at low Earth orbit. By adjusting the low-order aberrations of the primary mirror shape and adjusting the positions and attitudes of certain mirrors and the focal plane, the optical system’s image quality corrupted by gravity discharge and temperature variation is increased to the undegraded level in the average sense in the full field of view.

Broadband SIW Horn Antenna Loaded With Offset Double-Sided Parallel-Strip Lines

A novel substrate integrated waveguide horn antenna loaded with offset double-sided parallel-strip lines (DSPSL) is proposed in this letter. The severe mismatch between horn aperture and free space leads to narrow impedance bandwidth when the substrate thickness is much smaller than the wavelength (typically less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\lambda _0}$</tex-math></inline-formula> /6). Two groups of periodic offset DSPSL structure are placed in front of the horn aperture, which acts as an impedance transformer to realize smooth transition. Good impedance matching can be easily realized by modifying the constitutive parameters of the offset DSPSL structure. Measured results show that the impedance bandwidth can be improved up to 20.5% from 14 to 17.2 GHz with a low profile less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\lambda _0}$</tex-math></inline-formula> /13. Stable end-fire radiation patterns are obtained across the bandwidth.

Tandem-L: A Technical Perspective on Future Spaceborne SAR Sensors for Earth Observation

Tandem-L is proposed as a spaceborne synthetic aperture radar (SAR) mission developed and operated by the German Aerospace Center in cooperation with several Helmholtz research centers and the German space industries. The mission concept comprises two fully polarimetric radar satellites providing monostatic and bistatic SAR imagery. A key feature of these SAR sensors is the employment of large lightweight unfurlable mesh reflectors fed by digital feed arrays. The main advantage of this new SAR system concept is the provision of large antenna apertures in space and flexible operation via reconfigurable feed electronics. By this, it becomes possible to map, for the first time, a continuous 350-km wide swath with a 7-m azimuth resolution with excellent noise equivalent sigma zero and ambiguity suppression. This paper shall give an overview on the technical aspects of the Tandem-L SAR instrument and antenna design. In particular, after a short review of the SAR system requirements, the concept of reflector SAR systems is outlined and the operation principle is presented. General guidelines for the design of array-fed reflector antennas with application to SAR imaging are given. Then, the optimization approach of the feed array design is detailed with a specific emphasis on a fixed beamforming concept in azimuth. In this context, also the problem of cross-pol pattern mitigation is addressed. These optimization steps are shown to be crucial for achieving the performance requirements in quad-pol acquisitions. Beamforming in elevation is performed onboard the spacecraft via digital hardware. This paper presents the beamforming architecture on receive for Tandem-L, which would apply in general for instance also to planar multielevation beam SAR antennas with Scan-On-Receive capabilities. Tandem-L is operated as a staggered SAR, which means varying the pulse repetition interval from pulse to pulse. In this context, the major design challenges are presented. Moreover, the impact of pulse staggering on the imaging performance is discussed. Tandem-L’s SAR performance is presented by means of numerical simulations showing that the performance requirements imposed by the scientific user community could be met. The final part of this paper addresses options for high azimuth resolution imaging as well as a beamforming method for enhanced range ambiguity suppression.

Characteristics of Coal and Cleat Attributes in Ulak Lebar and Surroundings Area, Lahat Regency, South Sumatra

The research conducted in Ulak Lebar Area, Lahat Regency, South Sumatra. The focus of this study is on the Muara Enim Formation as it is known as the main coal-bearing formation in South Sumatra Basin. The research is aimed to determine the characteristics of coal based on its physical properties, also knowing the cleat development at the coal seam in the research area. The methods used such as literature study, field observation and laboratory test. Field observations included the observations of coal megascopic appearance and cleat attributes analysis such as the measurements of cleat orientation, aperture, spacing and length. The analysis result of cleat at LP 2A and LP 2B shows a relatively similar such as spacing value ranged between 6 - 9 cm and 5.5 – 8.6 cm while the aperture value ranged from 0.05 - 0.2 cm, and 0.05 - 0.25 cm, whereas at LP 67 the spacing value ranged from 1.5 to 3.3 cm and the aperture valued between 0.1 - 0.3 cm. A rather significant difference between the cleat spacing values of LP 67 towards both previous observation stations is indicated by the geological processes occurred during the coalification process thus affected the development of cleats in coal, the geology structure control is characterized by a relatively large aperture and smaller spacing values. The laboratory test conducted is maceral analysis, shows the coal in the research area is dominantly composed of vitrinite (74,8%) and the vitrinite reflectance value is 0,40 indicating the coal classified as sub-bituminous rank.

Wrinkling of transversely loaded spinning membranes

Spinning membrane structures provide a mass-efficient solution for large space apertures. This paper presents a detailed study of the wrinkling of spinning circular membranes loaded by transverse, uniform loads. Experimental measurements of the angular velocities at which different membranes become wrinkled, and of the wrinkling mode transitions that occur upon spin down of the membrane, are presented. A theoretical formulation of the problem is presented, from which pairs of critical angular velocities and critical transverse loads are determined. A general stability chart is presented, which identifies the stability limits in terms of only two dimensionless parameters, for any membrane. The transition between bending dominated behavior and in-plane dominated behavior is identified, and it is shown that in the bending-dominated case the critical non-dimensional transverse load is independent from the non-dimensional angular velocity.

大口径空间主反射镜拼接化结构技术综述

大口径空间主反射镜拼接化结构技术综述

A numerical approach for assessing effects of shear on equivalent permeability and nonlinear flow characteristics of 2-D fracture networks

Abstract Hydro-mechanical properties of rock fractures are core issues for many geoscience and geo-engineering practices. Previous experimental and numerical studies have revealed that shear processes could greatly enhance the permeability of single rock fractures, yet the shear effects on hydraulic properties of fractured rock masses have received little attention. In most previous fracture network models, single fractures are typically presumed to be formed by parallel plates and flow is presumed to obey the cubic law. However, related studies have suggested that the parallel plate model cannot realistically represent the surface characters of natural rock fractures, and the relationship between flow rate and pressure drop will no longer be linear at sufficiently large Reynolds numbers. In the present study, a numerical approach was established to assess the effects of shear on the hydraulic properties of 2-D discrete fracture networks (DFNs) in both linear and nonlinear regimes. DFNs considering fracture surface roughness and variation of aperture in space were generated using an originally developed code DFNGEN. Numerical simulations by solving Navier–Stokes equations were performed to simulate the fluid flow through these DFNs. A fracture that cuts through each model was sheared and by varying the shear and normal displacements, effects of shear on equivalent permeability and nonlinear flow characteristics of DFNs were estimated. The results show that the critical condition of quantifying the transition from a linear flow regime to a nonlinear flow regime is: 10−4 〈 J

Acoustic field approximation with sparse arrays through optimization

In a previous work, the objective of approximating the acoustic field from an arbitrary continuous aperture with constrained sparse transducer arrays was established. Results from this showed limited accuracy due to the constraints and called for further consideration. The aim of this research is to extend the analysis and increase the accuracy of the field approximation through multi-variable optimization. System parameters such as transducer excitation level and location within the aperture space are varied. The acoustic coupling between transducers is also taken into account. Results from the optimizations will provide the system parameters that minimize error between compared radiation patterns.

Image degradation characteristics and restoration based on regularization for diffractive imaging

The diffractive membrane optical imaging system is an important development trend of ultra large aperture and lightweight space camera. However, related investigations on physics-based diffractive imaging degradation characteristics and corresponding image restoration methods are less studied. In this paper, the model of image quality degradation for the diffraction imaging system is first deduced mathematically based on diffraction theory and then the degradation characteristics are analyzed. On this basis, a novel regularization model of image restoration that contains multiple prior constraints is established. After that, the solving approach of the equation with the multi-norm coexistence and multi-regularization parameters (prior’s parameters) is presented. Subsequently, the space-variant PSF image restoration method for large aperture diffractive imaging system is proposed combined with block idea of isoplanatic region. Experimentally, the proposed algorithm demonstrates its capacity to achieve multi-objective improvement including MTF enhancing, dispersion correcting, noise and artifact suppressing as well as image’s detail preserving, and produce satisfactory visual quality. This can provide scientific basis for applications and possesses potential application prospects on future space applications of diffractive membrane imaging technology.

Monte Carlo simulation of a Knudsen effusion mass spectrometer sampling system.

Knudsen effusion mass spectrometry (KEMS) shows improved performance with the "restricted collimation" method of Chatillon and colleagues, which consists of two apertures between the Knudsen cell orifice and the ionizer. These apertures define the shape and position of the molecular beam independently of the sample and effusion orifice and as a result reduce background and improve sampling from the Knudsen cell. Modeling of the molecular beam in restricted collimation allows optimization of the apertures' diameters and spacing. Knudsen flow is easily simulated with a Monte Carlo method. In this study a Visual Basic for Excel (VBA) code is developed to simulate the molecular beam originating from a vaporizing condensed phase in a Knudsen cell and passing through the cell orifice and the two apertures. The code is able to calculate the transmission coefficient through the cell orifice, through the cell orifice and the first aperture, and through the cell orifice and first and second apertures. Also calculated are the angular distributions of the effusate density emerging from the cell and average number of collisions with the orifice walls. This code allows the geometry (aperture spacing and diameters) of the sampling system to be optimized for maximum transmission. The calculated effusate distributions and low average number of orifice wall collisions illustrated the advantages of restricted collimation. Calculated transmission factors are also compared to literature values calculated via the analytical method of Chatillon and colleagues. Published in 2017. This article is a U.S. Government work and is in the public domain in the USA.

Analysis of an antenna system design for a synthetic L- and P-band aperture radar

The article presents an analysis of options for designs of the antenna system for a synthetic aperture space radar (SAR), with acceptable performance characteristics in L- and P-bands. At the first stage the analysis of the current state of producing spacecraft with L- and P-band radars that use quite large folding antenna structures is carried out. It is shown that to obtain high performance of SAR in L- and P-band large folding reflectors with the diameter of up to15 meters are used. Further, variants of antenna systems are discussed and compared to determine the best combination in terms of the characteristics obtained and design complexity. An antenna system is proposed in the paper that uses antennae of the Yagi-aerial type as antenna array elements. The possibility of creating a space system consisting of several unified small spacecraft with L- and P-band SAR on board is shown.

A large ring deployable mechanism for space satellite antenna

With the development of space technology, new demands for satellite communications services, space and earth observations drive the requirement for large aperture space antennas. In order to meet the missions, this paper presents a kind of novel single mobility deployable ring mechanism based on a set of planar six-bar linkages. The ring mechanism has very high deploy/fold ratio, which is suitable for building large scale satellite deployable antenna. The mobile assembly of the ring structure and synchronous movement compatibility conditions are investigated, then the synchronization movement of the six-bar linkage modules can be realized by using close-loop cable and dual slider-crank mechanisms, which ensure the single mobility of the whole ring mechanism. Two types of driving mode for the deployable ring mechanism including cable driven and torsion spring driven are studied, and the high rigidity driving joints are designed. Finally, a ground experimental prototype of a 3.9 meter in diameter is fabricated to show the feasibility of the proposed mechanism, the deployment accuracy are also tested and the results show the good repeatability for the ring deployable mechanism.

Technology gap assessment for a future large-aperture ultraviolet-optical-infrared space telescope.

The Advanced Technology Large Aperture Space Telescope (ATLAST) team identified five key technology areas to enable candidate architectures for a future large-aperture ultraviolet/optical/infrared (LUVOIR) space observatory envisioned by the NASA Astrophysics 30-year roadmap, "Enduring Quests, Daring Visions." The science goals of ATLAST address a broad range of astrophysical questions from early galaxy and star formation to the processes that contributed to the formation of life on Earth, combining general astrophysics with direct-imaging and spectroscopy of habitable exoplanets. The key technology areas are internal coronagraphs, starshades (or external occulters), ultra-stable large-aperture telescope systems, detectors, and mirror coatings. For each technology area, we define best estimates of required capabilities, current state-of-the-art performance, and current technology readiness level (TRL), thus identifying the current technology gap. We also report on current, planned, or recommended efforts to develop each technology to TRL 5.

Numerical simulation of closest vane thermal performance in large aperture Space Solar Telescope

Since large aperture & small field of view (FOV) solar telescopes operate in special environment, thermal design is a key to achieve excellent optical performance. A thermal-scattering integrated design on vanes in large aperture & small FOV solar telescopes is proposed to obtain high signal to noise ratio and high spatial resolution. The present study is aimed to investigate the importance of additional thermal effect on the primary mirror caused by closest vane with various geometrical parameters. Three series of thermal models of Space Solar Telescope (SST) are developed to investigate the effects on the primary mirror temperature for its original thermal design after adding closest vane. Thermal variation based on three different geometrical parameters for these models are analyzed and the importance of the additional thermal effects of closest vane are identified and discussed. Suggestion is proposed to optimize three geometrical parameters of closest vane in thermal-scattering integrated design of SST.

超大口径空间光学遥感器的应用和发展

超大口径空间光学遥感器的应用和发展

Self-synchronization and spatial diversity of passive time reversal communication.

Time reversal (TR) achieves spatial and temporal focusing in complex environments. In the context of communications, passive (uplink) TR is equivalent to active (downlink) TR with the communications link being in opposite directions but with the same theoretical performance. The benefit of passive TR, however, is its natural self-synchronization, making TR communications robust and readily extended to cases involving temporal diversity and synthetic aperture communications. Self-synchronization is examined analytically and demonstrated with shallow water experimental data. In addition, the impact of spatial diversity on communications performance is investigated in terms of the aperture and element spacing of the 64-element vertical receive array.

Acoustic direction finding using a spatially spread tri-axial velocity sensor

This paper shows how a triad of orthogonally oriented uniaxial velocity sensors may be spatially separated, yet facilitate direction finding of incident emitters via closed-form subspace-based parameter estimation algorithms, while extending the triad's spatial aperture in three-dimensional space to enhance the resolution of the azimuth/elevation direction of arrival estimates.

Microwave absorption enhancement of rectangular activated carbon fibers screen composites

The radar absorbing materials composited of activated carbon fibers (ACF) as wave-absorber and glass fibers (GF) as impedance modifier were successfully prepared. The structure parameters of rectangular ACF felt screen, the influences of GF-piece and the reflection properties were investigated. It was observed that the effective bandwidth (the reflection loss below −10 dB) increased with the aspect ratio (a/b) increasing, but decreased with aperture spacing (δ) increasing. A double absorption peak (16.1 dB and 14.2 dB) for the optimized configuration (a/b = 3, δ = 5 mm, a = 30 mm) appeared at 5.8 GHz and 16.9 GHz, and a wide absorption bandwidth (4–17.6 GHz) could be achieved. When a 30 mm × 15 mm rectangular aperture was filled by GF-piece of 20 mm × 10 mm, the effective bandwidth was improved to 12.1 GHz. The composites with a wide absorption bandwidth could be achieved via designing the proper element parameters and adjusting the composition of ACF and GF.

Improving coherent field imaging quality by suppressing the influence of transmitting aperture spacing error

Coherent field imaging is based on the assumption of equal transmitting apertures spacing and equal spectrum of laser, and high-resolution image is reconstructed by iteratively computing the frequency spectrum. However, the inevitable transmitting aperture spacing error of laser is a key factor to affect the coherent field imaging quality in the application. Aiming at the problem of degrading imaging quality caused by the transmitting aperture spacing error, we discuss the mechanism of influence of aperture spacing error on imaging quality and propose a novel method of improving imaging quality from the perspective of suppressing the influence of transmitting aperture spacing error. Firstly, the mechanism of the influence of aperture spacing error on imaging quality and laser echo spectrum is analyzed in detail. Then we derive a frequency spectrum error propagation model. Based on the model, the iterative effect of frequency spectrum error is investigated and the trend of variation in imaging quality with frequency spectrum error is given. We propose a theoretical equation, in which the transmitting aperture spacing error has no influence on frequency spectrum nor imaging quality. To solve the above equation, an optimized method of linear programming is proposed and the optimized matrix of aperture spacing error is obtained. In practice, the influence of aperture spacing error on imaging quality can be largely counteracted by reasonably allocating aperture spacing error according to the optimized spacing error matrix. The correctness and validity of the theoretical model are verified by a simulation experiment. The results show that the Strehl ratio of imaging quality index can be improved by about 100% through using the proposed method, the greater the aperture spacing error, the higher the Strehl ratio of imaging quality index obtained by the method will be. In addition, the method is easy to use practically and less costly as well. Finally, it is concluded that the proposed method can easily and effectively counteract the degrading effect of aperture spacing error on imaging quality. The research can provide a theoretical basis for improving imaging quality and reasonably assigning transmitter aperture spacing accuracy of coherent field imaging telescope.

傅里叶望远镜激光发射系统性能分析

The performance of Fourier telescope transmitting system directly affects the image resolution and quality. The parameters and performance requirements of laser are analyzed. The design outline of fiber laser is presented. Meanwhile, the point is put forward that the laser coherence length need to be at least 1.6 times of the residual optical path. The stability of laser power and frequency is simulated. To ensure the stability of the laser, some methods are put forward from the points of design, algorithm and practice. Furthermore, combined with the results of simulation and experiment, the influences of the transmitter performance on image quality is analyzed, from the aspects of transmitting aperture layout, aperture amount, position precision and the beam pointing error. And point out that baseline redundancy, image resolution, target spectrum and image quality are the factors should be synthetically considered when arranging transmitter array. The formula for calculating the number of transmitting apertures is obtained based on the two-dimensional sampling theorem and the statistical analysis results. The transmitting aperture number is calculated for the Fourier telescope whose resolution would be 5 cm for objects in 1000 km low earth orbit. In addition, it is concluded that the transmitting aperture position error should be less than 5% of the minimum aperture spacing. ©, 2015, Chinese Optical Society. All right reserved.