Reference Point Source Research Articles

Improved On-Orbit MTF Measurement Method Based on Point Source Arrays

The modulation transfer function (MTF) is a key characteristic used to assess the performance of optical remote sensing satellite sensors. MTF detection can directly measure a sensor’s two-dimensional (2D) point spread function (PSF); therefore, it has been applied to various high-resolution remote sensing satellites (e.g., Pleiades) using point sources. However, current point source methods mainly use 2D Gaussian functions to fit the discrete digital number (DN) of the point source on the image to extract the center of the point source and fit the PSF after encrypting multiple point sources; thus, noise robustness is poor and measurement accuracy varies widely. In this study, we developed a noise-resistant on-orbit MTF detection method based on the object space constraint among point source arrays. Utilizing object space constraint relationships among points in a point source array, a homography transformation model was established, enabling accurate extraction of sub-pixel coordinates for each point source response. Subsequently, aligning the luminosity distribution of all point sources concerning a reference point source, the encrypted PSF was obtained and then fitted to obtain the MTF. To validate the method, Gaofen-2 (GF-2) satellite images were used to conduct an in-orbit imaging experiment on the point source array of the Chinese Zhongwei remote sensing satellite calibration site. Compared with the Gaussian model methods, the proposed method yielded more accurate peak positions for each point source. Standard deviations of peak position constant ratios in along- and cross-track directions improved by 2.8 and 4.8 times, respectively. The root-mean-square error (RMSE) of the collinearity test results increased by 92%, and the noise resistance of the MTF curve improved by two times. Dynamic MTF values at the Nyquist frequency for the GF-2 panchromatic band in along- and cross-track directions were 0.0476 and 0.0705, respectively, and MTF values in different directions were well distinguished.

½U low-cost laser guide star payload

We present all aspects of the development of a ½U laser guide star CubeSat payload. The payload, AMS Beacon, was integrated into the Agile Microsat (AMS) 6U platform developed by MIT Lincoln Laboratory and will function as a space-based point source reference for high-angle-rate adaptive optics (AO) control. In addition to a 500-mW onboard laser, it carries a retro-reflector and a photodiode (PD) with a 1 kHz readout. The system, launched successfully in May 2022 into low-Earth orbit, utilizes commercial components, fixed optics, and high-precision body-pointing enabled by a bus-provided GPS receiver, among other innovations, to keep the design compact and the costs low. To our knowledge, it is the first payload to be built as a test platform for high-angle-rate AO on orbital targets. The system architecture offers opportunities for future cost reductions and a strong foundation for payload proliferation across multiple satellites. The capabilities of the host spacecraft combined with the Beacon payload can also serve as a roadmap for integrating thrusting capabilities with an artificial guidestar, which would be critical in enabling various astronomical applications.

A novel virtual point detector model for high purity germanium detectors

A novel virtual point detector model was developed for cylindrical high purity germanium detectors that are widely used in gamma-ray spectrometry. According to this model, the full energy peak efficiency response of these detectors can be computed by replacing the detector with a virtual point detector located at the center of the detector crystal. In fact, the novel model makes the transfer of the FEP efficiency response from a reference point source to a given point source, both sources being located on a line that passes through the virtual point detector. The model was tested and provided accurate results.

Cross-correlation of phase fluctuations measured with a point source and an incoherent luminous object

A high degree of cross-correlation of radiation wavefront aberrations, measured from a point reference source and from an extended object (a system of several luminous points), has been for the first time revealed based on simultaneous measurements of phase fluctuations in optical waves propagating in a turbulent atmosphere. The experimental data are consistent with analytical calculations. The high degree of cross-correlation (above 0.97) of measured phase fluctuations proves that an extended incoherent source can be used as a reference source to ensure the operation of an adaptive focusing system.

Phaseless inverse source scattering problem: Phase retrieval, uniqueness and direct sampling methods

Similar to the obstacle or medium scattering problems, an important property of the phaseless far field patterns for source scattering problems is the translation invariance. Thus it is impossible to reconstruct the location of the underlying sources. Furthermore, the phaseless far field pattern is also invariant if the source is multiplied by any complex number with modulus one. Therefore, the source can not be uniquely determined, even the multi-frequency phaseless far field patterns are considered. By adding a reference point source into the model, we propose a simple and stable phase retrieval method and establish several uniqueness results with phaseless far field data. We proceed to introduce a novel direct sampling method for shape and location reconstruction of the source by using broadband sparse phaseless data directly. We also propose a combination method with the novel phase retrieval algorithm and the classical direct sampling methods with phased data. Numerical examples in two dimensions are also presented to demonstrate their feasibility and effectiveness.

Reconstruction of apertured Fourier Transform Hologram using compressed sensing

Fourier Transform Holography (FTH) encodes object information as the Fourier spectrum in an interference pattern with the help of a reference point source. Hence, a single Fourier transform (FT) operation is capable to provide a complete object from a digitally reconstructed FTH. However, high intensity values in the central region of the FTH demands central blocking due to the limited dynamic range of a detector in a variety of applications. The central blocking causes the loss of frequency content and hence affects the reconstruction result. Still, it can be properly recovered using signal processing techniques such as Compressive Sensing (CS) or Sparse signal processing. This paper investigates the role of CS for centrally blocked FTH using various kind of beam stops and provide a detailed comparison between usual FT reconstruction and technique equipped with CS. We also provide a detail procedure to apply the CS reconstruction in the FTH and compared the results with and without CS. It is established that an object can be recovered from limited size of a recorded hologram with the constraint that object has to be sparse in known basis. Quantitative analysis has been carried out to compare the quality of reconstruction using inverse Fourier transform (IFT) and CS techniques. Quality of reconstruction is assessed by evaluating measures such as the overall visibility, efficiency and PSNR of reconstruction. The finding highlights that the CS reconstruction is better than that of IFT reconstruction. Both simulation and experimental results demonstrate the effectiveness and robustness of the CS-based approach.

Study of the germanium dead layer influence on HP(Ge) detector efficiency by Monte Carlo simulation

The dead layer thickness provided by the manufacturer increases with the aging of the HPGe detector; the increase of dead layer thickness leads to the decrease in the detector’s efficiency, not only due to gamma rays attenuation in the dead layer but also due to the reduction of the active volume of the detector. In this work, the dead layer influence on HPGe detector efficiency was studied by Monte Carlo simulation. The detector model was developed using Monte Carlo N Particle (MCNP5) code for adjustment of the dead layer thickness; the adjustment was performed according to a specific irradiation configuration by collimation approach using three reference point sources: 241Am (59.5 keV), 137Cs (661.6 keV) and 60Co (1173.2 keV; 1332.5 keV). The calculated efficiencies were compared to the measured intrinsic efficiency ones for these point sources; a good agreement between Monte Carlo and measurements results was found after the experimental validation. The results confirm the variation in the dead layer thickness according to aging of the detector; the average change of dead layer is in order of 1.30 ± 0.05 mm after 9 years.

Retrieval of acoustic sources from multi-frequency phaseless data

This paper is concerned with the inverse source problem of reconstructing an unknown acoustic excitation from phaseless measurements of the radiated fields away at multiple frequencies. It is well known that the non-uniqueness issue is a major challenge associated with such an inverse problem. We develop a novel strategy to overcome this challenging problem by recovering the radiated fields via adding some reference point sources as extra artificial sources to the inverse source system. This novel reference source technique requires only a few extra data, and brings in a simple phase retrieval formula. The stability of this phase retrieval approach is rigorously analyzed. After the reacquisition of the phase information, the multi-frequency inverse source problem with recovered phase information is solved by the Fourier method, which is non-iterative, fast and easy to implement. Several numerical examples are presented to demonstrate the feasibility and effectiveness of the proposed method.

Метод определения радиуса сферической поверхности

The theoretical positions of the pairwise distance method have been developed as applied to the determination of the radius of a spherical surface. To implement the method, the reference points are placed on the investigated surfaces and linear distances are measured between them. The implementation of the method for determining the radius of a spherical surface from distances measured between 4 points located on the surface is presented. The specified number of points is the minimum necessary to solve the problem. An analytical expression is obtained for the radius of a spherical surface through the distances between pairs of points. The root-mean-square deviation of the error in the estimate of the radius caused by the errors in the measurement of distances is calculated. Optimal configurations of 4 points on the sphere are determined, which ensure the minimum dispersion of the estimate of the radius. For these optimal configurations, the opposite edges of tetrahedra formed by the corresponding points are equal to each other. The method was applied in a complex of works on the evaluation of the parameters of the alignment of a television-based measurement system for the angular position of a dynamic stand with a gas spherical bearing, equipped with reference point sources placed along the envelope of the spherical surface. Determination of the radius of such a spherical surface is complicated by the fact that it is discontinuous, covers a part that is less than half the sphere, there is no access to the center of the sphere, which is the center of rotation spherical bearing of the dynamic stand. The known indirect methods for determining the radius of a spherical surface, based on the results of linear or angular measurements with the help of special surface measuring devices based on measurements directly on the surface, the radius of curvature of which is determined, also prove to be unacceptable due to the absence in this case installation measuring base for the measuring device. The method makes it possible to exclude the use of expensive coordinate measuring machines to obtain a radius estimate, since a simple measuring instrument can be used to measure the pairwise distances.

High precision three-dimensional iterative indoor localization algorithm using code division multiple access modulation based on visible light communication

To solve the problem of positioning accuracy affected by mutual interference among multiple reference points in the traditional visible light communication positioning system, an iterative algorithm of received signal strength (RSS) based on code division multiple access (CDMA) coding is proposed. Every light-emitting diode (LED) source broadcasts a unique CDMA modulation identification (ID) code, which is associated with geographic position. The mobile terminal receives a mixed light signal from each LED reference point. By using the orthogonality of the spreading codes, the corresponding ID position information and the intensity attenuation factor of each LED reference point source can be available. According to the ID information and signal attenuation intensity, the location area of each LED and the distance between the receiver end and each LED can be determined. The three-dimensional (3-D) position of the receiver can be obtained by using the iterative algorithm of RSS triangulation. The experimental results show that the proposed algorithm can achieve a positioning accuracy of 5.25 cm in a two-dimensional (2-D) positioning system. And in the 3-D positioning system, the maximum positioning error is 10.27 cm, the minimum positioning error is 0.45 cm, the average positioning error is 3.97 cm, and the proportion of the positioning error exceeding 5 cm is 25 % . With a very good positioning accuracy, this system is simple and does not require synchronization processing. What is more, it can be applied to both the 2-D and 3-D localization systems, which has a broad application prospect.

A New Method for Finding Optical Aberrations on the Basis of Analysis of the Object Hologram Without Additional Measurements

We propose a new method of compensating for the wavefront aberrations during the image processing. The method employs the digital-holography potential. The developed algorithms allow one to find the wavefront distortions caused by the optical-path nonuniformities during the interference recording of images without additional measurements (i.e., without using the reference point source and measuring the wavefront distortions). The possibility of decreasing the wavefront aberrations from tens to several radians using digital methods is demonstrated.

Transformation properties of dynamic χ(2) holograms generating at the difference frequency

The transformation properties of dynamic χ(2) holograms generating at the difference frequency are analyzed as applied to the position and scales of reconstructed images. Some regularities are established for the spatial localization and scales of images for different combinations of frequencies and positions of the reference and object point sources, involved in the formation of a holographic image. The formation of amplitude transparency images, reconstructed by dynamic χ(2) holograms with a decrease or conservation of the object beam frequency, and their transformations are experimentally demonstrated.

Нigh-precision determination of the angular position for point light source with CCD-arrays

Subject of study. A method of angular position measuring for point light source in the system with CCD arrays is proposed by converting of angle – time – code in low light conditions and at high signal / noise ratio. Assessment of the method potential accuracy is given. It is determined by instrumental irremovable random errors of measurement in terms of optimal processing of incoming information with a single reading it with CCD – arrays. Method. This work introduces an optoelectronic system circuit with CCD arrays with stretched sensing elements and a point of reference for angular position measuring. In this case the arrays have images projections of both the reference point source and the target point source, which angular position is measured with high precision. From the CCD array output the signals arrive at an optimal (or apt) linear filter, and then to the signal peak position detection circuit. The scheme provides minimum error due to the influence of noise. Pulse edges, corresponding to the signals maximum, make a time interval filled with high-frequency counting pulses. The number of pulses in this interval will correspond to the measured angular position of the target point source. Main results. Analysis of random errors has been carried out in terms of statistical decision theory. Analysis takes into account the spectral function of the signals defined by transfer functions of the optical system and a CCD array as an image analyzer. Research of measurement accuracy is carried out depending on the clock frequency of reading information from CCD arrays for different values of signal-to-noise ratio. It has been shown that even with a single readout with CCD array by the proposed opto-electronic circuit measuring error does not exceed 0.01 of the sensing element size. Practical significance. The results are usable in high-precision measuring opto-electronic systems of star sensors for determining the angular position of the low-power point light sources.

Imaging cold atoms with shot-noise and diffraction limited holography

We theoretically develop and experimentally demonstrate a holographic method for imaging cold atoms at the diffraction and photon shot noise limits. Aided by a double point source reference field, a simple iterative algorithm robustly removes the twin image of an 87Rb cold atom sample during the image reconstruction. Shot-noise limited phase shift and absorption images are consistently retrieved at various probe detunings, and during the laser cooling process. We consistently resolve less than 2 mrad phase shift ( attenuation) of the probe light, outperforming shot-noise limited phase-contrast (absorption) imaging by a factor of 4 or more if the same camera is used without pixel saturation. We discuss the possible extension of this work for precise phase imaging of dense atomic gases, and for off-resonant probing of multiple atoms in optical lattices.

Full-Wave Modeling of Near-Field Radar Data for Planar Layered Media Reconstruction

A new near-field radar modeling approach for wave propagation in planar layered media is presented. The radar antennas are intrinsically modeled using an equivalent set of infinitesimal electric dipoles and characteristic, frequency-dependent, global reflection, and transmission coefficients. These coefficients determine through a plane wave decomposition wave propagation between the radar reference plane, point sources, and field points. The interactions between the antenna and layered medium are thereby inherently accounted for. The fields are calculated using 3-D Green's functions. We validated the model using an ultrawideband frequency-domain radar with a transmitting and receiving Vivaldi antenna operating in the range 0.8-4 GHz. The antenna characteristic coefficients are obtained from near- and far-field measurements over a copper plane. The proposed model provides unprecedented accuracy for describing near-field radar measurements collected over a water layer, the frequency-dependent electrical properties of which were described using the Debye model. Layer thicknesses could be retrieved through full-wave inversion. The proposed approach demonstrated great promise for nondestructive testing of planar materials and digital soil mapping using ground-penetrating radar.

Pure-optical quadratic phase compensation in image-plane digital holographic microscopy

A pure-optical quadratic phase compensation method is proposed for the image-plane digital holographic microscope. Theoretical analysis testifies that the quadratic phase can be removed completely only when the reference point source is rightly located at the back focal plane of the microscope objective. The image-plane digital holographic microscopy is built, in which the well located sphere reference beam directly emitting from the tip of the fiber is utilized as the reference beam. Experimental results reveal how the quadratic phase curvature changes with the scanning of the reference point source, and make the adjustment of the reference beam visualized and operable. The effectiveness of the proposed method is demonstrated by the quantitative three-dimensional measurement of a microlens array and the morphological monitoring of living mouse fibroblast cells. The quadratic phase aberration can be compensated inherently without any computation, which greatly eases the computation load in the reconstruction of the digital hologram and helps to improve the real time of imaging system. Besides, the adjustment and alignment of the quadratic phase free image-plane digital holographic microscope is practicable and convenient.

Accurate Beam Prediction Through Characteristic Basis Function Patterns for the MeerKAT/SKA Radio Telescope Antenna

A novel beam expansion method is presented that requires employing only a few Characteristic Basis Function Patterns (CBFPs) for the accurate prediction of antenna beam patterns. The method is applied to a proposed design of the MeerKAT/SKA radio telescope, whose antenna geometry is subject to small deformations caused by mechanical or gravitational forces. The resulting deformed pattern, which is affected in a nonlinear fashion by these deformations is then sampled in a few directions only after which the interpolatory CBFPs accurately predict the entire beam shape (beam calibration). The procedure for generating a set of CBFPs - and determining their expansion coefficients using a few reference point sources in the sky - is explained, and the error of the final predicted pattern relative to the actual pattern is examined. The proposed method shows excellent beam prediction capabilities, which is an important step forward towards the development of efficient beam calibration methods for future imaging antenna systems.

Adaptive focusing of coherent radiation with the use of a fluctuating illuminating signal as a reference source

Adaptive focusing of a coherent beam in a turbulent atmosphere is considered. Distributions of the mean intensity of the field of a coherent laser beam focused in a turbulent medium by means of adaptive phase correction with the use of a point reference source are calculated. The source retains a random position on the object onto which laser radiation is focused. Results of adaptive focusing with moving and motionless reference sources are compared.

Effect of antenna‐medium coupling in the analysis of ground‐penetrating radar data

ABSTRACTPhysically‐based ground‐penetrating radar (GPR) data processing is essential for quantitative characterization of soils and materials. A novel near‐field GPR antenna model coupled with layered media Green's functions was used to investigate the effect of antenna‐medium coupling in the analysis of GPR data. The radar antennas are modelled using an equivalent set of infinitesimal electric dipoles and characteristic, frequency‐dependent, global reflection and transmission coefficients. These coefficients determine through plane wave decomposition, wave propagation between the radar reference plane, point sources and field points. We calibrated an actual commercial 400 MHz time‐domain antenna, from which synthetic GPR data sets were generated. We observed that, depending on the model configuration, antenna effects may affect the topography of the objective function in full‐waveform inverse problems. In addition, antenna‐medium coupling has a significant impact on the medium surface reflection, whether in terms of amplitude or propagation time (which usually defines the so‐called time zero). We also showed that an effective source cannot be used for simulating near‐field radar data as the antenna‐medium coupling strongly depends on medium properties. In this respect, numerical experiments demonstrated promising perspectives for simultaneous estimates of medium permittivity and conductivity from antenna‐medium coupling.

Focal depth of digital lensless Fourier transform micro-holographic system

Based on the Strehl criterion of coherent lens imaging system, the focal depth (FD) of lensless Fourier digital micro-holographic system is deduced. The FD expressions of the holographic systems with symmetrical and unsymmetrical reference point sources are obtained. The results show that the FD of lensless Fourier digital micro-holographic system is dependent not only on the optical wavelength and the numerical aperture of the recording system but also on the offset of the reference optical source, which is different from the FD of lensless Fourier digital micro-holographic system. The validities of the theoretical analysis are demonstrated by the computer simulations and experiments.