Star Tracker Research Articles

Observational evidence of third dredge-up occurrence in S-type stars with initial masses around 1 M⊙

Context. S stars are late-type giants with spectra showing characteristic molecular bands of ZrO in addition to the TiO bands typical of M stars. Their overabundance pattern shows the signature of s-process nucleosynthesis. Intrinsic, technetium (Tc)-rich S stars are the first objects on the asymptotic giant branch (AGB) to undergo third dredge-up (TDU) events. Exquisite Gaia parallaxes now allow for these stars to be precisely located in the Hertzsprung–Russell (HR) diagram. Here we report on a population of low-mass, Tc-rich S stars previously unaccounted for by stellar evolution models. Aims. Our aim is to derive parameters for a sample of low-mass, Tc-rich S stars and then, by comparing their location in the HR diagram with stellar evolution tracks, to derive their masses and to compare their measured s-process abundance profiles with recently derived STAREVOL nucleosynthetic predictions for low-mass AGB stars. Methods. Stellar parameters were obtained using a combination of HERMES high-resolution spectra, accurate Gaia Data Release 2 (Gaia-DR2) parallaxes, stellar-evolution models, and newly designed MARCS model atmospheres for S-type stars. Results. We report on six Tc-rich S stars lying close to the 1 M⊙ (initial mass) tracks of AGB stars of the corresponding metallicity and above the predicted onset of TDU, as expected. This provides direct evidence for TDUs occurring in AGB stars with initial masses as low as ∼1 M⊙ and at low luminosity, that is, at the start of the thermally pulsing AGB. We present AGB models producing TDU in those stars with [Fe/H] in the range −0.25 to −0.5. There is reasonable agreement between the measured and predicted s-process abundance profiles. For two objects however, CD −29°5912 and BD +34°1698, the predicted C/O ratio and s-process enhancements do not simultaneously match the measured ones.

Autonomous orbit determination using observations of a sodium guide‐star network

A new method of spacecraft orbit determination is developed based on position information from the observation of fluorescing streaks in the atomic sodium of Earth's upper atmosphere. This concept could be used for GPS constellation autonomous navigation (AutoNav). This new orbit determination method uses a spacecraft-mounted camera to observe fluorescing line segments in the sodium layer. The fluorescence is excited by ground-based sodium guide-star lasers. The system also uses a star tracker in order to reference the measured guide-star directions to inertial coordinates. The measured direction to an unknown point on a known guide-star segment at a known time defines a plane on which the spacecraft must lie. Seven or more independent measurements suffice to determine a Keplerian orbit and the spacecraft clock offset. Cramer-Rao analysis indicates that 1 - ?? position accuracies on the order of 20 m or better may be achievable for filtering intervals of several days.

An accuracy of better than 200 m in positioning using a portable star tracker

Using a portable star tracking system, the geographical position of the observer, latitude, longitude, and the North direction were obtained. In the previous work, the accuracy of latitude and longitude were obtained as 932 and 1510 m, respectively. The weakest point in the previous work was the installation of the charge-coupled device (CCD) on the inclinometer which had the largest uncertainty in the procedure of positioning. Therefore, a method was presented to calculate the angle between the CCD and the inclinometer. In this work, a new optimization method is applied to improve the accuracy of the calculation of observer position to 147 m in Latitude and 171 m in Longitude. In this method, 80 to 100 stars are used in each picture taken of the sky to apply the optimization method. To obtain the accuracy, 50 nights’ observations were recorded. On each night, 100 quite independent images were taken.

A Rotation-Invariant Additive Vector Sequence Based Star Pattern Recognition

A novel star pattern recognition technique for a “Lost-in-space” mode star tracker is presented in this paper. First, the two-dimensional (2-D) vectors connecting the stars are constructed in a rotation-invariant frame. Later, the additive property of 2-D vectors is integrated with the rotation-invariant frame to build a vector sequence for star identification. The proposed technique achieves an identification accuracy of 98.7% and has a run-time of only 12 ms for real-time testing on star images.

Protection Scheme for Star Tracker Images

Radiation effects in star trackers can cause malfunctions that may lead to the loss of communication with the satellite. This paper presents a scheme to protect star tracker images stored in memory against single event upsets (SEUs). Fault injection and resource utilization reports show that, with the protection technique, 99.98% of the SEUs can be corrected using 37% less memory space than single error correction Hamming codes. The remaining 0.02% has a negligible effect in the star identification algorithm.

A Hamming Distance and Spearman Correlation Based Star Identification Algorithm

This paper presents a novel star identification algorithm for a “lost-in-space” mode star tracker. The Spearman-correlation approach provides reliable recognition even when the captured images are swayed and biased from the onboard star pattern database. The hamming distance approach provides a shortlisted list of star IDs. Thus, the proposed combination of hamming distance and Spearman correlation provides a reliable and fast recognition. The achievable performance is evaluated by testing on simulated and real images.

Improved triangular-based star pattern recognition algorithm for low-cost star trackers

Star identification algorithms based on triangular-pattern are more suitable for low-cost star trackers since they require less star density in the field of view to operate effectively. In this paper, we propose a modified star pattern recognition algorithm based on the triangular-based algorithm of “LIEBE”. The main contribution of the proposed work is twofold. First, a new strategy for the selection of star triplets is proposed for database construction. Second, new selection criteria of the reference star are considered for pattern generation process. A sky simulation program is developed to assess mainly the robustness against different conditions of noise. The obtained results show an improvement in the overall identification rate, more robustness towards missing stars, and more efficiency towards magnitude noise. Furthermore, our proposed algorithm shows comparable robustness with the recently proposed triangular algorithms despite their reliance on more accurate camera and a validation process. To assess the algorithm performance, the algorithm is implemented on a prototype of Data Processing Unit (DPU) based on ARM Cortex-M4 processor. In this part, we discuss the major design decisions and we present the hardware architecture of DPU. The algorithm shows promising running time at a reduced on-board database when implemented on ARM platform.

The Relation between Outburst Rate and Orbital Period in Low-mass X-Ray Binary Transients

Abstract We have investigated the outburst properties of low-mass X-ray binary transients (LMXBTs) based on a comprehensive study of the outbursts observed in the past few decades. The outburst rates were estimated based on the X-ray monitoring data from Swift/BAT, RXTE/ASM, and MAXI and previous reports in the literature. We found that almost all LMXBTs with the orbital period below ∼12 hr showed only one outburst in these observations. There are systematic differences in the outburst rate between long-period ( hr) and short-period ( hr) systems. We infer that mass transfer rate is responsible for the systematic difference, since the disk instability model suggested that the mass transfer rate is a key factor affecting the quiescence time. The difference in outburst rate between long-period and short-period LMXBTs is probably due to the different mass transfer mechanism at different evolutionary stages of the donors. Based on the evolutionary tracks of single stars, we derived the critical orbital period for X-ray binaries that harbor a subgiant donor in various metallicities. The critical orbital period ( hr) is consistent with the above orbital period boundary obtained from the statistics of outburst rates. Furthermore, we found a negative correlation between the outburst rate and the orbital period in the samples for which the luminosity class of the donor star is III/IV. The best-fitting power-law index for the black hole subsamples is roughly consistent with the theoretical prediction for those systems with a donor star evolved off the main sequence.

Using the Galileo Solid-State Imaging Instrument as a Sensor of Jovian Energetic Electrons

We develop a technique to quantitatively characterize the high-energy electron environment using the Galileo spacecraft solid-state imaging (SSI) instrument and results from 3-D Monte Carlo particle transport simulations in Geant4, validating our findings with the Galileo energetic particle detector (EPD). We postprocess raw SSI images to obtain frames with only the radiation contribution. The camera settings (gain state, filter, and so on) are used to compute the intensity of the observed radiation hits. Simulating the response of the SSI instrument to mono-energetic electron environments, we find that the SSI is capable of detecting ≥10-MeV electrons (>90% of 11-MeV integral flux channel. We find agreement within $3\sigma $ of the EPD data for 43 out of 43 (100%) of the SSI images evaluated. 62% of fluxes are also within $1\sigma $ of the EPD data. This approach can be applied to other sets of imaging data (star trackers) in energetic electron environments, such as those found in geostationary Earth orbit.

Investigation into Star Tracker Algorithms using Smartphones with Application to High-Precision Pointing CubeSats

Investigation into Star Tracker Algorithms using Smartphones with Application to High-Precision Pointing CubeSats

Stabilization Control Mothed for Two-Axis Inertially Stabilized Platform Based on Active Disturbance Rejection Control With Noise Reduction Disturbance Observer

To improve the stability accuracy of two-axis inertially stabilized platform (ISP) for airborne star tracker application, a stable control method based on active disturbance rejection control (ADRC) is presented to handle the system nonlinearities, parameter uncertainties, and disturbances. A noise reduction method based on a modified disturbance observer (DOB) structure is proposed to suppress sensor noise and improve anti-disturbance capability and rapidity. Moreover, the robust stability of the proposed method is discussed. To illustrate the effectiveness of the proposed method, simulations and laboratory experiments are implemented. Finally, the vehicle tracking star experiments are performed. In addition, the contrast results show that for the two-axis ISP the presented controller has superior performance in anti-jamming ability, rapidity, and isolation.

Фотометрическая модель звёздного датчика ориентации

Фотометрическая модель звёздного датчика ориентации

Accurate and Robust Synchronous Extraction Algorithm for Star Centroid and Nearby Celestial Body Edge

Celestial body features are important navigation information in deep space exploration. This study provides a synchronous high-precision extraction algorithm for star centroid and nearby celestial body edges for a miniaturized independent optical navigation sensor, which combines the functions of a star tracker and a navigation camera. The image is filtered by a ring filter template to eliminate the interference information of background and improve the contrast between the target and the background. The second-order directional derivative and specific area characteristic method aim to roughly extract and distinguish the features (star centroid and the nearby celestial body edge). In local area template where feature points are located, the 1D energy deviation effect is proposed to extract the features of the two different light intensity distribution models. The accuracy and robustness of our algorithm are verified by simulation and ground-based experiments. The algorithm has certain reference significance for other types of dim target and edge detections, such as infrared detection, medical image, target measurement, and machine vision.

A Factor Graph Approach for Attitude Estimation of Spacecraft Using a Stellar Gyroscope

A stellar gyroscope is an attitude sensor that is composed of a micro star sensor and a micro-electro-mechanical system gyroscope. The attitude is the output by fusing the information of the two sensors. Due to the processing of the star image from the star sensor, a measurement delay is introduced and can affect the performance of the fusion of the two sensors. In this paper, a factor graph method that is based on a sliding window is investigated for dealing with the measurement delay. Temporary factors are calculated to temporarily occupy the places of the factors that are under processing, and sliding window incremental smoothing is only performed with a set of factors and states after the delayed measurement has arrived. To reduce the frequency of attitude calculation based on a star image, the keyframe strategy is adopted. Only in keyframes, the attitude is solved by using the star image and used as the measurement of the factor. In non-keyframes, only the star positions in the star images are determined, with a robust star tracking method that is aided by the gyroscope. Moreover, a high-rate low-latency output framework that is by a gyroscope is proposed for accelerating the attitude output rate. According to the results of the experiments on simulated data, the proposed method can overcome the problem of measurement delay and is more effective than the compared methods. According to the results of the experiments on real data, the proposed method can be used in applications.

AUTOMATED PARAMETRIC SYNTHESIS OF A PHOTOGRAPHIC LENS WITH REDUCED DISTORTION

Almost complete correction of distortion is required in a variety of optoelectronic devices in which optical systems create real images. The star trackers containing matrix image sensors are an example of such devices. Since the star trackers are installed on satellites or spacecrafts, it is desirable to avoid extra energy assumption due to necessity of applying digital processing techniques for correction of the image distortion. For such devices, it is reasonable to use the lenses which have distortion reduced to a practically zero level by optical means.Since the aberration of distortion has a nonlinear dependence on field, it can be corrected mostly in multi-lens systems. Unfortunately, the approach to optical design based on the 3rd order aberration theory does not take into account high-order aberrations. Another widely-used method based on local optimization requires to apply initial optical systems with already satisfactory image quality.This paper describes the implementation of automated optical design of lenses with reduced distortion by using of one of modern global optimization algorithms, namely, the adaptive Cauchy differential evolution algorithm.By using specialized optical design software, an experimental verification of the feasibility of the proposed approach has been fulfilled in a completely automated mode. The computer simulation of a parametric synthesis process has been done on an example of the orthoscopic lens, similar to one already patented. The lens, which was selected for a comparison study, has the focal length of100 mm, the angular field of view of 8.4°, the relative aperture of 1:1.8 and the working spectral range of 0.404 to 0.706 μm. The level of its residual relative distortion over the entire field does not exceed 0.02%. The obtained results indicate that the image quality of two found solutions exceeds the image quality of the prototype lens indicated in the patent.

Application of PIV and Delaunay Triangulation Method for Satellite Angular Velocity Estimation Using Star Tracker

The main objective of this paper is to apply a particle image velocimetry technique accompanied by Delaunay triangulation and spatial angles to consecutive images of star tracker in order to estimate the angular velocity of spacecraft. This method is independent of star identification or attitude matrix and can be performed parallel to conventional star tracker algorithms. It is computationally light and consumes a reasonable time to perform. An interrogation window containing two or more stars in the first image is selected and based on the peak of normalized cross correlation matrix, the same window in the second image is matched. In order to make sure that the matched windows are indeed the same, local recognition features, such as spatial angles for windows with two stars and Delaunay triangle planar angles for three or more stars, are compared. When matched, window center angular displacement is calculated in a fixed imaging time interval, and eventually, an angular rate is estimated. In order to evaluate the performance of the proposed method, Monte Carlo simulation of random night sky images is used. It is shown that for angular rates, about 1°/s accuracies better than 7e−5°/s are achieved. Accuracy decreases to 1e−3°/s for the rates of about 12°/s in 0.1-s time interval.

Global field-of-view imaging model and parameter optimization for high dynamic star tracker.

With the expansion of applications, star trackers break through the domain of traditional applications and operate under high dynamic environments. These new applications require high maneuverability and render the analyses and conclusions of previous traditional star trackers unsuitable. In order to resolve the limitation of the previous studies, we focus on the global field-of-view (GFOV) imaging performance of a high dynamic star tracker (HDST) in this paper. A GFOV imaging trajectory model is derived to correctly describe the different motions of stars imaged at different positions of focal plane. A comprehensive positional accuracy expression is obtained by analyzing the centroiding errors of stars in GFOV. On the basis of the proposed trajectory model and positional accuracy expression, a solution of GFOV optimal parameters is presented for the best performance in centroid estimation. Finally, comparative evaluations, numerical simulations, and a night sky experiment support the conclusions.

Preparation of Ni–P alloys super-black materials applied to baffle surface

ABSTRACTSuper-black materials refer to the materials whose reflectivity is less than 0.5% and have many significant applications in the aerospace field. Though super-black materials have excellent optical properties, most of them cannot be directly applied to the baffle of star trackers because of all kinds of limits. We presented a method to improve the preparation of super-black materials based on Ni–P alloys which helped this material better applied to star tracker baffle. This process chose the concentration of 10% oxalic acid solution to activate the aluminium surface and enlarged the plating area of Ni–P alloys film. Meanwhile, we anodised Ni–P alloys film in the non-oxidising acid solution instead of conventional blackening process. Consequently, the method improved the controllability of blackening process and avoided the time error. Thus, this method of preparing super-black materials could be applied to star tracker baffle.

On Increasing the Accuracy of Star Trackers to Subsecond Levels

Modern star trackers are based on photodetector arrays such as CCD or CMOS arrays. The accuracy of commercially available devices is ~1–3 arcseconds. However, the development of the space industry calls for higher orientation accuracies, which are needed in laser space communications, monitoring of near-Earth space and space debris, high-precision global mapping, and remote sensing of the Earth. The problems associated with enhancing the accuracy of modern star trackers are discussed.

GOCE star tracker attitude quaternion calibration and combination

We analyse the inter-boresight angles (IBA) measured by the star trackers on board the GOCE satellite and find that they exhibit small offsets of 7–9″ with respect to the ones calculated from the rotation of the star tracker reference frames to the satellite reference frame. Further, we find small variations in the offsets with a peak-to-peak amplitude of up to 8″, which correlate with variations of the star trackers’ temperatures. Motivated by these findings, we present a method for combining the attitude quaternions measured by two or more star trackers that includes an estimation of relative attitude offsets between star trackers as a linear function of temperature. The method was used to correct and combine the star tracker attitude quaternions within the reprocessing of GOCE data performed in 2018. We demonstrate that the IBA calculated from the corrected star tracker attitude quaternions show no significant offsets with respect to the reference frame information. Finally, we show that neglecting the star tracker attitude offsets in the processing would result in perturbations in the gravity gradients that are visible at frequencies below 2 mHz and have a magnitude of up to 90 mE. The presented method avoids such perturbations to a large extent.