Stellar Sensor Research Articles

Method of increasing the accuracy of the star sensor

A new algorithm for filtering distortions of the deep image of the starry sky is considered to improve the accuracy of determining the position of the star - the detection of the centroid. An increase in the accuracy of measuring the orientation parameters of the spacecraft by stellar sensors is being investigated. The method of calculations using the algorithm of exact estimation of the centroid is applied, restoring the function of the scatter of points from recorded astronomical images. Minimization of the effects of atmospheric turbulence and system noise on long-exposure images obtained by a ground-based telescope is proposed. The method was tested in the MATLAB environment for a real image of the deep sky recorded by the ground system.

ACCURACY COMPARISON OF ALGORITHMS FOR DETERMINATION OF IMAGE CENTER COORDINATES IN OPTOELECTRONIC DEVICES

Accuracy in determination of coordinates for image having simple shapes is considered as one of important and significant parameters in metrological optoelectronic systems such as autocollimators, stellar sensors, Shack-Hartmann sensors, schemes for geometric calibration of digital cameras for aerial and space imagery, various tracking systems. The paper describes a mathematical model for a measuring stand based on a collimator which projects a test-object onto a photodetector of an optoelectronic device. The mathematical model takes into account characteristic noises for photodetectors: a shot noise of the desired signal (photon) and a shot noise of a dark signal, readout and spatial heterogeneity of CCD (charge-coupled device) matrix elements. In order to reduce noise effect it is proposed to apply the Wiener filter for smoothing an image and its unambiguous identification and also enter a threshold according to brightness level. The paper contains a comparison of two algorithms for determination of coordinates in accordance with energy gravity center and contour. Sobel, Pruitt, Roberts, Laplacian Gaussian, Canni detectors have been used for determination of the test-object contour. The essence of the algorithm for determination of coordinates lies in search for an image contour in the form of a circle with its subsequent approximation and determination of the image center. An error calculation has been made while determining coordinates of a gravity center for test-objects of various diameters: 5, 10, 20, 30, 40, 50 pixels of a photodetector and also signalto-noise ratio values: 200, 100, 70, 20, 10. Signal-to-noise ratio has been calculated as a difference between maximum image intensity of the test-object and the background which is divided by mean-square deviation of the background. The accuracy for determination of coordinates has been improved by 0.5-1 order in case when there was an increase in a signal-to-noise ratio. Accuracy improvement due to increase of a diameter in a test-object is typical for large signal-to-noise ratios: 70 or more. The conducted investigations have made it possible to establish that the algorithm for determination of coordinates of the energy gravity center is more accurate in comparison with contour methods and requires less computing power (for the MatLab software package), which is related to discreteness while determining a contour.

Optimizing the Support of a Stellar Sensor

The mass of a support for a stellar sensor in a spacecraft is minimized in the light of requirements on the first eigenfrequency and the reactive force in the connectors. Topological and parametric optimization is employed. The finite-element method is adopted, within the capabilities of the Altair Optistruct system

A novel navigation method used in a ballistic missile

The traditional strapdown inertial/celestial integrated navigation method used in a ballistic missile cannot accurately estimate the accelerometer bias. It might cause a divergence of navigation errors. To solve this problem, a new navigation method named strapdown inertial/starlight refractive celestial integrated navigation is proposed. To verify the feasibility of the proposed method, a simulated program of a ballistic missile is presented. The simulation results indicated that, when multiple refraction stars are used, the proposed method can accurately estimate the accelerometer bias, and suppress the divergence of navigation errors completely. Specifically, in order to apply this method to a ballistic missile, a novel measurement equation based on stellar refraction was developed. Furthermore a method to calculate the number of refraction stars observed by the stellar sensor was given. Finally, the relationship between the number of refraction stars used and the navigation accuracy is analysed.

Nonlinear empirical mode predictive drift extraction on fiber optical gyroscope

The integration drift of inertial unit is a fatal error on navigation and attitude determination system. Many approaches have been developed to extract and eliminate it. A novel fiber optical gyroscope drift extraction algorithm was proposed in this paper to ameliorate the performance of fiber optical gyroscope by extracting the trend component as the compensation. In this algorithm, the attitude quaternion of stellar sensor and the output of fiber optical gyroscope were imported into a nonlinear empirical mode predictive algorithm to extract the drift component of gyroscope. Combing the advantages of predictive filter and empirical mode decomposition in nonlinear signal processing, our proposal is more precise in drift extraction and data approximation. Software simulation and hardware verifications on gyroscope were launched, in which the results had proven the capability of the algorithm.

Stellar Sensor Assistant Fiber Optic Gyroscope Drift Extraction

The fiber-optic gyroscope is used widely utilized as an important component of spacecraft inertial navigation system. Although it has several good characteristics over conventional gyroscope techniques, there is still some drift. To obtain high performance and diagnose faults in fiber-optic gyroscopes, error and drift must be found. Although many approaches have been proposed to filter drift in attitude determination, the solutions are in adequate. The stellar sensor has no integration drift making it very suitable for finding drift in fiber-optic gyroscopes. We proposed a novel fiber-optic gyroscope drift extraction algorithm using the stellar sensor to generate the vehicle reference rotation rate. Simulations of the algorithm are described an verify the proposed algorithm.

Stellar sensor based nonlinear model error filter for gyroscope drift extraction

The conventional gyroscope based spacecraft attitude determination approaches are afflicted by the integration drift of gyroscope. To ameliorate the performance of gyroscope, many compensation algorithms have been developed. A novel spacecraft attitude determination algorithm, which is based on the stellar sensor and nonlinear model error filter algorithm, is discussed in this paper. This algorithm uses the attitude quaternion obtained from stellar sensor to generate the attitude information of spacecraft, which could compensate the drift error of the gyroscope unit. This approach could be utilized both as the gyroscope error compensation in high precision integrate navigation system and as an attitude determination unit individually in low precision mini-satellite task. Two verifications of different satellite orbits have been launched and the performance of our approach was proved.

인공위성 자세결정 시스템을 위한 등가 오차모델

We introduce the error models for an attitude determination system(ADS) with gyroscopes and stellar sensor. The ADS error models are derived according to the definition of the reference frame and of the attitude error. The equivalent error models applicable to the attitude determination system with large attitude errors are presented. The simulation results show that the proposed error models improve performance of the attitude determination system.

A fast star sensor for balloon payloads

We developed a system to reconstruct the attitude of balloon borne spinning experiments, where high accuracy (about 1′) and high rotation speed (up to tens of degrees per second) are required. It is based on a stellar sensor, and gathers together hardware simplicity, cheap components, high resolution, and sensitivity. It is composed of an optical mirror (diameter 40 cm), an array of 46 fast and sensitive photodiodes, and low noise readout electronics. It was designed for the Archeops experiment, a balloon borne millimetric telescope whose goal is to generate high resolution maps of large regions of the sky, to study the temperature anisotropies of the cosmic background radiation.

ELISA, a Small Experiment For a Large Sub-millimeter Survey

This paper presents the scientific objectives and technical aspects of the balloon-borne Experiment for Large Infrared Survey Astronomy (ELISA). The emphasis is put upon the synergies existing between the ELISA project and future space missions, both with respect to scientific and technical aspects. ELISA is a small balloon project for an experiment dedicated to measure the Far-Infrared to Sub-millimeter continuum emission of Galactic dust over a large fraction of the sky, with unprecedented sensitivity and angular resolution. The primary mirror of the telescope, similar to the one used for the Top-Hat mission, will have a diameter of 1 m, ensuring an angular resolution of about 3.5'. Bolometer arrays will be used in four photometric bands centered at 170, 240, 400, and 650 μm and providing a 22' × 45' instantaneous field of view per channel. A liquid He cryostat will host the cold optics, including the secondary mirror of the telescope, as well as the detectors, which will be cooled to 0.3 K using an 3He close-cycle fridge. Mapping of the sky will be accomplished by rotating the gondola over a large azimuth range (up to 60° amplitude). The pointing of the experiment will be maintained to a constant elevation during the azimuth scans through a feed back loop using the signal from a large format, fast stellar sensor, operating day and night. The scientific goal of the experiment is to map the diffuse Sub-millimeter emission along a large fraction of the Milky Way. The astronomical data obtained will be used to derive the emission properties of the dust grains in the Interstellar Medium (ISM), such as their temperature and emissivity. It will also allow to systematically measure the polarization of the dust emission. It should also lead to the detection of a few thousand point sources such as newly formed stars and distant galaxies. In addition to these goals, the ELISA project will serve as a test bed for the detector technology that will be used for the HERSCHEL space mission to be launched in 2007. The ELISA data will be made available to help calibrate the observations of HERSCHEL and PLANCK and to plan the large-scale surveys to be undertaken with HERSCHEL. Owing to these objectives, 3 flights of the ELISA experiment, are foreseen in the period from 2004 to 2006. The ELISA project is carried out by an international collaboration including France (CESR, IAS, CEA, CNES), The Netherlands (SSD/ESTEC), Denmark (DSRI), UK (Cardiff Univ.).

The CCD stellar sensor of the MASCO telescope pointing system

We describe the development of a CCD stellar sensor capable of determining the pointing coordinates with an error smaller than 0.5′. This instrument is part of the attitude control system of the balloon-borne γ-ray MASCO (acronym in Portuguese for coded mask — MÁScara COdificada) telescope. It acquires an image of the sky, recognizes the most brilliant stars in the image, creates with them a “constellation”, and searches for this pattern in an on board star catalogue of the observed region. The searching algorithm is based on the analysis of the geometrical configuration of the “constellation” and considers the distances and angles among the stars and their relative brightness. This instrument consists in an inexpensive CCD camera coupled to a telephoto lens and a frame grabber which digitizes the images at 512×400 pixels resolution in a field of view of about 6°×5°. The implementation of interpolation algorithms allowed an accuracy in the determination of the pointing coordinates of 1 10 of a pixel, i.e., a few arcseconds. The tests carried out with this sensor showed the following results: 100% of correct matching of the point patterns in more than 1500 sky images, error dispersion in the determination of the pointing coordinates smaller than 30″ and dispersion smaller than 4″ in the determination of the star image's positions.

Development of an attitude control system for a balloon-borne gamma ray telescope

Abstract We describe the attitude control system employed by the MASCO balloon-borne gamma ray telescope and present the first laboratory tests of this system. The MASCO experiment is a low energy gamma ray imaging telescope that employs a MURA coded mask. Its angular resolution is 14′ over a 13° field of view, which requires a pointing accuracy of some arcminutes in order to allow the accomplishment of the scientific goals. The attitude control system was designed to provide pointing and stabilization of a 1400 kg, 7 m-high, 2 m-wide, 2 m-deep gondola that encompasses the telescope detector system and its associated electronics. The sensors used are an electronic magnetic compass, an encoder, a two-axis solar sensor, a sun-tracker, two accelerometers, a gyroscope and three CCD stellar sensors. A GPS system is also employed. The actuators are a reaction wheel for azimuth control, an azimuth motor located in the balloon-gondola decoupling mechanism for desaturation of the reaction wheel, and a telescope elevation motor with a harmonic drive reduction. The software architecture and the operation modes are also presented.

Gamma-telescope with very high angular and spectral resolution

New type of gamma-telescope with angular resolution about several seconds of arc and spectral resolution ΔE/E <0.1% is proposed. In this instrument a position sensitive detector is used together with random mask, crystal diffractor and stellar sensor. The scientific objectives and the possible ways to carry out the first experiment with this telescope are discussed.

Covariance analysis of a charge carrier device processing algorithm for stellar sensors

This paper addresses the problem of modeling a charge carrier device imager array, develops equations for propagating the signal, and develops equations to propagate the noise. Equations are used for a window processor with a comparator for stellar detection. Finally, a numerical example is presented that shows how the probability of detection varies as a function of the signal-to-noise ratio and charge-transfer inefficiency. Some surprising results are explained.

Testing of Stellar-Inertial Guidance Systems

The ever growing demands for improvement in performance of guidance equipment to be used in missiles and space vehicles has led to intensive interest in stellar-aided inertial reference systems. A stellar-inertial system uses star position information to correct for gyro drift and misalignment errors and serves as a means for updating position and velocity information generated by the guidance system computer. This capability becomes particularly attractive for extended space flights and for mobile ballistic missile systems. The design configurations assumed by stellar-inertial guidance systems may vary widely. Variations arise from the types of guidance equations dictated by the applications and also as a consequence of the instruments used as stellar sensors. The guidance problem may be solved by a variety of computational schemes, using explicit equations or schemes such as Q matrices. The star sensing devices are usually categorized by their detector mechanisms. The three main types used in systems now operational or under development are photomultipliers, vidicon tubes, and solid-state elements. The scanning methods used with these devices range from mechanical drives to electronic schemes using no moving parts. Each of these methods has merits and disadvantages which influence the guidance system designer in his selection. The intended application is the criterion in the type of computational scheme and components that are selected. This paper presents a survey of the above-mentioned aspects of stellar-inertial guidance equipment with a view toward establishing the validity and applicability of conventional inertial guidance test methods to the testing of stellar-inertial equipment.