Fine Sun Sensor Research Articles

OPTIMIZATION OF A SUN VECTOR DETERMINATION FOR PINHOLE TYPE SUN SENSOR

The sun vector is commonly used for defining a satellite attitude and many types of sensors exist for its determination. An attitude determination system is designed for each satellite project based on missions’ requirements. A fine pinhole sun sensor type was chosen and designed for HORYU-IV nanosatellite of Kyushu Institute of Technology. This sensor has a round-shaped hole and uses commercial off-the-shelf silicon photodiode, which consists of four small sensitive elements arranged close to each other. This type of sensors commonly uses look-up tables for providing high accuracy, which requires a large amount of data to be saved. Linear and polynomial methods for sun vector determination were considered instead of look-up tables to avoid having a large amount of data to be saved. Moreover, the influence of dead spaces between photodiodes on sensor accuracy was also investigated. Six real sun sensors and their theoretical models with different configurations were designed for investigating the difference between various calculating methods. The comparison of accuracies between proposed methods for real sun sensor models leads to two main findings: 1) on average, a polynomial method decreases error level of determined angle by 70% when compared with linear method; 2) accounting for gaps between photodiodes further decreases the average error of the angle determined by 15 % for polynomial, and by 6% for linear method when compared with methods not accounting for gaps.

IMPROVEMENT OF SUN ANGLE ACCURACY FROM IN-ORBIT DATA OF A QUADRANT PHOTODIODE SUN SENSOR

The sun vector is commonly used for defining a satellite attitude and many types of sensors exist for its determination. A fine pinhole sun sensor type was chosen and designed for HORYU-IV nanosatellite of Kyushu Institute of Technology. This sensor has a round-shaped hole and uses commercial off-the-shelf silicon photodiode, which consists of four small sensitive elements arranged close to each other. This type of sensors commonly uses look-up tables for providing high accuracy, which requires a large amount of data to be saved. Polynomial methods for sun vector determination were considered instead of look-up tables to avoid having a large amount of data to be saved. The influence of dead spaces between photodiodes on sensor accuracy was also investigated. The sensor was tested in space environment. It was found that its output signals went to saturation point. A method for the compensation of signals truncated by saturation was proposed. It was found that: 1) polynomial method provided 0.1deg accuracy for a sensor with ±5deg field of view; 2) accounting for gaps between photodiodes decreases the average error of the angle determined by 15%; 3) a method for compensation of truncated signals saves full sensor FOV with decreasing accuracy till 0.11deg.

Mean radius and shape of Pluto and Charon from New Horizons images

Approach images taken by the LORRI imaging system during the New Horizons spacecraft encounter have been used to determine the mean radii and shapes of Pluto and Charon. The primary observations are limb locations derived using three independent approaches. The resulting mean radii of Pluto and Charon are 1188.3±1.6km and 606.0±1.0km, respectively (2-σ). The corresponding densities are 1854±11kg/m3 and 1701±33kg/m3 (2-σ). The Charon radius value is consistent with previous Earth-based occultation estimates. The Pluto radius estimate is consistent with solar occultation measurements performed by the ALICE and Fine Sun Sensor instruments on New Horizons. Neither Pluto nor Charon show any evidence for tidal/rotational distortions; upper bounds on the oblateness are <0.6% and <0.5%, respectively.

정밀도 향상을 위한 2축 소형 정밀 태양센서의 모델링

태양센서는 제작이 용이하고 가벼워 위성의 자세센서로 많이 사용되며 센서 모델의 정밀도는 위성의 자세결정 정밀도에 큰 영향을 미친다. 따라서 본 논문에서는 정밀도 향상을 위한 2축 소형 태양센서 모델링 방식을 새롭게 제안하였다. 제안된 센서 모델링 방식은 기존 수학적 모델링 방식에서는 고려되지 않았던 슬릿 두께가 센서 출력에 영향을 주는 그림자 효과(shadow effect)를 고려한 물리적 모델링 방식이다. 또한 기존의 수학적 모델링 방식과의 비교를 통해 제안된 방식의 성능을 검증하였다. 새롭게 제안된 모델을 사용하면 기존 방식보다 정밀도가 약 29% 증가함을 확인할 수 있었다. 제안된 센서 모델은 현재 한국항공대학교 우주시스템연구실에서 개발 중인 HAUSAT-2 위성에 탑재될 <TEX>${\pm}60^{\circ}$</TEX>의 시야각을 갖는 2축 고정밀 태양센서를 대상으로 검증하였다. Sun sensors are frequently implemented by satellites for attitude sensing, due to its simple manufacturability and light weight. A modeling of sun sensors has an important effect on the accuracy of satellite attitude determination. This paper addresses a new modeling of a 2-axis miniature fine sun sensor with improved accuracy. Unlike other previous algebraic modeling methods, the newly suggested physical modeling method takes into account the shadowing effect of the slit thickness. It was shown that a newly proposed sun sensor modeling provides a substantial accuracy improvement of 29% compared to the generic algebraic modeling. The proposed sensor modeling was validated using 2-axis fine sun sensors with FOV(Field of View) of <TEX>${\pm}60^{\circ}$</TEX> mounted on HAUSAT-2 small satellite, currently under development by SSRL(Space System Research Lab.) at Hankuk Aviation University, Korea.

A new modeling and validation of two-axis miniature fine sun sensor

Sun sensors are frequently implemented by satellites for attitude sensing, due to its simple manufacturability and lightweight. A modeling of sun sensors has an important effect on the accuracy of satellite attitude determination. This paper addresses a new modeling of a two-axis miniature fine sun sensor with improved accuracy. Unlike other previous algebraic modeling methods, the newly suggested physical modeling method takes into account the shadowing effect of the slit thickness. It was shown that a newly proposed sun sensor modeling provides a substantial accuracy improvement of 29% compared to the generic algebraic modeling. The proposed sensor modeling was validated using two-axis fine sun sensors with FOV (field of view) of ±60° mounted on HAUSAT-2 small satellite, currently under development by SSRL (Space System Research Lab.) at Hankuk Aviation University, Korea.

ALEXIS Spacecraft Attitude Reconstruction with Thermal/Flexible Motions Due to Launch Damage

An attitude reconstruction algorithm has been developed and tested for the ALEXIS spacecraft, whose dynamics include significant thermal/flexible motions due to structural damage that occurred during launch. The ALEXIS science mission needs the reconstructed attitude to do off-line mapping of soft x-ray photons from telescopes' image planes aboard the spinning spacecraft into inertial coordinates. The attitude reconstruction algorithm takes inputs from a fine sun sensor and a horizon crossing indicator and processes them using a square-root-information filter/smoother. The filter/ smoother model includes Euler dynamics and randomwalk torques to approximate thermal/flexible effects. The launch damage left a solar array paddle free to flap under the influence of thermal inputs, which causes spin rate and direction changes. Despite the complexity and uncertainty of the dynamic model, the algorithm achieves a 0.25° 1-cr accuracy over 30-min. periods of orbit night, when attitude data is unavailable.

Solar motion-based method of attitude recovery - Application to International Ultraviolet Explorer

The International Ultraviolet Explorer is a geosynchronous orbiting telescope launched by NASA January 26, 1978, and operated jointly by NASA and the European Space Agency. The spacecraft was built with six gyroscopes to provide an inertial reference system for spacecraft attitude and control and with prime and redundant mechanical panoramic attitude sensors for coarse attitude determination. Following the failure of the fourth gyroscope in 1985, a new attitude control system was uplinked to the spacecraft using a fine sun sensor and the two remaining gyros for three-axis stabilization. By 1985 both panoramic attitude sensors had failed, and an alternate method developed for coarse attitude determination was not applicable to the newly installed control system. We present here a new method of attitude recovery that uses the relative motion of the sun with respect to the inertial reference provided by the remaining two gyros and that has been employed since 1985. This general technique should be of use to other spacecraft where weight is critical or there is a desire to avoid mechanical devices.

Science Observations with the IUE Using the One Gyro Mode

AbstractThe International Ultraviolet Explorer (IUE) is a geosynchronous orbiting telescope launched by the National Aeronautics and Space Administration (NASA) on January 26, 1978, and operated jointly by NASA and the European Space Agency. The science instrument consists of two spectrographs which span the wavelength range of 1150 to 3200 Å and offer two dispersions with resolutions of 6 Å and 0.2 Å. The spacecraft’s attitude control system originally included an inertial reference package containing 6 gyroscopes for 3-axis stabilization. The science instrument includes a prime and redundant Field Error Sensor (FES) camera for target aquisition and offset guiding. Since launch, 4 of the 6 gyroscopes have failed. The current attitude control system utilizes the remaining 2 gyros and a Fine Sun Sensor (FSS) for 3-axis stabilization. When the next gyro fails, a new attitude control system will be uplinked which will rely on the remaining gyro and the FSS for general 3-axis stabilzation. In addition to the FSS, the FES cameras will be required to assist in maintaining fine attitude control during target aquisition. This has required thoroughly determining the characteristics of the FES cameras and the spectrograph aperture plate as well as devising new target acquisition procedures. The results of this work are presented.

The Main Attitude Control System of the British X4 Satellite: In-Orbit-Performance

Miranda (X4) is the first British technological satellite to carry an active three-axis attitude control system. The short-term reference is obtained from a set of strap-down integrating rate gyroscopes. Control torquing is provided by a cold-gas stored-liquid propane Jet system. Novel signal processing is used to provide a high quality of limit-cycle operation. In two axes the inertial reference is updated by a fine sun sensor. In the third axis, normal operation involves solely the gyroscope reference, but provision is made to update to a stellar reference, using an experimental star sensor. Despite some malfunction of the gas pressure regulation, the satellite has achieved its experimental objectives. This paper describes briefly the attitude control system, and assesses its performance.