Sun Sensor Research Articles

Sliding Mode Control of a Water-Displacement Based Mechanism Applied to the Orientation of a Parabolic-Trough Solar Concentrator

In this paper the problem of guiding a parabolic trough solar concentrator through a system based on a water displacement mechanism is addressed. The actuator mechanism consists of a set of four on/off electrovalves in ”H-bridge” configuration, a small water pump, a sun sensor and two containers half-filled with water, installed on the side ends of the concentrator. To guide and balance the concentrator, a sliding mode controller (SMC) to enable / disable the opening and closing of the valves that control the direction and flow of water between the two containers is proposed. The validity of the implementation of the proposed law of control applied to the actuator mechanism is verified with stability analysis and simulation results.

Singular value decomposition based satellite attitude determination using different sensor configurations

Satellite's attitude can be determined by using several different ways. In order to use a recursive filter to determine the attitude especially after crucial intervals for the satellite including detumbling, eclipse, etc., single-frame methods can be used, and singular value decomposition (SVD) method has been selected because of its robustness for this paper. They can be placed for the initial values, or continuous input to the filter in addition to covariance matrix at each step. Also, sensor configurations depending on the position of the Sun are considered and used in the SVD method. Wahba's loss function is minimized with separating the basic matrix including the reference and model vectors into singular values because many researches show the SVD as the most robust algorithm after error analyses between several single-frame methods. Sun sensor, magnetometer, and horizon sensor are the selected common attitude sensors for small satellites in order to compare their attitude determination performances. For this purpose, absolute errors for different sensor configurations in the simulation environment in addition to the RMS errors are presented. The purpose of the paper is to present an attitude determination concept for a nanosatellite. Also, different sensor configurations are considered in the single-frame method with switching logic depending on the vector data. Horizon sensor and sun sensor are the optimum pair for the light side of the Earth on the other hand, during the eclipse; horizon sensor and magnetometer give the most accurate results. Study shows that the proposed algorithm can be performed for initial values to the Kalman filter, and in the transient phase before the filter tuned in practical satellite usage. In the eclipse period, sun-independent sensors are used and changing configurations provide better attitude estimation results to filtering techniques.

Initial In-flight Results: The Total Solar Irradiance Monitor on the FY-3C Satellite, an Instrument with a Pointing System

The total solar irradiance (TSI) has been recorded daily since October 2013 by the Total Solar Irradiance Monitor (TSIM) onboard the FY-3C satellite, which is mainly designed for Earth observation. The TSIM has a pointing system to perform solar tracking using a sun sensor. The TSI is measured by two electrical substitution radiometers with traceability to the World Radiation Reference. The TSI value measured with the TSIM on 2 October 2013 is \(1364.88~\mbox{W}\,\mbox{m}^{-2}\) with an uncertainty of \(1.08~\mbox{W}\,\mbox{m}^{-2}\). Short-term TSI variations recorded with the TSIM show good agreement with SOHO/VIRGO and SORCE/TIM. The data quality and accuracy of FY-3C/TSIM are much better than its predecessors on the FY-3A and FY-3B satellites, which operated in a scanning mode.

Photovoltaic panels as attitude sensors for artificial satellites

The sun is a useful geodetic reference for spacecraft operating near earth, where its high brightness in relation to other astronomical objects makes it easy to detect [1]. It also provides great power availability near earth, thus being the most reliable power supply for spacecraft orbiting earth. The orientation of the sun relative to the spacecraft body is an important piece of information for essential onboard functions such as attitude determination, safe operation, activation or deactivation of light-sensitive equipment, and orientation of photovoltaic (PV) panels for maximum power conversion [2]. Such relative orientation can be determined using a device known as a sun sensor.

Signal Conditioning for the Kalman Filter: Application to Satellite Attitude Estimation with Magnetometer and Sun Sensors.

Most satellites use an on-board attitude estimation system, based on available sensors. In the case of low-cost satellites, which are of increasing interest, it is usual to use magnetometers and Sun sensors. A Kalman filter is commonly recommended for the estimation, to simultaneously exploit the information from sensors and from a mathematical model of the satellite motion. It would be also convenient to adhere to a quaternion representation. This article focuses on some problems linked to this context. The state of the system should be represented in observable form. Singularities due to alignment of measured vectors cause estimation problems. Accommodation of the Kalman filter originates convergence difficulties. The article includes a new proposal that solves these problems, not needing changes in the Kalman filter algorithm. In addition, the article includes assessment of different errors, initialization values for the Kalman filter; and considers the influence of the magnetic dipole moment perturbation, showing how to handle it as part of the Kalman filter framework.

Flexible attitude control design and on-orbit performance of the ZDPS-2 satellite

ZDPS-2, the 2nd generation nano-satellites designed and manufactured by Microsat Research Center of Zhejiang University, were successfully launched by the new generation rocket CZ-6 on Sep 20th, 2015. They are two 20kg identical nano-satellites (ZDPS-2-A and ZDPS-2-B) with a large deployable mechanism with a truss structure. The main mission of these two satellites is on-orbit demonstration of several new technologies, including large lightweight and flexible deployable mechanisms, a custom-designed panoramic wide field of view micro camera Sun sensor and infrared Earth sensor, and a micro-propulsion system as well as dual-satellite formation flying technology. The satellite is a 25cm cube with a deployable mechanism, whose fundamental frequency is only approximately 0.13Hz. This brings several challenges to attitude control. Flexible structure dynamics and its influence on attitude stabilization must be taken into consideration. A robust control scheme is developed by fully utilizing environmental torques with limited control actuator capability. On-orbit performance verified this design. ZDPS-2 satellites have successfully achieved rate damping, sun-pointing control and 3-axis stabilized Earth-pointing control, with a control accuracy of 5° (3σ) and stability superior to 0.1°/s (3σ). This paper demonstrates the flexible attitude dynamics modeling, illustrates the attitude control system design of ZDPS-2 and presents the on-orbit performance as well.

Sun sensor design and test of a micro satellite

According to the requirement of small satellite, this paper designed a digital sun sensor which diaphragm is a V-shaped cross-section structure. Using Position Sensitive Detector (PSD) as the light detector, we designed the V-shaped cross-section structure based on the pinhole imaging principle. The sun sensor realized the accurate calculation for two axis sun angle of the sun sensor. The mechanical test, thermal test and testing of the sun sensor are designed and carried out. The mechanical test and thermal test results verify the stability of the sun sensor. Testing result shows that the detection angle can reach (120°)×(120°), and the attitude determination accuracy is better than 6” in the entire viewing field. The mass, volume and power consumption of the sun sensor is 0.177 kg, 78 mm×77 mm×21 mm and 0.25 W. The sun sensor has low power consumption, large viewing angle and high precision characteristics, which realized the sun sensor the miniaturization and meet the requirements of the micro satellite. Its performance has been verified in orbit.

A microfabricated sun sensor using GaN-on-sapphire ultraviolet photodetector arrays.

A miniature sensor for detecting the orientation of incident ultraviolet light was microfabricated using gallium nitride (GaN)-on-sapphire substrates and semi-transparent interdigitated gold electrodes for sun sensing applications. The individual metal-semiconductor-metal photodetector elements were shown to have a stable and repeatable response with a high sensitivity (photocurrent-to-dark current ratio (PDCR) = 2.4 at -1 V bias) and a high responsivity (3200 A/W at -1 V bias) under ultraviolet (365 nm) illumination. The 3 × 3 GaN-on-sapphire ultraviolet photodetector array was integrated with a gold aperture to realize a miniature sun sensor (1.35 mm × 1.35 mm) capable of determining incident light angles with a ±45° field of view. Using a simple comparative figure of merit algorithm, measurement of incident light angles of 0° and 45° was quantitatively and qualitatively (visually) demonstrated by the sun sensor, supporting the use of GaN-based sun sensors for orientation, navigation, and tracking of the sun within the harsh environment of space.

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.

A highly accurate wireless digital sun sensor based on profile detecting and detector multiplexing technologies

The advancing growth of micro- and nano-satellites requires miniaturized sun sensors which could be conveniently applied in the attitude determination subsystem. In this work, a profile detecting technology based high accurate wireless digital sun sensor was proposed, which could transform a two-dimensional image into two-linear profile output so that it can realize a high update rate under a very low power consumption. A multiple spots recovery approach with an asymmetric mask pattern design principle was introduced to fit the multiplexing image detector method for accuracy improvement of the sun sensor within a large Field of View (FOV). A FOV determination principle based on the concept of FOV region was also proposed to facilitate both sub-FOV analysis and the whole FOV determination. A RF MCU, together with solar cells, was utilized to achieve the wireless and self-powered functionality. The prototype of the sun sensor is approximately 10 times lower in size and weight compared with the conventional digital sun sensor (DSS). Test results indicated that the accuracy of the prototype was 0.01° within a cone FOV of 100°. Such an autonomous DSS could be equipped flexibly on a micro- or nano-satellite, especially for highly accurate remote sensing applications.

Micro digital sun sensor with linear detector.

In this paper, the design of a novel micro digital sun sensor is described. It relies on V-shaped slit and linear array CCD to measure sun-ray angle against two axes. A highly integrated microprogram control unit) is used to make a very simple and compact system. V-shaped slit can simplify algorithm and achieve a wider field of view. Error compensation and accurate calibration are employed to improve accuracy. Adaptive threshold and adjustable expose time further improve reliability. Experiments and flight validation show that the FOV (Field of View) of the sun sensor is ±65° × ± 65° and the accuracy is 0.1° in the whole FOV. It can work reliably at an update rate of 25 Hz, while the consumption is only 200 mW. This sun sensor is proved to have a good prospect in micro/nanosatellites.

Cascade closed-loop control of solar trackers applied to HCPV systems

High concentration photovoltaic (HCPV) modules require a high precision tracking system for reaching their highest conversion efficiency. One way to accomplish this goal is by using a closed-loop mechanism and a sun sensor to track the sunlight. This paper proposes a cascade control algorithm capable of achieving a sun tracking error of 1′ for its application in high concentration photovoltaic systems. The algorithm follows an inner loop-outer loop topology. The inner loop employs a Nonlinear Proportional-Proportional Integral (NP-PI) controller and the outer loop resorts on a Proportional Integral (PI) controller. A tuning procedure for setting up the cascade controller is also described. Experiments on a laboratory prototype compare the performance of the proposed cascade controller with a PI controller not resorting on an inner loop. These outcomes show that the proposed control law provides improved tracking accuracy with less actuator wear.

Gyro-free attitude and rate estimation for a small satellite using SVD and EKF

This paper describes the development of a gyroless attitude determination system that can rely on magnetometer and sun sensor measurements and achieve good accuracy. Vectors coming from the selected sensor data and developed models can be placed in Wahba's problem. The system uses Singular Value Decomposition (SVD) method to minimize the Wahba's loss function and determine the attitude of the satellite. In order to obtain the attitude of the satellite with desired accuracy an extended Kalman filter (EKF) for satellite's angular motion parameter estimation is designed. The EKF uses this attitude information as the measurements for providing more accurate attitude estimates even when the satellite is in eclipse. The “attitude angle error covariance matrix” calculated for the estimations of the SVD method is regarded as the measurement noise covariance for the EKF.The SVD and EKF algorithms are combined to estimate the attitude angles and angular velocities, respectively. Besides, the proposed algorithm and traditional approach using nonlinear measurements are compared and concluded that SVD/EKF gives more accurate results for most of the time intervals. The algorithm can be used for low-cost small satellites where using high power consuming, expensive, and fragile gyroscopes for determining spacecraft attitude are not reasonable.

Three-axis sun sensor for attitude determination

The field of attitude determination sensors is mature. The different sensor technologies have been known for decades, and existing product lines are available for all types of sensors. Table 1 shows existing attitude determination sensor technologies [1], [2].

RETRACTED: A semi-physical simulation platform of attitude determination and control system for satellite

A semi-physical simulation platform for attitude determination and control system is proposed to verify the attitude estimator and controller on ground. A simulation target, a host PC, many attitude sensors, and actuators compose the simulation platform. The simulation target is composed of a central processing unit board with VxWorks operating system and many input/output boards connected via Compact Peripheral Component Interconnect bus. The executable programs in target are automatically generated from the simulation models in Simulink based on Real-Time Workshop of MATLAB. A three-axes gyroscope, a three-axes magnetometer, a sun sensor, a star tracer, three flywheels, and a Global Positioning System receiver are connected to the simulation target, which formulates the attitude control cycle of a satellite. The simulation models of the attitude determination and control system are described in detail. Finally, the semi-physical simulation platform is used to demonstrate the availability and rationality of the control scheme of a micro-satellite. Comparing the results between the numerical simulation in Simulink and the semi-physical simulation, the semi-physical simulation platform is available and the control scheme successfully achieves three-axes stabilization.

Effects of light polarization and waves slope statistics on the reflectance factor of the sea surface.

Above-water radiometry depends on estimates of the reflectance factor ρ of the sea surface to compute the in situ water-leaving radiance. The Monte Carlo code for ocean color simulations MOX is used in this study to analyze the effect of different environmental components on ρ values. A first aspect is examining the reflectance factor without and by accounting for the sky-radiance polarization. The influence of the sea-surface statistics at discrete grid points is then considered by presenting a new scheme to define the variance of the waves slope. Results at different sun elevations and sensor orientations indicate that the light polarization effect on ρ simulations reduces from ∼17 to ∼10% when the wind speed increases from 0 to 14m s-1. An opposite tendency characterizes the modeling of the sea-surface slope variance, with ρ differences up to ∼12% at a wind speed of 10m s-1. The joint effect of polarization and the the sea-surface statistics displays a less systematic dependence on the wind speed, with differences in the range ∼13 to ∼18%. The ρ changes due to the light polarization and the variance of the waves slope become more relevant at sky-viewing geometries respectively lower and higher than 40° with respect to the zenith. An overall compensation of positive and negative offsets due to light polarization is finally documented when considering different sun elevations. These results address additional investigations which, by combining the modeling and experimental components of marine optics, better evaluate specific measurement protocols for collecting above-water radiometric data in the field.

Robust Change Vector Analysis (RCVA) for multi-sensor very high resolution optical satellite data

The analysis of rapid land cover/land use changes by means of remote sensing is often based on data acquired under varying and occasionally unfavorable conditions. In addition, such analyses frequently use data acquired by different sensor systems. These acquisitions often differ with respect to sun position and sensor viewing geometry which lead to characteristic effects in each image. These differences may have a negative impact on reliable change detection. Here, we propose an approach called Robust Change Vector Analysis (RCVA), aiming to mitigate these effects. RCVA is an improvement of the widely-used Change Vector Analysis (CVA), developed to account for pixel neighborhood effects. We used a RapidEye and Kompsat-2 cross-sensor change detection test to demonstrate the efficiency of RCVA. Our analysis showed that RCVA results in fewer false negatives as well as false positives when compared to CVA under similar test conditions. We conclude that RCVA is a powerful technique which can be utilized to reduce spurious changes in bi-temporal change detection analyses based on high- or very-high spatial resolution imagery.

Stabilization and Attitude Determination Methods for FalconSAT-3

FalconSAT-3 is a cadet-built and -operated satellite that was launched in 2007. In late 2014, cadets successfully stabilized the satellite for the first time through an operational test campaign using a -dot controller. This paper describes the many challenges in attitude stabilization and determination, and the solutions provided to achieve partial mission capability of two payloads with a magnetometer as the only sensor. Due to problems with onboard sun sensors, engineers have used solar panel currents to create a sun vector to perform a full three-axis attitude determination. This paper compares two different methods for determining the sun vector from solar panel currents and uses the triad method to perform a three-axis attitude determination. A discussion of the key lessons learned from the cadet-run satellite operations is also included.

Self powered solar tracker for Low Concentration PV (LCPV) systems

A design and performance of a new self powered LCPV solar trackers using bifacial solar cells and a concentrating mirror is described in the report. This design does not need additional power supply nor cables and so it is cheaper than classical PV power plants with many cables. The power supply bifacial PV panels are simultaneously used as sun sensors. The new tracker design with bifacial solar cells was successfully used at big MW range PV power plants. The very simple self powered tracker with concentrating mirror was successfully tested at Low Concentration PV (LCPV) systems. The tracking accuracy is less than ±1 angular degree.

Absolute Navigation Information Estimation for Micro Planetary Rovers

This paper provides algorithms to estimate absolute navigation information, e.g., absolute attitude and position, by using low power, weight and volume Microelectromechanical Systems-type (MEMS) sensors that are suitable for micro planetary rovers. Planetary rovers appear to be easily navigable robots due to their extreme slow speed and rotation but, unfortunately, the sensor suites available for terrestrial robots are not always available for planetary rover navigation. This makes them difficult to navigate in a completely unexplored, harsh and complex environment. Whereas the relative attitude and position can be tracked in a similar way as for ground robots, absolute navigation information, unlike in terrestrial applications, is difficult to obtain for a remote celestial body, such as Mars or the Moon. In this paper, an algorithm called the EASI algorithm (Estimation of Attitude using Sun sensor and Inclinometer) is presented to estimate the absolute attitude using a MEMS-type sun sensor and inclinometer, only. Moreover, the output of the EASI algorithm is fused with MEMS gyros to produce more accurate and reliable attitude estimates. An absolute position estimation algorithm has also been presented based on these on-board sensors. Experimental results demonstrate the viability of the proposed algorithms and the sensor suite for low-cost and low-weight micro planetary rovers.