Seon-Ho Lee

Gyroless attitude estimation of sun-pointing satellites using magnetometers

Deterministic attitude estimation methods based on vector observations are used for the coarse-attitude calculation of the sun-pointing satellites. The objective of the deterministic attitude estimation is to get the direction cosine matrix without considering the stochastic noises. The existing deterministic attitude estimation methods require two complete (three-axis) vector observations corresponding to two known reference vectors. This letter suggests a new deterministic attitude estimation method using only geomagnetic-field measurements on two axes for sun-pointing satellites in the sun-acquisition mode.

Time optimal attitude maneuver strategies for the agile spacecraft with reaction wheels and thrusters

Reaction wheels and thrusters are commonly used for the satellite attitude control. Since satellites frequently need fast maneuvers, the minimum time maneuvers have been extensively studied. When the speed of attitude maneuver is restricted due to the wheel torque capacity of low level, the combinational use of wheel and thruster is considered. In this paper, minimum time optimal control performances with reaction wheels and thrusters are studied. We first identify the features of the maneuvers of the satellite with reaction wheels only. It is shown that the time-optimal maneuver for the satellite with four reaction wheels in a pyramid configuration occurs on the fashion of single axis rotation. Pseudo control logic for reaction wheels is successfully adopted for smooth and chattering-free time-optimal maneuvers. Secondly, two different thrusting logics for satellite time-optimal attitude maneuver are compared with each other: constant time-sharing thrusting logic and varying time-sharing thrusting logic. The newly suggested varying time-sharing thrusting logic is found to reduce the maneuvering time dramatically. Finally, the hybrid control with reaction wheels and thrusters are considered. The simulation results show that the simultaneous actuation of reaction wheels and thrusters with varying time-sharing logic reduces the maneuvering time enormously. Spacecraft model is KOrea Multi-Purpose SATellite (KOMPSAT) -2 which is being developed in Korea as an agile maneuvering satellite.

Fully Magnetic Devices-Based Control for Gyroless Target Pointing of a Spinning Spacecraft

The use of magnetic devices is an attractive solution in the spacecraft design because of its simplicity, low cost, power efficiency, and reliability. A new method of pointing a spacecraft towards the target in the inertial reference frame using only magnetic devices is proposed-more specifically, utilizing only magnetometers and magnetic torquers (MTQs) to perform the functions of attitude sensors and actuators, respectively. Simulation was performed and results are presented illustrating the performance of the proposed algorithm under the conditions of measurement uncertainties and environmental disturbances.

Slowly varying bias of magnetometer based on orbit geometry

A new method is proposed to correct the induced magnetic disturbance using a set of measured magnetic field data from the three-axis-magnetometer on orbit. The magnetic field correction is based on the orbit geometry and its algorithm excludes the geomagnetic field model. The considered satellite is supposed to operate in the Earth-point mode and the Sun-point mode in accordance with the mission requirements. Particularly, the magnetic field correction is performed via static estimation based on an average method and dynamic estimation with an estimation law. Besides, the usefulness of the proposed method is investigated throughout both the simulation and the real telemetry data of KOMPSAT-1.

Design Criteria and Cluster Configuration Improvement of Single Gimbal Control Moment Gyros for Satellite

Nowadays, CMG(Control Moment Gyros) becomes one of the essential actuators for satellite attitude control. The method to define the key requirements of CMG is suggested to avoid CMG`s singularity problem for the limited envelope of angular momentum of 2H. Furthermore, the analysis and simulation are carried out to provide a necessary guideline when three CMGs are used for spacecraft control purpose. An improved configuration of redundant four CMG cluster, slightly different from the conventional configuration, is proposed not only to avoid the CMG singularity problem, but to improve agility about roll or pitch-axis.

On-orbit magnetic field correction of magnetometers and geometry-based attitude determination for small satellites

In this paper, the use of a magnetometer mounted on small satellites in a Low Earth Orbit (LEO) is considered for the purpose of the magnetic field correction of magnetometers and the geometry-based attitude determination. The induced magnetic disturbance is estimated using a set of the measured field data from a single magnetometer on orbit. Furthermore, the attitude determination algorithm, which has a simple structure without any filter dynamics, uses the magnetometer and sun sensor data.

Development of 100Nm-class Control Moment Gyroscopes for Industrial Applications

The control moment gyroscope(CMG) which is well known as an effective high-torque-generating device is applicable to space vehicles, airplanes, ships, automobiles, robotics, etc. for attitude stabilization and maneuver. This paper deals with the overall details of 100Nm-class CMG development for various industrial applications, and provides the activities and results associated with the CMG system-level requirement analysis, the motor subsystem design/manufacturing/integration, the construction of ground support equipment, and the performance test and evaluation. The performance test reveals that the CMG generates the torque output more than 120Nm in as-designed operation of spin motor and gimbal motor.

An Experimental Study on Micro-vibration Measurement Methods of a Reaction Wheel

A reaction wheel assembly(RWA) is the largest disturbance source that can induce high frequency micro-vibration on an optical payload of satellites. To ensure a tight pointing-stability budget of satellites, the RWA disturbance effect on spacecraft should be accurately analyzed and evaluated for whole design phases. For this purpose, the micro-vibration disturbance of RWA should be precisely measured. In the present study, two measurement methods on RWA micro-vibration disturbances are compared and investigated. One is a free run-down speed test and the other is a constant speed test. The micro-vibration data measured by the two methods are analyzed in terms of spectrum characteristics, static and dynamic imbalance values, and root sum square(RSS) values. The analysis results show that both methods can measure very similar results in time and frequency domains and that the free run-down speed method is more adequate in respects to wheel friction modeling, noise rejection of imbalance and RSS peak evaluation.

Error Analysis of Reaction Wheel Speed Detection Methods

Reaction wheel is one of the actuators for spacecraft attitude control, which generates torque by changing an inertial rotor speed inside of the wheel. In order to generate required torque accurately and estimate an accurate angular momentum, wheel speed should be measured as close to the actual speed as possible. In this study, two conventional speed detection methods for high speed motor with digital tacho pulse (Elapsed-time method and Pulse-count method) and their resolutions are analyzed. For satellite attitude maneuvering and control, reaction wheel shall be operated in bi directional and low speed operation is sometimes needed for emergency case. Thus the bias error at low speed with constant acceleration (or deceleration) is also analysed. As a result, the speed detection error of elapsed-time method is largely influenced upon the high-speed clock frequency at high speed and largely effected on the number of tacho pulses used in elapsed time calculation at low speed, respectively.

Geomagnetic field from KOMPSAT-1 magnetometer measurements

We estimated the geomagnetic field from KOMPSAT-1 Three-Axis Magnetometer (TAM) measurements that control the attitude. Initial processing involved transforming the TAMs telemetry data from EarthCentered Inertial Frame to Earth-Centered Earth-Fixed Frame and then to spherical coordinates. The magnetic field of the satellite body produces symmetric signature in the ascending and descending data sets and thus can be readily removed. To further eliminate the induced external fields and noise related to solar activity, spectral correlation filtering was applied for extracting these dynamic components. The ascending and descending data were then spectrally reconstructed to estimate the total magnetic field with minimum track-line noise. The KOMPSAT-1 field correlates with the Ørsted and IGRF2000 models at coefficients of 0.97 and 0.96, respectively. The spherical harmonic coefficients were then calculated from the KOMPSAT-1 model to degree and order of 19 by conjugate gradient inversion method. The KOMPSAT-1’s coefficients are strongly coherent with the Ørsted, CHAMP, and IGRF2000 coefficients through degree 7.