Engineering Test Satellite VI Research Articles

Multi-input Multi-output System Identification Using Impulse Responses

This paper presents a new algorithm for multi-input multi-output (MIMO) system identification in the time domain using impulse responses. The algorithm is suitable for the on-orbit system identification of spacecraft using the responses to thruster impulse inputs measured by typical satellite on-board sensors. The Eigensystem Realization Algorithm (ERA) realizes a multi-input multi-output (MIMO) system using asynchronous impulse responses in the time domain. Our new method identifies the input and output matrices of a MIMO collocated system by applying a recursive least-squares iteration scheme to refine the matrices obtained from conventional ERA. In this manner, the input matrix is considered to be constructed by the mode shape vectors and the actuator sensitivity matrix. A numerical simulation of an actual spacecraft, the Engineering Test Satellite-VI (ETS-VI), is performed to verify the algorithm. The nominal dynamics model of ETS-VI, which has six rigid body modes and fourteen elastic modes due to large flexible solar panels, is excited by six body-mounted thrusters, and the translational velocities and attitude rates are measured simultaneously. Our method successfully identifies all of the fourteen natural frequencies, damping ratios and mode shape vectors, confirming its validity.

加速度計出力を用いた柔軟構造衛星のシステム同定

This paper presents an application of single-input multi-output (SIMO) system identification in the time domain using accelerometer outputs that are typical of satellite on-board sensors.The algorithm is suitable for the on-orbit system identification of six degrees-of-freedom spacecraft motions using the responses to thruster impulse inputs. Our new method identifies the input and output matrices of a SIMO collocated system by applying a recursive least-squares iteration scheme to refine the matrices obtained from conventional ERA. In a numerical simulation of an actual spacecraft to verify the algorithm, a mathematical model of the Engineering Test Satellite-VI (ETS-VI) is excited by a body-mounted thruster, and the translational and rotational accelerometers are measured simultaneously. Our method successfully identifies all of the natural frequencies, damping ratios, and mode shape vectors except for pitch rotational motion that has low sensitivity, confirming its validity.

Multi-input Multi-output System Identification Using Impulse Responses

This paper presents a new algorithm for a multi-input multi-output (MIMO) system identification in the time domain using impulse responses. Eigensystem Realization Algorithm (ERA) realizes a single-input multi-output (SIMO) system using single impulse input, therefore we modify the method for identifying the system matrix of MIMO system. Moreover, a recursive least-square iteration scheme is developed to refine input and output matrices of the system. In this manner, the input matrix is considered to be constructed by the mode shapes and actuator sensitivity matrix. Numerical simulation for an actual spacecraft-the Engineering Test Satellite-VI (ETS-VI) is performed to verify the new algorithm. In the example, the nominal dynamics model of ETS-VI, which has six rigid body modes and fourteen elastic modes derived from large flexible solar panels, is excited by six body-mounted thrusters each, and three translational velocities and three attitude rates are assumed to be measured simultaneously. The proposed method identifies all of fourteen natural frequencies, damping ratios and mode shapes. The results of the numerical example show the validity of the new identification scheme.

Overview of the Attitude and Orbit Control Systems (AOCS) Developed for NASDA/JAXA Engineering Test and Applications Satellites

Dating back to the middle of the early 1990's, the Engineering Test Satellite VI (ETS-VI) was developed with a strong intention to establish the AOCS for the applications satellites. Although injection of the ETS-VI into the GTO was failed, the developed AOSC had proven its performance and at the same time revealed some technical issues to be solved for the real applications. After the ETS-VI, the ETS-VII was launched and completed the intended missions on the rendezvous and docking as well as the robotics experiments. To follow these tests satellites, the Advanced Earth Observation Satellite I and II were launched and the AOCS of these had proven its performance in spite of the fact that the satellites missions were terminated before the end of life because of the failures in the electric power systems. Now a small satellite called Micro-LabSat-1 is in operation to continue the AOCS experiments with some ambitious missions. This overview will cover the history and technical issues of the efforts made by NASDA (now JAXA) and the related industries. This will disclose the reality of the control technologies of Japan in these two decades.

Ground-to-satellite narrow-laser-beam pointing by use of a backscattered image

A technique for transmitting a narrow laser beam from a ground station to a satellite has been developed. The principle of pointing a laser beam to a distant target in a scattering medium by use of a backscattered laser beam image is described. We calculated the intensity distribution of the image by using a typical model of atmospheric coefficients. The method was applied to transmit a laser beam from a ground station to Engineering Test Satellite-VI. The accuracy of pointing the laser beam to the satellite was approximately 10 murad in this experiment.

In-orbit measurements of short term attitude and vibrational environment on the Engineering Test Satellite VI using laser communication equipment

Morio ToyoshimaNational Space Development Agency ofJapan2-2-1 SengenTsukuba, Ibaraki 305-8505JapanE-mail: toyoshima.morio@nasda.go.jpKenichi ArakiCommunications Research Laboratory4-2-1 Nukui-KitamachiKoganei, Tokyo 184-8795JapanE-mail: araki@crl.go.jpAbstract. Angular microvibrations of platform jitter on a three-axisattitude-stabilized satellite are measured in space using an onboard la-ser communication terminal. Stable optical tracking control allows themeasurement of relative angular variations from the reference line ofsight of the optical link. The tracking accuracy in this measurement isless than 1 mrad rms. The angular variation and the drift rate on thesatellite are measured at a sampling rate of 500 Hz, and these data aredownlinked to a ground station via a satellite-to-ground optical commu-nication link. The power spectral density of the satellite microvibration iscalculated by a Fourier transform analysis. The frequency spectrum ofthe gimbal mirror’s angular variation is also obtained when the coarsetracking control is operational. The rms value agrees with the residualcoarse tracking error estimated before launch of the satellite. These re-sults will be useful for the future design of a tracking control loop foroptical communication systems.

On-orbit system identification experiments on Engineering Test Satellite-VI

On-orbit system identification experiments were performed using the Engineering Test Satellite-VI (ETS-VI), launched in 1994. The purpose of the experiments is to check the validity of a mathematical model for ETS-VI which was previously obtained by ground experiments. Two kinds of system identification schemes are utilized. One is a traditional scheme based on polynomial black-box models such as the prediction error method, and the other is a subspace method based on state-space models. The purpose of this paper is to compare these system identification schemes using experimental data. It will be shown that the subspace method proves to be promising for the system identification of large space structures typified by ETS-VI.

Real-time trajectory estimation experiments with the COMETS spacecraft

NASDA has developed a new trajectory estimation program capable of estimation in real-time. NASDA conducted experiments to verify the performance of the program with Engineering Test Satellite VI(ETS-VI) in 1994, as well as with Communications and Broadcasting Engineering Test satellite (COMETS), in March and May 1998. This paper presents an overview of the real-time trajectory estimation experiments with COMETS.

Interference suppression by adaptive beamforming of satellite‐borne phased‐array antennas

This paper describes adaptive beam formation of a satellite-borne antenna for suppressing the uplink interference in satellite communication. The S-band phased-array antenna installed on the Engineering Test Satellite-VI (ETS-VI) was used in this experiment. The beamforming network in the satellite was controlled from the earth through a TT&C link. It was found that the installed antenna could form a designed beam pattern. It was also found that the actual interference on the S-band intersatellite link could be effectively suppressed. © 1998 Scripta Technica, Electron Comm Jpn Pt 1, 81(11): 58–66, 1998

Storm-time magnetic field variations observed by the ETS-VI satellite

To study the ring current structure in the inner magnetosphere, we have statistically examined the magnetic field data acquired by ETS-VI (the Engineering Test Satellite-VI). During a magnetic storm, the Dst index shows a rapid recovery of its amplitude for about 9 hours on average after the main phase and a subsequent long-lasting slow recovery. We have investigated this “two-step recovery” of the Dst index by obtaining magnetic field vectors and calculating the current structure in the inner magnetosphere for each magnetic storm phase determined by the Dst index. From this study, following results are obtained: (1) Throughout the storm-time, disturbed magnetic fields exhibit clear day-night asymmetry with strong peak in the nightside. (2) During the main phase, southward perturbed field components have a relative bump in the nightside region between ~2000 and ~0400 MLT and between ~4.0 and 6.4 RE (geocentric distances in Earth radii). (3) The initial rapid recovery of the Dst index is considerably influenced by the nightside currents flowing between ~1800 and ~0600 MLT and between 5.6 and 7.2 RE. These currents are thought to be mainly composed of the particles that escape the magnetosphere on the duskside flank, which are simulated in particle tracing in a realistic magnetosphere.

On-orbit attitude control experiments using ETS-VI by [formula omitted] control and two-degree-of-freedom control

The engineering Test Satellite VI (ETS-VI) is a three-axis-stabilized satellite launched by NASDA in 1994. On-orbit experiments, which consist of identification and attitude-control experiments, were planned as one of the ETS-VI on-board experimental programs, and were carried out in 1995. This experimental program was called ‘FLEX’ (FLexible EXperiments). The purpose of FLEX was to establish fundamental attitude-controller design methods for a flexible-structure satellite. In this paper, attitude controller design methods for FLEX are discussed, and some results of the attitude-control experiments are presented. The attitude controllers were designed using an H ∞ control method and a two-degree-of-freedom control method. H ∞ controllers were derived using the specified stability degree method. On the other hand, conventional two-degree-of-freedom control design methods could not be applied for the FLEX, because the program memory area was restricted to about 2.6 Kbytes. In this paper, feedforward control design methods which never increase the order of the controller were proposed and applied. Experimental results showed a rapid tracking response and high disturbance-attenuation performance, which have been anticipated by simulations on the ground. Noting that the on-orbit experiments presented here have not previously been carried out, the authors believe these results will be utilized for attitude controller design in the future.

ULF pulsations observed by the ETS-VI satellite: Substorm associated azimuthal Pc 4 pulsations on the nightside

The magnetic field data from the Engineering Test Satellite -VI (ETS-VI) have been analyzed to investigate the occurrence distributions of pulsations in Pc 3–5 frequency ranges in the magnetosphere. The observation of ETS-VI covered the invariant latitude (ILAT) range of 64.5°−69°ILAT near the geomagnetic equator (−10°−20° magnetic latitude) at all magnetic local time (MLT). Magnetic pulsations were selected by the Fast Fourier Transform method and checked by visual scanning if they have continuous waveforms. From the occurrence distributions of pulsations, we have found distinctive features in the following pulsations: (1) azimuthal Pc 5 pulsation; (2) azimuthal Pc 3 pulsation; (3) radial Pc 4 pulsation on the dayside; (4) azimuthal Pc 4 pulsations on the nightside. In respect of the first three types of pulsations (i.e., the azimuthal Pc 5 pulsation, the azimuthal Pc 3 pulsation, and the radial Pc 4 pulsation on the dayside), the results presented in this study confirm the previous results obtained by other satellite observations. The azimuthal Pc 4 pulsations on the nightside were observed in continuous waveforms lasting for about 10 minutes. Although the azimuthal Pc 4 pulsations on the nightside start at almost the same time as substorm onsets, they are different from Pi 2 pulsations in the magnetosphere. They are observed frequently in the MLT range of 23-04MLT with an occurrence peak at 01–02MLT. We suggest that the azimuthal Pc 4 pulsations on the nightside are excited through coupling to the fast mode Alfvén waves which were launched at substorm onset.

Flight data from solar cell monitor on Engineering Test Satellite-VI and ground test data

This paper describes the performance of NASDA's various space solar cells by comparing flight data and ground test data. We first present the flight data from the Solar Cell Monitor on Engineering Test Satellite-VI. We then describe the ground irradiation tests of solar cells performed in collaboration with the Japan Atomic Energy Research Institute. Then discuss the radiation effects on solar cells, and present other flight data of solar cells and optical data of the coverglass from other satellites in addition. Finally, we describe the future plan to obtain flight data on various solar cells.

On-orbit measurement of phased arrays in satellites by rotating element electric field vector method

This paper discusses the measurement of the excitation amplitude and phase of each element antenna of phased arrays in satellite orbit. In a phased array, the excitation amplitude and the phase of each element antenna deviate from the desired values, due to the nearby satellite structure, the error of attachment to the satellite, and thermal deformation in the orbit. The errors in the amplitude and the phase must be known for beam scanning and the synthesis of various antenna patterns. The rotating element electric field vector method is useful in measuring the amplitude and the phase of phased arrays. In this method, the amplitude and the phase of the element antenna can be determined simply by varying the phase of the element antenna by a phase-shifter and measuring the amplitude change of the electric field as a result of composition by the array. This is suited to the measurement of phased arrays in satellite orbit, where direct measurement of the phase is difficult. In this study, the above method is used to measure the excitation amplitude and phase of the phased array installed on Engineering Test Satellite VI in orbit. Based on the measured phase, the phase is corrected to realize the designed value. The accuracy of the beam directivity is examined and the validity of the measurements is evaluated. © 1998 Scripta Technica. Electron Comm Jpn Pt 1, 81(1): 1–13, 1998

Satellite interference location system using on‐board multibeam antenna

In satellite communication using a high-gain on-board antenna, there may be a problem with up-link interference, which depends on the directivity of the on-board antenna or the position of the satellite on the orbit. In order to cope with radio frequency interference, it is important to locate (specify) the position of the interference wave radiation source, as seen from the position of the satellite. Technical difficulties are that the characteristics of the interference wave are unknown and there is a limit in the installed weight, making it difficult to add dedicated on-board equipment for location. In a conventional location system, the wave is received simultaneously by two or more satellites. Correlation processing is applied on the ground to the interference waves in order to estimate the location of the radiating source based on the frequency and time differences. In this paper, the following location system is discussed. Angle measurement, based on the received amplitude comparison of multiple beams, is applied to the interference, overlapping with the communication signals received by the multibeam satellite antenna where the interference power ratio between the beams is measured on the ground. A feature of the system is that the source can be located using only a single satellite and no additional on-board equipment. An experimental system was constructed using the S-band multibeam phased array antenna on the experimental Engineering Test Satellite VI. The results of the experiment, for an interference source with known position and for a radiation source with the properties of a radar wave with an unknown position, are presented, together with a discussion of the estimation error and of its improvement. © 1997 Scripta Technica, Inc. Electron Comm Jpn Pt 1, 80(11): 22–33, 1997

On-Orbit Identification Experiments by Engineering Test Satellite-VI

Abstract On-orbit system identification experiments have been performed using Engineering Test Satellite-VI (ETS-VΙ) launched in 1994. The purpose of the experiments is to check the validity of a mathematical model for ETS-VΙ which was previously obtained by ground experiments. Two kinds of system identification algorithms are utilized. One is a prediction error method and the other is a subspace-based method.The purpose of this paper is to compare these system identification algorithms using experimental data. It will be shown that the subspace method is promising for system identification of large space structures.

Antenna pattern measurement of S-band active phased array on ETS-VI

Of the evaluation methods for antenna patterns of a satellite-borne antenna on an orbit, measurement using the spin of the satellite is effective for high-function antennas such as an antenna with a wide beam, a phased array, and a multibeam antenna because a wide-angle pattern can be acquired. By actually using the spin of the Engineering Test Satellite VI (ETS-VI), antenna patterns of a phased array for S-band intersatellite communication (SIC) were measured. It was not possible to carry out the antenna pattern measurement of the SIC phased array with a beam width of approximately 5° by using the attitude bias control of the satellite because the angle offset limit in the attitude bias of the satellite is only 2° in the three-axis attitude control condition of the ETS-VI. In addition, the attitude bias has an operational problem. By using the spin of the satellite in the measurement, a wide-angle antenna pattern was acquired. In an actual spin of the ETS-VI satellite, a small amount of attitude bias is added. Since the satellite's orbit is elliptical, the location of the ground station as seen from the satellite changes. Errors in the antenna pattern measurement due to these factors are also investigated. Also, the evaluation of the G/T using the antenna pattern and the noise power is described. © 1997 Scripta Technica, Inc. Electron Comm Jpn Pt 1, 80 (4): 29–38, 1996

On-Orbit Experiments of Frequency-Shaped H.INF. Control for Flexible Space Structure.

Flexible control experiments have been performed as one type of on-orbit experiment for the Engineering Test Satellite VI (ETS-VI) which was launched in August, 1994. The aim of the flexible control experiments is the feasibility study of new attitude control schemes for future flexible space structures. We have designed two types of controllers. One is a controller designed according to the Low Authority Control (LAC)/High Authority Control (HAC) scheme. The purpose of LAC is damping augmentation of structural vibration modes, while that of HAC is achievement of high control performance. The other controller is a HAC-only controller: this HAC controller is designed for high-performance attitude control of a rigid body, as well as for damping augmentation of the vibration modes. The HAC controllers in both cases are designed using H∞ control theory with the frequency-shaping technique. These control schemes have been tested on orbit by ETS-VI. The experimental results have been analyzed in both time and frequency domains. It is shown from the analysis that both control schemes can be accurately realized on orbit and perform well with respect to damping augmentation and attitude control.

Evaluation of system noise temperature of S-band intersatellite communications equipment onETS-VI satellite

The intersatellite communication antenna installed on a data relay satellite has a wide antenna beam scan range so that the existence of the background brightness temperature and the artificial interference noise in the antenna pointing direction affect the receiving system noise temperature. In the data relay satellite system, the receiving performance of the intersatellite link significantly influences the channel quality of the entire system. Hence, it is important to evaluate the antenna pointing direction dependence of the system noise temperature. In regard to the S-band intersatellite communications, a phased array antenna installed on the engineering test satellite VI (ETS-VI), the noise temperature was evaluated with attention to the characteristics of both spin condition and the three-axis control condition. The distribution of the system noise temperature agrees well with the results predicted by the background brightness temperature. On the other hand, distribution is found to be affected by the artificial noise dependent on the pointing direction. © 1997 Scripta Technica, Inc. Electron Comm Jpn Pt 1, 80 (1): 62–69, 1997

ETS-VI On-Orbit System Identification Experiments.

System identification and attitude control experiments have been performed as part of a series of bus experiments on the Engineering Test Satellite-VI, which was launched by an H-II rocket in August 1994. The aims of the experiments were to establish a pre-launch system dynamics modeling method, to evaluate typical on-orbit modal parameter identification methods and to develop robust and precise attitude controller design technology for future large flexible spacecraft and large space structures. In the on-orbit identification experiments, the central body of the satellite was excited by gas jet thrusters and the measured attitude and paddle accelerometer signals were downlinked by telemetry. In this paper, the identified parameters are compared with those of the pre-launch modeling and the accuracy of the pre-launch model is evaluated.