Thruster Noise Research Articles

Drag-free control design and in-orbit validation of TianQin-1 satellite

Drag-free control (DFC) is one of the key technologies for space-borne gravitational wave detectors. For the TianQin (TQ) project, non-gravitational disturbances on the spacecraft must be suppressed by DFC. TianQin-1 (TQ-1) is the first satellite for technology verification, whose main goal is to verify the disturbances reduction technologies. In this paper, the DFC requirement and preliminary analysis for the TQ project are introduced, and then the design and the experimental validation with the TQ-1 satellite is presented. In-orbit results show a successful validation of DFC and the residual acceleration of TQ-1 satellite has been suppressed to at 10 mHz versus thruster noise.

Drag-free design based on Embedded Model Control for TianQin project

With the ability to detect low-frequency gravitational waves (GWs), space-borne detectors will play an important role in exploring the universe in the future. The TianQin project proposed in China participates in this challenge by aiming to detect millihertz GWs. The TianQin GW detector consists of three spacecraft forming a triangular constellation. Each spacecraft carries a pair of test masses (TM) which, inside shielding cages, are left to free fall along the local geodesic. In this way, a pair of TMs on two separate spacecraft can be each other aligned to become the gravitational references of the GW detector, made by an inter-satellite laser interferometer. Each free-falling TM is limited by parasitic forces to be appropriately bounded. The relevant acceleration bound has been fixed to 10−15 m/s2/Hz within the TianQin measurement bandwidth (MBW) ranging from 0.1mHz to 0.1Hz. In turn, TM to cage fluctuations must be kept below 4nm/Hz to limit the stiffness coupling with the spacecraft displacement caused by non-gravitational disturbances, such as solar radiations and the thruster noise. Suppression of such disturbances calls for a challenging drag-free control technology, since the center of mass (CoM) of a single spacecraft cannot track the separated CoMs of two TMs simultaneously, and consequently tracking must be limited to three degrees of freedom (DoF): two non-orthogonal sensitive axes (one for each TM) and the perpendicular direction to their plane. Control design and simulated tests of this paper will be restricted to the drag-free control along the sensitive axes. The remaining nine DoFs of the two TMs (position and attitude) are controlled by electrostatic suspensions, not to be treated here, but accounted for in the simulated trials. The design of the two-DoF drag-free control relies on a model-based control methodology, the Embedded Model Control (EMC), capable of predicting and suppressing unknown disturbances within the required bandwidth and of decoupling the TM dynamics along the non-orthogonal sensitive axes. The paper starts with the nonlinear model of the TM to cage dynamics, followed by the relevant EMC design, restricted to the sensitive axes. Numerical simulations are employed to validate closed-loop performance and robust stability. Simulated results show that the residual TM to cage fluctuations can be kept below 3nm/Hz, which leaves a margin within the required spectral bound. The EMC methodology discussed in the paper can provide a reference for future developments and implementations.

First result of orbit verification of Taiji-1 hall micro thruster

The Hall Micro Thrusters (HMTs) use cold gas or accelerated plasma dual mode to provide ultra-precise spacecraft altitude control. They were operated in space for the first time as part of the demonstration payloads on Chinese Academy of Science’s (CASs) Taiji-1 spacecraft since September 2019. Hall Micro Thruster Assemblies (HMTAs) were the actuators in drag-free control, and will compensate the nonconservative force for gravity wave observatories. The HMTAs meet the requirements of operating at 5–100 [Formula: see text] N of thrust with 0.7 [Formula: see text] N resolution and [Formula: see text]0.6 [Formula: see text] N/Hz[Formula: see text] (0.01–1 Hz) noise to deliver the nanometer-level precision control as fast as 30 ms measured by Gravitational Reference Sensor (GRS). A transfer function model in z-domain was fit and used to filter HMTs cathode voltage to predict GRSs thrust noise response. Simulations of a single or dual-frequency disturbance and the corresponding compensation demonstrated that HMTAs could deliver the required thrust profile expected. The capability to meet the requirements of thruster noise in drag-free control is critical for future missions because the acceleration noise on test mass directly relates to the gravity wave signa l. Preliminary in-orbit verification of Taiji-1 has showed HMTAs’ great potential in future, and the data in the experiments are presented in this paper.

Astrodynamical middle-frequency interferometric gravitational wave observatory AMIGO: Mission concept and orbit design

AMIGO is a first-generation Astrodynamical Middle-frequency Interferometric Gravitational Wave (GW) Observatory. The scientific goals of AMIGO are to bridge the spectra gap between first-generation high-frequency and low-frequency GW sensitivities: to detect intermediate mass BH coalescence; to detect inspiral phase and predict time of binary black hole coalescences together with binary neutron star & black hole-neutron star coalescences for ground interferometers; to detect compact binary inspirals for studying stellar evolution and galactic population. The mission concept is to use time delay interferometry (TDI) for a nearly triangular formation of three drag-free spacecraft with nominal arm length 10,000 km, emitting laser power 2–10 W and telescope diameter 300–500 mm. The design GW sensitivity in the middle frequency band is [Formula: see text] Hz[Formula: see text]. Both geocentric and heliocentric orbit formations are considered. All options have LISA-like formations, that is, the triangular formation is [Formula: see text] inclined to the orbit plane. For all solar orbit options of AMIGO, the first-generation TDI satisfies the laser frequency-noise suppression requirement. We also investigate for each option of orbits under study, whether constant equal-arm implementation is feasible. For the solar-orbit options, the acceleration to maintain the formation can be designed to be less than 15 nm/s2with the thruster requirement in the 15 [Formula: see text]N range. AMIGO would be a good place to test the feasibility of the constant equal-arm option. Fuel requirement, thruster noise requirement and test mass acceleration actuation requirement are briefly considered. From the orbit study, the solar orbit option is the mission orbit preference. We study the deployment for this orbit option. After a last-stage launch from 300 km Low Earth Orbit (LEO), each S/C’s maneuver to an appropriate 2-degree-behind-the-Earth AMIGO formation in 95 days requires only a [Formula: see text]v of about 80 m/s.

LISA Pathfinder micronewton cold gas thrusters: In-flight characterization

The LISA Pathfinder (LPF) mission has demonstrated the ability to limit and measure the fluctuations in acceleration between two free falling test masses down to sub-femto-g levels. One of the key elements to achieve such a level of residual acceleration is the drag free control. In this scheme the spacecraft is used as a shield against any external disturbances by adjusting its relative position to a reference test mass. The actuators used to move the spacecraft are cold gas micropropulsion thrusters. In this paper, we report in-flight characterization of these thrusters in term of noise and artefacts during science operations using all the metrology capabilities of LISA Pathfinder. Using the LISA Pathfinder test masses as an inertial reference frame, an average thruster noise of ∼0.17 μN/Hz is observed and decomposed into a common (coherent) and an uncorrelated component. The very low noise and stability of the onboard metrology system associated with the quietness of the space environment allowed the measurement of the thruster noise down to ∼20 μHz, more than an order of magnitude below any ground measurement. Spectral lines were observed around ∼1.5 mHz and its harmonics and around 55 and 70 mHz. They are associated with the cold gas system itself and possibly to a clock synchronization issue. The thruster noise-floor exhibits an excess of ∼70% compared to characterization that have been made on ground on a single unit and without the feeding system. However this small excess has no impact on the LPF mission performance and is compatible with the noise budget for the upcoming LISA gravitational wave observatory. Over the whole mission, nominal, and extension, the thrusters showed remarkable stability for both the science operations and the different maneuvers necessary to maintain LPF on its orbit around L1. It is therefore concluded that a similar cold gas system would be a viable propulsion system for the future LISA mission.

Using an Electric Thruster as an Acoustic Transducer

The acoustic noise produced by thrusters is often viewed as undesirable and frequent efforts have been made to minimize this noise. This research investigates transmitting information by controlling the pulse width Modulation (PWM) noise produced by the thrusters. This research investigates using the thruster noise as the transmitter in a one-way acoustic data link. A 250 bit/s acoustic underwater communication link using a thruster as the signaling transducer was demonstrated at a range of 10m. A possible application as a navigation aid for docking is suggested.

Attitude reconstruction for theGaiaspacecraft

Context. The Gaia mission will produce a stereoscopic map of the Milky Way by collecting highly accurate positions, parallaxes and proper motions for about 1 billion stars. These astrometric parameters will be determined through the astrometric core solution of the Gaia mission which will employ about 10$^{8}$ primary sources (a subset of the observed sources with the best astrometric properties). The attitude of the spacecraft is reconstructed as part of the astrometric solution and provides the reference frame relative to which the astrometric measurements are obtained. This implies extreme demands on the accuracy of the attitude reconstruction. Aims: This paper presents an analysis of the capabilities and limitations of the Gaia attitude reconstruction, focusing on the effects on the astrometry of bright (V {lsim} 11) stars and the implications of employing cubic B-splines in the modelling of the attitude measurements. Methods: We simulate the attitude of the spacecraft using a realistic and very detailed model that considers not only physical effects but also technical aspects like the control system and thruster noise. We include the effect of shorter integration times for the bright stars on the effective attitude and we estimate the residual modelling noise in the reconstruction of the attitude. Results: We provide an analysis of the dependency of the residual modelling noise in the reconstructed attitude with respect to the following parameters: integration time, B-spline knot interval, micro-propulsion system noise, and number of observations per second. Conclusions: The final noise in the attitude reconstruction for Gaia is estimated to be {ap}20 {$μ$}as, and the main source will be the micro-propulsion system. However its effect on the astrometric performance will be limited, adding up to 7 {$μ$}as rms to the parallax uncertainties. This is larger than the 4 {$μ$}as from previous estimations and would affect the performance for the brightest (V {lsim} 11) stars.

Measurement of spray-on damping effectiveness and application to bow thruster noise on ships

Measurement of spray-on damping effectiveness and application to bow thruster noise on ships

LISA satellite formation control

The joint ESA–NASA Laser Interferometer Space Antenna (LISA) mission consists of a triangular formation of three satellites aiming at detecting gravitational waves. In linear approximation the LISA satellites describe a circle around a reference point, maintaining a fixed position with respect to each other. The reference point, the center of the triangle, orbits the Sun in a circular orbit, trailing the Earth at twenty degrees. In reality the distance between the satellites will vary by about one to two percent and the angle between the arms of the antenna will vary by about 0.5° over the course of one year for the nominal LISA satellite configuration. For measurement accuracy it is desirable that the pointing offset of the telescopes be kept small. This makes it necessary to actuate the telescopes or to control the formation. It was assumed that the LISA satellites are equipped with six μN engines that would allow to keep the two cubical proof masses within each satellite in almost perfect free fall. It was found that control forces up to about 700 μN are required for maintaining the absolute triangular LISA formation, leading to unacceptable excursions of the proof masses from free fall. However, these forces compensate predominantly very low frequency variations of the arm lengths and angles of the triangle, which are then to be compensated by the telescope actuators. The variations are outside the aimed LISA measurement bandwidth (10 −4–0.1 Hz). In addition, the effect of thruster noise, orbit determination errors and orbit injection errors was examined. The effect of these error sources on the arm lengths and orientation angles between the LISA satellites was assessed both in open loop and in closed loop, where the closed loop was based on a proportional-derivative (PD) controller. It was found that orbit determination errors of the order of a few km in position and a few mm/s in velocity lead to negligible closed loop control forces. In addition, orbit injection errors of the order of 10 km in position and 1 cm/s in velocity could be eliminated after a few days when thrusting at levels of 200–500 μN. Even for a high estimated level of thruster noise below 10 −4 Hz (equivalent to about 2 μN random thrusts once an hour), the closed loop controller effectively reduced the effect on the angles of the triangle to below 30 nrad.

Detection rate of positions of an unmanned submersible for underwater exploration

The acoustic position measuring systems for the unmanned research submersible are used in severe environment conditions : thruster noise and disturbances, signal reflections, etc. In this paper the detection probability of position based on statistical decision theory are examined using stochastic model for the acoustic channel, and some useful results are extracted in order to design high reliable system even though in such severe environmental conditions : 1) time window is effective, but do not need in case of high signal level.2) redundancy of one hydrophone is equivalent to 1 dB in S/N.3) threshold level should be higher than half of received signal.

The Detection Probability of Positions and The Position Control of an Unmanned Submersible for Underwater Exploration

The acoustic position sensing system has been used for the unmanned submersible, however position sensing is affected by environmental factors such as scattering in the sea, ambient noise, thruster noise and others. In this paper the idea of detection probability of positions is introduced and its description equation is derived from detection theory and discussed in terms of position sensing process and designing position sensing system for the unmanned submersible. The validation of the simulation model of motion and control of the vessel under consideration of position sensing process is examined in comparison with OSR-V system's experiments in the sea, and relationships between detection probability and motion and control of the vessel are examined by its simulation.

Use of Ballistic Arcs in Low Thrust Navigation

A technique is presented for improving navigation accuracy in the Solar Electric Encke Slow Flyby Mission proposed to encounter the comet Encke during its 1980 apparition. The effect of the dominant navigation error source, the high-level thruster noise, is reduced through the introduction of a ballistic coast arc for the purpose of orbit determination enhancement. The placement and duration of the arc is investigated with respect to its impact on final delivery error and on control effort required for trajectory correction. The relative contributions of the various individual navigation error sources to the total delivery error and the relative effectiveness of various data type combinations are analyzed and compared for a navigation strategy which includes a coast arc and one which does not. Also examined are the changes in delivery accuracy and control effort as a result of the time lag between the state estimation process and the thrust control program update.

Terminal Navigation Analysis for the 1980 Comet Encke Slow Flyby Mission

The initial results of a terminal navigation analysis for the proposed 1980 solar electric slow flyby mission to the comet Encke are presented. The navigation technique employs onboard optical measurements with the scientific television camera, groundbased observations of the spacecraft and comet, and groundbased orbit determination and thrust vector update computation. The knowledge and delivery accuracies of the spacecraft are evaluated as a function of the important parameters affecting the terminal navigation. These include optical measurement accuracy, thruster noise level, duration of the planned terminal coast period, comet ephemeris uncertainty, guidance initiation time, guidance update frequency, and optical data rate.