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On-ground Actuator Calibration for ISA-BepiColombo
Abstract
The key role of the Italian Spring Accelerometer (ISA) in the radio science measurements of the ESA BepiColombo mission to Mercury is to remove, a posteriori, the non-gravitational accelerations acting on the Mercury Planetary Orbiter (MPO) due to the very strong radiation environment around Mercury. This paper is devoted to describe the on-ground actuator calibration procedure along with the facility assembled to carry it out. Such a calibration is necessary to guarantee the accelerometer performance and hence to reach the very ambitious objectives of the Radio Science Experiment (RSE) of the ESA mission.
... Afterwards, the actuators are excited by a periodical signal at the same frequency, in order to generate a periodic displacement of the mass with the same amplitude. Further details on this topic can be found in [11,12]. ...
High sensitive triaxial accelerometers are used in several space missions to measure the non-gravitational accelerations acting on the spacecraft. Among these, the capacitive accelerometers developed for ESA missions JUICE and BepiColombo were designed to measure accelerations of the order of 3∙10 ⁻⁶ m/s ² with an accuracy level of 300 ppm in the frequency range (3·10 ⁻⁵ - 0.1) Hz. Despite the signal to be be measured is of the same order of magnitude of the seismic noise on the earth, an accurate on-ground calibration is needed.
The facility set-up at INRIM to this purpose is based on a simple principle: the base of the accelerometer is dinamically tilted by an angle α so that the sensor undergoes a component of the gravitational acceleration g proportional to angle α. In practice, several issues have to be addressed by the calibration facility, such as the seismic noise limiting the signal-to-noise ratio, the generation and the accurate measurement of the tilt angle. Furthermore, the calibration was performed taking into account different on-flight conditions such as different operating temperatures and possible deformation of the accelerometers during the launch. The experimental set-up and the calibration procedure are described in the paper. The measurement results and the uncertainty budget show that a relative accuracy of 240 ppm has been achieved.
... The accelerometers, or the inertial measurement unit (IMU), is calibrated for very low accelerations with a tilt table platform where the projection of the acceleration gravity vector parallel to the platform is used as a reference acceleration. The technique has been already used for the calibration of the accelerometers on board the ESA spacecrafts BepiColombo and JUICE with destinations of Mercury and Jupiter [38]. For higher accelerations, the device is calibrated using a facility built to the purpose making use of dynamic laser interferometry having an accuracy of at least 1 × 10 −4 m s −2 . ...
In the area of synthetic sensors for flow angle estimation, the present work aims to describe the verification in a relevant environment of a physics-based approach using a dedicated technological demonstrator. The flow angle synthetic solution is based on a model-free, or physics-based, scheme and, therefore, it is applicable to any flying body. The demonstrator also encompasses physical sensors that provide all the necessary inputs to the synthetic sensors to estimate the angle-of-attack and the angle-of-sideslip. The uncertainty budgets of the physical sensors are evaluated to corrupt the flight simulator data with the aim of reproducing a realistic scenario to verify the synthetic sensors. The proposed approach for the flow angle estimation is suitable for modern and future aircraft, such as drones and urban mobility air vehicles. The results presented in this work show that the proposed approach can be effective in relevant scenarios even though some limitations can arise.
... Another recent and unusual application of ACs was the calibration of ISA accelerometers (Italian Spring Accelerometer) for ESA BepiColombo mission [8,9]. The calibration consisted in tilting the accelerometer of a known angle α, thus generating an acceleration a = g 0 sinα, i.e. equal to the projection of the gravitational acceleration g 0 on the measurement axis. ...
An old photoelectric Hilger & Watts autocollimator (AC) has been modified at INRIM to perform accurate angle measurements on two axes in a range of ±250''. A CMOS camera was placed in the focus of the AC's optical system and a software implemented in LabVIEW processes on-line the image of the reticle projected on the camera. The data acquisition is triggerable at a selectable frequency up to some hertz to allow synchronization in critical applications. The AC's sensitivity was determined through a calibration with respect to the national angle standard. The combined standard uncertainty of the AC calibration results to be equal to 0.035''
... Angle measurement and the control of angle tilting units at the nanoradian uncertainty level are essential to numerous applications ranging from astrometry, space missions, and scientific experiments to industrial applications [1][2][3][4][5]. Precise angle measuring devices-such as angle encoders, angle interferometers, small angle generators and autocollimatorsare used to provide solutions to these demands. ...
Interpolation errors at small angular scales are caused by the subdivision of the angular interval between adjacent grating lines into smaller intervals when radial gratings are used in angle encoders. They are often a major error source in precision angle metrology and better approaches for determining them at low levels of uncertainty are needed. Extensive investigations of interpolation errors of different angle encoders with various interpolators and interpolation schemes were carried out by adapting the shearing method to the calibration of autocollimators with angle encoders. The results of the laboratories with advanced angle metrology capabilities are presented which were acquired by the use of four different high precision angle encoders/interpolators/rotary tables. State of the art uncertainties down to 1 milliarcsec (5 nrad) were achieved for the determination of the interpolation errors using the shearing method which provides simultaneous access to the angle deviations of the autocollimator and of the angle encoder. Compared to the calibration and measurement capabilities (CMC) of the participants for autocollimators, the use of the shearing technique represents a substantial improvement in the uncertainty by a factor of up to 5 in addition to the precise determination of interpolation errors or their residuals (when compensated). A discussion of the results is carried out in conjunction with the equipment used.
... Indeed, because of the stresses occurring at the take-off, the mechanical properties of the elastic hinges could change in an unknown way. The calibration strategy and the complete calibration process is described in detail in [12]. ...
Accelerometers for space applications are typically based on a capacitive readout. Here we present the design of an accelerometer based on a classical mass spring mechanical system with an interferometric readout. The heterodyne interferometer is completely fiber based with a 1064 nm diode laser source and has a subpicometer resolution at 1 Hz. We present and discuss the design and the characterization of the interferometer, the foreseen performances of the whole accelerometer and the characterization strategies of the same.
ISA (Italian Spring Accelerometer) is a high sensitivity accelerometer flying, as scientific payload, on-board one of the two spacecraft (the Mercury Planetary Orbiter) of BepiColombo, the first ESA mission to Mercury. The first commissioning phase (performed in the period November 2018 - August 2019) allowed to verify the functionality of the instrument itself as well as of the related data handling and archiving system. Moreover, the acceleration measurements gathered in this time frame allow to envisage the potentiality of such an instrument as a high-accuracy monitor of the spacecraft mechanical environment.
BepiColombo, the forthcoming ESA (cornerstone) mission to Mercury, will include a comprehensive set of experiments - called Radio Science Experiments (RSE) in order to measure the gravitational field and rotational state of the planet and its rotation and to perform precise tests of Einstein's theory of general relativity versus other metric theories of gravity. Among the onboard instruments, a fundamental role in the RSE will be played by ISA (Italian Spring Accelerometer). This paper is devoted to the description of the main on-ground ISA internal actuators calibration and the obtained accuracy which is necessary to guarantee the accelerometer performance in order to reach the very ambitious objectives of RSE.
The Italian Spring Accelerometer (ISA) is an high- sensitivity instrument designed to measure very small accelera- tions. As such, it is suitable for a number of applications, notably the measurement of non-gravitational accelerations acting on the surface of a spacecraft. ISA has been selected to fly on board the forthcoming BepiColombo mission for the exploration of the planet Mercury; it is part of the suite of instruments which will perform the so-called Radio Science Experiments (RSE) to study gravitational field, internal structure and rotational state of Mercury, as well as to verify some predictions of the general theory of relativity. In this work the concept and design of ISA are described, along with important calibration and performance evaluation activities.
The Italian Spring Accelerometer (ISA) will give a fundamental contribution to the Radio Science Experiments of BepiColombo mission, enabling substantial improvement of the knowledge of Mercury's orbit and rotation, and of the relativistic dynamics in the solar system. ISA is a three-axis accelerometer devoted to the measurement of the non-gravitational acceleration of Mercury Planetary Orbiter (MPO), whose knowledge is important in order to fully exploit the quality of the tracking data. ISA shall have an intrinsic noise level of in the Hz to Hz frequency range, to guarantee the fulfilment of the RSE scientific goals. A comprehensive presentation of ISA accelerometer is given, including details about its scientific and technological features, the updated measurement error budget, the ongoing experimental activities and foreseen calibration and science operations strategies.
Abstract. Mercury has always excited a great interest in the scientific establishment because
of its proximity to the Sun that represents, however, the principal obstacle for its
observations from Earth and spacecraft exploration. BepiColombo is, therefore, one of the
most challenging long-term planetary projects that foresees two spacecraft dedicated to
the exploration of Mercury’s environment. The mission allows better understanding of the
planet itself as well as the formation of our Solar System. The density of Mercury does not
conform to that of the other terrestrial planet and for this reason the evaluation of Mercury’s
interior structure is one of the fundamental objectives of the mission. The Mercury Orbiter
Radio Science Experiment (MORE) is one of BepiColombo’s investigations, designed to
provide an accurate estimation of Mercury’s gravity field and Love number k2 by means
of highly stable, multi-frequency radio links in X and Ka band. Gravity not only provides
crucial information on the interior structure of the planet, but also, allows a good orbit determination
of the spacecraft. After an introduction to the mission and the MORE experiment,
we report on numerical simulations aiming at a realistic assessment of the attainable accuracy
in the determination of Mercury’s gravity field. The best results are obtained with a
batch-sequential filter, which proves to cope well the complexity of the noise and dynamical
models.
At the angle metrology laboratory of the Istituto Nazionale di Ricerca Metrologica (INRiM, formerly IMGC), an instrument to generate very small angles has been recently developed. It is a sine-bar angle generator based on an elastic hinge and a piezocapacitive device. The facility can be used to calibrate precise angle measuring instruments (levels, autocollimators, etc) in a range of 120 µrad with an uncertainty of about 20 nrad and a sensitivity of fractions of a nanoradian. The working principle and metrological characterization are presented.
A method to amplify the rotation angle of a mirror, based on multiple reflections between two quasi-parallel mirrors, is presented. The method allows rotations of fractions of nanoradians to be measured with a simple setup. The working principle, the experimental setup, and the results are presented.
Despite the fact the Mercury is a small, rocky, burned planet, so close to the Sun that it is hard to be seen from Earth except during twilight, it has played a very important role in the history of astronomy and fundamental physics and plays a crucial role in the understanding of the formation and evolution of our Solar System.
The paper is focused on the estimate of the impact of the non-gravitational perturbations on the orbit of the Mercury Planetary Orbiter (MPO), one of the two spacecrafts that will be placed in orbit around the innermost planet of the solar system by the BepiColombo space mission. The key rôle of the Italian Spring Accelerometer (ISA), that has been selected by the European Space Agency (ESA) to fly on-board the MPO, is outlined. In the first part of the paper, through a numerical simulation and analysis we have estimated, over a time span of several years, the long-period behaviours of the disturbing accelerations produced by the incoming direct solar radiation pressure, and the indirect effects produced by Mercury’s albedo. The variations in the orbital parameters of the spacecraft, together with their spectral contents, have been estimated over the analysed period. The direct solar radiation pressure represents the strongest non-gravitational perturbation on the MPO in the very complex radiation environment of Mercury. The order-of-magnitude of this acceleration is quite high, about 10−6m/s2, because of the proximity to the Sun and the large area-to-mass ratio of the spacecraft. In the second part of the paper, we concentrated upon the short-period effects of direct solar radiation pressure and Mercury’s albedo. In particular, the disturbing accelerations have been compared with the ISA measurement error and the advantages of an on-board accelerometer are highlighted with respect to the best modelling of the non-gravitational perturbations in the strong radiation environment of Mercury. The readings from ISA, with an intrinsic noise level of about
10-9m/s2/Ö{Hz}10^{-9}\,m/s^{2}/\sqrt{Hz} in the frequency band of 3·10−5–10−1Hz, guarantees a very significant reduction of the non-gravitational accelerations impact on the space mission accuracy, especially of the dominant direct solar radiation pressure.
BepiColombo is an interdisciplinary mission to explore Mercury, the planet closest to the sun, carried out jointly between the European Space Agency and the Japanese Aerospace Exploration Agency. From dedicated orbits two spacecraft will be studying the planet and its environment. The scientific payload of both spacecraft will provide the detailed information necessary to understand the origin and evolution of the planet itself and its surrounding environment. The scientific objectives focus on a global characterization of Mercury through the investigation of its interior, surface, exosphere and magnetosphere. In addition, instrumentation onboard BepiColombo will be used to test Einstein's theory of general relativity. Major effort was put into optimizing the scientific return of the mission by defining a payload complement such that individual measurements can be interrelated and complement each other. This paper gives an in-depth overview of BepiColombo spacecraft composite and the mission profile. It describes the suite of scientific instruments on board of the two BepiColombo spacecraft and the science goals of the mission.
- David M Lucchesi
David M. Lucchesi, Valerio Iafolla Celestial Mech Dyn Astr (2006)
96:99-127 DOI 10.1007/s10569-006-9034-9