ISA accelerometer onboard the Mercury Planetary Orbiter: error budget

Abstract

We have estimated a preliminary error budget for the Italian Spring Accelerometer (ISA) that will be allocated onboard the Mercury Planetary Orbiter (MPO) of the European Space Agency (ESA) space mission to Mercury named BepiColombo. The role of the accelerometer is to remove from the list of unknowns the non-gravitational accelerations that perturb the gravitational trajectory followed by the MPO in the strong radiation environment that characterises the orbit of Mercury around the Sun. Such a role is of fundamental importance in the context of the very ambitious goals of the Radio Science Experiments (RSE) of the BepiColombo mission. We have subdivided the errors on the accelerometer measurements into two main families: (i) the pseudo-sinusoidal errors and (ii) the random errors. The former are characterised by a periodic behaviour with the frequency of the satellite mean anomaly and its higher order harmonic components, i.e., they are deterministic errors. The latter are characterised by an unknown frequency distribution and we assumed for them a noise-like spectrum, i.e., they are stochastic errors. Among the pseudosinusoidal errors, the main contribution is due to the effects of the gravity gradients and the inertial forces, while among the random-like errors the main disturbing effect is due to the MPO centre-of-mass displacements produced by the onboard High Gain Antenna (HGA) movements and by the fuel consumption and sloshing. Very subtle to be considered are also the random errors produced by the MPO attitude corrections necessary to guarantee the nadir pointing of the spacecraft. We have therefore formulated the ISA error budget and the requirements for the satellite in order to guarantee an orbit reconstruction for the MPO spacecraft with an along-track accuracy of about 166 Valerio Iafolla et al.1 m over the orbital period of the satellite around Mercury in such a way to satisfy the RSE requirements.

METRIC: A Dedicated Earth-Orbiting Spacecraft for Investigating Gravitational Physics and the Space Environment

“…In particular, they could be attainable with an adaptation of the ISA BepiColombo accelerometer to the proposed mission. Very important in this respect is the fact that METRIC would offer a stable environment for the instruments it will host; indeed, a relevant part of the accelerometer error budget in that case is given by spurious signals coming from the on-board environment itself (see, e.g., [26,51]). …”

Zero-g positioning for the BepiColombo ISA accelerometer

“…ISA is a three-axes accelerometer realised by assembling three single accelerometers, arranged to form an orthogonal reference frame inside the MPO [2,3,6,14]. The core of each sensor is manufactured from a single piece of aluminium A17075, carved through milling machine, to obtain a proof mass (sensing element) hanged on an external frame through a tiny foil-shaped spring (∼140 m in width), having elastic constant k t (Fig.…”

“…Applying the formulas previous introduced, (4)- (6), and, on the basis of hypotheses, approximating the angles with the shifts, we obtain respectively:…”

“…One of the main objectives of BepiColombo is to measure with very high accuracy the gravitational field of Mercury ( [2,6]). Information about the gravity of a planet is acquired from the knowledge of the motion of a spacecraft orbiting around it.…”

Parabolic approximation to EMA motion profiles

“…Regardless of the limitations of the driver, the validity of using a parabolic approach lies in the pseudo-sinusoidal nature of the motion profiles [9]- [11] and the good fit of parabolic curves to sinusoids (Fig. 3).…”