dedicated to imaging, radio tracking, spectrography, laser-altimetry and magnetospheric measurements. In particular, the Mercury Orbiter Radioscience Experiment (MORE) will have a key role in gravity field estimation, in characterisation of the planet’s interior status and the estimation of post-Newtonian parameters. MORE will use the radio tracking Ka-T three links tracking data and the Italian Spring Accelerometer (ISA) measurements.
measurements shall be considered as relative and not absolute accelerations. The main expected accelerations measured by ISA can be divided into non-conservative and conservative accelerations. So, respectively, accelerations that can cause a net move of S/C CoM and accelerations causing only oscillation of CoM from its nominal trajectory. The non-conservative accelerations are as follows:
• The direct solar radiation pressure (SRP)
• The visible albedo radiation pressure
1.2. The ISA payload The Italian Spring Accelerometer is a three-axis high-sensitivity accelerometer (onedimensional sensing elements per acceleration axis forming a quasi-independent orthogonal triplet). The ISA’s final scope is to measure the three components of the non-gravitational acceleration vector acting on the spacecraft body, Santoli et al. (2020). ISA measurement band extends from 3 × 10−5 Hz to 10−1 Hz. Hence, its
• The planetary thermal emission radiation pressure
• The S/C anisotropic thermal re-emission
• Outgassing
• Thrusters or tanks gas leaks On the other hand, the conservative accelerations are:

• Inertial rotations accelerations
• Gravity Gradient effects
• Appendices movements
• S/C Internal parts low band vibrations.
Since ISA is sensitive to Non-Gravitational Perturbations (NGPs), a relevant part of ISA science data are foreseen to be used in the context of the BepiColombo Radio Science Experiment (BC-RSE) to perform at best the Precise Orbit Determination (POD). The aim of the instrument is to continuously measure the accelerations acting on-board both in visibility of the ground-station and in out-of-pass conditions De Filippis et al. (2024). ISA is made of three sensing elements, designed as mechanical harmonic oscillators. Each one consists of a proof mass hanged from an external rigid frame through a flexural element (spring) with low elastic constant. The sensing elements are equipped with two pairs of pick-up and actuation plates. The pick-up plates are used to read the acceleration signal by capacitive sensing of the movement of the central mass. On the other hand, the actuation plates are used to generate electrostatic forces to centre the sensing mass with respect to the electrical zero of pick-up plates and to internally calibrate each element by acting on the mass with a known on-ground calibrated signal. (figure (2)). In order to better take under control electrical and thermal disturbances, it has been decided to decouple as much as possible the digital electronics and the power supply section from the detectors. This results in a configuration that foresees two independent boxes. Therefore, the ISA instrument configuration is based on two units: the ISA Detector Assembly (IDA) and the ISA Control Electronics (ICE) that interfaces the MPO for power supply and communication. IDA is made up by a Front End Electronics (FEE), placed at the bottom of the box, and by the three sensing elements assemblies, each enclosed in dedicated shields, in order to decouple as much as possible the sensing elements from the temperature
(a)
(b) Figure 2: Exploded view of one of the three ISA Sensing Element. a) The trapezoidal sensing moving mass b) Section of mass evidencing the blade shape spring.
environmental disturbances. At the same time, shields reduce the power consumption of the active thermal control system. The configuration is described in figure (3a). As depicted in figure (3b) accelerometers Center of Mass (CoM) are not located in same point and each sensing element has a Sensing Axis (SA), identified during the on-ground calibration as the direction in which a known acceleration external to the assembly gives the maximum signal. Hence, the three sensing elements SA, despite designed to point in the three space directions, does not represents an ideal orthogonal tern. To fulfill the need to address the measuring acceleration to a unique, fixed point on the assembly, reference frame, the ISA Instrument Line of Sight (ISA ILS) is introduced. ISA ILS is defined as the closer orthogonal tern to the non-orthogonal reference formed by the three sensing axes, and it is placed in the Y sensing element CoM. ISA ILS relation between ISA URF and MPO RF is then a very simple rotation, since, by design, the two reference frames are 90 anticlockwise Y-rotated, so the XILS ∼= −ZMPO, YILS ∼= YMPO and ZILS ∼= XMPO.