In-flight calibration system of imaging x-ray polarimetry explorer

The NASA/ASI Imaging X-ray Polarimetry Explorer, which will be launched in 2021, will be the first instrument to perform spatially resolved X-ray polarimetry on several astronomical sources in the 2-8 keV energy band. These measurements are made possible owing to the use of a gas pixel detector (GPD) at the focus of three X-ray telescopes. The GPD allows simultaneous measurements of the interaction point, energy, arrival time, and polarization angle of detected X-ray photons. The increase in sensitivity, achieved 40 years ago, for imaging and spectroscopy with the Einstein satellite will thus be extended to X-ray polarimetry for the first time. The characteristics of gas multiplication detectors are subject to changes over time. Because the GPD is a novel instrument, it is particularly important to verify its performance and stability during its mission lifetime. For this purpose, the spacecraft hosts a filter and calibration set (FCS), which includes both polarized and unpolarized calibration sources for performing in-flight calibration of the instruments. In this study, we present the design of the flight models of the FCS and the first measurements obtained using silicon drift detectors and CCD cameras, as well as those obtained in thermal vacuum with the flight units of the GPD. We show that the calibration sources successfully assess and verify the functionality of the GPD and validate its scientific results in orbit; this improves our knowledge of the behavior of these detectors in X-ray polarimetry

PixDD: a multi-pixel silicon drift detector for high-throughput spectral-timing studies

The Pixelated silicon Drift Detector (PixDD) is a two-dimensional multi-pixel X-ray sensor based on the technology of Silicon Drift Detectors, designed to solve the dead time and pile-up issues of photon-integrating imaging detectors. Read out by a two-dimensional self-triggering Application-Specific Integrated Circuit named RIGEL, to which the sensor is bump-bonded, it operates in the 0:5 — 15 keV energy range and is designed to achieve single-photon sensitivity and good spectroscopic capabilities even at room temperature or with mild cooling (< 150 eV resolution at 6 keV at 0 °C). The paper reports on the design and performance tests of the 128-pixel prototype of the fully integrated system

Calibration of the IXPE instrument

IXPE scientific payload comprises of three telescopes, each composed of a mirror and a photoelectric polarimeter based on the Gas Pixel Detector design. The three focal plane detectors, together with the unit which interfaces them to the spacecraft, are named IXPE Instrument and they will be built and calibrated in Italy; in this proceeding, we will present how IXPE Instrument will be calibrated, both on-ground and in-flight. The Instrument Calibration Equipment is being finalized at INAF-IAPS in Rome (Italy) to produce both polarized and unpolarized radiation, with a precise knowledge of direction, position, energy and polarization state of the incident beam. In flight, a set of four calibration sources based on radioactive material and mounted on a filter and calibration wheel will allow for the periodic calibration of all of the three IXPE focal plane detectors independently. A highly polarized source and an unpolarized one will be used to monitor the response to polarization; the remaining two will be used to calibrate the gain through the entire lifetime of the mission

The IXPE Instrument Calibration Equipment

The Imaging X-ray Polarimetry Explorer is a mission dedicated to the measurement of X-ray polarization from tens of astrophysical sources belonging to different classes. Expected to be launched at the end of 2021, the payload comprises three mirrors and three focal plane imaging polarimeters, the latter being designed and built in Italy. While calibration is always an essential phase in the development of high-energy space missions, for IXPE it has been particularly extensive both to calibrate the response to polarization, which is peculiar to IXPE, and to achieve a statistical uncertainty below the expected sensitivity. In this paper we present the calibration equipment that was designed and built at INAF-IAPS in Rome, Italy, for the calibration of the polarization-sensitive focal plane detectors on-board IXPE. Equipment includes calibration sources, both polarized and unpolarized, stages to align and move the beam, test detectors and their mechanical assembly. While all these equipments were designed to fit the specific needs of the IXPE Instrument calibration, their versatility could also be used in the future for other projects

Modeling of non-conservative forces for the Galileo-FOC spacecraft

Suboptimal modeling of direct Solar Radiation Pressure (SRP) is currently the main source of error in determining the orbit of any type of spacecraft of the Global Navigation Satellite System (GNSS). The complex shape of these satellites (bus and wings) combined with their particular attitude law – which requires the face of the satellite that collects the different antennas to continuously point to the nadir and deep space the face near which the atomic clocks are located, while at the same time the array of solar panels must continuously point towards the Sun for energy reasons – make the modeling of this perturbation and its optimal insertion into the Precise Orbit Determination (POD) process a non-trivial issue. We will present the results for the perturbative accelerations produced by solar and terrestrial (albedo and infrared) radiation in the case of a Box-Wing model built using the ESA Galileo metadata. The Yaw Steering law for the spacecraft attitude was also include in our model. The Box-Wing model and a 3D model of the satellite were also incorporated into the s/w COMSOL for a preliminary activity on the use of the Ray-tracing technique. Our final aim is, in fact, to build a Finite Element Model of the satellite and apply an ad hoc Ray-tracing for the calculation of the different perturbations related to the radiation pressure, also considering umbra and penumbra effects and multiple reflections. This activity is part of those of the Galileo for Science project (G4S 2.0) funded by ASI. The main objectives of G4S 2.0 are in the field of Fundamental Physics and a POD of the Galileo satellites based on a reliable dynamic model, in particular for Doresa and Milena, the two satellites in elliptical orbit, is of primary importance

The Galileo for science (G4S_2.0) project: non-gravitational perturbations models and precise orbit determination with SLR data

The G4S_2.0 project, funded by the Italian Space Agency, aims to test fundamental physics in the terrestrial field using the satellites of the Galileo FOC Constellation and, in particular, the two satellites GSAT0201 and GSAT0202 in eccentric orbit. Three Italian research centers are involved in the project: ASI-CGS in Matera, Politecnico di Torino and IAPS-INAF at Roma Tor Vergata. We present some of the ongoing activities at IAPS-INAF, which is Prime of the project. The activities undertaken have as main final objectives: i) the measurement of the relativistic precessions of the orbits of the satellites, ii) to place constraints on a possible violation of the Local Position Invariance (LPI) through an accurate measurement of the Gravitational Redshift, iii) to place constraints on the presence of dark matter in our Galaxy, and iv) to develop a new concept of accelerometer for a future generation of Galileo satellites. These activities therefore concern both the analysis of the orbits of satellites and those of the atomic clocks they carry. In particular, an activity is underway to improve the dynamic model of the orbits of satellites starting from the direct solar radiation pressure (SRP), the largest non-gravitational perturbation (NGP) on Galileo satellites and in general on all satellites of the Global Navigation Satellite Systems (GNSS) constellations. We have developed a Box-Wing (BW) model of the satellite using the information provided by ESA and a 3D model of the same to be used for a finite element analysis based on the ray-tracing technique, once all surfaces and elements of the satellite will be adequately characterized. We also started preliminary analyses of the orbits of Galileo FOC satellites based on laser tracking data. Our goal from this point of view is to improve the Precise Orbit Determination (POD) of the satellites by jointly using laser tracking data (in particular the Full Rate ones) and the microwave tracking data. For this goal, as underlined, a key role will be played by the improved dynamic model of the spacecraft: we plan to use increasingly sophisticated BW models up to the construction, if possible, of a finite element model (FEM) of the satellite. We will present the preliminary results obtained in the development of the spacecraft model and those obtained for the satellites orbit determination

The Galileo for science (G4S_2.0) project: precise orbit determination for fundamental physics and space geodesy

G4S_2.0 is a project funded by the Italian Space Agency aiming to perform a set of Fundamental Physics measurements using the two Galileo FOC satellites GSAT0201 (Doresa) and GSAT0202 (Milena). Indeed, the orbits of these satellites are characterized by a relatively high eccentricity, about 0.16, which represents a good prerequisite for a series of tests and measurements concerning the predictions of different theories of gravitation, as compared with the General Relativity (GR) ones. The main objectives include a new measurement of the gravitational redshift effect of the on-board atomic clocks --- thanks to its modulation with the orbital period due to the high eccentricity of the orbits --- and the measurement of the main precessions of relativistic origin, primarily the Schwarzschild one. To achieve these significant results, and possibly improve the current constraints of several theories of gravitation with respect to GR, it is of fundamental importance to take a step forward --- compared to the state of the art --- in the reliability of the dynamic model used for the orbits of the satellites and, as a direct consequence of this, in their precise orbit determination (POD). In this context, non-gravitational perturbations (NGPs) are the most subtle and difficult to model because of the complex shape of the Galileo satellites and their attitude law. In this regard, the main challenge is represented by a more refined and reliable model for the direct solar radiation pressure (SRP), the largest NGP on Galileo satellites, as well as on every satellite of every GNSS constellation. Our final goal is to build a finite element model (FEM) of the Galileo FOC spacecraft, as refined as possible, and apply a dedicated raytracing technique to it to compute the perturbing accelerations due to radiation pressure. In view of this, we have already developed a 3D-CAD model of the spacecraft. As an intermediate step, we have built a Box-Wing (BW) model based on the relatively poor information presently available on the geometrical and physical properties of the spacecraft. This BW model has been used to compute the perturbing accelerations due to the direct SRP and to the Earth's albedo and infrared radiation. The results obtained for the accelerations, to be included in the POD process, will be presented in various cases. Then, by computing the residuals in the orbital elements, it will be possible to verify the goodness of the POD results and observe the expected progressive improvement starting from the BW model towards the FEM one. The present analyses were made using the nominal attitude law of the Galileo FOC spacecraft; the application of this law will be discussed in the case of satellites in elliptical orbit. We finally highlight that the results of G4S_2.0 in terms of POD improvements are particularly useful for all applications of the Galileo FOC satellites in the fields of space Geodesy and Geophysics

The Galileo for science (G4S 2.0) project: the measurement of the gravitational redshift with the Galileo satellites Doresa and Milena.

G4S 2.0 is a new project funded by the Italian Space Agency which aims to perform measurements in the field of Fundamental Physics with the two satellites DORESA (E18) and MILENA (E14) of the Galileo-FOC constellation. Indeed, the orbits of these satellites are characterized by a relatively high eccentricity of about 0.16. After a general introduction to the main objectives of G4S 2.0, the preliminary activities developed at IAPS-INAF in Rome for the measurement of the gravitational redshift, based on the time bias analysis of accurate onboard atomic clocks, will be presented. This measurement will be a test for the validity of the local position invariance, one of the ingredients of Einstein’s principle of equivalence