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Human activities have been identified as critical contributors to climate changes. Modern industrial development and increasing urbanisation have been affecting the environment at an unprecedented scale since the 19th century. In the two last decades the process has become even faster, also due to the impressive development of largely populated countries like India and China. Historical data records testify a direct correlation between increase in atmospheric CO2 levels and Earth's temperatures but processes underlying climate regulation and changes are only partially known. The improvement of knowledge of atmosphere and climate processes needs the availability of complete and reliable data about atmospheric composition and properties and space-based observations play a primary role. Implementation of a demonstration mission based on an occultation technique at optical wavelengths is proposed. Observations in the infrared spectral range vest a particular importance because this band exhibits many absorptive spectral lines due to greenhouse gases, identified as the main responsible of global warming, thus H2O, CO2, CH4, N2O, O3, and others can be observed with high accuracy. It is expected that the mission will demonstrate technical feasibility of an optical payload for limb sounding observations and provide useful inputs to climatic benchmarking (greenhouse gases and wind profile, as well as atmospheric thermodynamic properties). The identification and the preliminary definition of the instrument architecture and the identification of the critical technologies have been among the main tasks. Possible design options for the laser transmitter and the receiver are discussed, considering available technological solutions and technical constraints. Potential technological criticalities are illustrated too. The creation of performance models, analytical and numerical, facilitates and addresses the payload design activity, both at instrument level and at general system level.

Measurement of the static and temporal variation of Earth’s gravity field yields important information on water storage, seasonal and sub-seasonal water cycles, their impact on water levels and delivers key data to Earth’s climate models. The satellite mission GOCE (ESA) and GRACE (US-GER) resulted in in a significant improvement on our understanding of the system Earth. On GRACE and GRACE Follow-On two satellites are following each other on the same orbit with approx. 200 km distance to each other. A microwave inter-satellite ranging system measures the variation of the intersatellite distance from which the gravity field is derived. In addition, on GRACE Follow-On, which has been launched May 22nd 2018, a laser interferometer is added as an experiment to demonstrate the capability of this system to improve the ranging accuracy by at least one order of magnitude. To significantly improve the gravity field measurement accuracy, ESA is investigating the concept of a ‘Next generation gravity mission’ (NGGM), consisting of two pairs of satellites and a laser interferometer as the sole inter-satellite ranging system. Based on the heritage of the development of the laser ranging interferometer for GRACE Follow-On and the former and ongoing studies for NGGM, several concepts for the laser metrology instrument (LMI) for NGGM, namely the on- and off-axis variants of the transponder and the retroreflector concept have been investigated in detail with respect to their application for an inter-satellite distance of approx. 100 km. This paper presents the results of the detailed tradeoff between different concepts, including laser link acquisition, ranging noise contributors, instrument performance analyses, technology readiness levels of the individual instrument units and an instrument reliability assessment.

The extreme resolution provided by laser interferometers can be exploited for improving the measurement of Earth’s gravity field from space. Here we describe the concept of a Next Generation Gravity Mission based on this optical metrology and some of the ongoing developments on the key technologies, carried out in preparation of an operational mission.

A 5-DOFs compact optical sensor, named COATS, was recently realized at INRIM. Now the sensor head architecture, conceived through a modular approach, is reconfigured in different ways to address specific problems with a simplified design and a dedicated solution, retaining the necessary degrees of freedom while removing those not requested by the specific case. Some of the possible configurations are described together with the range and performance limits of the same. Furthermore the possibility of integrating the roll angle metrology in the system is described, opening the possibility of realizing a 6-DOF COATS.

The European gravity mission GOCE (Gravity field and steady-state Ocean Circulation Explorer), which flew between 2009 and 2013 with extraordinary success, was the result of more than 20 years of system studies and technology developments directed by the European Space Agency (ESA). Already while GOCE was being developed, ESA started preparatory studies for a Next Generation Gravity Mission (NGGM). While GOCE aimed to provide a high resolution static map of the Earth’s gravity, the objective of NGGM is long-term monitoring of the time-variable gravity field with high temporal and spatial resolution, by satellite-to-satellite tracking, a technique flown with the US–German mission GRACE for the first time. With respect to GOCE, the new mission implies new measurement techniques and instrumentation, a new mission scenario and different spacecraft design drivers. Despite the differences, however, several achievements of GOCE (e.g. demonstration of long-duration wide-band drag compensation, ultra-sensitive accelerometers, stable non-cryogenic temperature control in low Earth orbit) stand as the basis upon which NGGM is being defined. Technologies and techniques specific of the new mission (laser metrology, multi-task control architectures, high performance propulsion) are undergoing parallel developments in anticipation of the NGGM implementation phase. The paper recalls the GOCE mission and heritage, and introduces the objectives, scenarios, concepts and challenges of the NGGM, highlighting commonalities and differences between the two missions.

The Laser Interferometer Space Antenna is a foreseen gravitational wave detector, which aims to detect $10^{-20}$ strains in the frequency range from 0.1 mHz to 1 Hz. It is a triangular constellation, with equal sides of $2,5 \times 10^9$ m, of three spacecraft, where heterodyne interferometry measures the spacecraft distances. The stray light from the powerful transmitted beam can overlap with the received one and interfere with the heterodyne signal. We investigated the contribution of random phase variations of the stray photons to the noise of the heterodyne signal. A balanced detection scheme more effectively mitigates this adverse effect than a separation of the frequencies of the transmitted and local radiation. In the balanced scheme, in order to limit the phase noise to picometer level, the incoherent power of the stray light must be kept below about 10 nW/W for an asymmetry of the recombination beam splitter of 1%.

A typical issue of modern space missions is the measurement of the relative attitude (orientation and position in space) of one part of the satellite with respect to the reference frame (main body) of the satellite or the relative attitude of two parts of the same satellite or even two or more satellites flying in formation.

The Laser Interferometer Space Antenna aims to measure picometer changes of the 2.5 × 106 km sides of a triangular constellation of satellites. Each spacecraft hosts two telescopes that simultaneously transmit and receive laser beams measuring the constellation arms by heterodyning the received wavefronts with local references. We report an end-to-end investigation of the measurement noise due to the interaction between the telescope jitters and wavefront aberrations. With provisional design parameters, to achieve the targeted sensitivity the root-mean-square aberrations must be less than λ/65.

We present an absolute interferometer based on a synthetic wavelength generated by mixing two fiber coupled extended cavity diode lasers. The synthetic wavelength scheme is simple and compact and based on fiber components. The measurement capability has been demonstrated up to 500 m in air and the accuracy is limited by the refractive index of air. The ultimate estimated accuracy in vacuum is of the order of few micrometers. The interferometer, originally designed for Synthetic Aperture Radar missions can be used whenever the distance between two or more parts of a satellite (e.g. an antenna fixed to a boom) or even the distance between satellites flying in formation has to be monitored or controlled.

The Laser Interferometer Space Antenna is a foreseen space-based gravitational wave detector, which aims to detect 10^20 strains in the frequency range from 0.1 mHz to 1 Hz. It is a triangular constellation of three spacecraft, with equal sides of 2,5 x 10^9 m, where every spacecraft hosts a pair of telescopes that simultaneously transmit and receive laser beams measuring the constellation arms by heterodyning the received wavefronts with local references. Due to the spacecraft and constellation jitters, the interfering (received and local) wavefronts become misaligned. We investigate analytically the coupling between misalignments and aberrations of the interfering wavefronts and estimate the relevant contribution to the noise of the heterodyne signal.

The Fringe Sensor Unit (FSU) is the central element of the Phase Referenced Imaging and Micro-arcsecond Astrometry (PRIMA) dual-feed facility and provides fringe sensing for all observation modes, comprising off-axis fringe tracking, phase referenced imaging, and high-accuracy narrow-angle astrometry. It is installed at the Very Large Telescope Interferometer (VLTI) and successfully servoed the fringe tracking loop during the initial commissioning phase. Unique among interferometric beam combiners, the FSU uses spatial phase modulation in bulk optics to retrieve real-time estimates of fringe phase after spatial filtering. A R=20 spectrometer across the K-band makes the retrieval of the group delay signal possible. The FSU was integrated and aligned at the VLTI in summer 2008. It yields phase and group delay measurements at sampling rates up to 2 kHz, which are used to drive the fringe tracking control loop. During the first commissioning runs, the FSU was used to track the fringes of stars with K-band magnitudes as faint as m_K=9.0, using two VLTI Auxiliary Telescopes (AT) and baselines of up to 96 m. Fringe tracking using two Very Large Telescope (VLT) Unit Telescopes (UT) was demonstrated. During initial commissioning and combining stellar light with two ATs, the FSU showed its ability to improve the VLTI sensitivity in K-band by more than one magnitude towards fainter objects, which is of fundamental importance to achieve the scientific objectives of PRIMA., 19 pages, 23 figures. minor changes and language editing. this version equals the published article

This paper summarizes the development of a successful project, LAUE, supported by the Italian Space Agency (ASI) and devoted to the development of long focal length (up to 100 m) Laue lenses for hard X--/soft gamma--ray astronomy (80-600 keV). The apparatus is ready and the assembling of a prototype lens petal is ongoing. The great achievement of this project is the use of bent crystals. From measurements obtained on single crystals and from simulations, we have estimated the expected Point Spread Function and thus the sensitivity of a lens made of petals. The expected sensitivity is a few $\times10^{-8}$ photons cm$^{-2}$ s$^{-1}$ keV$^{-1}$. We discuss a number of open astrophysical questions that can settled with such an instrument aboard a free-flying satellite., 17 pages, 18 figures, published in Proceedings of the SPIE, Volume 8861, id. 886106 17 pp. (2013)

We present the status of LAUE, a project supported by the Italian Space Agency (ASI), and devoted to develop Laue lenses with long focal length (up to 100 meters), for hard X--/soft gamma--ray astronomy (80-600 keV). Thanks to their focusing capability, the design goal is to improve the sensitivity of the current instrumention in the above energy band by 2 orders of magnitude, down to a few times $10^{-8}$ photons/(cm$^2$ s keV)., 9 pages, 9 figures, presented at the Space Telescopes and Instrumentation Symposium in Amsterdam, 2012: Ultraviolet to Gamma Ray Conference. Published in the Proceedings of the SPIE, Volume 8443, id. 84430B-84430B-9 (2012)

The technique called “Low-Low Satellite-to-Satellite Tracking” makes use of two-satellite loose formations for detecting the Earth’s gravity field and its space/time variations: the effect of the geopotential shows up as a variation of the inter-satellite distance, which is measured by a suitable metrology. Accelerometers are utilized on each satellite for measuring and separating the effect of the non-gravitational forces. Thales Alenia Space Italia (TAS-I) has studied for the European Space Agency a gravimetric mission of this kind, in which the inter-satellite distance variation is measured by a laser interferometer. The reference mission scenario that has been defined and studied consists of two satellites flying along the same circular orbit at 10 km relative distance and 325 km altitude. The formation control for this mission shall be designed to work in synergy with the drag-free control (necessary for providing quiet operational environment to the accelerometers), to not interfere with the scientific measurement and to minimize the use of the thrusters. Another control system is in charge of maintaining the fine pointing of the interferometer laser beam from one satellite to the other. This chapter summarizes the main results of the studies performed by TAS-I and its team on these subjects 1 2 3.

The Laser Metrology and Optic Active Control (LM&OAC) program has been carried out under ESA contract with the purpose to design and validate a laser metrology system and an actuation mechanism to monitor and control at microarcsec level the stability of the Basic Angle (angle between the lines of sight of the two telescopes) of GAIA satellite. As part of the program, a breadboard (including some EQM elements) of the laser metrology and control system has been built and submitted to functional, performance and environmental tests. In the followings we describe the mission requirements, the system architecture, the breadboard design, and finally the performed validation tests. Conclusion and appraisals from this experience are also reported.

We describe a software tool developed to simulate the behaviour of the angle between two lines of sight in a dual view telescope assembly (usually referred to as basic angle) due to optical misalignments induced by thermo-mechanical fluctuations. The tool applies to a variety of reflective optical designs. In principle, not only the basic angle behaviour can be simulated, but also other optical parameters. As a practical example, we present and discuss results obtained from application of our software to the case of the Gaia baseline optical design. We show that the final error can be severely degraded by fluctuations of the basic angle due to thermo-mechanical effects.