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.
This paper describes the internal metrology breadboard development activities performed in the frame of the EUCLID CEPA 9 RTP 9.9 “High Resolution Optical Satellite Sensor” project of the WEAO Research Cell by AAS-I and INETI. The Michelson Interferometer Testbed demonstrates the possibility of achieving a cophasing condition between two arms of the optical interferometer starting from a large initial white light Optical Path Difference (OPD) unbalance and of maintaining the fringe pattern stabilized in presence of disturbances.
The activities described in this paper have been developed in the frame of the EUCLID CEPA 9 RTP 9.9 “High Resolution Optical Satellite Sensor” project of the WEAO Research Cell. They have been focused on the definition of an interferometric instrument optimised for the high-resolution optical surveillance from geostationary orbit (GEO) by means of the synthetic aperture technique, and on the definition and development of the related enabling technologies. In this paper we describe the industrial team, the selected mission specifications and overview of the whole design and manufacturing activities performed.
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.
This paper describes the study of an interferometric instrument for the high-resolution surveillance of the Earth from geostationary orbit (GEO) performed for the EUCLID CEPA 9 RTP 9.9 “High Resolution Optical Satellite Sensor” project of the WEAO Research Cell. It is an in-depth description of a part of the activities described in. The instrument design, both optical and mechanical, is described; tradeoffs have been done for different restoration methods, based on an image generated using calculated point spread functions (PSF’s) for the complete FOV. Co-phasing concept for the optical interferometer has been defined together with the optical metrology needed. Design and simulation of the overall instrument control system was carried out.
The challenging constraints imposed on the Euclid telescope imaging performances have driven the design,
manufacturing and characterisation of the multi-layers coatings of the dichroic. Indeed it was found that the coatings
layers thickness inhomogeneity will introduce a wavelength dependent phase-shift resulting in degradation of the image
quality of the telescope. Such changes must be characterized and/or simulated since they could be non-negligible
contributors to the scientific performance accuracy. Several papers on this topic can be found in literature, however the
results can not be applied directly to Euclid’s dichroic coatings. In particular an applicable model of the phase-shift
variation with the wavelength could not be found and was developed. The results achieved with the mathematical
model are compared to experimental results of tests performed on a development prototype of the Euclid’s dichroic.
In the Euclid mission the straylight has been identified at an early stage as the main driver for the final imaging quality of the telescope. The assessment by simulation of the final straylight in the focal plane of both instruments in Euclid’s payload have required a complex workflow involving all stakeholders in the mission, from industry to the scientific community. The straylight is defined as a Normalized Detector Irradiance (NDI) which is a convenient definition tool to separate the contributions of the telescope and of the instruments. The end-to-end straylight of the payload is then simply the sum of the NDIs of the telescope and of each instrument. The NDIs for both instruments are presented in this paper for photometry and spectrometry.
In the context of the increasing demand in high-speed data link for scientific, planetary exploration and earth observation
missions, the Italian Space Agency (ASI), involving Thales Alenia Space as prime, the Polytechnic of Turin and other
Italian partners, is developing a program for feasibility demonstration of optical communication system with the goal of
a prototype flight mission in the next future.
We have designed and analyzed a ground level bidirectional Free Space Optical Communication (FSOC) Breadboard at
2.5Gbit/s working at 1550nm as an emulator of slant path link. The breadboard is full-working and we tested it back-toback,
at 500m and 2.3km during one month. The distances were chosen in order to get an equivalent slant path
cumulative turbulence in a ground level link. The measurements campaign was done during the day and the night time
and under several weather conditions, from sunny, rainy or windy. So we could work under very different turbulence
conditions from weak to strong turbulence. We measured the scintillation both, on-axis and off-axis by introducing
known misalignments at the terminals, transmission losses at both path lengths and BER at both receivers. We present
simulations results considering slant and ground level links, where we took into account the atmospheric effects;
scintillation, beam spread, beam wander and fade probability, and comparing them with the ground level experimental
results, we find a good agreement between them. Finally we discuss the results obtained in the experimentation and in
the flight mission simulations in order to apply our experimental results in the next project phases.
Within a Technology Research Program funded by the European Space Agency, a team led by Alenia Aerospazio has investigated and started the development of some technologies which are considered fundamental for the achievement of the scientific objectives of the future astrometric mission GAIA. The activities have been focused on the design of a two-aperture optical interferometer and of a system for the active stabilization of its configuration within few picometers. A laboratory prototype of the active stabilization system has been implemented and tested. The results achieved in the laboratory tests proved that the very challenging requirements imposed by the GAIA astrometric goal of 10 micro-arcsec accuracy can be fulfilled.