A novel technique to measure absolute distances is presented. It is based on a Michelson interferometer where two tuneable lasers are superposed to create a synthetic wavelength. Relative and absolute interferometry theories are merged together. Its experimental realization allows absolute distance measurements with subfringe resolution. Preliminary results are presented in this work.
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 22<sup>nd</sup> 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.
We propose a technique based on a transmission grating placed in front of an imaging system (e.g. a telescope) mounted on a frame that can be rotated around the optical axis. The grating creates, for each point of the source image (e.g. a star), at the focal plane, an image composed by the undistorted image of the star plus symmetrical dispersion images of several diffraction orders. The grating is rotated and several images are captured for different angular positions of the same. By analyzing the different images obtained for a different grating angle, it is possible to build the hyperspectral cube. The advantages of this method is its simplicity, extreme compactness and low cost making it suitable both for amateur astronomy and low budget science laboratory. We will present preliminary experimental results along with a discussion about the achievable spectral and spatial resolution and photon collection efficiency as a function of different type of gratings and of the number of the captured pictures. Furthermore, we present the result when the method is applied to extended non-punctiform light sources.
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.
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.
Within the ESA technology research project "Laser Interferometer High Precision tracking for LEO", Thales Alenia Space Italia is developing a laser metrology system for a Next Generation Gravimetric Mission (NGGM) based on satellite-to-satellite tracking. This technique is based on the precise measurement of the displacement between two satellites flying in formation at low altitude for monitoring the variations of Earth’s gravity field at high resolution over a long time period. <p> </p>The laser metrology system that has been defined for this mission consists of the following elements: <p> </p>• an heterodyne Michelson interferometer for measuring the distance variation between retroreflectors positioned on the two satellites; <p> </p>• an angle metrology for measuring the orientation of the laser beam in the reference frames of the two satellites; <p> </p>• a lateral displacement metrology for measuring the deviations of the laser beam axis from the target retro-reflector. <p> </p>The laser interferometer makes use of a chopped measurement beam to avoid spurious signals and nonlinearity caused by the unbalance between the strong local beam and the weak return beam. <p> </p>The main results of the design, development and test activities performed on the breadboard of the metrology system are summarized in this paper.
A novel hyperspectral imaging technique based on Fourier Transform analysis applied to a low finesse scanning Fabry-Perot (F-P) interferometer has been demonstrated in the visible and NIR regions. The technique allows the realization of a lightweight and compact instrument yet having high throughput with respect to classical instruments based on dispersive means. Experiments carried out in the visible region have demonstrated hyperspectral imaging capability with a spectral resolution of 2 nm @ 532 nm and an image resolution limited by the CCD (696x512 pixel). In the NIR (0,9-1,7 μm) region a spectral resolution of 8 nm @1064nm and an image resolution limited by the CCD (320x256 pixel) has been obtained. The potentiality in spectroscopic applications like remote gas sensing has been demonstrated as well as accurate thermal imaging capabilities. Unlike classical hyperspectral instruments, based on dispersive means or on tuneable band-pass filters, the efficiency of our F-P based system is very high (about 30% of the photons collected by the objective reach the CCD) allowing much faster and/or better quality hyperspectral images. In the present experiments the speed was by far limited by the acquisition speed of the CCD sensors. Furthermore the device is very compact and is placed between the objective and the CCD of a standard imaging system: in this configuration the field of view of the instrument is only limited by the same objective that in the present system is interchangeable. Because of its roughness, compactness, lightweight and luminous efficiency, the device is a good candidate for airborne or space borne hyperspectral applications.
In the last decades Hyperspectral Imager (HI) have become irreplaceable space-borne instruments for an increasing number of applications. A number of HIs are now operative onboard (e.g. CHRIS on PROBA), others are going to be launched (e.g. PRISMA, EnMAP, HyspIRI), many others are at the breadboard level. The researchers goal is to realize HI with high spatial and spectral resolution, having low weight and contained dimensions. The most common HI technique is based on the use of a dispersive mean (a grating or a prism) or on the use of band pass filters (tunable or linear variable). These approaches have the advantages of allowing compact devices. Another approach is based on the use of interferometer based spectrometers (Michelson or Sagnac type). The advantage of the latter is a very high efficiency in light collection because of the well-known Felgett and Jaquinot principles.
Acceleration measurements are needed to various levels of sensitivity for almost all space missions in the fields of fundamental physics, space geodesy, space exploration, as well as on the space station. Acceleration sensors have a “free” (or weakly coupled) test mass inside a cage rigid with the spacecraft, and yield their relative acceleration by reading the relative displacements (linear and angular, if needed) of the test mass with respect to the cage.
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.
Low-frequency noise measurements are usually performed by measuring the voltage across a dipole, or the current through a wire. Here we demonstrate the feasibility of the measurement of the noise <i>power</i> flowing through a line connecting two dipoles. A sampling wattmeter, with spectral display capabilities, having a bandwidth in the range 1 Hz - 10 kHz and sensitivity better than 10<sup>-22</sup> W/Hz is here described, and employed to measure the noise power flowing between two resistors at different temperatures. Possible applications of the device include noise thermometry, noise measurement of active devices under load condition, investigations of excess noise below thermal threshold.