Gilles Metris

Quantum Physics Exploring Gravity in the Outer Solar System: The SAGAS Project

We summarise the scientific and technological aspects of the Search for Anomalous Gravitation using Atomic Sensors (SAGAS) project, submitted to ESA in June 2007 in response to the Cosmic Vision 2015–2025 call for proposals. The proposed mission aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a Solar System escape trajectory in the 2020 to 2030 time-frame. SAGAS has numerous science objectives in fundamental physics and Solar System science, for example numerous tests of general relativity and the exploration of the Kuiper belt. The combination of highly sensitive atomic sensors and of the laser link well adapted for large distances will allow measurements with unprecedented accuracy and on scales never reached before. We present the proposed mission in some detail, with particular emphasis on the science goals and associated measurements and technologies.

MICROSCOPE, testing the equivalence principle in space

The test of the equivalence principle can be performed in space with orders of magnitude better resolution than in the laboratory, because of the outstanding steady and soft environment of the in-orbit experiment. The expected new experimental results will contribute to the unification of the four interactions, demonstrate the existence of extra scalar interaction or participate in the research for a quantum gravity theory. The MICROSCOPE space mission is being developed within the framework of the Cnes scientific program with the objective of testing the universality of free fall with a 10−15 accuracy. The concept and the design of the experiment are discussed and the major performance drivers of the room temperature instrument are pointed out. The launch of the drag-free satellite is scheduled for late 2004. By its specific technology demonstration, the mission will open the way to even more accurate acceleration measurements for other space missions in fundamental physics.

MICROSCOPE Mission: First Results of a Space Test of the Equivalence Principle

According to the weak equivalence principle, all bodies should fall at the same rate in a gravitational field. The MICROSCOPE satellite, launched in April 2016, aims to test its validity at the 10^{-15} precision level, by measuring the force required to maintain two test masses (of titanium and platinum alloys) exactly in the same orbit. A nonvanishing result would correspond to a violation of the equivalence principle, or to the discovery of a new long-range force. Analysis of the first data gives δ(Ti,Pt)=[-1±9(stat)±9(syst)]×10^{-15} (1σ statistical uncertainty) for the titanium-platinum Eötvös parameter characterizing the relative difference in their free-fall accelerations.

Odyssey: A Solar System Mission

The Solar System Odyssey mission uses modern-day high-precision experimental techniques to test the laws of fundamental physics which determine dynamics in the solar system. It could lead to major discoveries by using demonstrated technologies and could be flown within the Cosmic Vision time frame. The mission proposes to perform a set of precision gravitation experiments from the vicinity of Earth to the outer Solar System. Its scientific objectives can be summarized as follows: (1) test of the gravity force law in the Solar System up to and beyond the orbit of Saturn; (2) precise investigation of navigation anomalies at the fly-bys; (3) measurement of Eddington’s parameter at occultations; (4) mapping of gravity field in the outer solar system and study of the Kuiper belt. To this aim, the Odyssey mission is built up on a main spacecraft, designed to fly up to 13 AU, with the following components: (a) a high-precision accelerometer, with bias-rejection system, measuring the deviation of the trajectory from the geodesics, that is also giving gravitational forces; (b) Ka-band transponders, as for Cassini, for a precise range and Doppler measurement up to 13 AU, with additional VLBI equipment; (c) optional laser equipment, which would allow one to improve the range and Doppler measurement, resulting in particular in an improved measurement (with respect to Cassini) of the Eddington’s parameter. In this baseline concept, the main spacecraft is designed to operate beyond the Saturn orbit, up to 13 AU. It experiences multiple planetary fly-bys at Earth, Mars or Venus, and Jupiter. The cruise and fly-by phases allow the mission to achieve its baseline scientific objectives [(1) to (3) in the above list]. In addition to this baseline concept, the Odyssey mission proposes the release of the Enigma radio-beacon at Saturn, allowing one to extend the deep space gravity test up to at least 50 AU, while achieving the scientific objective of a mapping of gravity field in the outer Solar System [(4) in the above list].

The microscope mission

Abstract The MICROSCOPE mission had been selected at the end of 1999 by the French space agency Cnes for a launch scheduled in 2004. The scientific objective of the mission is the test of the Equivalence Principle (EP) up to an accuracy of 10−15 with its well-known manifestation, the universality of free fall. This principle, at the origin of general relativity, is only consolidated by experimental results and presently with an accuracy of several 10−13. The micro-satellite developed by Cnes weighs less than 120 kg and is compatible with a low-cost launch like ASAP ARIANE V. The instrument is composed of two differential electrostatic accelerometers operating at finely stabilised room temperature. Each accelerometer includes two cylindrical and concentric test masses, made of platinum or titanium alloys. The experiment consists in controlling the two masses in the same orbital motion. Because of the drag compensation system of the satellite including field effect electrical thrusters, this motion is quite purely gravitational. The electrostatic control forces used in the differential accelerometers are finely measured. The principle of the experiment is presented, the configuration of the instrument and of the satellite is detailed with regard to the present development status. The specifications for the major parameters of the experiment are detailed.

Lunar laser ranging in infrared at the Grasse laser station

For many years, lunar laser ranging (LLR) observations using a green wavelength have suffered an inhomogeneity problem both temporally and spatially. This paper reports on the implementation of a new infrared detection at the Grasse LLR station and describes how infrared telemetry improves this situation. Our first results show that infrared detection permits us to densify the observations and allows measurements during the new and the full Moon periods. The link budget improvement leads to homogeneous telemetric measurements on each lunar retro-reflector. Finally, a surprising result is obtained on the Lunokhod 2 array which attains the same efficiency as Lunokhod 1 with an infrared laser link, although those two targets exhibit a differential efficiency of six with a green laser link.

Regression analysis with missing data and unknown colored noise: application to the MICROSCOPE space mission

It is usual in the data analysis of physical measurements to deal with highly correlated noise. In addition, outliers, saturation events or data transmission losses can arise, leading to interruptions in the measured time series. We investigate the impact of missing data on the performance of linear regression analysis involving the fit of modeled or measured time series. We show that data gaps can significantly alter the precision of the regression parameter estimation in presence of colored noise, due to the frequency leakage of the noise power. We present a regression method which cancels this effect and enables us to estimate the parameters of interest with a precision comparable to the complete data case, even if the noise power spectral density (PSD) is not known a priori. The method is based on an autoregressive (AR) fit of the noise, which allows us to build an approximate generalized least squares estimator approaching the minimal variance bound. The method, which can be applied to numerous processing of similar data, is tested on simulated measurements of the MICROSCOPE space mission, whose goal is to test the Weak Equivalence Principle (WEP) with a precision of 10 −15 . The challenge of the data processing is to find a WEP violation signal around a well defined frequency in data samples disrupted by deterministic perturbations, colored noise and data gaps. We test our method with different gaps patterns and noise of known PSD and find that the results agree with the mission requirements. We show that it also provides a test of significance to assess the uncertainty of the measurement.

Gaussian regression and power spectral density estimation with missing data: The MICROSCOPE space mission as a case study,

We present a Gaussian regression method for time series with missing data and stationary residuals of unknown power spectral density (PSD). The missing data are efficiently estimated by their conditional expectation as in universal Kriging based on the circulant approximation of the complete data covariance. After initialization with an autoregressive fit of the noise, a few iterations of estimation/reconstruction steps are performed until convergence of the regression and PSD estimates, in a way similar to the expectation-conditional-maximization algorithm. The estimation can be performed for an arbitrary PSD provided that it is sufficiently smooth. The algorithm is developed in the framework of the MICROSCOPE space mission whose goal is to test the weak equivalence principle (WEP) with a precision of 10−15. We show by numerical simulations that the developed method allows us to meet three major requirements: to maintain the targeted precision of the WEP test in spite of the loss of data, to calculate a reliable estimate of this precision and of the noise level, and finally to provide consistent and faithful reconstructed data to the scientific community.

Determination of the Equivalence Principle Violation Signal for the MICROSCOPE Space Mission: Optimization of the Signal Processing

The MICROSCOPE space mission aims at testing the Equivalence Principle (EP) with an accuracy of 10−15. The test is based on the precise measurement delivered by a differential electrostatic accelerometer on-board a drag-free microsatellite which includes two cylindrical test masses submitted to the same gravitational field and made of different materials. The experiment consists in testing the equality of the electrostatic acceleration applied to the masses to maintain them relatively motionless at a well-known frequency. This high precision experiment is compatible with only very little perturbations. However, aliasing arises from the finite time span of the measurement, and is amplified by measurement losses. These effects perturb the measurement analysis. Numerical simulations have been run to estimate the contribution of a perturbation at any frequency on the EP violation frequency and to test its compatibility with the mission specifications. Moreover, different data analysis procedures have been considered to select the one minimizing these effects taking into account the uncertainty about the frequencies of the implicated signals.

Validation of the in-flight calibration procedures for the MICROSCOPE space mission

Abstract The MICROSCOPE space mission aims to test the Equivalence Principle with an accuracy of 10 - 15 . The drag-free micro-satellite will orbit around the Earth and embark a differential electrostatic accelerometer including two cylindrical test masses submitted to the same gravitational field and made of different materials. The experience consists in testing the equality of the electrostatic acceleration applied to the masses to maintain them relatively motionless. The accuracy of the measurements exploited for the test of the Equivalence Principle is limited by our a priori knowledge of several physical parameters of the instrument. These parameters are partially estimated on-ground, but with an insufficient accuracy, and an in-orbit calibration is therefore required to correct the measurements. The calibration procedures have been defined and their analytical performances have been evaluated. In addition, a simulator software including the dynamics model of the instrument, the satellite drag-free system and the perturbing environment has been developed to numerically validate the analytical results. After an overall presentation of the MICROSCOPE mission, this paper will describe the calibration procedures and focus on the simulator. Such an in-flight calibration is mandatory for similar space missions taking advantage of a drag-free system.