Internal fuselage noise: comparison of computations and tests on a Falcon airframe

An unequipped Falcon 2000 airframe, a complex heterogeneous structure reduced to the fuselage alone, was subjected to vibro-acoustic characterization within the framework of a cooperative effort by Aerospatiale, Dassault-Aviation, and Onera. Onera's Structural Dynamics and Coupled Systems Department measured the structure's vibro-acoustic responses for four mechanical excitation configurations, and analysed three structural configurations: fuselage alone without floor, with floor, and with floor and internal wall. The resulting database of vibratory and acoustic level measurements is used for validating numerical models in the low- and medium-frequency range, as well as statistical energy analysis (SEA) models in the medium- and high-frequency range.The database was analysed by Dassault-Aviation and Onera. We present here the computation/test comparisons and the limitations of the methods in use. All of the data and comparisons validate the predictive vibro-acoustic response models and methods for a Falcon fuselage subjected to a mechanical excitation.     

Influence of time variable geopotential models on precise orbits of altimetry satellites,global and regional mean sea level trends

During the last decade a significant progress has been reached in the investigation of the gravity field of the Earth. Besides static, also time variable geopotential models have been recently created. In this paper we investigate the impact of the recent time variable geopotential models on altimetry satellite orbits and such altimetry products based on these orbits, as global and regional mean sea level trends. We show that the modeling of time variable gravity improves the orbit solutions, at least for the GRACE period where time variable gravity is sufficiently accurately observed by this mission. Our analysis includes six geopotential models jointly developed by GFZ German Research Centre for Geosciences and Space Geodesy Research Group (CNES/GRGS) Toulouse: the stationary model EIGEN-GL04S, a stationary version of EIGEN-6S (EIGEN-6S_stat), a corrected version of EIGEN-6S and three enhanced versions of EIGEN-6S called EIGEN-6S2, EIGEN-6S2A and EIGEN-6S2B. By “stationary” we mean “containing periodic parameters such as annual and semi-annual variations, but no secular (drift) terms”. We computed precise orbits for the radar altimetry satellites ERS-1, ERS-2, TOPEX/Poseidon, and Envisat over 20 years between 1991 and 2011. The orbit, single-mission and multi-mission altimetry crossover analyses show that the time variable models EIGEN-6S_corrected, EIGEN-6S2 and its two precursors EIGEN-6S2A/B perform notably better than the stationary models for the GRACE period from 2003 onwards. Thus, using EIGEN-6S2 and EIGEN-6S2A/B we have got 3.6% smaller root mean square fits of satellite laser ranging observations for Envisat, as when using EIGEN-GL04S. However, for the pre-GRACE period 1991–2003, the stationary geopotential models EIGEN-GL04S and EIGEN-6S_stat as well as EIGEN-6S2 having no drift terms for degree 3–50 at this time interval perform superior compared to EIGEN-6S_correct and EIGEN-6S2A/B which contain drifts for this period. We found, that the time variable geopotential models have a low (0.1–0.2 mm/yr) impact on our results for the global mean sea level trend. However, we found strong East/West differences up to 3 mm/yr in the regional mean sea level trends when using orbits of all four satellites based on time variable and stationary geopotential models. We show that these differences are related to the relative drifts of the centers-of-origin between the orbit solutions based on the time variable and stationary geopotential models. From the results of our detailed study, we conclude that the final version of the time variable gravity field model EIGEN-6S2 performs best for the four satellites tested. This model provides the most reliable and mission-consistent sea level estimates for the whole time period from 1992 to 2010. This model is of maximum spherical harmonic degree and order 260 and contains time series for drifts as well as annual and semiannual variations of the spherical harmonic coefficients for degree 2–50.     

From Titan’s tholins to Titan’s aerosols: Isotopic study and chemical evolution at Titan’s surface

In the present work, we focused on the possible isotopic fractionation of carbon during the processes involved in the formation of Titan’s tholins. We present the first results obtained on the ¹²C/¹³C isotopic ratios measured on Titan’s tholins synthesized in laboratory with cold plasma discharges. Measurements of isotopic ratio ¹²C/¹³C, done both on tholins and on the initial gas mixture (N₂:CH₄ (98:2)) used to produce them, do not show any evident deficit or enrichment in ¹³C relatively to ¹²C in the synthesized tholins, compared to the initial gas mixture. This observation allows to go further in the analyses of the ACP experiment data, including part of the Cassini–Huygens mission.     

Simulation of Ambiguity Effects in Doppler Tracking of Pioneer Probes

Various models have been proposed to interpret the anomalous Pioneer Doppler data. We present in this paper a simulation tool aiming at determining signatures that could be searched in the data reduction process in order to discriminate between these different explanations. We discuss preliminary results on the seasonally modulated anomalies and compare two interpretations corresponding to a constant anomalous acceleration on one hand, to an anomalous curvature in the second sector of the gravity law on the other hand. Though the second sector interpretation could naturally induce large modulated anomalies, the adjustment of the initial conditions partly compensate these modulations and thus produces a signature resembling that of a constant acceleration. The difference between the predictions of the two interpretations is in fact close to the rms level of the residuals after the data analysis.     

Current Capabilities of MEMS Capacitive Accelerometers in a Harsh Environment

One of the general trends of the MEMS sensors business is the utilization of the technology to satisfy harsh environment requirements (temperature, shock, vibration, environment security). The conjunction of material (standard silicon, SiC or SOI), with complex micromachining techniques and advanced assembly techniques are the key to provide robust sensors with a minimum concession on specification. The goal of this paper is to present progress on gun hard (>20,000 g) and wide temperature range MEMS accelerometers (-120degC to +180degC). Concrete solutions and results (out of more than 500 tested products) will be presented and discussed in detail     

CNES/GRGS 10-day gravity field models (release 2) and their evaluation

GRACE, designed to monitor temporal variations in the fluid mass at the surface of the Earth, is still operating and providing invaluable data 7 years after launch. One hundred and ninety-nine satellite-only geopotential solutions to degree and order 50 were recomputed per 10-day interval for the period 29 July 2002–27 May 2008 using an improved data editing and solution regularization procedure. These release 2 solutions are significantly improved compared to release 1 solutions, the noise over deserts and oceans in the form of North–South striping being reduced by 20–40%. This is thanks to the tailored regularization of each individual Stokes coefficient applied in the solution procedure, and to a time-variable reference model containing mean annual, semiannual and secular variations for degrees 2–50 towards which the variations per 10-day solution are constrained. It may attenuate signals of the order of a few percent, whereas this always occurs when applying a Gaussian smoother even with a half-width smoothing radius as small as 300 km. The uncertainty of an individual point in the time series of a basin expressed in equivalent water height inferred from the 10-day solutions is approximately 20 mm. Comparison of these 10-day solutions to monthly GRACE project solutions (CSR, GFZ and JPL) shows substantial differences. Even for the largest basin, the Amazon, a 15% difference in annual amplitude is found between CNES release 2 and CSR versus GFZ and JPL. The mass-loss estimates for East and West Greenland vary by 100%. Sometimes clear outliers are detected in the GFZ and JPL solutions when a particular basin is studied, which have to be eliminated. In view of the large differences detected between the time series for specific basins, it is hazardous to draw conclusions based on a single solution.     

Modeling PSBL high speed ion beams observed by Cluster and Double Star

On October 8, 2004, the Cluster and Double Star spacecraft crossed the near-Earth (12–19 R_E) magnetotail neutral sheet during the recovery phase of a small, isolated substorm. Although they were separated in distance by ∼7 R_E and in time by ∼30 min, both Cluster and Double Star observed steady, but highly structured Earthward moving >1000 km/s high speed H⁺ beams in the PSBL. This paper utilizes a global magnetohydrodynamic (MHD) simulation driven by Wind spacecraft solar wind input to model the large-scale structure of the PSBL and large-scale kinetic (LSK) particle tracing calculations to investigate the similarities and differences in the properties of the observed beams. This study finds that the large-scale shape of the PSBL is determined by the MHD configuration. On smaller scales, the LSK calculations, in good qualitative agreement with both Cluster and Double Star observations, demonstrated that the PSBL is highly structured in both time and space, on time intervals of less than 2 min, and spatial distances of the order of 0.2–0.5 R_E. This picture of the PSBL is different from the ordered and structured region previously reported in observations.     

ODYSSEY, Orbit Determination Software for the Pioneer Data Analysis

The Pioneer anomaly refers to the difference between the computed trajectories of the Pioneer 10 and 11 spacecrafts and their actual trajectories as observed through Doppler tracking. This difference has been described by the Jet Propulsion Laboratory (JPL) as a constant anomalous acceleration. In order to perform an independent analysis, specific trajectography software, named ODYSSEY, has been developed. The paper will focus on the models implemented in this software and on the results obtained. The existence of a constant anomalous acceleration is confirmed with properties similar to those reported by JPL. Time dependent components of the anomaly are also found and discussed.