Improvement of BepiColombo's radio science experiment through an innovative Doppler noise reduction technique

The Mercury Orbiter Radio science Experiment (MORE), onboard the ESA/JAXA BepiColombo mission to Mercury, is designed to estimate Mercury’s gravity field, its rotational state, and to perform tests of relativistic gravity. The state-of-the-art onboard and ground instrumentations involved in the MORE experiment will enable to establish simultaneous X/X, X/Ka and Ka/Ka-band links, providing a range rate accuracy of 3 µm/s (at 1000 s integration time) and a range accuracy of 20 cm. The purpose of this work is to show the improvement achievable on MORE’s performance by means of the Time-Delay Mechanical Noise Cancellation (TDMC) technique. The TDMC consists in a combination of Doppler measurements collected (at different times) at the two-way antenna and at an additional, smaller and stiffer, receive-only antenna that should be located in a site with favorable tropospheric conditions. This configuration could reduce the leading noises in a Ka-band two-way link, such as those caused by troposphere and ground antenna mechanical vibrations. We present the results of end-to-end simulations and estimation of Mercury’s gravity field and rotational state considering the TDMC technique. We compare results for a two-way link from NASA’s DSS-25 (in Goldstone, CA) or from ESA’s DSA-3 (in Malargue, Argentina), while we assume APEX as the receive-only antenna. We show that in best-case noise conditions, the TDMC technique allows to obtain a factor-of-two accuracy gain on both global and local parameters, considering DSA-3 as two-way antenna. Such improvement in the scientific objectives of MORE is of geophysical interest as it could provide a constraint on the interior structure of Mercury

El origen del sistema de relaciones laborales en el Uruguay

The purpose of this paper is the identification of issues that help to explain the origin of a Labor Relations Systems in this country. It includes the analysis of the different components and their links with the social, economic and political context.Labor relations, collective bargaining

Report on first inflight data of bepicolombo’s mercury orbiter radio-science experiment

BepiColombo’s Mercury Orbiter Radio-science Experiment (MORE) was conceived to enable extremely accurate radio tracking measurements of the Mercury Planetary Orbiter to precisely determine the gravity field and rotational state of Mercury, and to test theories of gravitation (e. g. Einstein’s Theory of General Relativity). The design accuracy of the radio tracking data was 0.004 mm/sec (at 1000 s integration time) for range-rate measurements and 20 cm for range (at a few seconds of integration time). These accuracies are attained due to a combination of simultaneous two-way microwave links at X (7.2-8.4 GHz) and Kaband (32-34 GHz) to calibrate the dispersive plasma noise component. In this letter, we present the first analysis of range and range-rate data collected by ESA’s deep space antenna (DSA) during the initial cruise phase of BepiColombo. The novel 24 Mcps pseudo-noise (PN) modulation of the Ka-band carrier, enabled by MORE’s Ka-band Transponder (KaT), built by Thales Alenia Space Italy, provided two-way range measurements to centimeterlevel accuracy, with an integration time of 4.2 s at 0.29 astronomical units. In tracking passes with favorable weather conditions, range-rate measurements attained an average accuracy of 0.01 mm/s at 60 s integration time. Data from 20 to 24 May 2019 were combined in a multi-pass analysis to test the link stability on a longer timescale. The results confirm the noise level observed with the single-pass analysis and provide a preliminary indication that the MORE PN ranging system at 24 Mcps is compatible with the realization of an absolute measurement, where the need to introduce range biases in the orbital fit is much more limited than in the past. We show that in the initial cruise test the BepiColombo radio link provided range measurements of unprecedented accuracy for a planetary mission, and that, in general, all target accuracies for radio-metric measurements were exceeded

Determination of Jupiter’s mass from Juno radio tracking data

Juno has been in a highly elliptical (average eccentricity e=0.95), nearly polar, 53-day orbit around Jupiter since July 2016. The solar-powered, spin-stabilized spacecraft hosts a complete suite of scientific investigations (gravity science, magnetospheric science, particles and fields analysis) aimed at shedding light into the planet’s interior and formation [1]. The gravity science investigation relies on Doppler tracking of Juno at Ka-band (32–34 GHz) as it flies by close to Jupiter’s top clouds (∼4000  km altitude on average). Measurements of the spacecraft’s radial velocity (Doppler) with respect to NASA Deep Space Station (DSS) 25 are fitted in the least-square sense to accurate dynamic models of the Juno’s motion to reconstruct its trajectory and estimate Jupiter’s gravity field coefficients [2]. After the first two pericenter passages dedicated to the gravity investigation, Juno had already revealed a north–south asymmetry of the gravity field, which has been tied to the presence of zonal winds at ∼3000  km depth [3,4]. The most recent analysis of the gravity science dataset (hereafter called GRAV), after 11 dedicated perijoves at the midpoint of Juno’s mission, has decreased the uncertainties on the gravity field coefficients, while providing also a determination of the gas giant’s tidal response and pole dynamics [5]. Before the end of the mission scheduled in July 2021, Juno might also unveil fine features of Jupiter’s gravity, such as its frequency-dependent tidal response, the presence of normal modes within the planet, and the depth of the Great Red Spot [6–8]. The GRAV data are sensitive to Jupiter’s gravity field; however, due to the limited timespan of the observations (6–8 h) compared with the 53-day orbital period, it is not possible to decorrelate the monopole component of Jupiter’s gravity field (proportional to the planet’s mass times the gravitational constant GMJ) from the low-degree components [9]. An improvement over the current estimate of GMJ, based on observations of the motion of Jupiter’s satellites from the latest ephemeris reconstruction (GMJ=126,686,534.196±2.7  km3/s2, where the quoted uncertainty is 1-σ) [10], would be relevant for spacecraft navigation, planetary ephemerides generation, and general relativity experiments in the solar system [11]. Before the arrival of the JUICE and Europa Clipper missions to the Jovian system in the 2030s, Juno will be the only spacecraft capable of such a determination

El origen del sistema de relaciones laborales en el Uruguay

The purpose of this paper is the identification of issues that help to explain the origin of a Labor Relations Systems in this country. It includes the analysis of the different components and their links with the social, economic and political context. El objetivo de este documento es identificar las condicionantes que contribuyen a explicar el surgimiento de un Sistema de Relaciones Laborales en el país. Incluye el análisis de las características de los componentes sistema y sus interrelaciones con el contexto social, económico y político.Labor relations, collective bargaining

Analysis of Cassini Altimetric Crossovers on Titan

The Cassini spacecraft performed several flybys of Saturn’s largest moon, Titan, collecting valuable data. During several passes, altimetric data were acquired. Here, we focus on altimetric measurements collected by Cassini’s radar when flying over the same region at different epochs in order to correlate such measurements (crossovers) and investigate differences in altimetry. In our study, we assess altimetric errors associated with three distinct methods for extracting topography from Cassini’s radar data: the maximum likelihood estimator (MLE), the threshold method, and the first moment technique. Focusing on crossover events, during which Cassini revisited specific areas of Titan’s surface, we conduct a detailed examination of the consistency and accuracy of these three topography extraction methods. The proposed analysis involves closely examining altimetric data collected at different epochs over identical geographical regions, allowing us to investigate potential errors due to the variations in off-nadir angle, relative impact, uncertainties, and systematic errors inherent in the application of these methodologies. Our findings reveal that the correction applied for the off-nadir angle to the threshold and first moment methods significantly reduces the dispersion in the delta difference at the crossover, resulting in a dispersion of the order of 60 m, even lower than what is achieved with the MLE (~70 m). Additionally, an effort is made to assess the potential of Cassini for estimating the tidal signal on Titan. Considering the altimetric errors identified in our study and the relatively low number of crossovers performed by Cassini, our assessment indicates that it is not feasible to accurately measure the tidal signal on Titan using the currently available standard altimetry data from Cassini. Our assessment regarding the accuracy of the Cassini altimeter provides valuable insights for future planetary exploration endeavors. Our study advances the understanding of Titan’s complex landscape and contributes to refining topographical models derived from Cassini’s altimetry observations. These insights not only enhance our knowledge of Saturn’s largest moon but also open prospects for Titan surface and interior exploration using radar systems

Feasibility of an innovative technique for noise reduction in spacecraft doppler tracking

Precise measurements of spacecraft range-rate enabled by two-way microwave links are used in radio science experiments for planetary geodesy. The final accuracies in the gravity field recovery depend almost linearly on the Doppler noise in the link. In this work, we present results of simulations carried out to evaluate the improvement attainable in Doppler measurements using an innovative noise-cancellation technique proposed by Armstrong et al. [1], using two mission profiles: a representative low-altitude Venus orbiter and the BepiColombo spacecraft. The Time-Delay Mechanical Noise Cancellation (TDMC) technique involves a combination of Doppler measurements collected (at different times) at the two-way antenna and at an additional, smaller and stiffer, receive-only antenna that should be located in a site with favorable tropospheric conditions. This configuration could reduce the leading noise sources in a Ka-band two-way link, such as tropospheric and antenna mechanical noises. We considered a two-way link either from NASA's DSS 25 (in Goldstone, CA) or from ESA's DSA-3 (in Malargüe, Argentina) antennas. Moreover, we selected the 12-m Atacama Pathfinder EXperiment (APEX) in Chajnantor (Chile) as the three-way antenna and developed its noise model according to atmospheric data and mechanical stability specifications available in literature. For an 8-hour Venus orbiter tracking pass in Chajnantor's winter/nighttime conditions, the Allan deviation of fractional frequency fluctuations Δf/f0(a measure of the link's frequency stability, thus accuracy) of the simulated TDMC observable at 10-s integration time is 4.5×10-14, to be compared to 1.5×10-13for the two-way link. For BepiColombo, we obtained 1×10-13and 2.6×10-13, respectively for the TDMC and two-way links. If successfully implemented, the use of this noise-reducing technique could be valuable for planetary geodesy missions, where the accuracy in the estimation of high-order gravity harmonic coefficients is limited by tropospheric and antenna mechanical noises, difficult to reduce at the short integration times of interest. The improved orbit determination could be also beneficial whenever high navigation accuracies are required