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Deciphering the genesis and evolution of the Martian polar caps can provide critical understanding of the climate system of Mars and allow us to apply the lesson learned about planetary climate change on other terrestrial planets. In this work, we present a novel mission concept that can support autonomous navigation of rovers devoted to scientific investigations of these regions. We propose and analyse a constellation of 5 small satellites with coverage over Mars polar regions, focusing on the positioning performance that can be obtained. Ground support can be heavily reduced by employing an inter-satellite link (ISL) communication architecture. Moreover, this concept may provide excellent range rate accuracies in the ISL via the implementation of radio link architectures able to suppress the adverse effects of on board clock instabilities. The constellation would use 5 satellites deployed on three quasi-circular high-altitude polar orbits. The configuration is composed by a main spacecraft in polar orbit and four spacecraft symmetrically located on the two inclined orbits. The periodical synchronisation of the constellation clocks is granted by the main spacecraft, the only element of the constellation communicating with Earth. We describe the overall architecture of the constellation and report on the results of our numerical simulations in different operative scenarios. We show that excellent orbital accuracies can be obtained for the whole constellation using a batch-sequential filter that can be easily implemented on board, thus enabling a high degree of navigational autonomy. Furthermore, we analyse the effects of non-gravitational accelerations acting on the satellites and their modelling (based on the SmallSats design being developed in Argotec for Mars/Moon constellations), and assess their effects against the on board limited computational resources. Another crucial aspect that has been investigated is the optimal arc length of the batch sequential filter, as a trade-off between the degradation of accuracy in the dispatched ephemerides and the accuracy in the dynamical model. The assessment of the achievable positioning accuracy is also fundamental to evaluate the feasibility of a future positioning system providing a global coverage of the planet.

Navigation of deep-space probes is accomplished through a variety of different radio observables, namely Doppler, ranging and Delta-Differential One-Way Ranging (Delta-DOR). The particular mix of observations used for navigation mainly depends on the available on-board radio system, the mission phase and orbit determination requirements. The accuracy of current ESA and NASA tracking systems is at level of 0.1 mais at 60 s integration time for Doppler, 1 to 5 m for ranging and 6 to 15 nrad for Delta-DOR measurements in a wide range of operational conditions. The ASTRA study, funded under ESA's General Studies Programme (GSP), addresses the ways to improve the end-to-end accuracy of Doppler, ranging and Delta-DOR systems by roughly a factor of 10. The target accuracies were set to 0.01 mm/s at 60 s integration time for Doppler, 20 cm for ranging and 1 nrad for Delta-DOR. The companies and universities that took part in the study were the University of Rome Sapienza, ALMASpace, BAE Systems and Thales Alenia Space Italy. The analysis of an extensive data set of radio-metric observables and dedicated tests of the ground station allowed consolidating the error budget for each measurement technique. The radio-metric data set comprises X/X, X/Ka and Ka/Ka range and Doppler observables from the Cassini and Rosetta missions. It includes also measurements from the Advanced Media Calibration System (AMCS) developed by JPL for the radio science experiments of the Cassini mission. The error budget for the three radio-metric observables was consolidated by comparing the statistical properties of the data set with the expected error models. The analysis confirmed the contribution from some error sources, but revealed also some discrepancies and ultimately led to improved error models. The error budget reassessment provides adequate information for building guidelines and strategies to effectively improve the navigation accuracies of future deep space missions. We report both on updated error budget for radio-metric observables and the system configurations proposed for the upgrade of ESA's tracking and orbit determination systems.

The RC-SIM project aimed at the analysis of existing and new techniques for the exploitation of radio-communication signals from interplanetary probes for remote sensing of the surface and other geophysical parameters of planets and moons. Main objectives of the project have been to investigate the physical parameters which can be obtained from the use, as a remote sensing instrument, of the radio-communication systems on-board interplanetary probes; to analyse the added value provided by this approach compared to other types of instruments also supplying surface and other geophysical parameters; to develop a prototype simulator generating quantitative results of the different aspects involved in the radio communication link. After reviewing the previous and foreseen experiments on this field and the state-of-the-art of radio-communication systems, three target scenarios were considered: * Titan scenario, defining an orbiter around Titan and a balloon overflying its surface in different configurations for the realization of bistatic radar experiments to retrieve the dielectric constant and the root-mean-square (r.m.s.) surface slope. * Martian lander, to determine the Martian rotational state by using the Radio Frequencies (RF) signal coming from a lander on Mars to an Earth station. * Moon interferometric mission: this scenario involves a network of 3-4 widely spaced landers on the Moon and aims to accurate determination (to 0.2 mm) of lunar tides and librations. The most relevant models selected to simulate these scenarios have been: accurate Mars and Moon rotational state models; Titan surface model of interaction and reflection of RF signal; error models (atmospheric, solar plasma, transponder ageing …); retrieval models. The simulator has been developed according to pre-selected scientific requirements that may come from future planetary missions to assess the potential benefits of the use of telecom links for geophysical investigation. The results show that state-of-the-art architecture of radio-communication systems may provide significant improvements in the knowledge of several geophysical parameters. The flexible simulator setup allows analysing different configurations and models, as well as performing sensitivity analyses in future missions’ context.

In its tour of the Saturnian system, the spacecraft Cassini is carrying out measurements of the gravity field of Titan, whose knowledge is crucial for constraining the internal structure of the satellite. In the five flybys devoted to gravity science, the spacecraft is tracked in X (8.4 GHz) and Ka band (32.5 GHz) from the antennas of NASA’s Deep Space Network. The use of a dual frequency downlink is used to mitigate the effects of interplanetary plasma, the largest noise source affecting Doppler measurements. Variations in the wet path delay are effectively compensated by means of advanced water vapor radiometers placed close to the ground antennas. The first three flybys occurred on February 27, 2006, December 28, 2006, and June 29, 2007. Two additional flybys are planned in July 2008 and May 2010. This paper presents the estimation of the mass and quadrupole field of Titan from the first two flybys, carried out by the Cassini Radio Science Team using a short arc orbit determination. The data from the two flybys are first independently fit using a dynamical model of the spacecraft and the bodies of the Saturnian system, and then combined in a multi-arc solution.

Cassini's last look at Saturn's rings During the final stages of the Cassini mission, the spacecraft flew between the planet and its rings, providing a new view on this spectacular system (see the Perspective by Ida). Setting the scene, Spilker reviews the numerous discoveries made using Cassini during the 13 years it spent orbiting Saturn. Iess et al. measured the gravitational pull on Cassini, separating the contributions from the planet and the rings. This allowed them to determine the interior structure of Saturn and the mass of its rings. Buratti et al. present observations of five small moons located in and around the rings. The moons each have distinctive shapes and compositions, owing to accretion of ring material. Tiscareno et al. observed the rings directly at close range, finding complex features sculpted by the gravitational interactions between moons and ring particles. Together, these results show that Saturn's rings are substantially younger than the planet itself and constrain models of their origin. Science , this issue p. 1046 , p. eaat2965 , p. eaat2349 , p. eaau1017 ; see also p. 1028

The gravity harmonics of a fluid, rotating planet can be decomposed into static components arising from solid-body rotation and dynamic components arising from flows. In the absence of internal dynamics, the gravity field is axially and hemispherically symmetric and is dominated by even zonal gravity harmonics J_(2n) that are approximately proportional to q^n, where q is the ratio between centrifugal acceleration and gravity at the planet’s equator. Any asymmetry in the gravity field is attributed to differential rotation and deep atmospheric flows. The odd harmonics, J_3, J_5, J_7, J_9 and higher, are a measure of the depth of the winds in the different zones of the atmosphere. Here we report measurements of Jupiter’s gravity harmonics (both even and odd) through precise Doppler tracking of the Juno spacecraft in its polar orbit around Jupiter. We find a north–south asymmetry, which is a signature of atmospheric and interior flows. Analysis of the harmonics, described in two accompanying papers, provides the vertical profile of the winds and precise constraints for the depth of Jupiter’s dynamical atmosphere.