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International audience; An X/Ka-band (8.4/32 GHz) celestial reference frame has been constructed using single baselines from the combined NASA and ESA Deep Space Networks for approximately 100 sessions each of ∼24-hour duration. The frame solution has dramatically improved with respect to the last reported frame due to the inclusion of Southern NASA-ESA baselines, routine 2-Gbps data rates, and correction of instrumental delays by recently deployed Ka-band phase calibration tones. Comparisons with the S/X-band (2.3/8.4 GHz) ICRF-2 reference frame will be presented showing increasing agreement for 525 common sources. About 135 sources are located in the south polar cap (δ < −45◦) which became accessible for first time with the addition of the ESA station in Malargüe, Argentina to our project’s network. There is evidence for systematic errors at the 100 μas level. The known sources of error will be discussed.Frame tie precision with Gaia has been estimated in about ±7 μas (1-σ, per 3-D rotation com- ponent) using measured X/Ka position uncertainties and simulated Gaia uncertainties. Compared to X-band, Ka-band allows access to more compact radio source morphology and reduced core shift which should reduce these systematic errors compared to a tie of Gaia to S/X-band VLBI. However, there is a great deal of uncertainty in the offset between optical and radio centroids from effects such as optical host galaxy asymmetry which may ultimately limit the frame tie accuracy.

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.

ESA Deep Space Antennas are currently equipped with precise meteorological stations, capable to measuring the atmospheric parameters. Thanks to mathematical models both dry and wet components are estimated in order to remove the troposphere delay from Doppler data for deep space probe navigation purposes. Although the mentioned technique for the estimation of the troposphere delay offers high precision in the estimation of the dry component, this is not true for the wet one. In particular, the estimation of the wet component using meteorological data leaves an uncalibrated wet delay which reduces the precision in the Orbit Determination (OD) process of ESA spacecrafts (S/C). A novel GPS-based software (S/W) code has been developed at the Radio Science Laboratory of the University of Bologna in Forlì, aimed at the estimation of the troposphere delay, in particular the wet component. Thanks to the acquisition of raw data from the GPS receiver installed close to the ESA Deep Space Antenna in Cebreros, Spain, the local troposphere path delay has been estimated with this new algorithm. The obtained results have been feed into the ESOC’s interplanetary orbit determination software, and the quality of the range-rate residuals compared to those obtained with standard tropospheric calibrations. To this aim, range-rate data from the ESA deep space probe Venus Express, as acquired at the Cebreros Deep Space Antenna, while the spacecraft was close quadrature with the Earth and the Sun. This paper describes the data analysis performed on the acquired S/C range-rate and GPS-receiver measurements and the conclusions and recommendations for further studies.

Deep space exploration missions are aimed at acquiring information about the solar system and a significant communication capacity has to be planned to transfer data for such very large distances. Terrestrial atmospheric impairments on the space-to-Earth propagating signal are the major responsible for the signal degradation thus reducing the channel temporal availability. In this work weather forecast models, coupled with microphysically-oriented radio-propagation models, are described in order to evaluate atmospheric effects at Ka-band. Estimation data return techniques are summarized and numerical results in a simulated operational scenario are illustrated in terms of received data volume using the BepiColombo mission as a baseline example.

Media propagation noises are amongst the main error sources of radiometric observables for deep space missions, with fluctuations of the tropospheric excess path length representing a relevant contributor to the Doppler noise budget. Microwave radiometers currently represent the most accurate instruments for the estimation of the tropospheric delay and delay rate along a slant direction. A prototype of a tropospheric delay calibration system (TDCS), using a 14 channel Ka/V band microwave radiometer, has been developed under a European Space Agency contract and installed at the deep space ground station in Malargüe, Argentina, in February 2019. After its commissioning, the TDCS has been involved in an extensive testbed campaign by recording a total of 44 tracking passes of the Gaia spacecraft, which were used to perform an orbit determination analysis. This work presents the first statistical characterization of the end-to-end performance of the TDCS prototype in an operational scenario. The results show that using TDCS-based calibrations instead of the standard GNSS-based calibrations leads to a significant reduction of the residual Doppler noise and instability.

The Mercury Orbiter Radio Science Experiment (MORE) of the ESA mission BepiColombo will provide an accurate estimation of Mercury’s gravity field and rotational state, improved tests of general relativity, and a novel deep space navigation system. The key experimental setup entails a highly stable, multi-frequency radio link in X and Ka band, enabling two-way range rate measurements of 3 micron/s at nearly all solar elongation angles. In addition, a high chip rate, pseudo-noise ranging system has already been tested at 1-2 cm accuracy. The tracking data will be used together with the measurements of the Italian Spring Accelerometer to provide a pseudo drag free environment for the data analysis. We summarize the existing literature published over the past years and report on the overall configuration of the experiment, its operations in cruise and at Mercury, and the expected scientific results.