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This paper illustrates the design, implementation and testing of a prototype of an off-line correlator for large dish antenna array, in the frame of the ESA-funded study “Prototype of off-line Correlator for Arraying of Large Aperture Antennas” In order to improve the performance of downlink reception, a four 35-m antenna array is a valid alternative to a single large 70-m dish, offering the same performance with remarkable advantages, such as being more fail-safe against the failure of components. The concept is to acquire two or more replicas of the same signal at different Ground Stations and combine them after delay and phase correction, due mainly to media and clock offset. After a first study phase of the best combination and correlation methods, as well as of the requirements driven by the analysis of the error sources of delay and phase misalignment, the tool has been designed, implemented and successfully tested. Together with the correlator, a simulator has also been developed, in order to better test the performance of the combination tool. Tests with simulated, static (generated in ESOC) and real mission data have been carried out. The improvement in the Signal-to-Noise Ratio has been demonstrated, and the information bits have been successfully reconstructed after the combination of the deep space signal acquired in Cebreros and Malargue stations from Exomars and Mars Express spacecraft, endorsing the tool for an off-line use in the near future.

The objective of this study, named AWARDS (Advanced microWAve Radiometers in Deep space Stations), is the preliminary design of a transmission Media Calibration System (MCS) to be located at an ESA Deep Space Antenna (DSA) site. The crucial aspect is the capability to accurately retrieve the tropospheric path delay along the line-of-sight of the deep space probe in order to allow precise tropospheric calibration of deep space observables (range and range-rate) with particular reference to the BepiColombo spacecraft and its primary DSA at Cebreros (ES). The study focuses on two main aspects which lead to the preliminary design of the Mercury Orbiter Radioscience Experiment (MORE) MCS: the characterization of current microwave radiometers (MWRs) available at ESA/ESTEC and the atmospheric fluctuation effects on the MCS error budget, in terms of the Allan standard deviation (ASD). In the course of the study, further critical aspects have been identified (effects of Sun contamination, effects of ground noise emission), and mitigation strategies have been proposed. The final outcome is a preliminary design of the MWR (and the entire MCS) to be deployed at the ESA/ESTRACK (ESA Tracking station network) sites and being compliant with MORE requirements.

The move towards higher frequency bands like 26GHz or 32GHz for scientific satellite data downlinks calls for the need to better characterise the atmosphere at the sites where ground antennas are located. For this reason ESA is undertaking the deployment of microwave radiometers at its Deep Space station locations. This paper reports the results of the first two years of site characterisation at the ESA Deep Space antenna 2 located in Cebreros, Spain.

The estimation of the path delay due to water vapor is a crucial aspect for the calibration of the Doppler observables of a deep space probe. The Advanced Water Vapor Radiometer (AWVR), developed by the Jet Propulsion Laboratory (JPL, NASA), already proved its capability to accurately estimate the path delay during the entire Cassini mission. Here, from the AWVR measurements, a scalar Sky Status Indicator (SSI) was developed as a criterion for selecting the radiometric path delay estimations in the orbit determination process. Results indicate that the use of such index allows a reduction of Doppler residual root mean square. © 2014 IEEE.

In a suggested radio propagation experiment using a deep space antenna, accurate calibration of the propagation delay through the Earth's atmosphere is essential. One or two microwave radiometers can be used for this purpose. Differences in precise locations of the radiometer(s) and antenna to be calibrated leave a residual wet path delay value. We computed the Allan Standard Deviation (ASD) of this residual, as well as the one resulting from different pointing positions in the plane of the sky, by simulations. Pointing offsets, e.g., to avoid solar radiation into the radiometer beam, lead in general to an increased ASD. However, for many observation geometries a deliberate pointing offset can compensate for the location differences. In the case studied we found a reduction of the ASD with up to 45% compared to the ASD obtained for a zero pointing offset. The size of the calculated ASD depends strongly on the model parameters used, e.g., the turbulence strength parameter Cn^2, which has a significant natural variation over a year.

Turbulence is an important effect for different applications including different aspects of electromagnetic propagation in the atmosphere, especially for frequencies in the microwave part of the spectrum. Our parameter of interest is an integrated effect, namely the propagation delay of a signal propagating through the Earth's atmosphere to a receiver on the ground. A simulation method has been implemented in order to predict the statistics of atmospheric turbulence. For systems where calibration for the wet delay is done by means of radiometric measurements there is an uncalibrated remainder of the variability due to imperfections of the method: (1) the radiometer location, which may not be identical to the location of the receiving antenna that is subject to calibration, (2) a pointing offset on the sky, and (3) the radiometer beam that may sense another volume of the air than does the original signal. The size of the remainder depends on the assumptions made about the atmospheric variability when the instrument is calibrated. The analysis shows that the overall performance of the system to a significant level depends on the uncertainty in the true range of variation for the turbulence strength parameter C n 2.

The main focus of this study is the characterization of internal noise of the Microwave Radiometer (MWR) to be located at an ESA Deep Space Antenna (DSA) site, to accurately retrieve the tropospheric path delay along the line-of-sight of the deep space probe. This would enable precise tropospheric calibration of radiometric observables (range and range-rate) with particular reference to the BepiColombo SIC and the Cebreros (S) DSA. The characterization of current MWRs technologies has been based on a detailed analysis of data collected at Cabauw (NL) remote sensing site with the available instruments.

This paper describes the techniques currently in use and those proposed for a precise estimation of the Earth troposphere path delay for Deep Space probe tracking purposes. The different proposed techniques could be selected according to the to the mission goals and tracking accuracy requirements. In this paper, they are sorted by an increasing accuracy but also level of complexity.

Radio occultation technique performed with GNSS observables acquired by low Earth orbit satellites offer a great amount of data to carry out experiments and measurements in the framework of Earth survey. A dedicated GNSS-based radio occultation experiment has been designed to be installed onboard the microsatellite ALMASat-EO mission, developed at the Microsatellite Laboratory of the II Faculty of Engineering of the University of Bologna. The experiment, named ALASE (ALMASat Atmosphere Sounding Experiment) will offer a large amount of occultation events in order to perform scientific measurements of the atmosphere profile and climatology studies. In order to evaluate the feasibility of the experiment, it has been simulated with a dedicated ALMA GNSS Scenario Simulator Software (ALMAG3S) specifically developed for this purpose. The scenario software includes all GNSS satellites position in a precise post processing and pseudo real time environment. The modeled GNSS observables are perturbed considering the main error sources: satellite clock, receiver clock, instrumental errors, relativistic errors and propagation models. ALMAG3S models the errors and the GNSS satellites position by using the IGS products.

For a numberof political, social and economic reasons, the State has been taking over, during the last few decades, a number of functions that have made it, in point of fact, the protagonist of the country's collective life.

In the Microsatellite Laboratory of the University of Bologna a project was started in 2003, aimed at the development of a multipurpose microsatellite platform named ALMASat. The launch into orbit of the first prototype ALMASat-1 is currently foreseen by the end of 2008. One of the natural applications of the modular ALMASat platform is in the field of space-based Earth observation. At this aim, a spin-off company named ALMASpace was established in 2006, with the goal of designing and demonstrating the feasibility of a low-cost Earth observation mission based on the technological achievements obtained by the ALMASat project. This paper summarizes the main characteristics of the ALMASat-1 spacecraft and describes the current status of the design and development of the ALMASat-EO (Earth Observation) spacecraft bus and optical payload whose launch into orbit is currently scheduled at the end of 2010.