OFF-A: software offline correlator for antenna array

The concept of antenna array is to combine two or more signals, acquired at different antennas, to increase the SNR or reduce the station and transmission time. In the former case, the increase in the SNR allows for a larger data volume, since a higher data rate can be used for the downlink transmission. In the latter case, the same amount of data can be acquired, saving station and transmission time. The project was triggered by the mission EnVision, selected by ESA as the fifth medium-class mission in the Agency’s Cosmic Vision plan. However, it will bring flexibility to a large variety of ESA missions. An offline software correlator for antenna array (OFF-A) has been developed, accessible through a web-based GUI, to be installed in a central location (ESOC) where the open loop data are transferred after being recorded at two deep space antennas over a long baseline. The data (up to 40 Msps) are processed in a correlator/combiner, that adjusts for delay and phase offsets between the two signals before the combination. The combined signal is then demodulated (PCM(NRZ-L)/PSK/PM, PCM/PM/SP-L and GMSK BT=0.5) and decoded (Turbo Code with dimension k=8920 and available rates 1/2, 1/4 and 1/6), and the telemetry frames provided to mission control system. The software nature of the tool presents attractive characteristics (scalability, high independence of HW platform, easy evolution and deployment, evolution to a cloud service), but at the same time it opens to a technological challenge to reach good performances in terms of execution time. Different technologies and solutions were put in place to reach the goal, such as internal parallelization using either CPUs or Single Instruction Multiple Data (SIMD), global parallelization among the services, state-of-the-art machines (but still commercial to reduce the costs). This led to a tool characterized by extreme efficiency and speed. The system is deployed on six machines, where several services run under the orchestration of Kubernetes. This assures the best level of parallelization and performances. The tool has been tested against a large variety of data, including simulated data, static data generated in ESOC facilities and data acquired from the Solar Orbiter and ExoMars Trace Gas Orbiter missions. Results are above expectations, with quasi-real time performances at 20 Msps symbol rate, suitable for today’s deep space spacecraft. Real time applications for collocated stations using the software tool for the antenna array are the follow up of this project.