The Solar Orbiter X-Band TT&C New Features: GMSK with PN Regenerative Ranging and DDOR Semaphores Implementation

Solar Orbiter is an ESA mission in cooperation with NASA scheduled for launch in February 2020. The Solar Orbiter spacecraft will enter, after a multi-year cruise phase, into an elliptical orbit around the sun with a perihelion of around 0.3 AU and an increasing inclination of up to 35° to return images of the solar polar regions and to probe the plasma of the inner heliosphere.As all deep space missions, Solar Orbiter has a severely power limited data downlink, which means that optimizing the science data return of the mission within the given constraints is of paramount importance.The Solar Orbiter mission operations centre has consequently optimized the concept, technique and duration of the required ranging transmissions to improve the science data return while meeting the orbit determination requirements for flying the mission through multiple planetary gravity assists.Solar Orbiter TT&C subsystem is derived from the successful Bepi Colombo design that implemented for the first time the CCSDS recommended Pseudo Noise (PN) regenerative ranging with residual carrier telemetry modulation schemes like NRZ-L/PSK/PM or SP-L/PM. In order to provide the required mission science data return and, due to the bandwidth limitation of the SP-L/PM modulation, GMSK (Gaussian Minimum Shift Keying) modulation was included in the Solar Orbiter design allowing increased telemetry data rates up to 1 Mbps while NRZ-L/PSK/PM and SP-L/PM were kept for lower data rates.Successful in-flight utilization of GMSK modulation with the Herschel and Planck spacecraft in 2009, Gaia in 2013 and ExoMars TGO in 2016, has increased the interest to investigate the possibility of utilizing GMSK modulation simultaneously with PN regenerative ranging. As such ESA started to investigate this technique for Lagrange and Deep Space missions as part of the CCSDS standardization activities. The development activities started in 2012 and the first tests with a ground station modem breadboard [7] to demonstrate the concept feasibility were performed in 2014 at the ESOC reference station in Darmstadt.Being GMSK a suppressed carrier modulation, it does not allow to use at the same time the ESA standard phase modulated ranging signals (based on a residual carrier) so that dedicated ranging sessions were needed in the missions flown so far. Alternating sessions with residual carrier modulation of low rate telemetry plus ESA standard ranging and sessions with suppressed carrier GMSK modulation of high rate telemetry had to be performed in each communication pass in order to allow gathering of the required ranging data for orbit determination while meeting the scientific data volume requirements.To avoid such inefficient mode switching, the use of simultaneous GMSK telemetry modulation with PN regenerative ranging was studied in CCSDS and a recommend standard was published. Solar Orbiter adopted and implemented it both on board and on ground.The Bepi Colombo transponder also implemented the generation of Differential Delta One-way Ranging (DDOR) signals used to improve the navigation during gravity assist manoeuvres (GAMs) and during fly-bys. Solar Orbiter will also rely on Earth and Venus GAMs during its 10 years extended mission, so DDOR signals were also inherited in the design. The additional inclusion of in flight programmable modulation indexes brought to the creation of a semaphores modulation scheme that will be used to provide fundamental spacecraft health status information in case it would enter in survival mode during which only limited communication capabilities are possible.This paper will present the implementation of these new features and report on the performance results obtained during the Radio Frequency compatibility tests between the RF suitcase including the on-board TT&C engineering models and the ESTRACK ground station at ESOC.