Assessment of the Robustness of the Mars 2020 Terminal Descent Sensor in the Event of Beam Failure

The Mars 2020 entry, descent, and landing architecture resembles closely that of MSL. Due to the inherent similarity between both missions, Mars 2020 will make use of heritage technology and spare parts of Mars Science Laboratory (MSL), including the Terminal Descent Sensor. The Terminal Descent Sensor collects instantaneous altitude and velocity measurements of the vehicle using a six narrow-beam line of sight Doppler altimeter/velocimeter. For MSL, it was shown that the system is robust enough for one beam to be temporarily obscured by the jettisoned heatshield. In this paper, we further evaluate the system's robustness and EDL performance in the event of beam failure. This assessment was done by means of Monte Carlo dispersion analysis. In each simulation it is assumed that one of the six beams does not collect measurements. Further analysis was conducted to evaluate system performance by employing a measurement collection scheme that leads to a lower loss of measurements. Analysis demonstrated that in the event of beam failure the TDS provides degraded altitude and velocity measurements. The largest degradation in performance is seen when the nadir pointing beam does not collect altitude measurements, resulting in timeline degradation, higher altitude errors, and a higher vertical velocity at touchdown. Results showed, however, that measurements provided by the TDS to the navigation filter are sufficient to still ensure a survivable touchdown if the system employs the original beam sequences or opts to use an alternate sequence in the event that a hardware failure is discovered during the inter-planetary cruise to Mars.