SSim: NASA Mars Rover Robotics Flight Software Simulation

Each new Mars rover has pursued increasingly richer science while tolerating a wider variety of environmental conditions and hardware degradation over longer mission operation duration. Sojourner operated for 83 sols (Martian days), Spirit for 2208 sols, and Opportunity is at 5111 sols, and Curiosity operation is ongoing at 2208 sols. To handle this increase in capability, the complexity of onboard flight software has increased. MSL (also known as Curiosity), uses more flight software lines of code than all previous missions to Mars combined, including both successes and failures[1]. MSL has more than 4,200 commands with as many as dozens of arguments, 54,000 parameters, and tens of thousands of additional state variables. A single high-level command may perform hours of configurable robotic arm and sampling behavior. Incorrect usage can result in the loss of an activity or the loss of the mission. Surface Simulation ("SSim") was developed to address the challenge of making full and effective use of many capabilities of MSL, while managing complexity and risk. SSim is software that performs rapid context sensitive simulation of flight software. NASA Mars missions are comprised of three phases: several months of Cruise, a brief but exciting Entry Descent and Landing (EDL), and a Surface mission that typically lasts as long as the hardware survives. SSim is meant for use during the surface phase when the mission fulfills its primary objectives. The focus of SSim on MSL was the robotic flight software, including rover mobility and navigation, robotic arm manipulation, and sample acquisition, processing, and delivery. It can execute behaviors in simulation a thousand times faster than they execute in real time on the flight compute element. SSim is used by rover drivers to develop and validate command sequences throughout the planning cycle. SSim has been used to plan all of the Curiosity robotic operations since landing and is expected to continue to be used for the remaining life of the rover. Due to the impact of SSim on MSL, the Mars 2020 mission plans to increase the scope of SSim during flight operations, simulating not only rover planner operations, but all surface operations, including the instrument, power, thermal and telecommunication behavior. SSim is part of the Rover Sequencing and Visualization (RSVP) suite of Rover Planning tools [2]. In the paper we provide an overview of SSim architecture, design, implementation, and usage on MSL, as well as an overview of plans for Mars 2020.