Catenary Model of InSight SEIS Tether for Instrument Deployment

Abstract

NASA's InSight Mars Lander, which landed on Mars in November 2018, completed the first precision robotic instrument placement on a planetary surface as part of its instrument deployment phase. Using the Instrument Deployment Arm (IDA), InSight deployed the two key science payload elements, a seismometer (SEIS) and heat flow probe (HP3), as well as a wind and thermal shield (WTS) placed over the seismometer. SEIS and HP3 are tethered to the lander for both power and communications purposes. Placement accuracy of the instrument deployment was essential for successful realization of the mission's core science objectives. In order to meet the requirements for placement accuracy, we worked to understand and mitigate the factors contributing to placement error. Those factors include but are not limited to: the IDA position control error, the error in the computation of the digital elevation model (DEM), the targeting error in selecting the desired placement site, and each instruments ground interaction. In particular, the SEIS placement accuracy was greatly affected by the placement of its tether on the terrain during the instrument deployment. The SEIS tether is a stack of six flex cable belts attached at one end to the lander deck. At the other end, the tether is looped and attached to the instrument with a load shunt assembly (LSA) designed to isolate signals detected by the instrument from any noise created by the tether due to thermoelastic deformation, atmospheric events, etc. Additionally, the tether contains a field joint and a pinning mass (which grounds the tether and is used to manipulate the tether with the robotic arm post-placement). In our validation and verification (V&V) activities, we determined that the interaction of the SEIS tether on the terrain greatly affected our instrument placement accuracy. In order to understand and minimize this effect, we created a mathematical model of the SEIS tether and then used that model to develop the deployment sequences. Ultimately, our sequences deployed both the tether and the instrument on the Martian surface, taking into consideration the whole system for a successful deployment. This paper will discuss the motivation and details of the implementation of the SEIS tether model and the ways in which the SEIS tether impacted placement accuracy in certain configurations. It will also show results from testing this model on Earth and deploying SEIS to the Martian surface on sol 22 of InSight's mission.