The GOCE Ion Propulsion Assembly - Lessons Learnt from the First 22 Months of Flight Operations

Abstract

The Gravity Field and steady state Ocean Circulation Explorer (GOCE) is a mission implemented in the " Earth Explorer " line of research oriented ESA-Earth observation missions. The objective of the mission is to produce high-accuracy, high resolution, global measurements of the Earth´s gravity, leading to improved gravity field and geoid models for use in a wide range of applications. The GOCE spacecraft was launched successfully in March 2009 with a nominal 2 year mission. The spacecraft is flying in a near-circular, sun-synchronous dawn-dusk orbit, with 96.5° inclination and an altitude of around 260 km. During scientific measurement the Ion Propulsion Assembly is used to provide drag free control of the satellite. Two QinetiQ T5 ion thrusters are used, one in operation and one in redundancy, each capable of throttling over a thrust range 0.6–20.6mN at a resolution of 12 µ µ µ µN. The telemetry data generated by the spacecraft presents a good opportunity to investigate long term in-orbit performance of this type of propulsion system and to provide lessons learnt to improve future missions. This paper presents an evaluation of the in-flight Ion Propulsion Assembly data from the GOCE spacecraft. The evaluation covers the variation of thruster performance and operating parameters with thrust level and through time, including comparisons with commissioning and ground test data. Nomenclature ESA = European Space Agency GOCE = Gravity and Ocean Circulation Explorer GSE = Ground Support Equipment IPA = Ion Propulsion Assembly IPCU = Ion Propulsion Control Unit ITA = Ion Thruster Assembly PXFA = Proportional Xenon Feed Assembly XST = Xenon Storage Tank