The T5 ion propulsion assembly for drag compensation on GOCE

Abstract

Due to the nature of the gravity field measurements to be made by GOCE, the satellite must fly in a near-circular, sun-synchronous, dawn-dusk orbit at an altitude of around 250 km. At this altitude the residual air drag is significant, and must be actively compensated by onboard thrusters. In addition, the gradiometer instrument is sensitive to linear acceleration, which must also be precisely compensated for by the thrusters. The dominant component of the drag force is in the primary flight axis, requiring a propulsion system capable of continuous throttling between 1 and 20 mN, with a thrust resolution of 12 μN, a response rate up to 2.5 mN/s, and a thrust vector stability of better than ±0.2 degrees. In addition to the drag compensation role, the propulsion system will also be operated at a fixed high thrust during orbit-raising for the long eclipse season. These stringent requirements can only realistically be met using a highly controllable ion propulsion system, which on GOCE is known as the ion propulsion assembly (IPA). The QinetiQ T5 Kaufman-type ion thruster assembly (ITA) is the heart of this system, and is ideally suited for this mission, having been designed for a nominal thrust of between 15-25 mN, and including solenoid magnets which allow the operating parameters to be efficiently and accurately controlled over the required thrust range. As such the QinetiQ T5 ion thruster represents an enabling technology for the mission. This paper presents an overview of the IPA system and its components, and is intended to provide the GOCE scientific users with an understanding of the technology, its performance, and the challenges, which have been overcome to provide it.