Orbit Control Manoeuvre Strategy for EarthCARE

Abstract

The Earth Clouds, Aerosol and Radiation Explorer (EarthCARE) mission is a European Space Agency (ESA) Earth Observation mission developed in cooperation with JAXA and scheduled to launch end 2016. As with other ESA missions, the spacecraft will perform control of its orbit with respect to a reference orbit. This maintenance will have to be performed in such a way that also meets the demanding calibration requirements and schedules from the spacecraft instruments, while at the same time minimising cost of mission operations by limiting the time on console and in preparation activities for engineers and controllers. This paper will discuss, from an operational perspective, the strategies considered for triggering, planning and executing the orbit control operations for the EarthCARE mission, given the challenging constraints applied. The spacecraft orbit will be polar at approximately 400 km altitude, with requirements to maintain a ground track of ± 25 km from the reference orbit in routine operations. The drag environment around Earth has a high level of variation due based on unpredictable solar activity and at 400 km altitude the level of drag is severe. This means that the required frequency of manoeuvres could be anywhere from once per day up to once per month, depending on the conditions, deadband and operational constraints. The four instruments on the spacecraft have extensive and varied calibration operations, including periodic operations, location-triggered operations and operations involving active ground assets. Many of these imply constraints on routine orbit maintenance activities, beyond the basic science requirement of constraining the spacecraft’s actual orbit to within a deadband of 25km to the reference orbit in routine operations. The spacecraft design prohibits retrograde manoeuvres during routine operations and the in-plane and out-of-plane manoeuvres have restrictions on when in the orbit or during which period of the year respectively, they can be performed. Along with these physical factors, operations need to take into account the need to minimise effort on ground while still allowing sufficient validation and preparation time to operate safely. On a practical level, the operations concept has been designed to have staff on console during working hours, to only conduct 1 or 2 commanding passes per day with the spacecraft and to have routine operations planned at least 2 weeks in advance of execution. This paper will outline all of these different factors and constraints that make the planning and operation of orbit control manoeuvres on EarthCARE a complex challenge. The paper will then go on to discuss the options already used by other ESA Earth Observation missions and some new proposals for both triggering the need for an orbit control manoeuvre and the way in which it is then interleaved with a plan that has already been agreed and loaded on board the spacecraft. The pros and cons of each of the options and their potential applicability to the EarthCARE mission will be discussed. A summary will also be provided on the chosen strategy of other ESA Earth Observation missions and the reasons for their adoption of a given strategy.