Scheduling Science Campaigns for the Rosetta Mission : A Preliminary Report

Rosetta is an ESA cornerstone mission that will reach the comet 67P/Churyumov-Gerasimenko in 2014 and will escort the comet for approximately 1 year offering the most detailed study of a comet ever undertaken by humankind. The Rosetta orbiter has 11 scientific instruments (4 remote sensing) and the Philae lander to make complementary measurements of the comet nucleus, coma (gas and dust), and surrounding environment. The ESA Rosetta Science Ground Segment is developing a science planning and scheduling system that includes an automated scheduling capability to assist in developing science plans for the Rosetta Orbiter. While automation is a small portion of the overall Science Ground Segment (SGS) as well as the overall scheduling system, this paper focuses on the automated and semi-automated scheduling software and process. Prior to arrival at the comet, the Rosetta mission is developing skeleton plans, which are pre-developed plans to conduct sets of measurement campaigns. Automated planning is being developed to support the skeleton planning process by generating plans for given trajectories and science campaigns. During encounter operations, segments of the skeleton plan, referred to as “bones,” will be refined and possibly swapped (for different bone segments). The plan segments will be refined and made operational as part of a long term planning process. Subsequent adaptation as additional information becomes available and earlier observations of the comet will alter science measurement priorities. When these plans are made operational, automated planning will assist the science planning team in adapting and adjusting science plans. In this usage, the automated science planner can be used to evaluate alternative trades by changing the science campaign and observation priorities, constraints, and parameters and generating alternate plans. Introduction Rosetta is an extremely ambitious mission by the European Space Agency [ESA, Factsheet] to conduct the most detailed exploration of a comet ever performed. The Rosetta spacecraft was launched in March 2004 and has circled the sun almost four times in a ten-year journey to comet 67P/Churyumov-Gerasimenko. Its trajectory has included Mars (2007) and three Earth (2005, 2007, 2009) flybys. Its path has also included a flyby of the Steins (2008) and Lutetia (2010) asteroids. The Rosetta spacecraft was approximately 3000kg at launch and is approximately 2.8 x 2.1 x 2.0 meters with two 14 m long solar panels with a total of 64 meters squared of solar panel area for power generation. Science planning for the Rosetta mission is extremely complex with each of the eleven science instruments conducting multiple science campaigns and presenting numerous operational constraints on the spacecraft to achieve their science measurement including geometry, illumination, position, spacecraft pointing, instrument mode, timing, and observation cadence. Because of the challenges in effectively planning science instrument operations, ESA has a highly skilled team of liaison scientists and instrument operations engineers who work with the instrument teams using the SGS to develop science plans for the Rosetta mission. Copyright © 2013. All rights reserved. Portions of this work were carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. In order to smooth science planning during operations, significant elements of the science operations are pre-planned by derivation of so called bones of the skeleton plan. In effect, a bone plan is a segment of science plan that is pre-derived for a specific context (e.g. operations and science phase of the missions). By preplanning a range of likely contingencies in advance it is expected that a significant portion of escort operations will be adapting already developed and checked plans rather than constructing and vetting plans from scratch. However this strategy requires the pre-generation of the skeleton bones. During escort operations, the current baseline plan will be modified and adapted to science, comet, and spacecraft conditions. In some cases this may result in significant modifications to the plan. Because this is the first extended mission to a comet, there is much to be learned about the dynamic cometary environment. Therefore the science planning team must be prepared for the possibility that previously developed plans will have to be significantly modified. The overall science planning process involves managing significant planning related information to support the planning process. For example, ESA is developing the Observation Manager (OBM) software to enable development and storage of science campaigns, observations, and scheduling rules and constraints. Additionally, campaign management and tracking is required to maintain an overall situational awareness of how past, current, and planned observations will and will not satisfy requirements for science campaigns. Additionally, the SGS team is leading the development of the pointing, slewing, and simulation capabilities for the SGS. The Rosetta mission is developing an automated science scheduling capability to support both skeleton plan development and operational plan refinement. While this scheduling system, called the Rosetta SGS Scheduling Component (RSSC) is but one part of the overall Science Ground Segment, this paper focuses on the RSSC because of the target audience. Readers interested in other components of the SGS are directed to other papers. In the remainder of this paper we first describe the two operational usages of automated science planning: (1) skeleton plan generation and (2) refinement of plans as they progress through long term planning, medium term planning, and short term planning. We then briefly survey some of the challenges in automating Rosetta science planning. Finally we describe the current state of the implementation of the automated scheduling component of the Rosetta SGS. Rosetta Science Planning Rosetta science planning can be broken into two types of planning: (1) skeleton plan generation and (2) operational plan development. Skeleton Plan Generation Skeleton plan generations involves considering a reference spacecraft trajectory in the context of specific spacecraft and comet conditions, and science priorities. From the perspective of automated scheduling, an input trajectory, spacecraft state, exogenous conditions (such as downlinks), and science campaigns with priorities. The scheduler can be used by the mission science team to enhance exploration of possible science plans by repeatedly running the scheduler with variations of trajectory, exogenous conditions, and science campaigns. Operational Planning In operational planning, a reference science plan already exists as s proposed bone from the skeleton plan. As this plan progresses from the long term planning cycle to the medium term and short term planning cycle it is adapted based on updates to starting conditions, exogenous events, spacecraft state, and changing science priorities. In this context automated scheduling accepts and input schedule and uses it as a guide to generate a new schedule accommodating input updates as will occur during operations. As a schedule progress closer to execution, certain aspects of the schedule become harder or impossible to change. At a relatively early phase the trajectory is frozen. Next the rough spacecraft pointing is frozen, only allowing for minor changes to reflect navigation updates. Finally, observation activities themselves are frozen only allowing minor parameter changes. Scheduling Constraints In Rosetta science planning there are a significant number of constraints and preferences that must be accommodating in generating science instrument schedules. In this section we describe a number of these constraints and how they are handled. Science Campaign Definition Rosetta science is organized into a number of science themes relating to the scientific questions to be answered by science measurements/observations. Science campaigns are sets of observations that are directed at collecting data to enable the science team to answer these questions and refine relevant theories and models. Three primary structures exist for scheduling unit observations. “Repeat while repetition” requires scheduling of an observation (or set of observations) a number of times with temporal relationships among adjacent observations. “Repeat/insert while obs/window” enables scheduling of observations while a condition is met, such as a geometric configuration (observation opportunity) or concurrent with another observation. “Start/end when Start/end” enables scheduling of one type of observation with a defined temporal relation to a different type of observation. Another complexity in science campaigns is campaign expansion into schedulable observations. For example, a science campaign may be to map the surface of the nucleus of the comet at a pre-specified spatial resolution, at two varying illumination conditions. The spatial coverage may be represented by expanding the campaign to replicate over a list of point targets and restriction on the distance to the comet. The iteration over the varying illumination conditions is handled by expansion of the previous target set replicating a request for each illumination condition. In general, these expansions are handled by replicating the observation requests over all of the point instances and the cross product of the applicable conditions. This results in an exhaustive enumeration of the observation requests that is then input to the scheduler. Monitoring campaigns are somewhat different. These campaigns are active over extended periods of time and intend to achieve a specified duration level. Monitoring campaigns may interrupted to acquire competing observations that have incompatible pointing or state constraints. Monitor