EUCLID SKY SCANNING TOOL

EUCLID SKY SCANNING TOOL 5TH INTERNATIONAL CONFERENCE ON ASTRODYNAMICS TOOLS AND TECHNIQUES ESA/ESTEC, NOORDWIJK, THE NETHERLANDS 29 MAY 1 JUNE 2012 Francesco Cacciatore (1), Mariano Sánchez (2), Alberto Anselmi (3) (1) Deimos-Space, Ronda de Poniente, 19, Portal 2-2º, 28760 Tres Cantos (Madrid), Spain, francesco.cacciatore@deimos-space.com (2) Deimos-Space, Ronda de Poniente, 19, Portal 2-2º, 28760 Tres Cantos (Madrid), Spain, mariano.sanchez@deimos-space.com (3) Thales Alenia Space Italia, Turin, Italy, alberto.anselmi@thalesaleniaspace.com ABSTRACT In the EUCLID mission study, a relevant role was played by the design and analysis of the Sky Mapping strategy. A tool has been developed by DEIMOS-Space with the purpose of providing the mission team and scientific groups with the means for designing a complete observation scenario and assessing its performances. The tool allows two observation modes: basic and flexible (or modified). In the basic observation mode the scanning execution is almost completely controlled by the simulator itself, while in the flexible observation mode the S/C attitude is completely controlled by the user. The tool architecture is based on a Fortran-coded simulation engine, and a MATLAB coded simulation environment and postprocessing. In addition, a MATLAB Graphical User Interface (GUI) has been developed in order allow the user a direct feedback of the effect introduced on the mapping schedule by changes in the strategy parameters. 1. LIST OF SYMBOLS AND ACRONYMS DS ESST FOV LOS SAA WES λ min ψ scan X FOV Y FOV Δθ 0 λ threshold Ω Deep Survey Euclid Sky Scanning Tool Field of View Line of Sight Sun Aspect Angle Wide Extragalactic Survey Minimum observed ecliptic latitude Yaw scanning step FOV x-side length FOV y-side length Nominal ecliptic longitude step Ecliptic latitude limit to avoid singularities FOV lateral overlapping factor 2. INTRODUCTION The EUCLID Sky Scanning Tool was created in the frame of the Definition Study of EUCLID Medium Class Mission - Cosmic Vision 2015-2025 (EUCLID), which had the main objective of providing all the elements to enable a further down-selection process as part of the overall Cosmic Vision Plan, which is relative to the elaboration and implementation of future ESA science missions. The ESST (EUCLID Sky Scanning Tool) software was initially developed with the purpose of creating a tool capable of carrying out simulations and generating results in line with the analyses performed in the mission study at the time of the tool creation. Nevertheless, the evolution of the tool design resulted in a code able to simulate general sky observation conditions. The sky-mapping strategy, at the time of tool creation, had to allow the completion of the observation of the celestial caps with galactic latitude above 30º, covering a total area of approximately 20000 deg 2, in a mission time which ideally should not exceed a 7-year lifetime. The observation of the galactic caps corresponds to the so-called Wide Extragalactic Survey (WES), which shall be coupled to a Deep Survey (DS), aimed at performing an additional detailed coverage of the ecliptic pole areas with high number of re-visits. The EUCLID mission shall perform observations of the celestial sphere making use of a spacecraft located in an orbit about the L2 point of the Earth-Sun system. The selected orbit, used as baseline for the mapping strategy analyses, is a quasi-lissajous type with maximum Sun- S/C-Earth angle 30º approximately. Sky-scanning operations are assumed to start when the trajectory crosses for the second time the Sun-L2 direction after arriving at L2 (point located at null Y and positive X in plot available in Figure 1)

Figure 1: EUCLID reference trajectory about Sun-Earth L2 2.1. Sky Scanning Basics In this paper the nomenclature used during the execution of the study will be adopted for the description of the ESST features. The basic elements composing the sky scanning strategies designed are the Field of View (FOV), the strip and the patch. A FOV is the sky area seen with a single image taken by the S/C instruments. A strip is the portion of sky which can be observed with a number of subsequent fields acquired between two Sun s of the S/C X axis. A patch is a geometrical shape (with the highest possible regularity) in which strips are organized. The following rotations convention will be used: - X axis: Yaw rotation - Y axis: Pitch rotation - Z axis: Roll rotation During the execution of the study, the work on the skymapping strategy has been based on the definition and simulation of the so-called basic and modified (or flexible) observation strategies. In the basic scanning strategy the solar arrays always point to the Sun, and the spacecraft rotates about its x- axis (the S/C-Sun direction) to observe a strip. The time between two subsequent Sun re-pointings of this stepand-stare approach is determined by the length of the FOV longitudinal dimension and by the required lateral overlapping of fields, and is directly connected to the extension of the observed strip: the longer the time, the longer the time available for imaging, and the longer the strip observed. During the mission, the motion around the Sun allows the observation of strips with increasing ecliptic longitude. At each ecliptic longitude, the strips begin for the first year at the ecliptic latitudes corresponding to the galactic caps limits (±30º); for subsequent years the strips start where the previous strip taken at the same longitude ended. Two main sources of inefficiency can be identified in the basic strategy: (a) the increasing overlap of strips for ecliptic latitudes close to the poles, and (b) the existence of dead zones corresponding to those longitudes (i.e. times) where there is no available latitude for observation. In order to cope with such drawbacks, a modified strategy was defined, assuming a relaxation of the constraint on the null Sun Aspect Angle (SAA) of solar array up to a maximum value of 30º. The relaxation of the constraint allows observing areas of the sky located at different longitudes from the one available for observation at a given time with the standard operation mode (rotation of the S/C about the S/C-Sun direction). As a consequence, it was possible to exploit the aforedefined dead zones and transform them in flexible zones. In the modified strategy, the observation was performed as for the basic strategy in those areas of the sky where the ecliptic latitude was low enough to guarantee a limited overlap between contiguous strips (optimal scanning areas), while the polar zones were observed during the flexible times by a pitch rotation of the S/C; such flexible times are located in correspondence of the equinoxes. In order to reduce the mission duration as much as possible, the duration of the flexi-mode areas were tailored specifically for each mission scenario analysed, extending the portions of the mission in which a relaxation of the null SAA constraint was allowed. Over the execution of the EUCLID study the modified (flexible) observation mode acquired a growing relevance, and thus a consistent effort was devoted in the codification of the tool to the improvement of the functionalities connected with such observation mode. Specifically, in the evolution from the initial prototype, a higher degree of control has been progressively given to the user in the definition of the flexi-intervals commands, while at the same time enhancing the usertool interface. The final result of such effort was the creation of a Graphical User Interface, which, exploiting a built-in simplified model of the problem, allows realtime design of a complete observation strategy with contained user time dedication. 3. SIMULATOR ARCHITECTURE The Euclid Sky Scanning Tool architecture is based on two main blocks: a Fortran-coded simulation engine, and a MATLAB coded simulation environment. The simulation management is carried out by the MATLAB code, which handles both the simulation engine execution and the output post-processing. The simulator engine architecture is constituted by a set of interconnected functional blocks, each one of which is composed by one or more Fortran routines, and is devoted to perform a specific task. The following blocks can be identified: - Simulation preparation: this block prepares the execution of the simulation, consisting of input acquisition, trajectory data loading, calculation execution and sky map data output printout. - Sky scanning loop: this block performs the loop simulating the observation mission. At each

observation step the reference attitude is computed, and then the observation mode (basic or modified) is selected and applied; the loop is closed with the S/C state update and stop condition check. At the beginning of each year, a check is made to identify the sky areas in which basic modes observations can be taken. - Observation acquisition: this block performs the operations needed to carry out the sky observations, and can be divided in two sub-blocks according to the two observation modes defined for EUCLID (basic and modified). In both modes three main steps are taken, each one related to a specific simulator functionality: computation of rotation commands for correct LOS targeting, computation of time available before next observation and of interpolated S/C state, and strip acquisition 4. BASIC SCANNING MODE In the basic scanning mode the observation execution is almost completely controlled by the simulator itself. The scanning starts at the border of the galactic caps, and proceeds towards the interior of the caps with a band-like pattern. Each new observed band, composed by the lateral juxtaposition of subsequent strips, starts where the previous year strips ended. Therefore, in the basic scanning mode, for both the northern and southern galactic cap, the strip acquisition starts at lower ecliptic latitude (in module), and moves towards the ecliptic poles. In this scanning mode the only motion allowed for observation is the yaw (x-axis) rotation; in order to successfully implement the basic scan, the simulator must be able to compute at each time the rotation needed to target the telescope LOS (Line of Sight) at the correct point of the sky. If at the first year of mission, the LOS must be targeted at the boundary of the selected (northern or southern) galactic cap; for each ecliptic longitude, however, there are two rotations that target to the cap border: the correct rotation to be selected is the one that points the telescope at the higher ecliptic latitude. For the following years of mission, the sky area to be observed depends on the previous year of observation. The information needed to compute the correct rotations is stored and updated year by year in a dedicated set of arrays. After targeting the LOS to the correct sky area, and selecting the correct rotation direction, the strip acquisition is simulated. The length of the observed strip is determined by the number of fields that can be observed between two Sun re-pointings of the S/C x-axis. The ecliptic longitude step for re-pointing is directly determined by the FOV size and by the desired fields overlapping, and is constant, unless using the Adaptive Strip Length option. The time needed by the S/C to advance of a certain ecliptic longitude step depends on its velocity around the Sun. For a vehicle orbiting around libration point L2, the angular velocity relative to the Sun is a combination of the angular velocity of L2 (which equals the velocity of the Earth system barycentre around the Sun, and is almost constant), and of the orbital velocity relative to L2; this last velocity will add or subtract to the velocity of L2 depending on the position along the orbit, thus on the epoch. As a consequence, the time available between re-pointings will not be constant even for constant angular step, and therefore the number of FOVs that can be observed in a strip will depend on the time of the year at which the strip is observed, with a periodicity which follows the periodicity of the S/C orbit about L2, that for the case considered in the current phase of the EUCLID study is approximately six months. Figure 2: FOV shape definition scheme With the FOV setup represented in Figure 2, the ecliptic longitude step for re-pointing and the Yaw rotation step are computed as specified in Eqn. (1), where Δθ 0 is the nominal ecliptic longitude step, ψ scan is the Yaw scanning step, and Ω is the lateral overlapping factor. Δθ 0 Δψ scan = X = Y FOV ( 1 Ω) ( 1 Ω) FOV (1) The observed galactic cap selection (northern/southern) is controlled by the user, which must define a set of S/C ecliptic longitude intervals, and specify for each interval the cap to be scanned. This feature is meant to allow the user to control the straylight avoidance strategy, by selecting the observed cap that for each ecliptic longitude interval maximises the Earth and Moon straylight angles, defined as the minimum angular distance between the telescope LOS direction and the bodies limb. Over the evolution of the study a constraint was placed on the straylight angle to avoid pollution of the sky observations by light reflected by Earth and Moon; the maximisation of such angle acquired a relevant role in the definition of the sky scanning strategy. The experience gained by working on the simulator prototype has shown that when switching the observed cap, a gap in the observation is generated due to the pattern in which the strips are laid on the sky in the

basic mode step-and-stare strategy along the observation year. In order to solve this issue, a fixed Roll (z-axis) rotation is permitted for each ecliptic longitude interval; the magnitude of such rotation must be defined by the user, and shall be enough to compensate for the arising gap. The entity of the above mentioned gap increases with the implementation of the Adaptive Strip Length strategy.wrong gap compensation would lead to a malfunctioning of the simulator, which in such case would not be able to target the LOS at the correct sky area. In the basic mode the length of a strip is determined by the number of FOVs that can be observed between two solar panels re-pointings. The strip acquisition is therefore interrupted when the number of FOVs that can be acquired in such time interval is reached. An option has been introduced in the simulator to allow the user to decide whether the acquisition of a strip shall be interrupted when the galactic cap s border is reached, or if the scan shall continue until the full number of observable FOVs is reached. 4.1. Adaptive Strip Length If the Adaptive Strip Length option is activated, the time available to perform the scan is computed before starting the acquisition of each strip. This time, which is the time between S/C x-axis re-pointing towards the Sun, controls the number of FOVs that compose each strip, and is a function of the observed ecliptic latitude. Specifically, the formula controlling the ecliptic longitude step for re-pointing is given in Eqn. (2), where Δθ indicates the ecliptic longitude step to be used, λ min is the minimum (in module) observed ecliptic latitude, and λ threshold is a limit to avoid singularities in the computation. if else Δθ λ min < λ λ adaptive Δθ = threshold = λ threshold 0 λ adaptive cos( λ = λ min adaptive 5. MODIFIED OBSERVATION MODE The modified observation mode, also called flexible observation mode, was introduced so that the relaxation of the constraint on the solar panels SAA made possible the introduction of a pitch (y-axis) rotation, which allows the observation of sky areas which would not be available for imaging otherwise. The S/C attitude for this observation mode is completely controlled by the user, which shall define for each year of observation the number of flexi-time intervals. For each interval, the following set of data shall be defined: - Initial and final S/C ecliptic longitude - Yaw (x-axis) rotation at start of strip acquisition; this value must be defined for the two ecliptic ) (2) longitude extremes of the interval and, if needed, at a middle longitude point. For each strip observed in the flexi-interval under consideration, the initial yaw rotation is obtained by interpolating on the S/C ecliptic longitude at the time of observation, assuming linear or parabolic variation in the defined interval. - Yaw rotation direction. After defining the initial yaw rotation for a strip, it is necessary to specify whether the FOVs shall be acquired with increasing or decreasing rotations around the x-axis. - Pitch rotations at the extremes (and middle, if needed) of the defined interval. The pitch rotation at each S/C ecliptic longitude is obtained interpolating between the specified values, assuming linear or parabolic variation in the defined interval. - Roll rotations at the extremes (and middle, if needed) of the defined interval. The roll rotation at each S/C ecliptic longitude is obtained interpolating between the specified values, assuming linear or parabolic variation in the defined interval. - Yaw, Pitch and Roll variation types: rotation shapes assumed for interpolation, which can be linear or parabolic - Number of fields of view per strip. The number of fields of view per strip controls the length of the strip, whereas the other parameters control the location of the observations. All rotational commands are defined as relative to the S/C Reference Attitude, which is recomputed at each observation step (thus, at the beginning of each strip acquisition), and is defined as follows: - X-axis towards Sun - Y-axis parallel to ecliptic and such that z-axis is directed towards northern ecliptic pole - Z-axis bi-normal to the other axes According to the above list, freedom is given to the user to define the attitude of the S/C by tilting around all of the three rotation axes. This is meant to allow the user not only to take observations to complete the Wide Survey, but also to include in the strategy observations aimed at completing the Deep Survey, which may not need any pitch rotation. In this sense, the flexi-times are not intended anymore only as the times where a SAA constraint relaxation is allowed, but as those intervals in which images are no acquired following the standard Basic mode law. The simulator does not actively control the maximum solar panels SAA obtained with a given command combination, nor the angle between the LOS and the Sun direction. Simultaneous Roll (z-axis) and Pitch (yaxis) rotations shall be used carefully, taking into account the mentioned constraint, whose evolution can be monitored through the post-processing suite s graphical output. In the case in which a maximum SAA violation is detected, an on-screen warning is provided to the user during the execution. In the basic scanning mode the number of fields of view

per field and the lateral separation of strips were directly connected, and it was impossible to define arbitrarily the strip length without violating the lateral overlapping requirement. In the flexi scanning intervals, the Pitch and Roll rotations can be used to control independently the number of FOVs per strip and the strips lateral overlapping, which in this case cannot be controlled by the simulator, and therefore shall be taken into account by the user in the strategy design process. For a flexi interval, increasing the number of FOVs per strip will increase the length of the strips, but will reduce their number, due to the increased re-pointing step. The Yaw rotation step, as in the basic scanning mode, is kept constant, and is computed taking into account the overlapping requirement and FOV size in the direction of the scan. During the simulation, the software checks for each S/C ecliptic longitude which type of observation mode shall be employed. If the ecliptic longitude considered falls into one of the defined flexi-mode intervals, the interval parameters are retrieved, and used to compute the commands to be employed for strip acquisition, which is then performed by a dedicated routine. 6. POSTPROCESSING AND OUTPUT A post-processing utility has been coded in MATLAB with the purpose of allowing the user to generate the plots used to assess the characteristics of the sky scanning strategy simulated. The post-processing utility is able to generate the following set of plots: - S/C rotations commands: for each year of mission, the evolution of Yaw, Roll and Pitch angles is shown, as well as the solar panels SAA - Observation efficiency: curve showing the evolution of the observed area vs time along the scanning mission - Auxiliary flexi-times design: set of plots showing for each year of mission, the evolution of Yaw, Roll and Pitch angles vs the S/C ecliptic longitude at which observations are taken - Observation strategy in Ecliptic Sky: layout of the observations for the selected years of mission projected in the ecliptic reference frame - Observation strategy in Galactic Sky: layout of the observations for the selected years of mission projected in the galactic reference frame - Visit Count: map of the number of times a sky area has been observed - Earth Stray-light constraint: constraint map in ecliptic sky. - Moon Stray-light constraint: constraint map in ecliptic sky. - Solar panels SAA map - Slew count curves: curves showing the number of FOV, strip and hemisphere slews over the mission duration - Slew amplitude curves: curves showing for each year the slew amplitude in all three rotation axes, for each slew performed. - FOV number: set of plots showing, for each year of mission, the number of FOVs per strip vs the epoch of the observations Figure 3 shows an example of the output provided by the post-processing utility. Plots relative to the observation strategy representation in the ecliptic sky, the number of revisits over the observed area, and the evolution of the observation efficiency are reported. Figure 3: Post-processing output for example observation scenario 7. GRAPHICAL USER INTERFACE A Graphical User Interface (GUI) has been developed in MATLAB for ESST in order to allow the user to quickly design an observation strategy making use of flexi intervals, with a direct feedback of how changing one of the interval parameters affects the layout of the strips on the celestial sphere. As mentioned previously, in the flexible observation intervals the S/C attitude is completely controlled by the user, who is free to specify a number of parameters controlling the shape of the S/C rotations over time. In

the GUI the flexi-times definition is exploited in its widest sense: no basic mode is available, and the observation strategy shall be completely designed by the user. The GUI computation engine makes use of a simplified model to simulate the S/C attitude at strip acquisition, and the strip acquisition process itself. The main difference between the full FORTRAN simulator engine and the GUI engine is that the GUI does not consider the effects of the motion of the S/C about its trajectory around L2. Due to such simplifications, inherent of the nature of the GUI, slight differences are expected between the outcomes of the simulation of the same case with full FORTRAN engine and the GUI engine. The simulation of a sky mapping strategy with the GUI is based on the definition of each single patch composing the strategy; each patch can be defined and stored, and then modified during the design process. The parameters defining the attitude control law to be followed in a patch can be controlled through a set of dedicated sliders, thus providing the user with a real time feedback of the disposition of the FOV on the celestial sphere for given controls. In order to take into account the different relevance of certain sky areas for the purposes of the scientific observations to be take in the mission, user-defined sky priority maps can be loaded and represented. The prioritisation defined will be taken into account in the computation of the weighted observation efficiency, which is a performance index accounting for the evolution of observed area vs mission time of the sky scanning strategy under analysis The GUI allows the generation of a set of auxiliary plots, useful in the assessment of the coverage performances and characteristics achieved by the strategy being designed. Each auxiliary plot can be generated for a single patch only, or for the full set of stored patches. The available plot types are: - X-axis Sun Aspect Angle map - Efficiency (observed area vs time) - Attitude rotational commands In addition the GUI engine allows the user to represent the sky scanning strategy in a map projection different from the basic latitude-longitude rectangular sky used in the main GUI window to represent the observation strategy under study. The specific projections made available to the user have been selected in order to be representative of different projection types. Finally, a sky scanning scenario defined through the GUI can be easily simulated with the full ESST Fortran engine, thanks to a GUI functionality which allows the translation of a GUI scenario to a corresponding ESST input file. The main window of the graphical user interface of ESST is represented in Figure 4. 8. SKY SCANNING EXAMPLES An example case has been computed assuming a FOV with size 0.7014x0.765 deg, dwell time per FOV 3000 sec, and lateral overlapping 2.5%. Adaptive strip length has not been employed, and the flexi-times have been exploited to achieve complete coverage of the galactic caps within 6 years of mission. The outcome of such example exercise is provided in Figure 3. As expected, the designed strategy shows an increase of the revisits per sky area towards the ecliptic poles. The exploitation of the flexible times allows maintaining a good observation efficiency throughout the full mission (the observed area curve grows at an almost constant ratio). The strategy was designed respecting the maximum SAA constraint of 30º. Figure 4: ESST GUI main window Figure 5: EUCLID Consortium strategy example (from [1])

The EUCLID Scientific Consortium has been so far the main user of the ESST. The tool and its GUI have been exploited for the generation of refined observation schedules which take into account the sky priorities as well as operational constraints like the acquisition of calibration FOVs. The output provided by ESST for a reference scenario defined by the EUCLID Consortium is reported in Figure 5, and has been extracted from [1]. 9. CONCLUSIONS AND FUTURE WORK The creation of the EUCLID Sky Scanning Tool is the result of the work carried out during the EUCLID Phase AB study. The need for a tool devoted to the simulation of the sky observation condition has aroused along the evolution of the study, as the accuracy level of the observation strategy assessment increased. The ESST, started initially as internal prototype at Deimos-Space, has been developed with the purpose of allowing the user a quick and detailed design of complex scenarios. Thanks to its characteristics, the tool has been taken as reference for the observation strategy definition by the mission scientific consortium. Further developments have been identified in the field of optimisation and system support. Concerning the first topic, the aim is to investigate techniques to allow the tool to refine an initial user-defined design, taking into account a given set of mission constraint, and maximising a suitable performance index for the observation schedule. The second development field mentioned is meant to provide the system design of direct information on how the system characteristics affect the design feasibility, and viceversa how the design characteristics can impact on the refinement of the system design, for example in terms of attitude control (and related delta-v budget), memory management and data download, and thermal control of the S/C. 10. REFERENCES 1. J. Amiaux & R.S. & C., Euclid Survey is: feasible, EC meeting Bologna, September 2011