Sino-InSpace: A Digital Simulation Platform for Virtual Space ... - MDPI

International Journal of

Geo-Information Article

Sino-InSpace: A Digital Simulation Platform for Virtual Space Environments Liang Lyu 1,2 , Qing Xu 1,3 , Chaozhen Lan 1, *, Qunshan Shi 1 , Wanjie Lu 1,2 , Yang Zhou 1 and Yinghao Zhao 1,2 1

2 3

*

Zhengzhou Institute of Surveying and Mapping, Zhengzhou 450052, China; [email protected] (L.L.); [email protected] (Q.X.); [email protected] (Q.S.); [email protected] (W.L.); [email protected] (Y.Z.); [email protected] (Y.Z.) Science and Technology on Information System Engineering Laboratory, Nanjing Research Institute of Electronics Engineering, Nanjing 210007, China Collaborative Innovation Center of Geo-Information Technology for Smart Central Plains, Zhengzhou 450001, China Correspondence: [email protected]; Tel.: +86-130-1451-1234

Received: 14 July 2018; Accepted: 3 September 2018; Published: 8 September 2018

Abstract: The implementation of increased space exploration missions reduces the distance between human beings and outer space. Although it is impossible for everyone to enter the remote outer space, virtual environments could provide computer-based digital spaces that we can observe, participate in, and experience. In this study, Sino-InSpace, a digital simulation platform, was developed to support the construction of virtual space environments. The input data are divided into two types, the environment element and the entity object, that are then supported by the unified time-space datum. The platform adopted the pyramid model and octree index to preprocess the geographic and space environment data, which ensured the efficiency of data loading and browsing. To describe objects perfectly, they were abstracted and modeled based on four aspects including attributes, ephemeris, geometry, and behavior. Then, the platform performed the organization of a visual scenario based on logical modeling and data modeling; in addition, it ensured smooth and flexible visual scenario displays using efficient data and rendering engines. Multilevel modes (application directly, visualization development, and scientific analysis) were designed to support multilevel applications for users from different grades and fields. Each mode provided representative case studies, which also demonstrated the capabilities of the platform for data integration, visualization, process deduction, and auxiliary analysis. Finally, a user study with human participants was conducted from multiple views (usability, user acceptance, presence, and software design). The results indicate that Sino-InSpace performs well in simulation for virtual space environments, while a virtual reality setup is beneficial for promoting the experience. Keywords: virtual space environments; Sino-InSpace; simulation platform; visualization scenario; multilevel application; user evaluation

1. Introduction The astounding advances in aerospace technology are making the dream of space exploration come true. Since the launch of the first artificial satellite by the Soviet Union in 1957, human beings have been constantly exploring the vastness of the universe, which includes the near-earth space [1], the moon, Mars, and asteroids [2]. Chinese space activities started with the launch of Dong Fang Hong I in 1970; since then, a number of BeiDou navigation satellites, resource remote sensing satellites, and F¯engyún meteorological satellites had been sent into operation. In addition, China’s progress in

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deep space exploration is also noteworthy [3]. The Chang’E lunar exploration program, which involves achieving lunar orbit, soft-landing on the moon, and returning a lunar sample has now entered its third stage [4]. The first Chinese Mars exploration mission has also been formally approved, and a Mars probe is planned for launch in the second half of 2020. With the possible opportunities and new avenues that space exploration promises, especially considering the problems of environmental pollution and resource depletion, many countries have set up ambitious space programs [5–7]. For human beings, outer space is a relatively “new world” and difficult to reach currently. However, virtual environments, initially viewed as “mirror worlds”, can supply computer-based digital spaces that we observe, participate in, and experience in person [8]. With the increasing scope and scale of human space activities, it is essential to construct a digital simulation platform for virtual space environments (VSEs). The traditional geographical information disciplines primarily study the Earth’s surface including commercial (e.g., ArcGIS, MapInfo, and SuperMap) or open source geographic information system (GIS) platforms [9]; these platforms focus on processing, expressing, and analyzing the surface geographical data that are closely related to daily production and life. With the data acquired by deep space probes, the visualization of the planets’ surface map based on GIS has been studied by some organizations such as the ‘Multilingual Maps of the Terrestrial Planets and their Moons’ [10], ‘Lunar Web Mapping Application’ [11], ‘Mars Global GIS Mapping Application’ [12], and Gazetteer of Planetary Nomenclature [13]; however, these maps are often relatively simple and lack effective three-dimensional (3D) visualization. Since the concept of a digital globe was proposed in 1998 by U.S. Vice President Al Gore, a large number of platforms such as Google Earth [14], World Wind [15], ArcGIS Earth [16], DEPS/CAS [17], and GeoGlobe [18] have been developed, providing a new means to understand the Earth. Furthermore, some similar digital visualization platforms such as Moonworld [19], Sino-VirtualMoon [20], Google Mars [21], JMARS [22], and ADVISER [23,24] have been developed, which can intuitively display the images of planetary surfaces and depict information regarding their topography, geology, and landing point through virtual reality (VR) while also supporting collaborative simulation. However, these platforms are often limited to a single planet and lack the capability to simulate the entire process of complex space-based activities. Due to the specialization of space science, there are not many mature aerospace mission simulation platforms thus far. Systems Tool Kit (STK) [25], developed by AGI, is a leading commercial software used for analysis in the aerospace field, which can support space exploration simulation including design, testing, launch, operation, and task application. Recently, the company also introduced a simplified open-source, web-based version of the platform called Cesium [26]. In addition, World Wind, General Mission Analysis Tool (GMAT) [27], SaVi [28], 3DView [29], and Cosmographia [30] can offer certain capabilities for track design, simulation, and scientific data visualization; moreover, these were made open source for the public. These aforementioned platforms focus on space activity simulation and have inadequate features for the analysis and visualization of a planet’s surface information. In addition, these platforms require professional knowledge to operate, and would not be widely accepted by the general public such as the case of Google Earth [31]. The virtual geographic environments (VGEs) aim to provide integrated tools to understand the world better and have been widely used in multiple fields, such as air pollution experiments, silt dam planning, and group behavior simulation [8,32–39]. In fact, VSEs can be regarded as the extension of VGEs in the field of outer space, and some ideas derived from VGEs can also be used for reference. A digital simulation platform called Sino-InSpace was developed for the abstract representations, visualizations with multiple viewpoints, and analytical understanding of VSEs from Earth to the outer planets. Using this platform, space activities and the access and analysis of geological information can be supported. In addition, it can provide simulation technology for the development of VGEs, the improvement of public universe cognition, and space science exploration. The remainder of the paper is organized as follows. The orientation and data source of Sino-InSpace are analyzed in Section 2. Section 3 illustrates the key technologies including time-space datum, data organization


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of 27 organization, and design of the visualization engine. 3Three types of application modes were designed, and related case studies are introduced in Section 4. Then, Section 4. Then, user study was withconducted participants was conducted to evaluate theplatform performance of the a user study withaparticipants to evaluate the performance of the in Section 5. platform in Section 5. Finally, we discuss the features and future work of the platform in Section 6. Finally, we discuss the features and future work of the platform in Section 6.

2. Platform Orientation and Data Source Analysis Compared Compared to the majority of the digital earth and VGE platforms, Sino-InSpace focuses on visualization andanalysis analysis a larger scaleincluding space including the construction the virtual visualization and on aon larger scale space the construction of the virtualofinterplanetary interplanetary space environment, digital planets, space environment/weather monitoring, space space environment, digital planets, space environment/weather monitoring, space mission launch, mission launch, space objects collision landing area space objects collision warning, landingwarning, area selection, and soselection, on (Figureand 1). so on (Figure 1).

Figure 1. Platform orientation.

Complete VGEs be divided into four the data environment, modeling environment, Complete VGEscancan be divided intosubtypes: four subtypes: the data environment, modeling expression environment, and collaborative environment [8,35,40,41]. The data environment is environment, expression environment, and collaborative environment [8,35,40,41]. The data fundamental for the other three Therefore, data source ofthe VSEs is analyzed and environment is fundamental forsubtypes the other[42]. three subtypesthe [42]. Therefore, data source offirst VSEs is divided into two types: the environment element and the entity object element (Table 1). analyzed first and divided into two types: the environment element and the entity object element

(Table 1).

Table 1. Data source of virtual space environments. Data Type

Table 1. Data source of virtual space environments. Content

Basic geographic Remote sensing images on the surface of the planet, digital Data Type Content environment elevation model (DEM), and vector and feature data. Remote sensing images on the surface of the planet, Environment Basic geographic The position, distribution, temperature and intensity information digital elevation model (DEM), and vector and feature element environment Physical space of physical space environment such as middle and upper data. environment/weather atmosphere, geomagnetic field, ionosphere, radiant zone, and so Environment The position, distribution, temperature anddebris. intensity on. The track and size information of space element information of physical space environment such as Physical space Astronomical catalogues of stars containing accurate position, Background star middle and upper atmosphere, geomagnetic field, proper motion, level, and color. environment/weather ionosphere, radiant zone, and so on. The track and size The size, shape, texture, rotation speed, etc. of planets. Planet in the solar information of space debris. Astronomical ephemerides used to calculate the position and system Astronomical catalogues stars containing accurate Entity object velocity of of planets. Background star element position, proper motion, level, and color. Orbit elements, trajectories, attitudes, 3D models, and attributes of Spacecraft The size,artificial shape, texture, rotation speed, etc. of planets. satellites and spacecraft. Planet in the solar system Astronomical ephemerides used to calculate the position The location, model, capability, and attribute data of all kinds of and velocity of planets. Entity object Ground facility ground facilities such as optical telescopes, radar, launching and element Orbit elements, trajectories, attitudes, 3D models, and measurement, and control stations. Spacecraft attributes of artificial satellites and spacecraft. The location,the model, capability, attribute to data all The environment elements are the basis for building virtual space. and In addition theofimage, Ground facility kinds of ground facilities such as optical telescopes, digital elevation model (DEM) and other basic geographic data, space activities also need to consider radar, launching and measurement, and control stations.

The environment elements are the basis for building the virtual space. In addition to the image, digital elevation model (DEM) and other basic geographic data, space activities also need to consider


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the space environment such as the atmosphere, magnetic field, ionosphere, and space debris around the spacecraft during the flight. With all kinds of earth observation systems, we have accumulated considerable geographical environment data. For the Moon, Mars, asteroids, and other celestial bodies, we can also obtain rich remote sensing images and topographic data through detectors, and these data enhance the cognition of the planets. The data source of the space environment includes two types: (i) data collected by sensing instruments [43,44] and (ii) data calculated by empirical models (e.g., NRLMSISE-00 empirical model [45], IGRF-12 model [46], and IRI2012 model [47]) developed using sensory data. With these data sources, datasets such as the position distribution, intensity, and density of various elements are achieved. Entity objects refer to the targets of direct or indirect participation in space activities, and related elements include the following types. (i) Background star. Sino-InSpace adopts the Tycho-2 catalogue [48] to calculate the current position, brightness, and size of stars, which are then described using the point-based rendering. (ii) Planet in the solar system. The data includes the radius, eccentricity, texture, rotation speed, etc. The speed and position are calculated by deploying the NASA Jet Propulsion Laboratory Development Ephemeris (JPL DE) 405 [49]. (iii) Spacecraft. As the main participants of space activities, the platform focuses on the expression of its position, posture, form, and attribute information. (iv) Ground facility. They are usually used as auxiliary parts of space activities including observation equipment, data transmission and receiving equipment, launch sites, and so forth. Compared with environment elements, entity object elements focus on the extraction of overall information, e.g., location, form, capability, etc. The storage and management of the above data can be carried out in a variety of ways such as files, databases, network services, etc., which are not the focus of the platform construction. To build VSEs, the platform mainly solves the problems of data organization, target modeling, scene organization, and visualization under the unified time-space datum. In addition, taking into account the needs of low-, medium-, and high-level users, three application modes (application directly, visualization development, and scientific analysis) were designed correspondingly. The difficulty of the application mode increased gradually, while the activity that can be simulated will be more complex. Taking teachers who are engaged in popular science education of the moon as an example, they may be considered as low-level users and always do some simple work in the application directly mode. In this case, the platform is used to display the shape, topography, and famous craters of the planet directly without any development. Accordingly, medium- and high-level users usually have further needs regarding application and research, such as individualized customization, real-time data monitoring, analysis of a space event, etc. Current functions of the platform are not enough, so these demands can be realized based on the reserved interfaces and modules in the other two modes. 3. Key Technologies The data source elements that Sino-InSpace can accommodate increases considerably in category, quantity, and scale when compared with most of the current simulation platforms for the earth space. Therefore, based on a unified space-time datum, Sino-InSpace needs proper strategies to satisfy the requirements of visualization representation and analysis. It should be noted that Sino-InSpace was developed using the programming languages C++ and Qt, while the rendering engine was realized on an OpenGL 3.0 platform independently. Consequently, the cross-platform features were ensured. 3.1. Time-Space Datum Time and space domains are basic attributes of an object. Thus, accurate space-time datum is a prerequisite to depict and cognize an object; these datums are maintained by many specialized research institutes [50–52]. Due to the different sources of data and means of obtaining these data in practical applications, the spatial-temporal coordinate format of the data and model interfaces vary widely. Therefore, it is extremely essential for Sino-InSpace to clarify the time-space reference and perform mutual conversions.

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observations of of the sun to stars, and then to the introduction of the International Atomic From observations Time (TAI) in 1967, the development of the time system has been guided by the pursuit of higher The current current time time system system primarily primarily consists consists of of Sidereal Sidereal Time Time (ST), (ST), Universal Universal Time Time (UT), (UT), accuracy. The Ephemeris Time (ET), Dynamical Time (DT), and Atomic Time. In addition, other time references Ephemeris have been defined, such as Coordinated Universal Universal Time (UTC), which has been adopted within the Satellite System System Time Time Sino-InSpace, Global Positioning System Time (GPST), and BeiDou Navigation Satellite (BDT) for convenience. The conversion between time systems when the data are imported or exported for convenience. The conversion between time systems when the data are imported or is accomplished with the help the Standards of Fundamental Astronomy (SOFA) Library ANSI exported is accomplished withof the help of the Standards of Fundamental Astronomy (SOFA)for Library C [53]. for ANSI C [53]. Considering the thespace spacedatum, datum, primary scope of human activity is currently thecircle; earth Considering thethe primary scope of human activity is currently in thein earth circle; therefore, we considered a coordinate reference system centered on earth. the earth. Nevertheless, therefore, we considered a coordinate reference system centered on the Nevertheless, the the activities at the solar system scale needed supported a coordinatesystem systemwith withaalarger largerscale. scale. activities at the solar system scale needed to to bebe supported byby a coordinate These large-scale coordinate systems [54–56] primarily included the International Celestial Reference Reference System (PCRS), International Terrestrial Reference System System (ICRS), (ICRS),Planetary PlanetaryCentroid Centroid Reference System (PCRS), International Terrestrial Reference (ITRS), Station Coordinate System, and Satellite Coordinate System, as shown in Figure 2. System (ITRS), Station Coordinate System, and Satellite Coordinate System, as shown in Figure 2. In addition, the transitional coordinate system was used to meet the needs of coordinate system conversion. For Forexample, example, orbit prediction ofEarth-center an Earth-center satellite is mostly conversion. thethe orbit prediction of an satellite is mostly carriedcarried out in out the in the J2000 coordinate system (there is a small deviation matrix between it and the Geocentric J2000 coordinate system (there is a small deviation matrix between it and the Geocentric Celestial Celestial Reference System (GCRS)). However, substellar point calculation and coverage analysis are Reference System (GCRS)). However, substellar point calculation and coverage analysis are often often carried in ITRS. Therefore, transformation between thesesystems systemsrequires requiresthe the celestial carried out inout ITRS. Therefore, the the transformation between these intermediate reference system and terrestrial intermediate reference system. The conversion between reference systems in Sino-InSpace is also supported by the SOFA SOFA Library Library [53]. [53]. Heliocentric celestial reference system ICRS

Planet centroid celestial reference system Geocentric celestial reference system

PCRS

Deviation matrix Precession– nutation

Celestial intermediate reference system Earth rotation

Space

ITRS

reference system Station coordinate system

Geocentric coordinate systems

Polar motion

Reference-ellipsoid-centric coordinate system Station equator coordinate system Station orthogonal coordinate system Satellite orbit coordinate system

Satellite coordinate system

Terrestrial intermediate reference system

Satellite inertial coordinate system Satellite centroid coordinate system

Legend Commonly used coordinate system Transitional coordinate system

Space datum datum of the Sino-InSpace platform. Figure 2. Space

3.2. Environment Data Organization 3.2.1. Organization of Geographical Environment Data Geographical environment data is the basis of human activities on the surface of a planet, which includes all kinds of data collected by probes, primarily remote images of the planet surface, DEMs, vectors, and toponymic data, among others. To improve the efficiency of data loading and rendering,


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3.2. Environment Data Organization 3.2.1. Organization of Geographical Environment Data Geographical environment data is the basis of human activities on the surface of a planet, which includes all kinds of data collected by probes, primarily remote images of the planet surface, DEMs, among others. To improve the efficiency of data loading and ISPRS Int.vectors, J. Geo-Inf.and 2018,toponymic 7, x FOR PEERdata, REVIEW 6 of 27 rendering, the strategy of the multiresolution pyramid model is generally adopted by most digital the strategy of[57,58] the multiresolution by system most digital planet planet systems depending on apyramid structuremodel knownisasgenerally a discreteadopted global grid (DGGS) [59]. systems [57,58] depending on a structure known as a discrete global grid system (DGGS) [59]. The The traditional approach discretizing a solid planet is to use the latitude/longitude coordinate system, traditional approach discretizing a solid planet is to use the latitude/longitude coordinate system, which was also adopted in the early stages of the platform development (see Figure 3). which was also adopted in the early stages of the platform development (see Figure 3). 0

Col

9 90

4 Lat

Row

L0 0 -180

Lon

-90 180

Ln: (Lon, Lat) L1 L2

 (Lat + 90) × 2 n  Row =   36    (Lon + 180) × 2 n  Col =   36  

Index: \ Data Folder\ Data Set Name\ Ln\ Row\ Col.abc

Figure 3. 3. Geographic data segmentation and index index method. method.

For example, considering consideringthe theMars Marsimages imagesand andDEMs, DEMs, the size pyramid set512 to the size of of thethe pyramid tiletile waswas set to ◦ ◦ 512 ×pixels, 512 pixels, the first divided 5 ×and 10, and the row increased from to 90 N and × 512 the first layerlayer waswas divided intointo 5 × 10, the row increased from 90° 90 S toS90° N and the the column increased from to 180 E. Then, tile was segmented layer-by-layer, the column increased from 180°180 W◦toW180° E. ◦Then, eacheach tile was segmented layer-by-layer, and and the row row and column number (Row, the coordinate Lat) Ln could level could be calculated and column number (Row, Col) Col) at theatcoordinate (Lon,(Lon, Lat) of theofLnthe level be calculated using using the formula specified in Figure 3. The used three-level storagewith withthe thenaming naming convention, the formula specified in Figure 3. The tiletile used three-level storage ‘Ln \Row\Col’, which forfor querying andand loading. ForFor feature data,data, the point region (PR) ‘Ln\Row\Col’, whichwas wasconvenient convenient querying loading. feature the point region quadtree index index [60] was created based on the location and recorded the eXtensible (PR) quadtree [60] was created based on the coordinates location coordinates and using recorded using the Markup Language file. (XML) file. eXtensible Markup(XML) Language 3.2.2. 3.2.2. Discrete Discrete Space Space Environment Environment Data Data Index Index The rich elements andand covers a vast area;area; for example, takingtaking the earth The space spaceenvironment environmentcontains contains rich elements covers a vast for example, the space environment as an example, elements such as the middle and upper atmosphere, the geomagnetic earth space environment as an example, elements such as the middle and upper atmosphere, the field, and thefield, ionosphere in the range of the tensrange of kilometers tens of thousands kilometers. geomagnetic and theare ionosphere are in of tens oftokilometers to tens ofof thousands of For space environment elements with a wide distribution and large density under a static state, whether kilometers. For space environment elements with a wide distribution and large density under a static it is the clippingitin process the nearest neighbor in the query operation, the conventional state, whether is the the visual clipping in theorvisual process or the nearest neighbor in the query operation, traversal methodtraversal does notmethod meet the requirements. spheroid degenerated-octree grid (SDOG) the conventional does not meet the The requirements. The spheroid degenerated-octree uses volumetric of the sphere that can provide reference [61,62]. The octree spatial grid a(SDOG) uses discretization a volumetric discretization of the sphere thata can provide a reference [61,62]. The subdivision method was adopted to construct the index for the original version of Sino-InSpace octree spatial subdivision method was adopted to construct the index for the original version of Sino(see Figure 4).Figure 4). InSpace (see


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Root node

A

Level 0 1

B

E D B

C

A

2

C

DE

3

Figure4.4.Space Spaceenvironment environmentoctree octreesubdivision subdivisionmethod. method. Figure

Anoctree octreeisisaa tree tree data data structure structure involving involving spatial spatial level level recursive recursive subdivision. subdivision. The Thesubdivision subdivision An of the the space space environment environment data data was was based based on on the the simple simple cylindrical cylindrical projection projection plane plane and and height height of direction. The cube represented by the root node included all the data to be processed. The root node direction. The cube represented by the root node included all the data to be processed. The root node wassubdivided subdividedinto intoeight eightsub subnodes nodesrecursively, recursively,and andthe thenonempty nonemptynode nodecontinued continuedtotobe bedivided. divided. was The maximum maximum recursive depended onon thethe resolution of various environmental data, The recursive depth depthofofan anoctree octree depended resolution of various environmental display requirements, and query accuracy. data, display requirements, and query accuracy. 3.3.Entity EntityObject ObjectModel ModelDesign Design 3.3. Satellites,rockets, rockets,and andprobes probesare areused usedto tocarry carryout outspace spacemissions missionssuch suchas aslaunch, launch,observation, observation, Satellites, and detection, and become the actual participants in space activities. Furthermore, these man-made and detection, and become the actual participants in space activities. Furthermore, these man-made spacetargets targetswill willbecome becomethe themain mainbody bodyof ofVSEs VSEsthat thatneeds needsto tobe besupported supportedand andexpressed expressedbecause because space of the inability of a large number of human beings to enter into space at present. Figure 5 shows the of the inability of a large number of human beings to enter into space at present. Figure 5 shows the four kinds of information required for describing a working BeiDou navigation satellite: (i) Attribute; four kinds of information required for describing a working BeiDou navigation satellite: (i) Attribute; thiscontains containsthe thename, name,id, id,category, category, and andcountry countryof ofthe theobject. object.(ii) (ii)Structure; Structure;this thismainly mainlydescribes describesthe the this geometric form and the change. In addition to the model construction of the target, it also includes the geometric form and the change. In addition to the model construction of the target, it also includes change of overall form following thethe action of the components, such as the directional rotation of the the change of overall form following action of the components, such as the directional rotation of solar panels during the operation. (iii) Position; determining the location of the target in the space-time the solar panels during the operation. (iii) Position; determining the location of the target in the spacecoordinate system. The motion of the of space a certaina dynamic model. (iv) Attitude; time coordinate system. The motion the target space often targetfollows often follows certain dynamic model. (iv) according to the missions’ need, the attitude of the satellite should be adjusted frequently such as the Attitude; according to the missions’ need, the attitude of the satellite should be adjusted frequently direction of the satellite antenna to the earth needs to ensure the visibility of the communication link. such as the direction of the satellite antenna to the earth needs to ensure the visibility of the Therefore, a unified modela unified was designed, which was consisted of the attribute, ephemeris, geometric communication link.object Therefore, object model designed, which consisted of the attribute, model, andgeometric behavior model, is shown in Figure 6. is shown in Figure 6. ephemeris, model, which and behavior model, which Based on the multiple ephemeris segment, the ephemeris elementsare areused usedto to determine determinethe the Based on the multiple ephemeris segment, the ephemeris elements locationof ofthe thetarget targetobject objectat atany anytime. time.Each Eachephemeris ephemerissection sectioncontains containsaadescription descriptionofofthe thetimeline, timeline, location coordinate system, and orbital information. In addition, three forms of orbits are supported in coordinate system, and orbital information. In addition, three forms of orbits are supported in SinoSino-InSpace: Kepler, two-line element (TLE), and discrete track points. The first two orbits have InSpace: Kepler, two-line element (TLE), and discrete track points. The first two orbits have their own their own prediction model whichtois calculate used to calculate the position. the description the shape, prediction model which is used the position. For theFor description of theofshape, the the geometric model is composed of components that are constituted by some primitives based on geometric model is composed of components that are constituted by some primitives based on the the tree structure; the primitives are the smallest parts of the model consisting of dots, lines, planes, tree structure; the primitives are the smallest parts of the model consisting of dots, lines, planes, and and textures. In addition, the primitive canbe also be a model file constructed by commercial software textures. In addition, the primitive can also a model file constructed by commercial software (e.g., (e.g., 3ds Max). The behavior model is used to describe the movements of the overall and internal 3ds Max). The behavior model is used to describe the movements of the overall and internal

components of the target object at different stages. Each phase contains a description of the timeline,

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components of and the target object at different phase contains a description timeline, action object, behavior type, which stages. includesEach three types of operations such ofasthe translation, action object, and behavior type, which includes three types of operations such as translation, action object, and behavior type, which includes three types of operations such as translation, rotation, rotation, and scaling. rotation, and scaling. and scaling.

Figure5.5. Description Description for for aaBeiDou BeiDounavigation navigationsatellite. satellite. Attribute Attribute information information does does not not generally generally Figure Figure 5. Description for a BeiDou navigation satellite. Attribute information does not generally change.Structure Structureinformation informationisispartially partiallyrelated relatedto totime timebecause becausesometimes sometimesthe thecomponents componentsneed needto to change. change. Structure information is partially related to time because sometimes the components need to adjust.Position Positionand andattitude attitudeinformation informationisisalways alwaysrelated relatedtototime. time. adjust. adjust. Position and attitude information is always related to time. Object Object Attribute Attribute

Ephemeris Ephemeris

Ephemeris1 Ephemeris2 ... Ephemeris1 Ephemeris2 ...

EphemerisN EphemerisN

Timeline Timeline

Coordinate Coordinate system system

Orbit Orbit

Kepler Kepler

TLE TLE

Track Track

Geometric model Geometric model Component1 Component2 Component1 Component2 ComponentN ComponentN Primitive1 Primitive1

Primitive2 Primitive2

PrimitiveN PrimitiveN Dot Dot

Line Line

Plane Plane

Texture Texture

Behavior model Behavior model Stage1 Stage1

Stage2 Stage2

... ...

StageN StageN

Timeline Timeline

Action object Action object

Behavior Behavior type type

Translate Translate

Rotate Rotate

Scale Scale

Figure6.6. Entity Entity object object model model design. design. The The action action object object derives derives from from the thecomponent component or orgeometric geometric Figure Figure 6. Entity object model design. The action object derives from the component or geometric model modelitself. itself. model itself.

3.4. 3.4.Visualization VisualizationScenario ScenarioOrganization Organization 3.4. Visualization Scenario Organization The Thevisualization visualizationscenario scenariorefers refersto tothe theorganization organizationof ofthe thegeographical geographicalenvironment, environment,entity entityand and The visualization scenario refers to activities, the organization of the geographical environment, entity and space environment elements, and space which belong to the geographical scene. Traditional space environment elements, and space activities, which belong to the geographical scene. space environment and space activities, belong to and the temporal geographical scene. cartography and GIS elements, are commonly to express thetowhich static information relationship. Traditional cartography and GIS areused commonly used express the static information and temporal Traditional cartography and GIS are commonly used to express the static information and temporal The visualization focuses on multilevel information regarding time, relationship. Thescenario visualization scenario focusesand on multidimensional multilevel and multidimensional information relationship. The visualization scenario focuses on multilevel andothers multidimensional information location, participants, events, processes, and phenomena, among [38,63]. The visualization regarding time, location, participants, events, processes, and phenomena, among others [38,63]. The regarding time, location, participants, events, processes, and phenomena, amongand others The scenario organization this study was based onwas twobased levels: modeling data[38,63]. modeling, visualization scenario in organization in this study onlogical two levels: logical modeling and data visualization scenario organization in this study was based on two levels: logical modeling and data as shown inasFigure 7. in Figure 7. modeling, shown modeling, as shown in Figure 7.


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(a)

Logical model

(b)

Geographical environment data configuration Space environment data configuration Entity object data configuration

Data model Geometric model

Ephemeris

Behavior model

Figure 7. 7. Scenario modeling based based on on Unified Unified Modeling Modeling Language Language (UML) (UML) and and Figure Scenario organization organization and and modeling eXtensible Markup Language (XML). (a) UML diagram of the visualization scenario. (b) XML data of eXtensible Markup Language (XML). (a) UML diagram of the visualization scenario. (b) XML data of the visualization scenario. Timeline is used as an example; its logical model includes the start time, the visualization scenario. Timeline is used as an example; its logical model includes the start time, end time, time, and and time time step. step. The The datatype datatype of of the the start start time time and and the the end end time time is is defined as dateTime, dateTime, while while end defined as that of of the the time time step step is is Double. Double. XML XML content content was was filled filled according according to to the the field field data data type, type, which which can can be be that stored in in the the local local file file to to be be accessed accessed later. later. Furthermore, Furthermore, Timeline Timeline was was also also the the part part of of the the Ephemeris Ephemeris and Action Action classes classes with with the the corresponding corresponding relationship relationshipof of11to to1. 1. and

The and relationship of the scenario data were by defining The static staticstructure structure and relationship of visualization the visualization scenario datadescribed were described by the specific using theusing Unified Modeling Language (UML) (UML) diagram. As in the previous case, defining theterms specific terms the Unified Modeling Language diagram. As in the previous SpaceScenario contains three main SpaceEnvironment, and Object. The type, case, SpaceScenario contains threenodes: main GeoEnvironment, nodes: GeoEnvironment, SpaceEnvironment, and Object. structure, data constraints, and the correspondence and association between different levels of the The type, structure, data constraints, and the correspondence and association between different levels hierarchy were described. In GeoEnvironment, the path givesgives the file information for the of the hierarchy were described. In GeoEnvironment, the path thesystem file system information for config file of data,data, which contains the details of pyramid tiles (e.g., region, tile size, the config filethe of geographical the geographical which contains the details of pyramid tiles (e.g., region, tile size, tile level, and storage location). In SpaceEnvironment, the path and model indicate the source of the space environment data. The former refers to the directory of the environment data file and the latter is the model used for calculation. The Object node has three child nodes: Model, EphemerisList,


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tile level, and storage location). In SpaceEnvironment, the path and model indicate the source of the space The REVIEW former refers to the directory of the environment data file and the ISPRS Int. J.environment Geo-Inf. 2018, 7, xdata. FOR PEER 10 of 27 latter is the model used for calculation. The Object node has three child nodes: Model, EphemerisList, andActionList. ActionList.These Thesenodes nodesdepict depictthe thegeometric geometric model, ephemeris, behavior model in Section and model, ephemeris, andand behavior model in Section 3.3. 0. Model, In Model, filepath theofpath of the object geometric 3D InitialTranslate, model file. InitialTranslate, In filepath meansmeans the path the object geometric 3D model file. InitialAngle, InitialAngle, and InitialScale describe state of the object geometric 3D model. In and InitialScale describe the initial state ofthe the initial object geometric 3D model. In EphemerisList, several EphemerisList, several Ephemeris mayone be contained eachHere, one represents a track. Here, the Ephemeris nodes may be containednodes and each representsand a track. the orbit type of “BEIDOU orbitsatellite type ofis“BEIDOU 1D” satellite is TLE. ActionList more thanrecords one Action node, 1D” TLE. ActionList also comprises more thanalso one comprises Action node, which the change which recordsrotation, the change translation, rotation, and size. of translation, andofsize. 3.5. 3.5. Visualization Visualization Engine Engine Design Design The abstract data datamakes makesititdifficult difficulttotomeet meet final needs of the users. However, The use of abstract thethe final needs of the users. However, the the visualization technology describe data graphically to provide users with a visual outlook visualization technology cancan describe thethe data graphically to provide users with a visual outlook of of these data, which can help them understand and recognize the current space activities. Faced these data, which can help them understand and recognize the current space activities. Faced with with complex, massive spatial data, efficient data scheduling andflexible flexibleoperational operationalcontrol control are key complex, massive spatial data, efficient data scheduling and key problems problems in in the the visualization visualization process. process. Therefore, Therefore, Sino-InSpace Sino-InSpace divides divides the the visualization visualization engine engine into into two two parts: parts: the the data data engine engine and and rendering rendering engine, engine, as as shown shown in in Figure Figure8. 8.

Index calculation and updating

Mouse

Visible area calculation

Progressive data scheduling

Data engine

Viewpoint control Data request

Rendering Required data is included

Y

engine Buffer data reading

N Multithread data reading

Buffer updating

User interaction

Keyboard

Gesture

Time control

Data loading

Real-time Behavior control

Script driver Non-real-time

Figure Figure 8. 8. Visualization Visualization engine engine design. design.

The The data data engine engine is is responsible responsible for for the the rapid rapid indexing indexing and and scheduling scheduling of of data. data. Depending Depending on on the the visual cone parameters delivered by the rendering engine, the visible area is first calculated and visual cone parameters delivered by the rendering engine, the visible area is first calculated and then, then, the that need to to be be updated areare determined. To improve the the tiles, tiles,environment environmentdata, data,ororgraphic graphicnames names that need updated determined. To improve rendering of visualization, the data are progressively scheduled from near to far based on the distance the rendering of visualization, the data are progressively scheduled from near to far based on the to the viewpoints; in addition, certain part of the data is reserved forisvisualization. distance to the viewpoints; inaaddition, a certain part buffer of thememory data buffer memory reserved for For the visualization process, first, we determined whether the data buffer contained the required visualization. For the visualization process, first, we determined whether the data buffer contained data; if the data were not available in the buffer, they were imported by multithreading, and the data the required data; if the data were not available in the buffer, they were imported by multithreading, buffer was then updated. After receiving the data transmitted by the data engine, the rendering engine and the data buffer was then updated. After receiving the data transmitted by the data engine, the graphically renders the environment and entity elements in the visual scene. rendering engine graphically renders the environment and entity elements in the visual scene. The enginealso alsosupports supports user interactions as mouse-, keyboard-, and gestureThe rendering rendering engine user interactions suchsuch as mouse-, keyboard-, and gesture-based based interactions as real-time or non-real-time drivers to flexibly control the viewpoint, interactions as wellasaswell real-time or non-real-time script script drivers to flexibly control the viewpoint, time, time, and entity target behavior. A script file can be opened and edited directly in text, and each and entity target behavior. A script file can be opened and edited directly in text, and each line line represents represents aa command commandand andhas hasaafixed fixedformat formatas asfollows: follows: Command identification {[Parameter identification Parameter]} Command identification {[Parameter identification Parameter]}

Table22shows showssome someof ofthe thecommands commandsand andparameters. parameters. These These commands commands support support the the switch switch of of Table viewpoints, control of simulation speed, text display, etc. A simple example is given in Figure 9. viewpoints, control of simulation speed, text display, etc. A simple example is given in Figure 9.


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Table 2. Some commands and parameters. Command Identification Command Identification

Parameterand parameters. Table 2. Some commands Command Meaning Parameter Meaning Identification Parameter center Central target name. Parameter Meaning Command Meaning Setting the position Identification Position of viewpoint in the center body x, y, z and perspective Setting the positionof center Central target name. coordinate system observation andthe perspective of Position of viewpoint Attitude of sight in the center center body body the observation (data x, y, z (data can be obtained coordinate system qw, qx, qy, qz coordinate system represented by the canfrom be obtained from the platform). Attitude of sight in the center body coordinate the platform). quaternion. qw, qx, qy, qz system represented by day. the quaternion. jd Julian

setviewpoint setviewpoint

settime

settime

Setting the time and step of the Setting the time and step simulation. of the simulation.

wait wait

print print

image image

The script continues The script continues to to execute after execute waiting after for a waiting certain for a certain length length of time.of time. Printing the message message on Printing the the screen. screen. on the

Illustration Illustration

jd

ts

ts

duration duration

Julian Time step. Theday. positive number represents the forward simulation, Time step. The positive number representsthe the forward simulation, the negative negative number represents thenumber backward represents the backward simulation, 0 simulation, and 0 represents the and pause. represents the pause. The unit is second. The unit is second. The The length of the to run The unit length ofscript the script topause. run pause. The is second. unit is second.

text text

Text hint screen. Text hint onon thethe screen.

id id state state file file

Picture identification Picture identification 0—hide, 1—show. 0—hide, 1—show. File path. File path. The pixel position of the image on the x, y x, y The pixel position of the image on the screen. screen. width, height The width and height of the image. width, height The width and height of the image.

Figure 9. A simple example of the script file.

4. 4. Design Design of of Platform Platform Application Application Modes Modes and and Case Case Study Study By and human By supporting supporting geo-visualization, geo-visualization, geo-simulation, geo-simulation, geo-collaboration, geo-collaboration, and human participation, participation, VGEs provide open virtual environments that correspond to the real world to assist computer-aided VGEs provide open virtual environments that correspond to the real world to assist computer-aided geographic platform forfor VSEs, Sino-InSpace aims to satisfy the geographic experiments experiments[8]. [8].As Asa adigital digitalsimulation simulation platform VSEs, Sino-InSpace aims to satisfy multilevel requirements of theof application. As shown in Figure three10, modes supplied the multilevel requirements the application. As shown in10, Figure threearemodes are including supplied application directly (without more development), visualization development, and scientific analysis. including application directly (without more development), visualization development, and

scientific analysis.


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Figure 10. 10. Design Design of of platform platform application application modes. modes. Figure

4.1. Application Directly 4.1. Application Directly Sino-InSpace provides an executable software to serve low-level users directly. It has an accurate Sino-InSpace provides an executable software to serve low-level users directly. It has an accurate starry sky and can integrate massive environment data from different planets. Therefore, it is possible starry sky and can integrate massive environment data from different planets. Therefore, it is possible for children to study the solar system and for the public to understand the appearance of other planets. for children to study the solar system and for the public to understand the appearance of other In addition, complex simulation and deduction of space missions can be conducted through the planets. In addition, complex simulation and deduction of space missions can be conducted through scenario construction and script edit. Finally, the virtual reality setup combined with Sino-InSpace in the scenario construction and script edit. Finally, the virtual reality setup combined with Sinoour laboratory was introduced and used to evaluate the performance of the platform. All case studies InSpace in our laboratory was introduced and used to evaluate the performance of the platform. All in this section were conducted with a Dell workstation (Intel Core i7 CPU, 8G RAM, Nvidia Quadro case studies in this section were conducted with a Dell workstation (Intel Core i7 CPU, 8G RAM, K2000 graphics card, and Microsoft Windows 10 × 64 operating system). Nvidia Quadro K2000 graphics card, and Microsoft Windows 10 × 64 operating system). 4.1.1. Application Main Window 4.1.1. Application Main Window The application main window is shown in Figure 11 and offers a friendly operation interface The main window is shown in Figure 11 and offers a friendly operation includingapplication the Scenario Editor, 2D and 3D view, Time Control, and Viewpoint Control. interface Scenario including the Scenario Editor, 2D and 3D view, Time Control, and Viewpoint Control. Editor Editor provides the structure of scenarios constructed with satellites, spacecraft,Scenario environment provides the structure of scenarios constructed with satellites, spacecraft, environment configurations, and so on. Entity elements including properties, ephemerides, geometric models, configurations, and so on. elements ephemerides, geometric models, and and actions (mentioned inEntity Section 3.3) canincluding be edited properties, further. Multidimensional visualization enables actions (mentioned in Section 0) can be edited further. Multidimensional visualization enables the the presentation of scenarios and phenomena to be more dynamic [64]. The platform provides 2D and presentation of scenarios and phenomena to be more dynamic [64]. The platform provides 2D and 3D view windows. The mouse and keyboard can be used for smooth panning and zooming through 3D The andThe keyboard can be used forto smooth panning and zooming through dataview withwindows. six degrees ofmouse freedom. Viewpoint Tool helps add, edit, delete, and switch between data with six degrees of freedom. The Viewpoint Tool helps to add, edit, delete, and switch between different viewpoints; this is convenient for flexible observations. Furthermore, the Time Tool contains different viewpoints; this is convenient flexible observations. the Time Toolbrowsing contains the function of acceleration, deceleration,for going ahead, going back,Furthermore, and time setting to satisfy the function of acceleration, deceleration, going ahead, going back, and time setting to satisfy the event at different speeds and times. browsing the event at different speeds and times.


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Figure 11. The The operation operation window window of Sino-InSpace Sino-InSpace in the application directly mode. (a) (a) The main includes the theScenario ScenarioEditor, Editor,2D 2Dand and view, Time Control, Viewpoint Control; window includes 3D3D view, Time Control, andand Viewpoint Control; (b) (b) Trajectory setting window; (c) Geometric model setting window; Action design window. Trajectory setting window; (c) Geometric model setting window; andand (d) (d) Action design window.

4.1.2. Visualization and Deduction 4.1.2. Visualization and Deduction Sino-InSpace provides a flexible and visible platform for the massive achievement not only Sino-InSpace provides a flexible and visible platform for the massive achievement not only on on earth but also on other planets. In support of a perfect time-space datum framework, stars, earth but also on other planets. In support of a perfect time-space datum framework, stars, planets in planets in the solar system, and geographical and space environment data are integrated, organized, the solar system, and geographical and space environment data are integrated, organized, and and displayed as shown in Figure 12. Excellent data organization and a good visualization engine displayed as shown in Figure 12. Excellent data organization and a good visualization engine guarantee the fluent running of the platform. The time delay mainly originates from the initial data guarantee the fluent running of the platform. The time delay mainly originates from the initial data loading and the real-time calculation (e.g., orbit prediction and empirical space environment models). loading and the real-time calculation (e.g., orbit prediction and empirical space environment models). The time required to load the necessary data in the illustrations is no more than 3 s, and the frame rates The time required to load the necessary data in the illustrations is no more than 3 s, and the frame are all greater than 30 ftps, is sufficiently satisfactory for observation. rates are all greater than 30 which ftps, which is sufficiently satisfactory for observation. Advance deductions are often carried out to enhance the of theof mission. As shown Advance deductions are often carried out to enhance understanding the understanding the mission. As in Figure 13, a launching mission deduction of the Shenzhou spacecraft was designed. The trajectory shown in Figure 13, a launching mission deduction of the Shenzhou spacecraft was designed. The and actionand data of the CZofrocket and manned spacecraft were added the related object model. trajectory action data the CZ rocket and manned spacecraft wereto added to the related object Then, the script file was edited and loaded to drive the mission execution, and the viewpoint was model. Then, the script file was edited and loaded to drive the mission execution, and the viewpoint switched to the perspective. Meanwhile, some illustration andand texttext hints appeared on the was switched tointerested the interested perspective. Meanwhile, some illustration hints appeared on screen occasionally. the screen occasionally.

ISPRS Int. J. Geo-Inf. 2018, 7, 373 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW

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Figure 12. visualization of and data. Figure 12. Integration and visualization of constellation the constellation and environment data.(Tycho-2 (a) Stars Figure 12. Integration Integration and and visualization of the the constellation and environment environment data. (a) (a) Stars Stars (Tycho-2 catalogue, number: >2.5 million) and planets in the solar system (NASA Jet Propulsion Laboratory (Tycho-2 catalogue, >2.5and million) and the solar system (NASA Jet Propulsion catalogue, number:number: >2.5 million) planets in planets the solarin system (NASA Jet Propulsion Laboratory Development Ephemeris (JPL (b) all features for moon 8990) (c) Laboratory Development (JPL 405); (b) all named for the moon (number: Development Ephemeris Ephemeris (JPL DE) DE) 405); 405); (b)DE) all named named features for the thefeatures moon (number: (number: 8990) [65]; [65]; (c) geomagnetic field generated from the Tsyganenko 96 model [66], which includes the solar-windgeomagnetic field generated from the Tsyganenko model [66],96which the solar-wind8990) [65]; (c) geomagnetic field generated from the 96 Tsyganenko modelincludes [66], which includes the controlled region and Birkeland currents, and the interconnection of the controlled magnetopause, magnetopause, region 11region and 221 and Birkeland currents, and and the the interconnection of of thethe solar-wind-controlled magnetopause, 2 Birkeland currents, interconnection magnetospheric and solar wind fields at the boundary; and (d) geographical data from the southeast magnetospheric and solar wind fields at the boundary; and (d) geographical data from the southeast magnetospheric and solar wind fields at the boundary; and (d) geographical data from the southeast area of China (image resolution: 30 m, DEM resolution: m, data amount: >200 GB). area China (image resolution:30 30m, m,DEM DEMresolution: resolution: 90 90 area of of China (image resolution: 90 m, m, data dataamount: amount:>200 >200GB). GB).

Figure 13. Launching mission deduction of the spacecraft. (a) Rocket firing. The Figure Launching mission deduction the Shenzhou Shenzhou spacecraft. (a)firing. Rocket The Figure 13. 13. Launching mission deduction of theof Shenzhou spacecraft. (a) Rocket Thefiring. illustrations illustrations introduced the stage name and the capacity of the CZ rocket. (b) Rocket booster illustrations stage name andofthe of the rocket. (b) Rocket booster introduced theintroduced stage namethe and the capacity the capacity CZ rocket. (b) CZ Rocket booster separation. The separation. The action executed 114 ss later; the of expansion and separation. The separation separation114 action executed 114process later; (c,d) (c,d) the process process of solar solar panel panel and separation action executed s later; (c,d) the of solar panel expansion and expansion orientation. The orientation. The solar as of spacecraft model was and rotated orientation. solar panel panel as aspacecraft a component component of the the was translated translated rotated solar panel as aThe component of the model wasspacecraft translatedmodel and rotated accordingand to the action according to according to the the action action sequence sequence setting. setting. sequence setting.


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4.1.3. Virtual Virtual Reality Reality Setup 4.1.3. Setup Virtual and reality have received a amount of of attention attention by by researchers researchers Virtual and augmented augmented reality have received a considerable considerable amount in the last decade and provide new ways to perceive the world [67–69]. Virtual reality is defined in the last decade and provide new ways to perceive the world [67–69]. Virtual reality is defined asas a a computer-generated scenariothat thatsimulates simulatesexperience experiencethrough throughsenses sensesand and perception perception [70], [70], while computer-generated scenario while augmented reality reality creates creates an an environment environment where where digital digital information information is is inserted inserted in in aa predominantly predominantly augmented real-world view [71]. The well-known basic components of a VR immersive application include input input real-world view [71]. The well-known basic components of a VR immersive application include devices (responsible (responsiblefor forinteraction), interaction),output outputdevices devices (for feeling of immersion), software devices (for thethe feeling of immersion), andand software (for (for a proper control and synchronization of the whole environment). For the better simulation and a proper control and synchronization of the whole environment). For the better simulation and presence for for VSEs, VSEs, aa simple simple VR VR setup setup was was constructed constructed in in our our Space Space Situational Situational Awareness Awareness (SSA) (SSA) presence shared laboratory laboratory based based on on Sino-InSpace. Sino-InSpace. The The standard standard mouse mouse and and keyboard keyboard allow allow users users aa relatively relatively shared easy and normal way of manipulation of a virtual scenario. In addition, Sino-InSpace supports easy and normal way of manipulation of a virtual scenario. In addition, Sino-InSpace supports aa variety of of stereoscopic observations (e.g., (e.g., red red and and green, green, split split screen, screen, and and blinking) blinking) to to produce produce an an variety stereoscopic observations immersive environment. The first two types have no special requirement for the hardware, and the immersive environment. The first two types have no special requirement for the hardware, and the virtual experience experience is is poor poor accordingly. accordingly. The The blinking blinking mode mode is is most most frequently frequently used used with with the the help help of of virtual Nvidia 3D 3D vision vision technology (Figure 14). 14). The The output output devices devices consist consist of of aa stereoscopic graphic card card Nvidia technology (Figure stereoscopic graphic (Quadro K2000), synchronous signal transmitters, 3D glasses, Panasonic projectors, and a projection (Quadro K2000), synchronous signal transmitters, 3D glasses, Panasonic projectors, and a projection screen. The The equipment supports up up to to tens which screen. equipment supports tens of of people people viewing viewing stereo stereo content content simultaneously, simultaneously, which could better better assist assist in could in collaborative collaborative and and multiperspective multiperspective space space science science research research and and education. education.

Figure 14. Blinking vision technology. technology. The Blinking stereoscopic stereoscopic observations observations with with the the help of Nvidia 3D vision synchronous synchronous signal signal transmitters transmitters control control the the switch switch of of two two lenses lenses of of 3D glasses, glasses, while while the the 3D view window provides a left or right frame.

4.2. Visualization Visualization Development Development 4.2. Compared with with the the “Application Directly” mode, mode, the the “Visualization mode was Compared “Application Directly” “Visualization Development” Development” mode was proposed to develop the individualized customizable software. Figure 15 presents the core components proposed to develop the individualized customizable software. Figure 15 presents the core of Sino-InSpace. The SSimAstroCore componentcomponent is mainly used to provide mathematical functions components of Sino-InSpace. The SSimAstroCore is mainly used to provide mathematical and model calculations, e.g., coordinate transformation, matrix operation, orbit prediction, space functions and model calculations, e.g., coordinate transformation, matrix operation, orbit prediction, environment model calculations, and so on, arewhich the basis the basis platform. Theplatform. SSim2DView space environment model calculations, andwhich so on, areofthe of the The and SSim3DView components, as the visualization engines, are responsible for the data loading SSim2DView and SSim3DView components, as the visualization engines, are responsible for the data and rendering. The SSimScenario component parses the scenario file and extracts types of loading and rendering. The SSimScenario component parses the scenario file and different extracts different elements the nextfor step. SSimMainApp component is the interface between the external program types of for elements theThe next step. The SSimMainApp component is the interface between the external program and internal components and is the most frequently used component in “Visualization Development” mode. It supplies encapsulated User Interface (UI) widgets shown in


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and internal components is the most frequently used component in “Visualization Development” Figure 11 which can be and embedded in the new program conveniently. In addition to the related mode. It supplies encapsulated User Interface (UI) widgets shown in Figure 11 which can be the embedded Figure 11 such whichascan be embedded the objects new program addition to related functions environment datainand adding conveniently. and time andInviewpoint control, local in the new program conveniently. In addition to the adding related functions environment datalocal and functions environment data and objects and time such and as viewpoint control, scenario filesuch openaswas also developed. objects adding and time and viewpoint control, local scenario file open was also developed. scenario file open was also developed. Time Control Viewpoint Control Time Control Viewpoint Control

SSim2DView SSim2DView

Mathematical calculation Mathematical calculation

Rendering Rendering

SSimMainApp SSimMainApp

Time Control Viewpoint Control Time Control Viewpoint Control

SSim3DView SSim3DView

Mathematical calculation Mathematical calculation

SSimAstroCore SSimAstroCore

Rendering Rendering

Orbit Prediction SSimScenario Space Environment Model Caculation Scenario construction Orbit Prediction SSimScenario Space Environment Model Caculation Figure 15. Core component diagram of Sino-InSpace. Arrows indicate dependencies. Figure 15. 15. Core Core component component diagram diagram of of Sino-InSpace. Sino-InSpace. Arrows Arrows indicate indicate dependencies. dependencies. Figure Scenario construction

Taking the “Comprehensive Situation System” in the China BeiDou Navigation Satellite System Situation China Navigation Satellite Taking the“Comprehensive “Comprehensive Situation System” in the thedeveloped China BeiDou BeiDou Navigation Satellite System System (BDS)Taking data the center as an example (FigureSystem” 16), it in was based on Sino-InSpace in the (BDS) as 16), itit was developed on in (BDS) data data center center as an an example example (Figure 16), and was developed basedlocation on Sino-InSpace Sino-InSpace in the the “Visualization Development” mode.(Figure The offline online location based data, device situation, “Visualization Development” mode. The offline and online location data, location device situation, “Visualization offline andpassed online into location location BeiDou satelliteDevelopment” situation, andmode. map The service were the data, platform. The SSimMainApp BeiDou situation, and were passed into the The SSimMainApp BeiDou satellite satellite situation, and map map service were passed intoobjects theplatform. platform. The their SSimMainApp component provides the interface for service the access to these entity and updates trajectory component provides the interface for the these and updates their trajectory component provides thetime. interface for map the access access tofollowing these entity entity objects andService updates their standard, trajectory and state changes in real For the serviceto the objects Web Map (WMS) and state in real For thetiles mapunder service Map Service (WMS) andcomponent state changes changes inobtain real time. time. the WebEngine the will the image thefollowing control of Data (Section 0). In standard, addition, component will obtain the tiles under the control ofofviewpoint Data (Section 3.5). thesoftware component will obtain theimage image tiles under the(time control DataEngine Engine (Section 0). In addition, the style was customized. Some UI widgets and control tools) were invoked the software style was customized. Some UI widgets (time and viewpoint control tools) were invoked the software style was customized. Some UI widgets (time viewpoint tools) were invoked directly. As the platform was developed under Qt languages, a readablecontrol and declarative language, directly. As was under QtQt languages, a readable andand declarative language, the directly. Asthe theplatform platform wasdeveloped developed under languages, a readable declarative the Qt Meta-Object Language (QML) [72] is often adopted to create fluidly animated andlanguage, visually Qt Language [72] is often toeasily createto fluidly and visually appealing theMeta-Object Qt Meta-Object Language (QML) [72] isadopted often adopted createanimated fluidly animated and visually appealing applications. It(QML) allows components to be reused and customized within a user applications. It allows components to be easily reused customized within a user interface. appealing applications. It allows components to be and easily reused and customized within a user interface. interface.

Figure 16. Comprehensive Comprehensive situation situation system system based based on on Sino-InSpace. Sino-InSpace. Qt Qt Meta-Object Meta-Object Language Language (QML) (QML) was used the interactivesituation operationsystem and information display. Figure 16.for Comprehensive based on Sino-InSpace. Qt Meta-Object Language (QML) display. was used for the interactive operation and information display.

usually has a specific and given geometry geometry that that can be created or combined in The entity object usually advance. However, some invisible changing parameters such as sensor communication The However, entity object usually has or a or specific and given geometry canrange, be range, created or combined in advance. some invisible changing parameters such asthat sensor communication link, advance. someboundary invisible or changing as sensor range, communication link, and spaceHowever, environment also need aparameters visible andsuch dynamically generated geometry. It is and space for environment boundary also need a visible dynamically geometry. It is impossible any visualization platform to integrate alland shapes, includinggenerated Sino-InSpace. Therefore, impossible for any visualization platform to integrate all shapes, including Sino-InSpace. Therefore,


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link, and boundary also ISPRS Int. J.space Geo-Inf.environment 2018, 7, x FOR PEER REVIEW

need a visible and dynamically generated geometry. It 27 is 17 of impossible for any visualization platform to integrate all shapes, including Sino-InSpace. Therefore, it provides the “BaseGeometry” class in the SSim3DView component, which can be inherited for Any geometry geometry drawn drawn with with a standard OpenGL language can be added to the geometric rendering. Any model (Figure (Figure6)6)ofofany any entity object. function maximizes the extensibility of the platform. entity object. ThisThis function maximizes the extensibility of the platform. Figure Figure 17 illustrates a visualization result for the Van Allen radiation belt in cooperation with the 17 illustrates a visualization result for the Van Allen radiation belt in cooperation with the National National SpaceCenter Science(NSSC) Centerand (NSSC) and Chinese Academy of Sciences (CAS). The distribution Space Science Chinese Academy of Sciences (CAS). The distribution and fluxand of fluxradiation of the radiation belt particles were calculated the AE8/AP8 [73], which was expressed the belt particles were calculated by theby AE8/AP8 modelmodel [73], which was expressed with with boundary and colors, respectively. boundary lines lines and colors, respectively.

Figure 17. 17. Van Van Allen Allen radiation radiation belt belt provided provided by by the the AE8/AP8 AE8/AP8 model. Figure model.

4.3. Scientific Scientific Analysis Analysis 4.3.

As a platform for data import and visualization, visualization, Sino-InSpace also opens interfaces to export calculation and query results. The SSimMainApp component (Figure 15) provides most of the output information (time, viewpoint, viewpoint, mouse mouse position, position, interfaces, mainly divided into three types: system information etc.), environment data (position, distribution, distribution, temperature, temperature, intensity, intensity, etc.), and entity object data (attribute, structure, latter two two types types are are meaningful meaningful for forscientific scientificanalysis. analysis. structure, position, and attitude). The latter The analysis of space objects collision warning was carried out as shown in Figure 18. The orbit and position likely approach and even collision events outside the position data data were wereobtained obtainedtotoestimate estimatethe themost most likely approach and even collision events outside platform [74]. [74]. In contrast, the result could could be validated throughthrough the reappearance of collision scenario. the platform In contrast, the result be validated the reappearance of collision The distances in three orthogonal directions directions (radial, in-track, and cross-track) between two risk targets scenario. The distances in three orthogonal (radial, in-track, and cross-track) between two were plotted as curves. The closest time was clear with the help of visual analysis. risk targets were plotted as curves. The closest time was clear with the help of visual analysis. To support the Chinese Mars Exploration Project 2020, China’s Ministry of Science and Technology funded research on planetary surface precise landing guidance and control (Grant No. 2012CB720000-G) during the twelfth five-year plan. The landing area selection is one of the most important scientific objectives. Based on the strategy of the multiresolution pyramid model (Figure 3), the altitude could be obtained in real time. Then, terrain analysis tools (Figure 19) containing point, distance, area measurement, and profile-map were developed, which supported mouse selection and manual input. It played a role in the terrain and geomorphology analysis of the Mars surface.

(attribute, structure, position, and attitude). The latter two types are meaningful for scientific analysis. The analysis of space objects collision warning was carried out as shown in Figure 18. The orbit and position data were obtained to estimate the most likely approach and even collision events outside the platform [74]. In contrast, the result could be validated through the reappearance of collision scenario. The distances two ISPRS Int. J. Geo-Inf. 2018, 7, 373in three orthogonal directions (radial, in-track, and cross-track) between 18 of 27 risk targets were plotted as curves. The closest time was clear with the help of visual analysis.


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Figure 18. Space objects collision warning analysis and report output. The distance curves are drawn with ECharts [72], a powerful, interactive charting and visualization library for browsers.

To support the Chinese Mars Exploration Project 2020, China’s Ministry of Science and Technology funded research on planetary surface precise landing guidance and control (Grant No. 2012CB720000-G) during the twelfth five-year plan. The landing area selection is one of the most important scientific objectives. Based on the strategy of the multiresolution pyramid model (Figure 3), the altitude could be obtained in real time. Then, terrain analysis tools (Figure 19) containing point, distance, area measurement, and profile-map were and developed, whichThe supported and Figure 18. Space objects collision warning analysis report output. distancemouse curvesselection are drawn manual input. It played a role in the terrain and geomorphology analysis of the Mars surface. with ECharts [72], a powerful, interactive charting and visualization library for browsers.

Figure Terrain and geomorphologyanalysis. analysis.The The crater closest map was Figure 19.19. Terrain and geomorphology closest to toCeti CetiChasma. Chasma.The Thebase base map was derived from Mars Colorized Viking Mosaic [75] and the resolution is 232 m. The source of DEM is MOLA derived from Mars Colorized Viking Mosaic [75] and the resolution is 232 m. The source of DEM is MEGDRs [76] and is 463ism. of segmented geographical data exceeds 15 MOLA MEGDRs [76]the andresolution the resolution 463The m. amount The amount of segmented geographical data exceeds GB. 15 GB.

5. Platform Evaluation 5. Platform Evaluation Despitethe thefact factthat thatSino-InSpace Sino-InSpace provides provides real-time real-time interactive Despite interactive experiences experiencesvia viacomputer computer simulations,little littleisis known known about potential users perceive VSEs VSEs effectively and how user– simulations, aboutwhether whether potential users perceive effectively and how system interaction contributes to system usability. Many Human–Computer Interaction (HCI) user–system interaction contributes to system usability. Many Human–Computer Interaction (HCI) studies have compared learning performance, and subjective ratings with applications different studies have compared learning effects,effects, performance, and subjective ratings with different applications and tasks [77], which provide grounds for an evaluation of the system. and tasks [77], which provide grounds for an evaluation of the system. The different approachesofofmeasurement measurement fall fall into into two two general measures The different approaches general categories: categories:subjective subjective measures and objective measures. Here, a user study with human participants was conducted and objective measures. Here, a user study with human participants was conductedusing using postquestionnaires, which are the most common technique in subjective measurement. Usability, postquestionnaires, which are the most common technique in subjective measurement. Usability, user acceptance, and presence are considered based on previous research [78,79] to understand and user acceptance, and presence are considered based on previous research [78,79] to understand and evaluate critical contributions. Each questionnaire contains 30 items regarding usability (10 items), evaluate critical contributions. Each questionnaire contains 30 items regarding usability (10 items), user acceptance (10 items), and presence (10 items) as listed in Appendix A. The experience in stereo user acceptance (10 items), and presence (10 items) as listed in Appendix A. The experience in stereo 3D (blinking), nonstereo 3D (naked-eye), and 2D conditions were tested together using the virtual 3D (blinking), nonstereo 3D (naked-eye), and 2D conditions were tested together using the virtual reality setup. In addition, the latter two application modes related to development were also studied reality In addition, latter two application related to development wereconsidering also studied fromsetup. the perspective of the software design [80]. Eachmodes questionnaire consists of 10 items correctness (2 items), robustness (2 items), flexibility (1 item), reusability (2 items), efficiency (1 item), reliability (1 item), and usability (1 item) as listed in Appendix B. All the answers for each question are marked on a five-point scale except for user acceptance (seven-point scale).


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from the perspective of software design [80]. Each questionnaire consists of 10 items considering correctness (2 items), robustness (2 items), flexibility (1 item), reusability (2 items), efficiency (1 item), reliability (1 item), and usability (1 item) as listed in Appendix B. All the answers for each question are marked on a five-point scale except for user acceptance (seven-point scale). 5.1. Hypotheses The performance of Sino-InSpace was measured by two questionnaires from different perspectives. However, all questions are in the same and positive direction. The higher score indicates better performance. Naturally, we expect that performance in an upper level and multidimensional condition contributes to a better experience. Accordingly, the hypotheses are: Usability, user acceptance, and presence in three conditions (i.e., stereo 3D, nonstereo 3D, and 2D) will be at a good level (>80%). In other words, usability and presence ratings will exceed 40, while the user acceptance rating will exceed 56. Usability, user acceptance, and presence ratings will decrease in the sequence of stereo 3D, nonstereo 3D, then 2D conditions. The software design rating of the platform will exceed 40. 5.2. Participants and Procedure In testing the first two hypotheses, the participants were composed of 27 undergraduate students, 12 graduate students, and six professors from a large university in China who had never used Sino-InSpace. In terms of gender, 10 were female and 35 were male. Their ages were between 19 and 54 years. The same procedures and instruments were used for stereo 3D (blinking), nonstereo 3D (naked-eye), and 2D conditions. The participant numbers were balanced among the three groups. A brief instruction for the mouse and keyboard control, time, and viewpoint control, and scenario and script file edit were first provided. There were 10 min for each participant to become familiar with the environment and operations. Then, three tasks relating to key technologies of the platform were carried out. Multiscale Environment Experience: Participants travelled between different planets in the solar system freely. They evaluated the efficiency of data loading, the effect of visualization, and the flexibility of the interactive process. Scenario Construction: A simple scenario, such as the one Figure 7 described, was assumed. Participants were instructed to complete the whole process of construction (new-built, elements adding, entity object model edit, save, etc.) and evaluated the usability of the platform. Deduction Script Design: Based on the above scenario, participants designed the deduction script (such as in Figure 9) as they expected to test the applicability of the visualization engine. A paper questionnaire (Appendix A) was administered immediately after each experiment. Participants were asked to rate their response or agreement with the statements with regard to how it describes their experience. Finally, the results were collected in the computer for statistical analysis. Considering the long learning cycle of platform development, 14 participants from our cooperative institutes and companies overall, who had developed related applications using Sino-InSpace, took part in the study for hypothesis 3. An email questionnaire (Appendix B) was sent to them and returned after being filled out based on the actual experience. Then, the ratings were gathered for statistical analysis. 5.3. Evaluation Result All participants successfully completed designated tasks. Overall, 59 cases were collected for analysis, which was performed with SPSS 17.0. First, the internal consistency of the measurement scales was assessed using Cronbach’s alpha [81]. All questions are in the positive direction and the scores of Cronbach’s alpha are as follows, usability = 0.91, user acceptance = 0.85, presence = 0.93, and software design = 0.91. The scores for the four variables all exceeded 0.80 (the generally accepted level), which indicated that measures of the associated constructs in the study were internally consistent.


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To determine the distribution of variables, normality tests were first conducted. Table 3 shows the ISPRS Int. J. Geo-Inf. x FOR PEER REVIEW 20 of 27 Shapiro–Wilk (SW)2018, test7,statistics and probabilities of the different measures in the three conditions. Software design was also tested (W = 0.973, p = 0.904). However, the data across different conditions conditions did fit well with a normal distribution (p > 0.05), which provided a premise and foundation did fit well with a normal distribution (p > 0.05), which provided a premise and foundation for the for the mean test analysis. mean test analysis. Table 3. Normality test results on stability, user acceptance, and presence. Table 3. Normality test results on stability, user acceptance, and presence. Stereo 3D Nonstereo 3D 2D Measure Stereo 3D Nonstereo 3D 2D W p W p W Measure W p W p W p Usability 0.983 0.984 0.958 0.657 0.957 Usability 0.983 0.984 0.958 0.657 0.957 0.648 User acceptance User 0.989 0.999 0.984 0.989 0.984 0.989 0.999 0.984 0.989 0.984 0.991 acceptance Presence Presence 0.969 0.969 0.837 0.969 0.836 0.984 0.837 0.969 0.836 0.984 0.973

p 0.648 0.991 0.973

Hypothesis 1 and hypothesis3 3predicted predictedthat thatratings ratings would would be condition. Hypothesis 1 and hypothesis be at ataagood goodlevel levelinineach each condition. Considering the number of cases ( 0.05) usability and presence in the stereo 3D and nonstereo 3D conditions, as shown in Table 4 and Figure of usability and presence in the stereo 3D and nonstereo 3D conditions, as shown in Table 4 and 20. However, ratings of user acceptance in three conditions and measures in the 2D condition were Figure 20. However, ratings of user acceptance in three conditions and measures in the 2D condition significantly lower (p < 0.05) than the expected values. The results indicated that more effort should were significantly lower (p < 0.05) than the expected values. The results indicated that more effort be devoted to creating user-friendly application interactions and interfaces. Meanwhile, there is no should be devoted to creating user-friendly application interactions and interfaces. Meanwhile, there high-level perception of VSEs in the 2D condition. For software design, participants gave a high is no high-level perception of VSEs 2D condition. Forthe software participants a high rating (M = 40.27, SD = 1.94, T = 53,inpthe = 0.70), which proves ability design, of Sino-InSpace as angave extensive rating (M = 40.27, SD = 1.94, T = 53, p = 0.70), which proves the ability of Sino-InSpace as an extensive future application. future application. Table 4. Usability, user acceptance, and presence mean comparisons using a lower-tail one-sample t-test. Table 4. Usability, user acceptance, and presence mean comparisons using a lower-tail one-sample Stereo 3D Nonstereo 3D 2D t-test. Measure M SD T p M SD T p M SD T p Stereo 3D Nonstereo 3D 2D Usability 1.51 0.92 40.27 3.54 0.29 0.61 32.20 3.75 −8.07 0.00 Measure 41.13 2.90 M SD T p M SD T p M SD T p User 45.67 5.08 −7.88 0.00 45.00 5.18 −8.22 0.00 44.47 4.66 −9.59 0.00 Usability 41.13 2.90 1.51 0.92 40.27 3.54 0.29 0.61 32.20 3.75 −8.07 0.00 acceptance User acceptance 45.67 5.08 −7.88 0.00 45.00 5.18 −8.22 0.00 44.47 4.66 −9.59 0.00 Presence 3.40 3.403.18 3.181.00 1.00 39.5339.533.16 Presence 42.80 42.80 3.16 −0.57 −0.57 0.290.29 33.60 33.60 3.09 3.09 −8.02 −8.02 0.00 0.00

Figure 20.20. Usability, useracceptance, acceptance, presence and standard Figure Usability, user andand presence meanmean valuesvalues and standard deviationsdeviations in differentin different conditions. conditions.


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To test the differences among the stereo 3D, nonstereo 3D, and 2D condition, as predicted in hypothesis 2, a one-way analysis of variance (ANOVA) was carried out (Table 5). The homogeneity of variance was tested first, and the sample data all satisfied the requirements of homogeneity (p > 0.05). Combined with the mean values (Figure 20), the differences between the conditions were significant regarding usability and presence following the direction predicted (p < 0.001). However, user acceptance was not differentiated by the three conditions. Table 5. Usability, user acceptance, and presence mean differences based on one-way analysis of variance. Homogeneity Test Measure Usability User acceptance Presence

ANOVA

L

p

F

p

0.306 0.041 0.042

0.738 0.960 0.959

31.265 0.219 31.451

0.000 0.804 0.000

In general, Sino-InSpace achieves a highly positive evaluation in usability and presence, which reflects the advantage in practicability and immersion of the simulation. Moreover, a VR setup with 3D view is more beneficial for potential users in comparison with conventional plane views. However, participants perceived a medium-level of user acceptance, which explains the willingness to adopt the platform. Therefore, further research should be focused on enriching the interactive means and attempting to add more sensory elements (e.g., auditory, gustation, smell, and tactile). Although we looked deeply at the performance by means of questionnaires, there are still limitations associated with this research that affect the ability to generalize the results. In the future, more and various users should be gathered for the evaluation; meanwhile, some objective measuring tasks and methods reflecting the characteristics of the platform will be designed and conducted. 6. Discussion With the development and progress of space technology, outer space is becoming a new living space for human beings. Thus, the expansion of VGEs to VSEs has become imperative. In this study, we independently designed a digital simulation platform for VSEs, known as Sino-InSpace. Based on the analysis of the platform orientation, the details of key technologies were introduced. To meet the needs of multilevel users, multilevel application modes were accordingly designed, each of which had related case studies. These studies demonstrated not only the visual effect but also the quantitative performance in a practical application. The efficiency of the platform can also be verified with the supplementary materials (Video S1). Furthermore, the subjective evaluations with human participants confirmed the value of the platform and the benefits of the VR setup from multiple perspectives. Sino-InSpace is a product of multidisciplinary integration and has distinctive technical features in comparison with existing platforms. These are as follows: (i) the platform transcends the concept of traditional digital Earth and enriches the paradigm of VGEs. With more frequent interplanetary activities in the future, Sino-InSpace has an advantage over simulation platforms restricted to a single planet in that it supports the integration, expression, and analysis of spatial activity data within the solar system. (ii) It supports multidisciplinary dynamic collaborative deduction. Practitioners from different disciplines (e.g., astronomy, space environment science, geography, etc.) can participate in the platform to conduct scientific simulation experiments throughout secondary development. In contrast, most similar platforms serve a single disciplinary field. (iii) Sino-InSpace considers multilevel users early in its design process and supports applications of direct application, visualization development, and scientific analysis. These modes can support diversified needs, which improves popularization greatly. The platform is still in the preliminary stage (version 1.0), and the following aspects will be considered in future work. (i) Considering the platform involves multidisciplinary interactions and


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various users, the simulation calculation modules need to be accessed using different operating systems, programming languages, and data interfaces. To address these issues, we will use Service-Oriented Architecture (SOA), register professional modules of different fields with a backstage service center, provide data support simultaneously for multiple stages through an accessible data interface, and improve the simulation efficiency and interoperability. (ii) Based on the concept of the Pan-spatial Information System [82], our platform will be modified to gradually increase the support for indoor (e.g., space station and lunar base) and underground space (e.g., planetary geological survey) in the future. (iii) Based on the concept of geographic scenes, the platform will improve the description for the semantics and relationships of scene elements and constantly enrich the content for scene modeling; in addition, with the aid of VR and AR equipment (e.g., HTC Vive, Oculus Rift, and HoloLens), we will try to use visual and auditory perception methods to enhance the user’s ability to understand space environments. Supplementary Materials: The following are BC9EIrSkT7gh8mUMtVUfA, Video S1: Sino-InSpace.

available

online

at

https://pan.baidu.com/s/1-

Author Contributions: L.L. and Q.X. conceived and designed the research; L.L., C.L., Q.S., and W.L. developed the Sino-InSpace together. Y.Z. and Y.Z. participated in data collection, application debugging and platform evaluation. L.L. and Y.Z. wrote the paper. Funding: This research was funded by the National Program on Key Basic Research Project of China (Grant No. 2012CB720000-G), the National Natural Science Foundation of China (Grant Nos. 41701463 and 41371436), and the National Key R&D Program of China (Grant Nos. 2016YFB0801301 and 2017YFC1200300). Acknowledgments: We would like to thank all the individuals who contributed in the development of the Sino-InSpace platform including the students who assisted in improving the platform by testing, reporting bugs, and adding functions. We have to extend our heartfelt thanks to all of participants who took part in evaluating the platform. We are also grateful to our users for their patience and suggestions and the reviewers for their helpful comments on this manuscript. We thank Yanxia Cai from NSSC for instruction in the AE8/AP8 model. Conflicts of Interest: The authors declare no conflict of interest.

Appendix A Usability Please indicate your response by selecting just ONE of the numbers using the 5-point scale below. Almost Never 1

Some of the Time 2

About half of the Time 3

Most of the Time 4

Almost Always 5

Is the platform user friendly? Does this platform provide sufficient content for understanding the solar system? Does this platform provide precise description for elements in the solar system? Are you satisfied with the efficiency of data loading? Are you satisfied with the effect of visualization? Does this platform provide huge and high-resolution geographical information of planetary surface? Does this platform improve your cognition of the abstract space environment? Is it convenient for you to construct a needed scenario? Does the script meet your needs for deduction? Assuming you are a space science researcher, would you intend to use this platform for simulation and public education? User Acceptance Please indicate your response by selecting just ONE of the numbers using the 7-point scale below.


Extremely Unlikely 1

Quite Unlikely 2

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Slightly Unlikely 3

Neither 4

Slightly Likely 5

Quite Likely 6

Extremely Likely 7

My interaction with this platform is clear and understandable. Learning to operate this platform was easy for me. I find it easy and fluent to go anywhere I intend to go. I find it easy to control and switch the observation viewpoint. I find it flexible to control the simulation time. The platform enables me to accomplish the addition of environment data quickly. It is easy for me to remember how to construct a normal scenario. It is easy for me to learn how to edit a script of deduction. I find it easy to recover from errors encountered while using the platform. The platform provides helpful guidance in performing tasks. Presence Please indicate how much you agree or disagree with each of the following statements by circling just ONE of the numbers using the 5-point scale below. Strongly Disagree 1

Disagree 2

Neutral 3

Agree 4

Strongly Agree 5

To maintain the validity of the copyright owned by the UK Independent Television Commission, 10 items from ITC-SOPI [83] cannot be published in this journal. Appendix B Software Design Please answer the following questions by circling the relevant number. Does this platform provide sufficient functions and interfaces for your projects? Not at all

1

2

3

5

Extremely sufficient

5

Always right

5

Very well

4

5

Extremely stable

4

5

Not at all

4

5

Very strong

5

Extremely clear and readable

4

Does this platform provide the correct functions and interfaces? Always wrong

1

2

3

4

How well do you think the platform performed in fault tolerance? Very poor

1

2

3

4

Are the components of platform stable enough for further development? Not at all

1

2

3

Does adding or modifying a function influence another? Almost always

1

2

3

Is the coupling between different components strong? Very low

1

2

3

Is the description of functions and interfaces clear and legible? Not at all

1

2

3

4

To what extent can the execution efficiency of the platform meet the needs of simulation? Not at all

1

2

3

4

How often, on average, does the platform crash or make an error?

5

A lot


Almost always

1

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2

3

4

5

Almost never

5

Extremely easy

To what extent do you feel that the platform is easy to develop? Not at all

1

2

3

4

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