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 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.) 2 Science and Technology on Information System Engineering Laboratory, Nanjing Research Institute of Electronics Engineering, Nanjing 210007, China 3 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 Fengy¯ ún meteorological satellites had been sent into operation. In addition, China’s progress in
ISPRS Int. J. Geo-Inf. 2018, 7, 373; doi:10.3390/ijgi7090373 www.mdpi.com/journal/ijgi ISPRS Int. J. Geo-Inf. 2018, 7, 373 2 of 27 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 Section2. Section3 illustrates the key technologies including time-space datum, data organization ISPRS Int. J. Geo-Inf. 2018, 7, 373 3 of 27
ISPRSand preprocessing, Int. J. Geo-Inf. 2018 visualization, 7, x FOR PEER scenarioREVIEW organization, and design of the visualization engine. 3 Three of 27 types of application modes were designed, and related case studies are introduced in Section4. Then, Sectiona user study 4. Then, with a participants user study waswith conducted participants to evaluatewas conducted the performance to evaluate of thethe platform performance in Section of the5. platformFinally, we in discussSection the5. Finally, features we and discuss future the work features of the and platform future in work Section of the6. platform in Section 6.
2. Platform Orientation and Data Source Analysis Compared to the majority of the digital earthearth and VGE platforms, Sino-InSpace focuses on visualization andand analysis analysis on aon larger a larger scale spacescale including space including the construction the construction of the virtual of interplanetary the virtual interplanetaryspace environment, space digital environment, planets, spacedigital environment/weather planets, space environment/weather monitoring, space monitoring, mission launch, space missionspace objects launch, collision space objects warning, collision landing warning, area selection, landing and area so selection, on (Figure and1). so on (Figure 1).
Figure 1. Platform orientation.
Complete VGEsVGEs can can be dividedbe divided into fourinto subtypes: four subtypes: the data environment,the data environment, modeling environment, modeling environment,expression environment, expression andenvironment, collaborative and environment collaborative [ 8,environment35,40,41]. The [8,35,40,41]. data environment The data is environmentfundamental foris fundamental the other three for subtypes the other [42 three]. Therefore, subtypes the [42]. data Therefore, source of the VSEs data is analyzedsource of firstVSEs and is analyzeddivided into first two and types: divided the environmentinto two types: element the en andvironment the entity element object and element the (Tableentity 1object). element (Table 1). Table 1. Data source of virtual space environments.
Data TypeTable 1. Data source of virtual space environments. Content
DataBasic Type geographic Remote sensing images on theContent surface of the planet, digital 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 element digital elevation model (DEM), and vector and feature Physicalenvironment space of physical space environment such as middle and upper data. environment/weather atmosphere, geomagnetic field, ionosphere, radiant zone, and so Environment on.The The position, track and distribution, size information temperature of space and debris. intensity 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, environment/weather proper motion, level, and color. 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 Entity object system Astronomical catalogues of stars containing accurate Background star velocity of planets. element position, proper motion, level, and color. Orbit elements, trajectories, attitudes, 3D models, and attributes of Spacecraft The size,artificial shape, texture, satellites rota andtion spacecraft. speed, etc. of planets. Planet in the solar system Astronomical ephemerides used to calculate the position The location, model, capability, and attribute data of all kinds of Entity object Ground facility ground facilities such asand optical velocity telescopes, of planets. radar, launching and element Orbit elements,measurement, trajectories, and control attitudes, stations. 3D models, and Spacecraft attributes of artificial satellites and spacecraft. The environment elements are the basis forThe building location, the model, virtual capability space., and In addition attribute todata the of image, all digital elevation model (DEM)Ground and facility other basic geographickinds of ground data, facilities space activities such as optical also need telescopes, 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
ISPRS Int. J. Geo-Inf. 2018, 7, 373 4 of 27 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. ISPRS Int. J. Geo-Inf. 2018, 7, 373 5 of 27 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 5 of 27
From observations of the sun to stars, and then to the introduction of the International Atomic Time (TAI) in 1967, the development of the time systemsystem has been guided by the pursuit of higher accuracy. The The current current time time system system primarily primarily consis consiststs of of Sidereal Sidereal Time Time (ST), (ST), Universal Universal Time Time (UT), (UT), Ephemeris Time (ET), Dyna Dynamicalmical Time (DT), and Atomic Time. In addition, other time references have been defined, defined, such as Coordinated Universal Time (UTC), which has been adopted within the Sino-InSpace, Global Positioning Sy Systemstem Time (GPST), and BeiDou Navigation Satellite Satellite System System Time Time (BDT) forfor convenience.convenience. The The conversion conversion between between time time systems systems when when the data the are data imported are imported or exported or exportedis accomplished is accomplished with the helpwith ofthe the help Standards of the St ofandards Fundamental of Fundamental Astronomy Astronomy (SOFA) Library (SOFA) for Library ANSI forC[ 53ANSI]. C [53]. Considering the the space space datum, datum, the the primary primary scope scope of human of human activity activity is currently is currently in the inearth the circle; earth therefore,circle; therefore, we considered we considered a coordinate a coordinate reference reference system system centered centered on the on earth. the earth. Nevertheless, Nevertheless, the activitiesthe activities at the at thesolar solar system system scale scale needed needed to tobe besu supportedpported by by a coordinate a coordinate system system with with a alarger larger scale. scale. These large-scale coordinate systems [54–56] [54–56] primarily included the International Celestial Reference System (ICRS),(ICRS), Planetary Planetary Centroid Centroid Reference Reference System Syst (PCRS),em (PCRS), International International Terrestrial Terrestrial Reference Reference System System(ITRS), Station(ITRS), CoordinateStation Coordinate System, System, and Satellite and Satell Coordinateite Coordinate System, System, as shown as in shown Figure in2 .Figure 2. In addition, addition, the transitional transitional coordinate system wa wass used to meet the needs of coordinate system conversion. For For example, example, the the orbit orbit prediction prediction of an of Earth-center an Earth-center satellite satellite is mostly is mostly carried carried out in outthe J2000in the coordinate J2000 coordinate system system(there is (there a small is adeviation small deviation matrix between matrix betweenit and the it Geocentric and the Geocentric Celestial ReferenceCelestial Reference System (GCRS)). System (GCRS)). However, However, substellar substellar point calculation point calculation and coverage and coverage analysis analysis are often are carriedoften carried out in out ITRS. in ITRS. Therefore, Therefore, the thetransformati transformationon between between these these systems systems requires requires the the celestial intermediateintermediate reference system and terrestrial inte intermediatermediate reference system. The conversion conversion between between reference systems in Sino-InSpace is also supported by the SOFASOFA LibraryLibrary [[53].53].
Heliocentric celestial reference system Planet centroid celestial ICRS reference system Deviation Geocentric celestial matrix Celestial intermediate reference system Precession– reference system nutation PCRS Earth rotation Geocentric coordinate Polar motion systems Terrestrial intermediate Space ITRS reference system Reference-ellipsoid-centric reference coordinate system system Station equator coordinate Station coordinate system system Station orthogonal coordinate system
Satellite orbit coordinate Legend system Commonly used Satellite coordinate Satellite inertial coordinate coordinate system system system Transitional coordinate Satellite centroid system coordinate system
Figure 2. SpaceSpace datum datum of the Sino-InSpace platform.
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,
ISPRS Int. J. Geo-Inf. 2018, 7, 373 6 of 27
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,ISPRS Int. vectors, J. Geo-Inf. and2018, toponymic7, x FOR PEER data, REVIEW among others. To improve the efficiency of data loading 6 ofand 27 rendering, the strategy of the multiresolution pyramid model is generally adopted by most digital planetthe strategy systems of[ 57the,58 multiresolution] depending on apyramid structure model known is as generally a discrete adopted global grid by systemmost digital (DGGS) planet [59]. Thesystems traditional [57,58] approach depending discretizing on a structure a solid known planet as is toa discrete use the latitude/longitudeglobal grid system coordinate(DGGS) [59]. system, The whichtraditional was alsoapproach adopted discretizing in the early a solid stages planet of the is platform to use the development latitude/longitude (see Figure coordinate3). system, which was also adopted in the early stages of the platform development (see Figure 3). 0 9 Col 90 4 Row Lat
L0 0
-90 -180Lon 180
Ln: (Lon, Lat)
L1 (Lat+× 90) 2n Row = 36 L2 +×n = (Lon 180) 2 Col 36
Index: \ Data Folder\ Data Set Name\ Ln\ Row\ Col.abc
Figure 3. Geographic data segmentation and index method.method.
For example, considering considering the the Mars Mars images images and and DEMs DEMs,, the the size size of ofthe the pyramid pyramid tile tilewas was set to set 512 to ◦ ◦ 512× 512× pixels,512 pixels, the first the firstlayer layer was wasdivided divided into into5 × 10, 5 × and10, andthe row the rowincreased increased from from 90° 90S toS 90° to 90N andN and the thecolumn column increased increased from from 180° 180 W ◦toW 180° to 180 E. ◦Then,E. Then, each each tile was tile wassegmented segmented layer-by-layer, layer-by-layer, and andthe row the rowand column and column number number (Row, (Row, Col) Col)at the at coordinate the coordinate (Lon,(Lon, Lat) of Lat) the of Ln the level Ln could level couldbe calculated be calculated using usingthe formula the formula specified specified in Figure in Figure 3. The3. The tile tile used used three-level three-level storage storage with with the the naming convention,convention, ‘Ln‘Ln\Row\Col’,\Row\Col’, which which was was convenient convenient for for querying querying and and loading. loading. For For feature feature data, data, the pointthe point region region (PR) quadtree(PR) quadtree index index [60] was [60] created was created based on based the location on the coordinateslocation coordinates and recorded and usingrecorded the eXtensibleusing the MarkupeXtensible Language Markup (XML)Language file. (XML) file.
3.2.2. Discrete Space Environment DataData IndexIndex The spacespace environmentenvironment contains contains rich rich elements elements and an coversd covers a vast a vast area; area; for example, for example, taking taking the earth the spaceearth environmentspace environment as an example, as an example, elements elements such as the such middle as the and middle upper atmosphere,and upper atmosphere, the geomagnetic the field,geomagnetic and the field, ionosphere and the are ionosphere in the range are in of the tens rang of kilometerse of tens of to kilometers tens of thousands to tens of ofthousands kilometers. of Forkilometers. space environment For space environment elements with elements a wide distribution with a wide and distribution large density and underlarge density a static state,under whether a static itstate, is the whether clipping it inis thethe visualclipping process in the or visual the nearest process neighbor or the nearest in the query neighbor operation, in the query the conventional operation, traversalthe conventional method traversal does not method meet the does requirements. not meet the Therequirements. spheroid degenerated-octreeThe spheroid degenerated-octree grid (SDOG) usesgrid a(SDOG) volumetric uses discretizationa volumetric discretization of the sphere thatof th cane sphere provide that a can reference provide [61 a,62 reference]. The octree [61,62]. spatial The subdivisionoctree spatial method subdivision was method adopted was to constructadopted to the construct index forthe theindex original for the version original of version Sino-InSpace of Sino- (seeInSpace Figure (see4). Figure 4).
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Root node Level 0
A B 1 E D B C C 2 A 3 D E FigureFigure 4. 4. SpaceSpace environment environment octree octree subdivision subdivision method. method.
AnAn octree octree is is a a tree tree data data structure structure involving involving spat spatialial level level recursive recursive subdivision. subdivision. The The subdivision subdivision ofof 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 direction.direction. The The cube cube represented represented by by the the root root node node included included all all the the data data to to be be processed. processed. The The root root node node waswas subdivided subdivided into into eight eight sub sub nodes nodes recursively, recursively, and and the the nonempty nonempty node node continued continued to to be be divided. divided. TheThe maximum maximum recursive depthdepth ofof an an octree octree depended depended on on the the resolution resolution ofvarious of various environmental environmental data, data,display display requirements, requirements and, queryand query accuracy. accuracy.
3.3.3.3. Entity Entity Object Object Model Model Design Design Satellites,Satellites, rockets, rockets, and and probes probes are are used used to to carry carry out out space space missions missions such such as as launch, launch, observation, observation, andand detection, detection, and and become become the the actual actual participants participants in in space space activities. activities. Furthermore, Furthermore, these these man-made man-made spacespace targets targets will will become become the the main main body body of of VSEs VSEs that that needs needs to to be be supported supported and and expressed expressed because because ofof the the inability inability of of a a large large number number of of human human beings beings to to enter enter into into space space at at present. present. Figure Figure 5 shows the fourfour kinds kinds of of information information required required for for describing describing a a working working BeiDou BeiDou navigation navigation satellite: satellite: (i) (i) Attribute; Attribute; thisthis contains contains the the name, name, id, id, category, category, and and country country of of th thee object. object. (ii) (ii) Structure; Structure; this this mainly mainly describes describes the the geometricgeometric form form and the change.change. InIn additionaddition toto the the model model construction construction of of the the target, target, it alsoit also includes includes the thechange change of overallof overall form form following following the the action action of theof the components, components, such such as theas the directional directional rotation rotation of theof thesolar solar panels panels during during the the operation. operation. (iii) (iii) Position; Position; determining determining the the location location of the of the target target in the in the space-time space- timecoordinate coordinate system. system. Themotion The motion of the of space the targetspace oftentarget follows often follows a certain a dynamiccertain dynamic model. (iv)model. Attitude; (iv) Attitude;according according to the missions’ to the missions’ need, the need, attitude the of attitude the satellite of the should satellite be adjustedshould be frequently adjusted frequently such as the suchdirection as the of thedirection satellite of antenna the satellite to the antenna earth needs to the to ensure earth theneeds visibility to ensure of the the communication visibility of link.the communicationTherefore, a unified link. objectTherefore, model a unified was designed, object mode whichl was consisted designed, of the which attribute, consisted ephemeris, of the attribute, geometric ephemeris,model, and geometric behavior model,model, whichand behavior is shown model, in Figure which6. is shown in Figure 6. BasedBased on on the the multiple multiple ephemeris ephemeris segment, segment, the the ephemeris ephemeris elements elements are are used used to to determine determine the the locationlocation of of the the target target object object at at any any time. time. Each Each ephe ephemerismeris section section contains contains a adescription description of of the the timeline, timeline, coordinatecoordinate system, system, and and orbital orbital information. information. In addi In addition,tion, three three forms forms of orbits of orbits are supported are supported in Sino- in InSpace:Sino-InSpace: Kepler, Kepler, two-line two-line element element (TLE), and (TLE), discrete and discretetrack points. track The points. first two The orbits first twohave orbits their own have predictiontheir own predictionmodel which model is used which to is calculate used to calculate the position. the position. For the For description the description of the ofshape, the shape, the geometricthe geometric model model is composed is composed of components of components that thatare constituted are constituted by some by some primitives primitives based based on the on treethe structure; tree structure; the primitives the primitives are th aree smallest the smallest parts parts of the of model the model consisting consisting of dots, of dots,lines, lines,planes, planes, and textures.and textures. In addition, In addition, the primitive the primitive can also can be also a mo bedel a model file constructed file constructed by commercial by commercial software software (e.g., 3ds(e.g., Max). 3ds Max).The behavior The behavior model model is used is used to descri to describebe the the movements movements of ofthe the overall overall and and internal internal components of the target object at different stages. Each phase contains a description of the timeline,
ISPRS Int. J. Geo-Inf. 2018, 7, 373 8 of 27 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 8 of 27 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 8 of 27 componentsaction object, of and the targetbehavior object type, at different which stages.includes Each three phase types contains of operations a description such ofas the translation, timeline, action object, and behavior type, which includes three types of operations such as translation, actionrotation, object, andand scaling. behavior type, which includes three types of operations such as translation, rotation, rotation, and scaling. and scaling.
FigureFigure 5.5. DescriptionDescription forfor aa BeiDouBeiDou navigationnavigation satellite.satellite. AttributeAttribute informationinformation doesdoes notnot generallygenerally Figure 5. Description for a BeiDou navigation satellite. Attribute information does not generally change.change. StructureStructure informationinformation isis partiallypartially relatedrelated tototime timebecause becausesometimes sometimesthe thecomponents componentsneed needto to change. Structure information is partially related to time because sometimes the components need to adjust.adjust. PositionPosition andand attitudeattitude informationinformation isis alwaysalways related related to to time. time. adjust. Position and attitude information is always related to time.
Object Object
Attribute Ephemeris Geometric model Behavior model Attribute Ephemeris Geometric model Behavior model
Ephemeris1 Ephemeris2 ... EphemerisN Component1 Component2 Stage1 Stage2 ... StageN Ephemeris1 Ephemeris2 ... EphemerisN Component1 Component2 Stage1 Stage2 ... StageN ComponentN ComponentN
Primitive1 Primitive2 Behavior Coordinate Primitive1 Primitive2 Timeline Action object Timeline Coordinate Orbit Behaviortype Timeline system Orbit Timeline Action object system PrimitiveN type PrimitiveN
Kepler TLE Track Dot Line Plane Texture Translate Rotate Scale Dot Line Plane Texture Kepler TLE Track Translate Rotate Scale
FigureFigure 6.6. EntityEntity objectobject modelmodel design.design. TheThe actionaction objectobject derivesderives fromfrom thethe componentcomponent oror geometricgeometric Figure 6. Entity object model design. The action object derives from the component or geometric modelmodel itself.itself. model itself. 3.4.3.4. VisualizationVisualization ScenarioScenario OrganizationOrganization 3.4. Visualization Scenario Organization TheThe visualizationvisualization scenarioscenario refers refers toto thethe organizationorganization ofof thethe geographicalgeographical environment,environment, entityentity andand The visualization scenario refers to the organization of the geographical environment, entity and spacespace environment environment elements, elements, and and space space activities, activities which, belongwhich tobelong the geographical to the geographical scene. Traditional scene. space environment elements, and space activities, which belong to the geographical scene. cartographyTraditional cartography and GIS are commonlyand GIS are used commonly to express used the to static express information the static andinformation temporal and relationship. temporal Traditional cartography and GIS are commonly used to express the static information and temporal Therelationship. visualization The scenariovisualization focuses scenario on multilevel focuses and on multidimensionalmultilevel and multidimensional information regarding information time, relationship. The visualization scenario focuses on multilevel and multidimensional information location,regarding participants, time, location, events, participants, processes, events, and phenomena,processes, and among phenomena, others [among38,63]. others The visualization [38,63]. The regarding time, location, participants, events, processes, and phenomena, among others [38,63]. The scenariovisualization organization scenario inorganization this study wasin this based study on was two based levels: on logical two levels: modeling logical and modeling data modeling, and data visualization scenario organization in this study was based on two levels: logical modeling and data asmodeling, shown in as Figure shown7. in Figure 7. 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 organization and modeling based on UnifiedUnified ModelingModeling LanguageLanguage (UML) andand eXtensible MarkupMarkup LanguageLanguage (XML).(XML). ((aa)) UMLUML diagramdiagram ofof thethe visualizationvisualization scenario.scenario. ((b)) XMLXML datadata ofof thethe visualizationvisualization scenario. Timeline is used as an example; its logical model includes the start time, end time, and time step. The datatype of the start timetime and the end time is defined defined as dateTime, while thatthat ofof thethe timetime stepstep isis Double.Double. XML content was filledfilled according toto thethe fieldfield datadata type,type, whichwhich cancan bebe storedstored inin thethe locallocal filefile toto bebe accessedaccessed later.later. Furthermore,Furthermore, Timeline waswas alsoalso thethe partpart ofof thethe EphemerisEphemeris and ActionAction classesclasses withwith thethe correspondingcorresponding relationshiprelationship ofof 11 toto 1.1.
The staticstatic structure structure and and relationship relationship of the of visualizationthe visualization scenario scenario data were data described were described by defining by thedefining specific the termsspecific using terms the using Unified the Unified Modeling Modeling Language Language (UML) (UML) diagram. diagram. As in theAs in previous the previous case, SpaceScenariocase, SpaceScenario contains contains three main three nodes: main GeoEnvironment,nodes: GeoEnvironment, SpaceEnvironment, SpaceEnvironment, and Object. and The Object. type, structure,The type, structure, data constraints, data constraints, and the correspondenceand the correspondence and association and association between between different different levels oflevels the hierarchyof the hierarchy were described.were described. In GeoEnvironment, In GeoEnvironment the path, the path gives gives thefile thesystem file system information information for thefor configthe config file offile the of geographicalthe geographical data, data, which which contains contai thens detailsthe details of pyramid of pyramid tiles (e.g.,tiles (e.g., region, region, tile size, 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,
ISPRS Int. J. Geo-Inf. 2018, 7, 373 10 of 27 tile level, and storage location). In SpaceEnvironment, the path and model indicate the source of theISPRS space Int. J. environmentGeo-Inf. 2018, 7, x data. FOR PEER The REVIEW former refers to the directory of the environment data file and 10 of the 27 latter is the model used for calculation. The Object node has three child nodes: Model, EphemerisList, andand ActionList.ActionList. TheseThese nodes nodes depict depict the the geometric geometric model, model, ephemeris, ephemeris, and and behavior behavior model model in Section in Section 3.3. In0. Model,In Model, filepath filepath means means the path the of path the object of the geometric object geometric 3D model file.3D InitialTranslate,model file. InitialTranslate, InitialAngle, andInitialAngle, InitialScale and describe InitialScale the initial describe state ofthe the initial object geometricstate of the 3D model.object geometric In EphemerisList, 3D model. several In EphemerisEphemerisList, nodes several may be Ephemeris contained no anddes each may one be contained represents and a track. each Here,one represents the orbit type a track. of “BEIDOU Here, the 1D”orbit satellite type of is “BEIDOU TLE. ActionList 1D” satellite also comprises is TLE. ActionList more than also one comprises Action node, more which than records one Action the change node, ofwhich translation, records rotation,the change and of size. translation, rotation, and size.
3.5.3.5. Visualization EngineEngine DesignDesign TheThe use of abstract abstract data data makes makes it itdifficult difficult to tomeet meet the the final final needs needs of the of theusers. users. However, However, the thevisualization visualization technology technology can can describe describe the thedata data grap graphicallyhically to provide to provide users users with with a visual a visual outlook outlook of ofthese these data, data, which which can can help help them them understand understand and and recognize recognize the thecurrent current space space activities. activities. Faced Faced with withcomplex, complex, massive massive spatial spatial data, data, efficient efficient data data scheduling scheduling and and flexible flexible operational operational control control are keykey problemsproblems inin thethe visualizationvisualization process.process. Therefore,Therefore, Sino-InSpaceSino-InSpace dividesdivides thethe visualizationvisualization engineengine intointo twotwo parts:parts: thethe datadata engineengine andand renderingrendering engine,engine, asas shownshown inin FigureFigure8 8..
Mouse Index calculation Visible area and updating calculation Viewpoint control User Keyboard Progressive data Data interaction scheduling Data engine request Rendering Time control Gesture engine Required data Y is included Buffer data reading Data loading Real-time N Behavior Multithread data Script driver Buffer updating control reading Non-real-time
FigureFigure 8.8. Visualization engineengine design.design.
TheThe datadata engineengine isis responsibleresponsible forfor thethe rapidrapid indexingindexing andand schedulingscheduling ofof data.data. DependingDepending onon thethe visualvisual conecone parametersparameters delivereddelivered byby thethe renderingrendering engine,engine, thethe visible area is firstfirst calculated and then, thethe tiles,tiles, environmentenvironment data, data, or or graphic graphic names names that that need need to to be be updated updated are are determined. determined. To improveTo improve the renderingthe rendering of visualization, of visualization, the data the aredata progressively are progressively scheduled scheduled from near from to farnear based to far on based the distance on the todistance the viewpoints; to the viewpoints; in addition, in aaddition, certain part a certain of the datapart bufferof the memorydata buffer is reserved memory for is visualization.reserved for Forvisualization. the visualization For the process,visualization first, weprocess, determined first, we whether determined the data whether buffer the contained data buffer the contained required data;the required if the data data; were if the not data available were not in the available buffer, in they the were buffer, imported they were by multithreading,imported by multithreading, and the data bufferand the was data then buffer updated. was Afterthen updated. receiving After the data receiving transmitted the data by the transmitted data engine, by thethe renderingdata engine, engine the graphicallyrendering engine renders graphically the environment renders and the entityenvironmen elementst and in theentity visual elements scene. in the visual scene. TheThe rendering engine engine also also supports supports user user interactio interactionsns such such as mouse-, as mouse-, keyboard-, keyboard-, and gesture-based and gesture- basedinteractions interactions as well as as well real-time as real-time or non-real-time or non-real-time script scriptdrivers drivers to flexibly to flexibly control control the viewpoint, the viewpoint, time, time,and entity and entity target target behavior. behavior. A script A script file filecan can be beopened opened and and edited edited directly directly in in text, text, and and each lineline representsrepresents aa commandcommand andand hashas aa fixedfixed formatformat asas follows:follows:
CommandCommand identification identification {[Parameter{[Parameter identification identification Parameter]}Parameter]}
TableTable2 2 shows shows some some of of the the commands commands and and parameters. parameters. These These commands commands support support the the switch switch of of viewpoints,viewpoints, controlcontrol ofof simulationsimulation speed,speed, texttext display,display, etc. etc. AA simple simple example example is is given given in in Figure Figure9 .9.
ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 11 of 27 ISPRS Int. J. Geo-Inf. 2018, 7, 373 11 of 27 Table 2. Some commands and parameters.
Command Parameter Command MeaningTable 2. Some commands and parameters. Parameter Meaning Identification Identification Command Parametercenter Central target name. CommandSetting the Meaning position Parameter Meaning Identification Identification Position of viewpoint in the center body and perspective of x, y, z Setting the position center Centralcoordinate target name.system setviewpointsetviewpoint andthe perspective observation of PositionAttitude of viewpoint of sight in the center bodybody the(data observation can be obtained (data x, y, z qw, qx, qy, qz coordinatecoordinate system systemrepresented by the canfrom be obtained the platform). from the platform). Attitude of sight in thequaternion. center body coordinate qw, qx, qy, qz jd system representedJulian by day. the quaternion. Setting the time and jdTime step. Julian The day.positive number settime step of the Setting the time and step Time step.represents The positive the forward number simulation, represents the the settime simulation. forward simulation, the negative number of the simulation. ts ts negative number represents the backward representssimulation, the backward and 0 represents simulation, the andpause. 0 represents the pause. The unit is second. The unit is second. TheThe scriptscript continuescontinues to to execute after TheThe length length of the of script the script to run to pause. run pause. The unit The waitwait execute after waiting for durationduration waiting for a certain unitis second. is second. a certainlength length of time. of time. Printing thethe messagemessage on printprint texttext Text Text hint hint on on the the screen. screen. on the screen.screen. idid PicturePicture identification identification state 0—hide, 1—show. state 0—hide, 1—show. image Illustration file File path. image Illustration file File path. The pixel position of the image on the x, yx, y The pixel position of the image on the screen. screen. width,width, height height The The width width and and height height of theof the image. image.
Figure 9. A simple example of the script file.file.
4. Design of Platform Application Modes and Case Study By supportingsupporting geo-visualization,geo-visualization, geo-simulation, geo-collaboration, and human participation, VGEsVGEs provideprovide openopen virtual environments that correspondcorrespond to the real world to assist computer-aided geographic experimentsexperiments [ 8[8].]. As As a a digital digital simulation simulation platform platform for for VSEs, VSEs, Sino-InSpace Sino-InSpace aims aims to satisfy to satisfy the multilevelthe multilevel requirements requirements of the of application. the application. As shown As shown in Figure in 10Figure, three 10, modes three are modes supplied are includingsupplied applicationincluding application directly (without directly more (without development), more visualizationdevelopment), development, visualization and development, scientific analysis. and scientific analysis.
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Figure 10. Design of platform application modes. 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 Sino- our 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 includingThe application the Scenario main Editor, window 2D andis shown 3D view, in Figure Time Control,11 and offers and Viewpointa friendly operation Control. interface Scenario includingEditor provides the Scenario the structure Editor, 2D of and scenarios 3D view, constructed Time Control, with and satellites, Viewpoint spacecraft, Control. Scenario environment Editor providesconfigurations, the andstructure so on. of Entity scenarios elements constructed including properties,with satellites, ephemerides, spacecraft, geometric environment models, configurations,and actions (mentioned and so on. in Entity Section elements 3.3) can includ be editeding properties, further. Multidimensional ephemerides, geometric visualization models, enables and actionsthe presentation (mentioned of scenarios in Section and 0) phenomenacan be edited to befurther. more dynamicMultidimensional [64]. The platformvisualization provides enables 2D andthe presentation3D view windows. of scenarios The mouse and phenomena and keyboard to canbe mo bere used dynamic for smooth [64]. The panning platform and zoomingprovides through2D and 3Ddata view with windows. six degrees The of mouse freedom. and The keyboard Viewpoint can be Tool used helps for to smooth add, edit, panning delete, and and zooming switch betweenthrough datadifferent with viewpoints; six degrees thisof freedom. is convenient The Viewpoint for flexible T observations.ool helps to add, Furthermore, edit, delete, the and Time switch Tool between contains differentthe function viewpoints; of acceleration, this is convenient deceleration, for going flexible ahead, observations. going back, Furthermore, and time setting the Time to satisfy Tool browsing contains the eventfunction at different of acceleration, speeds and deceleration, times. going ahead, going back, and time setting to satisfy browsing the event at different speeds and times.
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Figure 11. TheThe operation window of Sino-InSpace in the application directly mode. ( (aa)) The main window includes includes the the Scenario Scenario Editor, Editor, 2D 2D and and 3D 3D view, view, Time Time Control, Control, and and Viewpoint Viewpoint Control; Control; (b) Trajectory(b) Trajectory setting setting window; window; (c) Geometric (c) Geometric model model setting setting window; window; and and (d) (Actiond) Action design design window. 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 loadingloading and and the the real-time real-time calculation calculation (e.g., (e.g., orbit orbit prediction prediction and and empirical empirical space space environment environment models). models). The time requiredrequired toto load load the the necessary necessary data data in in the th illustrationse illustrations is no is moreno more than than 3 s, and3 s, theand frame the frame rates ratesare all are greater all greater than 30than ftps, 30 whichftps, which is sufficiently is sufficiently satisfactory satisfactory for observation. for observation. Advance deductionsdeductions areare often often carried carried out out to enhanceto enhance the understandingthe understanding of the of mission. the mission. As shown As shownin Figure in 13Figure, a launching 13, a launching mission deductionmission deduction of the Shenzhou of the Shenzhou spacecraft spacecraft was designed. was designed. The trajectory The trajectoryand action and data action of the data CZ of rocket the CZ and rocket manned and manned spacecraft spacecraft were added wereto added the related to the related object model.object model.Then, the Then, script the filescript was file edited was edited and loaded and loaded to drive to drive the mission the mission execution, execution, and theand viewpoint the viewpoint was wasswitched switched to the to interestedthe interested perspective. perspective. Meanwhile, Meanwhile, some some illustration illustration and and text text hints hints appeared appeared on theon thescreen screen occasionally. occasionally.
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FigureFigureFigure 12. 12.12. IntegrationIntegration andand and visualizationvisualization visualization ofof thethe of constellationconstellation the constellation andand environmentenvironment and environment data.data. ((aa)) StarsStars data. (Tycho-2(Tycho-2 (a) Stars (Tycho-2catalogue,catalogue, catalogue, number:number: number: >2.5>2.5 million)million) >2.5 andand million) planetsplanets and inin th planetsthee solarsolar in systemsystem the solar (NASA(NASA system JetJet PropulsionPropulsion (NASA Jet LaboratoryLaboratory Propulsion LaboratoryDevelopment Development Ephemeris Ephemeris(JPL DE) 405); (JPL (b)) DE) allall namednamed 405); ( featuresbfeatures) all named forfor thethe features moonmoon (number:(number: for the moon8990)8990) [65];[65]; (number: ((cc)) 8990)geomagneticgeomagnetic [65]; (c) geomagnetic fieldfield generatedgenerated field fromfrom generated thethe TsyganenkoTsyganenko from the 9696 Tsyganenko modelmodel [66],[66], 96 whichwhich model includesincludes [66], which thethe solar-wind-solar-wind- includes the controlled magnetopause, region 1 and 2 Birkeland currents, and the interconnection of the solar-wind-controlledcontrolled magnetopause, magnetopause, region 1 region and 2 1 andBirkel 2 Birkelandand currents, currents, and andthe theinterconnection interconnection of ofthe the magnetospheric and solar wind fields at the boundary; and (d) geographical data from the southeast magnetosphericmagnetospheric and and solar solar wind wind fields fields at at thethe boundary;boundary; and ( (dd)) geographical geographical data data from from the the southeast southeast area of China (image resolution: 30 m, DEM resolution: 90 m, data amount: >200 GB). areaarea of of China China (image (image resolution: resolution: 30 30 m, m, DEM DEM resolution:resolution: 90 m, data data amount: amount: >200 >200 GB). GB).
FigureFigureFigure 13. 13.13.Launching LaunchingLaunching mission missionmission deduction deductiondeduction of the ofof Shenzhou thethe ShenzhouShenzhou spacecraft. spacecraft.spacecraft. (a) Rocket ((aa)) firing. RocketRocket The firing.firing. illustrations TheThe illustrations introduced the stage name and the capacity of the CZ rocket. (b) Rocket booster introducedillustrations the introduced stage name the and stage the capacityname and of the the capacity CZ rocket. of the (b) CZ Rocket rocket. booster (b) Rocket separation. booster The separation. The separation action executed 114 s later; (c,d) the process of solar panel expansion and separationseparation. action The executedseparation 114 action s later; executed (c,d) the114 processs later; ( ofc,d solar) the process panel expansion of solar panel and expansion orientation. and The orientation. The solar panel as a component of the spacecraft model was translated and rotated solarorientation. panel as aThe component solar panel of theas spacecrafta component model of the was spacecraft translated model and rotated was translated according and to therotated action according to the action sequence setting. sequenceaccording setting. to the action sequence setting.
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4.1.3. Virtual Reality Setup Virtual and augmented reality have received a considerable amount of attention by researchers in the last last decade decade and and provide provide new new ways ways to to perceive perceive the the world world [67–69]. [67–69]. Virtual Virtual reality reality is isdefined defined as as a computer-generateda computer-generated scenario scenario that that simulates simulates experi experienceence through through senses senses and and perception perception [70], [70], while augmented reality creates an environment where digi digitaltal information is inserted in a predominantly real-world view [71]. [71]. The The well-known well-known basic compon componentsents of a VR VR immersive immersive application application include input devices (responsible (responsible for for interaction), interaction), output output devices devices (for (for the the feeling feeling of immersion), of immersion), and andsoftware software (for (fora proper a proper control control and and synchronization synchronization of ofthe the whol wholee environment). environment). For For the the better better simulation simulation and presence for VSEs, a simple VR setup was constructed in our Space Situational Awareness Awareness (SSA) shared laboratory based on Sino-InSpace. The The standard standard mouse and keyboard keyboard allow users users a a relatively relatively easy and normal way of manipulationmanipulation of aa virtualvirtual scenario.scenario. In addition,addition, Sino-InSpace supports a variety of stereoscopic observatio observationsns (e.g., red and green, split screen, and blinking) to produce an immersive environment. environment. The The first first two two types types have have no special requirement for the hardware, and the virtual experience is poor accordingly. The blinking mode is most frequently used with the help of Nvidia 3D vision technology (Figure 14).14). The output devices consist of a stereoscopic graphic card (Quadro K2000), synchronous signal transmitters, 3D glasses, Panasonic projectors, and a projection screen. The equipment supports up up to tens of peoplepeople viewing stereo content simultaneously, which could better assist in collaborative and multiperspective space science research andand education.education.
Figure 14. BlinkingBlinking stereoscopic stereoscopic observations observations with with the help of Nvidia 3D vision technology.technology. The synchronous signal transmitters control control the the switch switch of two lenses of 3D glasses, while the 3D view window provides a left or right frame.
4.2. Visualization Development Compared with the “Application Directly” mode mode,, the “Visualization Development” mode was proposed toto develop develop the the individualized individualized customizable customi software.zable software. Figure 15Figure presents 15 thepresents core components the core componentsof Sino-InSpace. of Sino-InSpace. The SSimAstroCore The SSimAstroCore component comp is mainlyonent used is mainly to provide used to mathematical provide mathematical functions functionsand model and calculations, model calculations, e.g., coordinate e.g., coordinate transformation, transformation, matrix operation,matrix oper orbitation, prediction, orbit prediction, space spaceenvironment environment model calculations,model calculat andions, so on,and which so on, are which the basis are of the the basis platform. of the The platform. SSim2DView The SSim2DViewand SSim3DView and SSim3DView components, components, as the visualization as the visu engines,alization are engines, responsible are responsible for the data for the loading data loadingand rendering. and rendering. The SSimScenario The SSimScenario component component parses the parses scenario the scenario file and extractsfile and differentextracts different types of typeselements of forelements the next for step. the The next SSimMainApp step. The SSimMainApp component is thecomponent interface betweenis the interface the external between program 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
ISPRS Int. J. Geo-Inf. 2018, 7, 373 16 of 27 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 16 of 27 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 16 of 27 Figureand internal 11 which components can be andembedded is the most in the frequently new program used component conveniently. in “Visualization In addition to Development” the related functionsmode.Figure It11 supplies suchwhich as encapsulated canenvironment be embedded User data Interface in and the objectsnew (UI) widgetsprogram adding shown conveniently.and intime Figure and 11In viewpoint whichaddition can tocontrol, be the embedded related local scenarioinfunctions the new file such program open as was environment conveniently. also developed. data In addition and objects to the adding related functionsand time suchand asviewpoint environment control, data local and objectsscenario adding file open and was time also and developed. viewpoint control, local scenario file open was also developed. Time Control Viewpoint Control <
Rendering Time Control Mathematical <
Rendering
Rendering
Scenario construction <
Scenario construction <
Figure 16. ComprehensiveComprehensive situation system based on Sino-InSpace. Sino-InSpace. Qt Qt Meta-Object Meta-Object Language Language (QML) (QML) wasFigure used 16. for Comprehensive the interactive situation operation system and information based on Sino-InSpace. display. Qt Meta-Object Language (QML) was used for the interactive operation and information display. The entity object usually has a specific specific and give givenn geometry that can be created or combined in advance.The However,entity However, object some some usually invisible invisible has ora orspecificchanging changing and parameters give parametersn geometry such such as thatsensor as sensorcan range, be range,created communication communication or combined link, in andadvance. space However, environment some boundary invisible or also changing need a parameters visible and such dynamically as sensor range,generated communication geometry. Itlink, is impossibleand space forenvironment any visualization boundary platform also need to integrate a visible all and shapes, dynamically including generated Sino-InSpace. geometry. Therefore, It is impossible for any visualization platform to integrate all shapes, including Sino-InSpace. Therefore,
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ISPRSlink, andInt. J. space Geo-Inf. environment 2018, 7, x FOR PEER boundary REVIEW also need a visible and dynamically generated geometry. 17 of It 27 is 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 rendering. Any geometry drawn with a standard OpenGL language can be added to the geometric model (Figure (Figure 6)6) of of any any entity entity object. object. This This function function maximizes maximizes the extensibility the extensibility of the ofplatform. the platform. Figure 17Figure illustrates 17 illustrates a visualization a visualization result for result the Van for theAllen Van radiation Allen radiation belt in cooperation belt in cooperation with the National with the SpaceNational Science Space Center Science (NSSC) Center and (NSSC) Chinese and ChineseAcademy Academy of Sciences of Sciences (CAS). The (CAS). distribution The distribution and flux and of theflux radiation of the radiation belt particles belt particles were calculated were calculated by the by AE8/AP8 the AE8/AP8 model model [73], which [73], which was expressed was expressed with withboundary boundary lines linesand colors, and colors, respectively. respectively.
Figure 17. Van AllenAllen radiationradiation beltbelt providedprovided byby thethe AE8/AP8AE8/AP8 model.
4.3. Scientific Scientific Analysis As a platform for data import and visualization,visualization, Sino-InSpace also opens interfaces to export calculation and query results. The SSimMainApp component (Figure 1515)) provides most of the output interfaces, mainly divided into three types: syst systemem information (time, viewpoint, mouse position, etc.), environment data (position, distribution, temp temperature,erature, intensity, intensity, etc.), and entity object data (attribute, structure, position, and attitude). The la lattertter two two types types are are meaningful meaningful for for scientific scientific analysis. analysis. The analysis of space objects collision warning was carried out as shown in Figure 1818.. The orbit and position datadata werewere obtainedobtained to to estimate estimate the the most most likely likely approach approach and and even even collision collision events events outside outside the platformthe platform [74]. [74]. In contrast, In contrast, the result the result could could be validated be validated through through the reappearance the reappearance of collision of collision scenario. Thescenario. distances The distances in three orthogonal in three orthogonal directions directio (radial, in-track,ns (radial, and in-track, cross-track) and cross-track) between two between risk targets two wererisk targets plotted were as curves. plotted The as curves. closest timeThe closest was clear time with was the clear help with of visualthe help analysis. 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 (Figure3), 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.
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it provides the “BaseGeometry” class in the SSim3DView component, which can be inherited for rendering. Any geometry drawn with a standard OpenGL language can be added to the geometric model (Figure 6) of any entity object. This function maximizes the extensibility of the platform. Figure 17 illustrates a visualization result for the Van Allen radiation belt in cooperation with the National Space Science Center (NSSC) and Chinese Academy of Sciences (CAS). The distribution and flux of the radiation belt particles were calculated by the AE8/AP8 model [73], which was expressed with boundary lines and colors, respectively.
Figure 17. Van Allen radiation belt provided by the AE8/AP8 model.
4.3. Scientific Analysis As a platform for data import and visualization, Sino-InSpace also opens interfaces to export calculation and query results. The SSimMainApp component (Figure 15) provides most of the output interfaces, mainly divided into three types: system information (time, viewpoint, mouse position, etc.), environment data (position, distribution, temperature, intensity, etc.), and entity object data (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 ISPRSscenario. Int. J. Geo-Inf. The distances2018, 7, 373 in three orthogonal directions (radial, in-track, and cross-track) between 18two 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,Figure 18.areaSpace measurement, objects collision and profile-map warning analysis were anddeveloped, report output. which Thesupported distance mouse curves selection are drawn and manualwith ECharts input. It [72 played], a powerful, a role in interactive the terrain charting and geomorphology and visualization analysis library of for the browsers. Mars surface.
FigureFigure 19. 19.Terrain Terrain and and geomorphology geomorphology analysis. analysis. TheThe crater closest to to Ceti Ceti Chasma. Chasma. The The base base map map was was derivedderived from from Mars Mars Colorized Colorized Viking Viking Mosaic Mosaic [75] [75 and] and the theresolution resolution is 232 ism. 232 The m. source The of source DEM is of MOLA DEM is MOLAMEGDRs MEGDRs [76] and [76 ]the and resolution the resolution is 463 is m. 463 The m. am Theount amount of segmented of segmented geographical geographical data exceeds data exceeds 15 15 GB.
5. Platform5. Platform Evaluation Evaluation DespiteDespite the the fact fact that that Sino-InSpace Sino-InSpace providesprovides real-timereal-time interactive experiences experiences via via computer computer simulations,simulations, little little isis known about about whether whether potentia potentiall users users perceive perceive VSEs VSEseffectively effectively and how and user– how user–systemsystem interaction interaction contributes contributes to tosystem system usab usability.ility. Many Many Human–Computer Human–Computer Interaction Interaction (HCI) (HCI) studiesstudies have have compared compared learning learning effects, effects, performance, performance, and subjective and subjective ratings withratings different with applicationsdifferent andapplications tasks [77], and which tasks provide [77], which grounds provide for an grounds evaluation for an of evaluation the system. of the system. TheThe different different approaches approaches of of measurement measurement fallfall intointo two general categories categories:: subjective subjective measures measures andand objective objective measures. measures. Here,Here, a user study wi withth human human participants participants was was conducted conducted using using postquestionnaires,postquestionnaires, which which are are the the mostmost commoncommon techniquetechnique in in subjective subjective measurement. measurement. Usability, 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 AppendixA. 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 realityfrom setup. the perspective In addition, of thesoftware latter twodesign application [80]. Each modes questionnaire related toconsists development of 10 items were considering also studied 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).
ISPRS Int. J. Geo-Inf. 2018, 7, 373 19 of 27 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 AppendixB. 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 Figure7 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 Figure9) as they expected to test the applicability of the visualization engine. A paper questionnaire (AppendixA) 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 (AppendixB) 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. ISPRS Int. J. Geo-Inf. 2018, 7, 373 20 of 27
To determine the distribution of variables, normality tests were first conducted. Table3 shows the Shapiro–WilkISPRS Int. J. Geo-Inf. (SW) 2018 test, 7, statistics x FOR PEER and REVIEW probabilities of the different measures in the three conditions. 20 of 27 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 MeasureW p W p W p W p W p W p Usability 0.983 0.984 0.958 0.657 0.957 0.648 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 0.991 0.989 0.999 0.984 0.989 0.984 0.991 acceptance Presence Presence 0.969 0.9690.837 0.837 0.9690.969 0.8360.836 0.9840.984 0.973 0.973
HypothesisHypothesis 1 and 1 and hypothesis hypothesis 3 3 predicted predicted that that ratingsratings would be at at a a good good level level in in each each condition. condition. ConsideringConsidering the the number number of of cases cases (<30) (<30) and and the the unknownunknown variance, lower-tail lower-tail one-sample one-sample t-tests t-tests were were usedused to compareto compare the the means means with with the the expectedexpected values. Participants Participants perceived perceived a great a great level level (p > (0.05)p > 0.05) of 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 Table4 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 norating high-level (M = 40.27, perception SD = 1.94, of VSEsT = 53, in p the= 0.70), 2D condition.which proves For the software ability design,of Sino-InSpace participants as an gave extensive a high ratingfuture (M application.= 40.27, SD = 1.94, T = 53, p = 0.70), which proves the ability of Sino-InSpace as an extensive 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 UsabilityMeasure 41.13 2.90 1.51 0.92 40.27 3.54 0.29 0.61 32.20 3.75 −8.07 0.00 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 acceptanceUsability 41.13 2.90 1.51 0.92 40.27 3.54 0.29 0.61 32.20 3.75 −8.07 0.00 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 PresencePresence 42.80 42.803.40 3.403.18 3.181.00 1.0039.53 39.53 3.16 3.16 −0.57−0.57 0.29 0.29 33.60 33.60 3.09 3.09 −−8.028.02 0.00 0.00
FigureFigure 20. 20. Usability,Usability, user user acceptance, acceptance, and and presence presence meanmean values valuesand standard and standard deviations deviations in different in differentconditions. conditions.
ISPRS Int. J. Geo-Inf. 2018, 7, 373 21 of 27
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 (Table5). 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 ANOVA Measure L p F p Usability 0.306 0.738 31.265 0.000 User acceptance 0.041 0.960 0.219 0.804 Presence 0.042 0.959 31.451 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 ISPRS Int. J. Geo-Inf. 2018, 7, 373 22 of 27 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 available online at https://pan.baidu.com/s/1- BC9EIrSkT7gh8mUMtVUfA, Video S1: Sino-InSpace. 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 Some of the Time About half of the Time Most of the Time Almost Always 1 2 3 4 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. ISPRS Int. J. Geo-Inf. 2018, 7, 373 23 of 27
Extremely Quite Slightly Slightly Extremely Neither Quite Likely Unlikely Unlikely Unlikely Likely Likely 1 2 3 4 5 6 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 Disagree Neutral Agree Strongly Agree 1 2 3 4 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? Extremely Not at all 1 2 3 4 5 sufficient Does this platform provide the correct functions and interfaces? Always 1 2 3 4 5 Always right wrong How well do you think the platform performed in fault tolerance? Very poor 1 2 3 4 5 Very well Are the components of platform stable enough for further development? Extremely Not at all 1 2 3 4 5 stable Does adding or modifying a function influence another? Almost 1 2 3 4 5 Not at all always Is the coupling between different components strong? Very low 1 2 3 4 5 Very strong Is the description of functions and interfaces clear and legible? Extremely Not at all 1 2 3 4 5 clear and readable To what extent can the execution efficiency of the platform meet the needs of simulation? Not at all 1 2 3 4 5 A lot How often, on average, does the platform crash or make an error? ISPRS Int. J. Geo-Inf. 2018, 7, 373 24 of 27
Almost Almost 1 2 3 4 5 always never To what extent do you feel that the platform is easy to develop? Extremely Not at all 1 2 3 4 5 easy
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