CC BY
32
УДК 629.7
Joël Van Cranenbroeck
Business development Manager - GNSS Networks and Geodetic Monitoring
Leica Geosystems, Heinrich-Wild-Strasse, Heerbrugg, 9435
Switzerland
GNSS NETWORKS AS THE FUNDAMENTAL INFRASTRUCTURE FOR BUILDING THE DIGITAL CITIES
Abstract. In keeping with the philosophy and spirit of the Digital Earth initiative, more and more organisations are building digital cities with the express aim of providing a comprehensive range of spatial related services to industry and the public.
As technology advances and the number of applications requiring position attributes increases, greater numbers of users are demanding spatial data services with higher accuracy than ever before. These services are expected to provide high-quality, high-accuracy and high-reliability coordinates and corrections via various communication channels and protocols, for use across entire metropolitan areas. The construction of an infrastructure system that possesses these characteristics is the primary objective of all organisations wanting to succeed and benefit from digital cities.
Since GNSS technology has come to the forefront of modern surveying technologies thanks to accuracies better than one metre and a few centimetres, through DGNSS and RTK-GNSS technologies respectively, the user community have developed new applications to use these convenient and efficient technologies in a productive manner. Consequently GNSS Reference Station networks have caught the attention of authorities for their scalability, flexibility, stability and reliability whilst supporting spatial services. As a result, those organisations responsible for the construction, operation and management of digital cities, are becoming aware of the important role that GNSS reference station networks can play as the most important infrastructure component in their digital cities.
Thanks to the establishment of a GNSS reference station network, with their inherent high accuracy services and corrections based on a consistent geodetic coordinate datum, the fundamental coordinate control infrastructure for a digital city is already in place. Not only do surveying, GIS and mapping activities benefit from this fundamental infrastructure in terms of faster data acquisition and position updates, but also many other real-time applications, such as structural monitoring and vehicle navigation, that are becoming prevalent in urban areas.
Leica Geosystems, as a pioneer of GNSS in the geomatics industry, has identified the benefits of reference station networks and responded by improving the performance as well as the usability of reference networks to fulfill the needs of digital city applications. The new generation of Leica GNSS Network products is composed of a series of GNSS receivers for all reference station applications and GNSS Spider - the most powerful GNSS network configuration and management software incorporating advanced Network RTK algorithms utilizing the new Master-Auxiliary Concept. From the communication channels supported to the usability of the software system, from the core algorithms to the GNSS data and correction formats, from the sensor and product configuration to the management of rover users, Leica’s GNSS Reference Station network portfolio can help ensure that digital cities are built to last.
Keywords: GNSS, Reference Station network, Digital Cities.
1. Introduction
Digital Earth (DE), as first promoted by Gore (1998) was envisioned as a computerised, multi-dimensional, multi-scale, multi-temporal and multi-layer information facility. DE concepts are already strongly rooted in the visions of
many governments as being the means of being the sole platform for the management and display of spatial information. Not long after, China’s State Bureau of Surveying and Mapping (SBSM) embarked on Digital China - an ambitious mapping project to digitally map the entire nation. The realisation of Digital China would provide decision support in city planning and natural disaster prevention - two activities important to growing economies in over 600 Chinese cities. Already, the SBSM has created three digital map series of the country with a level of detail comparable to map scales of 1:4 million, 1:1 million and 1:250,000. 2005 should see the completion of a digital map series equivalent to a 1:50,000 map scale. Initiatives at the local level, are already generating detailed digital maps of cities, for example the city of Weihai in East China’s Shandong Province has already produced a detailed 3D digital city map at 1:500 scale, see Rediff (2004).
This specific focus on compiling a detailed representation of an individual city can be described as building a ‘Digital City’. Authorities in China are also working to build similar models of strategically important features such as coastlines and waterways as featured in the ‘Digital River’ initiative. Together such digital cartographic representations can be combined with demographic and environmental data to form Digital China.
The benefit of building these models is that they form the most efficient basis for administering and operating these resources. Given the rapid growth of China’s economy, and thus demand for natural, man-made and human resources, having such an overview makes extremely good sense. With the establishment of Digital China, particularly with the integration of demographic data, the visionary concepts of e-government, e-city management and others, become reality.
One major challenge within DE applications is that of data access and data fidelity (correctness). This paper focuses on the specialized activity relating to the definition of the reference frame for this datum - that is with the establishment of a GNSS Reference Station Network (RSN) that essentially defines the coordinates datum over the area that it covers, i.e. the Digital City - and the associated benefits that a RSN brings to the entire DC project.
2. Foundations for Digital Cities
A digital city can only be of benefit to its ‘digital architects’ and users if the data it contains is accurate and reliable - a true representation of the real world environment it portrays. At all times, the accuracy of the position information is the defining characteristic of the digital city’s worth. In terms of spatial accuracy, the specifications for digital cities can range from large scale mapping at the metre level for asset management and environmental applications, through decimetre and centimetre for cadastral surveying and engineering construction, up to millimetre level high-precision engineering and deformation monitoring tasks. The currency of the data can be described as its temporal accuracy - at which time did it have the attribute have that spatial accuracy.
Using a real world example of Kunming in Yunnan Province of China, the ongoing construction and population of new Kunming demanded the establishment
of a representative location model, namely a digital city model of Kunming, see Wu (2005). There in Kunming the construction of an active control network system was necessary to support surveying specifically for the construction of the City Spatial Data Infrastructure, to aid acceleration of construction in the city through improved survey control, to provide active control given that the majority of traditional survey control points are seriously damaged, and finally to provide a dynamic real-time correction service to support all users.
Kunming City’s needs in terms of city planning, construction and management demanded that the updating of digital topographic maps using GIS could take place in real-time, navigation and tracking of special vehicles, along with intelligent City traffic management and decision making.
To realise this active network, it is necessary to establish a system that provides a consistent coordinate reference datum over the area of interest. Every country that has been, or is, embarking on a redefinition of their national reference frame has done so making use of GNSS technology, typically in the form of a Reference Station Network.
A network of such continuously operating GNSS reference stations (CORS) is more efficient than a traditional triangulation and traverse network. The stations can be set-up at convenient locations where they are needed. Network geometry is not as critical as with traditional networks, and the accuracy is higher and more consistent.
Typically, local network operators define their own local datum covering their area of responsibility. Considering China, there are over 2000 counties, many of which have their own incumbent local datum. This is fine when working on a regional county level, but impractical and incorrect for production use on a national level.
By default, the information (whether data, corrections or final positions) supplied by the RSN is provided in a consistent global satellite datum, typically WGS84 (in some cases ITRF2000 or later). This datum and its parameters are well-known, many national reference frames are derived from it, and satellite surveying systems are well equipped to cope with this datum, and transformations based upon it. Once the datum is defined, all surveying of digital city features can be referenced to the geodetic datum, including geoid height if necessary. The designs of future projects (buildings, bridges, roads, etc.) can all then be represented in the digital city’s datum providing accurate and reliable datum-consistent position information.
3. Technologies in Reference Station Networks
Since GNSS technology has come to the forefront of modern surveying technologies thanks to accuracies better than one metre and a few centimetres, through DGNSS and RTK-GNSS technologies respectively, GNSS Reference Station Networks have caught the attention of authorities for their scalability, flexibility, stability and reliability whilst supporting spatial services. Users and operators in Digital Cities make use of different systems for populating and maintaining the geospatial data within the DC. GNSS is of course one such well-
established tool used worldwide, and under its conditions, allows for the rapid and efficient collection of specific features / attribute data. One of the main advantages of reference stations; both in their individual and network realisations, is their usage for supporting multiple applications, through both post-processing and realtime services. For example, the introduction of single-site RTK-GNSS into conventional surveying activities has yielded improvements in survey productivity of over 30%.
Leica Geosystems, as a pioneer of GNSS in the geomatics industry, has identified the benefits of Reference Station Networks and responded by improving the performance as well as the usability of reference networks to fulfill the specific needs of digital city applications.
The new generation of Leica GNSS Reference Station Network products is composed of a series of GNSS receivers for all reference station applications and GNSS Spider - a powerful GNSS network configuration and management software incorporating advanced Network RTK algorithms utilizing the new Master-Auxiliary Concept (MAC), see Leica (2005a). This concept, the basis for the forthcoming open RTCM standard for network RTK corrections, is a revolutionary new approach to network RTK that essentially extends the range and accuracy compared to conventional single-site RTK.
The RTCM network messages offer a truly open standardized format that enables efficient and accurate network RTK in both broadcast and two-way mode. Leica (2005c) shows that the theoretical advantages of the Master-Auxiliary Concept translate into true benefits for the rover user in terms of increased accuracy, performance and reliability. The statistical analysis of all tests clearly showed that the best performance was achieved by combining Leica GNSS Spider with Leica GNSS 1200 rovers utilizing MAX corrections. The individualized version of the MAX, known as i-MAX, which is also available from the Leica GNSS Spider reference station software gives almost similar high level performance as MAX but with the advantage of using the lower bandwidth RTCM 3.0 format that can also be used by older (legacy) receivers which are incapable of supporting the new network messages.
Seamless support of legacy and older GNSS receivers is an extremely desirable quality within RSN software ensuring that the valuable correction information afforded by the network can be used by the highest number of users operating in the digital city.
Broadband networks, including the Internet, provide the communications backbone to today’s society and are firmly anchored as the main communications channel for digital cities. Today’s communication technologies including Internet, cell phones, spread-spectrum radios etc, can support all manner of users and applications, and should do so if they want to become a part of the digital city’s extended infrastructure. Users want to have access to all of the data products available over the entire area of the network all of the time, over as many different media as possible, and digital city architects must recognise this. Portals in the digital city platform are typically web-based allowing multiple access by multiple
users for multiple applications. Clearly these components of the digital city’s RSN infrastructure should also be capable of this.
Even with the runaway success of GNSS as a surveying tool, it is limited by factors such as satellite visibility and communications coverage. Consequently Reference Station Networks should also be flexible in their ability to support surveying and mapping systems using conventional measurement sensors.
A recent groundbreaking innovation in data collection is the SmartStation concept - the integration of terrestrial and satellite data collection systems, see Leica (2005b). This concept is especially useful in metropolitan areas where satellite visibility can be limited, wherein the SmartStation can be established quickly and easily like any conventional total station system.
Use of a SmartStation unit removes the need to traversing to propagate coordinates from distant control points by providing RTK GNSS positioning of the total station, without the surveyor needing any specialised GNSS know-how. The proliferation of GNSS Reference Station Networks means that in many cases a surveyor can simply connect into the omnipresent network and receive a real-time correction stream in order to fix its position accurately and reliably within the preferred datum; coordinate transformations are just an additional step to be made on the survey system.
No longer is it necessary for surveyors to undertake time consuming traversing to propagate control into the survey area: comparisons show that topographic surveys can be completed over 30% faster than conventional survey, whilst maintaining an accuracy of better than 15mm in both horizontal and vertical components. The time savings were achieved through reduced reconnaissance and eliminating a control traverse to propagate control to the survey area. Following that, the individual components (TPS & GNSS) can be separated to individual survey teams.
4. Data Fusion and Application Fusion
Such integrated measurement systems exploit the synergies of data attribute collection and are ideally suited when building and maintaining the digital city model. Coupled with GIS-data collection products such as MobileMatriX (which is fully interoperable with Arc GIS products) and the GS20/SR20 GNSS/GIS data collectors, aerial photography systems to machine guidance systems, HDS (highdefinition scanning) to Railroad engineering, a Reference Station Network can provide consistent real-time control for all spatial positioning systems supporting multiple-applications.
Thanks to the establishment of a GNSS Reference Station Network, with their inherent high accuracy services and corrections based on a consistent geodetic coordinate datum, the fundamental coordinate control infrastructure for a digital city is already in place. Not only do surveying, GIS and mapping activities benefit from this fundamental infrastructure in terms of faster data acquisition and position updates, but also many other real-time applications, such as structural monitoring and vehicle navigation, that are becoming prevalent in urban areas.
This Data Access and Availability service is a prime example of the benefits that Reference Station Networks afford, not only in Data Fusion, but also Application Fusion.
The Kunming GNSS CORS System, running GNSS Spider, provides multilevel, multi-aspect social service for all users on two levels: the first is a Positioning information service directly supporting those users requiring spatial information, be it data or correction products. The second service is Non-spatial but equally important - examples include forecasting and environmental protection information. For example, weather forecasting trends can be derived from information contained within the magnitude of tropospheric refraction afforded on those GNSS signals received at the reference station locations, at a level of accuracy 30% to 40% higher than the traditional way, see Wu (2005).The integration of the GNSS CORS System and Digital Maps can be applied to any fields requiring spatial position information, such as city planning, land-use management, city management, police and fire control, landscaping, forestation, intelligent traffic management and decision, through to vehicle location and monitoring. The tracking of public transport vehicles (such as buses, trams, trains) requires GNSS infrastructure/components - all of which can be provided via RSN infrastructure. Nominally, precision agriculture does not typically exist in a digital city, rather the countryside but nevertheless makes use of the very same infrastructure; just another example of the interoperability and application fusion that a RSN brings to today’s society. Equally, the digital city architects should not neglect the importance of Reference Station Networks for upcoming applications such as Assisted GNSS and LBS (Location-based Services).
In terms of data access, the amount of data associated with GNSS RSN is relatively small compared with geospatial imaging datasets; in contrast, the importance of RSN data is arguably much more important than most others -without the absolute coordinate control (datum) afforded by these GNSS positions, all other data (imaging included) is not truly georeferenced.
In summary, from the communication channels supported to the usability of the software system, from the core algorithms to the GNSS data and correction formats, from the sensor and product configuration to the management of rover users, the use of Reference Station Networks allows authorities to define the geodetic datum of the digital city area, provides services that will allow the survey and measurement of the digital city, and no less important, allowing the digital architects (comprising government and developers) and to expand the digital city when new projects arise.
5. Building a Digital City Reference Station Infrastructure
It is worth stating here that the establishment of a Digital City platform must be flexible - each digital city is unique, and will evolve in its own distinct manner; for example, some provinces want to run the DC portal from a central location, whereas others prefer to implement a decentralized approach.
Consequently the Reference Station Network software should be flexible enough to adapt with the digital city’s current and future demands. In some
regions, it is planned to establish a decentralized Reference Station Network, whereby the provincial authority will establish its provincial network with a single ‘province-owned’ reference station in each city, obligating the city authority with the responsibility to densify their own city network. At all times, data and information will be overseen by the provinces.
The Reference Station Network software that will form the underlying datum for the DC must have the flexibility to adapt to the province’s preference. Designed as a Client/Server architecture, Leica GNSS Spider has the flexibility and scalability to be installed as a centralized system, or a decentralized distributed network, depending on the specific requirements.
Data access is of concern to the administrators of digital cities, and the security and traceability of all users is also very important. GNSS Spider has been designed from the outset with security and integrity in mind - all activities and events taking place within the software are recorded for archival purposes. All users requesting real-time data streams must authenticate themselves prior to authorization by GNSS Spider. Traceability of all operator actions is also a requirement for authorities working with demographic information. Regarding incoming data access for other GNSS signals, as well as the future GALILEO, the GNSS Spider platform is well-designed to accommodate and support these constellations once they achieve acceptable operational capability.
One very important aspect that needs to be discussed and clarified immediately is the definition of a standard description for every Digital City’s coordinate reference datum, and subsequently the use of GNSS data and services afforded by that network, and indeed across all of the digital cities that are being built in China. In addition, the archival of data, information, corrections, users logs etc are all ‘secure’ datasets that must be well controlled.
To be serious about building digital cities, the responsible authorities must not ignore the considerable benefits afforded by reference station network infrastructure. With the definition of such an open and public standard, the authorities supporting the digital architects can be guaranteed that each and every GNSS network will satisfy the demanding needs of high-accuracy, high-reliability and high-tech applications that operate in a digital city environment.
6. Conclusions
It is clear that the combined value of the Digital Earth and its Digital Cities to the world community is immense, not only in socio-economic terms, but also for crisis management and disaster mitigation.
Reference Station Networks and their services provide the geodetic coordinate control needed for high-precision positioning as well as supporting a wide variety of applications, for both real-time and post-processing activities. Such a network’s permanence as fundamental infrastructure means that it can support the building of a digital city as well as its maintenance, update and expansion.
The construction of Digital Cities, summarized as Digitalization, is the trend of modern city development in China. The successful construction of the Kunming GNSS CORS System is already making a remarkable contribution to the city
planning, construction and management of new Kunming, accelerating the progress of the population of Digital Kunming City, thus enhancing information management across the whole city.
With the continuing construction of digital cities, the responsible authorities and digital architects must quickly and clearly define open and public standards as to how Reference Station Networks and their beneficial services, should be created, populated and maintained - this will help to ensure that the considerable benefits afforded by reference station network infrastructure are utilized efficiently and productively.
REFERENCES
Gore, A., 1998, The Digital Earth: Understanding our planet in the 21st Century, http://www.digitalearth. gov/VP 19980131.html
Leica Geosystems, (2005a). An introduction to the philosophy and technology behind Leica Geosystems' SpiderNET revolutionary Network RTK software and algorithms. Leica Geosystems AG, Heerbrugg, Switzerland, June 2005.
www.leica-geosystems.com/common/shared/downloads/inc/downloader.asp?id=5367 Leica Geosystems, (2005b). The integration of GPS and Total Station Technologies, SmartStation. Leica Geosystems AG, Heerbrugg, Switzerland, February 2005.
http://www.leica-geosystems.com/common/shared/downloads/inc/downloader.asp?id=5037 Leica Geosystems, (2005c). Advances in ambiguity resolution for RTK applications using the new RTCM V3.0 Master-Auxiliary messages. Leica Geosystems AG, Heerbrugg, Switzerland, In Proc. ION-GNSS 2005, Long Beach, California, September 2005.
http://www.leica-geosystems.com/s-e/en/downloads/lgs page catalog.htm?cid=3228 Rediff., 2004, China's awesome digital mapping plan , http://inhome.rediff.com/money/2004/nov/18china.htm
Wu, L, 2005, , GPS Continuously Operating Reference Stations in Kunming, Kunming Surveying and Mapping Institute, Presentation to Yunnan Province, Kunming, PR China, 9th July 2005.
© Joel Van Cranenbroeck, 2008
