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CORS Networks

Rokubun, ICGC

Nowadays, the Continuous Operating Reference Station (CORS) networks positioning services are amongst the most used ones when the highest positioning accuracy is needed. In this situation, from private world-wide corporations to quite small companies, or from continental to national and regional mapping companies, are currently deploying and updating reference stations networks and derived services, to fulfill their users positioning needs.

CORS networks started to be established in the 80s and 90s and it was in the late 90s and early 00s when real time services started their implementation and continuous growth. This expansion was firstly based on terrestrial communications like RDS (Radio Data System) and satellite communications like WAAS (Wide Area Augmentation Systems), and lastly moved to IP (Internet protocol) based communications.

Nowadays, RDS based services have been mostly abandoned while WAAS and IP coexist and complement each other, evolving and moving forward. At the very beginning, CORS networks were mainly established for the computation and maintenance of the regional/international reference systems, and in the provisioning of data to surveyors in order to facilitate accurate differential post-processing capabilities. This has been the situation up to the first decade of 2000, when the technological boom has incredibly joined together different disciplines, in an increasing need of accuracy in real time and far exceeding the original main goals of surveying and reference frame maintenance which, obviously, haven’t been forgotten.

CORS networks are currently based on three main pillars:

  1. The GNSS reference stations are the core main pillar, as they can also act as the 2 other pillars, dealing with the computation of solutions and acting as a service provisioning platform. However, GNSS reference stations are usually focused only on gathering data from GNSS constellations and sending them to the computation facilities.
  2. The network control center collects data from all stations belonging to the same network or area, and processes everything together in order to obtain a unique and combined solution, in order to cover the area where to provide the positioning services. This control center also takes care of managing users, storing data, processing user requests....
  3. The service provisioning platforms are usually web services, based on different standard protocols, accessible via the world wide web or via other kinds of telecommunication systems, but always as much accessible as possible, to provide corrections to GNSS users. These platforms have to be understood as the user front-end for the CORS network and its control center.

The data representation

These corrections or observations computed on the control center, thanks to data from GNSS reference stations, can be provided in 2 different representations:

  • OSR (Observation State Representation) is the approach commonly used in RTK or NRTK services. In this representation, a CORS network continuously observes the different GNSS constellations, computes the whole network and adjusts it to the already known coordinates of stations, in order to determine observations errors and to derive the corresponding corrections. These computed corrections are the information sent to the final users for their interpolation on its “un-accurate” positions, to finally obtain “accurate” positions. In this regard, and back to RTK and NRTK services definition, the main differences between both concepts rely on the usage of one single station or a whole network of stations, respectively, to compute the corrections to be provided to users.

  • SSR (State Space Representation) is the approach commonly used in PPP services. In this representation, a CORS network also continuously observes the different GNSS constellations, computes the whole network and adjusts it to the already known coordinates of stations, in order to estimate the errors on the satellite clocks and orbits, atmosphere, biases… These estimated errors are the information sent to the users, so they can apply them to improve their own observations, to be able to compute accurate positions.

In summary, OSR provides the actual ranges and carrier-phase measurements (i.e. observations) of nearby receiver(s) to apply differential GNSS techniques, while SSR provides corrections on the necessary products delivered by the GNSS broadcast parameters (i.e. corrections on satellite orbits, clocks, ionospheric delay, code biases) to compute the position more accurately.

Furthermore, for the RTK and NRTK services, both based on the OSR representation, different concepts also apply and have to be considered:

  • VRS (Virtual Reference Station) was developed by Terrasat in the late 90s and currently belongs to Trimble. The correction flow distribution starts with a user sending its own and approximate position coordinates to the platform providing corrections. The network control center generates a virtual station on the received coordinates and interpolates corrections at this new station, which are then sent back to the user. The user just needs to apply the corrections, as if a GNSS station set by himself in a close benchmark was providing them to him. It has the main advantage that corrections are computed on the server at the network control center, so high computation capabilities are not required on the user side. However, it requires the user to be able to send its approximate position at the very beginning and update it regularly. For the described data flow, a bidirectional communication link is obviously required. As it is the same differential concept of installing a GNSS station in a close benchmark and applying its corrections, there is no need to deal with specific formats or additional software capabilities. As it was previously mentioned, this technique poses serious limitations for scalability in the number of simultaneous users, since for each user the network control center has to compute ad-hoc corrections.

  • FKP (Flächen Korrektur Parameter) was developed by Geo++ and MAC (Master Auxiliary Concept) should be considered as an evolution to improve the transmission of corrections to final users. Albeit these two techniques send different information to the final user, both work on the principle that data from several nearby stations are sent to the final user to obtain an accurate position estimate. FKP is based on the computation of corrections considering a whole set of stations in the network, so all the information is collected together and sent to the user, who is in charge of the combination with its own observations, in order to obtain the corrections to be applied and so to compute its accurate final position. Opposed to the VRS, it has the main advantage that it does not require a bidirectional communication link, as all the information is broadcasted by the service provisioning platform and the user is the one in charge to interpolate it for its own position. However, the receiver must be able to deal with this specific format and requires higher computation capabilities than VRS, as the final interpolation and adjustment is performed at the user side.

Mapping agencies

The CORS networks are usually managed by private companies or by mapping agencies, depending on territories and political or economical strategies. Having said this, it is also important to emphasize that, in some areas, private and public networks offering quite similar services can coexist perfectly, even resulting in symbiotic fruitful collaborations amongst them. Usually, in any of the above mentioned representations or concepts, the mapping agencies are always involved directly or indirectly. Even in the situation that agencies are not providing services by its own means, they usually have the responsibilities for the observation, computation and publication of the reference frames, having to provide accurate and reliable access to the official reference systems. This means that mapping agencies are obviously directly involved in the CORS networks whenever providing services by themselves, but also indirectly involved whenever private companies are providing the services, as these companies will always need to rely on the official reference frame computations and coordinates maintained by mapping agencies.

This last consideration of general public mapping agencies involvement, for most of the cases, is even more important whenever different service providers coexist in the same territory. In this way, and considering the high positioning accuracy achievable with the different available techniques and the expansion of service providers all over the world, the reference systems computations accuracy and reference frame materialization have become a critical consideration for the required coherence among different services, to guarantee compatibility and so coexistence in a feasible way.

Furthermore, and considering the high position accuracy achievable, it has been the technological evolution which has led to real time services providing decimetric and centimetric accuracies, and repeatabilities at the same order of magnitude. This technical evolution obviously considers the GNSS constellations updating/upgrading, the inclusion of new observables and frequencies for the constellations integration, new launches for increasing the satellite availability even in harsh conditions… Usage has grown rapidly in recent years so, thanks to this new technology, agencies and companies working in the same or close areas have started to share data in real time, mainly to have spare stations to cover failure situations and to better fit the computation algorithms with data from enveloping stations. Obviously, all this has required more computation and storage resources but, also thanks to the new available technologies, most of the drawbacks have been successfully overcome with new and more powerful servers, and with new and faster communication technologies.

The Catalan CORS network

ICGC operates the active GNSS CatNet network, which currently has 16 permanent GNSS stations. 11 of the stations are anchored in bedrock with geodynamic capabilities, for the reference frame monitoring, and the remaining 5 are installed in buildings to appropriately densify the network for an homogeneous RTK service provisioning.

The CatNet network, together with its data and positioning services, is fully and solely managed from ICGC headquarters. CatNet GNSS stations are operating 24x7 and covering the whole territory of Catalonia (as shown in the figure below) with an inter-distance small enough to provide, as mentioned before, reliable NTRIP services everywhere.

Figure 1. Distribution of CORS receivers of the CatNet GNSS network

Real time positioning services based on CORS networks

Nowadays, the usage of real time positioning services based on CORS networks have gone beyond their geodetic and topographic historical uses. Based on mapping agencies expertise in providing this kind of real time positioning services based on CORS networks, in the last decade there has been a growth of 50%, 60%, 70% and even more than 100% per year in the amount of service hours provided. This increase in demand is currently addressed by assigning more hardware and software resources to the network control center but, in the current and upcoming days, service providers must consider new uses such as drone navigation, precision farming, smartphone positioning, autonomous driving, machine guidance, sports monitoring, emergency servicing, fleet management… And it is not only needed to consider new uses, but also that some of them are potentially mass-market uses involving thousands or hundreds of thousands users simultaneously.

For sure, all of the abovementioned new uses will be growing in the upcoming years, but it is pretty clear that LBS (Location Based Services) based on smartphones and automated driving will be among the most used and with the greatest growth potential. At this point, and taking into consideration the technical details provided in the previous paragraphs, it can be concluded that lack of scalability will be one of the biggest issues to deal with, and it seems quite clear that it is not a matter of resources on the network control center but a matter of developing a new approach, for example, when considering that:

  • Current RTK services, for example, require a continuous Internet connection in real time, which is not available all over the territory. In this way, PPP or WAAS services based on satellite communication could provide a feasible solution. However, PPP services still have long solution convergence times that cannot be accepted by most users, and WAAS are not always capable of providing the required accuracy by the users. These drawbacks could be properly addressed when considering the help of RTK services abovementioned. Furthermore, RTK services are usually in need of approximate coordinates from the user, which means that a bidirectional link is needed, so broadcasted solutions are not feasible. However, when considering the SSR approach, one directional links are perfectly suitable. This is leading to the conclusion that all techniques have their pros and cons, and here is where the UNION UUPE will take care of exploiting the concept of undifferentiated and uncombined processing of GNSS measurements and data sources, allowing to go beyond a smart combination of PPP - RTK techniques, to overcome most of the drawbacks.

  • VRS concept requires bidirectional communication systems and a unique virtual reference station to be created for each user, and the corrections need to be computed continuously for the whole duration of each user connection, but this is not conceivable for mass-market applications. Some alternatives currently available in the market, such as FKP or MAC, do not require bidirectional communication, but are neither a viable solution, as they also face the same problem of overloading the server computation requirements on the network control center. The UNION Fixed VRS network concept will enable proper area coverage and one-way operation of GNSS CORS networks, thus ensuring service scalability to mass-market applications without overloading servers.

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