GPS Network Services For Supporting Surveying Tasks.

Joël van Cranenbroeck
Director GPS Reference Station Program,
Leica Geosystems AG,
Heinrich-Wild-Strasse CH-9435 Heerbrugg, Switzerland
Tel: (32) 8141 2602, Fax: (32) 8141 2602
Email: Joel.VanCranenbroeck@leica-geosystems.com


Introduction
It is well recognized today that a reference network comprised of permanent GPS stations provides the fundamental infrastructure required to meet the needs of professional GPS users in many areas of surveying and mapping. Examples of applications are found in survey control work, densification of existing geodetic networks, acquisition of data for GIS applications, cadastral operations, determination of fiducial points for photogrammetric work, monitoring of engineering works, mapping of utility corridors, assets, etc. In fact, the number of applications benefiting from the establishment of permanent networks is growing daily and in addition there is a new driving force, the non-survey community, who are demanding service.

The widespread use of RTK GPS and DGPS techniques has encouraged decision-makers to look for ways to replace traditional geodetic networks with networks of permanent GPS reference stations and save money. For example, a tighter control of the networks can be achieved from the data supplied by permanent reference stations both in post-processing and in real-time. With streaming data, the influence of those spatial and temporal errors affecting GPS measurements can be estimated in real-time. This in turn means the quality of the transmitted corrections is improved and the range of RTK GPS increased.

New methods have also been introduced to increase the performance and throughput of permanent networks investigated by the academic research centres as the University of Hannover in Germany, the University of Calgary in Canada and the University of New South Wales University in Australia. Private firms have become synonymous in the surveying community for their respective approaches in implementing local area corrections and so-called virtual reference station techniques.

Additionally, discussions taking place within the RTCM organization aim to introduce new standards for the broadcast of GPS corrections specifically for Network RTK messages. Leica has been actively involved with the working group responsible for this message standardization.

Can the cost associated with permanent GPS networks be recovered?

The majorities of permanent GPS networks have been, and will continue to be, for some time to come, initiatives primarily from government agencies.

These government entities have been able to justify the costs of implementing GPS networks by citing the approach of "preventable costs"; similar to the strategy used to finance the establishment of classical geodetic networks decades earlier. The return on the original investment is not measured in nominal terms of hard revenue but in keeping the costs borne by the industry lower than the alternative. This approach also encourages network standardization and avoids the appearance of a patchwork of private, customized networks for project-specific purposes.

The net result of these “free”, but limited, services has been to give the user the impression that the distribution of differential GPS corrections should remain free of charge, and that the cost of establishing and maintaining the networks, and providing services should be assumed by the network owners.

This statement is supported by the marked decrease in the number of paying users for the GPS correction services provided by companies such as DCI, and even Omnistar just a few years ago. They have since struggled to remain competitive in the face of the U.S. presidential decision to turn off Selective Availability on the GPS signal, and broadcast free signals.

Even today, agencies marketing and selling GPS RTK network corrections are facing an uphill battle in trying to convince potential users to subscribe to their GPS corrections services. The primary reason is the disproportionate cost (needed basically for re-covering the huge communication fixed costs) for the offered services with regard to the value service and the limited number of paying customers available. Unfortunately public services are not by nature concerned by the cost recovering, they are just providing the utility.

It may be useful to compare the present situation with that of cellular phone service providers several years ago. There is no denying that these companies are now seeing healthy profits from the various levels of wireless service they offer today. However, when the products were first introduced to the public, the companies gambled on the presupposed reliability and variety of services to lure the customers, and offset an often complex and costly infrastructure. Evidence that their investments paid off can be found in the steady increase in the number of users over the years and the attraction for new service offerings being rolled out on a regular basis. These services are indeed new applications that users have been willing to pay for.

A new generation of support services
A new generation of services based on GPS networks is probably the solution to definitively support the setup of those infrastructures.

It is necessary to address much more specifically the basic requests of the GPS network providers and to recognized the value services the users want to benefit and to support.

Decision-makers wish to control the quality of services based on the type of products their networks provide and want to identify their users even in real time to monitor usage and charge users for services.

They are also committed to providing GPS network solutions in the appropriate reference system. Coordinate transformations should not been seen purely as side products: the very purpose of permanent GPS networks is to offer a complete integrated datum-consistent solution.

We must add also that the software applications used to manage permanent GPS networks could, and even should, incorporate functions to monitor usage and/or charge users for services.

The subject of increased “data integrity” is also creating considerable interest among GPS network owners. What if they could provide a service that bypassed the problems that users routinely encounter in processing their own data?

A reliable network service providing high quality and high fidelity solutions would no doubt generate significant revenue.

A justify and paradigm shift
Instead of broadcasting corrections and placing the onus of obtaining a final solution on the user and his equipment a service provider can organize a specific service by taking advantage of an existing network systems infrastructure to compute his coordinates pre-broadcast in the required reference system. Final (position) solutions for all logged users would be simply computed as a by-product of the continuous network processes – all the time within the quality and integrity criterion implemented at the network administrator level. After all, there exist already a number of web-based services used to generate coordinates from the post-processing of data sent by the user. It is just an extension of this functionality to real-time processing.

This concept is certainly worth investigating! Currently, providers of GPS corrections have no control on the quality of the results computed by the user. This makes it difficult for them to justify charging for their services.

The very concept of virtual reference stations has spawned from the unarguable fact that rover-based RTK solutions exhibit inherent and historic biases, particularly of distance-dependency. This has led to the development of an even greater number of solution algorithms to tackle this problem. The current approach proposed by the RTCM standards committee will only compound the difficulties by forcing the burden of computational work onto the rover firmware. Overall this situation leaves providers in a weakened position to charge for their services since they do not have any control on the quality of the solutions generated in the field.

Therefore, we believe that a paradigm shift is required in order to meet the expectations of future network operators.

Conclusions
This new mode of distributing GPS RTK solution contradicts the data flow in classical RTK by asking the field user to transmit the GPS observations to the control centre. This facility can select the optimal combination of stations to compute the best possible position before returning the result to the user.

The advantages of this approach are clearly evident. A GPS Network provider can exercise control over the generated products and, as a result, place a commercial value on the service, especially as the typical user is released from the obligation of learning complicated GPS surveying techniques. Safeguards, and thus integrity, can also be easily implemented into the distribution service; if the number of satellites is too low, the geometry unfavourable, or the multi-path effects detrimental, a message can be sent back to the user warning them that the provided solution is not optimal and that it may not meet their specifications. With the critical processes (legal minefields) of traceability and integrity looming on the horizon of positioning services, these grey areas are becoming much darker and need to be considered in any long-term plans by deciders involved with GPS positioning services.

An added benefit to this approach is the decreased burden placed on the rovers by removing the need for field calculations, thus encouraging the development of a new generation of less costly rovers. These no-frills (stripped-down) receivers are already beginning to hit the market.

The same data may be transformed into a specific GIS format, or also used as input to topographic processing software for real-time visualizations. Other services may be incorporated such as the transmission of coordinate files, or portions of digital maps, etc.

As a further evolution, it may even be desirable to incorporate total stations in the network infrastructure for handling the data processing either in real-time or in a post-processed fashion. We can envision the natural extension of these services to civil engineering works and elsewhere.

This concept is simply based on the efficient use of available communications infrastructure and software solutions allowing decision-makers to completely fulfil their functions and responsibilities, effectively, by implementing a complete new solution to their users.

There is no doubt that other emerging technologies will take part of the solution. The Internet based services have already demonstrated their capabilities to exchange data in full duplex mode and in real time. The rapid growth of the third generation of wireless communication media will fit perfectly the request to have Internet access from the field.

Finally, all the elements are in place for providing to the professional surveyors, and for the emerging new non-surveyors community well identified services with clear values. The services associated with a GPS Network infrastructure are the key to entering in a digital world of information where the location will be as natural as the time of day on your wrist.