Imaginary reference stations.

A new level of technology built upon the foundation of GPS has been quietly under construction and is rapidly maturing into the next generation of GPS usage. In some places now, the user of a GPS rover can perform centimeter level positioning without a dedicated RTK GPS base station. This next-generation RTK GPS user is accessing RTK corrections made available through a network of GPS reference stations-sometimes for free, sometimes on a subscription basis. In any event, the RTK surveying is done without the accompanying five-figure investment in base station hardware, computers, post-processing software and conversion experts. A surveyor only needs to possess a simple GPS rover with an integrated GSM cellular modem or radio to acquire real-time RTK corrections from a GPS infrastructure provider.

Figure 1. This concept illustrates a rover sending its position to an access server, which authenticates the user and passes the data to the central server. Each rover then obtains corrected data over the same data link used to input its position to the central processing server.

How It Works

The user in the field begins with a pole-, ATV- or utility vehicle-mounted GPS receiver or rover. The rover calculates the user's current position within a few meters. Communi-cation via cellular modem transfers the GPS- (or GPS+GLONASS-) derived location of the rover to a dedicated processor that serves the GPS network. Depending on the networked RTK technology being used by the provider, the computer calculates a unique set of RTK corrections based on the user's position using data provided by reference stations throughout the network. This creates an "imaginary" or "virtual" reference station near the user or creates RTK corrections for different parts of the network, which the rover integrates into its precise position based on its coarsely determined position.

The network consists of a number of permanent GPS base stations situated to provide the desired area of coverage. Networks can be for a small town, an entire state or even a collection of states. They can be expanded easily to cover more geographic areas with the addition of more reference stations. These stations send raw data to a central server, which logs the incoming information. The server then uses this data to develop models to reduce or eliminate the effects of clock errors, orbital (ephemeris) errors, ionospheric and tropospheric errors. A unique set of RTK corrections will be created for each user in the network. These will be delivered to users as virtual reference station corrections in one manifestation of the RTK network, minimizing the ppm error contribution due to the effects of spatial decorrelation. This is error that creeps into the computations as one moves farther from a reference station. The result of this modeling is that the systematic errors that cause the ppm specification inherent in all RTK GPS systems are substantially eliminated or reduced.

Figure 2. This "virtual" reference network shows the virtual station created nearby from which the user obtains corrected data.
Once a user accesses this system and sends his or her location in, the server develops the imaginary reference station nearby. This location is used as the point from which real-time corrected data is received. Very precise measurements are obtained in real-time for the user. Figure 1 on page 61 illustrates the concept of a rover sending its position to an access server, which authenticates the user and passes the data to the central server. Each rover then obtains corrected data over the same data link used to input its position to the central processing server. Figure 2 on page 61 illustrates the virtual station created nearby from which the user obtains corrected data.

Regardless of the technology solution for networked RTK systems, the intention is to provide users with consistent precision through the entire network through access to corrected data for the applicable area. The central processing server handles importation of raw data from the reference stations as well as storage of the incoming raw data files and conversion to the standard and compressed RINEX formats. It also estimates systematic errors, calculates and models the correction data in a standard format such as that developed by the RTCM (Radio Technical Commission for Maritime Services). The server also handles quality assurance and transmission of data back to users in the field. The resulting measurements are obtained as though the GPS reference station were right there beside the user.

Who Is Currently Involved

Leica Geosystems, Thales Navigation/Geodetics Incorporated, Topcon and Trimble all offer this solution in varying states of readiness across the world. Their technical and operational approaches to providing networked RTK GPS corrections vary. Some government agencies, departments of transportation and private organizations have also pioneered this effort. In some cases, rather than developing their own technology and operational processes, they have adopted the systems offered by the above manufacturers. In fact, there are 70 or more systems in varying stages of operation around the world. North Carolina's Geodetic Survey is a leader in the technology; other pioneers can be found in Georgia and Washington. There are also the end users who purchase the required equipment to tap into this resource (if permitted), and provide location and positioning services.

An Interview with an Implementer

The Virginia Department of Transportation (VDOT) is currently implementing a GPS network system using Leica Geosystems rovers and temporary base stations in the field. They have also recently acquired Trimble products for the CORS permanent stations. Jim Jenigen, a geodetic survey engineer at VDOT, says that his organization believes that GPS is destined to be the benchmark system over time, eventually replacing conventional ground control systems such as monumentation for horizontal and vertical control.

Currently, fixed monuments are tied to the HARN (High Accuracy Reference Network) system. A surveyor using RTK GPS near a HARN point or a fixed monument can set up his or her (temporary) reference station on these points to survey within the working range of the RTK GPS system. This range varies by manufacturer and the age of the technology, but is often a 10 to 20 km radius. If working beyond the technology's range for RTK, the surveyor must perform a regular static GPS survey to establish control for the RTK reference station. If surveyors had access to networked RTK corrections that are continuously on tap, similar to the way we access the Internet with wireless technology, the effort and expense of temporary RTK base stations would be eliminated.

GPS reference networks improve on the CORS (Continuously Operating Reference Station) system, which currently delivers a stream of GPS data hourly to the National Geodetic Survey (NGS), which then makes it available on the Internet. With the CORS system, a surveyor performing static or rapid static work can obtain the raw data, process it and then make corrections based on where his project data resides. This can be done after the field work, or with modern telecommunications and processing technologies in the field, but not in real-time. With the GPS reference network for RTK, a surveyor can determine RTK positions at the project location in real-time.

Jenigen indicated that VDOT's first phase involves only one base station in operation now that broadcasts a base line correction in VDOT's Richmond district. It has a 30-km radius of access and a 1-3 cm range of accuracy. VDOT is in the process of creating a network in the district and adding several Virginia counties to the network for RTK corrections. These stations will be tied together so that a virtual reference network can be established. Ultimately, the stations will be able to provide an adjusted RTK capability for attaining positional accuracy. Computations and post-processing will be done on-the-fly.

VDOT's proposed implementation schedule begins with VDOT district surveyors obtaining access to the base line correction data. Phase II will then include the private consultant community. Currently, consultants perform their own data collection and post-processing for static surveys, and set up their own RTK base stations. If they can obtain access to this data from the VDOT signal, their costs to complete their surveys should be reduced, hopefully passing on some of those savings to the state. In addition, VDOT's cost for hardware for RTK base stations would be reduced substantially for outfitting district surveyors where the network is in place. Phase III would likely allow the general public to obtain access. VDOT's system requires a CDMA (Code Division Multiple Access) modem system where the user would provide members of the public with a static IP address for the modems to provide controlled access through the state's firewalls to the network's data server. The public could then also use rovers with no need for base stations of their own.

Michigan DOT employees tie into the local reference station network, saving them money and providing quality monitoring and control.

Interviews with The Manufacturers

Leica Geosystems
At Leica Geosystems, Andrew Hurley, product manager, and James Stowell, director of integrated GPS systems, say implementation for networked RTK GPS reference stations use their unique algorithms for RTK corrections. In contrast to the virtual reference stations, Leica's system determines high accuracy corrections that allow RTK corrections to be usable at distances of up to 30 km from the base station, allowing a wider spacing of reference stations in the network. Hurley and Stowell believe that Leica's approach is one that produces high accuracy and that potential customers should perform their research to identify which methodology would work best for them.

Leica has numerous reference station networks established around the world including North America. These networks range in size from four stations to 35 stations. They have two successful systems in place in the United States, one in Michigan and the other in Texas. The Michigan system resulted when the Michigan Depart-ment of Transportation (DOT) suffered budgetary cutbacks in the surveying division and began using outside consultants for survey work. In their opinion, this resulted in a loss of their ability to maintain quality control since each consultant had to establish his or her own geodetic control. Thus, the Michigan DOT put in its own RTK corrections infrastructure and required those consultants to tie into the local reference station network. They set the standard for control and continuously monitor its quality. The network has produced significant cost savings since the DOT no longer needs to perform the level of quality assurance that it did before the network was established. The system created in Texas includes six stations with 60 contractors as control users.

The Leica system is completely open and can be used by any rover. Leica's reference station solution is comprised of hardware (the new GRX1200 series) and software (GPS Spider). With the introduction of the new GRX1200 Series Reference Station receivers comes new technology: Leica's SmartTrack. This new technology ensures clean, high accuracy code and phase measurements with excellent signal-to-noise ratios to generate RTK data that is broadcast to a rover. On the rover side, the GPS1200 series RTK receivers also employ SmartTrack, and employ SmartCheck technology that allows continuous checking of the ambiguity solution resulting in RTK at 30 km without compromising reliability of the results. The reference stations themselves are in a monitored network where quality assurance is performed continuously. In this manner, Leica's system achieves centimeter accuracies using the actual reference station instead of a virtual reference station.

Leica, along with some other manufacturers, espouses the concept that the solution should be an open reference station system. They hope that these solutions will allow for users with rovers from multiple manufacturers to be able to obtain service from providers. Leica believes that as this technology grows and is implemented, it will one day be transparent to the engineering and surveying community.

Thales Navigation/Geodetics Incorporated Inc.
Thales Navigation offers a complete hardware/software solution that includes the Thales Micro-Z CGRS and iCGRS reference stations and open platform RTD (Real-Time Dynamics) software from Geodetics Inc. The RTD software suite is designed to control GPS reference station networks including Thales' iCGRS, which is the first direct Internet-connected reference station in the industry.

Dr. Yehuda Bock, president and CEO of Geodetics Inc., describes the heart of the system: the proprietary Epoch-by-Epoch (EBE) technology, that provides instantaneous (single-epoch) client- or server-side initialization/re-initialization with five or more satellites for a rigorous, independent network adjustment of the client's position. In conventional terms, this is equivalent to instantaneous multiply determined RTK solutions.

The RTD server centrally controls a network of GPS base stations through a full suite of data communication, processing and archiving functions. The EBE technology operates on the server to monitor (including deformation) and maintain the integrity of the base station network using Precise Instantaneous positioning. Data from multiple base stations are made available to clients through the Internet for real-time positioning with centimeter-level accuracies.

Geodetics' client-side RTD Rover application provides Precise Instantaneous Network (PIN) positioning using data from multiple reference stations and the rover receiver to create a rigorous network solution. Further, it allows extended range for multiple static/kinematic and moving clients through innovative single epoch-based treatment of ionospheric and tropospheric effects. RTD Rover is compatible with existing GPS equipment from all leading GPS hardware manufacturers, and does not require in-receiver RTK capability.

Topcon's scalable solution is explicitly designed to grow geographically as well as to provide increasing levels of functionality as the system expands.
Topcon has a multi-phased approach to this technology. Dave Young, senior product manager for Topcon's Network Products, explained that Topcon's approach is to start with the basic functionality required to connect a network of reference stations and provide users with access to raw and RINEX data for post-processing as is done in CORS-type networks. Topcon will also provide RTK correction data from each reference station in the network. The scalable solution is explicitly designed to grow geographically as well as to provide increasing levels of functionality as the system expands.

Topcon's first and second levels of network solutions include its unique use of both NAVSTAR and GLONASS satellites. Phase I consists of the TopNET CORS software package used to establish a system of one or many reference stations. This application is the central communication and data collection system that also ties reference stations to the central processing server. The data collected will be available for post-processing and can be sent to FTP sites in either raw or RINEX formats. The CORS sites will also allow users to obtain local RTK correction data using established communication means such as CDMA and GPRS (General Packet Radio Service) cell phone access, just two of several cell phone technologies. Although cellular technologies will be used, radios could still be maintained for localized broadcasting of RTK correction data from each reference station.

The second phase will utilize the infrastructure already established to communicate with the reference stations and then centralize communication, allowing users to receive RTK correction data from individual reference stations through a single communication server. The central server will still control and manage the reference stations and monitor the data provided for both RTK operation and post-processing. This additional level of functionality will augment the first phase.

Topcon's third phase includes the implementation of the models used to provide the corrected RTK data based on the data received from the network of reference stations. A unique set of RTK corrections will be created for each user in the network. These will be delivered to users as virtual reference station corrections.

Future development will likely comprise a point-to-multipoint solution that differs from the virtual reference station solution in that it computes regional models instead of a point-to-point based model. Such a solution will be suitable for one-way communication links. Topcon anticipates that product enhancement will continue as new techniques are developed and new signals in space become available.

The most advanced level of Trimble's GPS network is enhanced by a modeled network solution where data is streamed to a central control server(s) from the reference station array once per second. The control center uses the streamed reference station data to model the systematic errors in the GPS system that most greatly affect RTK performance.
Trimble has trademarked the term "VRS" for Virtual Reference Station. Jeff Hamilton, Trimble's portfolio manager for infrastructure, explained that in general terms there are three types of GPS networks. The first and simplest level is a series of dual-frequency GPS receivers and geodetic antennae located in an array that collect and store GPS measurements for post-processing purposes. In this case the intent of the user would be to perform his own post-processing. He would occupy an unknown point for a period of time and acquire raw data, download it and format it to RINEX. As an example, the user's data file could be uploaded to the NGS website where he could use the Online Positioning User Service (OPUS). NGS will process the data and return it with a corrected coordinate for the point, or the user could perform the post-processing in software supplied by the manufacturer of his GPS equipment.

Another type of GPS network performs the same functions as the simplest level system, but the receivers are connected to a central computer and GPS measurements from the receivers are sent to the central computer that generates the post-processing files. In this type of system, single base RTK corrections, similar to those mentioned by VDOT, may also be made available to the user via radio, cellular modem or wireless packet-switched communication by the central computer or the individual GPS reference stations.

The third level of GPS network is to add the additional enhancement of a modeled network solution where data is streamed to a central control server(s) from the reference station array once per second. The control center uses the streamed reference station data to model the systematic errors in the GPS system that most greatly affect RTK performance: ionosphere, troposphere and satellite orbital reporting errors. The result of this modeling is that the systematic errors that cause the ppm specifications, published by all manufacturers of RTK GPS, are eliminated or substantially reduced. According to Hamilton, in a properly designed Trimble VRS network, the RTK user can get ubiquitous RTK accuracy anywhere in the network, and at least standard single-base RTK accuracy outside the network.

Hamilton said that, in general, there are two classes of users that are interested in GPS infrastructure solutions: the network operator (perhaps a DOT customer or even a private provider offering services via subscription) and the end user. With regard to the network operator, the system might consist of the Trimble NetRS network reference station along with the Trimble Zephyr Geodetic antenna. The software that would accompany these would be the GPSNet and RTKNet applications that are installed on an operator's central server.

The end user would then use a rover such as the Trimble R8 with Survey Controller field software. He accesses and logs into the system with a username and password and can quickly get to work. Hamilton also mentioned that other manufacturers' equipment could work with Trimble's scalable GPS infrastructure software.

An Interview with an End User

Walt Sampsell, associate director of surveys for Bowman Consulting Group, discussed the end user benefits that he anticipates receiving as GPS networks for RTK corrections mature. His company has locations throughout the mid-Atlantic region and Sampsell works in the northern Virginia office. Bowman performs both private and public work, and Sampsell's area of responsibility includes the Fairfax and Prince William counties. He indicated that essentially all of his work in these counties is required to be on the NAD 83 datum. As such, his firm has typically computed the project data in this datum for performing control, boundary or stakeout operations prior to heading into the field.

Due to the work done in the office in preparing the data sets, coordinates and scale factors, Sampsell's field crews are set up to take advantage of this system. With a regional RTK GPS network, all of the data would be from NGS certified-traceable base stations, which would be more likely to yield consistent and reproducible surveyed positions.

He noted an example of how this translates into profits. When performing boundary work, the surveyor has positions for the boundaries, and the coordinates are often on the plats from the CAD system. Having real-time, corrected positional field data would allow direct location of boundary corners without the cumulative error that can tend to creep into the traditional techniques on large projects.

Sampsell also sees other user benefits to the technology. One is how it re-invigorates the personnel in the industry. Being on the leading edge of technology creates an excitement that could attract employees to the business or enhances the retention of existing staff. Another benefit is the proprietary edge that a company gains when it is on the pioneering side of technology.

Potential Fee Structures

Depending on the system and who owns or maintains it, a variety of fee structures can be developed that will allow the public to access these networks. The fees may be used to upgrade and improve functionality, and expand the networks. Governmental systems may be free or have nominal fees. Private systems may set up subscriptions for users that will have a daily or monthly charge depending on the user's access requirements. This might be in the range of a charge per month roughly equivalent to flat-rate cellular telephone bills, or if there is heavy usage, perhaps an annual fee. An alternative for the part-time user might be a nominal service charge that could get a user up and running, followed by a schedule of minute-by-minute payments similar to cell phone access.

Liability and Data Integrity

Liability and concern for data integrity will not disappear simply because we obtain our data from a centralized source. The fact that we have the potential for a regional network model of GPS data really does not seem to alter the "caveat emptor" philosophy ("buyer beware"). As with any new technology, surveyors and their clients will have to devise new methods for checking and verifying, and other quality assurance functions. It is likely that the manufacturers will do research and provide suggestions for such procedures. Although there is a growing standard acceptance of the data, scrutiny and a high level of approval should still be maintained. Sampsell indicated a strong desire that licensed surveyors manage, monitor or certify these regionally computed data sets and models.

Virtual Network Benefits

The benefits to the user community of post-processed, corrected GPS networks include:
  • Negating the need to establish temporary RTK reference stations
  • Providing the ability to use a GPS network system that creates real-time corrections that uses data observed at many stations in the network
  • Developing "local continuously operating reference sites" in combination with the creation of GPS RTK networks, which will systematically reduce and minimize errors
  • Allowing for RTK positioning at much further distances from base stations than standard techniques
  • Providing reliability, improved accuracy and faster initialization times

  • Fee structures that will allow for a strong return on investment for the user when purchasing GPS-related equipment