- SPECIAL REPORTS
- THE MAGAZINE
In an effort to create a GPS use network consistent across jurisdictional lines, Washoe County in northwestern Nevada implemented a regional GPS reference network in 1998. Today, with the advancement of a wide-area setup for RTK solutions, the county is successfully providing precise positioning using a new class of instantaneous, real-time GPS positioning algorithms. The technology allows users to do real-time GPS positioning (at distances up to 24 miles) from a base station with existing wireless services. With this network, the county displays a progressive nature in implementing the solutions of tomorrow-solutions that eliminate the need for an RTK GPS base station in many projects.
Establishing the SystemIn 1998, Washoe County, in conjunction with the Regional Base Map Committee for the counties of Sparks and Washoe and the city of Reno, established five regional GPS base stations, adding two more stations in 2004-and still more planned for later expansion of the network. This initiative was in response to the aftermath of severe flooding of the Truckee River the previous year. Administrators realized they couldn't provide disaster assistance agencies such as FEMA with maps of their respective jurisdictions or with maps that neatly aligned with neighboring jurisdictions. They further realized their profound problem could only be solved through collaboration under an inter-local agreement.
The Washoe County Public Works Department has been using the regional base station network for survey and photogrammetric control; GIS inventory of existing utilities and structures; aerial photogrammetry checks; and to prepare maps to support precise geospatial projects such as topographic surveys, and survey and subdivision plat checks. With the network solution, the integrity of static and real-time kinematic GPS positioning across county lines is not compromised because the data is computed in one system and resides in a single database. Further, the Washoe County Public Works and GIS Departments have developed a highly accurate GIS database by using the regional base stations for map control and feature positioning.
Improving the SystemThe Washoe County Real-Time Network (WCRTN) project began in 2003 when Jack Holmes, PLS, surveyor for Washoe county, Gary Beekman, GIS coordinator for Washoe County, and Victor Erickson, PLS, field survey party chief for Washoe County, saw a need to create or to improve the regional GPS network by adding the capability for RTK surveying. This improvement now allows surveying crews to position themselves with long baselines, using RTK survey methods, at centimeter level accuracy. Further, this added capability makes the expanded region's GIS databases homogeneous with significantly reduced field data collection costs.
The WCRTN currently covers approximately 1,200 square miles in northwestern Nevada inhabited by a population of about 400,000. The seven continuously operating GPS reference stations located in Washoe and Lyon counties and Carson City provide the corrections for the region. Expanded area coverage is planned as more base stations are added in Churchill, Lyon, Mineral and Pershing counties in the near future. All sites sample data at a 1 Hz interval (every second) and the raw data are streamed in real-time to the GPS network server located in the Washoe County Complex in Reno, Nev. This central repository provides the regional surveying and engineering firms with one location for information that they can use for real-time surveying.
The WCRTN utilizes Geodetics Inc. Real-Time Dynamic (RTD) software with Epoch-by-Epoch (EBE) technology to control the real-time flow of data from each of the network stations, to monitor the integrity of the reference network, and to serve correction data to users via TCP/IP. The WCRTN utilizes six dual-frequency Trimble 4700 receivers with Trimble micro-centered L1/L2 antennas with ground planes and one Trimble NET RS GPS receiver with Zephyr geodetic antenna.
RTD software operates on the GPS network server to monitor (and maintain) the integrity of the base station network using Precise Instantaneous (ÏGPS) positioning. This monitoring activity can also detect earth crust deformations (such as during earthquakes) or movement of base station antenna locations. Data from WCRTN is made available to clients through the Internet for real-time positioning with survey-grade accuracies.
Geodetics' open platform RTD software system is a fully automated server-client application designed for real-time, high-precision GPS sensor tracking. The heart of the system is Geodetics' Epoch-by-Epoch (EBE) technology. It provides single-epoch client and/or server-side initialization/re-initialization with five or more satellites for a rigorous independent network adjustment of the client's position. This is equivalent to multiply determined RTK solutions with single-epoch initialization and re-initialization.
For the land surveyor, the WCRTN sites are extremely stable and provide a quality geodetic control reference frame. Use of Geodetics' client-side RTD rover application provides land surveyors with Precise Instantaneous Network (PIN) positioning in the field, utilizing data from multiple reference stations in the network and the rover receiver to create a network solution at the client. The RTD rover provides single-epoch-based treatment of ionospheric and tropospheric effects. Like the WCRTN server, the client-side RTD rover is platform-independent and is compatible with existing GPS equipment from all leading GPS hardware manufacturers, and does not require in-receiver RTK capability, thereby enhancing legacy equipment.
Benefits of the SystemBenefits of the network include cost and time savings for both static and dynamic surveying. For static surveying, the regional base station sites have become Washoe County's primary GPS control network. Because each site is a control station continuously collecting GPS data, these sites can take the place of having to find and make GPS observations on local control stations. Using WCRTN requires less personnel and equipment, which can save money and time. Also, GPS RINEX and DAT data files are generated and can be downloaded at www.co.washoe.nv.us/pubworks/ gps_basestations.htm.
A primary benefit obtained using WCRTN sites for RTK surveying is that they can replace temporary RTK base stations set up by surveyors for RTK projects. This simplifies RTK surveying in Washoe and adjacent counties. No longer do users have to find a GPS control point, set up a GPS receiver and radio to operate as a base station. This method requires only one GPS receiver-the rover-reducing the additional cost of a GPS receiver and radio equipment along with survey crew time for site setup. It also removes the potential of having incorrect values entered for the base station site, or the additional equipment being damaged or stolen. Radio line of sight from base to rover is no longer an issue. The drawback, depending on the cellular carrier, is poor wireless network coverage.
Testing the SystemWashoe County tested Geodetics software using a Trimble 4800 receiver connected to a Juniper Allegro data logger running Geodetics' RTD rover and Carlson's SurvCE. Internet connectivity to the server was obtained with the Verizon Wireless Aircard 555. Surveys were conducted along U.S. Highway 50 between Carson City and Silver Springs, Nev. Horizontal positions tied with long baselines of up to 25 miles were found to be within 0.1 ft (3 cm) of positions determined with static GPS. Similar results were obtained at Incline Village, Nev. in the Lake Tahoe Basin. These results are well within the vendor's specifications for 1s horizontal positioning of Â±(10 mm + 0.2 mm/km).
Operational problems encountered during testing were predictable, including an inability to properly position due to canopy or lack of open horizon and the occasional unavailability of data modem coverage by the individual cellular providers. The former is a problem encountered with all precise GPS; the latter makes it important to look at each cellular provider's coverage. It might be necessary to have more than one provider in certain areas to ensure data coverage. Washoe County has employed two cellular carriers.
To utilize the WCRTN system, a survey-grade, dual-frequency GPS receiver must be used. WCRTN supports an unlimited number of users with access to the host server database from all the base stations. The rover must have a wireless data modem to receive the base station data, which is transmitted in RTCM 2.2, 2.3, CMR and CMR+ formats. Each WCRTN site/format is assigned an IP address and a unique port number.
"For the first time, surveyors, engineers, GIS professionals, environmentalists, structural and safety engineers, geologists, seismologists, emergency services personnel, and other users can determine network-based geodetic-quality GPS positions on an Epoch-by-Epoch basis," says Jack Holmes. "It's like going from horse and buggy to a whole new era."
Manufacturer Information:Geodetics Inc.: www.geodetics.com
Juniper Systems Inc.: www.junipersys.com
Carlson Software: www.carlsonsw.com