Real-Time Kinematic (RTK) GPS technology provides a productive and accurate method of establishing three-dimensional positioning in a wide variety of surveying and mapping applications.
First, a few basics. The conventional approach to RTK requires a base station as well as a rover to accurately determine new positions. A base station is set up at the project site over a known point. This base station includes a radio link that is used to establish communication with a GPS rover. After initialization, the rover is used to accurately establish positions over new points in real time with just a few seconds of GPS data collection.
While RTK is very productive in the conventional application, it requires a significant capital investment in equipment upfront: the base station, rover with a data collector, radio links between the two and appropriate software. Most project sites also require the use of additional personnel to guard the base, to protect it from being stolen, or otherwise moved or interrupted in any way during the survey. This is an inefficient use of trained surveying personnel that significantly increases the cost of the field data collection.
Technology now provides us with new options for RTK surveying in the form of a reference station network, also known as a Virtual Reference Station (VRS) network. An initialization and constant communication with the rover is still required as with any RTK surveying, but cell phone technology has replaced the radio for this Internet-based communication, and the network eliminates the need for a local base station at the project site. With reference station network technology, a virtual base is created at the project site without the capital investment required for a physical base or the labor costs for someone to monitor the unit onsite.
A reference station network is created by installing multiple fixed base stations in a given area much like any CORS application. GPS data is continuously collected at each of these fixed base stations and this data is transmitted via broadband communication over the Internet to a central server that quickly determines a precise model for atmospheric (ionosphere and troposphere) and ephemeris corrections covering the entire network. A surveyor using a rover working within the network can tie into the central server via cell phone technology with a username and password; conduct a quick initialization (typically ranging from a few seconds to less than a minute), in effect generating a virtual reference station right next to the rover; and collect points within the vicinity of that reference station much like more conventional RTK applications.
The benefits of reference station network technology are many. One major benefit, of course, is a large reduction in equipment costs. Eliminating the need for a base station and radio communication (which can be somewhat problematic in certain project conditions) at the jobsite is significant. Moreover, the effort that can be required to establish known points at the project site in areas without published monuments can also be eliminated. Secondly, by removing the need for field personnel to watch over the base station, the field effort for most projects is cut in half. And finally, equipment setup and initialization at the jobsite is greatly simplified.
Public Sector UseJoseph Scolari III, PLS, chief of survey for the Philadelphia District of the U.S. Army Corps of Engineers recognized the benefits of a RTK network early on. The Corps began with the concept and planning for its system in 2001 and installed the first base station two years ago. It now has 12 stations online in the network. “We recognized it would be a great way to improve efficiency and cost effectiveness for the things that we do in surveys,” Scolari states. “The system has proven to do just that. But the benefits go well beyond cost savings; accuracy is also a major benefit gained from our VRS. We have found discrepancies in control set by some of our previous surveying contractors using the VRS. The surveying methods enabled by our system ensure accurate positioning on a consistent datum.” The Corps surveyors often perform accuracy checks with their VRS and consistently find three-dimensional differences on published control monuments of a tenth of a foot or better.
The Philadelphia Corps has used its VRS in many different applications. “We have used the system for pre- and post-storm beach monitoring, for photo control, in some limited applications of hydrographic surveying and to verify construction quantities submitted by contractors,” Scolari notes. “While we have not yet used the system for the reduction of airborne GPS data acquired during photo acquisition for mapping projects, we anticipate this use in the future.”
The 12 stations operated by the Philadelphia District are not yet part of the NGS CORS network but will become federal cooperative stations before long. To avoid competition with the private sector, the VRS is available only for Philadelphia Corps projects and no other commercial applications.
A Cooperative ApproachWestern Data Systems, a GPS equipment rental specialist with offices in Texas and Oklahoma, announced an interesting approach to a VRS in January 2004 and installed the first base station soon thereafter. Known today as the Texas RTK Cooperative Network, this concept has now grown to a total network of 54 base stations, with six more slated to come online very soon. The surveying community has realized the advantages and the network currently has more than 200 users. This coverage area blankets much of the state of Texas, and the growth is now extending into Oklahoma.
As a cooperative, it is owned by the surveying community for the benefit of the surveying community, and operated at cost by Western Data Systems. Michael Hefer, president of Western Data Systems, shared his firm’s thought process at the beginning of the network implementation. “We knew networks would be dominating down the road and wanted to bring this powerful technology to the Texas surveying community,” he says. “The big advantages include increased productivity, less upfront capital cost since project base stations are not required, and the ability to share data between the field and office without leaving the project site. The accuracy is very good, similar to any other RTK application.” Western Data Systems was sure the network would be a success, and initially put up more than $300,000 for the hardware, software and bandwidth to get the concept off the ground.
There are two ways to take advantage of the cooperative RTK network. A firm can host a new base station for the network and in doing so receive five memberships at no cost, along with free renewals for those five memberships for the next three years. A membership is defined as a single rover. For those who do not host a base station in the network, the initial cost of a membership is $2,000 with an annual renewal fee of $500. These costs are relatively insignificant when compared to the additional equipment and personnel costs required in the conventional application of RTK surveys.
Western Data Systems also has a large collection of rental equipment. The company rents Trimble receivers that include the rover, data collector and a GSM chip that provides the cell phone technology necessary for the Internet connection that enables the service. Users then have all the technology at their fingertips to make use of the Texas network.
Private Sector UseBrian Wright, RPLS, is survey manager for Bohannan Huston Inc.’s Garland, Texas office. The firm has used a VRS network established by the Dallas District of the Texas Department of Transportation on multiple DOT projects, and the VRS services of Western Data Systems for commercial projects. “We have used the VRS network for a variety of uses from right of way mapping, photo control, in boundary surveys, some construction staking, and for setting project control that is subsequently used by conventional surveys,” Wright says. To maximize accuracy, Bohannan typically works within six miles of its virtual reference point after initialization and reinitializes if the project calls for the crews to move outside of that range.
“For most projects, we prefer to use a two-person crew for safety reasons,” Wright says. “But instead of tying up one of our receivers as a base station and losing the productivity of one of the crew members to simply guard the base, we are able to double our productivity by having both members collect data with two rovers. That productivity gain is significant. The VRS has made us more efficient in both the field and the office, thereby reducing costs to our clients.”
Reference Station Network Benefits to ConsiderThere are also ancillary benefits that come along with the establishment of the fixed base stations required for the reference station network. For example, the base stations operated by the Dallas District of the Texas Department of Transportation are also a part of the national CORS network. As such, they are very valuable for applications outside of the VRS network. As an example, these stations are currently being used to control aerial flights over the district to provide new high-resolution orthophoto updates. The cost savings to the DOT on this significant project alone is measured in the tens of thousands of dollars. Similarly, the Philadelphia District of the Corps uses its base stations to control static and rapid static GPS networks.
Technology has given us significant added surveying capabilities over the last few years. The application of reference station network technology in RTK surveying is a great example. The gains in productivity, reliability and the associated decrease in project cost are considerable. It is something you should evaluate for your professional practice if you haven’t put it in place yet.