By Michael W. Michelsen Jr.

• What Is GPS and How Does It Work?
  
• A Matter of Accuracy
  
• GPS in Construction, Excavation, and Grading
  
• GPS in Action
  
• The Future of GPS
• Following the Fleet
  
• Thinking Globally, Grading Locally: What Is Local Positioning System?
  

The space age is finally coming down to earth. The global positioning system (GPS) has hit the world of construction in a big way. In fact, according to GPS makers, grading and excavation applications of GPS technology promise to be a boon to both industries. Further, GPS is taking a business largely run by a competitive, low-bid system and turning the focus to a technologically based design-build agenda.

What Is GPS and How Does It Work?

GPS has been in existence since the 1970s, when it was created by the Department of Defense (DOD) to allow military ships, aircraft, and ground vehicles and personnel to determine their exact location anywhere in the world, in any weather. Originally called the Navigation Satellite Time and Ranging Positioning System (NAVSTAR GPS), it is now composed of 28 satellites that blanket the globe with location-finding radio signals that can help ascertain one's location, frequently to within centimeters.

GPS satellites are positioned in space so that they orbit the Earth every 12 hours. At any given time, six to 10 satellites are typically in view from anywhere on Earth. Each GPS satellite transmits a low-power, continuous signal that can be detected and decoded by a GPS receiver. Information contained on the satellite signal enables the receiver to compute satellite coordinates and make range measurements to all satellites in view. Measurements from four or more satellites are then used to derive the receiver's three-dimensional coordinates (i.e., latitude, longitude, and height) and time.

No matter where your machines are working, GPS can provide their operators with centimeter-level accurate sitework information and keep you in the loop as well.

A Matter of Accuracy

The achievable accuracy of GPS depends on the quality of the GPS receivers being used and the techniques used to process the GPS measurements. The two greatest sources of error in the computation of the GPS position are selective availability (SA), which is an intentional degradation of the transmitted GPS signal by the DOD and the Earth's atmosphere. In stand-alone, or autonomous, mode, a user can expect to achieve a horizontal accuracy of 100 m and a vertical accuracy of 156 m 95% of the time.

The accuracy of GPS can be improved greatly by employing a technique known as differential GPS, or DGPS. A GPS receiver (termed a reference station) is placed at a known location. This receiver then makes measurements and computes corrections for all GPS satellites in view. The corrected data are then used by one or more GPS receivers (termed rovers) to remove most of the errors resulting from SA and the atmosphere. For real-time applications, a radio link is usually used to transmit the correction data from the reference station to the rovers. Typical accuracy for DGPS positions range from 0.3 to 5 m, depending on the quality of the GPS receivers, the processing technique used, the distance between the reference and the rover, and the quality of the radio link.

Even better accuracy can be achieved by employing an advanced differential processing technique known as real-time kinematic (RTK). RTK processing allows users to obtain centimeter-level accuracy of a stationary or moving GPS rover using data transmitted from a reference station. In 1992, Trimble of Sunnyvale, CA, introduced the first commercial systems with RTK processing for land surveyors. These early systems required the user to occupy a known static point for a certain period of time (the initialization) prior to beginning the kinematic portion of the survey. In 1994, Trimble removed the requirement for a static initialization period by introducing the first commercially available GPS receivers with an on-the-fly RTK (OTF) initialization capability. Receivers with RTK and OTF functionality are the engines for high-precision centimeter-level machine guidance and control systems.

GPS in Construction, Excavation, and Grading

GPS technology offers a plethora of inherent components that are advantageous to the construction industry. The advent of GPS to the construction industry came by way of breakthroughs in surveying and mapping. Surveyors were (and still are) attracted to the use of GPS because it offers accurate, timely information regardless of earthly obstacles or weather. With a GPS unit, a line of sight is no longer needed for precise positioning. GPS allows surveying and mapping calculations to be conducted by one person, freeing other crew members for other work, and eliminates inaccuracy from lost or unintentionally moved stakes. With GPS, surveyors can log and direct topographical, detail, stake-out, boundary, and local control data to the home base within minutes and can concurrently receive information on their progress.

GPS eliminates the need for constant surveying and mapping updates on the job site because the GPS data, in conjunction with computer-aided drafting (CAD) and geographic information system (GIS) technology, can keep track of a project's progress. These combinations can also be effective in initial project study and evaluation. Project progress can be assessed and monitored throughout the construction process. Mapping the location of environmental obstacles on construction areas provides necessary information for decision-makers about environmental issues for a specific project. Precision in these early stages of a project can help ensure accuracy throughout the construction process. GPS technology has improved productivity more than 100% and usually pays for itself after only a short period of use.

Do you know how beneficial it would be to locate and communicate with every piece of equipment in a fleet at any time of the day and any place in the world? Using GPS and wireless communication, contractors can acquire real-time location of a machine, historical data (where the piece of equipment was yesterday and how it performed), tracking, and monitoring information. In a GPS-equipped piece of machinery, the operator can swipe his employee card to validate operation, which will automatically transmit start-up information-such as location, mileage to date, gas level, water level, oil level, and other data-to the home base for assessment and recording. The same information is posted at the time of shutdown and can be retrieved throughout the day.

The advantages of this integral system are practically endless. Contractors can get a better estimate of an employee's billable hours and productivity rate. If the Occupational Safety & Health Administration (OSHA) shows up on the job site, fines are prevented because OSHA requirements for precheck have been documented. If at the initial startup a problem is detected at home base, a technician can either radio the foreman and communicate this potential problem or easily navigate to this equipment's field site (via GPS) to remedy the problem. Technicians and parts departments become more efficient and cost-effective with this proactive dissemination of data. And if a piece of equipment is lost or stolen, not only can it be found, it can also be retrieved.

GPS has proven its value in structural placement and bridge construction. Large structural pieces, such as bridge sections, can be placed with precision and accuracy using GPS positioning to follow strict design blueprints. Coastal and remote placement of structures do not require line-of-sight contact and can be closely monitored using continuous positioning data from the GPS constellation. Unlike traditional survey means-i.e., laser, long plumb-bobs, and theodolites-the GPS accuracy does not lessen with height, thus making it the most accurate method for controlling verticality of tall structures. Once erected, a tall structure can be monitored using a GPS-based system to compare actual position with design location, calculating any shifts or changes from external forces such as wind. Implementing GPS in these construction operations means faster project completion, reduced construction costs, and more efficient monitoring upon completion of a project.

Another successful application of GPS is groundwork construction. With an onboard GIs database, including such information as the location of existing pipelines and utility cables, trenches can be dug without any danger to existing underground networks. The new information is recorded as installed, eliminating the need for "as-built" surveying. High-precision GPS also allows piles to be placed accurately without the need for stakeout surveys. Onboard guidance computers navigate the operator to a selected pile and, using tilt meters and depth monitors, ensure the most accurate placement without the need for continuous staking. In grading operations, the installation of centimeter-accuracy guidance on-board earthmoving equipment lets the operator visualize the position of the cut-and-fill surfaces in relation to the project design. GPS provides better control and continuous monitoring capabilities, reducing the amount of time that expensive machines stand idle or the possibility of having to move material more than once.

A major bonus to advanced GPS technology is that systems have been built with ease of use in mind. Software installed in the home base essentially computes all information received directly into specific programs, eliminating inaccuracy from human error. Onboard and handheld GPS tools are easy to operate and read. The ease with which these programs can be implemented makes GPS technology not only attractive for management but for construction employees as well.

GPS technology in construction applications can increase productivity, reduce costs and downtime, and make the construction process more organized and precise. GPS is quickly becoming one of the most productive tools for construction management. Using GPS-based information systems, design surfaces and requirements can be assessed more accurately and quickly, then directly communicated to a piece of equipment. Real-time data transmissions from machines allow managers to monitor field operations and progress within minutes. Immediate assessment of equipment health and usage means an increase in productivity all around. These revolutionary technologies suggest major improvements in the construction process. Providing accurate, timely information for everyone involved in the construction process, GPS technology is contributing to the ultimate construction goal of lower costs, higher production, and greater accuracy.

GPS in Action

Mike Hoselton, an estimating manager at L.H. Lacy Company in Dallas, TX, describes the difference between his work before and after GPS as "the difference between daylight and dark."

He should know. A many-year L.H. Lacy veteran, Hoselton is responsible for projects costing from a few thousand to several million dollars. In business since 1919, Lacy's projects have been built for municipal, state, and federal governments, as well as for private industry. The company was responsible for, among other projects, the construction of the Nolan Ryan Expressway.

"Grading before GPS involved a lot of surveying and staking," Hoselton explains. "As a grader got closer and closer to the correct grade, there had to be grade checkers pulling off and showing the operators when they were getting close. And a smaller contractor doesn't even have this luxury."

According to Hoselton, smaller contractors without grade checkers would be forced to use operators who perform their own grade-checking functions. "GPS for a small contractor would allow elimination of all the tools and work involved for the operator. All those tools are incorporated in the SiteVision equipment. The operator never has to leave the machine to determine whether he's getting close or has achieved the grade he needs."

SiteVision GPS is an earthmoving grade-control system that allows operators to "see" design surfaces, grades, and alignments from inside the cab. Machine position versus site plan is displayed on a color monitor mounted inside the cab in front of the operator, providing quick location of embankments and pad corners. Light bars mounted in the operator's field of view guide operators to cut or fill along alignments. Road alignments and cul-de-sacs are located and defined with minimal assistance from grade setters or stakes.

By placing the site plan in the cab, SiteVision GPS enables operators to bring the site up to consistent and accurate grade that makes finishing faster, more efficient, and more predictable. Machine operators find SiteVision GPS very easy to learn and use because it is simple and intuitive.

"At the beginning of the shift, the foreman transfers the project data from the engineer's CAD files to a SiteVision unit that is on a V6 Cat dozer," Hoselton describes. "These data are only in 2D, so all contour lines are set at zero. Fortunately, we can remedy this problem by calibrating the elevation data from our survey tools. This gives us a 3D image of the control points on the job we're working on. All of these data are put into the SiteVision unit on a PC card, then the work begins."

In actual operation, the display in front of the operator shows a map with the machine position on it, while the light bars indicate how much the left and right corners of the cutting edge should be moved relative to the design alignment. With a single key press, the operator can switch between the plan-view map and the blade-orientation display. The blade-orientation display shows a picture of the blade position relative to the planned design shown.

"It's almost like a combination of grading and connecting the dots," notes Hoselton. "The SiteVision always shows the operator where he's at as well as how close he is to where he needs to be on alignment."

L.H. Lacy is currently utilizing the system on a project in North Lakes, TX. In addition to roadwork, the company is grading for more than 500 housing units.

"In a lot of ways, GPS-and SiteVision in particular-is going to eliminate laser beams," states Hoselton. "SiteVision does everything laser beams do and more. And there's no cutoff. If I'm depending on a laser and another machine passes in front of me, I'm cut off until I can reestablish the contact. GPS works from the sky, which eliminates this problem. GPS will also follow a flat grade or it will follow a sloping grade. Our North Lakes project is a good example of this application. There are two-to-one grades and three-to-one grades. The SiteVision product is working just fantastic. It does it all for us. We're looking forward to the day when we can put it on all of our machines. For example, we could use a scraper to haul dirt while the dozer is keeping everything in line. The only change I can foresee is the need to change the icon on the screen from a dozer to a maintainer. They can all run off the same base station, which is smart and economical. Another thing that's important to us is interoperability. All of our GPS equipment works together, regardless of what we want it to do. The limits of our GPS are never the reason we are unable to do something. In fact, our GPS allows us to do more things more efficiently, economically, and accurately than ever before. We have taken our dozer and parked it on the ground since starting it up in the morning. We went up to the front of the machine with our survey rover and took a reading at the cutting edge of the dozer. The x, y, and z were within two-hundredths on both sides of the cutting edge, from what the computer in the machine told us the coordinates were compared to what we said they were. The accuracy level of this equipment is phenomenal."

Hoselton selected Trimble equipment, citing reputation of the company. "It's the biggest and the best, and accuracy of the equipment is only getting better. And the local distributor, McCuley Instruments [in Dallas], is always there when we need them."

The Future of GPS

The current design/build trend in the industry requires a continuous focus on the engineering aspects of the construction process. GPS technology is instrumental in harmonizing the design and build to occur simultaneously. By curbing time spent on data collection and dissemination, GPS allows managerial, technical, and operational teams to concentrate on their specific job requirements. In an age where efficiency and quality are tantamount to the overall construction outcome, GPS provides the technical foundation on which to build a successful construction business. Being a part of this revolutionary trend is an opportunity that construction businesses should not pass up.

A John Deere product scheduled for market release in May 2000 incorporates both GPS and cellular technology to help owners and operators of large fleets-such as contractors and rental agencies-track every piece of their equipment over the Internet. Designed specifically for construction equipment, DeereTrax tells owners where each item is and how long it's been operating. Owners can have each piece of equipment report in at scheduled intervals-i.e., once a day or some specified operational interval-and record its location and engine hours on a secure server, which the owner can access through the Internet with any Web browser. Owners can also poll equipment in real time to find out up-to-the-minute locations. Knowing exactly how many hours the machine has been in operation helps to schedule preventive maintenance. Through a feature called Integration Manager, owners can download the data to preventive-maintenance-scheduling software packages. "When you look at preventive maintenance as a means of lowering the overall total cost of ownership, our system can be one of the tools to help people manage that aspect of their business much better," remarks Tom Budan, DeereTrax's manager of construction. The DeereTrax hardware comes as a kit-a communication box, a cellular antenna, a GPS antenna, and wiring harnesses to hook it all together-and uses any 12- or 24-volt alternator as a power source. DeereTrax works on any brand and type of equipment from backhoes to skid-steer loaders to excavators and can be installed in as little as an hour. Along with the hardware, customers purchase a two-, three-, or four-year service contract that allows them to "contact" each piece of equipment 35 times a month through regularly scheduled reporting, real-time polling, or a combination of both. Customers will be able to allow equipment dealers access to the information from the fleet; based on the hours of operation, a dealer who has a contract to perform preventive maintenance on the equipment would know exactly when to schedule it. Although the cellular technology currently limits the system to North America, other parts of the world having different cellular standards will also be examined. Other future capabilities? "Being able to do diagnostics from remote locations over the computer is an area we need to look at," notes Budan. He adds that DeereTrax might also eventually be able to send an alarm if the machine's overheating or experiences some sort of malfunction.

GX
March / April 2000