Surveyors Dial Up for Accuracy

July 23, 2002
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Combining GPS reference stations and cellular phones offers great benefits.



“Working with GPS remains a challenge. Sometimes it does miracles for you, and sometimes it leaves you thinking and waiting for the right combination of satellites to show up,” says Jan Noppen, GPS Department Manager for Ford Engineering Inc. (FEI) in San Antonio, Texas.

It doesn’t have to be this way. Improving GPS-driven field survey productivity depends on one thing—the ready availability of a consistent control network. Without it, conventional GPS surveys can include tedious research to find existing monuments, regularly interrupted communication signals and often duplication of effort.

The answer, for resourceful companies like FEI, appeared in the form of two relatively new technologies—the cellular phone and a privately owned base reference station. This past year, FEI in coordination with Leica Geosystems (Norcross, Ga.), combined the flexibility of a cell phone/GPS communication system with the reliability of a “universal” base reference station.

The combination has provided FEI with unprecedented 24/7 GPS control. Through this network, an FEI surveyor in the San Antonio area can, at anytime and anywhere, simply dial into the reference station to receive RTK corrections at real state plane coordinates. No need to set additional control. No interrupted signals.

Currently, FEI surveyors use GPS RTK techniques via a cell phone connection for 95 percent of their GPS survey activities. “More than that, it’s opened new doors of opportunity for us to grow our business,” Noppen says. “We’ve been able to speed field data gathering, maximize RTK GPS techniques and even market the service to other surveyors in the area.”

Cell phone-equipped rovers "communicate" with FEI's Cooperative CORS station to collect shots quicker.

In Search of Stability

FEI is a 23-year-old surveying and engineering firm with clients that range from the Texas Department of Transportation to local public and private entities in south Texas. The firm has increasingly relied on GPS to support its activities and the ongoing need for accessible control. Typically, FEI relied on monuments established by federal and state authorities. As in other fast-growing urban areas, these monuments are increasingly hard to find and use.

In search of improved customer service, quality, accuracy, better security and stability, FEI investigated GPS reference stations. First introduced in the late 1990s, reference stations are typically used by large public and private entities as a point-to-point real-time method of extending control for urban development. Academic and scientific organizations also rely on these large GPS networks to study such things from plate tectonics to tracking rodent habitats for disease control.

Today, advanced reference stations perform a dual function. Besides extending control by static observations, they broadcast an RTK signal, providing continuous high accuracy position data to multiple users such as surveyors, public works agencies and utilities.

For FEI, it seemed a wise investment that could support its client base today—and create the opportunity for unforeseen services in the future. Just one station has the ability to eliminate the need for extensive physical control point monumentation, averts radio modem frequency congestion and provides data positioning in more reliable state plane coordinates.

In April 2001, FEI installed an RS500 Reference Station system by Leica Geosystems on a 9’ pillar on the roof of its office building in north San Antonio. The station includes a choke ring antenna designed by the Jet Propulsion Laboratory providing 0.5cm+0.5ppm in accuracy for static observations, a dual-frequency GPS receiver (Leica SR530) and communication equipment to accommodate an unlimited number of modems to support multiple landlines for cell phone dial-ins.

Continuously operating from this permanent and stable platform, the station tracks visible GPS satellites, then receives, stores and even transmits RTK/DGPS positions in WGS84 geodetic coordinates to local GPS users in RTK mode at 1cm+1ppm within the entire city of San Antonio.

During the initial RTK testing session from FEI’s permanent base station to a first order National Geodetic Survey (NGS) mark (HERNDON 1903) approximately 30 kilometers away, surveyors achieved position data within 0.05 feet and 0.11 feet in the orthometric height after averaging 26 consecutive RTK observations, taken every 30 seconds.

Soon after, FEI registered the station with the NGS and was approved for the Cooperative CORS (Continuously Operating Reference Stations) network program, which meets the high accuracy NGS standards and specifications for GPS surveying. As a requirement for being a Cooperative CORS station, the RINEX (Receiver INdependent EXchange) data for post-processing static GPS data is available free of charge on the FEI website every hour on the hour.

Looking to eliminate the conventional and cumbersome radio modem connection used for GPS RTK surveys, FEI took the system one step further, and extended the availability of the RTK corrections to a new wireless communication medium—namely a cell phone.

Just A Phone Call Away

For RTK operations, once a base reference station has been established, surveyors typically rely on a roving GPS receiver linked to a standard two-watt radio modem, which broadcasts a signal within a two-mile perimeter range. Most surveyors, however, require a wider perimeter of operation and acquire an amplifier that allows them to extend the signal to an operating range of about a 10-mile radius. To do that the operator needs a license from the FCC.

As most GPS users know, the FCC grants the licensed operator a range of frequencies mostly used by the police department, the fire department and/or other emergency services. When they “squawk” over the radio, the GPS packet is interrupted and ends up on the bottom of the priority list until the frequency is “clear” again. Great for community safety, disastrous for GPS operators.

Noppen recalls one job when he had to return to the base five times to change the base frequency and the rover to obtain clear communication and working production. “In the end, I bought a scanner to figure out which one of our 15 frequencies I received from FCC was free of traffic so I could go to work on any given day,” he says.

Cellular phone technology offers a speedier, reliable alternative. “When single cellular communication became available between one GPS base and one rover (only) in the field, it not only eliminated frequency interruption,” Noppen explains, “it extended my operating range three times further than the radio modem. It was also less expensive to operate, lighter to carry and more dependable to work within an entire metroplex and still maintain survey grade accuracy (1cm+1ppm).” Working again with Leica Geosystems, FEI introduced their cell phone-capable GPS receiver.

Once in the field, a surveyor turns on the receiver prompting it to dial the dedicated phone number at the base reference station. Within 10 seconds from satellite lock, the RS500 system resolves ambiguities at a distance up to 50 kilometers. “At 1cm+1ppm, we can guarantee our client 0.15 ft of accuracy within 30 kilometers from the base reference, which encompasses the entire metroplex of the city of San Antonio,” Noppen says.

The rover receiver can also be configured to immediately produce state plane coordinates and surface elevations, which cuts back on headaches of “calibrating” or “transforming” local coordinates to do a job with RTK. Noppen demonstrated the system’s accuracy and flexibility for the survey of the city of San Antonio’s newest Northeast Vehicle Maintenance Service Center. The coordinates from a computer-aided design program were input into the GPS rover with a cell phone. When “staking out” or navigating to a known coordinate, the GPS receiver adjusts for the distance and elevation referenced either from the north, from the sun, or from a known point or a line to the desired point. It continually matriculates to tell the observer where to go. Once within the predetermined range, it starts to beep.

When the rover hits the right spot, a tiny cross appears on the terminal screen. In addition, the cellular connection makes elevation information readily available. On the construction site the FEI team used the GPS to stake out building corners and then followed up with a total station to verify the data. The resulting square and diagonal distances did not exceed 0.01 ft from the projected design. Such desired accuracies can only be obtained with optimum satellite availability, which must be planned ahead of time.

“Bottom line,” Noppen emphasizes, “we can produce results faster and better than any traditional way of doing RTK, cut cost and be productive like we were never able to be before.”

This is clearly demonstrated on a recent project where FEI prepared a topographic profile on a 73-acre flat and open tract of land. “We rigged a frame attached perpendicularly to one of our survey vehicles to position the GPS rover receiver at a designated known height above the ground, and programmed the receiver to record a position and elevation automatically every 50 ft,” Noppen recalls. While driving the vehicle back and forth over the entire tract of land, the rover automatically collected a position and surface elevation every 50 ft. “Because each point collected was sounded by a beep, we also knew exactly when to make the next turn at each end of the tract,” he adds.

In the end, the receiver collected over 1,200 points and FEI finished the job in 3.5 hours. Conventionally, such jobs would require more than one survey team over a period of several days to accomplish such a task. In the future, FEI anticipates performing similar jobs by rigging the GPS rover on an ATV.

FEI’s GPS teams have also capitalized on the dual port capabilities available through the Leica SR530 receiver. Through this functionality, one port can be configured to receive a signal from a base station in the field with a radio modem in one project area while the other port can be set up to receive a signal from the continuously operating base station with the cell phone. The signal switch can be performed at the rover end without having to return to the base station. This option offers the user a crosstie position and elevation in RTK mode for control setting, verification and redundancy while increasing productivity.

Once this Leica system is set up and dialed up, productivity is better than traditional RTK processes.

An Expanding Business Opportunity

Now, FEI is offering its cellular-based lease agreement program to surveying firms, such as Sightline Surveying Inc., a local San Antonio-based surveying firm.

At the present time, any customer with a cell phone-capable receiver can dial into FEI’s base reference station for a monthly fee and receive RTK corrections. “The greatest benefit to surveyors—both in and out of FEI—is the ability to get to work immediately,” Noppen adds. “They don’t have to operate, maintain and safeguard a base reference if they are working in the San Antonio metroplex.”

Because the FEI reference station is NGS and CORS approved, clients get reliable coordinates. The signal is provided by the digital cell phone provider and captured by the local telephone company, which provides the dedicated phone land line number. FEI offers the first month of service for free to help defray the initial cost of connection for the purchase of the cell phone and a provider plan of their choice. Once the customer is set up, the user fee is accommodated on a monthly basis or contracted by project length.

Rick Shelley of Sightline Surveying Inc., the first subscriber to the FEI GPS network, says the greatest benefit is the availability of a permanent, known base station. “In the past, the search for monuments has been tedious at best, requiring many hours of Internet searches,” Shelley states. “With the dedicated phone connection, you can get out of the truck and go to work.” Shelley says he’s found additional savings in the dedicated access line, as opposed to utilizing the equipment independently.

For FEI, this RTK GPS offers the user or client consistent coordinates in an environment that is the same for everyone. “Neither the company nor I have any guarantee that this investment is going to take off as a service,” Noppen says. “It’s a lot of time and money invested, but in the interest of the client we are providing the best GPS service a client can hope to receive (for the money).”

A traditional RTK survey with a base/rover is estimated to cost approximately $90,000 annually to operate 200 days a year. It’s a reoccurring annual cost that meets the needs of one user or group. A GPS reference station, including hardware and antenna installation, has a one-time cost of about $25,000 and serves hundreds of users and applications.

Once operational, the fully automated system runs 365 days a year, 24 hours a day regardless of weather conditions. When costs are broken down, an RTK Cellular Lease Program can be less than the expense involved in purchasing the roving equipment, hiring a crew and inputting the labor hours to process the information. It also eliminates logistical deployment, security, setup and support of base reference in city limits such as amplifiers, power supplies, antenna poles, etc.

“Internally, it’s a sound business investment that has significantly improved our field productivity, thus helping our bottom line,” Noppen concludes. “We’ve reduced operating expenses by nearly 30 percent and increased production by better than 15 percent.

“Beyond our doors, we see it as a venture in the making.”

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