Keeping a watch on the entire U.S. coastline is a job of high order, one that defense and aerospace leader Raytheon has been overseeing since 1993 through its Relocatable Over-the-Horizon Radar (ROTHR) sites. Three continuously operating sites located in Texas, Virginia and Puerto Rico are staged to provide the country and surrounding areas with homeland security. The radar stations detect surface and air traffic within 500 miles inland and 1,500 miles off the coastline, and report to tactical forces defending our nation's interests and countering drug smuggling in the Caribbean Sea and South America.
As part of the complex system in Fort Allen, Puerto Rico, a network of 500 antennae over 6,000 feet was constructed in 1999 to receive radar signals reflected by incoming traffic that are emitted from a transmitting site about 100 miles away on an island. While sophisticated, it was determined some years later that the data collected by this system was not accurate due to lack of exactness in the coordinates of the antennae positions. It was believed that the antennae were misaligned when installed and that perhaps the masts on which they are mounted were sinking into the soil. Consequently, results from the radar reception signals had substantial noise data and contained only relative data rather than real data. This caused the positions of targets tracked by the radar to not have absolute positioning accuracy.
To support the purpose of the ROTHR site, a survey was required to establish and verify a single mathematical vertical base line; with it, each antenna's position could be compared to the base line to verify that it was no more than one inch out of alignment from its design location. Each antenna, which is about the size of a flag pole, was measured from its vertical center to determine if any variation existed within a six-month period. For this, the exact center locations of the antennae were needed. The U.S. Navy contracted Precision Measurements Inc. (PMI) of Virginia Beach, Va., in April 2004 to determine the exact center locations in X,Y,Z coordinates of each of the antennae for establishment of a new base line. PMI provides a full spectrum of land surveying services including hydrographic surveying, aerial mapping control, GPS, GIS and 3D laser scanning. It was this last technology that drove the ROTHR project.
"Previous surveys were done conventionally with a theodolite and levels to determine the center of each antenna," says Alfred Amago, project manager and 3D scanning department head at PMI. "We employed 3D laser scanning technology because we could define the exact center location of each antenna." The antenna position is defined as the center of the mast (in X and Y) and at the top of the insulator (in Z). The foundation position is defined as the top of the center of the concrete pile cap on which the four threaded anchor bolts were placed. The previous surveys conducted conventionally located each antenna's center by determining the X,Y,Z coordinates of the pier caps and then projected upward the height of the pole. This method was determined to be less accurate because single measurements of the anchor bolts on the pier caps were made from different locations, sometimes the northwest side, sometimes the southeast side, etc. Couple that with the fact that many of the poles were not vertical and the accuracy of the radar signals suffered. The scanning technology allowed Amago to model each antenna by putting a vertex on each one; after modeling the pipe size, he was able to get the real centers of each antenna.
"By employing 3D scanning technology, we were able to get the accuracies to the nearest 0.01 foot," says Ken Leitz, PMI's project manager and Amago's partner on the project. "The survey had to account for the earth's curvature by means of correction factors to form a line that is tangential in a position at the center of the array. Utilizing our Leica HDS2500 3D scanning system [Leica Geosystems HDS, San Ramon, Calif.] was the only way to capture the level of detail and accuracy we required in the allotted time frame." Leitz says they started at the center and computed the vertical correction to be 4.5 inches at the extreme ends, to the left and to the right, compensating for nine inches of earth curvature (see figure on facing page).
Benefits On SiteThe PMI crew was given two weeks to complete the project of determining the alignment of 500 antennae. As PMI was mindful of cost, they determined that the project could be undertaken within the time requested with a two-person crew. The high accuracy of the scanning technology supported this decision.
"We ran cost comparisons for this project [and] the scanner was about half the [cost of] conventional methods and provided much more detail in the final product," Leitz says.
The work site was blanketed with wire mesh on the ground plane of the antennae array, which received reflecting signals. This mesh, however, could endanger Amago and Leitz during an electrical storm, of which there are many in the area. At least twice, the two had to evacuate the area and return following the storms. This often meant working in the evenings; luckily, the scanning technology allowed the team to collect the data from a safe distance and in the darkness.
On TargetThe team began the project by setting survey spikes as traverse points on the ground for use as temporary targets for the scanner. The spikes were set out at every couple of antennae pair for a tight survey, Leitz reports. The temporary targets were all flagged and labeled for uniformity and clarity, and placed on both the north and south side of each antenna about every 25 feet. GPS surveys using dual-frequency Ashtech (Thales, Santa Clara, Calif.) Z-Extreme GPS receivers were employed to establish horizontal control on the 3D monuments. Using the Leica HDS2500 scanner, the team was able to acquire the locations of the temporary targets, which later enabled them to pull all the various scans together during global registration. They then used the GPS-derived data to georeference all the various scan clouds. They next employed a high-precision level run. Backsight and foresight distances were kept equal to eliminate refraction and earth curvature effect. The earth curvature correction was applied and elevation values adjusted accordingly. The team corrected the vertical values of additional scans to be on a plane instead of an arc. Utilizing the modeling features within the Leica Geosystems HDS Cyclone software, they derived the center locations of the antennae as well as the base elevations.
Complications and ObstaclesAs with most every project, the PMI team experienced some setbacks. As the temperature rose in the hot sun of Puerto Rico, the Leica HDS2500 scanning equipment stopped working. "The oppressive heat (well over 100 degrees and with the heat index, 120 degrees) combined with direct sunlight exposure proved to be too much for our HDS2500 scanner," Leitz says. "From 7 a.m. to 8 p.m. there wasn't much that could be done with the scanner due to the heat. During this downtime we performed level runs and had to wait until after dusk to scan again. Our experience with our HDS3000 [Leica's next generation following the HDS2500] is that we have been able to use it effectively even in excessively hot conditions. Our HDS3000 would have solved the heat problem we encountered and allowed us to work throughout the day rather than at night."
The rain caused equal strain on the crew. "[The rain] plagued us with wet muddy grounds that sank the target heights," Amago says. "To prevent the scan head from sinking, we bought three 4-inch PVC caps, which were placed under the tripod legs. This solved our problem."
Technology Aids the Nation"The 3D scanner has really revolutionized the [profession]," Amago says. "This task has been a privilege as well as a challenge because it is the first time we have utilized this new technology [on such a project] to establish antenna locations with the highest accuracy."
In the end, PMI was able to deliver the X,Y,Z coordinates and graphical charts on time to the Navy. Had it not been for the new technology of the 3D scanner, the firm might not have been able to finish the project in a mere 14 days or with the accuracy required.
PMI will return to the site at a future date to reobserve the antennae. They established NGS 3D monuments for vertical control, which consisted of beveled rods driven to refusal (typically 12 to 48 feet) and a datum point punched into the tops. They were set in grease-filled tubes (top 36 inches of rod) in sand in an 8-inch PVC tube, which was set in a sand base with concrete around the cap. PMI will verify the movement, then the contractor will be tasked to realign the antennae; PMI will then verify the base line for accurate positioning of the antennae. Perhaps then the Puerto Rico ROTHR site will be in a better position-literally-to aid our nation's forces protecting our coast and battling to keep out drug traffickers.