To gain access to the Federal Aviation Administration's radar tower in Lincolnton, which tracks aircraft from west of Birmingham, Ala., to miles off the Atlantic Seaboard, all you have to do is walk through an open gate.
Some terrorism experts and FAA officials say that despite the lack of security measures, the remoteness of the radar site's location makes it an uninviting terrorism target. Even if it were destroyed, whether by lightning or pipe bomb, they say there is so much redundancy in the nation's aviation radar system that air travelers and pilots wouldn't notice its absence.
But others note that the rate of bomb threats against U.S. airports has almost doubled in the past four years. And as security tightens around airports, they predict terrorists will turn to the more vulnerable remote radar facilities and electronic navigation aids that form the backbone of the U.S. airspace system.
"They certainly are more vulnerable, and they could cause a great deal of disruption in the aviation system if they were destroyed," said Peter Sederbergcq, a University of South Carolina professor who has written several books and articles on terrorism.
Doug Lemke, a professor at Florida State University who has studied guerrilla warfare in Vietnam and Central America, said the sites provide a "classic target for guerrilla groups. You take out a key communication or electronic node and you wreak havoc without causing any direct loss of life."
The "Golf Ball," as locals call the Lincolnton site, is one of the FAA's 115 remote radar stations that allow air traffic controllers to track aircraft as they travel between airports. Its powerful radar, which rotates within a white, ball-shaped dome on a 50-foot-tall tower, scans for air traffic within a 200-mile radius of Lincolnton.
The site, located off a gravel road on the outskirts of town, serves as the main backup facility to Augusta's Bush Field if that airport-based radar should fail. It is also a standby tool to guide aircraft to a safe landing when they lose their instruments in bad weather.
Despite its importance, the only thing standing between would-be saboteurs and the structure is an 8-foot-tall chain-link fence and signs that warn: "This facility is used in FAA air traffic control. Loss of human life may result from service interruption."
Thirty miles away, in rural Colliers, S.C., a squat, round building, with what could be described as a large upside-down ice cream cone stuck in the middle of the roof, sits unmanned in the middle of a field. It is one of the FAA's 1,025 VHF Omnidirectional Radio Ranges - or "VORs" - which send out aircraft navigation signals. The Colliers VOR also serves as one of the approach aids for aircraft landing in inclement weather at Bush Field.
Like the Lincolnton facility, the VOR sits in a field surrounded by dense woods, allowing a potential saboteur plenty of time to do his damage and escape unnoticed. Because they must have an obstacle-free area to send out their radio signals, VORs are often located far away from population centers - and regular security patrols.
FAA officials say similar sites have been the target of deliberate attacks before, such as the destruction of a VOR and other FAA installations in Puerto Rico a few years ago. Other VORs, isolated in the Kentucky mountains, are often shot at, supposedly by frustrated hunters.
Richard Thoma, who supervises FAA facility maintenance and security for eight Southern states and the Caribbean, said that the FAA often depends on local law enforcement patrols and intrusion alarms to thwart vandalism and terrorist attacks.
"Are we 100-percent protected? No. But neither is any public building. If we kept an armed guard at each of our thousands of remote facilities, it would be cost prohibitive. We assign the resources where we anticipate the greatest threat may be," Mr. Thoma said.
Terrorism experts say the sites won't be further protected until they start to be regularly attacked.
"The FAA's approach has always been `When a plane falls out of the sky, then we'll find out what the problem is,'¦" Dr. Sederberg said. "No one's ever blown up a remote radar station in the United States, that we know of, so until they do, you won't see any guards standing watch."
FAA officials say that while the remoteness of the facilities could be a weakness, they consider the facilities' out-of-the-way locations more of a security strength.
"Not very many people know where these places are. It's not a secret or anything, but it's not something we advertise, either," said Michael Gunn, manager of Bush Field's tower and radar facility.
Mr. Gunn discounts the possibility that a terrorist could easily destroy Bush Field's radar, which scans a 50-mile radius around Augusta and provides more detailed information than the Lincolnton radar. Because the radar is located well within the airport's perimeter, a saboteur would soon be detected, he says.
A more likely scenario is that a lightning strike could take out the airport radar, which went down only once in 1995, better than the average of 1.6 unscheduled outages in the nation's other radar facilities.
"When that happened, all we did is flip a switch and make a phone call, and within a minute, our screens were back up (with Lincolnton)," Mr. Gunn said.
If both the local and Lincolnton radars were inoperable, air traffic controllers are trained to keep track of traffic without radar.
"We just do what we did before radar. It may result in some delays, but planes will not fall out of the sky just because the radar goes down," Mr. Gunn said.
Even with both Bush Field and Lincolnton radars disabled, pilots would likely remain under radar coverage because of the FAA's redundant, overlapping system. Radar sites in Marietta, Ga., South Carolina, Tennessee and North Carolina overlap into the Augusta area, Lincolnton radar technician Bruce Ryder said.
"A terrorist would have to take out at least half a dozen radar sites at once to darken any significant part of Georgia," said Richard Dellinger, a former corporate pilot and FAA examiner who now teaches for Auburn University's aviation management program.
Mr. Dellinger said that while navigational aids such as the Colliers VOR are vulnerable, the destruction of a few of those facilities would not likely impair airline or corporate navigation.
"The airlines don't use VORs anymore; neither do most corporate operations, except for a backup," Mr. Dellinger said.
Mr. Dellinger said airliners and corporate aircraft now have several navigation systems available, including inertial navigation - which uses onboard laser gyros and computers to constantly tell pilots where they are - and satellite-based navigation, or GPS.
But that technology is cost-prohibitive for most small airplanes, including air ambulance helicopters, which continue to rely on ground-based navigation systems.
"If they aren't important, then what's the purpose of spending millions of dollars each year to maintain them?" Dr. Lemke said. "We've been lucky in this country so far, but with (the bombings in) Oklahoma City and Atlanta, I think people are starting to realize that we aren't invulnerable anymore."