Chicago (CNN) — They watch the skies.

From fence lines, parking lots and roadsides -- they're looking up. They've staked out their favorite watch-spots around Chicago O'Hare International Airport and given them funny-sounding names like USG and The Beeline.

It's unlikely you'll ever meet any of these para-police officers, wearing their bright orange vests and ID tags. But if you're one of the millions of travelers who fly into Chicago every year, you might want to thank them -- because they're helping the FBI, Transportation Security Administration and other authorities protect you from terrorists.

In the Windy City, they call themselves ORD Airport Watch, and we found them on a sunny day in May, gathered next to a parking lot atop a little mound of greenery nicknamed the Grassy Knoll.

Taking their name from ORD -- O'Hare's international airport code -- they're a diverse group of fiercely independent personalities who share a deep passion for airplanes. To put it simply: These folks are obsessed with photographing, tracking and documenting the movement of aircraft. That's what makes them plane spotters.

By paying attention to aircraft tail numbers, landing and takeoff conditions and other details, spotters are natural allies for police.

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In no time at all we meet an ex-British navy war vet, a former IT director for Donald Trump's now defunct shuttle, a former "Jeopardy" game show contestant, and a Bulgarian who isn't afraid to point his camera at a Russian air force base.

One hundred sixty-two of these citizen sentinels volunteered about 5,000 total hours last year helping to guard O'Hare, one of the busiest airports on the planet. Under police supervision, they undergo background checks and special training. But instead of Glocks or Smith & Wessons, these guys prefer to strap on Canons or Nikons.

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For years -- especially after 9/11 -- police and plane spotters at airports around the world have been at odds. Police would find themselves wasting time tracking "pesky" plane spotters to make sure they weren't terrorists. And spotters would waste time trying to keep one step ahead of the "pesky" police.

But programs like these aim to turn the police-plane spotter relationship into a mutually beneficial partnership. Minneapolis launched one of the nation's first such programs in 2008, with just 10 volunteers. ORD followed about two years ago. In the Southwest, spotters are ramping up an outfit in Phoenix.

As we stand about 1,000 yards from the end of O'Hare Runway 14 Right, the wind blows from the south on this particular day. That means air traffic controllers will be sending departing jets our way. Just seconds after their wheels slip the surly bonds of Earth, brightly painted machines weighing a half-million pounds each will soon be dangling just a few hundred feet above our heads.

"747!" someone shouts.

A hulking Lufthansa jumbo jet looms over the trees. Immediately every camera lens points upward.

Richard Carlson, group vice president, takes aim with his Canon 7D. Sporting an ORD Airport Watch baseball cap and shades, Carlson walks and talks with a classic, charismatic Chicago swagger. When Carlson mentions the plane he most wants to capture on camera, his tone goes reverent: Air Force One.

"It's like the holy grail of airplanes," says Carlson. "It's the most famous plane in the world."

From coast to coast and country to country, many of them turn to Twitter for information via hashtags like #spotters and #avgeek -- shorthand for aviation geek.

For ex-Trump Airlines IT chief Steve Bailey, the holy grail is a Boeing 747. And not just any 747.

Officer JoLynn Christianson, Minneapolis-St. Paul airport police “It's a win-win for both the airport and for spotters.”

Bailey -- who grew up around airports in England and Canada -- has been jonesing for years to get a shot of a specific 747 located in the middle of a Tunisian desert.

Two 747s -- remnants of the 1991 Persian Gulf War -- languish on a landing strip near the city of Tozeur, where Saddam Hussein ordered them moved to avoid a U.S.-led attack. Bailey has a photo of one of the two desert planes. All he needs is the other one, and then he'll possess the complete pair. Plane spotting shares aspects of birdwatching and collecting baseball cards.

High school librarian Ana Peso, 30, first fell for jumbo jets as a kid during a family flight to Spain. "I love the shape of them," she says. Last year, Peso appeared as a contestant on TV's "Jeopardy." Show host Alex Trebek asked her, "So you're a plane spotter. What does that mean?"

"I like to take photos of airplanes," Peso said. Sadly, Peso didn't win -- perhaps because there weren't enough questions about planes.

Clearly outnumbered, Peso is one of only 13 women in the group. "It's awkward, I guess," she says, looking around at all the men on the Grassy Knoll. She shrugs it off as if to say it's not a big deal.

Ivan Voukadinov pipes in with his story. The 25-year-old Bulgarian with a diamond stud in his left earlobe counts himself as a lover of military aircraft. And boy, did he prove it. Last summer he traveled thousands of miles from Chicago to Voronezh in western Russia to capture images of supersonic MiG-25 fighter jets -- a classic symbol of the Cold War.

A Russian soldier noticed Voukadinov as he was shooting photos along the facility's fence line. After Voukadinov gave a quick explanation, the soldier left him alone as long as he promised to keep a low profile.

"Sometimes you have to risk your freedom," he says with a smile.

In addition to cameras, other spotting tools include software that tracks incoming planes on a map. Aircraft equipment called ADS-B sends out electronic signatures which broadcast unique tail numbers. Special receivers allow plane spotters to pick up those signals and follow each plane on the map as it approaches the airport. Yes, there's a phone app for that. But Bailey uses a small portable box called a Kinetic SBS-1.

Another plane screams overhead. "A330! SAS!"

The signature bright blue tail of a Scandinavian Airlines Airbus A330 comes into view. The cameras go nuts.

Serious business

But don't get the wrong idea. These folks may be enjoying themselves, but sometimes it's all business.

In fact, it was a landing by that "holy grail" -- Air Force One -- that got a member kicked out of the group. ORD Airport Watch is all-volunteer. There are no minimum hours and no shifts. The members spot whenever they like, on their own personal schedules. But as informal as that may be, there are rules.

While spotting, members must carry their police-issued ID cards with them and wear their official orange vests. Edward M. Pio Roda/CNN

Members must undergo police background checks. They're trained to look for suspicious activity and how to report it immediately to police. While spotting, they must carry their police-issued ID cards with them and wear their official orange vests. They must log their spotting time and activity on the official ORD Airport Watch website.

Those who don't follow the rules suffer the consequences.

Recently Secret Service agents discovered an ORD spotter parked near the airport waiting to shoot photos of Air Force One. "He wasn't wearing his vest and he wouldn't identify himself," says group president Ian Hardie. "At the next board meeting it was decided he would be let go."

Hardie -- a confident, buttoned-down former air traffic controller with the British navy -- co-founded a similar airport security outfit in his native Scotland.

Ian Hardie, ORD Airport Watch president “This program should be replicated across the whole U.S.”

"This program should be replicated across the whole U.S.," says Hardie, who on 9/11 lost a distant cousin at the World Trade Center. After serving in wars in the Falkland Islands, Bosnia-Kosovo, Iraq and Afghanistan, Hardie doesn't like the idea of getting "blown up in my own backyard."

Frank Soto, village president of Bensenville, one of several Chicago-area communities surrounding the airport, credits the the program with helping to cut crime by 6% last year -- 54% since 2009.

But the focus rests clearly on spotting terrorists. FBI and TSA agents meet and coordinate with the group bimonthly. The Department of Homeland Security trains members to identify shoulder-launched surface-to-air missiles and how to report them.

"When a plane is leaving or landing is the most vulnerable time to take them down," says Bensenville Police Officer Joel Vargas, an early supporter and co-founder. "It's crucial to have friendly people on the ground looking for terrorists who may be casing a place."

"If you're a criminal driving around the airport and you see people with cameras standing around a parking lot, would you be stopping your car anywhere near there to commit your crime? No," says Bensenville Police Deputy Chief John Lustro.

The idea of shared responsibility between spotters and local police appeals to federal authorities. "TSA is proud to provide support and training to the Bensenville Police Department Airport Watch Program to help improve security at the Chicago O'Hare International Airport," a TSA representative said in a statement to CNN.

Almost every member has a story about why he or she fell in love with airplanes. Bob LaCursia, 51, grew up under an O'Hare flight path. The roar of jet engines reminds him of home. Jose Guillen, 25, caught the bug seven years ago from a family friend who worked at American Airlines. Marcus McElroy's boyhood flights on a DC-9 from Alabama to Atlanta propelled him toward aerospace engineering and nine years at NASA.

Now they're all spotters -- watching the skies and helping police.

These kinds of cooperative programs are rare in the U.S. O'Hare's program took a page from similar outfits in Ottawa and at Minneapolis-St. Paul International Airport, which -- when it launched in 2008 -- boasted being the first program of its kind in the United States.

"It's important to bring our community in and try to get them involved in the protection of the airport because that's the way we get a lot of the information that we're going to need to keep it safe," says Officer JoLynn Christianson, who heads the police/spotter watch program at Minneapolis-St. Paul airport. It's critical, she said, to include "people having the leisure time to be watching the aircraft and knowing the difference between something suspicious and something that wouldn't be."

Despite these successes in Minneapolis-St. Paul and Chicago, a 2-year-old spotter watch program in Phoenix has been slower to grow.