Nebraska farmer Bob Crumbliss has seen the light and knows he's headed in the right direction. You might even call it divine guidance from the heavens.

But the light, in this case, is from the light bar mounted in the cab of his self-propelled sprayer. And the heavenly guidance comes from satellite signals that keep him on track as he sprays.

It's the same technology that's used for computerized field maps, yield monitors and variable-rate applicators. In fact, some of the equipment is interchangeable.

With swath guidance systems, as they're known, a GPS receiver tracks your position and sends a signal to a light bar to keep you on track with your previous spray pass. In most cases, amber lights tell you that you're aligned correctly, red lights signal that you've drifted to the left of your track and green lights warn that you're too far to the right.

The high-tech guidance systems have replaced foam markers for farmers and commercial applicators. Some of the advantages are obvious.

"With drilled beans, you have no idea where you are, particularly when you have an 80' boom. The foam just won't hang up on the beans," Crumbliss says. "We're saving chemical and getting better weed control because we don't have as much overlap or as many skips.

"We plant all Roundup Ready beans, and it used to cost us $1/acre for a drift retardant. We'd spray a border around fields on calm days," he says. "Now when it gets windy, we get up at 3 a.m. to spray. And by 8:30 a.m., we're done. With the swath guidance you can spray all night when the air is calm."

But the biggest advantage of swath guidance, according to Crumbliss, is it takes away the mental strain of trying to follow a disappearing trail of foam 40' out from the cab. "It's worth it to me just for the mental stress relief," he says. "With foam you have to have complete concentration."

There is a downside to an electronic guidance system. "If you lose your satellite signal, you're in trouble," says the Edgar, NE, farmer. So foam markers are used as a backup on his self-propelled sprayer. Swath guidance isn't cheap, though. Crumbliss points out that the $15,000 he spent for his top-of-the-line system is roughly the cost of a set of row markers for a 40' drill.

The cost of a swath guidance system depends on how much you ask it to do, according to precision ag consultant Brad Rathje, Precision Systems Center, Waco, NE. "There are basically three different classes of systems," he says.

An entry-level system will cost you between $4,500 and $6,000, says Rathje. That buys you a light bar and GPS receiver, and has the ability to keep track of your previous pass and your current position. It has no ability to record or display field data.

"If you already have a yield monitor that uses a GPS receiver with sub-meter accuracy and a 5-hertz update rate, you can use it instead of buying another receiver," he says. "A light bar alone is under $2,000."

At the intermediate level, you add a screen to the system that shows you where you have already driven in a field. "You can see your passes on the screen, but the system doesn't have the ability to show you whether the sprayer was turned on or off," Rathje says. "It can show you where you've been but it can't tell you what you were doing."

An intermediate system costs from $6,000 to $8,000, Rathje says. A high-end swath guidance system, like Crumbliss uses, provides a display that records where you've been in the field, what happened at any given spot and stores that data. You can spray part of a field one day, come back the next and start again right where you left off. If your sprayer controller is GPS-compatible, you can use it to vary rates on the go. And depending on patent considerations, you can variable-rate several products simultaneously.

The high-end swath guidance systems cost between $12,000 and $16,000. But the extra cost is worth it to Crumbliss. "It's one area where you don't want to cut corners," he says.

"The biggest advantage of a high-end system is it's much easier to operate, which makes it easier to switch operators and still get the job done right," says Rathje. "I can train somebody to use a high-end system in two to three hours. It takes a full day of training to learn how to run one of the other systems."

The system you buy isn't necessarily tied to farm size, accord- ing to Rathje and Crumbliss. It's more a matter of what you want to accomplish with it. "Boom width is a more important criterion than farm size," Crumbliss says. The wider your spray boom, the more you have to gain with a swath guidance system.

"Any farmer who sprays 600-800 acres can justify an entry- level system," says Rathje. "Most farmers who buy them are just getting started with the technology and want to improve their application efficiency, but aren't really interested in doing variable-rate application. But I've got one customer who sprays 7,500 acres with just a basic system."

Farmers who choose an intermediate system likely will be in the 1,000-acre and bigger category, says Rathje. He works with several 12,000-acre farmers who find the mid-level technology does everything they need.

The high-end systems tend to be bought by commercial applicators and farmers like Crumbliss who want to maximize the control they have when they spray. "The biggest danger with this kind of technology is farmers who don't spend the time to research the different systems and understand their relative strengths," says Rathje. "It's easy to end up spending more than you really need. You need to make sure you have the flexibility to use the equipment with other applications. There's no need to put together a system that you can't move back and forth."