The GPS Evolution

Satellite signal availability and accuracy options have increased significantly. And over the course of the last seven years, the availability and accessibility of real-time kinematic (RTK) correction has moved rapidly from the new technology on the block to an investment that more and more end-users are able to afford.

We’re certainly closer to a vision that Mike Gomes, agriculture product manager for receiver manufacturer Topcon, discussed in a 2006 article in sister publication PrecisionAg Special Reports. “The idea is that global positioning becomes like a utility,” he said. “For instance, when you turn on a light, electricity makes it work; or when you turn on the water, it is clean and usable. Imagine there being no time of day issues, no issues with compatibility or satellites or your position on Earth. It’s just there, always on and ready to use.”

Inside agriculture, receiver technology has evolved to the point where it is built into most equipment, notes Paul Welbig, marketing manager at Raven Industries. “The reliability, cost, and usability of GPS technology has really reached a point where customers trust it and use it in more and more applications,” he says.

Evolution Of The Revolution
GPS has gone through an evolution almost simultaneously with, and arguably no less remarkable than, the Internet. Like the Internet, GPS was in its infancy back in 1995, and product developers were working on a receiver that provided accuracy good enough for agriculture applications. Initial products were only providing accuracy measured in meters, which didn’t fit the bill, says Sid Siefken, product manager for Trimble.

“The data farmers got from recording the yield monitor was jumping around 2 to 5 meters,” he explains. “So a farmer would get his data back and say, “I didn’t drive like that — this isn’t accurate enough for my application.”

A little more than a decade later, an end-user interested in georeferencing field operations using a GPS system has a wide range of options in receiver technology, from the very basic sub-meter system all the way down to the sub-inch accuracy level. These developments have come both in signal technology and at the receiver level.

There are essentially four sources of GPS signals available today. The Wide Area Augmentation System, built originally to replace the current system for coordinating commercial air traffic, is available at no cost from the federal government. The signal provides accuracy averaging around 6 inches — a bit more or a bit less depending on the receiver used.

OmniSTAR provides a subscription-based signal with three options depending on accuracy needs — VBS at the basic level, followed by XP and HP, providing accuracy as good as 3 inches at the top end. The Coast Guard beacon system also continues to be accessible to users within range of a beacon.

Finally, there’s real-time kinematic systems, which require a static ground receiver and “rover” receiver that provides true sub-inch accuracy. The other potential benefit with RTK is repeatability — if the static receiver is returned to the precise same location each time it is used, or is permanently installed in one location, it will provide guidance that is repeatable. That is, you’ll be able to return to the same spot in the field, year over year.

The big news with RTK systems is price point reduction and accessibility. A few years ago, you could expect to invest $30,000 at the low end to add RTK. Individual systems today come in at under $20,000, and where there are tower networks available to pick up a broadcast signal (for a subscription fee), potentially much less than that.

“Cost-effective and easy-to-use base stations have put the power of repeatability through use of RTK within the reach of more (users),” says Deane Malott, director of product marketing for AutoFarm.

Satellite Signal Improvements
While RTK systems have gotten the lion’s share of the press, satellite-based GPS — and the receivers that use the signal — also have been steadily improving over the years. One of the most significant improvements is the availability of a “dual-frequency” signal.

Dual-frequency receivers use two of the GPS frequencies, L1 and L2, explains John Pointon, marketing manager at OmniSTAR. “Up until a few years ago those were exclusively used by surveyors who wanted high accuracy, and because of cost the ag market used only single frequency L1. With L1, you really cannot get better than submeter accuracy.

“People who are serious about automatic steering and planting and strip-till need to be within an inch or two, and thus need a dual-frequency receiver,” he continues. “Dual frequency permits better compensation for atmospheric errors and helps with the complex calculations required for high accuracy positioning. This is really where the growth is in the market today.”

Of course, not everyone needs or wants that kind of accuracy. So manufacturers are offering a wide range of accuracy options that balance price and value expectations.

What should you consider when weighing up receiver options? Pay attention to the antenna design of the receiver. Antennas today are designed for specific purposes, and knowing their abilities and limitations will ensure you are getting what you expect. It’s also important to note that newer technology is better. A newer, higher quality antenna will deliver a cleaner signal that minimizes interference that is generated by reflected signals and obstacles in the path of a signal.

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