Thursday, March 19, 2015

Valley Single Frequency GPS, a Reliable and Durable Guidance System | by John Kastl

I get a lot of questions from growers with corner machines about the new Single Frequency GPS guidance option from Valley®. Many folks want to know what single frequency is, how it works and how it’s different from other guidance options, so let’ take a look at GPS and when it’s a good choice.

All GPS guidance systems require a highly accurate signal in order to ensure the machine travels consistently along the defined path. Any errors or interruptions in the signal can cause the machine to wander, resulting in a wider or inconsistent wheel track. If the interruption is severe, the machine will stop and wait until the signal comes back, resulting in uneven water application. 

Traditionally, the way to minimize these problems is to use a Dual-Frequency, Dual-Constellation, RTK GPS receiver and a fixed correction or reference station. Unfortunately, this type of system is expensive and requires an annual subscription fee to a local RTK Correction Service provider or the purchase of an additional costly reference station. However, thanks to advances in GPS technology, Valley has developed a cost-effective Single Frequency, Dual-Constellation, RTK GPS guidance system.

OK, Dual/Single Frequency…Dual Constellation…RTK. What the heck do all these terms mean and what difference do they make????

Good questions. Let’s start with RTK. RTK is short for Real Time Kinematic. It refers to a GPS system that uses a signal from a fixed reference station to provide real-time corrections to the GPS guidance unit. This is the most accurate type of GPS system available and can tell you your position down to plus or minus 3 inches.

Dual Constellation refers to the number of satellite systems being used for GPS guidance. The two largest, worldwide systems are the GPS constellation, operated by the U.S. government, a
nd GLONASS, operated by the Russian government. Each system has its own set of satellites orbiting the earth, providing position signals as they pass overhead. During a 24-hour period, the total number of satellites available at any point on the earth varies as these satellites pass by. 

For RTK accuracy, a minimum of five satellites are needed. Should the GPS receiver “see” fewer than five satellites, the guidance system will stop and wait; so, the more satellites available, the more reliable the guidance. That’s the benefit of dual constellation; there’s more satellites overhead, 24/7/365. You can see the difference for yourself at All Valley GPS guidance options use both these systems for maximum reliability, whereas others only use the GPS constellation.

Finally, let’s discuss single and dual frequency systems. Dual-frequency systems use both the GPS L1 (civilian) and the L2 (old military) frequency for fast start-up times and year-to-year accuracy. In addition, using two frequency bands makes it easy to compensate for transient interruptions in the GPS signal (cycle slip). However with the advances in GPS technology, such as advanced cycle slip correction, the new Valley single frequency system provides fast start-up times and track-on-track guidance accuracy during the growing season.

Single Frequency GPS guidance is a great choice for many growers with corners, but is especially useful in fields that have underground obstacles such as drain tile, gas lines, rocks or other obstructions that make burying a guidance wire difficult or expensive. It’s also a great solution in areas where frequent lightning strikes can damage buried wire guidance. Finally, unlike buried-wire guidance, if you ever plan to move the corner to another field, or expand the existing field by taking out a row of trees, you haven’t buried part of your investment in the ground. The entire guidance system is contained on the machine, making it easy to relocate or change the path. No need to re-bury wire, just have your dealer change the guidance path in the GPS guidance computer. And, best of all, single frequency GPS guidance costs about the same as installing buried wire.

Many growers tell me they don’t like to pay an annual subscription fee for their GPS guidance system. With Valley Single Frequency, no subscription fee is needed, because the reference station is included at no additional cost.

I recommend that you take a closer look at the Valley GPS Single Frequency Guidance option. You’ll see it’s a cost effective solution that provides reliable and durable guidance for your corner machines.

John Kastl
Product Manager - Equipment

John joined the Valley Irrigation Engineering Department in 2000 after having spent 11 years at General Electric Aircraft Engines. Today, he manages the equipment products for Valley (center pivots, corners, and gearboxes, to name a few), helping to develop the next generation of Valley Irrigation machines. John enjoys photography, home renovation, and travel. On his third birthday, John watched the first moon landing!