Frazzled by the sun
May 1, 2009 12:00 PM, By David Hest
This October 2003 solar flare, among the largest ever recorded, caused significant disruptions in global positioning systems. It occupied an area equal to about 15 Earths and fired off the largest X-ray flare ever recorded.
Over the next few years, intensified storm activity on the sun could put you in the market for an upgraded guidance system. That's because an expected cyclical increase in solar storms (also called sunspots) could reduce the accuracy of low-end GPS guidance systems to the point that they may not be accurate enough for some field operations.
The good news is that accuracy losses from higher-end systems using dual-frequency GPS radios will be minimal, say guidance company experts. So switching to dual-frequency systems will be an option.
A second piece of good news: Solar storms should increase gradually, which will give growers with systems based on the Wide Area Augmentation System (WAAS) and other single-frequency guidance systems time to evaluate whether a change is needed.
Just how much navigation system accuracy will degrade is unclear. But some industry observers speculate that performance from single-frequency WAAS systems could be as much as two to three times worse than normal when solar activity is at its peak.
“At this time it is unknown how much the performance from single-frequency WAAS systems will be affected when solar activity is at its peak,” says Jorge Heraud of Trimble Navigation, which markets both single- and dual-frequency GPS receivers. “But I have a concern that people who have been attempting to use WAAS for more precise applications [such as row-crop planting] may not be able to do it. Those people may be pretty dissatisfied with performance unless they upgrade to an OmniStar HP/XP or RTK [real-time kinematic] system.”
However, he notes that WAAS differential GPS (DGPS) has been upgraded since the most recent peak in sunspot activity several years ago. So it's difficult to predict exactly how single-frequency receivers will perform today compared to how they performed in the past. “The WAAS system has the ability to transmit ionosphere corrections more rapidly than in the past,” Heraud notes. “It is hard to know exactly how good or bad the situation will be.”
Sunspot basics
Although sunspots and their underlying intense magnetic activity have been documented for more than a century, the current 11-year cycle will be the first to have a significant impact on agricultural guidance systems. That's because the last cycle, which peaked in 2003 and 2004, occurred when far fewer ag guidance systems were on the ground than today.
The connection between storms on the sun and guidance systems on the Earth is tied to the impact of sunspot activity on the ionosphere. The ionosphere is the uppermost part of the atmosphere, so-named because its particles are ionized by solar radiation. Notably, this ionization, which varies over time (and is heightened by solar storms), affects the speed of radio waves transmitted by GPS satellites to the Earth.
Even under normal conditions, one of the major challenges facing GPS guidance systems is to correct for changes in radio-wave speeds caused by variations in the ionosphere. Doing this is critical, because the speed with which a GPS signal moves is used to calculate positions on the ground.
Typically, under good conditions, high-end single-frequency systems using good antennas and DGPS corrections, such as WAAS, can correct for errors well enough to produce accuracies as high as ±6 to 8 in. But when sunspot-induced changes in the ionosphere are more extreme, the accuracy of single-frequency systems has fallen off substantially in the past.
Dual-frequency systems, including those using StarFire2, OmniStar's HP and XP, and RTK corrections, are more accurate and better able to compensate for sunspot-induced changes. That's because built-in software can directly calculate errors in real time based on known differences between the speeds of the two radio frequencies.
Learning from history
The new sunspot cycle got off to a slow start in January 2008 and has been relatively docile since then. But that doesn't predict how the rest of the cycle will play out. In fact, the intensity of sunspot activity varies considerably among and within cycles, according to Max Huff, a senior vice president at OmniStar.
The intensity of solar disruptions in each cycle typically follows a bell-shaped pattern, with minimal activity on the front and back shoulders and maximum activity at the midpoint. In this go-round, that's likely to be sometime in 2012 to 2013.
Based on the last sunspot cycle, users of single-frequency systems can expect accuracy to drop for short periods beginning in the middle of the ramp-up phase of the cycle, according to Huff. OmniStar measured changes in the accuracy levels of its various correction signals during the peak of the last solar cycle. On at least one occasion, a 50% accuracy reduction (a doubling of the inaccuracy level) was observed for its VBS single-frequency correction, which is similar to WAAS.
Fortunately for the agricultural community in North America, the worst disruptions during the last cycle occurred in December. Accuracy degraded to levels typical of uncorrected GPS, or worse, but returned to normal within a few days.
The areas most affected by sunspots lie along the geomagnetic equator, so performance in the U.S. and Canada is less affected than near the equator or in Europe, where the geomagnetic equator lies further north. In those areas, accuracy sometimes dropped four- or fivefold at the peak of the last sunspot cycle, Huff says.
Whether early or late in the cycle, the biggest accuracy reduction with single-frequency systems will be in the morning, says Heraud, of Trimble. For example, a potential 200% early morning accuracy degradation could fall to 130 to 150% of normal by midday.
“Instances of inaccuracy would last for a day or so every now and then, probably less than 15 times in a year,” adds Jeff Farrar of Hemisphere GPS, whose Outback line of GPS guidance products relies on single-frequency receivers. “On rare occasions, accuracy could be reduced by five or six times, but that's typically only a day or so during a year.”
Coping strategies
As the new sunspot cycle begins to ramp up, guidance system manufacturers are beginning to inform dealers and customers about what is ahead.
John Deere, which markets both single- and dual-frequency GPS receivers, has provided a bulletin to dealers discussing the sunspot cycle and the implications for guidance systems, says Kyle Collins, a marketing representative with Deere's Ag Management Solutions group.
“I don't think we exactly know how much accuracy will be degraded at this point with single-frequency receivers,” he says. “Growers using a dual-band receiver [such as a StarFire iTC with SF1, SF2 or RTK correction] are going to be in pretty good shape.”
“Tremendous adoption of agricultural navigation systems occurred during a lull in the sunspot cycle,” notes Mike O'Connor, chief technology officer for AutoFarm, which also markets both single- and dual-frequency GPS receivers. “Now growers have high expectations, and when these solar events occur, things will seem strange.”
Whether or not growers should upgrade guidance systems depends on how they are using GPS. “If it is for applications that don't require much precision, there isn't much that needs to be done,” O'Connor says. “But if more accuracy is needed, it may be time for an upgrade.” Then they should consider dual-frequency RTK.
Hemisphere's Farrar concurs that Outback customers may want to consider adding an Outback Baseline X RTK module to improve performance during increased solar activity. Using “shift” and “snap-to” features also can assist with compensating for GPS drift.
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