More GPS brownouts ahead

Global positioning system (GPS) brownouts – periods when the number visible navigation satellites falls below critical thresholds – are a fact of life in the machine guidance world.

But brownouts could become more frequent in the decade ahead because of delays in developing replacements for the 30 or so aging GPS satellites currently in orbit, according to a United States Government Accountability Office (GAO) report issued in April 2009.

Currently, brownouts occur for an hour or two at a time many times a year, depending on your location. Most pass by unnoticed by farmers, since they occur outside the peak GPS usage window, notes Matt Darr, a precision ag specialist at Iowa State University.

At-risk satellites

By some estimates, almost half of current GPS satellites are at or approaching the point where the failure of a single critical system could take a satellite out of commission. Because it’s virtually impossible to predict when this might occur, GAO calculated probabilities that the number of operational GPS satellites will fall below 24. That’s the number of GPS satellites required to meet current performance standards.

Based on Department of Defense replacement launch projections, there’s at least a 10 percent chance this will occur from now through 2015, according to GAO. And if launches of the latest-generation GPS III satellites are delayed, as the GAO report projects, the chances of the GPS satellite constellation dropping below 24 increases to 90% from 2017 through 2019 – and remains relatively high well into the next decade.

In response to the GAO report, the U.S. Air Force, which operates the GPS system, said that the system is in no danger of failing. But in press reports, Air Force officials appeared to agree with GAO that there is some risk that performance from the aging GPS satellite fleet could deteriorate.

Congressional testimony in May 2009 by Brad Parkinson, a Stanford University scientist who spearheaded development of the GPS system while in the Air Force, appeared to corroborate GAO concerns. He suggested several ways of mitigating potential GPS satellite failures, including turning off military and intelligence-related technology on failed satellites. This could reduce power needs and breathe new life into failed satellites, he said.

Ag impact

If the number of GPS satellites drops below the 24-satellite threshold, guidance from both lightbars and autosteering systems would be affected. This is particularly the case when farming in areas that have trees along field boundaries or other obstructions that limit satellite visibility, says Darr.

“If you are operating in rough terrain conditions where you normally only have six visible satellites and one of those browns out, then you are at a high risk of losing your position fix,” he says. “You have to have a minimum of four satellites to generate a GPS position for use with any precision ag product.”

Reduced satellite numbers could reduce accuracy even when the minimum number of satellites is available, he notes. “As you have more satellites, your accuracy increases,” he says.

Real Time Kinematic (RTK) systems are also at risk because the rover only corrects for the satellites that are visible by both the rover and the base station. A reduced satellite count increases the potential for the number of common satellites to drop below the critical level.

This problem is minimized with the latest dual-frequency navigation systems, which can access the Russian Glonass satellite system in addition to GPS, he says. The additional Glonass satellites would make up for reduced number of GPS satellites, assuming the RTK base station also can access the Glonass constellation.