Solar event raises concerns about stability of GPS signals

 - June 5, 2007, 7:01 AM

A solar flare 10 times stronger than anything researchers had previously observed or predicted surprised scientists last December, not only because of its size and strength, but because of the effect it had on GPS receivers and other communications systems. Now scientists are looking at previous data and trying to understand how these flares affect satellite signals, in the hope that they can one day prevent further disruptions.

According to NOAA solar forecaster Bill Murtagh, a solar flare is a brief, intense release of energy from the sun. After the flare erupts, radio waves radiate throughout the electromagnetic spectrum and can be detected across all frequencies, including the L1 (1575.43 MHz) and L2 (1227.6 MHz) frequencies GPS receivers use. This is known as a solar radio burst.

The radio burst causes “background noise” that makes it difficult, if not impossible, for receivers to trace the GPS satellite signals. “It’s like trying to talk in a soft voice at a cocktail party,” explained Dr. Paul Kintner, professor of electrical and computer engineering at Cornell University and head of the Cornell GPS Laboratory. “As the noise level rises around you, you just can’t communicate.”

During the December 6 solar event, which occurred at 18:47 UTC, the noise level “increased to such a degree that the signals [from a number of satellites] could no longer be tracked,” Murtagh said. Kintner added that many receivers experienced an 80- to 90-percent signal loss; however, the loss occurred only on the sun side of the Earth. “The dark side is never affected,” Murtagh said.

Patricia Doherty, senior research scientist at the Institute for Scientific Research at Boston College, said that the radio burst caused many GPS receivers to fail for about 10 minutes at 19:40 UTC on December 6, nearly an hour after the initial solar flare. “By fail, I mean that the GPS receivers lost lock with enough satellites to prevent navigation calculations,” Doherty said.

More Powerful Flares Possible
The radio burst also affected the wide-area augmentation system (WAAS), “but with less intensity than other systems.” WAAS receivers lost a lock on satellite signals for “less than five minutes.”

According to the FAA, the flare caused radio frequency interference that lasted between an hour and an hour-and-a-half. In addition, the WAAS network “suffered some loss of availability of its vertical service–Lnav/Vnav and LPV approach services.” It did not, however, lose any availability to its Lnav or nonprecision approach service. “It is important to note that ionospheric activity and solar burst affect only the vertical guidance provided by WAAS,” an FAA spokeswoman said. “Lateral guidance is unaffected.”

“One possible reason that the WAAS system was more stable than other systems during the solar radio burst is that the WAAS GPS receivers are equipped with a technology that rejects interference,” Doherty said. “Apparently, these receivers detected the solar radio noise as interference and rejected most of it.”

According to one report, the radio burst had more of an effect on the L2 military GPS/WAAS signal than the stronger L1 civilian signal. “The WAAS ground system uses signals from both L1 and L2,” the FAA spokeswoman explained. “Interference from solar flares can be stronger on one frequency than on another, and either signal can be more affected depending on the flare.”

The December 6 solar flare was the strongest any scientist had ever observed, which accounted for the radio frequency interference and signal loss GPS receivers experienced. At the time, scientists had predicted that the largest solar flare would be around 100,000 solar flux units (sfu). By some estimates, the December flare was around 1 million sfu. “We didn’t think [a flare of that size] was possible,” Murtagh said. “It stunned us.”

“It’s like observing a Category 6 hurricane in February,” Kintner added. The Saffir-Simpson scale rates hurricanes up to Category 5, and the hurricane season begins June 1 and ends November 30. “We’re confused. We don’t know whether this was an anomaly or if our measurements were in error.”

Unfortunately, scientists have no way of predicting if, or when, a flare of that magnitude will happen again. Scientists make estimates about the frequency and size of sun flares that will occur based on the number of sun spots and whether the sun is in a period of minimum or maximum activity (each phase lasts about 11 years). There isn’t enough data to make accurate predictions. At best, scientists can “predict the potential is there,” Murtagh said.

The sun is currently in a period of minimum activity, which raises concern since scientists expect solar activity to increase over the next few years. The next cycle is expected to begin in March and peak in late 2011 or mid-2012, according to a panel of forecasters and solar experts at the NOAA Space Environment Center.

“We’re in the middle of a solar minimum, and we just experienced something 10 times stronger than anything we’ve ever seen,” Murtagh said. “Could the next flare be five times bigger? Ten times bigger? We just don’t know.”

“We need to look more closely at the potential impact of future solar radio bursts that might be a lot more powerful at higher levels of solar activity,” Doherty said. “The solar radio burst that occurred on December 6 was the biggest, baddest ever recorded–and it happened at solar minimum.”

According to Kintner, the solar flares and radio bursts that affect GPS signals occur in short bursts lasting no longer than 20 minutes. “If you can survive with 10- to 15-minute breaks in [GPS or WAAS] service, you’ll be OK,” Kintner said. “This is only an issue for people who need continuous service.”

In addition, disturbances in the ionosphere, especially during geo-magnetic storms, can also affect signals. However, these storms can usually be predicted and, as long as the storms are detected by the system, the integrity and safety of the system can be maintained, according to Doherty. “Solar storms cause the ionosphere to be highly irregular and unpredictable,” Doherty said. “The solar radio burst is a type of solar event that doesn’t make the ionosphere unstable; rather, it produces noise that weakens the GPS signal.”

Doherty added that system-wide GPS modernization is expected in 2013. The modernization will provide a second civilian frequency that will improve WAAS performance during magnetic storms.

According to the FAA, AOPA estimates that 40 percent of the 140,000 general aviation aircraft in use are equipped with GPS receivers. In addition, 51 percent of the airline fleet is equipped with GPS, according to a 2005 Mitre study. She added that 6,500 WAAS-capable avionics products have been sold to date. More are on the way as avionics manufacturers begin to introduce WAAS-certified flight management systems for business aircraft.

All GPS equipment approved for IFR use is required to include so-called receiver autonomous integrity monitoring (RAIM) capability. The RAIM system identifies erroneous GPS signals and notifies the pilot when insufficient satellites are available to support GPS functions. However, it does not correct the problem; it merely alerts the pilot if the GPS is not working properly.

Although the FAA “will for the foreseeable future maintain a minimum number of ground-based navigation aids as a backup to satellite-based navigation,” it has begun a slow phase-out of current navaids. Decisions about the future of the next-generation Category I, II and III landing systems will be made next year. “This will be followed by another decision in 2012 to begin to reduce the number of Category I ILSes. A limited number of ILSes will be retained at [35 Operational Evolution Plan] OEP airports,” the spokeswoman said.

There are currently 1,275 ILSes in the U.S., and phase-out is expected to begin in 2015, according to the FAA’s 2005 Federal Radionavigation Plan. There are also plans to reduce the number of VOR/DMEs by 70 percent. “[The] FAA has already begun to remove NDBs from service,” the spokeswoman said. A target date has not yet been set as to when all aircraft must be equipped with GPS.

In the meantime, some pilots have expressed concern about the FAA’s plans eventually to decommission current navaids. “I don’t think any of us [pilots] would like to see terminal area navaid systems discontinued,” said Jeff Beck, a Gulfstream pilot with Worldwide Aviation Logistics. “What if I had a GPS outage on approach [at] 200 feet while I was looking for the runway? I’m not sure I’d feel really comfortable with that; in fact, I would object to relying solely on GPS without backup.”

Amid the concern about the impact of these solar radio bursts, scientists have been working not only to understand the phenomenon, but also to educate and inform the public. “These bursts are short-lived, but [their existence] has to be known as we become more dependent on GPS,” Murtagh said. “Remember, the sky is not falling,” Kintner said. “However, if you require uninterrupted GPS service, you will need a Plan B to fall back on.”