Solar flare puts GPS off the air

Global Positioning System outage could occur again

• By Patrick Marshall
• Apr 16, 2007

It was like something out
of a sci-fi movie. Mysteriously,
on Dec. 6, 2006, Global Positioning
System devices suddenly
malfunctioned across
large swaths of the planet.

The good news is that the
cause was not invading aliens.
It was an intense burst of radio
energy, called a solar flare,
emitting from the sun's surface.
And there's more good news '
although the event temporarily
knocked out many GPS receivers,
no airplanes fell from
the sky, and no ships lost their
way at sea.

But the event nonetheless
generated concern among scientists.
Although they were
aware that radio bursts generated
by solar flares could affect
GPS equipment, they were surprised
that this large an event
occurred during a period of relatively
low solar-flare activity
and that its impact was as
strong as it was.

Serious business

'It's more serious than we
thought. We didn't think this
was going to happen until the
next solar maximum, which is
about 2011,' said Paul Kintner
Jr., professor of electrical and
computer engineering at Cornell
University and head of
Cornell's GPS Laboratory.
'We've been monitoring solar
flares for four years. [The Dec.
6 event] suggests that monitoring
has been inaccurate.
And we don't have a good historical
basis for predicting
what's going to happen, so
we're concerned.'

The radio bursts don't actually
damage equipment but only
interfere with transmissions
between GPS satellites and receivers.
Some GPS equipment
may be affected for about 30
minutes, Kintner said, 'but
that's really an upper limit.'
This means that the risks are
greater for some GPS users
than for others.

'This past event was sort of
two really intense pulses that
lasted three to four minutes
each, so this is not a major outage
if you're driving a car or
hiking,' Kintner said. 'It's really
a concern for people who
want uninterrupted operation.'

People, for instance, operating
floating offshore oil platforms.
Such platforms constantly
monitor themselves,
navigating with GPS to stay
within a about a meter over
where the pipe enters the sea
floor. 'They have very demanding,
precision GPS requirements,'
Kintner said. 'A few
minutes of outage could break
them.'

Airliners also depend on continuous
signals, employing the
WAAS ' Wide Area Augmentation
System ' for GPS services.
The system improves the
standard GPS signal to lower
the margin of error from about
15 meters to 2 meters. That's
good enough that WAAS is
used not only for in-flight reporting
of location and terminal
operations but also for
guided-approach services,
which bring aircraft down to
about 200 feet above a runway
before the pilot has to take
over.

How did WAAS fare through
the Dec. 6 event? According to
Patricia Doherty, senior research
scientist at the Institute
for Scientific Research at
Boston College and a consultant
to the Federal Aviation Administration,
the results were
mixed.

Although the nonprecision
services used for in-flight location
and terminal operations
were not affected, Doherty
said, the precision-guided-approach
system was compromised.

'The system still operated,
but we lost use of that guidedapproach
service for about 15
minutes,' she said.

Doherty said, however, that
people shouldn't worry about
airplanes falling from the sky.
'WAAS doesn't consider [such
outages] a problem because it
often loses use of that system
due to other operational reasons,
such as satellite maintenance.
It wasn't a real worry to
them,' she said. 'If a solar
radio burst hits, it's detected by
the ground system. The
ground system then alerts the
aircraft that you can't use the
system because there's a problem
with the GPS. There are
alternatives.'

If another large radio burst
happens, and the guided-approach
service is unavailable,
Doherty said, the most likely
result is that those airplanes
trying to land in areas where
visibility is very bad may have
to circle a while to get their
turn to land on one of the few
other runways that have non-
GPS blind navigation systems
in place.

'The reason that WAAS did a
lot better than some of these
other networks is that they
have been built with a very robust
receiver design,' she said.

'The receivers have something
called 'interference rejection' in
them. When a solar radio burst
bombards a receiver, it bombards
it with noise on the same
frequency as the GPS. The
WAAS receivers filter out external
noise. It's a lot more expensive,
absolutely.'

Best defense

Kintner said there are two
basic ways to keep solar radio
bursts from interfering with
GPS devices. One is to increase
the strength of the signal
broadcast by GPS satellites.
The other way is to
improve the performance of
GPS receivers so they can better
filter out interference.

Unfortunately, 'I don't think
either is likely to be very effective
by the next solar maximum,'
Kintner said, referring
to the period of heightened
solar flare activity expected in
2011.

A third strategy ' and one
scientists had been counting on
to help the situation until affordable
technology comes to
the rescue ' is to be able to
predict solar radio bursts so
those who depend on continuous
GPS can prepare to get
through a period without it.

That's why the Dec. 6 solar
event was so unsettling to
some scientists.

'The relationship between
sunspots and solar flares and
radio bursts is qualitative,' said
Kintner. 'So larger solar flares
don't necessarily produce larger
solar radio bursts. The relationship
is not one to one.
That's part of what we're trying
to understand.'

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