Relying on advanced computer simulations, the U.S. government
has concluded that the performance of the global positioning system (GPS) navigation
satellites is influenced by natural phenomena in the environment, such as solar
storms.
The electrons that are emitted by the sun can affect an observer's ability
on Earth to lock on to signals from GPS satellites and maintain that lock to
achieve accurate location, said Steve O. Ouzts, manager for applied technology
at Litton-TASC, Reading, Mass.
The company has conducted a series of simulations for the U.S. Space Command
on the environmental effects on GPS accuracy and GPS lock-on, Ouzts said in
an interview. "The Air Force took those simulations to study how the environment
can have an impact on military systems."
When the sun is active, it streams out electrons, which are captured in the
upper atmosphere-or ionosphere-of the Earth by the magnetic field, he explained.
"They typically stream to the dark side of the planet and accumulate there,
so you get a fairly active total electron content at nighttime."
The upshot for those users of GPS at night is the presence of "scintillation
effects or fade effects" on the signal from the satellite, said Ouzts.
"You might actually be unable to lock on to it if it were sufficiently
active in the ionosphere." More likely, however, is that there would be
some interference or noise that would cause the user to doubt the veracity of
the signal, the lock-on, and the positioning data, he said.
One piece of software, developed by TASC, models the ionosphere and its electron
content. That information is sent to a simulation federation through a piece
of software called TAOS (total atmospheric ocean space). Then the simulation
can determine how effectively the user will be able to employ GPS.
The notion that GPS is vulnerable to environmental elements is not new.
During the first night of Operation Desert Storm in 1991, there was significant
interference to the U.S. GPS-based communications capability, Ouzts said.
"You can minimize the impact by carefully selecting what time of day you'd
like to employ the GPS receiver and knowing what the total electron content
of the ionosphere will be at that time," he said. "You could potentially
plan around it, but only to a degree."
The electrons, he added, don't affect the satellites themselves but the line
of sight between the satellite and the receiver. "There's no specific activity
you could undertake on the satellite or the receiver to mitigate that. You can,
however, do frequency selection to work around it. Some frequencies are more
prone to scintillation effects than others."
Litton-TASC conducted this work largely under contract to the U.S. Space Command
but also with funding from the Defense Modeling and Simulation Office and the
Army Topographic Center. This is "ground-breaking" work for any contractor,
he said. The simulations were conducted on PC-platforms.