Researchers
who are working to fix global positioning system (GPS) errors have
devised software to take a more accurate measurement of
altitude—particularly in mountainous areas.
The
software is still under development, but in initial tests it enabled
centimeter-scale GPS positioning—including altitude—as often as 97% of
the time.
Researchers
hope the software will help to improve the vertical accuracy of
measurements in potentially hazardous regions at high altitudes, such as
areas of soft, loose land that may be prone to landslides. They also
claim that their software could be used to measure how quickly glaciers
at high altitudes are melting.
The
GPS is most commonly known for its ability to provide on-the-spot
locations for drivers, but this application is just one of many possible
uses, explained Dorota Grejner-Brzezinska, professor of civil and
environmental engineering and geodetic science at Ohio State University.
As the level of GPS precision increases, so do potential applications
for scientific research.
While
drivers are generally concerned with tracking their own location in two
dimensions on the earth’s surface, the third dimension of altitude has
always been available through GPS—just with lower accuracy than that of
the horizontal coordinates.
Recently,
Grejner-Brzezinska and her colleagues from the University of Warmia and
Mazury in Poland have developed software that will allow GPS to relay
locations to within a few centimeters’ accuracy, including altitude.
While this high level of precision is not necessary for driving
directions, it is necessary for recognizing small shifts in topsoil that
may lead to dangerously destructive landslides.
She explained that a lot is going on behind the scenes during a typical use of GPS.
GPS
satellites transmit information in the form of radio waves to the GPS
receiver held by the user. At the same time, the signals must also
travel to at least one other ground-based receiver to obtain a location
reference, which allows the user’s receiver in turn to accurately
calculate its own position in 3D. Before the satellite signals reach the
receivers, they must travel through Earth’s atmosphere, which results
in time delays that affect accuracy.
When
the user’s receiver and the reference receiver reside at drastically
different altitudes, however, each location experiences different
amounts of time delay, which complicates matters even further. So, in
mountainous regions where height differences can vary greatly over a
short distance, acquiring the altitude of locations to within a few
centimeters is difficult.
“Time
is the heart that drives GPS, so it is important that we have a
proficient method that accounts for delays from earth’s atmospheric
layers,” said Grejner-Brzezinska. “It would be ideal for all GPS signals
to travel in a straight line directly to their destination, but due to
electron interaction and refraction in the lower atmosphere, the
signal’s path is far from straight,” she continued.
Electron
interaction and tropospheric refraction effectively re-route the GPS
signal, which means that the signal travels an extra distance and
requires extra time, said Grejner-Brzezinska.
She
and her colleagues looked specifically at troposphere delays—those
caused by the lowest level of the atmosphere. Their study can be found
in a recent issue of the journal Measurement Science and Technology.
In
the past, scientists have tried to account for troposphere delays by
using basic models of Earth’s atmosphere, said Grejner-Brzezinska. But
these models may not fully account for changes in the weather or
temperature, which can have a significant effect on the amount of
interference the GPS signals experience on their way down to earth.
Not
only weather and temperature, but also the height difference between
two stations can greatly affect the accuracy of a GPS-based height
determination.
Using
ground station receivers located in the Carpathian Mountains in
Poland—a region known for its steep slopes—the researchers collected GPS
information over a 13-hour period.
They
looked at two pairs of receivers with different height changes. The
first pair was located 72 km apart and had a height difference
of 32 m. The second pair was 66 km apart with a total height difference
of 380 m.
“We
figured that the easiest scenario would be provided by the receivers
with 32-m height difference, and the most challenging one with a height
difference of 380 m,” said Grejner-Brzezinska.
Using
processing software developed originally in Grejner-Brzezinska’s lab at
Ohio State, and further expanded by her research collaborators at the
University of Warmia and Mazury in Poland, the researchers applied three
different methods to measure GPS accuracy for the receivers.
The
results showed that, out of the three methods of handling tropospheric
delay in GPS measurements that were tested, there was one that provided
an accurate location, including the height of the receivers, 97% of the
time.
“Of
the three methods we tested, the third and most accurate was also the
most complicated,” said Grejner-Brzezinska. “This method was developed
by our team, and required knowledge of three or four reference stations
in order to perform the calculations properly.”
The
other two methods did not require the use of multiple reference
points—just a single one—but their levels of accuracy did not match the
third method’s positioning capabilities.
Further
testing will follow. But this early study shows that GPS accuracy for
altitude estimation can be improved, and may lead to the precision
estimates that researchers need to analyze, for example, the stability
of mountaintops and glaciers with 10-minute temporal resolution.
This
research was funded by the European Space Agency Plan for European
Cooperating States project and a grant from the Polish Ministry of
Science and Higher Education. Grejner-Brzezinska’s
collaborators at the University of Warmia and Mazury include Pawel
Wielgosz, Slawomir Cellmer and Zofia Rzepecka.
Troposphere modeling for precise GPS rapid static positioning in mountainous areas
