ION: Centimeter-Level Accuracy - Is It Coming?
- From: Sam Wormley <swormley1@xxxxxxxxx>
- Date: Sat, 29 Sep 2007 23:23:21 GMT
ION: Centimeter-Level Accuracy - Is It Coming?
AFI, September 27, 2007
http://www.ngs.noaa.gov/CORS/Newsletter/092707.doc
What is the future of civilian GNSS?
Is it cars that drive and navigate by themselves, with the assistance
of onboard sensors and GPS? Accuracy well below one meter-perhaps
even the centimeter level, and all that implies-thanks to the
promulgation of new civil signals from GPS, GLONASS, Galileo, etc.?
Without the promulgation of post-processing and other tricks
currently used to improve accuracy?
The hopeful, qualified answer is yes, say speakers here in Ft. Worth,
Texas, at the 20th International Technical Meeting of the Institute
of Navigation's Satellite Division-2007 ION GNSS, for short. But that
bright future is dimmed somewhat by the problems and challenges of
crowded orbits and crowded frequencies.
As far as self-driving cars are concerned, that future has already
begun. ION's Tuesday evening plenary session opened with keynote
speaker Sebastian Thrun. Thrun, a professor at Stanford University,
headed up the team of students that won the 2005 DARPA Grand
Challenge, creating an autonomous vehicle that navigated 132 miles of
Southwest desert terrain in 6 hours and 53 minutes.
Thrun and his students' experience is a classic example of the
capabilities and limitations of GPS technology when it comes to
terrestrial navigation. In the 2004 Grand Challenge, every team's
autonomous vehicle failed within the first few miles, because of an
over-reliance on GPS.
"A lot of people thought it was easy. I have a car, I have a
computer, I have a GPS system," Thrun recalled of the 2004 contest,
which involved a course covering 2,700 GPS-based waypoints. As anyone
who has depended solely on a PND or handheld GPS unit knows, the
problem is that GPS data is not always accurate; particularly in
terms of exact precision, there can be errors to contend with.
Furthermore, it doesn't account for obstacles and variations in
terrain.
The answer for Thrun's team was to put a suite of laser sensors on
the roof of their vehicle constantly scanning the terrain ahead of
the vehicle, creating a 3D cloud of data points that described the
shape of the nearby terrain. To overcome the short-range
effectiveness of laser sensors, Stanford's winning system also used
digital cameras for long-range terrain perception. It would create a
terrain model based on the short-range laser data and apply that
model to the long-range image to predict the long-range path,
constantly updating it as new short-range data became available.
In essence, the answer for an autonomous vehicle was relative
positioning, not the absolute positioning of GNSS. Of course, the
vehicle still depended on GPS data to describe its intended path. As
Thrun noted, to actually determine where one currently is, one still
needs the absolute positioning of GPS.
The next step for Thrun's Stanford team and others answering DARPA's
call for autonomous vehicle technology is the 2007 Urban Challenge-a
16-mile course in an urban setting, complete with traffic. While that
contest will take place later this year, Thrun discussed what the
Stanford team is doing-namely, updating its system with a 360-degree
infrared sensor, supplemented with digital map data.
So what's the one thing the GNSS industry could do to help make
autonomous vehicles commonplace? Something that's been talked about
for decades? Thrun's short answer is 10cm accuracy-it was the lack of
extreme accuracy in GPS, coupled with dependency on it, that caused
the entire field of competitors in the 2004 Grand Challenge field to
fail, he reminded the audience.
"If you could do that, it would change the world," Thrun said of
centimeter-level GNSS accuracy. "Its impact would be fundamental to
all of us."
Several plenary speakers suggested during a question-and-answer
session following Thrun's speech that this level of accuracy might be
closer than we think. Given the multitude of civil signals that will
be available in the future, between the new signals being added to
GPS, and the availability of other civil signals from GLONASS (and
potentially Galileo and China's Compass), centimeter-level
positioning accuracy may be obtainable, said Günter Hein, of the
Institute of Geodesy and Navigation at the University FAF in Munich,
Germany.
The challenge, suggested ION fellow Gaylord Green, is capturing and
ensuring the accuracy of all of the necessary satellite ephemeris
data. "But it is coming," he agreed. Green, incidentally, as an
officer in the U.S. Air Force-now retired-was a member of the
original Air Force team that developed and implemented the GPS
system.
And as Hein and others have pointed out here, all those civil signals
on the same frequencies are going to make things rather crowded,
which could prove problematic in the future.
While it may seem to simply be a matter of getting enough spacecraft
in orbit and coordinating the data they provide to achieve the goal
of centimeter-level accuracy in GNSS, that's actually no small or
simple matter.
As U.S. Air Force Major General William Shelton informed the ION
plenary session audience, for one thing, GNSS costs money, and lots
of it. To date the United States has invested about $32 billion in
the Global Positioning System; it costs about a billion dollars a
year to administer and maintain it, he noted.
Shelton is Commander of the 14th Air Force (Air Forces
Strategic-Space), Air Force Space Command, as well as the Commander
of the Joint Functional Component Command for Space, U.S. Strategic
Command at Vandenberg Air Force Base, California.
While GNSS-and GPS in particular-may have been driven early on by
military applications, GPS was designed from the beginning to be a
dual-use system, Shelton observed, and it is civilian commercial
applications that have become a principal driver of the technology.
"The commercial aspect of space is what is absolutely exploding right
now," he acknowledged.
But this introduces a question of policy: just who protects
commercial interests in space, and who will act as a traffic cop,
keeping the increasingly crowded geosynchronous and near-earth orbits
orderly-and keeping in mind geo-political situations on the ground.
"That's a big policy question for us," Shelton said.
He suggested that it was going to take collaborative efforts of
everyone involved in the military and the private commercial sector
to protect the interests of all the participants in space, namely
GNSS. "We believe this is the policy of the future."
.
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