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6.4 Proprioceptive Sensors
tained by the Russian Federation Ministry of Defense. There are two types
of channels on Navstar, one public, called the Standard Positioning System,
and an encrypted signal, the Precise Positioning System. Until early in the
year 2000, the U.S. military actually introduced an error in the satellite mes-
sage as to where the satellite actually is, which could result in triangulation
SELECTIVE errors of up to 100 meters. The error was called selective availability,because it
AVAILABILITY made accurate positioning available only to those users selected by the U.S.
military. This was intended to prevent a hostile country from putting a GPS
receiver on a guided missile and precisely targeting where the President is
giving a talk. Selective availability was turned off in part because of the rise
of civilian uses of GPS, and because it led to interoperability with groups
working with the U.S. military who were using commercial, not military,
GPS.
Many inexpensive hand-held receivers sold to hunters and hikers attempt
to improve on localization by averaging or filtering the readings. This can
reduce the error down to 10-15 meters. Surveyors and GPS specialty compa-
nies such as Trimble and Rockwell have found a way to subtract the error in
the public channel and get performance near the Y-code’s rumored accuracy
DIFFERENTIAL GPS of centimeters. The method is called differential GPS (DGPS), where two GPS
(DGPS) receivers are used. One remains stationary, while the other is put on the ro-
bot. If the two receivers are observing the same satellites, then any sudden
change in position on the stationary “base” receiver is due to the induced
error and can be subtracted from the readings at the robot GPS. The ultimate
fix to the induced error will probably come in a few years due to the com-
mercial sector. A consortium of private companies is planning to launch a
new constellation of GPS satellites, Teledesic, which will emit accurate data
at all times in a format that can be decoded only by chips licensed by the
consortium. Teledesic is scheduled to go on-line in 2004.
GPS and DGPS are not complete solutions to the dead reckoning problem
in mobile robots for at least two reasons. First, GPS does not work indoors
in most buildings, especially offices or factories with large amounts of steel-
reinforced concrete. As with cellular phones, these structures interrupt the
reception of radio signals. Likewise, GPS may not work outdoors in major
URBAN CANYONS cities where skyscrapers act as urban canyons and interfere with reception.
Second, commercial DGPS systems cost on the order of $30,000 USD, which
is prohibitively high. Several web sites now offer free “do-it-yourself” DGPS
code to create a DGPS from two inexpensive receivers.