Space Station Using GPS in Attitude Control

The Global Positioning System, used in a wide variety of applications on
Earth, is performing a new task in space. It is determining the
attitude, position and speed of the International Space Station. This is
the first successful use of GPS data in attitude control of a
spacecraft, NASA officials and scientists believe. It is working well,
feeding information on the station’s attitude to systems that control
its orientation in space. GPS also is providing more precise speed and
position data than had been available. “As far as I know, no one else is
using GPS operationally for attitude determination,” said Johnson Space
Center’s Susan Gomez, SIGI chief engineer. SIGI stands for Space
Integrated Global Positioning System/Inertial Navigation System.

The
space station uses only the GPS part of that system. GPS antennas,
brought to the station on the S-Zero (S0) Truss in April, were the final
piece of the system. The antennas are situated in a 1.5- by 3-meter (5-
by 10-foot) rectangle on the truss – far enough apart to precisely
determine attitude, but close enough together to resolve other issues.
Also, data received by the antenna can be distorted by “multipath,”
degradation of the GPS signal because of the reflectivity of large
objects on the space station, including the 240-foot-long solar wings.

As the station is assembled in space, the multipath changes. Multipath
(the phenomena that causes “ghosting” on television sets) had to be
calculated using new techniques, including mathematic models verified by
ground tests and tests during space shuttle flights. The antennas feed
information to two GPS receivers in the U.S. laboratory Destiny through
100-foot cables. Because the length of the cables changes the GPS
signal, engineers developed special software for the receivers, which
came to the station in Destiny on STS-98 (station flight 5A) in February
2001. “This represents a considerable improvement in the accuracy and
immediacy of information available on the precise position of the ISS,”
said Bill Gerstenmaier, deputy Space Station Program manager. “Having
that information can improve the quality of data from some experiments
on the station.”

The GPS position of the station, which is moving at
about five miles a second, is accurate to within 100 meters and is
updated continuously. Previously the station’s position was determined
using ground tracking and other techniques. That information was quite
accurate, but generally was updated only once a day. Just before an
update, the actual and propagated position of the station could differ
by as much as 10,000 meters.

The Global Positioning System includes 24 satellites and one or more
orbiting spares. The 24 satellites are at an altitude of about 12,600
statute miles in six orbits (four satellites in each) inclined about 55
degrees to the equator.

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