GPS is designed so that
typically four to eight satellites are visible simultaneously at any time from
most locations in the world. The GPS satellites, at an altitude of 20 000 km,
transmit down to the Earth carrier signals at two L-band frequencies (1.227
and 1.575 GHz) which are modulated by a pseudo-random noise code. The two frequencies
enable the user to remove most of the signal delay originating in the ionosphere.
When four satellites are in view, the user has enough information to solve for
the station position and the clock offset from GPS time.
A worldwide network
of 30 stations is permanently operated for global applications of interest to
IERS, earth rotation and the terrestrial reference frame. Polar motion is determined
daily with a precision of +/-0.2 milliarcseconds. The high-frequency variations
of universal time are determined daily with a precision of +/-60 ms, the low
frequency accuracy being limited by the instability of the orbit orientation
due to unmodelled forces acting on the satellite.
A major strength of
GPS for IERS is the possibility of fine densification of the terrestrial frame
with a precision of +/-1 cm by connecting regional campaigns to the permanently
operated network. The use of GPS as the technique for determining precise orbits
in a number of present and future geoscience space missions also opens wide
possibilities of interconnecting a variety of reference systems.