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.