Why does the reference frame change for GPS orbits?
GPS orbits are computed from data collected by a global network of receivers coordinated by the International GPS Service for Geodynamics (IGS). The accuracy of the GPS orbits depends on many factors, including the accuracy of the coordinates of the data collection sites. The earth’s surface is not fixed and rigid like an egg shell. It consists of many sections, or plates, which move slowly over time in various directions and rates in a process called crustal motion. Scientists have been studying this movement for several reasons. This includes wanting to know where land masses are with respect to one another and where they will be in the future. Since IGS sites are located on these crustal plates, we must be able to estimate where the sites are when the data are collected. The International Earth Rotation Service (IERS) periodically computes the positions of the sites for a given date. The sites define the IERS, International Terrestrial Reference Frame (ITRF) and the date defines the
