Review of Random graphs: (second edition) by Béla Bollobás

A means of accurately determining the position of a selected location such as at the end of a center-pivot irrigation system by using two Global Positioning System (GPS) receivers. The first GPS receiver is located at a fixed, known position such as at the center of the pivoting irrigation system while the second GPS receiver is located on a moving portion of the irrigation system. The first GPS receiver compares the GPS receiver reported position against the actual position and reports by wire or radio link the position differences to the second GPS, where the differences are used to remove errors from the second receiver's measured position.