Navstar

NAVSTAR stands for Navigation System with Timing And Ranging. It is the United States Global Positioning System (GPS), a satellite-based navigation system that provides geolocation and time information to receivers worldwide. The system enables precise position, velocity, and time data for civilian and military users, functioning independently of weather or terrain.

The program was developed by the U.S. Department of Defense to provide all-weather, around-the-clock navigation. The first NAVSTAR satellites were launched in the late 1970s, with full global availability achieved in the 1990s. Civilian accuracy improved significantly after the deactivation of Selective Availability in 2000, which had previously degraded public signal accuracy.

GPS comprises three interacting segments: the space segment, the control segment, and the user segment. The space segment consists of a constellation of medium Earth orbit satellites with on-board atomic clocks and transmitters. The control segment includes ground stations that monitor satellite orbits and clock errors and upload corrections. The user segment comprises receivers that compute position and time from signals broadcast by multiple satellites. Each satellite broadcasts time-synchronized signals on several frequencies; civilians primarily use the L1 C/A signal at 1575.42 MHz, while newer signals include L2C and L5, with encrypted P(Y) and M-code signals for military use.

Modernization and ongoing development aim to improve accuracy, integrity, and resilience. GPS III satellites introduce stronger signals and new civil channels (such as L1C and L2), enhanced anti-jamming capabilities, and better compatibility with other global navigation satellite systems, maintaining GPS as a cornerstone of global positioning and timing infrastructure.