Satellite-based navigation and positioning is dominated by the Global Positioning System( GPS) in America and Europe. The GPS system can be used if the navigation device has a line of sight to at least three GPS satellites. This is not always the case. For example, GPS positioning is not possible in tunnels, and satellite reception is also not possible in mountain valleys, urban canyons and under water.
To overcome these limitations, the Defense Advanced Research Project Agency( DARPA) is developing an alternative to GPS-based satellite navigation with the PNT project, "Positioning, Navigation, and Timing."
The PNT architecture is intended to eliminate the coverage gaps, uncertainties, and limitations that the GPS system has. These include potential GPS signal degradation due to spoofing. The PNT project has several goals, such as making navigation largely independent of navigation satellites, rapidly integrating PNT sensors and components into PNT systems, and improving position determination by using commercial satellites and terrestrial radio transmitters. Thus, the PNT system relies on the global navigation system( GNSS), which has better position accuracy and availability compared to the GPS system. In addition, some services can rely on other value-added content with higher data volume and lower latency.
Other PNT activities are concerned with the development of high-precision gyrosensors in microsystem technology( MEMS), as well as clock generators, inertial measurement units, inertial measurement units( IMU), and atomic clocks.