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 address these limitations, the Defense Advanced Research Project Agency (DARPA) is developing an alternative to GPS-based satellite navigation
withthe PNT project, "Positioning, Navigation, and Timing."
The PNT architecture is intended to address the coverage gaps, uncertainties, and limitations that the GPS system has
. The PNTproject has several goals, including 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. Other PNT activities include the development of high-precision gyrosensors in microsystems technology (MEMS), as well as clock generators, inertial measurement units, inertialmeasurementunits(IMU), and atomic clocks.