In a post entitled “Resilient Navigation in Demanding Maritime Environments” dated 23 July 2015, the European Global Navigation Satellite Systems Agency discusses the dangers of depending upon just space signals and the need for resiliency.
“But with the increased reliance on GNSS comes a correspondingly heightened potential risk if anything should go wrong with the signal… It is challenges like this that drove the discussion at the recent TransNav 2015 conference.”
“Complementary PNT Systems – There are already several different and complementary sources of PNT information. GNSS provides a three-dimensional position, velocity and time solution when at least four satellites are in view for a given system, such as those belonging to GPS, Glonass, Beidou and Galileo (as from next year). If constellations are combined, such as GPS and Galileo, an extra (fifth) satellite is needed – three for the position, one for the GPS time correction and one for the Galileo time correction. Meanwhile, land-based radio-navigation systems can provide a two-dimensional position using at least three stations. These systems include e-LORAN, which is upgrading and replacing LORAN-C, using solid-state transmitters, precise timing (using atomic clocks) and a data channel to provide correction and integrity messages.”
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