where and has the usual issues inherent with any radio reception. Issues specific to GNSS have already been covered -it requires an antenna with a wide-angle view of the sky as well as protection against lightning and weather; it is sensitive to signal jamming and can suffer signal degradation problems due to poorly routed antenna cables. Resilience One radio-based navigation system that started during World War II is the LORAN system, which evolved with several generations of signals, finally ending up as LORAN-C. Over the last 10-20 years, LORAN has been undergoing decommissioning, and many of the remaining transmitters (at least outside the far east FERNS timing chain) were shut down at the end of 2015. However, during the time that its use was declining, there were proposals to upgrade this system into what is referred to as enhanced LORAN (eLORAN). eLORAN would use a similar approach to the LORAN-C system, with very high-power LF (90-110 kHz) transmitters broadcasting signals for PNT services. This would allow the users' receiver to have access to a signal millions of times stronger than GPS and available in demanding locations, such as inside buildings or even under- ground. Testing was carried out in the United States, and some pilot stations even became opera- tional in the United Kingdom. The idea was to use eLORAN as a backup to space-based GNSS systems in case of any outages caused by events such as jamming or space weather. It was planned for eLORAN to broadcast sig- nals that would allow receivers to recover UTC time with an accuracy of 50 ns, as well as meet the stratum 1 frequency standard. Such a system has the following very definite potential advantages for such a role: The high power (hundreds of kilowatts) would make it almost impossible to jam. The long wavelength would enable it to propagate very well around the earth and easily penet- rate buildings, making an outside antenna unnecessary. Its terrestrial base would (hopefully) make it more resistant to space-based weather events. Alas, the 