Pitfalls of navigation
The article by Daria and Alex Blackwell ‘Can You Rely on GPS?’ in the August edition contains a number of errors:
• Loran-c worked on time differences not triangulation. It was not just in the US, there were chains elsewhere; Britain used the technically similar Decca system.
• The proposed replacement is eloran (Enhanced Loran), which uses a different technology, critically including precise time signals and differential corrections. It provides a sensible PNT alternative to GPS, which a different GNSS system does not do as the transmissions are much more powerful and difficult to jam.
• GPS accuracy is usually better than 15m, but to achieve 10cm requires a professional receiver and time to take averages. Signals are prone not just to jamming and anomalies, but also to outages and onboard faults.
• UK maritime qualifications, for unlimited range, require competency in astronavigation, which has never been dropped from the syllabus, as it was in the US.
• It is the survey data that dates from the 1800s, not the charts. Reading NP100 would assist in understanding what is shown on modern charts, which is not for the reasons assumed by the authors.