NavList:

Maybe I've missed something in the thread about the reinstatement of celestial navigation at the USNA, but the biggest threat to GPS probably comes from jamming - and it's very real.

The vulnerabilities of all global satellite navigation systems (GNSS) have been well known for years. The root problem is that the radio signals from the satellites are extremely weak and have a very long way to travel. It’s bit like trying to spot a car headlight 14,000 miles away. To make matters worse all the 140-odd GNSS satellites soon to be in orbit operate in the same narrow frequency ranges.

Jamming GNSS transmissions is therefore very easy. Small, cheap jammers readily available on the Internet can block the signal over a few hundred meters. A powerful but still portable device installed at the top of a high mast could wipe out GNSS reception over a radius of many miles.

Another problem is spoofing: transmitting a fake GNSS signal that can make your receiver tell lies about where you are or what time it is - or even shut it down permanently. The North Korean regime has used these techniques to interfere with GNSS reception in South Korea, and there’s plenty of evidence of local jamming either caused intentionally by criminals, or just by accident.

Nature is of course also a threat. Down at sea level the electronics on board your boat may well go up in smoke if you get struck by lightning, while out in space solar storms can and do disrupt GNSS signals. Human error can’t be ignored either. A fault in the Russian GLONASS system that threw up positional errors of 50 kilometers or more in the North Sea seems to have been caused by someone loading the wrong data.

Of course we now have GNSS chips everywhere. Almost every piece of electronic equipment on board a modern ship or boat - radar, electronic chart displays, AIS, radio - is linked to a GNSS receiver. If the GNSS goes down all these systems are likely to go down with it. Not very funny if you’re passing through the Dover Straits in fog. And this isn’t just a problem for mariners. Many other critically important things - like mobile phones, stock markets, banking and broadcasting - rely on the GNSS time signature. The opportunities for criminals and terrorists, not to mention hostile nations, to cause chaos and destruction are all too obvious.

Here's what the US Coast Guard says : uscg.mil pdf.

I understand that the US military are taking steps to protect their operations against these threats. They haven’t abandoned GNSS, of course, but they employ many other systems to generate the position, navigation and timing data they need. These include tiny inertial sensors, and chip-sized atomic clocks. Just how effective these are is uncertain, but in any case such technology is not yet widely available.

So where does that leave the rest of us?

In October 2014 the General Lighthouse Authorities of the UK and Ireland announced the successful launch of a prototype maritime navigation system called eLORAN. Descended from the old LORAN-C, this ‘enhanced’, digital version can provide positional fixes within a radius of 10 meters and can meet the exacting pilotage needs of ships using UK ports. Best of all it’s completely independent of GNSS - and far more robust.

However the prototype network ceased operation on 31 December when the French and Norwegian governments closed down their transmitting stations on economy grounds. The UK transmitter is still operational and usefully continues to provide an accurate time signal, but there are no plans at present to restore the European eLORAN capability. See rin.org.uk article.

The good news is that the US Administration is aiming to address the vulnerabilities of the GPS timing signal by establishing a "timing-focussed" eLORAN capability. See rntfnd.org article.

In the meantime it probably is a good idea to brush up your celnav skills - just in case!

All the best

David