Black Point, Anglesey – courtesy Kris Williams

"And all I ask is a tall ship and a star to steer her by." When John Masefield wrote that line in his poem Sea Fever, it was a sentiment that would have been instantly recognised by generations of sailors who ploughed the seas for thousands of years before the advent of satellites. Today, in an age when even mobile phones come equipped with GPS, it is hard to imagine a time when the world's great navies sailed around the globe relying on the sun, the moon and the stars – and the skill of their navigators – to get them safely to their destination.

Celestial navigation (CelNav) has not been formally taught in the US Navy (USN) for at least ten years, finally disappearing from the curriculum at the last of the USN training establishments in 2006. However, as awareness of the potential vulnerability of high-tech navigation systems to a range of possible threats, including hacking, is growing, this is about to change: The Chief of Naval Operations has put CelNav – alternatively known as astro navigation – firmly back in the classroom as a core competency for navy officers.

The Naval Academy resumed teaching CelNav in the summer of 2015, with a course that has been adjusted to contain elements of both celestial navigation and the potential cyber threats on navigation systems.

"Naval Academy midshipmen are receiving three hours of instruction in the fundamentals, the theory and concepts, of celestial navigation as a part of our semester-long advanced navigation class, required of all 2nd class midshipmen," explains Judy Campbell of the US Naval Academy's Public Affairs Office.

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When these students graduate in 2017, they will be the first to leave the institution with an understanding of CelNav in more than a decade.

Preparing the fleet

Alongside the Naval Academy, the Naval Education and Training Command (NETC), which conducts training for quartermasters and navigators, and has responsibility for Naval Reserve Officer Training Command (NROTC) units, is also involved in the renaissance of this traditional skill.

"Celestial navigation will be incorporated into the new 'A School' training and we expect this new course to be ready in FY17."

NETC public affairs officer Lieutenant Commander Kate Meadows says that its continuous scrutiny of requirements for training and improvements for fleet readiness led to it the reintroduction of celestial navigation in 2011 for officer-rank navigators and assistant navigators. That course is taught at the NETC's Surface Warfare School in Newport, Rhode Island.

"We are currently re-building the training curriculum for our Quartermasters – the enlisted QM – rating. As part of this, celestial navigation will be incorporated into the new 'A School' training and we expect this new course to be ready in FY17," says Meadows.

The Quartermaster Journeyman school that is to begin next summer will also teach celestial navigation, and additionally, pilot courses for CelNav are due to start this autumn at three of the NROTC units – Philadelphia Consortium, University of Rochester and Auburn University.

When systems fail: the threat of hacking, spoofs and jams

The thinking behind the US Navy's decision to resume teaching this time-honoured maritime art is clear. global navigation satellite systems (GNSS) undeniably provide inputs of unrivalled accuracy into warship electronic chart display and information systems (WECDIS), but there is always the risk that one day, those inputs may just not be available.

A range of factors could cause such a GNSS denial, including aerial damage, accidental interference, malfunctioning tracking systems, the satellites suffering loss or damage and major natural events such as sunspot activity, solar flares or geomagnetic storms. However, the one possible cause that has, understandably, drawn most attention is hostile hacking, which presents a particularly insidious threat.

An abrupt loss or degradation of the GNSS signal typically sounds an alarm – and widely, often across a surprising range of on-board devices. Everything from GNSS receivers, automatic identification systems, global maritime distress and safety systems and WECDIS, through many kinds of timing devices, all the way to some types of echo-sounders which have GPS feeds, will notice and raise the alert.

A good hack, on the other hand, will not. By carefully managing what is known as a 'graceful degradation', or spoor, of position and time, the hacker can shift the vessel's apparent location away from the true one in a way that masks what has happened, and so avoid automatic detection.

Even without the relatively sophisticated equipment and expertise needed to pull off spoofing, because GNSS signals are of extremely low power, they are susceptible to simple low-tech jamming. In a presentation on the future of celestial navigation, given at the British Embassy in Washington in April 2013, Dr Steve Bell, Head of the HM Nautical Almanac Office at the UK Hydrographic Office (UKHO), revealed that, according to a two-year government study, a 1.5 watt transmitter can render GPS unavailable for 30km.

The implications of that are staggering. In 2008, a Trinity House vessel was subjected to a deliberate GPS and differential global positioning system jamming exercise off Flamborough Head. Speaking at the UKHO's Astro-Navigation Solutions for the Future meeting in 2012, Alan Peacock, a former Royal Navy advanced navigation specialist, and the man who completely re-vamped the Admiralty Manuals of Navigation, described how it fared. Even though the vessel "knew exactly when and where it was going to have its GPS jammed [it] was completely unable to navigate and had to stop in the water."

Feats of navigation

GNSS clearly needs a backup, and the case for celestial navigation in that role pretty much makes itself – it cannot be jammed, it cannot be spoofed and it is not controlled by any military or political organisation.

"It cannot be jammed, it cannot be spoofed and it is not controlled by any military or political organisation."

Of course, it is not a perfect system. In many parts of the world, the weather means that it is, at best, only intermittently available, its one minute of arc/one nautical mile accuracy does not even begin to approach the precision of GPS – and it obviously requires training and skill. But, as Mary Taylor of CelestialNavigation.Net says, "it can be life-saving when the chips are down."

Taylor highlights some truly extraordinary feats of celestial navigation: From the days before GPS, Frank Worsley, the navigation officer whose pinpoint accuracy during awful weather saved the lives of Shackleton's expedition and Captain Bligh of Bounty fame, navigating over 3,500 nm to safety. In the modern age, the father and son who sailed around Cape Horn and Steve Callahan, lost at sea for 76 days in a raft and finding his course with a sextant made out of a pencil.

"I knew some old-time celestial navigators in Maine, men who had sailed around the globe during the war, and they, and I, were really surprised – shocked – when CelNav was eliminated as a requirement," Taylor says. She is convinced that it is a skill that every offshore mariner should have – and it seems the US Navy has now come back to the same view.

While, unlike John Masefield, a star is never going to be all that the navy would ask for to steer its ships on their way, thanks to the reintroduction of celestial navigation to the US fleet, if some future systems failure or cyber attack should ever mean that is the only option available, things should not go too far adrift.