The controversy over GPS

Learning advanced navigation for the USCG captains' license -
is this obsolete?

Periodically, and seemingly with increasing frequency, a cruiser somewhere posts a rant about how technology has reached the stage where paper charts and traditional navigation have been rendered obsolete. Some of these cruisers are very experienced (and I would have thought they'd know better).

I feel the need to put forth my proposition for why reliance on one technology that has its flaws is a really undesirable position to be in and has potentially serious consequences for the safety of the crew. It also undermines good seamanship practices.

In preparing for the USCG captains license and master's examinations, one must prove proficiency in multiple forms of coastal navigation techniques. Learning dead reckoning, how to take noon sites, and celestial navigation all have their place alongside GPS in my book.

In fact, the US Naval Academy has recently decided to go back to teaching celestial navigation. There must a good reason why. And that reason is cyber attack. "After all, you can't hack a sextant," reports the Military Times.

Type of sailing

First, let me state that the Global Positioning System (GPS) alone may be fine for regional sailing in coastal waters that are relatively well known. If you can find your way to safe harbour without assistance, then chances are you can do so with GPS alone, as long as you don't rely on a single source of GPS. Of course, coastal waters are strewn with rocks and shallows which, without an accurate chart plotter or paper chart, could come out to bite you.

Not so at sea, for the most part. In the middle of the Atlantic, if your GPS fails, then you know that if you head either due east or due west you will eventually come upon land. Let's hope you can avoid the Azores or Bermuda, the only obstacles lurking in the open waters of the North Atlantic. As long as you don't run into land, you should be fine. Radar could be a good back up when approaching coasts.  After all, it's the landing that's usually more dangerous than the sailing across oceans.

But if you are sailing into new waters, you will certainly want some form of back up other than electronic. We saw what could happen when our boat was struck by lightning. So when we sailed across the Atlantic, we always had backups: two sextants, a Nautical Almanac, the required tables, a handheld navigation computer for automated sight reductions, star charts, and paper navigational charts. By using the equipment routinely as backup, we were assured we had not lost our ability to navigate by alternate means.

Let's look at some reasons why we shouldn't abandon the old ways.

Risk of lightning

Most sailors have not had the experience of a lightning strike, thankfully. We have. A previous vessel we had was struck twice by lightning, and it had a brush dissipator on the mast head. The second time, the vessel took a direct hit - fortunately we were not aboard. Although there was no visible damage to her structure, all the electronics, the wiring and electrical systems had to be replaced, including handhelds. Everything had been fried.

Consider that a study published in Science magazine concluded that climate change is causing an increase in frequency of lightning strikes. For every one degree of temperature increase, the number of lightning strikes will increase by 12%. It may not sounds like a lot, but in lightning prone regions, it could easily significantly increase the risk of being struck.

GPS anomalies

We live in an area of the world where GPS anomalies are common and even reported on charts and in cruising guides. Those are the known ones. We have personally found several areas that have not been identified as having anomalies but where our GPS units in tandem reported very strange findings.

In Maine, I was at the helm and watching the chart plotter while passing between two islands. My visual reference was in line with the electronic one. Suddenly, the chartplotter showed us miles from where we were and rapidly approaching the shore of one of the islands I was passing between. I quickly spun the boat around and tried to retrace our incoming track. Even though I could see we were no where near the island, something made me react to the chartplotter, just in case.

In Scotland, a similar thing happened except the GPS kept zig-zagging our course like a sewing machine while I was driving a straight line.  (And yes, I was at the helm again.) Our back up GPS did the same at that location. And of course it was in a narrow convoluted, rocky channel. We were told that there are regions there which have strong magnetic fields that can affect the compass significantly, but can they affect GPS readings?

Another time, a similar thing happened in Ireland, with zig-zags replacing our perfectly straight line. That made us think there was something else afoot here. But what? Someone testing a jamming device? Interference by some unknown source? We'll never know.

Having GLONASS, the Russian satellite navigation system activated in October 2011 which is accessible by many smartphones, can increase reliability of positioning system readings. Keep in mind that both GPS and GLONASS are military systems and can be turned off at any time.

GPS accuracy

DGPS (differential GPS) improves the location accuracy of GPS from the 15-meter nominal accuracy to about 10cm when implemented under optimal conditions. GLONASS is accurate to 2.8m under the best conditions (no cloud cover, tall structures, or radio interference). Using two systems that are independent of each other has the potential to improve on the reliability of either system alone.

The inaccuracy of GPS chartplotters is a whole different story. When you overlay GPS accuracy onto charts with inaccurate datum, you run into some problems. The datum in many parts of the world and the charts based on that datum can date back to the 1800s without update. They were far less precise than the charts generated through new surveys today. Charts have been off by 2 miles or more.  In our inlet, we see exactly what that means. Whereas by sight and dead reckoning we know we are exactly in the middle of the channel, the GPS chartplotter shows us traveling over land. The entire bay is offset by about 1/2 mile to the south.

We have sailed in places where the chart plotter showed us far away from where we knew we were by dead reckoning. Without paper charts and visual corroboration, we would end up on the rocks more often than not.

Of course if you can download google earth images and overlay them with charts, you'll have the best of both worlds but they won't always match up. And getting a google earth download mid-ocean requires a sizable investment.

Taking these chart inaccuracies into account, it is no real surprise that computer chart programs which update the charts as you move have become popular with commercial and recreational fisherman. They take position information from the boat's GPS and depth information from the sounder and combine it to make updates to the charts which are then displayed. The resulting charts are not only more accurate, but also a lot more detailed. Navionics Sonarcharts are an example. Crowd sourced bathymetry may be the future of cartography.

But it still doesn't eliminate what the US military worries about.

GPS blocking and hacking

Michael Peck in the National Interest Magazine reported on January 14, 2016 that the Pentagon is worried about our national GPS system being hacked.  Apparently that is very easy to do.  My husband, Alex, researched it and used it for the plot of his second thriller novel. I thought it was pretty cool that he imagined this device that could block the GPS signal and then replace it with a bogus one.  It turns out that he's not the first to have imagined it.

GPS blind spots, spoofing and hacking are all real threats to the system. It's only a matter of time.

GPS in a time of war

Keep in mind that GPS was developed by the US military as a device to assist in the highly accurate delivery of smart bombs and military navigation.  They opened it up to civilian use as a matter of safety when they realized the value for civilian airliners and shipping. But they reserve the right to block the signal to civilians if necessary. And they occasionally do shut down the DGPS signal: differential GPS improves the location accuracy from the 15-meter nominal GPS accuracy to about 10cm when best implemented. Maybe that's what accounted for our zig-zagging experiences.

US Government's position

Recent budget cuts in military defense spending have raised questions about whether the aging satellites currently in orbit that support the US GPS array can be replaced. Many GPS satellites are beyond their designed lifetime and are likely to become less reliable in the future. But the military have made it clear that alternatives are required.

Alternatively the US Navy has called for small business innovation research to develop a Positioning, Navigation and Timing (PNT) system that allows navigation by radio signals.

The US Air Force is looking at PNT using low flying mini-satellites that can still transmit when then main satellites are blocked.

Loran-C as backup

Loran-C was a brilliant US land-based system that was decommissioned as obsolete in 2010. It is a radio navigation system that triangulates position based on radio frequencies transmitted from towers on the ground.  The Navy has proposed reinstating the system noting that the technology can be used by civilian airliners if their GPS navigation is disrupted.  (I wish I had kept our old Loran-C unit!)

Will this happen?  It might. But it's only useful near the coasts as is technology that depends on mobile signals. There are no mobile signals out there in the middle of the ocean. Not yet.

What's a good backup system?

I was a girl scout, and like it or not, I subscribe to the motto: BE PREPARED.

We have redundant GPS systems on board. We have the primary chartplotter unit, a backup GPS unit, handheld GPS, two smartphones with Navionics navigation apps - one with GPS and GLONASS, and a dedicated laptop with navigation software and a GPS mouse which we could fire up if needed. Of course, the smartphones use mobile signals for GPS so they can only be used on coastal approach but that's when you really need them.

We do not have a tablet, although my new laptop is a hybrid with Windows 10 and touchscreen. Unfortunately, the navigation apps don't work on it yet.

Onboard we carry paper charts for all waters we will be visiting and guidebooks with chartlets for the local harbours. We do not carry paper charts of all the harbours; that would be too expensive. But the guidebooks we have tend to have very good representations of the hazards in harbours, and often better information than carried on old charts. The chartplotter has worked its magic in most cases.

When the European GPS system, Gallileo, is in place, at least there will be a backup system available as long as you have the receivers to pick up its satellite signals. Unlike US GPS and Russia's GLONASS, Galileo was conceived, developed and will be operated under civilian control.  It is not intended for military applications. As of December 2015, 12 of 30 satellites were in orbit. Galileo will start offering Early Operational Capability (EOC) from 2016, go to Initial Operational Capability (IOC) in 2017-18 and reach Full Operational Capability (FOC) in 2019. The complete 30-satellite Galileo system (24 operational and 6 active spares) is expected by 2020.

China is also developing a global navigation system which is expected to be operational in 2020, too.

And in fact, traditional Polynesian navigation is making a comeback. Check out this very cool site on wayfinding.

Whatever you choose, I think the question of whether you need backup or not will remain unanswered until the day the lights go out on Broadway. And then, you'll want everything you can get your hands on.


  1. The current challenge I'm having is finding someone to teach me celestial nav! Those who know it well enough to teach are becoming rare, it seems.


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