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Transformational Technologies 4 From Sextant to Satellite

Feb 28, 2008 At the time of our first long distance cruise in 1984, across the Gulf of Mexico from New Orleans to the Yucatan Peninsula, the only form of electronic navigation was Loran. It was not always reliable, and was subject to significant errors, especially close to land where you need the most accuracy. Because it utilized shore-based stations to send out its signals, its range was limited – by the time we reached the Yucatan (600 miles south of New Orleans), the signal was intermittent, and shortly thereafter we lost it. From then on, we relied on traditional navigation and piloting techniques (primarily a hand-bearing compass for inshore work). Loran was, nevertheless a startling advance over the sextant, which I carried and knew how to use in those days.

A year or two later we installed a first generation satellite-based navigation receiver, a forerunner of GPS. It had more-or-less worldwide coverage, but with very few satellites you had to wait a considerable time to get a fix, sometimes hours. Every once in a while it was wildly erroneous (it put us in the Indian Ocean on one occasion), and in fact it seemed to get increasingly inaccurate over a period of time, and then snap back to accurate fixes. It took me months to correlate this with the occasions on which we ran our engine (not so common in those days as it is now), and to realize that the longer we ran the engine, the more inaccurate the fixes. I eventually concluded that we had a defective cranking battery and that every time this was switched into the boat’s circuits it had some strange effect on the satnav’s internal clock, fractionally slowing it down to produce increasingly out-of-whack fixes.

In 1987 we were crossing the Mona Passage from the Dominican Republic to Puerto Rico. The satnav chose this time to go 11 hours without a fix which I needed to judge the strength of the current through the passage. This was the last time I used a sextant in earnest. I finally took the sextant off the boat in 2004.

The early GPS units began to make it into civilian markets in the late 80’s, but were horrendously expensive. The cheapest, a Magellan, retailed at over $4,000. We held off until the price dropped below $1,000, and then took the plunge. We sailed to Belize, Guatemala and Honduras, where the last widespread survey work was done in 1844. While we were there, two cruising boats were destroyed on the Belize reefs at night because the owners were using this new-fangled satellite-based navigation thinking they could take fixes off the GPS and plot them directly on charts that did not support that level of accuracy (at that time, some of the Belize reefs were charted at up to half a mile out).

The GPS accuracy was, in any case, no better than 30 meters, and sometimes much worse (we got the occasional fix a mile or more out). This was because the US Defense Department, which controls the system, was deliberately programming errors into the civilian signal. This was known as Selective Availability. It introduced a degree of uncertainty that forced a cautious navigator to back up the satellite-based system with traditional techniques. On one memorable occasion in Cuba, this saved us from putting our boat on a reef.

In 1991 we were in Belize, making a chart of a narrow channel. All of a sudden, we got spot-on accuracy from the GPS. We couldn’t understand it until we tuned in the BBC world service and discovered the Americans had attacked Iraq (the first Iraq war). The US army didn’t have enough GPS units programmed to receive the accurate military signal and so bought units in the civilian marketplace and temporarily suspended Selective Availability to give these units military-grade accuracy. Selective Availability was eventually ended by President Clinton, although it can be reinstated any time the US Defense Department so decides (freedom from the US Defense Department is the strongest argument for the independent European Galileo system).

Nowadays, of course, we have GPS with Wide Area Augmentation System (WAAS) or equivalent corrections that consistently delivers 2 meter accuracy on a constant basis – a higher level of accuracy than that used to produce almost all paper and electronic charts.

What an incredible technological achievement this has been. Like almost all modern sailors, I use it to full advantage. But I also recognize that it has taken something away from the sailing experience. There is no longer that mingled sense of apprehension, and then delight, when making a landfall after a long passage and discovering that you are where you thought you were (or at least, pretty close).

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