The Past

Sat 5 Dec 2009 13:08

We have 8 GPS receivers on board including the main plotter, 2 hand held, 1 emergency beacon, 1 for the AIS(Automatic Identification System), 1 for the satellite phone, 1 mobile phone, and 1 camera. Even if we had a complete power failure and a lightning strike, it is unlikely we would have to resort to astro-navigation.
A few days ago, when we reached half distance, as a token to days past, we attempted to fix our position using the method tried and tested by generations of seafarers over hundreds of years. We got the sextant out of the box and took a noonday sunsight for latitude, and an afternoon sight, two and a half hours later, for a position line crossing our course. By runnning the first line forward along our course by the distance travelled in the meantime, we could fix position.
Having taken the noon shot, I worked it out straight away, as the calculation is simple. The latitude given was 2 miles in error when compared to the boat's main GPS; not a bad result to get from the deck of a  small boat rolling down the trade wind in 3 metres of swell which frequently obscured the horizon. Encouraged by this we took the afternoon sight, in similar conditions. Because the computation for this line is complicated, I left it until the next day. So as to avoid carrying a large almanac and two or three sets of tables,I have  a condensed almanac and sight reduction tables, which covers 5 years. Some of the tables are used more than once in different ways and the process is tedious, especially when you are working below in a rolling boat. Suffice it to say that I didn't manage to produce the position line that day, so had a second go the following day. The resulting fix was 7 miles in error compared to the GPS. 7 miles out and 2 days late.
Not good you would say, but by coincidence our horizon distance is about 7 miles so we sail along in a little circle of sea 7 miles in radius, and there is nothing to see in it day after day. So we could mark a point on the small scale Atlantic chart, and when we do so we note that the thickness of a pencil line on the chart is about 5 miles so greater precision is not essential. Then we head towards Grenada, and the 7 miles error makes no difference to the course to steer which has been 260 for several days. When we get closer we will almost certainly see cloud streaming off the island before we sight it, and having sighted it, hopefully from 30 or 40 miles away, we could make final adjustments.
The principal errors incurred in this type of fixing can be summed up as instrument errors, sighting errors, chronometer errors and dead reckoning errors. On a bad day these errors would compound to give a discrepancy of 7 miles or more. On a good day they might cancel to give less than 2 miles. Before GPS no one really knew, so positions had to be treated with caution. There are also multiple opportunities for making errors in the calculations.
Star shots can give better results as three or more position lines can be gained simultaneously, but the star must be shot when you can see the star and the horizon is still visible ( a very small window of opportunity).
Oh yes, and everything depends on a clear sky! Nowadays, for a few tens of pounds ( much less than the cost of a sextant and tables), anyone can buy a small GPS which will produce a position which is not 7 miles out and 2 days late, but better than 7 metres in 2 minutes from switch on.
The wonders of modern technology.
"They don't know they're born!"
JF Dec 2009