Hi all -
Story one: In the 90's I worked on a (reefer-)vessel where we had two GPS's. One at the chartroom table and one port side front wheelhouse. I often noticed that the time display showed different values, one or two seconds if I remember correctly. I did not worry about it since I always used a mechanical chronomter (Rolex GMT Master which I bought in my earlier days as a Navigator long before I realized that there are other things in life worth spending money on) which I always checked against the ship's official chronometer before taking sights. Said official chronometer was of course checked daily by Sparkie). Did not think very much about the differencies in the GPS's timekeeping, then.
Story two: In 2010 I was teacher/NO on a sailing vessel belonging to Swedish Royal Navy. One of the courses was astronav. When we equipped the vessel with almanacs, sextants et cetera we also decided to get a real chronometer, old fashioned style in a very carefully padded mahogony box. This machine was electric (battery powered quartz), had all official documentation and certificates. The reason for buying it was of course to give a message to the cadets: In astro You dont play around with Time.
After a couple of days when we started it (which was much easier compared to mechanical chronometers, so the pedagogical value was somewhat lesser than we intended) and the cadets were using it practically 7/24.
Several times each day it was checked against another source - GPS! After another couple of days we noticed that the chronometer had drifted close to 20 seconds since last check (still compared with GPS). At this stage we were very disappointed with the chronometer and seriously planned to return it to the central store, who in turn certainly would have returned it to the supplier, at gunpoint literally. It was expensive and of course you do not expect such a clock to jump away like that.
I had brought along my handheld GPS (Garmin) (as well as my old friend Rolex) and checked the GPS times on the ships GPS vesus my handheld (and wristwatch) and to our great surprise there was a 20 second difference. Next step we checked against radioed time signals (Honolulu) and it showed that the electronic chronometer, my handheld Garmin and my Rolex were right, and the ships GPS was wrong (by 20 seconds).
Restarting the ships GPS solved the problem, for at time. After a few hours it had drifted away again. My only explanation to this is that the ship's GPS had two "clocks": One for strict internal use (position etc was very accurate which it certainly not would have been with incorrect time information) one for the displayed time. (Does anyone on the NavList have any other explanation?).
After that, needless to say, we checked our new chronometer against radiod time signals (thank You, Hawaii). It went perfectly well and did not lag or gain more than a second for 4 weeks, and after all, buying it was not such a bad idea after all.
I still check my mechanical wristwatch before using it for astrosights, but after this experience I check it by time signals, not by GPS.
(My handheld Garmin have never let me down, not even on time-presentation, but I have developed allergia against short-cuts when you really don't have to).
Best to you all -
Magnus Sjoquist
Sweden
Den 09 feb 2012 kl 11:09 skrev Jaap vd Heide <Jaap.vdHeide@xs4all.nl>:
LOL
That's what I have a Garmin Forerunner 305 for. It gives you latitude and longitude. Oh, and time too. ;-)
Geoffrey Kolbe wrote:
The Suunto X10 watch would be my choice.
It also gives temperature, pressure and altitude. The time on the
watch can be synchronized with GPS time, so it is always accurate to
the nearest second.
..... and, it gives your latitude and longitude so you can check on
your results ;-)
Geoffrey Kolbe
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