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» Online Classroom   »   » Public Discussion of Cel Nav   » What Time is It?

Author Topic: What Time is It?

 - posted September 12, 2018 09:50 AM      Profile for Randall           Edit/Delete Post 
Greetings. This is Randall of the Figure 8 Voyage (www.figure8voyage.com).

I'm a couple weeks from second launch and am wrestling with time. I hope you don't mind the random question, but I don't know anyone else to ask.

After losing my SSB radio after a very wet knockdown in the Indian Ocean last February, I used my GPS chartplotter time for sextant sights from there to home in San Francisco.

The GPS gives me time in GMT to the second and updated once a second, but I've noticed, now that I'm home, that this time differs by 5 seconds and more from highly accurate time apps on my phone (e.g. an app called "Clocks" and another called "Zulu Time." Both these agree with the highly accurate site https://time.is.).

So, I'm confused. How could the GPS chart plotter (Simrad, new last year) be wrong since it uses time signal difference to establish my coordinates? A 5-second error is going to put my position out from actual by a mile and more.

I used the chart plotter time for sights after losing my SSB in the Indian Ocean, and was chuffed that my workings were very often spot-on (<1 - 4 miles out) when compared to the GPS position, but a friend of mine joked with me recently that my sights simply agreed with the chart plotter rather than, necessarily, reality.

I am replacing the SSB, so will have access soon to time stamps, but am still mystified by these differences.

Thanks in advance for sharing if you know the answer,


Randall Reeves

From: San Francisco
David Burch

 - posted September 12, 2018 10:57 AM      Profile for David Burch           Edit/Delete Post 
Good day Randall,

I am not sure what might be the cause of any specific source having wrong time display. We have a video online on how to get accurate time by several methods including showing that they simultaneously give the same times.


We also have a note showing that $10 Casio F91-W watches are dependable.


Also I might note that GPS going forward could likely be the primary external source of time as the NIST is trying to discontinue the US time broadcasts altogether. This could happen in 2019 if they get their wish. Their 2019 budget removes support for it, and the NWS also wants to discontinue the storm warnings on these NIST broadcasts as well.

I would refer to Pub 117 Radio Aids to Navigation for alternative sources of time broadcasts worldwide. msi.nga.mil.


From: Starpath, Seattle, WA

All times are Pacific  
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