davec
New member
Why should you not use the time from the GPS in reducing sights?
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reducing sights with GPS time
- Thread starter davec
- Start date
Twister_Ken
Well-known member
Eh?
If you've got a working GPS, why would you want to reduce a sight?
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If you've got a working GPS, why would you want to reduce a sight?
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tome
New member
There is no reason why you cannot use time from GPS.
However, many people would regard the use of sights as a back-up in case of GPS failure in which case it would not be sensible to rely on GPS time. Far better to use GPS to rate your chronometer(s) whilst it is working, then use the chronos for sight reductions.
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However, many people would regard the use of sights as a back-up in case of GPS failure in which case it would not be sensible to rely on GPS time. Far better to use GPS to rate your chronometer(s) whilst it is working, then use the chronos for sight reductions.
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alex_rogers
New member
This question keeps coming up and the answers are never clear cut (are they ever!). Apparently some GPS receivers don't show the correct time - there is a correction which has to be applied to convert GPS time to UTC and some manufacturers don't apply it to the time that is displayed on the receiver. The difference is small - supposedly up to 8 seconds - but it would make a difference.
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tome
New member
There's currently a 13 second difference between 'GPS Time' and UTC, but this difference is broadcast by the satellites and I'm quite certain that all GPS receivers apply the correction to give UTC. The 13 leap-seconds will increase by about 1 sec every 18 months.
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Keen_Ed
Active member
Re: Eh?
Practice?
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Practice?
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alant
Active member
On 6th January 1980, GPS time was declared & made equal to UTC at that date.
A 'leap' second occurred on 31 December 1979, so GPS time remained equal to UTC until 30th June 1980.
GPS time is now about 12 seconds ahead of UTC.
For a sight reduction, you need to use UTC, Zulu or GMT, so GPS time is inaccurate.
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A 'leap' second occurred on 31 December 1979, so GPS time remained equal to UTC until 30th June 1980.
GPS time is now about 12 seconds ahead of UTC.
For a sight reduction, you need to use UTC, Zulu or GMT, so GPS time is inaccurate.
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stephenh
Active member
Try this :
http://gpsinformation.net/main/gpstime.htm
sorry you'll have to cut and paste I'm feeling lazy...
Stephen
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http://gpsinformation.net/main/gpstime.htm
sorry you'll have to cut and paste I'm feeling lazy...
Stephen
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tome
New member
You're falling for a common myth here. GPS receivers correct for leap seconds (it's actually 13 at the moment) and display true UTC. It's perfectly safe to use for sight reductions.
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BrendanS
Well-known member
but as has been pointed out many times, most recently by Tome in this thread, the GPS sets are corrected for the difference (the information to do this is contained in the almanac which is updated continously from information in the satellite signals)
The time shown on the display is UTC (possibly a second or two out in some sets, due to delays in processing the display signal)
<hr width=100% size=1>Utinam logica falsa tuam philisophiam totam suffodiant
The time shown on the display is UTC (possibly a second or two out in some sets, due to delays in processing the display signal)
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tome
New member
GPS- Clarification (I hope)
The leap-second UTC correction is actually transmitted every 12.5 secs as part of the navigation message. Given that it only changes every 18 months, this means that you could be a maximum of 1 second out (for up to 12.5 minutes) if you hadn't switched on your GPS receiver for a long time or were unlucky enough to encounter an update between switching off and back on again. Hardly a major problem, and who would be doing sight reductions within 12.5 minutes of leaving the dock anyway? Perhaps this is a good argument for switching on your 'spare' GPS receivers from time to time so that they are updated also.
There can be delays in outputting the time to the screen as this is not the highest priority of the GPS receiver. Most GPS receivers update time on a second epoch (and NMEA data on a 2 second epoch) and this means that they can be up to an epoch out in refreshing time. This will be fairly consistent. If you can honestly say you have a better instrument on board for time-keeping I'd be most surprised. Don't forget that after a year or 2, your GPS time would be no further adrift.
Use it to check your watches and clocks, by which I mean note the drift of your clocks over an extended period. If you subsequently experience a total power failure or loss of GPS you will be very confident in knowing your time from your clock and it's previously measured drift. It's sufficient to note your clock differences from GPS at the end of each trip in your log.
GPS is a complex and brilliantly devised system at heart. It isn't easy to fully understand, and this has fuelled a lot of mythology.
If anyone wants to see x-channel sight reductions plotted against GPS time and position, I have an article which I could post or mail.
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The leap-second UTC correction is actually transmitted every 12.5 secs as part of the navigation message. Given that it only changes every 18 months, this means that you could be a maximum of 1 second out (for up to 12.5 minutes) if you hadn't switched on your GPS receiver for a long time or were unlucky enough to encounter an update between switching off and back on again. Hardly a major problem, and who would be doing sight reductions within 12.5 minutes of leaving the dock anyway? Perhaps this is a good argument for switching on your 'spare' GPS receivers from time to time so that they are updated also.
There can be delays in outputting the time to the screen as this is not the highest priority of the GPS receiver. Most GPS receivers update time on a second epoch (and NMEA data on a 2 second epoch) and this means that they can be up to an epoch out in refreshing time. This will be fairly consistent. If you can honestly say you have a better instrument on board for time-keeping I'd be most surprised. Don't forget that after a year or 2, your GPS time would be no further adrift.
Use it to check your watches and clocks, by which I mean note the drift of your clocks over an extended period. If you subsequently experience a total power failure or loss of GPS you will be very confident in knowing your time from your clock and it's previously measured drift. It's sufficient to note your clock differences from GPS at the end of each trip in your log.
GPS is a complex and brilliantly devised system at heart. It isn't easy to fully understand, and this has fuelled a lot of mythology.
If anyone wants to see x-channel sight reductions plotted against GPS time and position, I have an article which I could post or mail.
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Close hauled
Member
Why not just use a 24 pound sterling Casio digital watch which I have found to change less than 1 second a wek?
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BrendanS
Well-known member
no reason, but that wasn't the question. Also, as Tome says, can check watch drift against GPS so you know how far watch is out over a period
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alant
Active member
Re: GPS- Clarification (I hope)
The website mentioned by stephenh explains this quite well, but also Joe Mehaffey comments "your time may be off by perhaps 12 seconds until the complete NAV MESSAGE is received by the GPS". I think most people could live with a few nanosecs difference. Do you know how Selective Availability affects this difference?
Interesting that the RYA Ocean Theory course still makes a point that GPS time should not be used.
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The website mentioned by stephenh explains this quite well, but also Joe Mehaffey comments "your time may be off by perhaps 12 seconds until the complete NAV MESSAGE is received by the GPS". I think most people could live with a few nanosecs difference. Do you know how Selective Availability affects this difference?
Interesting that the RYA Ocean Theory course still makes a point that GPS time should not be used.
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tome
New member
Re: GPS- Clarification (I hope)
I think Joe is wrong to suggest that time may be out by 12 secs until the complete nav message is received, although that would be the case if you had a receiver which hadn't been switched on since 1980 or if the internal memory failed. GPS receivers retain almanac and nav data (otherwise it would take too long to acquire an initial position) so the most you are likely to be out is 1 sec for up to 12.5 mins.
SA makes very little difference to timing accuracy and can be ignored for the purposes of this discussion.
I'm guessing that the reason the RYA says that GPS time shouldn't be used is because they regard sight reductions as a backup to GPS in the event of failure.
I have facilities here to check the absolute timing accuracy of GPS receivers to 1/1000 of a second, but don't have access to every type of GPS receiver. I'll do some tests on my Garmin handheld later and post the results.
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I think Joe is wrong to suggest that time may be out by 12 secs until the complete nav message is received, although that would be the case if you had a receiver which hadn't been switched on since 1980 or if the internal memory failed. GPS receivers retain almanac and nav data (otherwise it would take too long to acquire an initial position) so the most you are likely to be out is 1 sec for up to 12.5 mins.
SA makes very little difference to timing accuracy and can be ignored for the purposes of this discussion.
I'm guessing that the reason the RYA says that GPS time shouldn't be used is because they regard sight reductions as a backup to GPS in the event of failure.
I have facilities here to check the absolute timing accuracy of GPS receivers to 1/1000 of a second, but don't have access to every type of GPS receiver. I'll do some tests on my Garmin handheld later and post the results.
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tome
New member
Correction
In my earlier post <The leap-second UTC correction is actually transmitted every 12.5 secs> should read every 12.5 minutes.
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In my earlier post <The leap-second UTC correction is actually transmitted every 12.5 secs> should read every 12.5 minutes.
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G
Guest
Guest
In fact .....
nearly all digital watchs ..... even your fiver jobs keep bloody good time and often accurate enough .......
Remember a Ships Chronometer is NOT an accurate KEEPER of time - it is when kept regularly wound a steady keeper on ERROR in time per day ..... that is the reason for checking the time signal, to ascertain its error - NOT to correct it physically. When you check your average Casio or Korean / Taiwanese digital - you often find it keeps better time than most .....
<hr width=100% size=1>Nigel ...
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nearly all digital watchs ..... even your fiver jobs keep bloody good time and often accurate enough .......
Remember a Ships Chronometer is NOT an accurate KEEPER of time - it is when kept regularly wound a steady keeper on ERROR in time per day ..... that is the reason for checking the time signal, to ascertain its error - NOT to correct it physically. When you check your average Casio or Korean / Taiwanese digital - you often find it keeps better time than most .....
<hr width=100% size=1>Nigel ...
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G
Guest
Guest
Amazing ....
Many of us at sea on ships used Transit SatNav sets to do our Compass errors / sights etc. No-one ever caused such a stir as now happens with GPS .... maybe its a product of its popularity - Transit was NBG for most people !!!!
And anyway - what accuracy do you think you expect from sights ? Before the advent of SatNav ...... we all used DR's / EP's to work our sights, and remember we are talking about from a Ships Bridge - where waves and swell didn't make the horizon so changeable as on a yacht ...... we reckoned about 5 miles for Stars and 10 miles for Noon ........... OK - SatNav derived position to use for calculation increased the accuracy marginally .....
So where is the point ????
<hr width=100% size=1>Nigel ...
<A target="_blank" HREF=http://www.qqbaltic.com/index.html>http://www.qqbaltic.com/index.html</A> For all those disbelievers ! /forums/images/icons/cool.gif
Many of us at sea on ships used Transit SatNav sets to do our Compass errors / sights etc. No-one ever caused such a stir as now happens with GPS .... maybe its a product of its popularity - Transit was NBG for most people !!!!
And anyway - what accuracy do you think you expect from sights ? Before the advent of SatNav ...... we all used DR's / EP's to work our sights, and remember we are talking about from a Ships Bridge - where waves and swell didn't make the horizon so changeable as on a yacht ...... we reckoned about 5 miles for Stars and 10 miles for Noon ........... OK - SatNav derived position to use for calculation increased the accuracy marginally .....
So where is the point ????
<hr width=100% size=1>Nigel ...
<A target="_blank" HREF=http://www.qqbaltic.com/index.html>http://www.qqbaltic.com/index.html</A> For all those disbelievers ! /forums/images/icons/cool.gif
tome
New member
Re: Amazing ....
Nigel
Transit SatNav used an external clock (we used a rubidium standard) so doesn't really come into this.
I think all the points made by the various posters have been valid and have merited a sensible reply. I can't quite see where you are coming from with your outburst.
Tom
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Nigel
Transit SatNav used an external clock (we used a rubidium standard) so doesn't really come into this.
I think all the points made by the various posters have been valid and have merited a sensible reply. I can't quite see where you are coming from with your outburst.
Tom
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alant
Active member
Re: Amazing ....
Nigel,
"and remember we are talking about from a Ships Bridge"
There are some recommendations, that because of refraction/parallax errors etc, that it's better to be lower down when taking a sight, even suggesting that deck level on a merchant vessel was sometimes preferred. Can you verify if this method was used, rather than always from Bridge level?
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Nigel,
"and remember we are talking about from a Ships Bridge"
There are some recommendations, that because of refraction/parallax errors etc, that it's better to be lower down when taking a sight, even suggesting that deck level on a merchant vessel was sometimes preferred. Can you verify if this method was used, rather than always from Bridge level?
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