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GPS time compared to Atomic Clock


forresth
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I received one of those fancy watches for Christmas that syncs to the cesium beam clock signals every night.

 

I have compared it to another one of the desk models of similar clock that I have. Which was accurate to the clock with the ball dropping on NY eve (within the television signal delay)

 

I noticed that my Magellan Meridian (Marine) time is off by about 2.5 minutes.

 

I have tried different time formats and it is different in all settings.

 

Anybody notice this effect?

 

I will re-initialize the GPS to see if that makes a difference but I was just wondering what is going on.

 

Maybe all those Leap Seconds have added up.

 

Forrest

 

I just tried the initialize on the GPS...still off by about 2.5 mins.

I also found that the leap seconds should only be about 14 seconds of error according to http://tycho.usno.navy.mil/gpstt.html .

Edited by forresth
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I noticed that my Magellan Meridian (Marine) time is off by about 2.5 minutes.

I have tried different time formats and it is different in all settings.

 

Don't know what you mean that you've tried different time formats. But, on my Merigold, when I switch from Local AM/PM for UTC and then back to Local AM/PM, it gives opportunity to reset the current time.

 

Setup > Enter > Time format > Enter > UTC > Enter > Time Format > Enter > Local AM/PM (or Local 24 Hrs) > Enter > use Enter and rocker pad to set current time > Done

 

Edit: Just tried it. Oops. You're right, time is off by about 2.5 minutes and resets itself to being off by 2.5 min. <shrug>

Edited by cliff_hanger
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I guess I should have mentioned that I have tried Local AM/PM, Local 24HR , and the UTC instead of saying "tried all time formats". I have had this thing for more than 5 years so I am pretty knowledgeable about its operation and quirks. I also have the latest firmware (ver 5.35) for my model. I guess I could try to load the Marine firmware but I really do not think that will make a difference.

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This is a well know thing that happens to Meridians and other Magellans. My Meridian has been off as much as 2.5 minutes, but now is within 2 seconds (of accurate clock using radio signal corrections.) Taking batteries out of the unit for a few hours can reset it.

My eXplorist has been off as much as 40 seconds, but is also within about 2 now.

 

This time will also affect your tracklogs so be aware of this if you are geocoding photos. Just set your camera to GPS time, or make the adjustment in your coding/tagging program. I always sync my camera's time before taking photos.

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I’ve read that GPS time is a few seconds ahead of “real time” because of the decaying rotation of the Earth. That article estimated approx. 1 second per year. The satellites were in sync back in 1980 when they were launched but never recalibrated. I don’t know if that is true. Anybody else heard of this?

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Has nothing to do with the decaying rotation of the earth.

"GPS, Global Positioning System time, is the atomic time scale implemented by the atomic clocks in the GPS ground control stations and the GPS satellites themselves. GPS time was zero at 0h 6-Jan-1980 and since it is not perturbed by leap seconds GPS is now ahead of UTC by 14 seconds."

 

This does not mean that UTC and GPS time is not synced. In fact the GPS is the world's most reliable and accurate method of time transfer, and of course it would have to be accurate to work. One nanosecond (one-billionth of a second) delay equals one foot of position accuracy off.

 

What you see on the display clock of your GPS is not the internal clock, but I am surprised that Magellan has done such a crappy job of syncing them.

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Leap seconds are totally accounted for in GPS time. It's a well understood problem.

 

330/ST/Meridians keep time like a Burger King watch. http://www.gpsfaqs.org/faqs/magellan/explo...usage.html#time

 

 

Robert,

 

That links says a full memory clear would be required to get the clocks back in sync. But, wouldn't a full memory clear also erase the WAAS satellite numbers and restore the ones that are stored in firmware? Since PRN122 and 134 were turned off in July and replaced by 135 & 138, wouldn't the fully cleared GPS then be looking for non-existent satellites? (http://groups.yahoo.com/group/Magellan_Meridian/message/42123)

 

So, for some (like me), it might become a choice between having WAAS or having an accurate display clock?

 

Edit: fixed spelling

Edited by cliff_hanger
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So, for some (like me), it might become a choice between having WAAS or having an accurate display clock?

 

Yes, the choice may be dealing with Magellan's defective time handling or their defective WAAS sat handling.

 

Passdump, the Garmins handle time sensibly. They have a local clock of reasonable accuracy that they use when they don't have a sat fix and they synchronize that clock to the crazy accurate GPS clocks when they do. Having that internal clock lets them know the expected constellation when they power on.

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Leaving the batteries out for a few hours did not work.

The Clearing of Memory did fix the problem. I wonder why the several times that I used Initialize, did not?

This must mean that the Magellan firmware stores a correction value in the non volitile RAM and re-uses this when the GPS sends the stream down. If this value gets corrupted, it is forced to keep the bogus value until a memory clear is issued.

 

I wonder if I could somehow clear by using the "Hidden Menu" rather than a full memory clear. Because I wiped out all my tracks and database of locations. Luckily I use "Easy GPS" to keep all the data points, and the tracks are on the SD card.

 

I suppose this is another one of the quirks I have to live with (or figure out).

 

And here I was thinking of getting one of the new Triton 500's.....Hmmmm.

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