An Arcane Question Re: Atomic Time

[+jacques0]

I have long regarded my gps to be an accurate source of exact time, since that is what the entire system is based upon. In fact, somewhere along the way I ran across a website that said having a gps with you is sort of like carrying an atomic clock.

 

At the same time (no pun intended) I have always kept my car's clock synchronized to broadcasters' announcements of time, thinking that they are probably tied to atomic clocks. In fact, any station I listen to announces a time that corresponds exactly to the time on the my car's clock/radio.

 

I have a small utililty on my computer by which I can log on to a time server (e.g., Boulder) and synch. my computer's clock with an atomic clock. [by this time you're probably thinking I'm very anal about time. No...that's not the case. I just play with software]

 

Finally, when taping shows on my vcr, I use the above-mentioned time synchs.

 

Now....back to the subject at hand. The time shown on my gpsr is 4 minutes (+/- 8 seconds) behind the time shown on the car radio and on the atomic-clock-corrected computer. And so, the question: has anyone else noticed this, and does anyone have any explanation?

 

(p.s. For the record, for the last 25 years I haven't even worn a watch!)

Edited April 24, 2006 by jacques0

[CenTexDodger]

How long has it been since your GPS has gotten a good, solid signal? It will not be very accurate if it has not recieved a signal in a while.

 

Also, I would not put that much faith in the Radio station's broadcast of the time. I have been in radio stations that the DJ just looks at the clock on the wall for the time.

[+jacques0]

I normally get 8-12 satellites locked, with a 7-13 foot accuracy, always with WAAS. Sometimes 13 satellites. As far as the radio stations, I agree to a point, except that advertisers insist on their spots being aired at precise times. When I repeatedly get the exact time reported by many stations I can't help but conclude that there are some standards involved.

 

I should mention, too, that the atomic-clock-corrected time on the computer matches the time on the car clock/radio, while the gpsr time trails by 4 minutes.

 

Not that all this matters that much, since my gps gives me great locating/positioning info. I can walk right up to geocaches. This time thing just intrigues me, that's all.

Edited April 24, 2006 by jacques0

[CenTexDodger]

Yeah, sounds interesting. I guess I have never double checked the time on mine! I just assumed that it was right.

[OuttaHand]

Radio stations quite often use a 5- or 8-second delay. They almost ALWAYS use one for callers they put on the aair. But they might also use one for the deejays as well. The purpse is so they can "dump" someone who swears or says somethin off-color.

So if they are running a delay, and the dejaa announces the time according to that, then the time announcement will also have the delay.

[robertlipe]

The problem is that you have an explorist 200 which shares this problem with every other Magellan handheld released this century.. See "longstanding defects" in my FAQ below.

[Neo_Geo]

The problem is that you have an explorist 200...

As a known Magellan-basher in this forum, my first thought was, "He's probably got a Magellan."

[+Renegade Knight]

The GPS clocks only need to be accurate relative to each other to calculate your position. Not relative to other non GPS system clocks. I don't know if they like to keep the time fresh or not by callibrating the GPS system to the official time clocks. Odds are they do because it would be handy for the military applications GPS was invented for.

Edited April 24, 2006 by Renegade Knight

[+jacques0]

The problem is that you have an explorist 200 which shares this problem with every other Magellan handheld released this century.. See "longstanding defects" in my FAQ below.

 

Well, Robert...Score two points for you. Yes I do, indeed, have an explorist 200 (my goodness, you keep good records. or else you've dug back through my post history). And yes, a reset-all restored my gps time to the internet-atomic clock time and..........at the same time (coincidentally or otherwise) moved the WAAS station from the west coast to the east coast (I had been attributing that to the movement of one of the sat's).

 

It's interesting that despite the above, I've been able to zero in on caches. I'll be curious to see whether the reset changes that.

 

Thanks for the insight.

[+cameltrekor]

I think you got your answer above (its your GPSr) 'cause GPS time is pretty accurate.

[+Sputnik 57]

Interesting link:

The GPS epoch is 0000 UT (midnight) on January 6, 1980. GPS time is not

adjusted and therefore is offset from UTC by an integer number of

seconds, due to the insertion of leap seconds. The number remains

constant until the next leap second occurs. This offset is also given

in the navigation (NAV) message and your receiver should apply the

correction automatically. As of January 1, 2006, GPS time is ahead

of UTC by fourteen (14) seconds.

[+ctgreybeard]

As of this moment my GPSr and my computer are about 1/2 second apart. The computer synchs to NTP (which is frequently referenced to GPS) but, because it's Windoze, it doesn't do the best job of synching.

 

My Linux system, which is synched to NTP also and does a much better job, is closer to 1 ms difference.

 

Many NTP systems these days use GPS as their reference source, it is closer to "real time" than reception from Boulder AFAIK.

 

Bill W

[planewood]

As of this moment my GPSr and my computer are about 1/2 second apart. The computer synchs to NTP (which is frequently referenced to GPS) but, because it's Windoze, it doesn't do the best job of synching.

 

My Linux system, which is synched to NTP also and does a much better job, is closer to 1 ms difference.

 

Many NTP systems these days use GPS as their reference source, it is closer to "real time" than reception from Boulder AFAIK.

 

Bill W

 

My 60cx shows about 1 sec, or less, behind my puter time which is sync'd to Denver.

[+jacques0]

I guess my question is answered (thanks RobertLipe) so I'll close the topic. Thanks to all the respondents.