FullCircle

[Stingo]

The weather station that you posted on the 25/11/06, arrived today! Perfect timing really, because I was going to buy a ships clock and barometer for the new boat today. Now I don't need to.

 

[phatcat1]

Not bad, what does 6 months postage cost?

 

[Stingo]

£3.12

 

[FullCircle]

Well, thats my reputation sort of in the clear then, a bit.Y ou can take back all the comments about your Postal comrades now then. /forums/images/graemlins/laugh.gif

Or perhaps I addressed it to Stingo, Durban South America and it has arrived in Europe via a Catamaran in Sable D'Olonnes and forwarded? Coincidence? You decide!

It is worthwhile doing a long term check against another quartz item,as mine differed by 17 seconds over 3 months, but it could have been the other piece of kit, or a share of the 2 inaccuracies.
The baro and humidity bits were pretty much singing off the same hymn sheet all winter, and the outside sensor showed no signs of distress having been perched on the roof of my shed all winter.

 

[tome]

If you have a hand-held GPS it is very good for rating clocks as GPS time doesn't drift at all

 

[FullCircle]

Not so tome, even a caesium resonator has an observation adjustment in its programme. /forums/images/graemlins/grin.gif


The resonator is used as a passive instrument to calibrate the quartz clocks, usually at intervals of a few days; and it is estimated that the clocks calibrated in this way provide at all times the atomic unit of frequency and time interval with a standard deviation of ± 2 parts in 1010. The quartz clocks are also calibrated in terms of astronomical time and the results are compared for the period from June 1955 to June 1956. For operational purposes the frequency of the resonance was taken as 9 192 631 830 c/s which was the value obtained in terms of the unit of uniform astronomical time made available by the Royal Greenwich Observatory in June 1955. The value is being determined in terms of the second of ephemeris time, which has now been adopted by the International Committee of Weights and Measures as the unit of time, but to obtain the accuracy required the comparison must be extended over a long interval in view of the difficulties associated with the astronomical measurements.

 

[Oen]

Has anyone suggested that you need to get out more???

 

[Pinnacle]

Just goes to show what imminent matrimony does for you.............



/forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

 

[Niander]

haha lol

 

[CPD]

[ QUOTE ]
Well, thats my reputation sort of in the clear then . . . . . .

[/ QUOTE ]

Well it was 5 minutes ago Jim ! /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

 

[Bob]

John

Great boat - Great builder.

Fair winds!

 

[tome]

That's because a caesium resonator drifts (abeit ver slowly). A gps receiver doesn't drift as time is one of the unknowns it is solving as part of the fix

If you had access to the clock, even a modest GPS receiver is capable of outputting time to an absolute accuracy of better than a micro-second without any drift

There is usually a delay in outputting time to the display as it is not the highest priority, but this is generally less than half a second and will not vary much

Hence, as I mentioned previously, gps is a very good instrument for rating onboard clocks

 

[FullCircle]

I know, I know, but with ignoring the radii of the infinite allegedly straight line, there are no circumstances where time is an absolute, especially from any given point in space. Therefore, I propose that there can be no point at which drift cannot be said to be present. /forums/images/graemlins/tongue.gif

 

[webcraft]

[ QUOTE ]
If you had access to the clock, even a modest GPS receiver is capable of outputting time to an absolute accuracy of better than a micro-second without any drift

[/ QUOTE ] - true, but to get UTC you have to deduct the leap-seconds (currently 9)

So UTC = GPS time - 9 sec

 

[tcm]

mm, i think you can award yourself an 8 on the Nerd scale for that.

 

[BrendanS]

The GPS signal contains the UTC offset, which allows the receiver to correct to UTC. However, as the UTC offset is only transmitted every 12 minutes or so, you'd better leave the receiver for 15 minutes after it first gets a fix after power on, to ensure it has received the offset signal! /forums/images/graemlins/grin.gif

 

[davel]

Bugger !
There's always a downside isn't there?
/forums/images/graemlins/smile.gif

 

[tome]

Sorry Webcraft, no cigar for you

The leap seconds are broadcast as part of the Nav message from the sats so the gps receiver automatically converts to UTC. You don't need to do any conversion as it displays UTC

Which is just as well as the current number is 14, not 9!

 

[sarabande]

You are confusing Euclidean geometry with Einsteinian relativity. Across a time warp (think rubber sheet matrix with an indentation) any GPS system would have no difficulty in locating itself in at least two points in space, each using four degrees of freedom (x,y,z, and time).

Self referentially, a satellite always knows where it is; it's the perturbations of the Earth in heliocentric orbit that make it giddy and give out false signals.

 

[tome]

Brendan

The receiver should remember the leap secs so unless you haven't powered it up for about 18 months (roughly the interval between leap sec updates) it should work correctly right away

BTW, leap sec updates are always done midnight 30/6 or 31/12