[time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

Peter Marczinowski peter.laboe at gmail.com
Tue May 24 01:01:29 UTC 2016


Hi Tom,
IMHO in your point "2)" you should not mirror the -104 to +104, but add 180
degrees instead resulting in a final longitude of +76 after 12 hours. In
all other respects I agree.
Peter


Am Dienstag, 24. Mai 2016 schrieb Tom Van Baak :

> Hi Skip,
>
> > Any help in understanding this behavior?  Thanks in advance.
>
> Yes, GPS satellites do repeat every ~12 hours in orbit around the mass of
> the earth -- but -- you and the earth turns 180 degrees during those 12
> hours. So you're no longer where you should be when the 1st repeat occurs.
> Instead you have to wait yet another 12 hours for the earth to get back to
> the place where you were, in time to see the 2nd repeat. Now when you hear
> "get back, get back, get back to where you once belonged", you'll think of
> GPS satellites instead of the Beatles.
>
> So the LH plots are correct. Here's another take:
>
> 1) Say it's 6 PM MDT in Denver at lat/lon +39/-104 and you see a pattern
> of N satellites in the sky.
> 2) Tomorrow morning at 6 AM MDT that same pattern will be in the sky --
> not for you -- but for some guy at 6 PM lost in Inner Mongolia at lat/lon
> +39/+104.
> 3) Tomorrow evening at 6 PM MDT that same pattern will again be in the
> sky, this time for you in Denver.
>
> /tvb
>
> ----- Original Message -----
> From: "Skip Withrow" <skip.withrow at gmail.com <javascript:;>>
> To: "time-nuts" <time-nuts at febo.com <javascript:;>>
> Sent: Monday, May 23, 2016 3:43 PM
> Subject: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)
>
>
> > Hello Nuts,
> >
> > I am attaching a capture from Lady Heather of a 3-day run.  You can see
> the
> > temperature vary by 7C over each day.  The TB is being run open loop and
> > another GPSDO 10MHz input to the unit instead of the unit's oscillator.
> >
> > I expected the purple line to repeat every 12 hours based on the GPS
> > constellation being the same (which maybe it kind of does), but there is
> > definitely a 24 hour repeat.  What is really weird is that the number of
> > satellites that LH sees also repeats on a 24 hour cycle, not 12 (bottom
> > trace).
> >
> > Any help in understanding this behavior?  Thanks in advance.
> >
> > Skip Withrow
> >
>
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