Hi Magnus,

> Hi Skip and Tom,
> Yes... almost. The thing is that the GPS orbits is a few minutes shy of 12 
> hours

Right. I think that's why he picked the subject line: "12 hours (-2 minutes)".

Since you're interested in this level of detail, there are papers about GPS 
orbits, repeat times, sidereal time, and orbital maneuvers:

http://www.kristinelarson.net/wp-content/uploads/2015/10/ig0806_gnss-solutions.pdf
http://www.kristinelarson.net/wp-content/uploads/2015/10/gpsrep.pdf

These details turn out to be more important in the geodetic community than the 
T&F community. We tend to average a lot, but they have embraced high-rate 
kinematic GPS receivers as zero-drift seismometers. Some more papers:

http://xenon.colorado.edu/igs5_revised.pdf
ftp://ftp.ngs.noaa.gov/pub/abilich/papers/Bilich2008_Denali.pdf

This issue of "just a bit less than 12 hours" caught my eye because I ran 
across it in ADEV plots. See:

http://leapsecond.com/pages/sidereal/index.htm
http://leapsecond.com/pages/sidereal/sv.htm
http://leapsecond.com/pages/sidereal/14years.htm

The 14years.htm page nicely shows the occasional "station keeping" orbital 
maneuvers of each GPS SV.

/tvb


----- Original Message ----- 
From: "Magnus Danielson" <mag...@rubidium.dyndns.org>
To: <time-nuts@febo.com>
Cc: <mag...@rubidium.se>
Sent: Monday, May 23, 2016 11:57 PM
Subject: Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)


> Hi Skip and Tom,
> 
> Yes... almost. The thing is that the GPS orbits is a few minutes shy of 
> 12 hours, since they is aligned to sidereal time, so the pattern shift 
> on the sky and it takes half a year to repeat exactly... if it where not 
> for orbital changes.
> 
> Cheers,
> Magnus
> 
> On 05/24/2016 02:01 AM, Tom Van Baak wrote:
>> 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.with...@gmail.com>
>> To: "time-nuts" <time-nuts@febo.com>
>> 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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