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

[Michael Wouters]

> Multipath on GPS normally requires a couple of things:
>

> 3) The signal path length has to be close enough that the normal firmware 
> does not reject
> the solution.
>

Expanding on this a bit, because it's relevant to the "aircraft
causing multipath" question, the pseudo random noise ranging method
allows discrimination in the time domain against sources of multipath
further away than 300 m at worst, and much better in practice.

At the L1C 1 MHz chip rate, the correlation function for the received
PRN is zero more than 1 us == 300 m away. In practice, you're
digitizing much faster than 1 MHz and you can use a more closely
spaced set of correlators, perhaps improving by a factor of 10.

This doesn't work though when a reflection is the only signal you are
seeing,  as in Bob's point (1).

(The effect of multipath when there is a line of sight signal also
visible is to distort the correlation function, which is triangular in
the ideal case. This introduces errors into the interpolation used to
improve the resolution of the PRN ranging.)

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

[Attila Kinali]

On Wed, 25 May 2016 09:59:33 -0500
David  wrote:

> I was designed as a transmitter hunting antenna sacrificing size and
> gain for minimum side lobes.  You can see a photo of it on the front
> cover of October 1995 73 magazine.

For those who are also wondering what it looks like:
https://archive.org/details/73-magazine-1995-10

Attila Kinali

-- 
Reading can seriously damage your ignorance.
-- unknown
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Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

[David]

On Wed, 25 May 2016 19:33:37 +0200, you wrote:

>
>> Le 25 mai 2016 à 16:59, David  a écrit :
>> 
>> I was designed as a transmitter hunting antenna sacrificing size and
>> gain for minimum side lobes. 
>
>I’ve got to see your selfie.  

It was not a selfie.  Joe Moell, K0OV, took the photograph on the
monthly Catalina Transmitter Hunt and without me knowing, submitted it
to 73.

When I was working through various antenna designs, that is the one I
stopped at because we had no way to test anything better; the side
lobes where lower than the reflections at our antenna test range which
was as barren a parking lot on top of a hill that we could find.
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Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

[Mike Cook]

> Le 25 mai 2016 à 16:59, David  a écrit :
> 
> I was designed as a transmitter hunting antenna sacrificing size and
> gain for minimum side lobes. 

I’ve got to see your selfie.  


"The power of accurate observation is commonly called cynicism by those who 
have not got it. »
George Bernard Shaw

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

[David]

On Wed, 25 May 2016 06:34:46 -0700, you wrote:

>On 5/24/2016 7:26 PM, David wrote
>>The 2 meter
>> directional antenna I ultimately designed was good enough to not only
>> track airlines by their reflected RF, but it could see reflections and
>> shadows from nearby objects like street lights and trees which
>> ultimately limited outside performance testing.
>
>As someone who operated amateur radio moonbounce on 2-meters back in the day 
>when you actually had to hear the signals, I find this really hard to believe.
>
>Wes  N7WS

I was designed as a transmitter hunting antenna sacrificing size and
gain for minimum side lobes.  You can see a photo of it on the front
cover of October 1995 73 magazine.
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Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

[Wes]

On 5/24/2016 7:26 PM, David wrote

   The 2 meter
directional antenna I ultimately designed was good enough to not only
track airlines by their reflected RF, but it could see reflections and
shadows from nearby objects like street lights and trees which
ultimately limited outside performance testing.
As someone who operated amateur radio moonbounce on 2-meters back in the day 
when you actually had to hear the signals, I find this really hard to believe.


Wes  N7WS
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Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

[Don Lewis]

Still do, to this day!

With my OTA HD TV and living in the vicinity of ABIA in Austin, TX.

When the wind is out of the north, and they take off to the north darn 
planes!


Don
N5CID






-Original Message- 
From: billriches

Sent: Wednesday, May 25, 2016 5:34 AM
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

Reminds me of the 1950's living in Wildwood, NJ.  We had a TV antenna on the 
roof to pick up stations from Philadelphia - 80 miles away.  When ever an 
airplane flew over you would see flutter and distorted sound!


73,

Bill, WA2DVU
Cape May

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

[Bob Camp]

Hi

Multipath on GPS normally requires a couple of things:

1) The satellite you are trying to lock on to needs to be obscured. Being below 
the 
local horizon is one normal way for this to happen. 

2) The satellite signal needs to be reflected off of something that does not 
put in enough 
dopler to take it out of whatever the doppler correction “stuff” is happy with 

3) The signal path length has to be close enough that the normal firmware does 
not reject
the solution. 

4) The reflected signal has to be strong enough to be out of the noise in the 
normal demodulation
bandwidths. This includes odd things like polarization. 

5) The geometry between the objects (sat included) has to hold long enough for 
it to contribute to 
a multi second solution. Anything below a few seconds is normally rejected by 
the GPSDO firmware. 

I’m sure there are a few other qualifiers. Bottom line is that it’s easier to 
get multi path off of a 
stationary object. A shallow angle fixed geometry setup can give you a problem 
for quite a while. 

Bob



> On May 24, 2016, at 10:26 PM, David  wrote:
> 
> On Tue, 24 May 2016 16:15:15 -0700, you wrote:
> 
>> 
>> kb...@n1k.org said:
>>> The glitches are to narrow (short duration) and far to regular for the
>>> ionosphere to be the issue.  
>> 
>> Is multipath from a large airliner in a landing pattern likely to cause that 
>> sort of problems?
>> 
>> I'm 20+ miles off the end of SFO, but it's common to see large planes going 
>> over and turning to line up for a landing.  On my one-of-these-days list is 
>> to grab the airline location data and see if it correlates with GPS glitches.
> 
> Back when I did a lot of transmitter hunting, I listened to multipath
> from airliners from 2 meters to 23 centimeters.  The 2 meter
> directional antenna I ultimately designed was good enough to not only
> track airlines by their reflected RF, but it could see reflections and
> shadows from nearby objects like street lights and trees which
> ultimately limited outside performance testing.
> 
> As you can calculate from the geometry, the flutter started out fast
> and decreased in frequency until there was a slow null/peak and then
> it reversed.  If the same happened with GPS, then maybe the receiver
> could briefly lock onto the reflection from the plane producing a
> different solution for a short time.
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Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

[billriches]

Reminds me of the 1950's living in Wildwood, NJ.  We had a TV antenna on the 
roof to pick up stations from Philadelphia - 80 miles away.  When ever an 
airplane flew over you would see flutter and distorted sound!

73,

Bill, WA2DVU
Cape May

-Original Message-
From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of David
Sent: Tuesday, May 24, 2016 10:26 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

On Tue, 24 May 2016 16:15:15 -0700, you wrote:

>
>kb...@n1k.org said:
>> The glitches are to narrow (short duration) and far to regular for 
>> the ionosphere to be the issue.
>
>Is multipath from a large airliner in a landing pattern likely to cause 
>that sort of problems?
>
>I'm 20+ miles off the end of SFO, but it's common to see large planes 
>going over and turning to line up for a landing.  On my 
>one-of-these-days list is to grab the airline location data and see if it 
>correlates with GPS glitches.

Back when I did a lot of transmitter hunting, I listened to multipath from 
airliners from 2 meters to 23 centimeters.  The 2 meter directional antenna I 
ultimately designed was good enough to not only track airlines by their 
reflected RF, but it could see reflections and shadows from nearby objects like 
street lights and trees which ultimately limited outside performance testing.

As you can calculate from the geometry, the flutter started out fast and 
decreased in frequency until there was a slow null/peak and then it reversed.  
If the same happened with GPS, then maybe the receiver could briefly lock onto 
the reflection from the plane producing a different solution for a short time.
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---
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Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

[David]

On Tue, 24 May 2016 16:15:15 -0700, you wrote:

>
>kb...@n1k.org said:
>> The glitches are to narrow (short duration) and far to regular for the
>> ionosphere to be the issue.  
>
>Is multipath from a large airliner in a landing pattern likely to cause that 
>sort of problems?
>
>I'm 20+ miles off the end of SFO, but it's common to see large planes going 
>over and turning to line up for a landing.  On my one-of-these-days list is 
>to grab the airline location data and see if it correlates with GPS glitches.

Back when I did a lot of transmitter hunting, I listened to multipath
from airliners from 2 meters to 23 centimeters.  The 2 meter
directional antenna I ultimately designed was good enough to not only
track airlines by their reflected RF, but it could see reflections and
shadows from nearby objects like street lights and trees which
ultimately limited outside performance testing.

As you can calculate from the geometry, the flutter started out fast
and decreased in frequency until there was a slow null/peak and then
it reversed.  If the same happened with GPS, then maybe the receiver
could briefly lock onto the reflection from the plane producing a
different solution for a short time.
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Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

[Bob Camp]

HI

The gotcha with anything local is the sidereal day repeat pattern The local 
stuff would have to 
be very solar oriented to be slipping at exactly that rate over a few months of 
data.  My guess 
is that when *this combo* gets into *that position* and *this multi path* 
happens … you get a 
glitch. It’s not there for long, but it is there. 

Credit where credit is due … some guy named Tom (who also drives around with 6 
5071’s on 
a regular basis … handing them out to strangers) spotted the sidereal day time 
slip. I had been 
chasing it as if it was 24 hours and some weird local “train goes by an 
midnight” sort of thing. 

Bob


> On May 24, 2016, at 7:15 PM, Hal Murray  wrote:
> 
> 
> kb...@n1k.org said:
>> The glitches are to narrow (short duration) and far to regular for the
>> ionosphere to be the issue.  
> 
> Is multipath from a large airliner in a landing pattern likely to cause that 
> sort of problems?
> 
> I'm 20+ miles off the end of SFO, but it's common to see large planes going 
> over and turning to line up for a landing.  On my one-of-these-days list is 
> to grab the airline location data and see if it correlates with GPS glitches.
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
> ___
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Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

[Hal Murray]

kb...@n1k.org said:
> The glitches are to narrow (short duration) and far to regular for the
> ionosphere to be the issue.  

Is multipath from a large airliner in a landing pattern likely to cause that 
sort of problems?

I'm 20+ miles off the end of SFO, but it's common to see large planes going 
over and turning to line up for a landing.  On my one-of-these-days list is 
to grab the airline location data and see if it correlates with GPS glitches.

-- 
These are my opinions.  I hate spam.



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

[Bob Camp]

HI

If you have a good antenna location and always have more sat’s in view than the 
receiver can use, 
the constellation shift may not be a big deal at some level. No matter how good 
the antenna, day / night
ionosphere compared to the estimated numbers they broadcast will be an issue. 

Is the level that the local multi path and the constellation "gets you" higher 
than the level that the ionosphere 
correction falls apart? Without a lot of data, you will have a hard time 
sorting that out. 

All of that said, I have seen “glitches” in long term plots that space out at 
the expected “just short of 24 hours” 
period. In my location and with the gear I’m running … it’s more likely the 
constellation than the ionosphere. 
The glitches are to narrow (short duration) and far to regular for the 
ionosphere to be the issue. 

Bob



> On May 23, 2016, at 11:52 PM, Skip Withrow  wrote:
> 
> Hello Bob,
> 
> Good point.  Ionosphere between day and night could very well explain it.
> 
> Thanks,
> Skip Withrow
> 
> On Mon, May 23, 2016 at 5:37 PM, Bob Camp  wrote:
> Hi
> 
> Well, *maybe* there is a 24 hour component in the GPS constellation :)
> 
> Indeed a lot of stuff repeats at the 24 hour point. The ionosphere is a bit 
> different at midnight than at noon.
> 
> Bob
> 
> > On May 23, 2016, at 6:43 PM, Skip Withrow  wrote:
> >
> > 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
> >
> > 
> > Virus-free.
> > www.avast.com
> > 
> > <#DDB4FAA8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
> > ___
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> > To unsubscribe, go to 
> > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> > and follow the instructions there.
> 
> 

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

[Tom Van Baak]

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" 
To: 
Cc: 
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" 
>> To: "time-nuts" 
>> 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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Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

[Magnus Danielson]

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

[Brooke Clarke]

Hi Skip:

The GPS orbit is 12 sidereal hours.  It sure would be nice if LH supported that 
time frame.
The ground track repeats for each satellite, so LH could also have a separate 
elevation mask for each of them.

--
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
The lesser of evils is still evil.

 Original Message 

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


Virus-free.
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Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

[Peter Marczinowski]

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" >
> To: "time-nuts" >
> 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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Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

[Tom Van Baak]

Thanks to Hal and Peter for catching my sign/180 degree error.
This means Skip's nocturnal evil twin lives near the border of China / 
Tajikistan / Kyrgyzstan instead of Inner Mongolia. Here are the contrasting 
maps:

https://www.google.com/maps/@39.68,-104.902,600782m/data=!3m1!1e3
https://en.wikipedia.org/wiki/Denver

vs.

https://www.google.com/maps/@39.68,75.09,600782m/data=!3m1!1e3
https://en.wikipedia.org/wiki/Kashgar

Attached are the predicted GPS SV counts for the next 24 hours for each 
location. You can see they are almost identical. GPS sister cities.
Skip, let me know if these predictions sort of match what you actually observe.

/tvb

- Original Message - 
From: Peter Marczinowski 
To: Tom Van Baak ; Discussion of precise time and frequency measurement 
Sent: Monday, May 23, 2016 6:01 PM
Subject: Re: [time-nuts] I thought GPS repeated every 12 hours (-2 minutes)


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

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

[Bob Camp]

Hi

Well, *maybe* there is a 24 hour component in the GPS constellation :)

Indeed a lot of stuff repeats at the 24 hour point. The ionosphere is a bit 
different at midnight than at noon.

Bob

> On May 23, 2016, at 6:43 PM, Skip Withrow  wrote:
> 
> 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
> 
> 
> Virus-free.
> www.avast.com
> 
> <#DDB4FAA8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
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