Terrestrial Tides and Land Movement
I did some idle searching trying to see if there was a relationship between terrestrial tides and timing receivers. I couldn't find anything useful, but I did discover that the Jersey Village area, about 2 miles northeast of me, is sinking about 2 inches a year. So, my question is what effect do either of these, terrestrial tides or this local sinkage, have on timing accuracy?
Bob - AE6RV
Hi
Tides are complicated once you get on land. They aren’t as simple as you might think when you are on the sea. There are people out there
who are “Tide Nuts” and every bit as obsessive as Time Nuts.
Most of the time and most of the places, you get roughly a third of a meter change in altitude due to on tides on solid land. If you look at the speed
of light as 3 ns / meter - you get about 1 ns from tides.
Bob
On May 16, 2015, at 12:41 AM, Bob Stewart bob@evoria.net wrote:
I did some idle searching trying to see if there was a relationship between terrestrial tides and timing receivers. I couldn't find anything useful, but I did discover that the Jersey Village area, about 2 miles northeast of me, is sinking about 2 inches a year. So, my question is what effect do either of these, terrestrial tides or this local sinkage, have on timing accuracy?
Bob - AE6RV
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On 05/16/2015 12:41 AM, Bob Stewart wrote:
I did some idle searching trying to see if there was a relationship between terrestrial tides and timing receivers. I couldn't find anything useful, but I did discover that the Jersey Village area, about 2 miles northeast of me, is sinking about 2 inches a year. So, my question is what effect do either of these, terrestrial tides or this local sinkage, have on timing accuracy?
If you're interested in finding out more about this sinkage and plate
movement, we are affiliated with the EarthScope system and have one of
their instruments here onsite. The system map is at
http://www.earthscope.org/science/maps/current-status-map/ and details
about the plate boundary observatory component are at
http://pbo.unavco.org/ with the instrument at our site at
http://pbo.unavco.org/station/overview/P779
Positional changes of 2 inches per year should create sub-nanosecond
differences in signal arrival times. The survey receivers used by the
PBO utilitize several techniques for measuring these very small changes
in position; however, as you can see by looking at the position data
(such as http://pboshared.unavco.org/timeseries/P779_timeseries.png ),
it's a long-term averaging that is optimized for position, not timing,
data, and the precision in the z axis is not as good as that in the x
and y axes by a factor of four or more. For any given GPS frame, the
uncertainty of arrival time is more determined by ionospheric effects
than by absolute position. Meteor scatter and multipath also play a role.
Now I am by no means a GPS expert, but I would think a receiver
optimized for timing would need different techniques than a receiver
optimized for survey, but I reserve the right to be wrong.
Hi Bob:
There may be two factors here.
One is the "sinking" that here in California is do to pumping out ground water. It's measured by the GRACE satellite
system.
https://en.wikipedia.org/wiki/Gravity_Recovery_and_Climate_Experiment
PS It's beginning to look like water is similar to oil, once you pump it out it can take a very long time until you can
pump more.
The other relates to "Love Numbers".
https://en.wikipedia.org/wiki/Love_number
PS not related to dating.
The APPOLO lunar ranging observatory uses a gravity meter to measure how much their bed rock mountain top moves due to
Earth Tides.
http://www.prc68.com/I/UkiahObs.shtml#Lunar_Ranging
Mail_Attachment --
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html
Bob Stewart wrote:
I did some idle searching trying to see if there was a relationship between terrestrial tides and timing receivers. I couldn't find anything useful, but I did discover that the Jersey Village area, about 2 miles northeast of me, is sinking about 2 inches a year. So, my question is what effect do either of these, terrestrial tides or this local sinkage, have on timing accuracy?
Bob - AE6RV
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On Sat, 16 May 2015 04:41:15 +0000 (UTC)
Bob Stewart bob@evoria.net wrote:
I did some idle searching trying to see if there was a relationship between
terrestrial tides and timing receivers. I couldn't find anything useful, but
I did discover that the Jersey Village area, about 2 miles northeast of me,
is sinking about 2 inches a year. So, my question is what effect do either
of these, terrestrial tides or this local sinkage, have on timing accuracy?
I guess you are looking for the relationship between earth movement
and GPS/GNSS time transfer? If so, solid earth tides (as they are
commonly called in the timing community) do not become an issue until
a lot of other factors are removed first. First you need a system
that reduces multipath to a minimum, then one that can measure the delay
induced by the ionosphere directly (ie an dual/tripple frequency GNSS
receiver). I am not sure whether tropospheric delay or solid earth tides
come next, my guess would be that both are in the same order of magnitude.
The size of solid earth tides can be up to 30cm (i think i read somewhere
that someone measured 50cm, but not sure), mostly in vertical direction
(horizontal to vertical has about a factor 10 difference in amplitude),
where GNSS precision is quite low (compared to horizontal precision).
It has to be corrected in precise GNSS augmentation systems like IGS
(see [1, page 12]).
I am not sure whether anyone accounts for continental drift in timing
applications. I would guess that at least people in VLBI have to.
Given that most GNSS high precision time transfer is used rather locally
(a couple of 100km) and that few people are running it for more than
a couple of months without recalibrating the system, i'd say that the
drift rates (which are between 2.5cm(Arctic) and 15cm(Chile) per year)
do not induce much error/jitter.
Attila Kinali
[1] "A guide to using international GNSS service (IGS) products",
by Jan Kouba, Version 2.1, 2009
https://igscb.jpl.nasa.gov/igscb/resource/pubs/UsingIGSProductsVer21.pdf
--
< av500> phd is easy
< av500> getting dsl is hard
Hi
If you believe the 3 ns / M applies in this case, tides will give you about
1 ns or so. If the geometry of the motion (vertical) and the orientation
of the sat’s is not same / same, the impact will be a bit less.
On an L1 system without some sort of ionosphere “help” and working just
off of broadcast data, you can / will / may see > 10 ns per day. Troposphere
is the same sort of thing only a bit less per day.
There are a whole raft of interesting relativity issues that get into GPS. Again
the L1 stuff does not do much beyond the in-system corrections.
The biggie is ionosphere ….
Bob
On May 24, 2015, at 8:02 AM, Attila Kinali attila@kinali.ch wrote:
On Sat, 16 May 2015 04:41:15 +0000 (UTC)
Bob Stewart bob@evoria.net wrote:
I did some idle searching trying to see if there was a relationship between
terrestrial tides and timing receivers. I couldn't find anything useful, but
I did discover that the Jersey Village area, about 2 miles northeast of me,
is sinking about 2 inches a year. So, my question is what effect do either
of these, terrestrial tides or this local sinkage, have on timing accuracy?
I guess you are looking for the relationship between earth movement
and GPS/GNSS time transfer? If so, solid earth tides (as they are
commonly called in the timing community) do not become an issue until
a lot of other factors are removed first. First you need a system
that reduces multipath to a minimum, then one that can measure the delay
induced by the ionosphere directly (ie an dual/tripple frequency GNSS
receiver). I am not sure whether tropospheric delay or solid earth tides
come next, my guess would be that both are in the same order of magnitude.
The size of solid earth tides can be up to 30cm (i think i read somewhere
that someone measured 50cm, but not sure), mostly in vertical direction
(horizontal to vertical has about a factor 10 difference in amplitude),
where GNSS precision is quite low (compared to horizontal precision).
It has to be corrected in precise GNSS augmentation systems like IGS
(see [1, page 12]).
I am not sure whether anyone accounts for continental drift in timing
applications. I would guess that at least people in VLBI have to.
Given that most GNSS high precision time transfer is used rather locally
(a couple of 100km) and that few people are running it for more than
a couple of months without recalibrating the system, i'd say that the
drift rates (which are between 2.5cm(Arctic) and 15cm(Chile) per year)
do not induce much error/jitter.
Attila Kinali
[1] "A guide to using international GNSS service (IGS) products",
by Jan Kouba, Version 2.1, 2009
https://igscb.jpl.nasa.gov/igscb/resource/pubs/UsingIGSProductsVer21.pdf
--
< av500> phd is easy
< av500> getting dsl is hard
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and follow the instructions there.
Attila,
Timing people account for everything that's important. A continental drift of an inch per year acts like a slow phase change over time, which by definition, is a frequency offset. So an inch per year is at most 1/12 * 1e-9 / (365*86400) or 3e-18. For the current precision with which UTC/TAI is calculated this is too small to worry about.
The other way to think of the frequency offset is simply the ratio of speed-of-continent vs. speed-of-light. A continent is slow, about 1e-9 m/s and light is fast, 3e8 m/s. This ratio is about 3e-18.
Note that an inch-per-year is about a nanometer-per-second. I'm also told fingernails grow about an inch a year. How's that for a rule of thumb (literally).
There's a nice (1 inch) 25 mm per year interactive drift map here:
http://www.unavco.org/software/visualization/GPS-Velocity-Viewer/GPS-Velocity-Viewer.html
The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house. Just think of continental drift as a slow moving car.
/tvb
See also:
http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/14/1.GPS_Background.pdf
http://www.unavco.org/education/resources/educational-resources/tutorial/how-quickly-are-we-moving-gps-tutorial.pdf
----- Original Message -----
From: "Attila Kinali" attila@kinali.ch
I am not sure whether anyone accounts for continental drift in timing
applications. I would guess that at least people in VLBI have to.
Given that most GNSS high precision time transfer is used rather locally
(a couple of 100km) and that few people are running it for more than
a couple of months without recalibrating the system, i'd say that the
drift rates (which are between 2.5cm(Arctic) and 15cm(Chile) per year)
do not induce much error/jitter.
Attila Kinali
Tom said: "The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house."
I don't get that. What's the purpose of doing a survey when you move your antenna if this the case?
Bob
From: Tom Van Baak <tvb@LeapSecond.com>
To: Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Monday, May 25, 2015 12:29 PM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
Attila,
Timing people account for everything that's important. A continental drift of an inch per year acts like a slow phase change over time, which by definition, is a frequency offset. So an inch per year is at most 1/12 * 1e-9 / (365*86400) or 3e-18. For the current precision with which UTC/TAI is calculated this is too small to worry about.
The other way to think of the frequency offset is simply the ratio of speed-of-continent vs. speed-of-light. A continent is slow, about 1e-9 m/s and light is fast, 3e8 m/s. This ratio is about 3e-18.
Note that an inch-per-year is about a nanometer-per-second. I'm also told fingernails grow about an inch a year. How's that for a rule of thumb (literally).
There's a nice (1 inch) 25 mm per year interactive drift map here:
http://www.unavco.org/software/visualization/GPS-Velocity-Viewer/GPS-Velocity-Viewer.html
The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house. Just think of continental drift as a slow moving car.
/tvb
See also:
http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/14/1.GPS_Background.pdf
http://www.unavco.org/education/resources/educational-resources/tutorial/how-quickly-are-we-moving-gps-tutorial.pdf
----- Original Message -----
From: "Attila Kinali" attila@kinali.ch
I am not sure whether anyone accounts for continental drift in timing
applications. I would guess that at least people in VLBI have to.
Given that most GNSS high precision time transfer is used rather locally
(a couple of 100km) and that few people are running it for more than
a couple of months without recalibrating the system, i'd say that the
drift rates (which are between 2.5cm(Arctic) and 15cm(Chile) per year)
do not induce much error/jitter.
Attila Kinali
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Hi
Add to that the fact that not everybody is moving at an inch per year. Around here the magic number
is in the 1.5 to 2 mm per year range. It’s enough to be worth correcting survey results vs benchmarks
every few years. It’s not enough to get into an L1 timing system any time soon ….
Bob
On May 25, 2015, at 1:29 PM, Tom Van Baak tvb@LeapSecond.com wrote:
Attila,
Timing people account for everything that's important. A continental drift of an inch per year acts like a slow phase change over time, which by definition, is a frequency offset. So an inch per year is at most 1/12 * 1e-9 / (365*86400) or 3e-18. For the current precision with which UTC/TAI is calculated this is too small to worry about.
The other way to think of the frequency offset is simply the ratio of speed-of-continent vs. speed-of-light. A continent is slow, about 1e-9 m/s and light is fast, 3e8 m/s. This ratio is about 3e-18.
Note that an inch-per-year is about a nanometer-per-second. I'm also told fingernails grow about an inch a year. How's that for a rule of thumb (literally).
There's a nice (1 inch) 25 mm per year interactive drift map here:
http://www.unavco.org/software/visualization/GPS-Velocity-Viewer/GPS-Velocity-Viewer.html
The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house. Just think of continental drift as a slow moving car.
/tvb
See also:
http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/14/1.GPS_Background.pdf
http://www.unavco.org/education/resources/educational-resources/tutorial/how-quickly-are-we-moving-gps-tutorial.pdf
----- Original Message -----
From: "Attila Kinali" attila@kinali.ch
I am not sure whether anyone accounts for continental drift in timing
applications. I would guess that at least people in VLBI have to.
Given that most GNSS high precision time transfer is used rather locally
(a couple of 100km) and that few people are running it for more than
a couple of months without recalibrating the system, i'd say that the
drift rates (which are between 2.5cm(Arctic) and 15cm(Chile) per year)
do not induce much error/jitter.
Attila Kinali
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Hi
If you happen to need precise time on a moving platform, then GPS can
do that as well. There are a number of military systems that have this need.
There are also some things like mobile direction finding by TDOA that have
multiple use cases.
Bob
On May 25, 2015, at 2:09 PM, Bob Stewart bob@evoria.net wrote:
Tom said: "The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house."
I don't get that. What's the purpose of doing a survey when you move your antenna if this the case?
Bob
From: Tom Van Baak <tvb@LeapSecond.com>
To: Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Monday, May 25, 2015 12:29 PM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
Attila,
Timing people account for everything that's important. A continental drift of an inch per year acts like a slow phase change over time, which by definition, is a frequency offset. So an inch per year is at most 1/12 * 1e-9 / (365*86400) or 3e-18. For the current precision with which UTC/TAI is calculated this is too small to worry about.
The other way to think of the frequency offset is simply the ratio of speed-of-continent vs. speed-of-light. A continent is slow, about 1e-9 m/s and light is fast, 3e8 m/s. This ratio is about 3e-18.
Note that an inch-per-year is about a nanometer-per-second. I'm also told fingernails grow about an inch a year. How's that for a rule of thumb (literally).
There's a nice (1 inch) 25 mm per year interactive drift map here:
http://www.unavco.org/software/visualization/GPS-Velocity-Viewer/GPS-Velocity-Viewer.html
The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house. Just think of continental drift as a slow moving car.
/tvb
See also:
http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/14/1.GPS_Background.pdf
http://www.unavco.org/education/resources/educational-resources/tutorial/how-quickly-are-we-moving-gps-tutorial.pdf
----- Original Message -----
From: "Attila Kinali" attila@kinali.ch
I am not sure whether anyone accounts for continental drift in timing
applications. I would guess that at least people in VLBI have to.
Given that most GNSS high precision time transfer is used rather locally
(a couple of 100km) and that few people are running it for more than
a couple of months without recalibrating the system, i'd say that the
drift rates (which are between 2.5cm(Arctic) and 15cm(Chile) per year)
do not induce much error/jitter.
Attila Kinali
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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Hi Bob,
How is this achieved? Is there a coupled dead-reckoning system that updates the timing location, or something else?
Bob
From: Bob Camp kb8tq@n1k.org
To: Bob Stewart bob@evoria.net; Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Monday, May 25, 2015 1:53 PM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
Hi
If you happen to need precise time on a moving platform, then GPS can
do that as well. There are a number of military systems that have this need.
There are also some things like mobile direction finding by TDOA that have
multiple use cases.
Bob
On May 25, 2015, at 2:09 PM, Bob Stewart bob@evoria.net wrote:
Tom said: "The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house."
I don't get that. What's the purpose of doing a survey when you move your antenna if this the case?
Bob
From: Tom Van Baak tvb@LeapSecond.com
To: Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Monday, May 25, 2015 12:29 PM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
Attila,
Timing people account for everything that's important. A continental drift of an inch per year acts like a slow phase change over time, which by definition, is a frequency offset. So an inch per year is at most 1/12 * 1e-9 / (365*86400) or 3e-18. For the current precision with which UTC/TAI is calculated this is too small to worry about.
The other way to think of the frequency offset is simply the ratio of speed-of-continent vs. speed-of-light. A continent is slow, about 1e-9 m/s and light is fast, 3e8 m/s. This ratio is about 3e-18.
Note that an inch-per-year is about a nanometer-per-second. I'm also told fingernails grow about an inch a year. How's that for a rule of thumb (literally).
There's a nice (1 inch) 25 mm per year interactive drift map here:
http://www.unavco.org/software/visualization/GPS-Velocity-Viewer/GPS-Velocity-Viewer.html
The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house. Just think of continental drift as a slow moving car.
/tvb
See also:
http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/14/1.GPS_Background.pdf
http://www.unavco.org/education/resources/educational-resources/tutorial/how-quickly-are-we-moving-gps-tutorial.pdf
----- Original Message -----
From: "Attila Kinali" attila@kinali.ch
I am not sure whether anyone accounts for continental drift in timing
applications. I would guess that at least people in VLBI have to.
Given that most GNSS high precision time transfer is used rather locally
(a couple of 100km) and that few people are running it for more than
a couple of months without recalibrating the system, i'd say that the
drift rates (which are between 2.5cm(Arctic) and 15cm(Chile) per year)
do not induce much error/jitter.
Attila Kinali
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
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Hi Bob,
You only need a survey if your timing receiver is running in zero-D mode. If you move the antenna more than some practical threshold you should adjust the fixed position or maybe just do another survey.
If you plan to move a lot, or if your application is mobile, or are on a slippery slope, or you just don't want to bother with a time-consuming survey, then run the timing receiver in 3D mode. As I said it will perform "almost as well". If you normally get, say 9 SV, I predict the timing accuracy difference is maybe only 10 or 20%.
You're still building a homebrew GPSDO, right? Collect a day of performance data in 0-D and then a day in 3-D and see what difference there is in RMS timing residuals (or in ADEV). I wonder if your GPSDO can even measure the difference.
/tvb
----- Original Message -----
From: "Bob Stewart" bob@evoria.net
To: "Discussion of Precise Time and Frequency Measurement" time-nuts@febo.com
Sent: Monday, May 25, 2015 11:09 AM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
Tom said: "The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house."
I don't get that. What's the purpose of doing a survey when you move your antenna if this the case?
Bob
Hi Tom,
I'm not sure what you mean by this 0D vs 3D. I'm using an LEA-6T. There are three time transfer modes 0=disabled, 1=survey-in, 2=fixed mode. Are you suggesting that I set it to disabled to run a test? I've never tried that. I know that if I monitor the output while doing a survey, the phase is all over the place.
I'm in the middle of a therm correction test at the moment, but the plots at the link below are essentially where I am in development. The PRS-45A is driving the reference and start inputs of the 5370A. I'm using a somewhat modified PID system.
http://evoria.net/AE6RV/Status/
Development is essentially over, at this point except for details, like adding code for the M12+ receiver. I can post a link to the schematic, if there's interest, but I've decided that the code will remain proprietary. This has turned into something a lot bigger than a $25 GPSDO engine.
Bob
From: Tom Van Baak tvb@LeapSecond.com
To: Discussion of Precise Time and Frequency Measurement time-nuts@febo.com
Sent: Monday, May 25, 2015 3:09 PM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
Hi Bob,
You only need a survey if your timing receiver is running in zero-D mode. If you move the antenna more than some practical threshold you should adjust the fixed position or maybe just do another survey.
If you plan to move a lot, or if your application is mobile, or are on a slippery slope, or you just don't want to bother with a time-consuming survey, then run the timing receiver in 3D mode. As I said it will perform "almost as well". If you normally get, say 9 SV, I predict the timing accuracy difference is maybe only 10 or 20%.
You're still building a homebrew GPSDO, right? Collect a day of performance data in 0-D and then a day in 3-D and see what difference there is in RMS timing residuals (or in ADEV). I wonder if your GPSDO can even measure the difference.
/tvb
----- Original Message -----
From: "Bob Stewart" bob@evoria.net
To: "Discussion of Precise Time and Frequency Measurement" time-nuts@febo.com
Sent: Monday, May 25, 2015 11:09 AM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
Tom said: "The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house."
I don't get that. What's the purpose of doing a survey when you move your antenna if this the case?
Bob
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How is this achieved? Is there a coupled dead-reckoning system that updates the timing location, or something else?
Hi BobS,
No need for dead-reckoning -- GPS gives you the location. And the time.
Take a step back and remember how GPS works. You have sats flying above that essentially send coordinated time down. The receiver solves N equations in 4 variables (X,Y,Z,T). The solution is 3D position and time. In general, the larger N is (the number of sats received) the more accurate the position and time calculation, by sqrt(N). GPS has always worked this way, whether the receiver is moving or stationary.
In fact, "stationary" is a bit misleading. You may think of your home as 0 MPH and your car as 60 MPH or a plane at 500 MPH, but the sats themselves are moving at 8700 MPH and this motion is relative. Since the equations are already working doppler frequencies and velocities of thousands of miles an hour there's not much difference between what you call a stationary receiver and a moving receiver.
What you are probably thinking about is the "zero-D", or "overdetermined solution" timing trick that was created in the 90's(?). The idea is simply that if you already know X,Y,Z then the receiver only has to solve N equations in 1 variable (T). This gives you a more accurate time than in the 3D case. The other advantage is that the receiver will still output time even if N drops below 4.
Many of the cheap GPS/1PPS receivers we use these days (Sparkfun, Adafruit, Parallax) don't even bother with a 0-D mode. So theoretically they should work just as well in a car as at home.
Does anyone on the list already have 0D vs. 3D timing data for our favorite receivers like M12, ublox6, TBolt, etc.? As long as the survey position is ideal, we would expect 0D to outperform 3D. The question is, by how much.
/tvb
Hi
If I was going to do it today, I’d do an “all in view” solution with GPS / GLONASS and whatever
else I could pick up (WAAS etc). I’d grab the data at L1 / L2 / L5. Even in motion, the location
solution is going to be pretty good.
If it was a system that made money for people, there are “for pay” services that will feed
you up to date data (WAAS on steroids …). You can get an on the fly solution that is as
good as the normal fast “self survey position” done by most L1 timing receivers.
That said, even if you don’t get all that fancy, every single position your gizmo calculates comes
with a time solution. Even the cheapest GPS still has an amazingly good time solution
buried somewhere down in it’s math. It may be 10 ns, but that’s still a lot better than my wrist
watch. It’s also better than your un-compensated portable clock ….
Bob
On May 25, 2015, at 3:02 PM, Bob Stewart bob@evoria.net wrote:
Hi Bob,
How is this achieved? Is there a coupled dead-reckoning system that updates the timing location, or something else?
Bob
From: Bob Camp kb8tq@n1k.org
To: Bob Stewart bob@evoria.net; Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Monday, May 25, 2015 1:53 PM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
Hi
If you happen to need precise time on a moving platform, then GPS can
do that as well. There are a number of military systems that have this need.
There are also some things like mobile direction finding by TDOA that have
multiple use cases.
Bob
On May 25, 2015, at 2:09 PM, Bob Stewart bob@evoria.net wrote:
Tom said: "The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house."
I don't get that. What's the purpose of doing a survey when you move your antenna if this the case?
Bob
From: Tom Van Baak <tvb@LeapSecond.com>
To: Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Monday, May 25, 2015 12:29 PM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
Attila,
Timing people account for everything that's important. A continental drift of an inch per year acts like a slow phase change over time, which by definition, is a frequency offset. So an inch per year is at most 1/12 * 1e-9 / (365*86400) or 3e-18. For the current precision with which UTC/TAI is calculated this is too small to worry about.
The other way to think of the frequency offset is simply the ratio of speed-of-continent vs. speed-of-light. A continent is slow, about 1e-9 m/s and light is fast, 3e8 m/s. This ratio is about 3e-18.
Note that an inch-per-year is about a nanometer-per-second. I'm also told fingernails grow about an inch a year. How's that for a rule of thumb (literally).
There's a nice (1 inch) 25 mm per year interactive drift map here:
http://www.unavco.org/software/visualization/GPS-Velocity-Viewer/GPS-Velocity-Viewer.html
The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house. Just think of continental drift as a slow moving car.
/tvb
See also:
http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/14/1.GPS_Background.pdf
http://www.unavco.org/education/resources/educational-resources/tutorial/how-quickly-are-we-moving-gps-tutorial.pdf
----- Original Message -----
From: "Attila Kinali" attila@kinali.ch
I am not sure whether anyone accounts for continental drift in timing
applications. I would guess that at least people in VLBI have to.
Given that most GNSS high precision time transfer is used rather locally
(a couple of 100km) and that few people are running it for more than
a couple of months without recalibrating the system, i'd say that the
drift rates (which are between 2.5cm(Arctic) and 15cm(Chile) per year)
do not induce much error/jitter.
Attila Kinali
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Hi
Let’s step back a bit:
Your module is accurate to maybe 2 ns over a short period of time and something in the 10 to 20 ns range
over 24 hours. The 2 ns comes from a variety of issues. The 20 ns comes mainly from the ionosphere.
One example of a part of the 2 ns - the satellite positions in orbit are only known to some level of accuracy. The
data your module gets is a broadcast data set that represents a “best guess” made yesterday to fit the fight
path today.
A GPS that does an on the fly position solution to 3M is not that exceptional these days. You certainly see
claims of performance at the 1M or even 0.5M level. Yes people do fudge the numbers. Yes they do use WAAS
and the like to improve things.
A 3M error is at most a 10 ns timing error. It’s more likely to be a 3-4 ns timing error. That’s not all that big compared to the numbers above.
Am I fudging the numbers a bit? Of course I am. There is an implicit assumption that the mobile platform has
a good view of the sky. Dropping below 4 sats would not happen in this case. I’d also upgrade the LEA-6T to
an LEA-8T (or similar). More sats is going to give you a better position solution.
The bottom line is that an L1 GPSDO that wanders 10 to 20 ns per 24 hours compared to a Cesium or maser is
doing pretty well. Taking that to 8 to 18 ns vs 12 to 22 is a worthy thing to do. Proving that a specific implementation better … maybe not so easy. I’ve seen a lot of plots of well respected eBay GPSDO’s that wander
30 ns or more (peak to peak) per day one day and 20 ns p-p the next….
Bob
On May 25, 2015, at 5:28 PM, Bob Stewart bob@evoria.net wrote:
Hi Tom,
I'm not sure what you mean by this 0D vs 3D. I'm using an LEA-6T. There are three time transfer modes 0=disabled, 1=survey-in, 2=fixed mode. Are you suggesting that I set it to disabled to run a test? I've never tried that. I know that if I monitor the output while doing a survey, the phase is all over the place.
I'm in the middle of a therm correction test at the moment, but the plots at the link below are essentially where I am in development. The PRS-45A is driving the reference and start inputs of the 5370A. I'm using a somewhat modified PID system.
http://evoria.net/AE6RV/Status/
Development is essentially over, at this point except for details, like adding code for the M12+ receiver. I can post a link to the schematic, if there's interest, but I've decided that the code will remain proprietary. This has turned into something a lot bigger than a $25 GPSDO engine.
Bob
From: Tom Van Baak tvb@LeapSecond.com
To: Discussion of Precise Time and Frequency Measurement time-nuts@febo.com
Sent: Monday, May 25, 2015 3:09 PM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
Hi Bob,
You only need a survey if your timing receiver is running in zero-D mode. If you move the antenna more than some practical threshold you should adjust the fixed position or maybe just do another survey.
If you plan to move a lot, or if your application is mobile, or are on a slippery slope, or you just don't want to bother with a time-consuming survey, then run the timing receiver in 3D mode. As I said it will perform "almost as well". If you normally get, say 9 SV, I predict the timing accuracy difference is maybe only 10 or 20%.
You're still building a homebrew GPSDO, right? Collect a day of performance data in 0-D and then a day in 3-D and see what difference there is in RMS timing residuals (or in ADEV). I wonder if your GPSDO can even measure the difference.
/tvb
----- Original Message -----
From: "Bob Stewart" bob@evoria.net
To: "Discussion of Precise Time and Frequency Measurement" time-nuts@febo.com
Sent: Monday, May 25, 2015 11:09 AM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
Tom said: "The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house."
I don't get that. What's the purpose of doing a survey when you move your antenna if this the case?
Bob
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Hi Bob,
Thanks for taking the time to explain the 4ns and 20ns wanders. I have just been calling them "constellation errors" without being able to explain it better than that. I've also wondered how much of the 20ns, if any, is attributable to the PRS-45A.
I still don't have the antenna located in a position suitable for precision timing. The power of the HOA is not to be trifled with. That, and the sky is full of power lines and other junk around here.
Bob
From: Bob Camp <kb8tq@n1k.org>
To: Bob Stewart bob@evoria.net; Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Monday, May 25, 2015 6:01 PM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
Hi
Let’s step back a bit:
Your module is accurate to maybe 2 ns over a short period of time and something in the 10 to 20 ns range
over 24 hours. The 2 ns comes from a variety of issues. The 20 ns comes mainly from the ionosphere.
One example of a part of the 2 ns - the satellite positions in orbit are only known to some level of accuracy. The
data your module gets is a broadcast data set that represents a “best guess” made yesterday to fit the fight
path today.
A GPS that does an on the fly position solution to 3M is not that exceptional these days. You certainly see
claims of performance at the 1M or even 0.5M level. Yes people do fudge the numbers. Yes they do use WAAS
and the like to improve things.
A 3M error is at most a 10 ns timing error. It’s more likely to be a 3-4 ns timing error. That’s not all that big compared to the numbers above.
Am I fudging the numbers a bit? Of course I am. There is an implicit assumption that the mobile platform has
a good view of the sky. Dropping below 4 sats would not happen in this case. I’d also upgrade the LEA-6T to
an LEA-8T (or similar). More sats is going to give you a better position solution.
The bottom line is that an L1 GPSDO that wanders 10 to 20 ns per 24 hours compared to a Cesium or maser is
doing pretty well. Taking that to 8 to 18 ns vs 12 to 22 is a worthy thing to do. Proving that a specific implementation better … maybe not so easy. I’ve seen a lot of plots of well respected eBay GPSDO’s that wander
30 ns or more (peak to peak) per day one day and 20 ns p-p the next….
Bob
Hi Bob S,
An HOA can be a daunting problem, but not one that cannot be
solved with a little guile.
Every house I have ever seen that has modern plumbing has a few
vent stacks on the roof. Would the HOA even notice if yours
sprouted another one dark weekend evening?
All the kit you need to add one to your roof is available at
your local big box store.
The usual bullet style antenna, sitting on top of a PVC vent
pipe would be invisible on most houses, particularly if it were
to be placed in the same vicinity as the rest.
Also, GPS bullet antennas are pretty well sealed, save for the
connector on the bottom. If you do a good job of sealing the
connector (eg. lots of wraps of electrical tape, followed by
a few of friction tape) there in no intrinsic reason you couldn't
safely mount yours on top of the main plumbing vent stack. Drill
a hole in the side at some convenient spot inside of the house,
and snake the coax up through the vent stack, mount the antenna
over the top, leaving adequate vent space. (OBTW, it isn't a
vent stack until it is above the highest fixture in the house.)
Although I don't have an HOA on my farm, I have my antenna mounted
that way on my radon mitigation pipe. I bent up a couple of
pieces of aluminum to make an open plug and put the antenna up
without ever setting foot on my roof. I simply snaked the cable
up and out the top of the radon pipe, and when I could reach it
from a window, installed the GPS antenna, and aluminum "plug",
and then pulled the antenna cable back through the pipe, causing
the plug and antenna to pop into the pipe mounting the antenna.
-Chuck Harris
Bob Stewart wrote:
Hi Bob, Thanks for taking the time to explain the 4ns and 20ns wanders. I have
just been calling them "constellation errors" without being able to explain it
better than that. I've also wondered how much of the 20ns, if any, is
attributable to the PRS-45A.
I still don't have the antenna located in a position suitable for precision
timing. The power of the HOA is not to be trifled with. That, and the sky is
full of power lines and other junk around here.
Bob