[time-nuts] Terrestrial Tides and Land Movement

[Bob Camp]

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 at 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 at LeapSecond.com>
> To: Discussion of Precise Time and Frequency Measurement <time-nuts at 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 at evoria.net>
> To: "Discussion of Precise Time and Frequency Measurement" <time-nuts at 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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