[time-nuts] Common-View GPS Network

[Bob Camp]

Hi

Unfortunately there's more to it than that. Unless both TBolts are looking at the same sats, they will get very different answers. Ideally you want to compare single sats, and have very precise station locations. That lets you eliminate a lot of geometric stuff and individual sat errors. Fancy sat orbit data also helps this.

The measurement stuff is pretty easy. Anything that will measure 100 to 200 ps is likely to be plenty good enough. There are a number of counters and boards that will do that. 

Then you get down to ionosphere stuff, and fiddly crustal tide shifts. There are lots of layers to this onion.

Bob

On Apr 15, 2013, at 8:27 PM, Lachlan Gunn <lachlan at twopif.net> wrote:

> My inclination is that one would need three things to participate in such a system---
> 
>   1. A GPS receiver. 
>   2. A local reference oscillator. 
>   3. A way to measure their relative phase.
> 
> I understand that the Thunderbolt can perform all of these functions, so expect that this would be the favoured option for those without an Rb oscillator.   I do not have one, so will move on to the next setup.  For development the ResT could work the same way, but it's oscillator isn't as good. So, we consider the next option.  
> 
> For those without a receiver that has itself a high-quality oscillator, the next option would be to use a local reference and build a separate phase-measurement circuit. I have already written a time-tagging system with an FPGA, which could be reworked into what we need. This is probably overboard, though, and might be achieved with a micro, possibly paired with a PICTIC for the final interpolation.
> 
> The next step would be to either transfer directly or to upload the phase data to a central location where it could be processed together to estimate  everyone's offset to the average at once.
> 
> A DMTD will be needed somewhere to evaluate performance, but this needs only to be done somewhere rather than everywhere. 
> 
> Since there has been some interest I'll start whacking some simulations together in the near future.
> 
> Thanks, 
> Lachlan Gunn
> 
> -------- Original message --------
> From: Mark Spencer <mspencer12345 at yahoo.ca> 
> Date:  
> To: time-nuts at febo.com 
> Subject: Re: [time-nuts] Common-View GPS Network 
> 
> 
> 
> I'd be interested in understanding what it would take to participate in this type of experiment.   I'm unlikely to have any free time for this over the next several months but if for example there was a particular GPS receiver that was well suited for this type of work that would be useful info for me.   
> (Recently at work I've started to become involved with precision GPS location systems and the claimed 3D accuracies that can be achieved using post processing seem very impressive.)
> Thanks in advance for any comments.
>  
>  
> Regards
> Mark Spencer
> ------------------------------
> 
> Message: 6
> Date: Mon, 15 Apr 2013 22:49:38 +0200
> From: Magnus Danielson <magnus at rubidium.dyndns.org>
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] Common-View GPS Network
> Message-ID: <516C67E2.80501 at rubidium.dyndns.org>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> Lachlan,
> 
> This would be fun to try with a few sites involved. It requires a fairly 
> decent GPS and antenna at each location, and loging and then 
> post-processing. Would be fun to see how well it can be made to perform.
> 
> Cheers,
> Magnus
> 
> On 04/15/2013 02:55 PM, Lachlan Gunn wrote:
>> Hello all.
>> 
>> 
>> 
>> Having spent some time working over the last year on GPS time stability
>> measurement, I'm keen to move onwards and upwards and have a go at
>> common-view time transfer.  While my receivers are in the post, I have
>> thinking about my next direction.  One thought that I have had is to try to
>> write some software that can be used for real-time common-view (public if
>> there is interest, but I am getting ahead of myself I think).
>> 
>> My question to those in the know is whether they have found common-view to
>> be useful over medium timescales (say, an hour or four).  My understanding
>> is that after a day or so the GPS signal itself becomes usable as a
>> standard, so building a network is probably not tremendously useful over
>> these sorts of time periods, but looking at such as figure 6 of [1],
>> common-view should still be useful between a few minutes and hours.  Has
>> anyone here tried using such a method to produce their own short-term time
>> scale, or is one better off just taking the simple route and tracking GPS
>> time directly?
>> 
>> 
>> 
>> Thanks,
>> 
>> Lachlan
>> 
>> 
>> 
>> 
>> 
>> [1] http://www.pttimeeting.org/archivemeetings/2008papers/paper45.pdf
>> 
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