[time-nuts] carrier phase tracking GPS receiver

[Didier Juges]

Magnus Danielson wrote:
> You can acheive much greater speedup by a combined frequency/phase approach.
> You will get a very accurate frequency error estimate, so you will very
> quickly be close enought to go into phase lock. At least if your clock isn't
> too noisy. So, the lock-in time should not be the major concern, but rather
> the behaviour of the full setup when running. Also, another classic trick is
> to vary the bandwidth, so you have a much wider bandwidth in the beginning and
> then step down towards your target bandwidth as some suitable conditions have
> been met prior to the step.
>
>   
My concern is that closing the loop faster by itself does nothing, if it 
is not stable enough. What I meant by being able to close the loop 
sooner was that the carrier phase data is actually better than the OCXO 
sooner after power up. I think you or Bruce said the Allan variance of 
the carrier phase signal could be as good as e-10 in 1 second, or 
something like that.
>> It seems that in this case, the acquisition time of the receiver will be 
>> the most significant delay.
>>     
>
> Your OCXO needs to heat up anyway. You can usually acheive a good GPS lock in
> that time. A full-fledge correction would probably require data collected over
> some time anyway, so don't fool yourself here.
>   
I was assuming the OCXO remains powered, which is fairly easy to do, but 
in a transportable application, you cannot guaranty continued visibility 
of the GPS satellites, or even continued connection of the GPS antenna, 
when the system has to be transported, and in a hobby application, I am 
not sure I can design a better set of algorithms to compensate for the 
OCXO variations than what the Trimble or others have come up with in 
their GPSDOs (in fact, I am sure I can't), so that holdover will 
probably not be optimized (read: suck).

Also, I am not sure what happens when you move a timing receiver, I 
guess the Thunderbolt for instance would have to do a new survey, here 
goes an hour at best...
>   
>> I guess as a result, it will become more important to have an algorithm 
>> that effectively filters out outliers. Are there any such things as 
>> hanging bridges with carrier phase receivers? (I hate asking that 
>> question...)
>>     
>
> Well, the resolution of carrier phase measures is below the noise level, so
> whatever errors there would be will be noised out. Also, it is not static as
> the satellite orbit and speed of change will practically ensure that there is
> no hanging bridge, or at least not for very long. So, effectively no.
>
>   
Good
> I was kind of expecting that question, it was just a matter of time before it
> came up. :)
>
> Cheers,
> Magnus
>
>   
Well, hopefully we are done with this one :-)

Didier