[time-nuts] need recomendation for a portable 10mhz reference oscilator

[SAIDJACK at aol.com]

In a message dated 12/11/2007 22:00:37 Pacific Standard Time,  
bg at lysator.liu.se writes:

>I am  surprised surveying mode is needed anymore. In a SA environment  it
>makes sense, but for me its hard to understand with the current  accuracy
>given by the GPS system.

--

>    Björn




Hi Bjoern,
 
even with a very good M12M or M12+ receiver we will typically see some  
variation of the averaged position over it's ~3 hour (default) auto-survey  
interval. I've seen altitude change by a meter or two for example.
 
Those couple of nanoseconds may be insignificant when compared to  older GPS 
receiver performance (VP Oncore etc), but if one has the time, why  not get 
the best position average possible.
 
I did also note that it does make a difference if the Auto Survey is done  
during the day, at dawn, or during the night etc. Might be caused by multipath  
etc. A 12 hour or 24 hour Auto Survey may reduce the Diurnal effects.
 
We recently added a command to our Fury firmware that allows the user  to set 
any time from 1s to 10,000s for the Auto Survey process to finish.  This was 
done because the differences are so small between any given  single-second 
position, and an auto-surveyed position (less than 5 meters  or so). The error 
due to this is much less than 100ns which is more than  adequate for a lot of 
applications.
 
We have some customers that want the unit to do only 3 to 5 minutes of Auto  
Surveying for a particular mobile military application, and the resulting  
<<100ns UTC accuracy is way better than their requirement.
 
In that case the Auto Survey time is insignificant when compared to the  OCXO 
warmup time.
 
It all depends on your particular accuracy requirement.
 
bye,
Said
 
 



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