[time-nuts] Building a GPSDO & trouble using Jupiter-T

Magnus Danielson magnus at rubidium.dyndns.org
Tue Jan 31 19:44:11 EST 2012


On 01/02/12 01:29, Chris Albertson wrote:
> On Mon, Jan 30, 2012 at 6:17 PM, Didier Juges<shalimr9 at gmail.com>  wrote:
>> You have to spend good money to get a GPS receiver capable of calculating
>> it's time and/or position more than once per second. I am not aware of that
>> being done for timing applications, but it is available for navigation GPS
>> receivers, such as those used to track race cars (for a race car, one
>> second is an eternity). I have seen navigation receivers capable of 10
>> fixes/second, I am sure there are better ones yet. They cost a lot of money.
>
> I'm pretty sure those GPS recievers that send out more frequent data,
> at say 2Hz or 5Hz are just interpolating.  It is not more accurate.
> The GPS sats only send a frame once over 6 seconds.
>
> They send at higher rate so that the system using the GPS does not
> need to know how to dead reckon and can have decent results for simply
> using last reported position

The frames does not relate to the rate of raw-data or solutions. You 
could track every 1 ms, as it would align with the rate of a full C/A 
code cycle. Typically these are integrated into complete sub-code 
symbols, of 20 ms or 50 bauds, so it is not unfair to see that rate of 
raw-data. The solution can be run as often if CPU time for all the 
calculations are there. The ephemeris data is designed such that it 
doesn't need to change often and is re-transmitted regularly, so those 
bits isn't immediately used for navigation solution by necessity. Their 
phase is however important.

Cheers,
Magnus




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