[time-nuts] how good an oscillator do you need for a GPS simulator

Magnus Danielson magnus at rubidium.dyndns.org
Sat Dec 17 21:22:01 EST 2011

Hi Jim,

On 12/18/2011 01:25 AM, Jim Lux wrote:
> On 12/17/11 2:56 PM, Magnus Danielson wrote:
>> On 12/17/2011 09:57 PM, Jim Lux wrote:
>>> L1 C/A
>>> But the real question isn't how to generate the signals (that's straight
>>> forward).. it's "how good does the oscillator have to be" to effectively
>>> test the receiver, in the sense of measuring it's timing performance.
>> A decent OCXO should be able to pull it off. Your receiver should
>> long-term follow your OCXO. Take one of these 40 dollar rubidiums if you
>> are worried.
>> Any drift of a good OCXO will be way within the bandwidth of the GPS
>> channels. This drift would show up as added drift of the GPS oscillator,
>> which is then being tracked and compensated.
> that is precisely what I was thinking.. I was just wondering if anyone
> had run across a reason why it wouldn't be the case. (short of actually
> doing the no doubt tedious analysis)

I can offer you several evidence of this:

1) None of the GPS simulators come with very special oscillator, but you 
may hook up your cesium if you need to for some reason.

2) A typical channel bandwidth typically measures in the Hz range. 
Tracking drift would not be too hard.

3) While we consider for all practical matter GPS time is stable and the 
GPS internal reference has incorrect frequency complete with drift, the 
GPS receiver uses the time-solution of the position to continuously 
correct the time, frequency and drift of the TCXO (or OCXO). Now, if we 
move a little of frequency error and drift over to the "GPS time" of the 
GPS simulator the receiver won't be able to say as long as the GPS 
simulator reference isn't drifting like a maniac so that the correction 
routines can't keep up with it.

So, there is my rough analysis for you.


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