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

[Brooke Clarke]

Hi Magnus:

Exactly.  The main problem with the Transit system was that the receiver needed a Cs clock for the system to work at 
all.  GPS removed that requirement.
It's my understanding that a GPS receiver that uses a Cs clock has much more capability.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Brooke4Congress.html


Magnus Danielson wrote:
> 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.
>
> Cheers,
> Magnus
>
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