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

Peter Bell bell.peter at gmail.com
Sat Dec 17 23:32:36 EST 2011


Maybe they used a Cs standard for the original experimental units, but
the first commercial Transit unit I saw (Magnavox MX700?) just had a
big OCXO in it - it was also all controlled by a HP2100 computer and
output the fix data onto a teletype.

The MX1102/1107 (which were pretty much standard equipment on big
ships for years) used a rather smaller OCXO, and the MX4102 used a
high-grade TCXO.

Regards,

Pete


On Sun, Dec 18, 2011 at 10:30 AM, Brooke Clarke <brooke at pacific.net> wrote:
> 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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